Metabacillus dongyingensis sp. nov. Is Represented by the Plant Growth-Promoting Bacterium BY2G20 Isolated from Saline-Alkaline Soil and Enhances the Growth of Zea mays L. under Salt Stress

Stress-tolerant Bacillus strains for enhancing tomato growth and biocontrol of Fusarium oxysporum under saline conditions: functional and genomic characterization

Soil salinity is a major limiting factor for agricultural crops, which increases their susceptibility to pathogenic attacks. This is particularly relevant for tomato (Solanum lycopersicum), a salt-sensitive crop. Fusarium wilt, caused by Fusarium oxysporum f. sp. lycopersici, is a significant threat to tomato production in both greenhouse and field environments. This study evaluated the salinity tolerance, biocontrol, and plant growth-promoting properties of Bacillus velezensis AF12 and Bacillus halotolerans AF23, isolated from soil affected by underground fires and selected for their resistance to saline conditions (up to 1000 mM NaCl). In vitro assays confirmed that both strains produced siderophores, indole-3-acetic acid (IAA), and proteases and exhibited phosphate solubilization under saline stress (100-200 mM NaCl). AF23 exhibited synergistic interactions with AF12, and inoculation with either strain individually or in combination significantly improved the growth of the Bonny Best tomato cultivar under 200 mM saline stress, leading to increased shoot and root weight, enhanced chlorophyll content, and higher total biomass. The biocontrol potential of AF12 and AF23 was evaluated in tomato plants infected with F. oxysporum. Both strains, individually or combined, increased shoot and root weight, chlorophyll content, and total biomass under non-saline conditions, promoting growth and reducing infection rates under saline stress (100 mM NaCl). Genomic analysis revealed that both strains harbored genes related to salt stress tolerance, biocontrol, and plant growth promotion. In conclusion, Bacillus strains AF23 and AF12 demonstrated strong potential as bioinoculants for enhancing tomato growth and providing protection against F. oxysporum in saline-affected soils.

Recruitment of specific rhizosphere microorganisms in saline-alkali tolerant rice improves adaptation to saline-alkali stress

Increasing annual soil salinization poses a major threat to global ecological security. Soil microorganisms play an important role in improving plant adaptability to stress tolerance, however, the mechanism of saline-alkali tolerance to plants associated with rhizosphere microbiome is unclear. We investigated the composition and structure of the rhizospheric bacteria and fungi communities of the saline-alkali tolerant (Oryza sativa var. Changbai-9) and sensitive (Oryza sativa var. Kitaake) rice grown in saline-alkali and non-saline-alkali soils. The results demonstrated that the saline-alkali tolerant rice enriched the rhizosphere bacteria taxa, including Hydrogenophaga, Pseudomonas, and Aeromonas, and fungi taxa, such as Chaetomium, Cladosporium and Tausonia, which may facilitate rice growth and enhance rice saline-alkali tolerance. Saline-alkali tolerant rice reduced the Na+/K+ ratio and improved rice yield by enhancing the stability of co-occurrence network associated with recruiting bacterial and fungal keystone species. The rhizosphere bacteria of the saline-alkali tolerant rice exhibited a markedly elevated expression of functions related to the saline-alkali tolerance, including the ABC transporter and the two-component system, compared to sensitive rice under saline-alkali stress. Overall, the saline-alkali tolerant rice responds to saline-alkali stress by recruiting keystone rhizosphere microorganisms to enhance rice saline-alkali tolerance. This study provides a theoretical basis for using specific microorganisms to improve plant tolerance in saline-alkali soils.

