Phytobeneficial Properties of Bacteria Isolated from the Rhizosphere of Maize in Southwestern Nigerian Soils

Molecular identification of endophytes from maize roots and their biocontrol potential against toxigenic fungi of Nigerian maize

Plants benefit from plant-associated microorganisms, of which endophytes are of particular interest as they are transmitted from generation to generation. This study characterises endophytes from maize roots and determines their biocontrol potential against toxigenic fungi in Nigerian maize. Maize roots were collected from farms in Lafia, and stored grain samples were collected from the six Northern States of Nigeria, from which endophytes and toxigenic fungal strains were isolated. Molecular identification employing 16SrRNA/internal transcribed spacer (ITS) sequences for isolated fungal endophytes was carried out, and mycotoxins produced by fungi were determined by high-performance liquid chromatography analysis. Biocontrol activity of the endophytes was determined using the dual culture confrontation test. Aspergillus and Fusarium genera were the prevalent isolated fungal species. Eight fungal endophytes were identified of which Trichoderma harzianum, Dichotomopilus erectus and Burkholderia spp. were the isolates with biocontrol activities, while 12 Aspergillus spp. were found to produce varying amounts of ochratoxin A and aflatoxin B1, respectively. T. harzianum showed the best inhibition (74%), followed by D. erectus (50%) and Burkholderia spp. (48%). T. harzianum showed poor inhibition of Aspergillus flavus (B7) at 30%. However, results from the Pakdaman Biological Control Index showed that T. harzianum has the best antifungal biocontrol activity of the three endophytes. The study concludes that antifungal biocontrol agents can be sourced from endophytes to obtain indigenous control activities that can check mycotoxin contamination of food and livestock feed, as well as elucidate possible metabolites for agricultural and industrial applications, which will help improve plant performance, increase crop yield and sustainability.

Growth enhancement and extenuation of drought stress in maize inoculated with multifaceted ACC deaminase producing rhizobacteria

Introduction

Maize is a major staple cereal crop grown and consumed globally. However, due to climate change, extreme heat and drought stresses are greatly affecting its production especially in sub-Saharan Africa. The use of a bio-based approach to mitigate drought stress is therefore suggested using plant growth-promoting rhizobacteria (PGPR).

Methods

This study investigated the abilities of 1-aminocyclopropane-1-carboxylate (ACC) deaminase producing PGPR Pseudomonas sp. MRBP4, Pseudomonas sp. MRBP13 and Bacillus sp. MRBP10 isolated from maize rhizosphere soil, to ameliorate the effect of drought stress in maize genotypes MR44 and S0/8/W/I137TNW//CML550 under two water regimes; mild drought stress (50% FC) and well-watered conditions (100% FC). The rhizobacterial strains were identified by 16S rRNA sequencing and biochemical tests, and evaluated for plant growth-promoting and abiotic stress tolerance traits.

Results and discussion

The synergistic effect of the bacterial strains had a highly significant (p < 0.001) effect on the total soluble sugar, soil moisture content and relative water content, which were enhanced under water-stress in the inoculated plants. Relative water content was significantly highest (p < 0.001) in maize plants co-inoculated with Pseudomonas sp. MRBP4 + Bacillus sp. MRBP10 (60.55%). Total chlorophyll content was significantly enhanced in maize seedlings sole inoculated with Pseudomonas sp. MRBP4, Pseudomonas sp. MRBP13, and co-inoculated with Pseudomonas sp. MRBP13 + Bacillus sp. MRBP10 by 15.91%, 14.99% and 15.75% respectively, over the un-inoculated control. Soil moisture content increased by 28.67% and 30.71% compared to the un-inoculated control when plants were inoculated with Pseudomonas sp. MRBP4 + Bacillus sp. MRBP10 and Pseudomonas sp. MRBP4 + Bacillus sp. MRBP10 respectively. The interactive effect of genotype × bacteria significantly enhanced biomass production. Leaf area was highest in maize plants co-inoculated with Pseudomonas sp. MRBP4 + Pseudomonas sp. MRBP13 (212.45 ± 0.87 cm2) under drought stress. Treatment of maize seeds with Pseudomonas sp. MRBP 4 + Pseudomonas sp. MRBP13 + Bacillus sp. MRBP10 significantly increased the root length (10.32 ± 0.48 cm) which enhanced survival of the maize seedlings. Bioinoculation of maize seeds with these strains could boost maize production cultivated in arid regions.

Rhizosphere Priestia species altered cowpea root transcriptome and enhanced growth under drought and nutrient deficiency

Priestia species isolated from the cowpea rhizosphere altered the transcriptome of cowpea roots by colonization and enhanced nutrient uptake, antioxidant mechanisms, and photosynthesis, protecting cowpea from drought and nutrient deficiency. Cowpea is a significant grain legume crop primarily grown in sub-Saharan Africa, Asia, and South America. Drought and nutrient deficiency affect the growth and yield of cowpea. To address this challenge, we studied the phyto-beneficial effects of stress-tolerant rhizobacteria on the biomass yield of cowpea under water- and nutrient-deficit conditions. Among the bacteria isolated, two rhizobacillus genotypes, C8 (Priestia filamentosa; basonym: Bacillus filamentosus) and C29 (Priestia aryabhattai; basonym: Bacillus aryabhattai) were evaluated for the improvement of seed germination and growth of cowpea under stress. Our study revealed that C8 protected cowpea from stress by facilitating phosphorus and potassium uptake, protecting it from oxidative damage, reducing transpiration, and enhancing CO₂ assimilation. A 17% increase in root biomass upon C8 inoculation was concomitant with the induction of stress tolerance genes in cowpea roots predominantly involved in growth and metabolic processes, cell wall organization, ion homeostasis, and cellular responses to phosphate starvation. Our results indicate a metabolic alteration in cowpea root triggered by P. filamentosa, leading to efficient nutrient reallocation in the host plant. We propose inoculation with P. filamentosa as an effective strategy for improving the yield of cowpea in low-input agriculture, where chemical fertilization and irrigation are less accessible to resource-poor farmers.

