Plant Beneficial Bacteria Research Articles

Short-term responses of identified soil beneficial-bacteria to the insecticide fipronil: toxicological impacts.

Pesticides including insecticides are often applied to prevent distortion posed by plant insect pests. However, the application of these chemicals detrimentally affected the non-target organisms including soil biota. Fipronil (FIP), a broad-spectrum insecticide, is extensively used to control pests across the globe. The frequent usage calls for attention regarding risk assessment of undesirable effects on non-target microorganisms. Here, laboratory-based experiments were conducted to assess the effect of FIP on plant-beneficial bacteria (PBB); Rhizobium leguminosarum (Acc. No. PQ578652), Azotobacter salinestris (Acc. No. PQ578649) and Serratia marcescens (Acc. No. PQ578651). PBB synthesized growth regulating substances were negatively affected by increasing fipronil concentrations. For instance, at 100 µg FIPmL-1, a decrease in indole-3-acetic acid (IAA) synthesis by bacterial strains followed the order: A. salinestris (95.6%) S. marcescens (91.6%) > R. leguminosarum (87%). Also, exposure of bacteria cells to FIP hindered the growth and morphology of PBB observed as distortion, cracking, and aberrant structure under scanning electron microscopy (SEM). Moreover, FIP-treated and propidium iodide (PI)-stained bacterial cells displayed an insecticide dose-dependent increase in cellular permeability as observed under a confocal laser microscope (CLSM). Colony counts (log10 CFU mL-1) and growth of A. salinestris was completely inhibited at 150 µg FIPmL-1. The surface adhering ability (biofilm formation) of PBB was also disrupted/inhibited in a FIP dose-related manner. The respiration loss due to FIP was coupled with a reduction in population size. Fipronil at 150 µgmL-1 decreased cellular respiration in A. salinestris (72%) S. marcescens (53%) and R. leguminosarum (85%). Additionally, biomarker enzymes; lactate dehydrogenase (LDH), lipid peroxidation (LPO), and oxidative stress (catalase; CAT) induced by FIP represented significant (p ≤ 0.05) toxicity towards PBB strains. Conclusively, fipronil suggests a toxic effect that emphasizes their careful monitoring in soils before application and their optimum addition in the soil-plant system. It is high time to prepare both target-specific and slow-released agrochemical formulation for crop protection with concurrent safeguarding of soils.

Insect-derived bacteria as biocontrol tool and a potent suppressor of plant pathogenic fungi in tomato cultivation

Sustainable agriculture is increasingly emphasized, focusing on microorganisms' role in maintaining soil fertility and inhibiting plant pathogens. Seeking novel sources of plant-beneficial bacteria, our study explores insects due to their established associations with plants and bacteria. The insect gut, hosting various bacteria, may hold microbes protecting against fungal infections, particularly plant pathogens. Traditional sources of plant growth-promoting bacteria are the rhizosphere and host plant tissues; however, insects serve as diverse bacterial reservoirs in the environment. This study aimed to identify insect-gut-derived bacteria with antifungal properties and cellulase enzyme production, predicting high plant tissue colonization abilities. Cellulase, crucial for breaking down cellulose, is essential for both industry and the environment. We sought to assess the potential of these bacteria as biocontrol agents against plant pathogenic fungi, with a focus on tomato plants. Bacterial isolates from insect bodies, including Lactococcus lactis, Pantoea ananatis, and Serratia liquefaciens, exhibited robust antifungal properties and cellulase activity. In vitro tests and glasshouse tests, confirmed their ability to inhibit the growth of plant-pathogenic fungi, indicating potential for biological control. Moreover, selected strains demonstrated high cellulase enzyme activity, vital for nutrient competition and rapid colonization of plant surfaces. The study introduces insect-gut-derived bacteria as promising biocontrol agents against plant pathogenic fungi. The identified strains, capable of inhibiting fungal growth and producing cellulase, offer sustainable alternatives to synthetic fungicides for protecting tomato plants. The findings advance agricultural practices by harnessing insect-associated bacteria, contributing to eco-friendly and efficient pest management strategies in modern agriculture.