Unraveling the microecological mechanisms of phosphate-solubilizing Pseudomonas asiatica JP233 through metagenomics: insights into the roles of rhizosphere microbiota and predatory bacteria

The effects of phosphate-solubilizing bacteria (PSB) on plant productivity are high variable under field conditions. Soil phosphorus (P) levels are proposed to impact PSB performance. Furthermore, the effect of exogenous PSB on rhizosphere microbial community and their functions are largely unexplored. Our study examined how different P background and fertilization affected the performance of PSB Pseudomonas asiatica JP233. We further conducted metagenomic sequencing to assess its impact on rhizosphere microbiota and functions, with a focus on genes related to soil P cycling. We found that JP233 could enhance P solubilization and tomato growth to different extent in both high and low P soils, irrespective of P fertilization. It was particularly effective in high P soil without extra fertilization. JP233 altered the rhizosphere microbial community, boosting taxa known for plant growth promotion. It also changed soil gene profiling, enriching pathways related to secondary metabolite biosynthesis, amino acids, carbon metabolism, and other key processes. Particularly, JP233 increased the abundance of most P cycle genes and strengthened their interconnections. Populations of certain predatory bacteria increased after JP233 inoculation. Our findings provide valuable insights into PSB's mechanisms for P solubilization and plant growth promotion, as well as potential adverse impacts of resident microbes on bioinoculants.

Growth promotion on maize and whole-genome sequence analysis of Bacillus velezensis D103

Root-associated microorganisms, particularly plant growth-promoting rhizobacteria (PGPR) from the Bacillus genus, play a crucial role in enhancing crop yield and health. In this study, a Bacillus strain was isolated from the rhizosphere soil of maize and identified as Bacillus velezensis D103. The primary objective of this research was to evaluate the potential of D103 as a PGPR. Laboratory tests demonstrated that D103 is capable of nitrogen fixation, inorganic phosphorus solubilization, potassium solubilization, and the synthesis of indole-3-acetic acid, ammonia, siderophores, amylase, protease, cellulase, β-1,3-glucanase, and 1-aminocyclopropane-1-carboxylate deaminase. Additionally, D103 exhibited swimming and swarming motility, biofilm formation, and an antagonistic activity against pathogenic fungi. Genome mining identified genes associated with growth promotion and biocontrol activities. In a hydroponics experiment, maize plants treated with a D103 suspension at a cell density of 103 CFU·mL-1 resulted in the most pronounced showed significant growth stimulation, with shoot length and total root length increasing by 43% and 148%, respectively. These results support the potential of D103 as an effective PGPR for promoting maize crop growth.

IMPORTANCE

In this study, we assessed the capacity of D103 to promote plant growth and examined the effects of hydroponic experiments inoculated with this strain on the growth of maize seedlings. We sequenced and analyzed the complete genome of D103, identifying several genes and gene clusters associated with plant growth promotion and resistance to pathogenic fungi, thus revealing the plant growth-promoting mechanisms of this strain. The isolation and characterization of new strains with beneficial traits are essential for expanding microbial resources available for biofertilizer production. Collectively, these findings highlight the promising potential of Bacillus velezensis D103 as a biofertilizer for agricultural applications.

Unveiling intraspecific diversity and evolutionary dynamics of the foodborne pathogen Bacillus paranthracis through high-quality pan-genome analysis

Understanding the evolutionary dynamics of foodborne pathogens throughout host-associated habitats is of utmost importance. Bacterial pan-genomes, as dynamic entities, are strongly influenced by ecological lifestyles. As a phenotypically diverse species in the Bacillus cereus group, Bacillus paranthracis is recognized as an emerging foodborne pathogen and a probiotic simultaneously. This poorly understood species is a suitable study model for adaptive pan-genome evolution. In this study, we determined the biogeographic distribution, abundance, genetic diversity, and genotypic profiles of key genetic elements of B. paranthracis. Metagenomic read recruitment analyses demonstrated that B. paranthracis members are globally distributed and abundant in host-associated habitats. A high-quality pan-genome of B. paranthracis was subsequently constructed to analyze the evolutionary dynamics involved in ecological adaptation comprehensively. The open pan-genome indicated a flexible gene repertoire with extensive genetic diversity. Significant divergences in the phylogenetic relationships, functional enrichment, and degree of selective pressure between the different components demonstrated different evolutionary dynamics between the core and accessory genomes driven by ecological forces. Purifying selection and gene loss are the main signatures of evolutionary dynamics in B. paranthracis pan-genome. The plasticity of the accessory genome is characterized by horizontal gene transfer (HGT), massive gene losses, and weak purifying or positive selection, which might contribute to niche-specific adaptation. In contrast, although the core genome dominantly undergoes purifying selection, its association with HGT and positively selected mutations indicates its potential role in ecological diversification. Furthermore, host fitness-related dynamics are characterized by the loss of secondary metabolite biosynthesis gene clusters (BGCs) and CAZyme-encoding genes and the acquisition of antimicrobial resistance (AMR) and virulence genes via HGT. This study offers a case study of pan-genome evolution to investigate the ecological adaptations reflected by biogeographical characteristics, thereby advancing the understanding of intraspecific diversity and evolutionary dynamics of foodborne pathogens.