Lysinibacilli: A Biological Factories Intended for Bio-Insecticidal, Bio-Control, and Bioremediation Activities

Microbes are ubiquitous in the biosphere, and their therapeutic and ecological potential is not much more explored and still needs to be explored more. The bacilli are a heterogeneous group of Gram-negative and Gram-positive bacteria. Lysinibacillus are dominantly found as motile, spore-forming, Gram-positive bacilli belonging to phylum Firmicutes and the family Bacillaceae. Lysinibacillus species initially came into light due to their insecticidal and larvicidal properties. Bacillus thuringiensis, a well-known insecticidal Lysinibacillus, can control many insect vectors, including a malarial vector and another, a Plasmodium vector that transmits infectious microbes in humans. Now its potential in the environment as a piece of green machinery for remediation of heavy metal is used. Moreover, some species of Lysinibacillus have antimicrobial potential due to the bacteriocin, peptide antibiotics, and other therapeutic molecules. Thus, this review will explore the biological disease control abilities, food preservative, therapeutic, plant growth-promoting, bioremediation, and entomopathogenic potentials of the genus Lysinibacillus.

Bacillus aryabhattai enhanced proline content, stabilized membrane and improved growth of cowpea under NaCl-induced salinity stress

AIMS

Salinity stress affects the growth of cowpea particularly at the stages of seed germination and early vegetative growth. This study examined the potential of particular stress-tolerant rhizospheric bacteria to improve the growth of cowpea under conditions of salinity stress.

METHODS AND RESULTS

Two rhizobacillus genotypes, Bacillus filamentosus-C8 and Bacillus aryabhattai-C29 were evaluated for their potentials to protect cowpea under NaCl-induced salinity stress. At 200 mM of NaCl concentration, control (non-inoculated) cowpea was affected, C8 was not able to significantly (p ≤ 0.05) alleviate the effects of salinity stress on cowpea growth while C29 significantly (p ≤ 0.05) reduced leaf wilting, increased chlorophyll content and improved the growth of cowpea plant under stressed condition. Interestingly, C29 significantly (p ≤ 0.05) induced high proline content and stabilized membrane by loss of electrolytes.

CONCLUSION

Our results indicate that stabilized membrane and enhanced proline content by Bacillus aryabhattai-C29 supported the growth of cowpea under salinity stress condition.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study revealed that rhizospheric bacteria screened for salinity stress tolerant have potential to be used as an effective bioprotectant for sustainable growth of cowpea under salinity stress condition.

Bacillus subtilis HG-15, a Halotolerant Rhizoplane Bacterium, Promotes Growth and Salinity Tolerance in Wheat (Triticum aestivum)

Certain plant growth-promoting bacteria (PGPB) reduce salt stress damage in plants. Bacillus subtilis HG-15 is a halotolerant bacterium (able to withstand NaCl concentrations as high as 30%) isolated from the wheat rhizoplane in the Yellow River delta. A qualitative and quantitative investigation of the plant growth-promoting characteristics of this strain confirmed nitrogen fixation, potassium dissolution, ammonia, plant hormone, ACC deaminase, and proline production abilities. B. subtilis HG-15 colonization of wheat roots, stems, and leaves was examined via scanning electron microscopy, rep-PCR, and double antibiotic screening. After inoculation with the B. subtilis HG-15 strain, the pH (1.08–2.69%), electrical conductivity (3.17–11.48%), and Na⁺ (12.98–15.55%) concentrations of rhizosphere soil significantly decreased (p < 0.05). Under no-salt stress (0.15% NaCl), low-salt stress (0.25% NaCl), and high-salt stress (0.35% NaCl) conditions, this strain also significantly increased (p < 0.05) the dry weight (17.76%, 24.46%, and 9.31%), fresh weight (12.80%, 20.48%, and 7.43%), plant height (7.79%, 5.86%, and 13.13%), and root length (10.28%, 17.87%, and 48.95%). Our results indicated that B. subtilis HG-15 can effectively improve the growth of wheat and elicit induced systemic tolerance in these plants, thus showing its potential as a microbial inoculant that can protect wheat under salt stress conditions.