Establishment and Effect of a Protist Consortium on the Maize Rhizosphere

Protists are abundant in the rhizosphere, and protist inoculation onto plants has revealed protists’ roles in plant growth and development, disease suppression, and relationships with plant-beneficial bacteria. In this study, we evaluated the establishment of an 18-member defined protist consortium after inoculation onto germinated maize seedlings, along with the effects on the bacterial populations in the rhizosphere. Inoculated protists established a community that could be detected in the rhizosphere 3 weeks after inoculation. Plants inoculated with the protist consortium developed a bacterial community structure similar to that of plants inoculated with protist-free rhizosphere bacteria but different from that of uninoculated plants. This indicates that the protist cultures introduced rhizosphere-competent bacteria to the seedling. We also observed a plant growth phenotype, as inoculation with rhizosphere bacteria suppressed plant growth in the absence, but not the presence, of protists. Maize inoculated with an antibiotic-treated protist had lower biomass than plants inoculated with an untreated protist, indicating that growth promotion by protists depended on bacteria in the protist culture microbiome. This study shows that inoculation of germinated seeds establishes protist and bacterial communities on maize roots and that plant-detrimental effects of rhizosphere bacteria can be mitigated by the presence of protists.

Volatile-mediated interspecific plant interaction promotes root colonization by beneficial bacteria via induced shifts in root exudation

BackgroundVolatile organic compounds (VOCs) released by plants can act as signaling molecules mediating ecological interactions. Therefore, the study of VOCs mediated intra- and interspecific interactions with downstream plant physiological responses is critical to advance our understanding of mechanisms underlying information exchange in plants. Here, we investigated how plant-emitted VOCs affect the performance of an interspecific neighboring plant via induced shifts in root exudate chemistry with implications for root-associated microbiota recruitment.ResultsFirst, we showed that VOCs emitted by potato-onion plants stimulate the growth of adjacent tomato plants. Then, we demonstrated that this positive effect on tomato biomass was attributed to shifts in the tomato rhizosphere microbiota. Specifically, we found potato-onion VOCs to indirectly affect the recruitment of specific bacteria (e.g., Pseudomonas and Bacillus spp.) in the tomato rhizosphere. Second, we identified and validated the compound dipropyl disulfide as the active molecule within the blend of potato-onion VOCs mediating this interspecific plant communication. Third, we showed that the effect on the tomato rhizosphere microbiota occurs via induced changes in root exudates of tomato plants caused by exposure to dipropyl disulfide. Last, Pseudomonas and Bacillus spp. bacteria enriched in the tomato rhizosphere were shown to have plant growth-promoting activities.ConclusionsPotato-onion VOCs—specifically dipropyl disulfide—can induce shifts in the root exudate of adjacent tomato plants, which results in the recruitment of plant-beneficial bacteria in the rhizosphere. Taken together, this study elucidated a new mechanism of interspecific plant interaction mediated by VOCs resulting in alterations in the rhizosphere microbiota with beneficial outcomes for plant performance.4wKeniGUxY9GPejLMpvDe9Video

The microbiome of continuous wheat rotations: minor shifts in bacterial and archaeal communities but a source for functionally active plant-beneficial bacteria

Continuous wheat cropping often leads to yield decline, with suspected causes including increased pathogen susceptibility, decreased root growth, or low nutrient use efficiency, though the exact causes remain unknown. Here, we investigated soil and root-associated bacterial and archaeal communities in wheat under rotation versus continuous cultivation, monitored fungal pathogen presence, and used beneficial bacteria to mitigate Gaeumannomyces tritici (Ggt) symptoms in greenhouse experiments. Sampling was conducted at two field sites with different soil textures over two years and at two plant developmental stages, capturing site-specific and seasonal fluctuations. By cultivation, 767 bacterial isolates were screened for plant-beneficial traits. Amplicon sequencing revealed minor effects on community structure due to wheat rotational position but pronounced effects related to site, year, and microhabitat. Rhizoplane isolates with plant-beneficial traits were mainly Flavobacterium, Microbacterium, Pedobacter, Pseudomonas, Stenotrophomonas, and Variovorax. Continuous wheat rotations showed the highest proportion of bacteria with fungal antagonistic potential, indicating plant roots’ recruitment of beneficial bacteria. Introducing Ggt in the greenhouse experiment altered the bacterial and archaeal community, confirming field study results and demonstrating that applying beneficial bacteria early in the season at the seedling stage can control Ggt and improve plant growth. This study highlights the dynamic nature of wheat's below-ground bacterial and archaeal community, influenced by soil type, season, and microhabitat, with wheat rotation playing a minor role. In addition, it confirms the potential of selected Bacillus and Pseudomonas isolates, whether used individually or in consortia, for microbe-assisted mitigation of yield decline in intensive wheat production.

