Phosphate solubilizing bacteria and their role in plant growth promotion

Phosphate-solubilizing bacteria facilitate rhizospheric processes of Bidens pilosa L. in the phytoremediation of cadmium-contaminated soil: Link between phosphorus availability and cadmium accumulation

Although cadmium (Cd) hyperaccumulators have been widely used in phytoremediation of Cd-contaminated soils, the relationship between soil phosphorus (P) uptake and Cd accumulation during phytoremediation remains unclear. In this study, a phosphate-solubilizing bacterium (PSB), Enterobacter sp., and the Cd hyperaccumulator B. pilosa L. were selected to address this knowledge gap. Our results show that Enterobacter sp. inoculation enhances P cycling processes in the rhizosphere of B. pilosa L., resulting in an increase in soil available phosphorus (AP), by 16.2-84.3 % in low-contaminated soil and by 17.6-64.8 % in high-contaminated soil. Inorganic P solubilization was the primary process driving the increase in AP content, contributing the most to soil P cycling. Moreover, Enterobacter sp. inoculation significantly promoted the growth of B. pilosa L., boosting total phosphorus, phospholipids, primary metabolic phosphorus, and Cd concentrations in plant tissues. Notably, a strong positive correlation was observed between soil AP and Cd concentrations in plant tissues. P-functional microbes in the rhizosphere, encoding genes such as gcd, ppa, and ppx-gppA, predominantly enhance P bioavailability in soils. Furthermore, in P-deficient and heavily contaminated soils, Proteobacteria replaced Actinobacteria as the predominant hosts for key genes involved in soil P cycling. This study provides valuable insights into the critical link between P availability and Cd accumulation, emphasizing the role of P cycling in enhancing Cd accumulation during phytoremediation mediated by PSB.

A new isolation methodology for phosphate solubilizing bacteria using a step-wise enrichment process

AIMS

Phosphate solubilizing bacteria (PSB) screened for their ability to solubilize Ca3(PO4)2 in a laboratory may not be effective in solubilizing phosphorus (P) in soils that contain different forms of P. We developed an efficient isolation method to obtain PSB from soils collected from diverse soil types that vary in P forms.

METHODS AND RESULTS

PSB isolated through enrichment in media containing AlO4P, Ca3(PO4)2 or FeO4P as sole P sources were compared with a previous collection of Pseudomonas- and Bacillus-like bacteria that solubilized P. There was a significant positive association (p < 0.05) between the number of culturable PSB and several properties from the original soils, including calcium, magnesium, potassium, total N, P, carbon concentrations, electrical conductivity, and silt content. The number of culturable PSB was inversely related to the sand content of the soil. Additionally, the most efficient PSB were obtained from sites with a high aridity index and from alkaline soils. After enrichment, there was a significant increase in PSB among all culturable bacteria. Isolates obtained after step-wise enrichment solubilized 1.8 to 2.2, 1.4 to 2.9, and 2.2 to 3.3 times more P from Ca3(PO4)2, AlO4P, and FeO4P, respectively, compared with strains selected using a taxonomic method. Isolates enriched using FeO4P produced predominantly tri- and di-carboxylates in vitro. Additionally, we identified an efficient P solubilizer that is closely related to Pseudomonas granadensis, a species not previously reported as a P solubilizer.

CONCLUSION

Using a step-wise enrichment method in culture media with sparingly soluble P increased the likelihood of isolating high-efficiency PSB from soil.

Characterization of halotolerant Kushneria isolates that stimulate growth of alfalfa in saline conditions

A key barrier to crop production is soil salinity, which is a serious and growing problem world-wide due to inadequate water drainage, saline ground water, or inadequate rainfall to wash away soil salts. There is substantial promise for plant-associated microbes isolated from halophytes (salt-tolerant plants) to enhance growth of salt-sensitive crop plants in salty soils. The objective of this study was to identify salt-tolerant bacteria from native halophytes and characterize their ability to stimulate the growth of alfalfa in salty soil conditions. Several halotolerant bacteria, including Kushneria, Halomonas, and Bacillus, were identified from the rhizosphere or roots of three halophyte species (Salicornia rubra, Sarcocornia utahensis, and Allenrolfea occidentalis) in a saline area south of Utah Lake, Utah, USA. Biochemical properties, including indole acetic acid production, biofilm formation, phosphate solubilization and siderophore production activities, which have been associated with plant growth promoting (PGP) activity, were characterized for several isolates. Selected strains were screened for the ability to stimulate growth of alfalfa in controlled laboratory experiments. Among these strains, two independent isolates of the genus Kushneria were found to have significant growth-promoting activity for inoculated alfalfa plants grown under saline conditions (0.205 M or 1.2% NaCl) that mimic common salinity levels of affected soils. Plants inoculated with a combination of two Kushneria strains that have salt-tolerant PGP (ST-PGP) properties exhibited a statistically significant increase in plant growth over uninoculated plants. A GFP marker confirmed presence of Kushneria in the roots of inoculated plants. Bacteria with ST-PGP activity will be a key resource to facilitate increased crop yield from land affected by salinity, and the data presented here for two Kushneria isolates are promising.

Organic acid release and microbial community assembly driven by phosphate-solubilizing bacteria enhance Pb, Cd, and As immobilization in soils remediated with iron-doped hydroxyapatite

Although iron-doped hydroxyapatite (Fe-HAP) and its composites have been reported to immobilize arsenic (As), lead (Pb), and cadmium (Cd), its practical application is limited by the inefficient release of iron and phosphate. In this study, Ochrobactrum anthropic, a phosphate-solubilizing bacterium isolated from a lead-zinc smelting site, was employed to enhance multi-heavy metal immobilization in Fe-HAP-amended soils. O. anthropic secreted low-molecular-weight organic acids, promoted phosphate (25.6 mg/L) and iron (14.2 mg/L) release from Fe-HAP, and minimally disrupted native bacteria. Compared to CK, the combination of 2 % O. anthropic (v/w) and Fe-HAP (Fe-to-HAP ratio of 1:1) significantly increased the residual fractions of Cd, Pb, and As by 109.09 %, 49.21 %, and 25.00 %, respectively. The combined treatment also improved available phosphorus, available nitrogen, and acid phosphatase activity by 233.24 %, 196.55 %, and 246.45 %, respectively. Furthermore, O. anthropic facilitated the recruitment of phylum Firmicutes and genera Acidovorax, Sedimentibacter, and Brevundimonas, shifting the bacterial community from specialists to generalists. Positive correlations were observed between residual fractions of Pb, Cd, As, well-crystallized As, and the abundance of Firmicutes, Acidovorax, and Sedimentibacte. These findings demonstrate the potential of an O. anthropic-driven Fe-HAP remediation strategy for the eco-friendly restoration of barren and polymetallic-contaminated soils.

Synergistic interaction of phosphate nanoparticles from fish by-products and phosphate-solubilizing bacterial consortium on maize growth and phosphorus cycling

Phosphate nanomaterials, such as hydroxyapatite/β-tricalcium nanoparticles (nHAs) derived from food industry by-products, offer a sustainable alternative to enhance P-use efficiency in agriculture. However, their limited solubility remains a challenge. This study first investigated the mechanisms of P solubilization of salmon and tuna bones (SnHAs and TnHAs) in fifteen strains of phosphate-solubilizing bacteria (PSB) by an in vitro system. Then, best-performing strains were assembled in a consortium and tested in vivo on maize. We hypothesized that combining nHAs and the PSB consortium inoculated as seed coating (SC) outperforms single treatments alone in promoting plant growth and P cycling, and ensures the establishment in plant-soil system without a bacterial reinforcement (BR) by an additional inoculum suspension. The synergistic effect of nHAs and PSB was proved, improving maize root (+22 %) and total plant biomass (+29 %), as well as P (+32 %) and K (66 %) uptake compared to single treatments. With nHAs and SC, P-use efficiency and recovery increased by 25 % and three-fold, respectively, compared to nHAs alone or with bacterial reinforcement. Consistently, root and substrate bacterial biomass were associated with nHAs plus SC, while nHAs alone or with PSB upregulated PHT1;1 and PHT1;2 transporter genes in maize. Finally, linking the in vitro and in vivo system, we demonstrated that propionic acid production and P-solubilization efficiency of PSB co-applied with nHAs are key drivers of maize growth and P uptake. Our findings indicated that co-applying nHAs and PSB through SC offers a sustainable strategy to improve maize P-use efficiency.

Taxonomic description of Micromonospora reichwaldensis sp. nov. and its biosynthetic and plant growth-promoting potential

Micromonospora strains proved to be a model organism for drug discovery and plant growth promotion (PGP). Strain DSM 115977 T was subjected to polyphasic taxonomic analysis and genome mining for biosynthetic gene clusters and PGP-associated genes in order to determine its taxonomic rank and assess its biosynthetic potential. The strain was found to form a novel species within the evolutionary radiation of the genus Micromonospora. The strain contained glucose, mannose, xylose, and ribose as whole-cell sugars and the isomer DL-diaminopimelic acid in its peptidoglycan. Strain DSM 115977T had iso-C15:0, iso-C16:0, C17:1cis 9, C17:0, iso-C17:0, and 10-methyl-C17:0 as fatty acid profile (>5%) and MK10-H4 and MK10-H6 as the predominant menaquinones (>10%). The polar lipid profile consisted of diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol, glycophosphatidylinositol, glycophospholipids, phosphoaminolipid, unidentified lipids, and phospholipids. The genome of the strain had a size of 7.0 Mbp with a DNA G + C content of 73.4%. It formed a well-supported sub-clade with its close phylogenomic neighbor, Micromonospora echinofusca DSM 43913T (98.7%). Digital DNA-DNA hybridization and average nucleotide identity derived from sequence comparisons between the strain and its close phylogenomic neighbors were below the thresholds of 70 and 95-96% for prokaryotic species demarcation, respectively. Based on these findings, strain DSM 115977T (Asg4T = KCTC 59188T) merits to be considered as the type strain of a new species for which the name Micromonospora reichwaldensis sp. nov. is proposed. Genome mining for biosynthetic gene clusters encoding specialized secondary metabolites highlighted its ability to produce potentially novel therapeutic compounds. The strain is rich in plant growth-promoting genes whose predicted products directly and indirectly affect the development and immune system of the plant.