Influence of Maize Residues in Shaping Soil Microbiota and Fusarium spp. Communities

Fusarium head blight (FHB) is a devastating fungal disease of small grain cereals including wheat. Causal fungal agents colonize various components of the field during their life cycle including previous crop residues, soil, and grains. Although soil and residues constitute the main inoculum source, these components have received much less attention than grains. This study aimed at disentangling the role of previous crop residues in shaping soil microbiota, including Fusarium spp. communities, in fields under wheat-maize rotation. Such knowledge may contribute to better understand the complex interactions between Fusarium spp. and soil microbiota. Dynamics of bacterial and fungal communities, with a special focus on Fusarium spp., were monitored in soils at 3 time points: during wheat cultivation (April 2015 and 2017) and after maize harvest (November 2016) and in maize residues taken from fields after harvest. Shifts in microbiota were also evaluated under mesocosm experiments using soils amended with maize residues. Fusarium graminearum and F. avenaceum were predominant on maize residues but did not remain in soils during wheat cultivation. Differences in soil bacterial diversity and compositions among years were much lower than variation between fields, suggesting that bacterial communities are field-specific and more conserved over time. In contrast, soil fungal diversity and compositions were more influenced by sampling time. Maize residues, left after harvest, led to a soil enrichment with several fungal genera, including Epicoccum, Fusarium, Vishniacozyma, Papiliotrema, Sarocladium, Xenobotryosphaeria, Ramularia, Cladosporium, Cryptococcus, and Bullera, but not with bacterial genera. Likewise, under mesocosm conditions, the addition of maize residues had a stronger influence on fungal communities than on bacterial communities. In particular, addition of maize significantly increased soil fungal richness, while bacteria were much less prone to changes. Based on co-occurrence networks, OTUs negatively correlated to Fusarium spp. were identified, such as those assigned to Epicoccum and Vishniacozyma. Altogether, our results allowed to gain a deeper insight into the complex microbiota interactions in soils, with bacteria and fungi responding differently to environmental disturbances.

N-acyl Homoserine Lactone Mediated Quorum Sensing Exhibiting Plant Growth-promoting and Abiotic Stress Tolerant Bacteria Demonstrates Drought Stress Amelioration

Multiple plant growth-promoting attributes with N-acyl homoserine lactone (AHL)-mediated quorum sensing exhibiting bacterial strains can help plants to withstand varying abiotic and biotic stress conditions for improving the plant health and productivity. In total, 306 bacterial isolates were isolated from diverse locations and sites. In our exploration, bacterial isolates were screened based on AHL production, plant growth-promoting attributes, abiotic stress tolerance, and antagonistic activity against phytopathogenic fungi. Among the screened 306 isolates, 4 (11VPKHP4, 7VP51.8, P51.10, NBRI N7) were selected based on their efficiency in AHL production, biofilm formation, enduring different abiotic stress conditions, exhibiting plant growth-promoting attributes, and antagonistic activity. Based on 16S rRNA gene sequencing analyses of the selected 4 isolates belong to Pseudomonas genera. Selected isolates 11VPKHP4, 7VP51.8, P51.10, and NBRI N7 were also proficient in biosurfactant production, emulsification, suggesting that all isolates fabricate emulsifiers. The plant growth promotion potential of selected 4 bacterial isolates showed significant growth enhancement in all the vegetative parameters of Zea mays under control as well as drought stress condition. Biochemical parameters and defense enzymes under drought stress conditions were also modulated in the PGPR treated plants as compared to their uninoculated respective controls. With quorum sensing, multiple PGPR attributes, stress tolerance, biofilm formation, and EPS production the selected isolates have the potential to facilitate enhanced plant growth, rhizosphere colonization, maintenance of soil moisture content under normal and diverse stresses.

Effect ofBacillus velezensis JC-K3 on Endophytic Bacterial and Fungal Diversity in Wheat Under Salt Stress

Plant growth-promoting bacteria (PGPB) can effectively reduce salt damage in plants. Currently, there are many studies on the effects of PGPB on the microbial community structure of rhizosphere soil under salt stress, but fewer studies on the community structure of endophytic bacteria and fungi. We propose that inoculation of endophytic bacteria into the rhizosphere of plants can significantly affect the microbial community structure of the plant's above-ground and underground parts, which may be the cause of the plant's "Induced Systemic Tolerance." The isolated endophytes were re-inoculated into the rhizosphere under salinity stress. We found that, compared with the control group, inoculation with endophytic Bacillus velezensis JC-K3 not only increased the accumulation of wheat biomass, but also increased the content of soluble sugar and chlorophyll in wheat, and reduced the absorption of Na in wheat shoots and leaves. The abundance of bacterial communities in shoots and leaves increased and the abundance of fungal communities decreased after inoculation with JC-K3. The fungal community richness of wheat rhizosphere soil was significantly increased. The diversity of bacterial communities in shoots and leaves increased, and the richness of fungal communities decreased. JC-K3 strain improved wheat's biomass accumulation ability, osmotic adjustment ability, and ion selective absorption ability. In addition, JC-K3 significantly altered the diversity and abundance of endophytic and rhizosphere microorganisms in wheat. PGPB can effectively reduce plant salt damage. At present, there are many studies on the effect of PGPB on the microbial community structure in rhizosphere soil under salt stress, but there are few studies on the community structure changes of endophytic bacteria and fungi in plants.

Kinetics, pathways and toxicity of hexabromocyclododecane biodegradation: Isolation of the novel bacterium Citrobacter sp. Y3

This research investigated the biodegradation kinetics, pathways and ecological risk of hexabromocyclododecane (HBCD) by a novel bacterium Citrobacter sp. Y3. Results showed the biodegradation followed a first-order model. The specific degradation rate constant of HBCD were obviously higher in batch experiments with combined carbon sources (k: 0.156-0.290 d^-1) than those using HBCD as the sole carbon source (k: 0.055 d^-1). Correspondingly, the degradation half-life became much shorter (T_1/2: 2.39-4.44 d vs T_1/2: 13.7 d). HBCD could be degraded through dehydrobromination and dehalohydroxylation, of which six possible degradation products were detected. To evaluate the ecological risk of HBCD biodegradation products, acute toxicity tests were assessed for the first time. The acute toxicity decreased slowly during treatment for 3-5 d and then decreased sharply. In general, treatment by Strain Y3 is not only a biodegradation process but also a detoxification process, thus it shows potential for bioremediation of HBCD contaminated sites.