Effect of Organic and Chemical Fertilizer on the Diversity of Rhizosphere and Leaf Microbial Composition in Sunflower Plant.

Applying organic manure to crops positively impacts the soil microbial community which is negatively impacted when chemical fertilizers are used. Organic manures also add new microbes to the soil in addition to influencing the growth of native ones. Metagenomic analysis of different organic manures, soil, and pot culture experiments conducted under various fertilizer conditions constitute the primary methodologies employed in this study. We compared the effect of two organic manure combinations and an inorganic fertilizer combination on microbial community of rhizosphere soil and leaves of sunflower plants. Metagenomic sequencing data analysis revealed that the diversity of bacteria and fungi is higher in organic manure than in chemical fertilizers. Each organic manure combination selectively increased population of some specific microbes and supported new microbes. Application of chemical fertilizer hurts many plant beneficial fungi and bacteria. In summary, our study points out the superiority of organic manure combinations in enhancing microbial diversity and supporting beneficial microbes. These findings enhance the profound influence of fertilizer types on sunflower microbial communities, shedding light on the intricate dynamics within the rhizosphere and leaf microbiome. Bacterial genera such as Bacillus, Serratia, Sphingomonas, Pseudomonas, Methylobacterium, Acinetobacter, Stenotrophomonas, and fungal genera such as Wallemia, Aspergillus, Cladosporium, and Penicillium constitute the key microbes of sunflower plants.

Wheat domestication alters root metabolic functions to drive the assembly of endophytic bacteria.

The domestication process progressively differentiated wild relatives from modern cultivars, thus impacting plant-associated microorganisms. Endophytic bacterial communities play vital roles in plant growth, development, and health, which contribute to the crop's sustainable development. However, how plant domestication impacts endophytic bacterial communities and relevant root exudates in wheat remains unclear. First, we have observed that the domestication process increased the root endophytic microbial community diversity of wheat while decreasing functional diversity. Second, domestication decreased the endophytic bacterial co-occurrence network stability, and it did significantly alter the abundances of core microorganisms or potential probiotics. Third, untargeted LC-MS metabolomics revealed that domestication significantly altered the metabolite profiles, and the abundances of various root exudates released were significantly correlated with keystone taxa including the Chryseobacterium, Massilia, and Lechevalieria. Moreover, we found that root exudates, especially L-tyrosine promote the growth of plant-beneficial bacteria, such as Chryseobacterium. Additionally, with L-tyrosine and Chryseobacterium colonized in the roots, the growth of wild wheat's roots was significantly promoted, while no notable effect could be found in the domesticated cultivars. Overall, this study suggested that wild wheat as a key germplasm material, and its native endophytic microbes may serve as a resource for engineering crop microbiomes to improve the morphological and physiological traits of crops in widely distributed poor soils.

Investigating protistan predators and bacteria within soil microbiomes in agricultural ecosystems under organic and chemical fertilizer applications

Organic farming can enhance biodiversity and soil health and is a sustainable alternative to conventional farming. Yet, soil protists especially protistan predators, have received inadequate attention, and their contributions to the sustainability of organic farming remained underexplored. In this study, we examined soil microbial communities from 379 samples, including both organic and chemically fertilized soils from China. Our findings revealed higher bacterial diversity and increases in plant-beneficial bacteria in organically farmed soils. Notably, organic farming systems facilitated dynamic predator-prey interactions, which may be disrupted by the application of chemical fertilizers. Additionally, organic farming enriched protistan predators, enhancing the relative abundance of functional PGPR, thus improving soil health. We further conducted a case study highlighting the critical role of organic matter in sustaining protistan predator populations and their interactions with bacteria. We propose the crucial contributions of organic inputs for supporting protistan predators and the interplay of predator-prey, ultimately enhancing soil functions and promoting agricultural sustainability.