IMPORTANCE

In view of the significant pharmaceutical, biotechnological, and ecological potentials of micromonosporae, it is particularly interesting to enhance the genetic diversity of this genus by focusing on the isolation of novel strain from underexplored habitats, with the promise that novel bacteria will lead to new chemical entities. In this report, modern polyphasic taxonomic study confirmed the assignment of strain DSM 115977T to a novel species for which the name Micromonospora reichwaldensis sp. nov. is proposed. The strain harbors in its genomic sequence several biosynthetic gene clusters for secondary metabolites and genes associated with plant growth-promoting features. The results of this study provide a very useful basis for launching more in-depth research into agriculture and/or drug discovery.

Functional and Genomic Potential of Burkholderia contaminans PB_AQ24 Isolate for Boosting the Growth of Bamboo Seedlings in Heavy Metal Contaminated Soils

The present study investigated the genomic and functional potential of Burkholderia contaminans PB_AQ24, a bacterial strain isolated from the municipal solid waste dumpsite, for boosting the growth of Dendrocalamus strictus (Male bamboo) seedlings. The isolated strain exhibited high potency for metal solubilization and ACC (1-Aminocyclopropane-1-carboxylate) deaminase activity. Its genome harbored diverse genes responsible for nitrogen and phosphorus utilization (trpABCDES, iaaH, acdS, pstABCS, phoAUD, pqqABCDE, kdpABC, gln, and nirBD) and also an abundance of heavy metal tolerant genes (ftsH, hptX, iscX-fdx-hscAB-iscAUR, mgtA, corA, and copC). Seeds priming experiments carried out in heavy metal contaminated soils (such as waste dumpsite soil (WDS), fly ash dumpsite soil (FAS) and natural garden soil (NGS control)) augmented with Burkholderia contaminans sp. PB_AQ24 showed 85% sustenance of seedlings in WDS and 80% in FAS. The study thus highlighted the potential features in isolated bacterial strain Burkholderia sp. PB_AQ24 (NCBI accession no. JAQOUG000000000), which could boost the growth of bamboo seedlings in heavy metal contaminated soils and may be applied for restoration and rejuvenation of contaminated sites.

Isolation, Characterization and Growth-Promoting Properties of Phosphate-Solubilizing Bacteria (PSBs) Derived from Peach Tree Rhizosphere

Microbial fertilizers have a significant role in promoting plant growth, resistance to environmental stresses, and soil remediation. Microbial fertilizers are mainly composed of beneficial microorganisms that contain specific functions. Focusing on the peach tree rhizosphere region, this study aimed to isolate and screen bacteria with efficient phosphate-solubilizing capacity for application in microbial fertilizers, as well as to dig deeper into the other properties of the strains to further explore the roles of these phosphate-solubilizing bacteria (PSBs) in terms of plant growth in order to provide valuable microbial resources for microbial fertilizer development. By collecting soil samples from peach tree rhizospheres, we initially screened 86 PSB strains using the plate method and determined the phosphate-solubilizing capacity (ranged from 0 to 14 μg/mL). Afterwards, 51 strains with strong phosphate-solubilizing capacity were selected for molecular identification; the strains belonged to 12 genera, with Bacillus and Burkholderia accounting for the majority. Concurrent evaluation of iron carriers and indoleacetic-3-acid (IAA) production capabilities identified strain WPD85 as exhibiting dual functionality with strong performance in both parameters. Subsequently, we combined the analysis of phosphate-solubilizing capacity and growth-promoting properties to select eight strains of PSBs; characterized them physiologically, biochemically, and molecularly; determined the biofilm-forming capacity; and conducted potting experiments. Notably, strain WPD103 exhibited exceptional biofilm-forming capacity (OD595 = 1.09). Of particular interest, strain WPD16 demonstrated both an elevated inorganic phosphate solubilization index (D/d = 2.99) and remarkable iron carriers production capacity, while peach seedlings treated with WPD16 exhibited 119% enhancement in plant height increment compared to the control. This study enhances our understanding of PSB traits and identifies Burkholderia sp. WPD16 as a strategic candidate for developing targeted microbial fertilizers, offering a sustainable solution to reduce reliance on chemical inputs in orchard management.

An Analysis of the Gut Microbiota of Fifth-Instar Antheraea Pernyi Larvae and a Functional Exploration of a Bacillus Subtilis Strain

This experiment investigated the role of the gut microbiota in the growth and development of Antheraea pernyi, isolated gut bacteria related to lignin degradation in oak trees, and preliminarily verified their lignin degradation ability, providing a new tool for the field of biodegradation. The changes in the structure of the gut microbial community of the oak silkworm Antheraea pernyi at different developmental stages were analyzed via high-throughput sequencing of the 16SrDNA genes of the gut microbiota of fifth-instar larvae. A series of biological analyses were conducted, including amplicon sequence variant (ASV), microbial community diversity, microbial community composition, species difference, and evolutionary analyses. Ralstonia, Achromobacter, Pseudomonas, Cupriavidus, Bacteroidetes, Enterococcus, and Bacillus were the dominant bacterial groups in the gut microbiota of the larvae, with Ralstonia comprising the highest proportion, thereby providing a reference basis for a deeper understanding of the functions of the gut microbiota in insect growth and development. In this study, we activated and identified the MYZ028 strain isolated from the fifth-instar larvae of Antheraea pernyi through molecular biology, and we determined that the strain belonged to Bacillus subtilis. In terms of performance, the strain exhibited a high lignin-degrading ability and could effectively degrade lignin in a short time and simple environment. Due to the complex and difficult nature of degrading the lignin structure, research on the degradation of lignin by composite strains can be explored in the field of biodegradation.

Endophytic bacteria-assisted cadmium removal in sunflower stalks: towards safe biomass recycling

Cadmium (Cd) contamination in agricultural soils is one of the major threats to food safety and environmental health. In a phytoremediation program for the extraction of Cd by plants, one critical challenge is the management of harvested biomass because of its highly contaminant content. This study investigates the use of endophytic bacteria to assist in Cd removal from sunflower stalks, aiming to make the biomass safer for reuse as fertilizer. Sixteen endophyte strains were isolated from sunflower plants grown in Cd-contaminated soils, out of which two strains, J14 and J15, namely Enterobacter roggenkampii and Kosakonia cowanii, respectively, showed the most potential for Cd removal. Under the optimized conditions, 42.03% and 37.99% Cd removal efficiency could be achieved by J14 and J15, respectively. More than 50% of some specific forms of Cd (F2, F4, F5) in sunflower stalks can be reduced during extraction. Importantly, the treatment with endophytes lowered Cd in contaminated biomass without significant reductions in the major plant nutrients (nitrogen, phosphorus, and potassium), thus rendering it safe for its reuse as fertilizer. This study offers a novel perspective on biomass contamination in phytoremediation, suggesting a new environmentally friendly approach for the recycling of polluted plant material towards safer and more economic phytoremediation practices.

Endophytes as bioenhancers of plant growth: An overview

The need for food production rises with the era of expanding population. As a result, there is more indirect demand for chemical pesticides and fertilizers. Serious environmental concerns result from the continuous and careless usage of chemicals. Additionally, they could make the land infertile. One of the finest substitutes for chemicals is to use microorganisms, particularly endophytes. Endophytes uses both direct and indirect mechanisms to encourage plant growth by increased mineral availability, resilience to biotic and abiotic stresses, synthesis of significant phytohormones. This review is focused on exploring the plant growth promoting effect of endophytes and its potential implications in the crop production.

Non-Rhizobial Endophytes (NREs) of the Nodule Microbiome Have Synergistic Roles in Beneficial Tripartite Plant-Microbe Interactions

Microbial symbioses deal with the symbiotic interactions between a given microorganism and another host. The most widely known and investigated microbial symbiosis is the association between leguminous plants and nitrogen-fixing rhizobia. It is one of the best-studied plant-microbe interactions that occur in the soil rhizosphere and one of the oldest plant-microbe interactions extensively studied for the past several decades globally. Until recently, it used to be a common understanding among scientists in the field of rhizobia and microbial ecology that the root nodules of thousands of leguminous species only contain nitrogen-fixing symbiotic rhizobia. With the advancement of molecular microbiology and the coming into being of state-of-the-art biotechnology innovations, including next-generation sequencing, it has now been revealed that rhizobia living in the root nodules of legumes are not alone. Microbiome studies such as metagenomics of the root nodule microbial community showed that, in addition to symbiotic rhizobia, other bacteria referred to as non-rhizobial endophytes (NREs) exist in the nodules. This review provides an insight into the occurrence of non-rhizobial endophytes in the root nodules of several legume species and the beneficial roles of the tripartite interactions between the legumes, the rhizobia and the non-rhizobial endophytes (NREs).

Four Decades of Bacillus Biofertilizers: Advances and Future Prospects in Agriculture

Over the past four decades, Bacillus biofertilizers, which are microbial formulations based on Bacillus species, have significantly contributed to sustainable agriculture by enhancing crop growth, improving soil health, and reducing the dependency on chemical fertilizers. Bacillus species, particularly known for their ability to promote plant growth, fix nitrogen, solubilize phosphorus, and produce growth-promoting substances such as phytohormones and antibiotics, have emerged as key players in the development of eco-friendly agricultural solutions. This research utilizes bibliometric analysis based on 3,242 documents sourced from the Web of Science database to map the development, key contributions, and innovation within the field from 1985 to 2023. This study identifies exponential growth in research output, particularly from 2003 onwards, indicating a robust interest and expanding research base predominantly in China, India, and the United States. We segmented the research timeline into three distinct phases, each marked by varying growth rates and research foci. This paper presents novel insights into the geographical and institutional distributions of research, highlighting the predominant role of developing countries in advancing Bacillus-based technologies. Key research hotspots have evolved from basic applications to complex interactions involving synthetic microbial communities and advanced multi-omics techniques. Our findings demonstrate a trend towards more strategic and technologically integrated approaches to developing Bacillus biofertilizers, reflecting broader shifts towards more sustainable agricultural systems. This study not only charts historical progress, but also proposes future research trajectories aimed at enhancing the application and effectiveness of microbial fertilizers across diverse ecosystems.