Bacterial Diversity and Community Structure in the Rhizosphere of Four Halophytes

The study of the rhizosphere microbial community in salinized soils aids in the elucidation of new and important microbial functional groups, which is of great importance in vegetation restoration and ecological reconstruction of salinized soil. The rhizosphere soil bacterial diversity and community structures of four halophytes, including Kalidium foliatum, Lycium ruthenicum, Karelinia caspia and Phragmites australis, typically distributed in the saline-alkaline land of Southern Xinjiang, China, were studied using an Illumina paired-end sequence platform. The study aims to reveal the alpha diversity, species composition, abundance and the differences of rhizosphere bacteria among the four halophytes, explore their correlation with environmental factors. The results showed that the highest bacterial species diversity was associated with P. communis, followed by K. foliatum, K. caspia, and L. ruthenicum. The species richness was the lowest for L. ruthenicum, while the others showed no significant difference. Firmicutes, Proteobacteria, Actinobacteria and Bacteroidetes were the most dominant phyla. And Bacillus and Halomonas were the most common dominant genera. The bacterial communities associated with K. foliatum and K. caspia were similar, while that of L. ruthenicum was significantly different from other halophytes. Soil total nitrogen and total phosphorus, soil organic matter, soil water content, electronic conductivity and pH were identified as the key factors affecting bacterial abundance associated with the assayed halophytes. These results indicate that the four halophytes evaluated in the present study have a similar rhizosphere bacterial community structure due to their being in the same region. However, the bacterial abundance is different among the plant species, and soil properties are the important factors driving the structures of bacterial communities.

Pepper Bacterial Spot Control by Bacillus velezensis: Bioprocess Solution

Pepper bacterial spot is one of the most severe plant diseases in terms of infection persistence and economic losses when it comes to fresh pepper fruits used in nutrition and industrial processing. In this study, Bacillus velezensis IP22 isolated from fresh cheese was used as a biocontrol agent of pepper bacterial spot, whose main causal agent is the cosmopolitan pathogen Xanthomonas euvesicatoria. After optimization of the cultivation medium composition aimed at maximizing of the antimicrobial activity against X. euvesicatoria and validation of the optimized medium at the scale of a laboratory bioreactor, in planta tests were performed. The results have showed significant suppression of bacterial spot symptoms in pepper plants by the produced biocontrol agent, as well as reduction of disease spreading on the healthy (uninoculated) pepper leaves. Furthermore, HPLC-MS (high pressure liquid chromatography–mass spectrometry) analysis was employed to examine antimicrobial metabolites produced by B. velezensis IP22, where lipopeptides were found with similar m/z values compared to lipopeptides from fengycin and locillomycin families. The bioprocess solution developed at the laboratory scale investigated in this study represents a promising strategy for production of pepper bacterial spot biocontrol agent based on B. velezensis IP22, a food isolate with a great perspective for application in plant protection.

Effects of Enterobacter cloacae HG-1 on the Nitrogen-Fixing Community Structure of Wheat Rhizosphere Soil and on Salt Tolerance

The present study investigated the physiological and biochemical characteristics of Enterobacter cloacae HG-1 isolated from saline-alkali soil. We further studied the effect of this strain on the salt tolerance of wheat and on the community structure of nitrogen-fixing bacteria in rhizosphere soil. We determined that the investigated strain had high nitrogen fixation activity and produced iron carriers, 1-aminocyclopropane-1-carboxylic acid deaminase, and plant hormones. The metabolites of this strain contained 2,3-butanediol, [R-(R*, R*)], 2-heptanone, and other growth-promoting and antibacterial substances. The strain was also highly salt-tolerant (10% NaCl). After the inoculation of wheat with the HG-1 strain, we recorded increases in root length, plant height, fresh weight, and dry weight of 19.15%, 18.83%, 16.67%, and 17.96%, respectively, compared with uninoculated plants (P < 0.05). Compared with the leaves of uninoculated plants, the proline concentration in the leaves of inoculated plants increased by 12.43% (P < 0.05), the malondialdehyde level decreased by 27.26% (P < 0.05), K+ increased by 20.69%, Ca2+ increased by 57.53% and Na+ decreased by 31.43% (all P<0.05). Furthermore, we detected that inoculation with the HG-1 strain did not affect the species composition of nitrogen-fixing bacteria in wheat rhizosphere soil at the phylum level. However, the average relative abundance of Proteobacteria was significantly increased, whereas the abundance of Verrucomiorobia was significantly decreased compared with uninoculated plants. At the genus level, we detected 32 genera in control samples and 27 genera in inoculated samples, and the species diversity and relative abundance of samples inoculated with the HG-1 strain decreased compared with uninoculated plants. Inoculated samples had lower abundances of Azospirillum, Rhodomicrobium, and Anabaena. Our study demonstrated that the inoculation of wheat with E. cloacae HG-1 could promote the growth of wheat under salt stress and increase salt stress tolerance. The results of this study investigating the interaction among soil, plants, and microorganisms supplement agricultural microbial databases and could provide a reference for the development of microbial-based saline soil improvement programs.