Harnessing plant-beneficial bacterial encapsulation: A sustainable strategy for facilitating cadmium bioaccumulation in Medicago sativa

Plant-beneficial bacteria (PBB) have emerged as a promising approach for assisting phytoremediation of heavy metal (HM)-contaminated soils. However, their colonization efficiency is often challenged by complex soil environments. In this study, we screened one rhizobacterium (Klebsiella variicola Y38) and one endophytic bacterium (Serratia surfactantfaciens Y15) isolated from HM-contaminated soils and plants for their high resistance to Cd and strong growth-promoting abilities. These strains were encapsulated individually or in combination with alginate and applied with Medicago sativa in Cd-contaminated soil pot experiments. The effectiveness of different bacterial formulations in promoting plant growth and enhancing Cd bioconcentration in M. sativa was evaluated. Results showed that PBB application enhanced plant growth and antioxidant capacity while reducing oxidative damage. Encapsulated formulations outperformed unencapsulated ones, with combined formulations yielding superior results to individual applications. Quantitative PCR indicated enhanced PBB colonization in Cd-contaminated soils with alginate encapsulation, potentially explaining the higher efficacy of alginate-encapsulated PBB. Additionally, the bacterial agents modified Cd speciation in soils, resulting in increased Cd bioaccumulation in M. sativa by 217–337 %. The alginate-encapsulated mixed bacterial agent demonstrated optimal effectiveness, increasing the Cd transfer coefficient by 3.2-fold. Structural equation modeling and correlation analysis elucidated that K. variicola Y38 promoted Cd bioaccumulation in M. sativa roots by reducing oxidative damage and enhancing root growth, while S. surfactantfaciens Y15 facilitated Cd translocation to shoots, promoting shoot growth. The combined application of these bacteria leveraged the benefits of both strains. These findings contribute to diversifying strategies for effectively and sustainably remediating Cd-contaminated soils, while laying a foundation for future investigations into bacteria-assisted phytoremediation.

Natural plant disease suppressiveness in soils extends to insect pest control

BackgroundSince the 1980s, soils in a 22-km2 area near Lake Neuchâtel in Switzerland have been recognized for their innate ability to suppress the black root rot plant disease caused by the fungal pathogen Thielaviopsis basicola. However, the efficacy of natural disease suppressive soils against insect pests has not been studied.ResultsWe demonstrate that natural soil suppressiveness also protects plants from the leaf-feeding pest insect Oulema melanopus. Plants grown in the most suppressive soil have a reduced stress response to Oulema feeding, reflected by dampened levels of herbivore defense-related phytohormones and benzoxazinoids. Enhanced salicylate levels in insect-free plants indicate defense-priming operating in this soil. The rhizosphere microbiome of suppressive soils contained a higher proportion of plant-beneficial bacteria, coinciding with their microbiome networks being highly tolerant to the destabilizing impact of insect exposure observed in the rhizosphere of plants grown in the conducive soils. We suggest that presence of plant-beneficial bacteria in the suppressive soils along with priming, conferred plant resistance to the insect pest, manifesting also in the onset of insect microbiome dysbiosis by the displacement of the insect endosymbionts.ConclusionsOur results show that an intricate soil–plant-insect feedback, relying on a stress tolerant microbiome network with the presence of plant-beneficial bacteria and plant priming, extends natural soil suppressiveness from soilborne diseases to insect pests.4oEYADM9UCsv5obDEsLZQ_Video

Enhancing spinach (Spinacia oleracea L.) resilience in pesticide-contaminated soil: Role of pesticide-tolerant Ciceribacter azotifigens and Serratia marcescens in root architecture, leaf gas exchange attributes and antioxidant response restoration