Insights from a Genome-Wide Study of Pantoea agglomerans UADEC20: A Promising Strain for Phosphate Solubilization and Exopolysaccharides Production

The genome sequence of Pantoea agglomerans UADEC20 is presented, which is a strain isolated from agricultural fields in northeast Mexico. The genome was assembled into 13 scaffolds, constituting a total chromosome size of 4.2 Mbp, with two of the scaffolds representing closed plasmids. The strain exhibits activity in phosphate solubilization and exopolysaccharide (EPS) production and secretion; therefore, we explored its biotechnological potential via its genome sequencing and annotation. Genomic analyses showed that a total of 57 and 58 coding sequences (CDSs) related to phosphate solubilization and EPS production were identified within its genome, in addition to a reduced number of CDSs related to drug resistance and phages. The comprehensive set of genes supporting phosphate solubilization, EPS synthesis, and secretion, along with its low virulence and antibiotic resistance levels, justify further research for its potential biotechnological application and possible use as a plant growth-promoting agent in the field. These findings suggest a unique genetic background in the P. agglomerans UADEC20 strain.

Effectiveness of salt priming and plant growth-promoting bacteria in mitigating salt-induced photosynthetic damage in melon

Seed priming and plant growth-promoting bacteria (PGPB) may alleviate salt stress effects. We exposed a salt-sensitive variety of melon to salinity following seed priming with NaCl and inoculation with Bacillus. Given the sensitivity of photosystem II (PSII) to salt stress, we utilized dark- and light-adapted chlorophyll fluorescence alongside analysis of leaf stomatal conductance of water vapour (Gsw). Priming increased total seed germination by 15.5% under salt-stress. NaCl priming with Bacillus inoculation (PB) increased total leaf area (LA) by 45% under control and 15% under stress. Under the control condition, priming (P) reduced membrane permeability (RMP) by 36% and PB by 55%, while under stress Bacillus (BS) reduced RMP by 10%. Although Bacillus inoculation (B) and priming (P) treatments did not show significant effects on some PSII efficiency parameters (FV/FM, ABS/RC, PIABS, FM), the BS treatment induced a significantly higher quantum efficiency of PSII (ΦPSII) and increased Gsw by 159% in the final week of the experiment. The BS treatment reduced electron transport rate per reaction center (ETO/RC) by 10% in comparison to the salt treatment, which showed less reaction centre damage. Bacillus inoculation and seed priming treatment under the stressed condition (PBS) induced an increase in electron transport rate of 40%. Salt stress started to show significant effects on PSII after 12 days, and adversely impacted all morphological and photosynthetic parameters after 22 days. Salt priming and PGPB mitigated the negative impacts of salt stress and may serve as effective tools in future-proofing saline agriculture.

Genome mining of Bacillus thuringiensis strain SY49.1 reveals novel candidate pesticidal and bioactive compounds

BACKGROUND

Bacillus thuringiensis SY49.1 (Bt SY49.1) strain has promising insecticidal and fungicidal activity against phytopathogens and pests. Therefore, we selected this strain for whole-genome sequencing (WGS), annotation and analysis, with the aim of identifying genes responsible for producing putative pesticidal toxins, antimicrobial metabolites and plant growth-promoting features.

RESULTS

Our results showed that the SY49.1 genome is 6. 32 Mbp long with a GC content of 34.68%. Genome mining revealed the presence of multiple gene inventories for the biosynthesis of bioactive compounds such as insecticidal delta endotoxins, secondary metabolites, and several plant growth-promoting proteins. Multiple sequence alignment revealed residue variations in the toxic core of Cry1Ab when compared with known Cry1Ab sequences from Bt nomenclature databases. This suggests that the cry1Ab of SY49.1 is a new kind of its group. Among the predicted secondary metabolites, we found a kurstakin with a predicted peptide that differs from the known kurstakin peptide available in the NORINE database. In addition, lipopeptides extracted from SY49.1 suppressed the growth of Verticillium dahliae and Fusarium oxysporum.

CONCLUSION

We anticipate that the complete genome of Bt SY49.1 may provide a model for properly understanding and studying antimicrobial compound mining, genetic diversity among the B. cereus group, and pathogenicity against insects. This is the first report on the WGS and mining of the Bt strain isolated from Turkey. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Amplicon Sequencing Analysis of Submerged Plant Microbiome Diversity and Screening for ACC Deaminase Production by Microbes

Submerged plants can thrive entirely underwater, playing a crucial role in maintaining water quality, supporting aquatic organisms, and enhancing sediment stability. However, they face multiple challenges, including reduced light availability, fluctuating water conditions, and limited nutrient access. Despite these stresses, submerged plants demonstrate remarkable resilience through physiological and biochemical adaptations. Additionally, their interactions with microbial communities are increasingly recognized as pivotal in mitigating these environmental stresses. Understanding the diversity of these microbial communities is crucial for comprehending the complex interactions between submerged plants and their environments. This research aims to identify and screen microbes from submerged plant samples capable of producing 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase and to explore microbial diversity through metagenomic analysis. Microbes were isolated and screened for ACC deaminase production, and metagenomic techniques, including co-occurrence network analysis, were used to examine microbial diversity and interactions within the communities. ACC deaminase-producing microbes can significantly enhance plant metabolism under stress conditions. The identification of the culturable bacteria revealed that most of these microbes belong to the genera Pseudomonas, Bacillus, and Acinetobacter. A total of 177 microbial strains were cultured, with molecular identification revealing 79 reductant, 86 non-reductant, and 12 uncultured strains. Among 162 samples screened for ACC deaminase activity, 50 tested positive. To further understand microbial dynamics, samples were collected from both natural sources and artificial pond reservoirs to assess the impact of the location on flood-associated microbiomes in submerged plants. Metagenomic analysis was conducted on both the epiphytic and endophytic samples. By exploring the overall composition and dynamics of microbial communities associated with submerged plants, this research seeks to deepen our understanding of plant-microbe interactions in aquatic environments. The microbial screening helped to identify the diverse microbes associated with ACC deaminase activity in submerged plants and amplicon sequencing analysis paved the way towards identifying the impact of the location in shaping the microbiome and the diversity associated with endophytic and epiphytic microbes. Co-occurrence network analysis further highlighted the intricate interactions within these microbial communities. Notably, ACC deaminase activity was observed in plant-associated microbes across different locations, with distinct variations between epiphytic and endophytic populations as identified through co-occurrence network analysis.

Enhancing Phosphate Uptake and Antifungal Activity in Tomato Plants via Bacillus licheniformis Mutagenesis: Evaluating Growth Parameters

The objective of the investigation was to improve phosphate solubilization in tomato plants by Bacillus licheniformis, a rhizobacterium that promotes plant growth. Ultraviolet (UV) radiation, Ethyl methanesulfonate (EMS) and Ethidium bromide (EtBr) mutagenesis produced twenty-one mutants. Phosphate solubilization was higher in the PM7 (physical mutant) (121.00 g mL-1) than in the wild type (82.00 g mL-1). PM7 showed high antifungal activity against Phytophthora capsici, Fusarium oxysporum and Dematophora necatrix besides increased siderophore production and HCN production. In a net-house experiment, PM7 improved root and shoot parameters, P assimilation and soil P availability in tomato plants. This study demonstrates the potential of PM7 as an effective rhizobacterium for enhancing nutrient availability and plant growth.

Enhancing carrot (Daucus carota var. sativa Hoffm.) plant productivity with combined rhizosphere microbial consortium

Background

Plant growth-promoting rhizobacteria (PGPR) are an integral part of agricultural practices due to their roles in promoting plant growth, improving soil conditions, and suppressing diseases. However, researches on the PGPR in the rhizosphere of carrots, an important vegetable crop, is relative limited. Therefore, this study aimed to isolate and characterize PGPR strains from the rhizosphere soil of greenhouse-grown carrots, with a focus on their potential to stimulate carrot growth.

Methods

Through a screening process, 12 high-efficiency phosphorus-solubilizing bacteria, one nitrogen-fixing strain, and two potassium-solubilizing strains were screened. Prominent among these were Bacillus firmus MN3 for nitrogen fixation ability, Acinetobacter pittii MP41 for phosphate solubilization, and Bacillus subtilis PK9 for potassium-solubilization. These strains were used to formulate a combined microbial consortium, N3P41K9, for inoculation and further analysis.

Results

The application of N3P41K9, significantly enhanced carrot growth, with an increase in plant height by 17.1% and root length by 54.5% in a pot experiment, compared to the control group. This treatment also elevated alkaline-hydrolyzable nitrogen levels by 72.4%, available phosphorus by 48.2%, and available potassium by 23.7%. Subsequent field trials confirmed the efficacy of N3P41K9, with a notable 12.5% increase in carrot yields. The N3P41K9 treatment had a minimal disturbance on soil bacterial diversity and abundance, but significantly increased the prevalence of beneficial genera such as Gemmatimonas and Nitrospira. Genus-level redundancy analysis indicated that the pH and alkali-hydrolyzable nitrogen content were pivotal in shaping the bacterial community composition.

Discussion

The findings of this study highlight the feasibility of combined microbial consortium in promoting carrot growth, increasing yield, and enriching the root environment with beneficial microbes. Furthermore, these results suggest the potential of the N3P41K9 consortium for soil amelioration, offering a promising strategy for sustainable agricultural practices.

Exploring microbial diversity and interactions for asbestos modifying properties

Asbestos poses a substantial environmental health risk, and biological treatment offers a promising approach to mitigate its impact by altering its chemical composition. However, the dynamics of microbial co-inoculation in asbestos bioremediation remain poorly understood. This study investigates the effect of microbial single cultures and co-cultures on modifying crocidolite and chrysotile fibers, focusing on the extraction of iron and magnesium. Seventy bacterial and eighty-three fungal strains were isolated from five diverse sites, characterized phylogenetically using the 16S rRNA gene and ITS region, respectively, and assessed for siderophore and organic acid production. Most bacterial strains were identified as Pseudomonas, while Penicillium predominated among fungal strains. Ten bacterial and 25 fungal strains were found to produce both organic compounds. Four microbial co-cultures (one bacterium-bacterium, two fungus-bacterium, and one fungus-fungus) exhibiting synergistic effects in plate assays, alongside their respective single cultures, were incubated with crocidolite and chrysotile. ICP-OES analysis revealed that in crocidolite, the co-culture HRF19-HRB12 removed more iron than their single cultures, while Penicillium TPF36 showed the highest iron removal. The co-culture of two Pseudomonas strains (HRB12-RB5) exhibited the highest magnesium concentration in the supernatant. In chrysotile, the co-culture HRB12-RB5 removed more iron than their individual cultures, with Penicillium TFSF27 exhibiting the highest iron concentration in the solution. Penicillium TFSF27 and the co-culture TFSF27-TPF36 demonstrated the highest magnesium removal. SEM-XRMA analysis showed a significant reduction in iron and magnesium content, confirming elemental extraction from the fibers' structure. This study significantly broadens the range of microbial strains capable of modifying asbestos fibers and underscores the potential of microbial co-cultures in asbestos remediation.