Drought tolerant Ochrobactrum sp. inoculation performs multiple roles in maintaining the homeostasis in Zea mays L. subjected to deficit water stress

Plant growth-promoting rhizobacteria (PGPR) improve plant health under various biotic and abiotic stresses. However, the underlying mechanisms of the protective effects of PGPR in deficit water stress (WS) remain less explored. This study aimed to characterize the role of Ochrobactrum sp. NBRISH6 inoculation on maize (Zea mays "Maharaja") under WS conditions using multiple approaches such as physiological, anatomical, metabolic, and molecular. The effect of NBRISH6 inoculation using maize as a host plant was characterized under greenhouse conditions in deficit water stress. Results from this study demonstrated that NBRISH6 significantly lowered the expression of genes involved in the abscisic acid cycle, deficit water stress-response, osmotic stress, and antioxidant enzyme activity (superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, and polyphenol oxidase). Phytohormones, i.e. indole acetic acid (IAA) and salicylic acid (SA) levels, intercellular CO₂ concentration, metabolites such as simple sugars, amino acids, aliphatic hydrocarbons, and the number of shrunken pith cells modulated in maize roots inoculated with NBRISH6. The NBRISH6 inoculation also improved the plant vegetative properties (root length, 33.80%; shoot length, 20.68%; root dry weight, 39.21%; shoot dry weight, 61.95%), shoot nutrients, xylem cells, root hairs, vapor pressure deficit (75%), intrinsic water-use efficiency (41.67%), photosynthesis rate (83.33%), and total chlorophyll (16.15%) as compared to the respective stress controls. This study provides valuable insights into mechanistic functions of PGPR in WS amelioration and promoting plant physiological response.

ACC deaminase-producing rhizosphere competent Bacillus spp. mitigate salt stress and promote Zea mays growth by modulating ethylene metabolism

Plant growth-promoting rhizobacteria (PGPR) are known for growth promotion and mitigating environmental stresses. Here, we examined the propitiousness of three indigenous salt-tolerant PGPR, i.e., Bacillus subtilis (NBRI 28B), B. subtilis (NBRI 33 N), and B. safensis (NBRI 12 M) for plant growth promotion and salt stress amelioration in Zea mays. Results of the in vitro plant growth-promoting attribute revealed NBRI 12 M demonstrated the highest values at 1 M salt (NaCl) concentration. Furthermore, the greenhouse experiment using three Bacillus strains confirmed plant growth-promoting and salt stress-ameliorating ability, through colonizing successfully and mitigating the adverse effects of ethylene by modulating 1-aminocyclopropane-1-carboxylic acid (ACC) accumulation, ACC-oxidase (ACO), and ACC-synthase (ACS) activities under salt stress. Bacillus sp. inoculation has also induced plant response for defense enzymes, chlorophyll, proline and soluble sugar under salt stress. Among three Bacillus strains, NBRI 12 M not only demonstrated higher values for plant growth-promoting (PGP) attributes but also the same was observed in the greenhouse experiment. Thus, the outcomes of this comparative study represent for the first time that salt-tolerant Bacillus strains exhibiting multiple PGP attributes under salt stress along with high rhizosphere competence can alleviate salt stress by reducing the stress ethylene level in the host plant.

Core and Differentially Abundant Bacterial Taxa in the Rhizosphere of Field Grown Brassica napus Genotypes: Implications for Canola Breeding

Modifying the rhizosphere microbiome through targeted plant breeding is key to harnessing positive plant-microbial interrelationships in cropping agroecosystems. Here, we examine the composition of rhizosphere bacterial communities of diverse Brassica napus genotypes to identify: (1) taxa that preferentially associate with genotypes, (2) core bacterial microbiota associated with B. napus, (3) heritable alpha diversity measures at flowering and whole growing season, and (4) correlation between microbial and plant genetic distance among canola genotypes at different growth stages. Our aim is to identify and describe signature microbiota with potential positive benefits that could be integrated in B. napus breeding and management strategies. Rhizosphere soils of 16 diverse genotypes sampled weekly over a 10-week period at single location as well as at three time points at two additional locations were analyzed using 16S rRNA gene amplicon sequencing. The B. napus rhizosphere microbiome was characterized by diverse bacterial communities with 32 named bacterial phyla. The most abundant phyla were Proteobacteria, Actinobacteria, and Acidobacteria. Overall microbial and plant genetic distances were highly correlated (R = 0.65). Alpha diversity heritability estimates were between 0.16 and 0.41 when evaluated across growth stage and between 0.24 and 0.59 at flowering. Compared with a reference B. napus genotype, a total of 81 genera were significantly more abundant and 71 were significantly less abundant in at least one B. napus genotype out of the total 558 bacterial genera. Most differentially abundant genera were Proteobacteria and Actinobacteria followed by Bacteroidetes and Firmicutes. Here, we also show that B. napus genotypes select an overall core bacterial microbiome with growth-stage-related patterns as to how taxa joined the core membership. In addition, we report that sets of B. napus core taxa were consistent across our three sites and 2 years. Both differential abundance and core analysis implicate numerous bacteria that have been reported to have beneficial effects on plant growth including disease suppression, antifungal properties, and plant growth promotion. Using a multi-site year, temporally intensive field sampling approach, we showed that small plant genetic differences cause predictable changes in canola microbiome and are potential target for direct and indirect selection within breeding programs.