This study unveils the detoxification potential of insecticide-tolerant plant beneficial bacteria (PBB), i.e., Ciceribacter azotifigens SF1 and Serratia marcescens SRB1, in spinach treated with fipronil (FIP), profenofos (PF) and chlorantraniliprole (CLP) insecticides. Increasing insecticide doses (25–400 μg kg−1 soil) significantly curtailed germination attributes and growth of spinach cultivated at both bench-scale and in greenhouse experiments. Profenofos at 400 μg kg−1 exhibited maximum inhibitory effects and reduced germination by 55%; root and shoot length by 78% and 81%, respectively; dry matter accumulation in roots and shoots by 79% and 62%, respectively; leaf number by 87% and leaf area by 56%. Insecticide application caused morphological distortion in root tips/surfaces, increased levels of oxidative stress, and cell death in spinach. Application of insecticide-tolerant SF1 and SRB1 strains relieved insecticide pressure resulting in overall improvement in growth and physiology of spinach grown under insecticide stress. Ciceribacter azotifigens improved germination rate (10%); root biomass (53%); shoot biomass (25%); leaf area (10%); Chl-a (45%), Chl-b (36%) and carotenoid (48%) contents of spinach at 25 μg CLP kg−1 soil. PBB inoculation reinvigorated the stressed spinach and modulated the synthesis of phytochemicals, proline, malondialdehyde (MDA), superoxide anions (O2•–), and hydrogen peroxide (H2O2). Scanning electron microscopy (SEM) revealed recovery in root tip morphology and stomatal openings on abaxial leaf surfaces of PBB-inoculated spinach grown with insecticides. Ciceribacter azotifigens inoculation significantly increased intrinsic water use efficiency, transpiration rate, vapor pressure deficit, intracellular CO2 concentration, photosynthetic rate, and stomatal conductance in spinach exposed to 25 μg FIP kg−1. Also, C. azotifigens and S. marcescens modulated the antioxidant defense systems of insecticide-treated spinach. Bacterial strains were strongly colonized to root surfaces of insecticide-stressed spinach seedlings as revealed under SEM. The identification of insecticide-tolerant PBBs such as C. azotifigens and S. marcescens hold the potential for alleviating abiotic stress to spinach, thereby fostering enhanced and safe production within polluted agroecosystems.

Quorum sensing-related activities of beneficial and pathogenic bacteria have important implications for plant and human health.

Eukaryotic organisms coevolved with microbes from the environment forming holobiotic meta-genomic units. Members of host-associated microbiomes have commensalic, beneficial/symbiotic, or pathogenic phenotypes. More than 100 years ago, Lorenz Hiltner, pioneer of soil microbiology, introduced the term 'Rhizosphere' to characterize the observation that a high density of saprophytic, beneficial, and pathogenic microbes are attracted by root exudates. The balance between these types of microbes decide about the health of the host. Nowadays we know, that for the interaction of microbes with all eukaryotic hosts similar principles and processes of cooperative and competitive functions are in action. Small diffusible molecules like (phyto)hormones, volatiles and quorum sensing signals are examples for mediators of interspecies and cross-kingdom interactions. Quorum sensing of bacteria is mediated by different autoinducible metabolites in a density-dependent manner. In this perspective publication, the role of QS-related activities for the health of hosts will be discussed focussing mostly on N-acyl-homoserine lactones (AHL). It is also considered that in some cases very close phylogenetic relations exist between plant beneficial and opportunistic human pathogenic bacteria. Based on a genome and system-targeted new understanding, sociomicrobiological solutions are possible for the biocontrol of diseases and the health improvement of eukaryotic hosts.

Sinomonas terricola sp. nov., a plant-beneficial bacterium isolated from litchi rhizosphere soil in Guangdong, China.