Mechanisms on salt tolerant of Paenibacillus polymyxa SC2 and its growth-promoting effects on maize seedlings under saline conditions

Soil salinity negatively affects microbial communities, soil fertility, and agricultural productivity and has become a major agricultural problem worldwide. Plant growth-promoting rhizobacteria (PGPR) with salt tolerance can benefit plant growth under saline conditions and diminish the negative effects of salt stress on plants. In this study, we aimed to understand the salt-tolerance mechanism of Paenibacillus polymyxa at the genetic and metabolic levels and elucidate the mechanism of strain SC2 in promoting maize growth under saline conditions. Under salt stress, we found that strain SC2 promoted maize seedling growth, which was accompanied by a significant upregulation of genes encoding for the biosynthesis of peptidoglycan, polysaccharide, and fatty acid, the metabolism of purine and pyrimidine, and the transport of osmoprotectants such as trehalose, glycine betaine, and K+ in strain SC2. To further enhance the salt resistance of strain SC2, three mutants (SC2-11, SC2-13, and SC2-14) with higher capacities for salt resistance and exopolysaccharide synthesis were obtained via atmospheric and room-temperature plasma mutagenesis. In saline-alkaline soil, the mutants showed better promoting effect on maize seedlings than wild-type SC2. The fresh weight of maize seedlings was increased by 68.10% after treatment with SC2-11 compared with that of the control group. The transcriptome analysis of maize roots demonstrated that SC2 and SC2-11 could induce the upregulation of genes related to the plant hormone signal transduction, starch and sucrose metabolism, reactive oxygen species scavenging, and auxin and ethylene signaling under saline-alkaline stress. In addition, various transcription factors, such as zinc finger proteins, ethylene-responsive-element-binding protein, WRKY, myeloblastosis proteins, basic helix-loop-helix proteins, and NAC proteins, were up-regulated in response to abiotic stress. Moreover, the microbial community composition of maize rhizosphere soil after inoculating with strain SC2 was varied from the one after inoculating with mutant SC2-11. Our results provide new insights into the various genes involved in the salt resistance of strain SC2 and a theoretical basis for utilizing P. polymyxa in saline-alkaline environments.

Metagenomic assembled genomes indicated the potential application of hypersaline microbiome for plant growth promotion and stress alleviation in salinized soils

Climate change is causing unpredictable seasonal variations globally. Due to the continuously increasing earth's surface temperature, the rate of water evaporation is enhanced, conceiving a problem of soil salinization, especially in arid and semi-arid regions. The accumulation of salt degrades soil quality, impairs plant growth, and reduces agricultural yields. Salt-tolerant, plant-growth-promoting microorganisms may offer a solution, enhancing crop productivity and soil fertility in salinized areas. In the current study, genome-resolved metagenomic analysis has been performed to investigate the salt-tolerating and plant growth-promoting potential of two hypersaline ecosystems, Sambhar Lake and Drang Mine. The samples were co-assembled independently by Megahit, MetaSpades, and IDBA-UD tools. A total of 67 metagenomic assembled genomes (MAGs) were reconstructed following the binning process, including 15 from Megahit, 26 from MetaSpades, and 26 from IDBA_UD assembly tools. As compared to other assemblers, the MAGs obtained by MetaSpades were of superior quality, with a completeness range of 12.95%-96.56% and a contamination range of 0%-8.65%. The medium and high-quality MAGs from MetaSpades, upon functional annotation, revealed properties such as salt tolerance (91.3%), heavy metal tolerance (95.6%), exopolysaccharide (95.6%), and antioxidant (60.86%) biosynthesis. Several plant growth-promoting attributes, including phosphate solubilization and indole-3-acetic acid (IAA) production, were consistently identified across all obtained MAGs. Conversely, characteristics such as iron acquisition and potassium solubilization were observed in a substantial majority, specifically 91.3%, of the MAGs. The present study indicates that hypersaline microflora can be used as bio-fertilizing agents for agricultural practices in salinized areas by alleviating prevalent stresses.