Root-tip cutting and uniconazole treatment improve the colonization rate of Tuber indicum on Pinus armandii seedlings in the greenhouse

The Chinese black truffle Tuber indicum is commercially valuable. The main factors influencing the success or failure of a truffle crop include the mycorrhizal colonization rate and host plant quality. The effects of a plant growth regulator (uniconazole) and plant growth management technique (root‐tip cutting) on T. indicum colonization rate and Pinus armandii seedling growth were assessed under greenhouse conditions. The results indicated that 10 mg l⁻¹ uniconazole or the combination of 5 mg l⁻¹ uniconazole and root‐tip cutting constitutes an effective method for ectomycorrhizal synthesis based on an overall evaluation of colonization rate, plant biomass, plant height, root weight, stem circumference and antioxidant enzyme activities (SOD and POD) of P. armandii. The abundance of Proteobacteria in the rhizosphere of colonized seedlings might serve as an indicator of stable mycorrhizal colonization. This research inspires the potential application of uniconazole and root‐tip cutting treatments for mycorrhizal synthesis and truffle cultivation.

Effect of aridity and dune type on rhizosphere soil bacterial communities of Caragana microphylla in desert regions of northern China

Understanding the response of soil properties and bacterial communities in rhizosphere soil to aridity and dune types is fundamental to desertification control. This study investigated soil properties and bacterial communities of both rhizosphere and bulk soils of Caragana microphylla from four sites with different aridity indices, and one site with three different types of dunes. All sites were located in the desert regions of northern China. The results indicated that compared with the bulk soil, the soil nutrient content of rhizosphere, especially the content of total phosphorus, was generally significantly improved in different desertification environments. The bacterial richness and diversity were also higher than those of bulk soil, especially in arid regions and fixed dunes. Firmicutes, Actinobacteria, Proteobacteria, and Acidobacteria were the most dominant phyla in all samples. The regression analyses showed that at different sites, soil total organic C, total N, Na⁺, and total P played key roles in determining the bacterial community structure while total organic carbon, electronic conductivity, pH and total phosphorus were the dominant factors at the different dunes. The results further revealed that the dominant phyla strongly affected by environmental factors at different sites were Acidobacteria, Gemmatimonadetes, and Actinobacteria among which, Acidobacteria and Gemmatimonadetes were negatively correlated with Na⁺ content. At different types of dunes, Actinobacteria, Planctomycetes, and Gemmatimonadetes were particularly affected by environmental factors. The increased abundance of Actinobacteria in the rhizosphere soil was mainly caused by the decreased soil pH.

Isolation and Characterization of Antagonistic Bacteria Paenibacillus jamilae HS-26 and Their Effects on Plant Growth

Soilborne pathogens affect plant growth and food production worldwide. The application of chemical fertilizers and pesticides to control plant diseases has harmful effects; fortunately, plant growth-promoting rhizobacteria can be used as a potential alternative strategy. Here, Paenibacillus jamilae HS-26 was selected for its highly antagonistic activity against several soilborne pathogens. The bacterium synthesized hydrolytic enzymes and released extracellular antifungal metabolites and volatile organic compounds—primarily, N, N-diethyl-1, 4-phenylenediamine, which was detected by gas chromatography-mass spectrometry and shown to inhibit fungal mycelial growth. Furthermore, HS-26 was useful for nitrogen fixation, phosphate and potassium solubilization, and siderophore and indoleacetic acid production. In vitro tests and pot experiments revealed that HS-26 considerably increased plant biometric parameters. Illumina MiSeq sequencing data showed a significant reduction in soilborne pathogens and increase in beneficial bacteria in the wheat rhizosphere after treatment with strain HS-26.

Combined Metabarcoding and Co-occurrence Network Analysis to Profile the Bacterial, Fungal and Fusarium Communities and Their Interactions in Maize Stalks

Fusarium Head Blight (FHB) is one of the most devastating diseases of cereals worldwide, threatening both crop production by affecting cereal grain development, and human and animal health by contaminating grains with mycotoxins. Despite that maize residues constitute the primary source of inoculum for Fusarium pathogenic species, the structure and diversity of Fusarium spp. and microbial communities in maize residues have received much less attention than in grains. In this study, a metabarcoding approach was used to study the bacterial, fungal and Fusarium communities encountered in maize stalks collected from 8 fields in Brittany, France, after maize harvest during fall 2015. Some predominant genera found in maize residues were cereal or maize pathogens, such as the fungal Fusarium, Acremonium, and Phoma genera, and the bacterial Pseudomonas and Erwinia genera. Furthermore, a high predominance of genera with previously reported biocontrol activity was found, including the bacterial Sphingomonas, Pedobacter, Flavobacterium, Pseudomonas, and Janthinobacterium genera; and the fungal Epicoccum, Articulospora, Exophiala, and Sarocladium genera. Among Fusarium spp., F. graminearum and F. avenaceum were dominant. We also found that the maize cultivar and previous crop could influence the structure of microbial communities. Using SparCC co-occurrence network analysis, significant negative correlations were obtained between Fusarium spp. responsible for FHB (including F. graminearum and F. avenaceum) and bacterial OTUs classified as Sphingomonas and fungal OTUs classified as Sarocladium and Epicoccum. Considering that isolates belonging to these taxa have already been associated with antagonist effect against different Fusarium spp. and/or other pathogenic microorganisms and due to their predominance and negative associations with Fusarium spp., they may be good candidates as biocontrol agents. Combining the use of Fusarium-specific primers with universal primers for bacteria and fungi allowed us to study the microbial communities, but also to track correlations between Fusarium spp. and other bacterial and fungal genera, using co-occurrence network analysis. Such approach could be a useful tool as part of a screening strategy for novel antagonist candidates against toxigenic Fusarium spp., allowing the selection of taxa of interest.