Microbial enzymatic indices for predicting composting quality of recalcitrant lignocellulosic substrates

Three different enzymes alkaline phosphatase, Urease and Dehydrogenase were measured during this study to monitor the organic matter dynamics during semi-industrial composting of mixture A with 1/3 sludge+2/3 palm waste and mixture B with ½ sludge+1/2 palm waste. The phosphatase activity was higher for Mix-A (398.7 µg PNP g-1 h-1) than Mix-B (265.3 µg PNP g-1 h-1), while Mix-B (103.3 µg TPF g-1d-1) exhibited greater dehydrogenase content than Mix-A (72.3 µg TPF g-1 d-1). That could contribute to the dynamic change of microbial activity together with high amounts of carbonaceous substrates incorporated with the lignocellulosic. The gradual increase in the dehydrogenase from the compost Mix-A implies that high lignocellulosic substrate requires gradual buildup of dehydrogenase activity to turn the waste into mature compost. A higher pick of urease with a maximum activity of 151.5 and 122.4 µg NH4-N g-1 h-1 were reported, respectively for Mix-A and B. Temperature and pH could also influence the expression of enzyme activity during composting. The machine learning well predicted the compost quality based on NH3/NO3, C/N ratio, decomposition rate and, humification index (HI). The root mean square error (RMSE) values were 1.98, 1.95, 4.61%, and 4.1 for NH+ 3/NO- 3, C/N ratio, decomposition rate, and HI, respectively. The coefficient of determination between observed and predicted values were 0.87, 0.93, 0.89, and 0.94, for the r NH3/NO3, C/N ratio, decomposition rate, and HI. Urease activity significantly predicted the C/N ratio and HI only. The profile of enzymatic activity is tightly linked to the physico-chemical properties, proportion of lignocellulosic-composted substrates. Enzymatic activity assessment provides a simple and rapid measurement of the biological activity adding understunding of organic matter transformation during sludge-lignocellulosic composting.

Isolation and characterization of non-rhizobial bacteria and arbuscular mycorrhizal fungi from legumes

This study investigates non-rhizobial endophytic bacteria in the root nodules of chickpea (Cicer arietinum L), faba bean (Vicia faba), and cowpea (Vigna unguiculata L. Walp), as well as arbuscular mycorrhizal fungi in the rhizospheric soil of chickpea and faba bean. Out of the 34 endophytic bacterial populations examined, 31 strains were identified as non-rhizobial based on nodulation tests. All strains were assessed for their plant growth-promoting (PGP) activities in vitro. The results revealed that most isolates exhibited multiple PGP activities, such as nitrogen fixation, indole-3-acetic acid (IAA) and ammonia (NH3) production, phosphate solubilization, and exopolysaccharide production. The most effective PGP bacteria were selected for 16S rRNA analysis. Additionally, a total of 36 species of native arbuscular mycorrhizal fungi (AMF) were identified. Acaulospora (100%) and Scutellospora (91.66%) were the most prevalent genera in Cicer arietinum L. and Vicia faba L. plants, respectively. Acaulospora also exhibited the highest spore density and relative abundance in both plants. Moreover, the root colonization of Cicer arietinum L. and Vicia faba L. plants by hyphae, vesicles, and arbuscules (HVA) was significant. The findings of this study provide valuable insights into non-rhizobial endophytic bacteria associated with legume root nodules and the diversity of AMF. These organisms have great potential for PGP and can be manipulated by co-inoculation with rhizobia to enhance their biofertilizer effectiveness. This manipulation is crucial for promoting sustainable agriculture, improving crop growth, and advancing biofertilizer technology.

Looking for Pathogens in Dust from North Africa Arriving in the French West Indies Using Metabarcoding and Cultivable Analysis

Periodically, the French West Indies receive dust originating from North Africa (NA). Microorganisms associated with desert dust can be transported over long distances through the atmosphere and could represent a means for the remote colonization of new habitats by putatively pathogenic microorganisms. The aim of this study was to determine the diversity and frequency of microbial agents (bacteria, eukaryotes) in NA dusts and the potential threat toward human and/or animal health by comparing microbial air composition during dust events and in control samples. In 2017 and 2018, 16 samples were collected during seven NA dust episodes and there were 9 controls. The microbial composition of the samples was characterized using a cultivable approach and by metabarcoding analyses (16S and 18S). A greater bacterial load and greater diversity were observed during the dust events, and some genera were significantly associated with the events. Some, such as Geodermatophilus, can be considered signature species of NA dust. No pathogenic species were found with the cultivable approach, whereas the metabarcoding analyses highlighted the presence of several potentially pathogenic species or known human pathogens such as Naegleria fowleri.

How do high phosphate concentrations affect soil microbial communities after a century of ecosystem self-reclamation?

The use of rock phosphate (RP) instead of soluble phosphate fertilizers is preferred for the development of more sustainable agriculture. However, the impact of high concentrations in RP on bacterial and fungal communities remains poorly documented. Thus, next-generation sequencing was used to characterize bacterial and fungal communities in the soils and roots of four plant species growing naturally in a self-restored ecosystem, on former open-pit phosphate mines where past exploitation generated locally a substantial phosphate enrichment of the soil. Our results show that bacterial communities are dominated by Actinobacteria and Proteobacteria phyla, while the Ascomycota and Basidiomycota phyla predominate in the fungal community. The alpha and beta diversities of both bacterial and fungal communities differ significantly between the root and soil compartments but are not significantly affected by RP inputs. However, Amplicon Sequence Variants (ASVs) indicative of RP-enriched soils have been identified; among them are bacteria representative of Streptomyces, Bacillus, Mycobacterium or Agromyces. Implications of these results open new ways of reflection to understand the microbial response following RP-inputs and long-term soil restoration, as well as to formulate microbial-based bioinoculants for sustainable agriculture applications based on microorganisms better adapted to high concentrations of RP.

Isolation and Characterization of Plant-Growth-Promoting Bacteria Associated with Salvinia auriculata Aublet

Salvinia auriculata Aublet is a floating aquatic plant, capable of absorbing the excess of nutrients and water contaminants and can be used in effluent treatment plants. The ability to survive in degraded areas may be related to the association with beneficial bacteria capable of promoting plant growth. However, little is known about the microbiota associated with this aquatic plant and its potential application to the aquatic environment. In this sense, this work aims to identify bacteria associated with S. auriculata that could be able to promote plant growth. Eighteen bacterial strains were identified by sequencing of the 16S rRNA gene, belonging to the genera Agrobacterium, Bacillus, Curtobacterium, Enterobacter, Pseudomonas, Siccibacter, and Stenotrophomonas. All isolates produced indole compounds, 12 fixed N2, and 16 solubilized phosphate. A new strain of Enterobacter (sp 3.1.3.0.X.18) was selected for inoculation into S. auriculata. For this purpose, 500 mL of nutrient solution and 1 g of the plant were used in the control and inoculated conditions. Enterobacter inoculation promoted a significant increase (p ≤ 0.05) in fresh plant biomass (17%) after 4 days of cultivation. In summary, the present study characterized 18 plant-growth-promoting bacteria isolated from S. auriculata with potential for biotechnological application, such as the production of bioinoculants or biomass resources, to protect or improve plant growth under conditions of stress.

Impact of floods and landslides on rhizosphere bacterial communities: a high-throughput 16S rRNA gene sequencing study

This study investigates the diversity and composition of soil bacterial communities in the rhizosphere of Attapadi and Nelliyampathy, prominent hill stations in Palakkad district, Kerala, India. The persistent flooding and landslides in 2018 and 2019 significantly impacted agricultural productivity in these regions. Utilizing high-throughput 16S rRNA gene sequencing (Illumina MiSeq), we conducted a comprehensive analysis of soil samples. Correlative assessments between soil parameters and microbial relative abundance at the phylum level revealed noteworthy positive associations. Notably, nitrogen (N) exhibited a positive relation with Crenarchaeota, Chloroflexi, Actinobacteriota, and Acidobacteriota; pH correlated with Firmicutes; organic carbon (OC) with WPS-2; and phosphorous with Proteobacteria. A total of 31,402 operational taxonomic units (OTUs) were identified, with the highest feature counts observed in undisturbed soils from Attapadi (AUD) and Nelliyampathy (NUD) (13,007 and 12,915, respectively). Disturbed soils in Nelliyampathy (ND) and Attapadi (AD) displayed a substantial decline in microbial diversity and composition, harbouring 1409 and 4071 OTUs, respectively. Alpha and beta diversity indices further underscored the more severe impairment of ND soils compared to AD soils. Interestingly, a majority of ND samples were landslide-affected (four out of five), while flood-affected soils accounted for four out of six AD samples. This indicates that landslides exert a more pronounced impact on microbial diversity and composition than floods. The observed decline in microbial count, composition, and diversity, even after 2 years of the disaster, raises concerns about potential threats to agricultural output. The findings emphasize the need for corrective measures, including the incorporation of microbial inoculum, to restore soil fertility in post-disaster landscapes.