IMPORTANCE

The strategic implementation of metagenomic assembled genomes (MAGs) in exploring the properties and harnessing microorganisms from ecosystems like hypersaline niches has transformative potential in agriculture. This approach promises to redefine our comprehension of microbial diversity and its ecosystem roles. Recovery and decoding of MAGs unlock genetic resources, enabling the development of new solutions for agricultural challenges. Enhanced understanding of these microbial communities can lead to more efficient nutrient cycling, pest control, and soil health maintenance. Consequently, traditional agricultural practices can be improved, resulting in increased yields, reduced environmental impacts, and heightened sustainability. MAGs offer a promising avenue for sustainable agriculture, bridging the gap between cutting-edge genomics and practical field applications.

From salty to thriving: plant growth promoting bacteria as nature's allies in overcoming salinity stress in plants

Soil salinity is one of the main problems that affects global crop yield. Researchers have attempted to alleviate the effects of salt stress on plant growth using a variety of approaches, including genetic modification of salt-tolerant plants, screening the higher salt-tolerant genotypes, and the inoculation of beneficial plant microbiome, such as plant growth-promoting bacteria (PGPB). PGPB mainly exists in the rhizosphere soil, plant tissues and on the surfaces of leaves or stems, and can promote plant growth and increase plant tolerance to abiotic stress. Many halophytes recruit salt-resistant microorganisms, and therefore endophytic bacteria isolated from halophytes can help enhance plant stress responses. Beneficial plant-microbe interactions are widespread in nature, and microbial communities provide an opportunity to understand these beneficial interactions. In this study, we provide a brief overview of the current state of plant microbiomes and give particular emphasis on its influence factors and discuss various mechanisms used by PGPB in alleviating salt stress for plants. Then, we also describe the relationship between bacterial Type VI secretion system and plant growth promotion.

Insights into Genomic Evolution and the Potential Genetic Basis of Klebsiella variicola subsp. variicola ZH07 Reveal Its Potential for Plant Growth Promotion and Autotoxin Degradation

The accumulation of autotoxins in soil causes continuous cropping obstacle stress in crops, and the bioremediation of autotoxins by microorganisms is an efficient process. In this study, strain ZH07 was isolated from the peanut rhizosphere and was found to be utilizing multiple autotoxins as its carbon sources. Based on its genomic characteristics and a phylogenetic analysis, ZH07 represents a member of Klebsiella variicola subsp. variicola. A comparative genomic analysis exhibited evolutionary dynamics exhibited by mobile genetic elements (MGEs), strain-specific genes, potential horizontal genes, and evolutionary constraints driven by purifying selection, which facilitated its genomic adaptation to rhizosphere soil. Genome mining revealed the potential genomic properties associated with plant growth promotion, such as nitrogen fixation, indole acetic acid synthesis, phosphonate solubilization and assimilation, siderophore production, and secondary metabolite synthesis. Moreover, abundant genes putatively responsible for the biodegradation of aromatic xenobiotics, including benzoic acid, cinnamic acid, vanillic acid, protocatechuic acid, phenylacetic acid, and p-hydroxybenzoic acid were also observed in the ZH07 genome. Compared to autotoxin stress alone, the combination of ZH07 application promoted peanut germination and seedling growth. Our analysis revealed the genetic adaptation of ZH07 to the rhizosphere environment and the potential genetic basis and effectiveness of the isolate to serve as a plant growth stimulator. IMPORTANCE Continuous cropping obstacles reduce the production and quality of agricultural products, and the application of rhizosphere beneficial microbes is an important strategy. Strain ZH07 showed autotoxin-degrading and plant growth-promoting capacities. The objectives of this study were to characterize its genomic evolution and the potential genetic basis of the autotoxin degradation and plant growth promotion. ZH07 represents a member of Klebsiella variicola subsp. variicola, based on genomic and phylogenetic analyses. Its genomic components have undergone different degrees of purifying selection, and the disparity in the evolutionary rate may be associated with its niche adaptation. A systematic analysis of the ZH07 genome identified the potential genetic basis that contributes to plant growth promotion and to aromatic xenobiotic biodegradation. This study demonstrates that plant growth-promoting rhizobacteria (PGPR) play important roles in autotoxin biodegradation and can be used as biofertilizers to enhance the growth of peanuts in response to continuous cropping obstacle stress.