Selecting lipopeptide-producing, Fusarium-suppressing Bacillus spp.: Metabolomic and genomic probing of Bacillus velezensis NWUMFkBS10.5

The results of this study indicate that the maize rhizosphere remains a reservoir for microbial strains with unique beneficial properties. The study sought to provide an indigenous Bacillus strain with a bioprotective potential to alleviate maize fusariosis in South Africa. We selected seven Bacillus isolates (MORWBS1.1, MARBS2.7, VERBS5.5, MOREBS6.3, MOLBS8.5, MOLBS8.6, and NWUMFkBS10.5) with biosuppressive effects against two maize fungal pathogens (Fusarium graminearum and Fusarium culmorum) based on 16S rDNA gene characterization and lipopeptide gene analysis. The PCR analysis revealed that lipopeptide genes encoding the synthesis of iturin, surfactin, and fengycin might be responsible for their antifungal activities. Few of the isolates also showed possible biosurfactant capability, and their susceptibility to known antibiotics is indicative of their eco‐friendly attributes. In addition, in silico genomic analysis of our best isolate (Bacillus velezensis NWUMFkBS10.5) and characterization of its active metabolite with FTIR, NMR, and ESI‐Micro‐Tof MS confirmed the presence of valuable genes clusters and metabolic pathways. The versatile genomic potential of our Bacillus isolate emphasizes the continued relevance of Bacillus spp. in biological management of plant diseases.

Isolation and characterization of antagonistic bacteria with the potential for biocontrol of soil-borne wheat diseases

AIM

The aim of this study was to identify efficient plant-beneficial rhizobacterium that has the potential to be developed as biocontrol agent for the control of wheat soil-borne diseases.

METHODS AND RESULTS

Rhizosphere soil samples were collected from a wheat field located in Taian City. Numerous bacteria were isolated and screened for antagonistic activity against soil-borne plant pathogenic fungi by performing dual-culture assays. Among them, XH-9 was selected for its highly antagonistic activity and others growth-promoting characteristics. Subsequently, the strain was identified as Bacillus amyloliquefaciens subsp. plantarum based on phylogenetic analysis of 16S rDNA sequence. Pot experiment indicated that XH-9 has good capacities for wheat, corn, and chili root colonization and considerably increased the biometric parameters of wheat seedlings. Quantitative real-time polymerase chain reaction experiments showed that the amount of Fusarium oxysporum associated with the XH-9 after treatment significantly decreased compared with control group.

CONCLUSIONS

Bacillus amyloliquefaciens subsp. plantarum XH-9 has the potential as biocontrol agent when applied in local arable land to prevent damage caused by F. oxysporum and other phytopathogens.

SIGNIFICANCE AND IMPACT OF THE STUDY

The development of biocontrol strategies for reducing the damage caused by plant pathogens is fully in accord with the current principles of sustainability.

Effect of reclamation of abandoned salinized farmland on soil bacterial communities in arid northwest China

Understanding the impact of reclamation of abandoned salinized farmland on soil bacterial community is of great importance for maintaining soil health and sustainability in arid regions. In this study, we used field sampling and 454 pyrosequencing methods to investigate the effects of 5-year reclamation treatments on soil properties, bacterial community composition and diversity. The four reclamation treatments are: abandoned salinized farmland (CK), cropland (CL), grassland (GL) and woodland (WL). We have found soil properties are significantly altered by abandoned salinized farmland reclamation. In particular, the lowest soil pH and electrical conductivity (EC) values are observed in CL (P<0.05). The dominant phyla are Firmicutes, Proteobacteria, Chloroflexi, Actinobacteria and Acidobacteria in all treatments. At the genus levels, the relative abundance of Bacillus, Lactococcus, Streptococcus and Enterococcus in CK, GL and WL is significantly higher than in CL. Bacterial diversity indices (i.e. ACE, Chao and Shannon) dramatically increase after the reclamation, with the highest in CL. Similar patterns of bacterial communities have been observed in CK, GL and WL soils, but significantly different from CL. Regression analyses indicate that the relative abundance of these phyla are significantly correlated with soil Fe, pH and EC. Results from non-metric multidimensional scaling (NMDS) and redundancy analysis (RDA) indicate that soil Fe content, EC and pH are the most important factors in shaping soil bacterial communities. Overall, results indicate that abandoned salinized farmland reclaimed for CL significantly decrease soil pH and EC, and increase soil bacterial community diversity. Soil Fe concentration, EC and pH are the dominant environmental factors affecting soil bacterial community composition. The important role of Fe concentration in shaping bacterial community composition is a new discovery among the similar studies.