Employment of pqqE gene as molecular marker for the traceability of Gram negative phosphate solubilizing bacteria associated to plants

Insoluble phosphorous compounds solubilization by soil bacteria is of great relevance since it puts available the phosphorus to be used by plants. The production of organic acids is the main microbiological mechanism by which insoluble inorganic phosphorus compounds are solubilized. In Gram negative bacteria, gluconic acid is synthesized by the activity of the holoenzyme glucose dehydrogenase-pyrroloquinoline quinine named GDH-PQQ. The use of marker genes is a very useful tool to evaluate the persistence of the introduced bacteria and allow to follow-up the effect of biotic and abiotic factors on these beneficial microorganisms in the soil. In previous studies we detected the presence of the pqqE gene in a great percentage of both non-culturable and culturable native soil bacteria. The objective of this study was to analyze the phylogeny of the sequence of pqqE gene and its potential for the study of phosphate solubilizing bacteria from pure and mixed bacterial cultures and rhizospheric soil samples. For this, the presence of the pqqE gene in the genome of phosphate solubilizing bacteria that belong to several bacteria was determined by PCR. Also, this gene was analyzed from mixed bacterial cultures and rhizospheric soil associated to peanut plants inoculated or not with phosphate solubilizing bacteria. For this, degenerate primers designed from several bacterial genera and specific primers for the genus Pseudomonas spp., designed in this study, were used. DNA template used from simple or mixed bacterial cultures and from rhizospheric soil samples was obtained using two different DNA extraction techniques. Results indicated that pqqE gene amplification product was found in the genome of all Gram negative phosphate solubilizing bacteria analyzed. It was possible to detect this gene in the DNA obtained from mixed cultures where these bacteria grew in interaction with other microorganisms and in that obtained from rhizospheric soil samples inoculated or not with these bacteria. The phylogenetic analysis indicated that pqqE gene is a conserved gene within related genera. In conclusion, pqqE gene could be a potential marker for the study of phosphate solubilizing bacterial populations.

Actinomycetes isolated from rhizosphere of wild Coffea arabica L. showed strong biocontrol activities against coffee wilt disease

Coffee, the second most traded commodity globally after petroleum and is the most exported cash crop of Ethiopia. However, coffee cultivation faces challenges due to fungal diseases, resulting in significant yield losses. The primary fungal diseases affecting coffee production include coffee berry disease, wilt disease (caused by Gibberella xylarioides), and coffee leaf rust. In this study, we aimed to isolate potentially antagonistic actinomycetes from the root rhizosphere of wild Coffea arabica plants in the Yayo coffee forest biosphere in southwestern Ethiopia. Soil samples were collected from the rhizosphere, and actinomycetes were selectively isolated and identified to the genus level by morphological, physiological, and biochemical characterization. These pure isolates were screened for their antagonistic activity against Gibberella xylarioides in vitro using a dual culturing method. Promising isolates demonstrating strong inhibition of fungal mycelial growth were further investigated through in vivo experiments using coffee seedlings. A total of 82 rhizobacteria were isolated. These isolates' inhibition of fungal mycelial growth varied from 0% to 83.3%. Among them, four isolates MUA26, MUA13, MUA52, and MUA14 demonstrated the highest percentage inhibition of fungal mycelial growth: 83.3%, 80%, 76.67%, and 73.3%, respectively. Seedlings inoculated with MUA13, MUA14, and MUA26 during the challenge inoculations (Rhizobacteria + Gibberella xylarioides) exhibited the lowest disease incidence compared to the infected fungi (P < 0.05). Notably, the seedlings inoculated with MUA26 demonstrated the highest disease control efficiency, reaching 83% (P < 0.05). MUA26 was found to produce extracellular enzymes, including chitinase, protease, and lipase, which acted as inhibitors. In summary, this study highlights that MUA26, among the actinomycete isolates, exhibited significant antagonistic activity against Gibberella xylarioides f.sp. coffea. Its efficacy in controlling coffee wilt disease, both in vitro and in vivo, positions it as a potential bioinoculant for managing coffee wilt disease.

Genome-based identification of phosphate-solubilizing capacities of soil bacterial isolates

Identifying genomic markers for phosphate-solubilizing bacteria (PSB) is vital for advancing agricultural sustainability. This study utilizes whole-genome sequencing and comprehensive bioinformatics analysis, examining the genomes of 76 PSB strains with the aid of specialized genomic databases and analytical tools. We have identified the pqq gene cluster, particularly the pqqC gene, as a key marker for (P) solubilization capabilities. The pqqC gene encodes an enzyme that catalyzes the conversion of precursors to 2-keto-D-gluconic acid, which significantly enhances P solubilization in soil. This gene's importance lies not only in its biochemical function but also in its prevalence and effectiveness across various PSB strains, distinguishing it from other potential markers. Our study focuses on Burkholderia cepacia 51-Y1415, known for its potent solubilization activity, and demonstrates a direct correlation between the abundance of the pqqC gene, the quantitative release of P, and the production of 2-keto-D-gluconic acid over a standard 144-h cultivation period under standardized conditions. This research not only underscores the role of the pqqC gene as a universal marker for the rapid screening and functional annotation of PSB strains but also highlights its implications for enhancing soil fertility and crop yields, thereby contributing to more sustainable agricultural practices. Our findings provide a foundation for future research aimed at developing targeted strategies to optimize phosphate solubilization, suggesting areas for further investigation such as the integration of these genomic insights into practical agricultural applications to maximize the effectiveness of PSB strains in real-world soil environments.

Differential Mechanisms of Microbial As(III) and Sb(III) Oxidation and Their Contribution to Tailings Reclamation

Mine tailings are extremely oligotrophic environments frequently contaminated with elevated As and Sb, making As(III) and Sb(III) oxidation potentially important energy sources for the tailing microbiome. Although they have been proposed to share similar metabolic pathways, a systemic comparison of the As(III) and Sb(III) oxidation mechanisms and energy utilization efficiencies requires further elucidation. In this study, we employed a combination of physicochemical, molecular, and bioinformatic analyses to compare the kinetic and genetic mechanisms of As(III) and Sb(III) oxidation as well as their respective energy efficiencies for fueling the key nutrient acquisition metabolisms. Thiobacillus and Rhizobium spp. were identified as functional populations for both As(III) and Sb(III) oxidation in mine tailings by DNA-stable isotope probing. However, these microorganisms mediated As(III) and Sb(III) oxidation via different metabolic pathways, resulting in preferential oxidation of Sb(III) over As(III). Notably, both As(III) and Sb(III) oxidation can facilitate nitrogen fixation and phosphate solubilization in mine tailings, with Sb(III) oxidation being more efficient in powering these processes. Thus, this study provided novel insights into the microbial As(III) and Sb(III) oxidation mechanisms and their respective nutrient acquisition efficiencies, which may be critical for the reclamation of mine tailings.

Novel cereal bran based low-cost liquid medium for enhanced growth, multifunctional traits and shelf life of consortium biofertilizer containing Azotobacter chroococcum, Bacillus subtilis and Pseudomonas sp

The present study was carried out to valorise cereal (rice and wheat) bran for the development of low-cost liquid consortium bioformulation. Different concentrations of bran-based liquid media formulations were evaluated for the growth of consortium biofertilizer cultures (Azotobacter chroococcum, Bacillus subtilis and Pseudomonas sp.). Among the bran-based formulations, wheat bran-based formulation WB5, exhibited the highest viable cell of 10.68 ± 0.09 Log10 CFU/ml and 12.63 ± 0.04 Log10 CFU/ml for Azotobacter chroococcum and Bacillus subtilis whereas for Pseudomonas sp., rice bran based bioformulation RB5 recorded maximum viability (12.71 ± 0.05 Log10 CFU/ml) after 72 h of incubation. RB51 and WB52liquid formulations were further optimized for enhanced shelf life using 5, 10 and 15 mM of trehalose, 0.05 and 0.1% carboxymethyl cellulose, and 0.5 and 1.0% glycerol. Following the peak growth at 72 h of incubation, a gradual decrease in the viable population of consortium biofertilizer cultures was observed in all the liquid formulations. The WB5 and RB5 formulations with 15 mM trehalose and 0.1% CMC, not only recorded significantly highest cell count of consortium biofertilizer cultures, but also maximally supported multi-functional traits i.e., phosphate and zinc solubilization, ammonia and IAA production up to 150 days. Further evaluation of seedling emergence and growth of wheat (PBW 826) under axenic conditions recorded WB5 amended with 15 mM trehalose-based consortium bioformulation to exhibit maximum emergence and growth of wheat seedlings. This low-cost liquid formulation can be used for large-scale biofertilizer production as a cost-effective liquid biofertilizer production technology.

Mangrove endophytic fungi: Biocontrol potential against Rhizoctonia solani and biofertilizers for fragrant rice cultivation

The mangrove ecosystem has emerged as a fascinating source for exploring novel bioresources which have multiple applications in modern agriculture. This study evaluates the potential applications of mangrove endophytic fungi (MEF), such as biocontrol agents against Rhizoctonia solani and as biofertilizers for improving the yield of fragrant rice variety Malaysian Rice Quality 76 (MRQ76). Through the antagonism assays, it is observed that among the 14 MEF studied, 4 fungal isolates (Colletotrichum sp. MEFN02, Aspergillus sp. MEFN06, Annulohypoxylon sp. MEFX02 and Aspergillus sp. MEFX10) exhibited promising antagonistic effect against the pathogen R. solani compared to the chemical fungicide (Benomyl). These isolates also revealed significant production of enzymes, phytochemicals, indoleacetic acid (40.96 mg/mL) and ammonia (32.54 mg/mL) and displayed tolerance to salt and temperature stress up to 2000 mM and >40 °C respectively. Furthermore, employing the germination and pathogenicity test, inoculation of these endophytes showed lower percentage of disease severity index (DSI%) against R. solani, ranging from (24 %-46 %) in MRQ76 rice seedlings. The in-vivo experiments of soil and seed inoculation methods conducted under greenhouse conditions revealed that these endophytes enhanced plant growth (8-15 % increase) and increased crop yield (≥50 %) in comparison to control treatments. The current findings provide valuable insights into eco-friendly, cost-effective and sustainable alternatives for addressing R. solani infection and improving the agronomic performance of the fragrant rice cultivar MRQ76, contributing to food security.