MsmR1, a global transcription factor, regulates polymyxin synthesis and carbohydrate metabolism in Paenibacillus polymyxa SC2

The multiple-sugar metabolism regulator (MsmR), a transcription factor belonging to the AraC/XylS family, participates in polysaccharide metabolism and virulence. However, the transcriptional regulatory mechanisms of MsmR1 in Paenibacillus polymyxa remain unclear. In this study, knocking out msmR1 was found to reduce polymyxin synthesis by the SC2-M1 strain. Chromatin immunoprecipitation assay with sequencing (ChIP-seq) revealed that most enriched pathway was that of carbohydrate metabolism. Additionally, electromobility shift assays (EMSA) confirmed the direct interaction between MsmR1 and the promoter regions of oppC3, sucA, sdr3, pepF, yycN, PPSC2_23180, pppL, and ydfp. MsmR1 stimulates polymyxin biosynthesis by directly binding to the promoter regions of oppC3 and sdr3, while also directly regulating sucA and influencing the citrate cycle (TCA cycle). In addition, MsmR1 directly activates pepF and was beneficial for spore and biofilm formation. These results indicated that MsmR1 could regulate carbohydrate and amino acid metabolism, and indirectly affect biological processes such as polymyxin synthesis, biofilm formation, and motility. Moreover, MsmR1 could be autoregulated. Hence, this study expand the current knowledge of MsmR1 and will be beneficial for the application of P. polymyxa SC2 in the biological control against the certain pathogens in pepper.

Insights into Evolutionary, Genomic, and Biogeographic Characterizations of Chryseobacterium nepalense Represented by a Polyvinyl Alcohol-Degrading Bacterium, AC3