Tackling maize fusariosis: in search of Fusarium graminearum biosuppressors

This review presents biocontrol agents employed to alleviate the deleterious effect of the pathogen Fusarium graminearum on maize. The control of this mycotoxigenic phytopathogen remains elusive despite the elaborate research conducted on its detection, identification, and molecular fingerprinting. This could be attributed to the fact that in vitro and greenhouse biocontrol studies on F. graminearum have exceeded the number of field studies done. Furthermore, along with the variances seen among these F. graminearum suppressing biocontrol strains, it is also clear that the majority of research done to tackle F. graminearum outbreaks was on wheat and barley cultivars. Most fusariosis management related to maize targeted other members of Fusarium such as Fusarium verticillioides, with biocontrol strains from the genera Bacillus and Pseudomonas being used frequently in the experiments. We highlight relevant current techniques needed to identify an effective biofungicide for maize fusariosis and recommend alternative approaches to reduce the scarcity of data for indigenous maize field trials.

Community structure and plant growth-promoting potential of cultivable bacteria isolated from Cameroon soil

Exploiting native plant growth-promoting rhizobacteria (PGPR) in Cameroonian agro-ecosystems provides a means to improve plant-microbe interactions that may enhance ecosystem sustainability and agricultural productivity in an environmentally eco-friendly way. Consequently, we aimed to investigate the community structure and functional PGPR diversity of maize grown in Cameroon. Native bacteria isolated from Cameroon maize rhizosphere soil were identified by partial 16S rRNA gene sequencing and screened for traits particularly relevant for Cameroon low-fertility soil conditions, such as their abilities to tolerate high concentrations of salt, and their plant growth- promoting potential. Genetic and functional diversity was characterized according to their phylogenetic affiliation. A total of 143 bacteria were identified and assigned to 3 phyla (Actinobacteria, Firmicutes and Proteobacteria), 13 families and 20 genera. Bacillus (31.5%), Arthrobacter (17.5%), and Sinomonas (13.3%) were the most abundant genera identified among all the isolates. Based on their in vitro characterization, 88.1% were salt tolerant at 2% NaCl, but only 16.8% could tolerate 8% NaCl, 50.4% solubilized phosphate, 10.5% possessed the nifH gene, and 19.6% produced siderophores. Six isolates affiliated to the most abundant genera identified in this work, Bacillus and Arthrobacter, carrying multiple or only single tested traits were selected to evaluate their growth- promoting potential in an in vitro maize germination assay. Three strains possessing multiple traits induced significantly increased hypocotyl and root length of maize seeds compared to non-inoculated control seeds. Our results indicate the potential of selected indigenous Cameroon rhizobacteria to enhance maize growth.

Characterisation of Pseudomonas spp. and Ochrobactrum sp. isolated from volcanic soil

Soil bacteria may have properties of plant growth promotion but not be sufficiently beneficial for plants under stress conditions. This challenge has led researchers to extend their searches into extreme environments for potential soil bacteria with multiple plant beneficial traits as well as abiotic stress tolerance abilities. In the current study, an attempt was made to evaluate soil bacteria from an extreme environment, volcano soils, based on plant growth promoting and abiotic stress mitigating characteristics. The screening led to the isolation of eight (NBRISH4, NBRISH6, NBRISH10, NBRISH11, NBRISH13, NBRISH14, NBRISH16 and NBRISH26) bacterial isolates capable of withstanding stresses, namely temperature (up to 45 °C), salt (up to 2 M NaCl) and drought (up to 60% Poly Ethylene Glycol 6000) in vitro. Further, the selected isolates were notable for their in vitro temporal performance with regards to survival (in terms of colony count), phosphate solubilisation, biofilm formation, auxin, alginate and exo-polysaccharide production abilities under abiotic stresses i.e. 40 °C temperature; 500 mM NaCl salt and drought (PEG) conditions. In vivo seed treatments of individual selected bacteria to maize plants resulted into significant enhancement in root and shoot length, root and shoot fresh and dry weight and number of leaves per plant. Overall, the plant growth promoting and abiotic stress tolerance ability was most evident for bacterial isolate NBRISH6 which was identified as an Ochrobactrum sp. using 16S rRNA based phylogenetic analysis.

Isolation and Screening of Bacteria for Their Diazotrophic Potential and Their Influence on Growth Promotion of Maize Seedlings in Greenhouses

Poor soil fertility is one of the major constraints for crop production. Nitrogen is the most limiting nutrient for increasing crop productivity. Therefore, there is a need to identify diazotrophic inoculants as an alternative or supplement to N-fertilizers for sustainable agriculture. In the current study, a number of free-living diazotrophic bacteria were isolated from soils collected from maize rhizosphere and from leaves and roots of maize within the KwaZulu-Natal Province, Republic of South Africa. Ninety-two isolates were selected for further screening because they were able to grow on N-free media containing different carbon sources. Isolates that were very slow to grow on N-free media were discarded. The isolates were screened in vitro for diazotrophic potential tests for ammonia production and acetylene reduction. Ethylene (C2H4) production was quantified and ranged from 4 to 73 nmoles of C2H4h(-1) culture(-1). The top 20 isolates were re-screened on maize seedlings, and eight isolates significantly (P = 0.001) enhanced some growth parameters of maize above the un-inoculated control. Isolates that showed significant effect on at least two growth parameters were identified at species or genera level. In conclusion, selected diazotrophic isolates may be potentially beneficial but they should be tested more in greenhouse and field conditions with maize to confirm their potential for application as biofertilizers.