Divergent responses of rhizosphere soil phosphorus fractions and biological features of Salix psammophila to fertilization strategies under cadmium contamination

Soil oligotrophy in areas heavily contaminated with heavy metals poses a significant challenge to vegetation establishment and phytoremediation processes. Phosphorus (P) cycling plays a critical role in global biogeochemical cycles, but there is limited understanding of its response to varying fertilization strategies and its correlation with phytoremediation effectiveness. This study primarily investigated the effects of various fertilization strategies, including nitrogen (N, 300 mg·kg-1), P (100 mg·kg-1), NP (combined N and P at 300 mg·kg-1 and 100 mg·kg-1, respectively), and HP (high P, 300 mg·kg-1) application, on rhizosphere soil P fractions and P-solubilizing microbial community (harboring phoD and phoC genes, respectively) of Salix psammophila under cadmium contamination. Application of NP significantly enhanced plant growth and cadmium accumulation, whereas HP inhibited cadmium bioaccumulation but promoted its translocation. Compared to untreated soil, N application promoted P cycling, leading to increases of 141.9 %, 60.4 %, and 10.3 % in Resin-Pi, diluted HCl-Pi, and conc.HCl-Pi, respectively. P application decreased organic phosphorus (Po) fractions by 24.4 % - 225.8 %, but N incorporation mitigated the declining trend in Po and augmented alkaline phosphatase activity. Fertilization strategies significantly regulated phoC- or phoD-harboring bacterial community structure, but their differential nutrient demands resulted in distinct responses. The phoD-harboring bacteria exhibited higher diversity and network complexity, with numerous biomarkers and fertilizer-sensitive OTUs discovered across treatments. Structural equation modeling (SEM) analysis indicated that phytoremediation efficiency was directly affected by Pi fractions, and phoD-harboring bacteria exhibited stronger associations with Pi fractions than phoC-harboring bacteria. In conclusion, our results reveal potential pathways through which fertilization strategies influence phytoremediation by affecting the structure of P-solubilizing microbial community. Furthermore, our study emphasizes the importance of combined N and P application in promoting Cd accumulation in plants, with high P levels appearing as an ideal fertilization strategy for phytoremediation targeting the harvest of aboveground biomass.

NaCl Modifies Biochemical Traits in Bacterial Endophytes Isolated from Halophytes: Towards Salinity Stress Mitigation Using Consortia

Bacterial endophytes (120) were isolated from six halophytes (Distichlis spicata, Cynodon dactylon, Eragrostis obtusiflora, Suaeda torreyana, Kochia scoparia, and Baccharis salicifolia). These halophiles were molecularly identified and characterized with or without NaCl conditions. Characterization was based on tests such as indole acetic acid (IAA), exopolysaccharides (EPS), and siderophores (SID) production; solubilization of phosphate (P), potassium (K), zinc (Zn), and manganese (Mn); mineralization of phytate; enzymatic activity (acid and alkaline phosphatase, phytases, xylanases, and chitinases) and the mineralization/solubilization mechanisms involved (organic acids and sugars). Moreover, compatibility among bacteria was assessed. Eleven halophiles were characterized as highly tolerant to NaCl (2.5 M). The bacteria isolated were all different from each other. Two belonged to Bacillus velezensis and one to B. pumilus while the rest of bacteria were identified up to the genus level as belonging to Bacillus, Halobacillus, Halomonas, Pseudomonas, Nesterenkonia, and three strains of Oceanobacillus. The biochemical responses of nutrient solubilization and enzymatic activity were different between bacteria and were influenced by the presence of NaCl. Organic acids were involved in P mineralization and nutrient solubilization. Tartaric acid was common in the solubilization of P, Zn, and K. Maleic and vanillic acid were only detected in Zn and K solubilization, respectively. Furthermore, sugars appeared to be involved in the solubilization of nutrients; fructose was detected in the solubilization tests. Therefore, these biochemical bacterial characteristics should be corroborated in vivo and tested as a consortium to mitigate saline stress in glycophytes under a global climate change scheme that threatens to exacerbate soil salinity.

Roseiterribacter gracilis gen. nov., sp. nov., a novel filterable alphaproteobacterium isolated from soil using a gel-filled microwell array device

Our previous studies indicate the abundant and diverse presence of yet-to-be-cultured microorganisms in the micropore-filtered fractions of various environmental samples. Here, we isolated a novel bacterium (designated as strain TMPK1T) from a 0.45-μm-filtered soil suspension by using a gel-filled microwell array device comprising 900 microwells and characterized its phylogenetic and physiological features. This strain showed low 16S rRNA gene sequence identities (<91%) and low average nucleotide identity values (<70%) to the closest validly described species, and belonged to a novel-family-level lineage within the order Rhodospirillales of Alphaproteobacteria. Strain TMPK1T exhibited small cell sizes (0.08-0.23 μm3) and had a high cyclopropane fatty acid content (>13%), and these characteristics were differentiated from other Rhodospirillales bacteria. A comprehensive habitability search using amplicon datasets suggested that TMPK1T and its close relatives are mainly distributed in soil and plant-associated environments. Based on these results, we propose that strain TMPK1T represents a novel genus and species named Roseiterribacter gracilis gen. nov., sp. nov. (JCM 34627T = KCTC 82790T). We also propose Roseiterribacteraceae fam. nov. to accommodate the genus Roseiterribacter.

Assessment of Plant Growth Promotion Potential of Endophytic Bacterium B. subtilis KU21 Isolated from Rosmarinus officinalis

The current study aimed to evaluate the plant growth-promoting (PGP) potential of endophytic strain Bacillus subtilis KU21 isolated from the roots of Rosmarinus officinalis. The strain exhibited multiple traits of plant growth promotion viz., phosphate (P) solubilization, nitrogen fixation, indole-3-acetic acid (IAA), siderophore, hydrogen cyanide (HCN), lytic enzymes production, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. The isolate also exhibited antagonistic activity against phytopathogenic fungi, i.e., Fusarium oxysporum, Fusarium graminiarum, and Rhizoctonia solani. The P-solubilization activity of B. subtilis KU21 was further elucidated via detection of glucose dehydrogenase (gdh) gene involved in the production of gluconic acid which is responsible for P-solubilization. Further, B. subtilis KU21 was evaluated for in vivo growth promotion studies of tomato (test crop) under net house conditions. A remarkable increase in seed germination, plant growth parameters, nutrient acquisition, and soil quality parameters (NPK) was observed in B. subtilis KU21-treated plants over untreated control. Hence, the proposed module could be recommended for sustainable tomato production in the Northwest Himalayan region without compromising soil health and fertility.

A smooth vetch (Vicia villosa var.) strain endogenous to the broad-spectrum antagonist Bacillus siamensis JSZ06 alleviates banana wilt disease

Fusarium wilt, caused by Fusarium oxysporum f. sp. cubense Tropical Race 4 (Foc TR4), poses a significant threat to banana production globally, thereby necessitating effective biocontrol methods to manage this devastating disease. This study investigates the potential of Bacillus siamensis strain JSZ06, isolated from smooth vetch, as a biocontrol agent against Foc TR4. To this end, we conducted a series of in vitro and in vivo experiments to evaluate the antifungal activity of strain JSZ06 and its crude extracts. Additionally, genomic analyses were performed to identify antibiotic synthesis genes, while metabolomic profiling was conducted to characterize bioactive compounds. The results demonstrated that strain JSZ06 exhibited strong inhibitory activity against Foc TR4, significantly reducing mycelial growth and spore germination. Moreover, scanning and transmission electron microscopy revealed substantial ultrastructural damage to Foc TR4 mycelia treated with JSZ06 extracts. Genomic analysis identified several antibiotic synthesis genes, and metabolomic profiling revealed numerous antifungal metabolites. Furthermore, in pot trials, the application of JSZ06 fermentation broth significantly enhanced banana plant growth and reduced disease severity, achieving biocontrol efficiencies of 76.71% and 79.25% for leaves and pseudostems, respectively. In conclusion, Bacillus siamensis JSZ06 is a promising biocontrol agent against Fusarium wilt in bananas, with its dual action of direct antifungal activity and plant growth promotion underscoring its potential for integrated disease management strategies.

Genomic insight of phosphate solubilization and plant growth promotion of two taxonomically distinct winter crops by Enterobacter sp. DRP3

AIMS

Study of rhizospheric microbiome-mediated plant growth promotional attributes currently highlighted as a key tool for the development of suitable bio-inoculants for sustainable agriculture purposes. In this context, we have conducted a detailed study regarding the characterization of phosphate solubilizing potential by plant growth-promoting bacteria that have been isolated from the rhizosphere of a pteridophyte Dicranopteris sp., growing on the lateritic belt of West Bengal.

METHODS AND RESULTS

We have isolated three potent bacterial strains, namely DRP1, DRP2, and DRP3 from the rhizoids-region of Dicranopteris sp. Among the isolated strains, DRP3 is found to have the highest phosphate solubilizing potentiality and is able to produce 655.89 and 627.58 µg ml-1 soluble phosphate by solubilizing tricalcium phosphate (TCP) and Jordan rock phosphate, respectively. This strain is also able to solubilize Purulia rock phosphate moderately (133.51 µg ml-1). Whole-genome sequencing and further analysis of the studied strain revealed the presence of pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase gdh gene along with several others that were well known for their role in phosphate solubilization. Further downstream, quantitative reverse transcriptase PCR-based expression study revealed 1.59-fold upregulation of PQQ-dependent gdh gene during the solubilization of TCP. Root colonization potential of the studied strain on two taxonomically distinct winter crops viz. Cicer arietinum and Triticum aestivum has been checked by using scanning electron microscopy. Other biochemical analyses for plant growth promotion traits including indole acetic acid production (132.02 µg ml-1), potassium solubilization (3 mg l-1), biofilm formation, and exopolymeric substances productions (1.88-2.03 µg ml-1) also has been performed.

CONCLUSION

This study highlighted the active involvement of PQQ-dependent gdh gene during phosphate solubilization from any Enterobacter group. Moreover, our study explored different roadmaps for sustainable farming methods and the preservation of food security without endangering soil health in the future.