Chryseobacterium spp. are Gram-negative rods found ubiquitously in the environment, with certain species being reported as having unusual degrading properties. Polyvinyl alcohol (PVA) is used widely in industry but causes serious global environmental pollution. Here, we report the complete genome sequence of a novel bacterium, AC3, that efficiently degrades PVA. As the representative genome of Chryseobacterium nepalense, key genomic characteristics (e.g., mobile genetic elements, horizontal genes, genome-scale metabolic network, secondary metabolite biosynthesis gene clusters, and carbohydrate-active enzymes) were comprehensively investigated to reveal the potential genetic features of this species. Core genome phylogenetic analysis in combination with average nucleotide identity, average amino acid identity, and in silico DNA-DNA hybridization values provided an accurate taxonomic position of C. nepalense in the genus Chryseobacterium. Comparative genomic analysis of AC3 with closely related species suggested evolutionary dynamics characterized by a species-specific genetic repertoire, dramatic rearrangements, and evolutionary constraints driven by selective pressure, which facilitated the speciation and adaptative evolution of C. nepalense. Biogeographic characterization indicated that this species is ubiquitously distributed not only in soil habitats but also in a variety of other source niches. Bioinformatic analysis revealed the potential genetic basis of PVA degradation in AC3, which included six putative genes associated with the synthesis of PVA dehydrogenase, cytochrome c, oxidized PVA hydrolase, and secondary alcohol dehydrogenase. Our study reports the first complete genome of C. nepalense with PVA-degrading properties, providing comprehensive insights into the genomic characteristics of this species and increasing our understanding of the microbial degradation of PVA. IMPORTANCE Although PVA is a biodegradable polymer, the widespread use of PVA in global industrialization has resulted in serious environmental problems. To date, knowledge of effective and applicable PVA-degrading bacteria is limited, and thus, the discovery of novel PVA biodegraders is pertinent. Here, we isolated a novel bacterial strain, AC3, which efficiently degraded PVA. The complete genome of AC3 was sequenced as the first genome sequence of the species C. nepalense. Comparative genomic analysis was performed to comprehensively investigate the phylogenetic relationships, genome-scale metabolic network, key genomic characteristics associated with genomic evolution, evolutionary dynamics between AC3 and its close relatives, and biogeographic characterization of C. nepalense, particularly regarding the potential genetic basis of PVA degradation. These findings could advance our understanding of the genomic characteristics of C. nepalense and PVA bioremediation.

Genomic Analysis of Endophytic Bacillus-Related Strains Isolated from the Medicinal Plant Origanum vulgare L. Revealed the Presence of Metabolic Pathways Involved in the Biosynthesis of Bioactive Compounds

Multidrug-resistant pathogens represent a serious threat to human health. The inefficacy of traditional antibiotic drugs could be surmounted through the exploitation of natural bioactive compounds of which medicinal plants are a great reservoir. The finding that bacteria living inside plant tissues, (i.e., the endophytic bacterial microbiome) can influence the synthesis of the aforementioned compounds leads to the necessity of unraveling the mechanisms involved in the determination of this symbiotic relationship. Here, we report the genome sequence of four endophytic bacterial strains isolated from the medicinal plant Origanum vulgare L. and able to antagonize the growth of opportunistic pathogens of cystic fibrosis patients. The in silico analysis revealed the presence of gene clusters involved in the production of antimicrobial compounds, such as paeninodin, paenilarvins, polymyxin, and paenicidin A. Endophytes’ adaptation to the plant microenvironment was evaluated through the analysis of the presence of antibiotic resistance genes in the four genomes. The diesel fuel degrading potential was also tested. Strains grew in minimum media supplemented with diesel fuel, but no n-alkanes degradation genes were found in their genomes, suggesting that diesel fuel degradation might occur through other steps involving enzymes catalyzing the oxidation of aromatic compounds.

Comparative Genomic Analysis of Vibrio cincinnatiensis Provides Insights into Genetic Diversity, Evolutionary Dynamics, and Pathogenic Traits of the Species

Vibrio cincinnatiensis is a poorly understood pathogenic Vibrio species, and the underlying mechanisms of its genetic diversity, genomic plasticity, evolutionary dynamics, and pathogenicity have not yet been comprehensively investigated. Here, a comparative genomic analysis of V. cincinnatiensis was constructed. The open pan-genome with a flexible gene repertoire exhibited genetic diversity. The genomic plasticity and stability were characterized by the determinations of diverse mobile genetic elements (MGEs) and barriers to horizontal gene transfer (HGT), respectively. Evolutionary divergences were exhibited by the difference in functional enrichment and selective pressure between the different components of the pan-genome. The evolution on the Chr I and Chr II core genomes was mainly driven by purifying selection. Predicted essential genes in V. cincinnatiensis were mainly found in the core gene families on Chr I and were subject to stronger evolutionary constraints. We identified diverse virulence-related elements, including the gene clusters involved in encoding flagella, secretion systems, several pili, and scattered virulence genes. Our results indicated the pathogenic potential of V. cincinnatiensis and highlighted that HGT events from other Vibrio species promoted pathogenicity. This pan-genome study provides comprehensive insights into this poorly understood species from the genomic perspective.