Diversity and Plant Growth Properties of Rhizospheric Bacteria Associated with Medicinal Plants

Microbes in the rhizosphere play a significant role in the growth, development, and efficiency of plants and trees. The rhizospheric area's microbes are reliant on the soil's characteristics and the substances that the plants release. The majority of previous research on medicinal plants concentrated on their bioactive phytochemicals, but this is changing now that it is understood that a large proportion of phytotherapeutic substances are actually created by related microorganisms or through contact with their host. The roots of medicinal plants secrete a large number of secondary metabolites that determine the diversity of microbial communities in their rhizosphere. The dominant bacteria isolated from a variety of medicinal plants include various species of Bacillus, Rhizobium, Pseudomonas, Azotobacter, Burkholderia, Enterobacte, Microbacterium, Serratia, Burkholderia, and Beijerinckia. Actinobacteria also colonize the rhizosphere of medicinal plants that release low molecular weight organic solute that facilitate the solubilisation of inorganic phosphate. Root exudates of medicinal plants resist abiotic stress and accumulate in soil to produce autotoxic effects that exhibit strong obstacles to continuous cropping. Although having a vast bioresource that may be used in agriculture and modern medicine, medicinal plants' microbiomes are largely unknown. The purpose of this review is to (i) Present new insights into the plant microbiome with a focus on medicinal plants, (ii) Provide information about the components of medicinal plants derived from plants and microbes, and (iii) Discuss options for promoting plant growth and protecting plants for commercial cultivation of medicinal plants. The scientific community has paid a lot of attention to the use of rhizobacteria, particularly plant growth-promoting rhizobacteria (PGPR), as an alternative to chemical pesticides. By a variety of processes, these rhizobacteria support plant growth, manage plant pests, and foster resilience to a range of abiotic challenges. It also focuses on how PGPR inoculation affects plant growth and survival in stressful environments.

IAA-producing plant growth promoting rhizobacteria from Ceanothus velutinus enhance cutting propagation efficiency and Arabidopsis biomass

Climate-induced drought impacts plant growth and development. Recurring droughts increase the demand for water for food production and landscaping. Native plants in the Intermountain West region of the US are of keen interest in low water use landscaping as they are acclimatized to dry and cold environments. These native plants do very well at their native locations but are difficult to propagate in landscape. One of the possible reasons is the lack of associated microbiome in the landscaping. Microbiome in the soil contributes to soil health and impacts plant growth and development. Here, we used the bulk soil from the native plant Ceanothus velutinus (snowbrush ceanothus) as inoculant to enhance its propagation. Snowbrush ceanothus is an ornamental plant for low-water landscaping that is hard to propagate asexually. Using 50% native bulk soil as inoculant in the potting mix significantly improved the survival rate of the cuttings compared to no-treated cuttings. Twenty-four plant growth-promoting rhizobacteria (PGPR) producing indole acetic acid (IAA) were isolated from the rhizosphere and roots of the survived snowbrush. Seventeen isolates had more than 10µg/mL of IAA were shortlisted and tested for seven different plant growth-promoting (PGP) traits; 76% showed nitrogen-fixing ability on Norris Glucose Nitrogen free media,70% showed phosphate solubilization activity, 76% showed siderophore production, 36% showed protease activity, 94% showed ACC deaminase activity on DF-ACC media, 76% produced catalase and all of isolates produced ammonia. Eight of seventeen isolates, CK-6, CK-22, CK-41, CK-44, CK-47, CK-50, CK-53, and CK-55, showed an increase in shoot biomass in Arabidopsis thaliana. Seven out of eight isolates were identified as Pseudomonas, except CK-55, identified as Sphingobium based on 16S rRNA gene sequencing. The shortlisted isolates are being tested on different grain and vegetable crops to mitigate drought stress and promote plant growth.

Agronomic advantage of bacterial biological nitrogen fixation on wheat plant growth under contrasting nitrogen and phosphorus regimes

Introduction

Given their remarkable capacity to convert atmospheric nitrogen into plant-accessible ammonia, nitrogen-fixing microbial species hold promise as a sustainable alternative to chemical nitrogen fertilizers, particularly in economically significant crops like wheat. This study aimed to identify strains with optimal attributes for promoting wheat growth sustainably, with a primary emphasis on reducing reliance on chemical nitrogen fertilizers.

Methods

We isolated free nitrogen-fixing strains from diverse rhizospheric soils across Morocco. Subsequently, we conducted a rigorous screening process to evaluate their plant growth-promoting traits, including nitrogen fixation, phosphate solubilization, phytohormone production and their ability to enhance wheat plant growth under controlled conditions. Two specific strains, Rhodotorula mucilaginosa NF 516 and Arthrobacter sp. NF 528, were selected for in-depth evaluation, with the focus on their ability to reduce the need for chemical nitrogen supply, particularly when used in conjunction with TSP fertilizer and natural rock phosphate. These two sources of phosphate were chosen to assess their agricultural effectiveness on wheat plants.

Results and discussion

Twenty-two nitrogen-fixing strains (nif-H+) were isolated from various Moroccan rhizospheric soils, representing Bacillus sp., Pseudomonas sp., Arthrobacter sp., Burkholderia sp. and a yeast-like microorganism. These strains were carefully selected based on their potential to promote plant growth. The findings revealed that the application of Rhodotorula mucilaginosa NF 516 and Arthrobacter sp. NF 528 individually or in combination, significantly improved wheat plant growth and enhanced nutrients (N and P) uptake under reduced nitrogen regimes. Notably, their effectiveness was evident in response to both natural rock phosphate and TSP, demonstrating their important role in wheat production under conditions of low nitrogen and complex phosphorus inputs. This research underscores the significant role of nitrogen-fixing microorganisms, particularly Rhodotorula mucilaginosa NF 516 and Arthrobacter sp. NF 528, in wheat production under conditions of low nitrogen and complex phosphorus inputs. It showcases their potential to reduce chemical nitrogen fertilization requirements by up to 50% without compromising wheat plant yields. Our study emphasizes the importance of bacterial biological nitrogen fixation in meeting the remaining nitrogen requirements beyond this reduction. This underscores the vital role of microbial contributions in providing essential nitrogen for optimal plant growth and highlights the significance of biological nitrogen fixation in sustainable agriculture practices.

Broad-spectrum pH functional chitosan-phosphatase beads for the generation of plant-available phosphorus: utilizing the insoluble P pool

Utilization of organic phosphates and insoluble phosphates for the gradual generation of plant-available phosphorus (P) is the only sustainable solution for P fertilization. Enzymatic conversions are one of the best sustainable routes for releasing P to soil. Phosphatase enzyme aids in solubilizing organic and insoluble phosphates to plant-available P. We herein report the preparation of highly functional chitosan beads co-immobilized with acid phosphatase and alkaline phosphatase enzymes via a glutaraldehyde linkage. The dual enzyme co-immobilized chitosan beads were characterized using Fourier-transform infrared (FTIR), thermogravimetric (TGA), and scanning electron microscopy-energy dispersive x-ray (SEM-EDX) analyses to confirm the immobilization. The co-immobilized system was found to be active for a broader pH range of ∼4-10 than the individually bound enzymes and mixed soluble enzymes. The bound matrix exhibited pH optima at 6 and 9, respectively, for acid and alkaline phosphatase and a temperature optimum at 50°C. The phosphate-solubilizing abilities of the chitosan-enzyme derivatives were examined using insoluble tri-calcium phosphate (TCP) for wide pH conditions of 5.5, 7, and 8.5 up to 25 days. The liberation of phosphate was highest (27.20 mg/mL) at pH 5.5 after the defined period. The residual soil phosphatase activity was also monitored after 7 days of incubation with CBE for three different soils of pH ∼5.5, 7, and 8.5. The residual phosphatase activity increased for all the soils after applying the CBE. The germination index of the Oryza sativa (rice) plant was studied using different pH buffer media upon the application of the CBE in the presence of tri-calcium phosphate as a phosphate source. Overall, the dual-enzyme co-immobilized chitosan beads were highly effective over a wide pH range for generating plant-available phosphates from insoluble phosphates. The chitosan-enzyme derivative holds the potential to be used for sustainable phosphorus fertilization with different insoluble and organic phosphorus sources.

Genomics analysis of three phosphorus-dissolving bacteria isolated from Torreya grandis soil

With the increasingly serious problem of phosphorus deficiency in the subtropical zone, chemical fertilizers are widely used. But it pollutes the environment. Phosphorus-solubilizing microorganisms (PSMs) are referred to as a new solution to this problem. We explored the phosphorus-dissolving characteristics of PSB strains isolated from the rhizosphere soil of Torreya grandis to provide a theoretical basis for selecting the strain for managing phosphorus deficiency in subtropical soils and also provides a more sufficient theoretical basis for the utilization of PSMs. From 84 strains, three strains exhibiting high phosphorus solubility and strong IAA producing capacity were selected through a series of experiments. The phosphate-solubilizing capacity of the three selected strains W1, W74, and W83 were 339.78 mg/L, 332.57 mg/L, and 358.61 mg/L, respectively. Furthermore, W1 showed the strongest IAA secreting capacity of 8.62 mg/L, followed by W74 (7.58 mg/L), and W83 (7.59 mg/L). Determination by metabolites, it was observed that these three strains dissolved phosphorus by secreting a large amount of lactic acid, aromatic acid, and succinic acid. The genome of these PSBs were sequenced and annotated in this study. Our results revealed that PSB primarily promotes their metabolic pathway, especially carbon metabolism, to secrete plenty organic acids for dissolving insoluble phosphorus.

Fungi beyond limits: The agricultural promise of extremophiles

Global climate changes threaten food security, necessitating urgent measures to enhance agricultural productivity and expand it into areas less for agronomy. This challenge is crucial in achieving Sustainable Development Goal 2 (Zero Hunger). Plant growth-promoting microorganisms (PGPM), bacteria and fungi, emerge as a promising solution to mitigate the impact of climate extremes on agriculture. The concept of the plant holobiont, encompassing the plant host and its symbiotic microbiota, underscores the intricate relationships with a diverse microbial community. PGPM, residing in the rhizosphere, phyllosphere, and endosphere, play vital roles in nutrient solubilization, nitrogen fixation, and biocontrol of pathogens. Novel ecological functions, including epigenetic modifications and suppression of virulence genes, extend our understanding of PGPM strategies. The diverse roles of PGPM as biofertilizers, biocontrollers, biomodulators, and more contribute to sustainable agriculture and environmental resilience. Despite fungi's remarkable plant growth-promoting functions, their potential is often overshadowed compared to bacteria. Arbuscular mycorrhizal fungi (AMF) form a mutualistic symbiosis with many terrestrial plants, enhancing plant nutrition, growth, and stress resistance. Other fungi, including filamentous, yeasts, and polymorphic, from endophytic, to saprophytic, offer unique attributes such as ubiquity, morphology, and endurance in harsh environments, positioning them as exceptional plant growth-promoting fungi (PGPF). Crops frequently face abiotic stresses like salinity, drought, high UV doses and extreme temperatures. Some extremotolerant fungi, including strains from genera like Trichoderma, Penicillium, Fusarium, and others, have been studied for their beneficial interactions with plants. Presented examples of their capabilities in alleviating salinity, drought, and other stresses underscore their potential applications in agriculture. In this context, extremotolerant and extremophilic fungi populating extreme natural environments are muchless investigated. They represent both new challenges and opportunities. As the global climate evolves, understanding and harnessing the intricate mechanisms of fungal-plant interactions, especially in extreme environments, is paramount for developing effective and safe plant probiotics and using fungi as biocontrollers against phytopathogens. Thorough assessments, comprehensive methodologies, and a cautious approach are crucial for leveraging the benefits of extremophilic fungi in the changing landscape of global agriculture, ensuring food security in the face of climate challenges.

Pseudomonas hefeiensis sp. nov., isolated from the rhizosphere of multiple cash crops in China

Three bacterial strains, FP250T, FP821, and FP53, were isolated from the rhizosphere soil of oilseed rape, licorice, and habanero pepper in Anhui Province, Xinjiang Uygur Autonomous Region, and Jiangsu Province, PR China, respectively. All strains were shown to grow at 4-37 °C and pH 6.0-9.0, and in the presence of 0-4.0 % (w/v) NaCl. Phylogenetic analyses based on 16S rRNA gene sequences or housekeeping genes (16S rRNA, gyrB, rpoB, and rpoD) and phylogenomic analysis showed that strains FP250T, FP821, and FP53 belong to the genus Pseudomonas, and are closely related to Pseudomonas kilonensis DSM 13647T, Pseudomonas brassicacearum JCM 11938T, Pseudomonas viciae 11K1T, and Pseudomonas thivervalensis DSM 13194T. The DNA G+C content of strain FP205T was 59.8 mol%. The average nucleotide identity and digital DNA-DNA hybridization values of strain FP205T with the most closely related strain were 93.2 % and 51.4 %, respectively, which is well below the threshold for species differentiation. Strain FP205T contained summed feature 3 (C16 : 1  ω6c and/or C16 : 1  ω7c), summed feature 8 (C18 : 1  ω7c and/or C18 : 1  ω6c) as major fatty acids, and diphosphatidylglycerol along with phosphatidylethanolamine and aminophospholipid as major polar lipids. The predominant isoprenoid quinone was ubiquinone-9. Based on these phenotypic, phylogenetic, and chemotaxonomic results, strain FP205T represents a novel species of the genus Pseudomonas, for which the name Pseudomonas hefeiensis sp. nov. is proposed. The type strain is FP205T (=ACCC 62447T=JCM 35687T).

Pseudomonas chlororaphis IRHB3 assemblies beneficial microbes and activates JA-mediated resistance to promote nutrient utilization and inhibit pathogen attack

Introduction

The rhizosphere microbiome is critical to plant health and resistance. PGPR are well known as plant-beneficial bacteria and generally regulate nutrient utilization as well as plant responses to environmental stimuli. In our previous work, one typical PGPR strain, Pseudomonas chlororaphis IRHB3, isolated from the soybean rhizosphere, had positive impacts on soil-borne disease suppression and growth promotion in the greenhouse, but its biocontrol mechanism and application in the field are not unclear.

Methods

In the current study, IRHB3 was introduced into field soil, and its effects on the local rhizosphere microbiome, disease resistance, and soybean growth were comprehensively analyzed through high-throughput sequencing and physiological and molecular methods.

Results and discussion

We found that IRHB3 significantly increased the richness of the bacterial community but not the structure of the soybean rhizosphere. Functional bacteria related to phosphorus solubilization and nitrogen fixation, such as Geobacter, Geomonas, Candidatus Solibacter, Occallatibacter, and Candidatus Koribacter, were recruited in rich abundance by IRHB3 to the soybean rhizosphere as compared to those without IRHB3. In addition, the IRHB3 supplement obviously maintained the homeostasis of the rhizosphere microbiome that was disturbed by F. oxysporum, resulting in a lower disease index of root rot when compared with F. oxysporum. Furthermore, JA-mediated induced resistance was rapidly activated by IRHB3 following PDF1.2 and LOX2 expression, and meanwhile, a set of nodulation genes, GmENOD40b, GmNIN-2b, and GmRIC1, were also considerably induced by IRHB3 to improve nitrogen fixation ability and promote soybean yield, even when plants were infected by F. oxysporum. Thus, IRHB3 tends to synergistically interact with local rhizosphere microbes to promote host growth and induce host resistance in the field.

Unlocking dynamic root phenotypes for simultaneous enhancement of water and phosphorus uptake

Phosphorus (P) and water are crucial for plant growth, but their availability is challenged by climate change, leading to reduced crop production and global food security. In many agricultural soils, crop productivity is confronted by both water and P limitations. The diminished soil moisture decreases available P due to reduced P diffusion, and inadequate P availability diminishes tissue water status through modifications in stomatal conductance and a decrease in root hydraulic conductance. P and water display contrasting distributions in the soil, with P being concentrated in the topsoil and water in the subsoil. Plants adapt to water- and P-limited environments by efficiently exploring localized resource hotspots of P and water through the adaptation of their root system. Thus, developing cultivars with improved root architecture is crucial for accessing and utilizing P and water from arid and P-deficient soils. To meet this goal, breeding towards multiple advantageous root traits can lead to better cultivars for water- and P-limited environments. This review discusses the interplay of P and water availability and highlights specific root traits that enhance the exploration and exploitation of optimal resource-rich soil strata while reducing metabolic costs. We propose root ideotype models, including 'topsoil foraging', 'subsoil foraging', and 'topsoil/subsoil foraging' for maize (monocot) and common bean (dicot). These models integrate beneficial root traits and guide the development of water- and P-efficient cultivars for challenging environments.

Endofungal bacteria and ectomycorrhizal fungi synergistically promote the absorption of organic phosphorus in Pinus massoniana

Ectomycorrhizal fungi (ECMFs) that are involved in phosphorus mobilisation and turnover have limited ability to mineralise phytate alone. The endofungal bacteria in the ectomycorrhizal fruiting body may contribute to achieving this ecological function of ECMFs. We investigated the synergistic effect and mechanisms of endofungal bacteria and ECMF Suillus grevillea on phytate mineralisation. The results showed that soluble phosphorus content in the combined system of endofungal bacterium Cedecea lapagei and S. grevillea was 1.8 times higher than the sum of C. lapagei and S. grevillea alone treatment under the phytate mineralisation experiment. The S. grevillea could first chemotactically assist C. lapagei in adhering to the surface of S. grevillea. Then, the mineralisation of phytate was synergistically promoted by increasing the biomass of C. lapagei and the phosphatase and phytase activities of S. grevillea. The expression of genes related to chemotaxis, colonisation, and proliferation of C. lapagei and genes related to phosphatase and phytase activity of S. grevillea was also significantly upregulated. Furthermore, in the pot experiment, we verified that there might exist a ternary symbiotic system in the natural forest in which endofungal bacteria and ECMFs could synergistically promote phytate uptake in the plant Pinus massoniana via the ectomycorrhizal system.

Pseudomonas taetrolens ULE-PH5 and Pseudomonas sp. ULE-PH6 Isolated from the Hop Rhizosphere Increase Phosphate Assimilation by the Plant

Most of the phosphorus incorporated into agricultural soils through the use of fertilizers precipitates in the form of insoluble salts that are incapable of being used by plants. This insoluble phosphorus present in large quantities in soil forms the well-known "phosphorus legacy". The solubilization of this "phosphorus legacy" has become a goal of great agronomic importance, and the use of phosphate-solubilizing bacteria would be a useful tool for this purpose. In this work, we have isolated and characterized phosphate-solubilizing bacteria from the rhizosphere of hop plants. Two particular strains, Pseudomonas taetrolens ULE-PH5 and Pseudomonas sp. ULE-PH6, were selected as plant growth-promoting rhizobacteria due to their high phosphate solubilization capability in both plate and liquid culture assays and other interesting traits, including auxin and siderophore production, phytate degradation, and acidic and alkaline phosphatase production. These strains were able to significantly increase phosphate uptake and accumulation of phosphorus in the aerial part (stems, petioles, and leaves) of hop plants, as determined by greenhouse trials. These strains are promising candidates to produce biofertilizers specifically to increase phosphate adsorption by hop plants.

Revealing the phosphate-solubilizing characteristics and mechanisms of the plant growth-promoting bacterium Agrobacterium deltaense C1

AIMS

This study explores the phosphate (Pi)-solubilizing characteristics and mechanisms of a novel phosphate-solubilizing bacterium, Agrobacterium deltaense C1 (C1 hereafter).

METHODS AND RESULTS

The growth-promoting effects of C1 were investigated by gnotobiotic experiments, and the Pi-solubilizing mechanism was revealed by extracellular metabolomics, liquid chromatography analysis, and reverse transcription quantitative polymerase chain reaction. Results showed that C1 significantly increased Arabidopsis biomass and total phosphorus (P) content under P deficiency. Under Ca3(PO4)2 condition, the presence of C1 resulted in a significant and negative correlation between available P content and medium pH changes, implying that Pi dissolution occurs through acid release. Metabolomics revealed C1's ability to release 99 organic acids, with gluconic acid (GA), citric acid, and α-ketoglutaric acid contributing 64.86%, 9.58%, and 0.94%, respectively, to Pi solubilization. These acids were significantly induced by P deficiency. Moreover, C1's Pi solubilization may remain significant even in the presence of available P, as evidenced by substantial pH reduction and high gcd gene expression. Additionally, C1 produced over 10 plant growth-promoting substances.

CONCLUSIONS

C1 dissolves Pi primarily by releasing GA, which enhances plant growth under P deficiency. Notably, its Pi solubilization effect is not significantly limited by available Pi.