A novel plant-beneficial bacterium strain, designated as JGH33T, which inhibited Peronophythora litchii sporangia germination, was isolated on Reasoner's 2A medium from a litchi rhizosphere soil sample collected in Gaozhou City, Guangdong Province, PR China. Cells of strain JGH33T were Gram-stain-positive, aerobic, non-motile, bent rods. The strain grew optimally at 30-37 °C and pH 6.0-8.0. Sequence similarity analysis based on 16S rRNA genes indicated that strain JGH33T exhibited highest sequence similarity to Sinomonas albida LC13T (99.2 %). The genomic DNA G+C content of the isolate was 69.1 mol%. The genome of JGH33T was 4.7 Mbp in size with the average nucleotide identity value of 83.45 % to the most related reference strains, which is lower than the species delineation threshold of 95 %. The digital DNA-DNA hybridization of the isolate resulted in a relatedness value of 24.9 % with its closest neighbour. The predominant respiratory quinone of JGH33T was MK-9(H2). The major fatty acids were C15 : 0 anteiso (43.4 %), C16 : 0 iso (19.1 %) and C17 : 0 anteiso (19.3 %), and the featured component was C18 : 3 ω6c (1.01 %). The polar lipid composition of strain JGH33T included diphosphatidylglycerol, phosphatidylglycerol, dimannosylglyceride, phosphatidylinositol and glycolipids. On the basis of polyphasic taxonomy analyses data, strain JGH33T represents a novel species of the genus Sinomonas, for which the name Sinomonas terricola sp. nov. is proposed, with JGH33T (=JCM 35868T=GDMCC 1.3730T) as the type strain.

Root-secreted nucleosides: signaling chemoattractants of rhizosphere bacteria.

The rhizosphere is a complex ecosystem, consisting of a narrow soil zone influenced by plant roots and inhabited by soil-borne microorganisms. Plants actively shape the rhizosphere microbiome through root exudates. Some metabolites are signaling molecules specifically functioning as chemoattractants rather than nutrients. These elusive signaling molecules have been sought for several decades, and yet little progress has been made. Root-secreted nucleosides and deoxynucleosides were detected in exudates of various plants by targeted ultra-performance liquid chromatography-mass spectrometry/mass spectrometry. Rhizobacteria were isolated from the roots of Helianthemum sessiliflorum carrying the mycorrhizal desert truffle Terfezia boudieri. Chemotaxis was determined by a glass capillary assay or plate assays on semisolid agar and through a soil plate assay. Nucleosides were identified in root exudates of plants that inhabit diverse ecological niches. Nucleosides induced positive chemotaxis in plant beneficial bacteria Bacillus pumilus, Bacillus subtilis, Pseudomonas turukhanskensis spp., Serratia marcescens, and the pathogenic rhizobacterium Xanthomonas campestris and E coli. In a soil plate assay, nucleosides diffused to substantial distances and evoked chemotaxis under conditions as close as possible to natural environments. This study implies that root-secreted nucleosides are involved in the assembly of the rhizosphere bacterial community by inducing chemotaxis toward plant roots. In animals, nucleoside secretion known as "purinergic signaling" is involved in communication between cells, physiological processes, diseases, phagocytic cell migration, and bacterial activity. The coliform bacterium E. coli that inhabits the lower intestine of warm-blooded organisms also attracted to nucleosides, implying that nucleosides may serve as a common signal for bacterial species inhabiting distinct habitats. Taken together, all these may indicate that chemotaxis signaling by nucleosides is a conserved universal mechanism that encompasses living kingdoms and environments and should be given further attention in plant rhizosphere microbiome research.

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

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. 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. 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.

Identification of synthetic consortia from a set of plant-beneficial bacteria.

The use of microbial inoculants in agriculture as biofertilisers and/or biopesticides is an appealing alternative to replace or reduce the practice of agrochemicals. Plant microbiota studies are revealing the different bacterial groups which are populating plant microbiomes re-energising the plant probiotic bacteria (PPB) translational research sector. Some single-microbial strain bioinoculants have proven valid in agriculture (e.g., based on Trichoderma, mycorrhiza or rhizobia); however, it is now recommended to consider multistrain consortia since plant-beneficial effects are often a result of community-level interactions in plant microbiomes. A limiting step is the selection of a fitting combination of microbial strains in order to accomplish the best beneficial effect upon plant inoculation. In this study, we have used a subset of 23 previously identified and characterised rice-beneficial bacterial colonisers to design and test a series of associated experiments aimed to identify potential PPB consortia which are able to co-colonise and induce plant growth promotion. Bacterial strains were co-inoculated in vitro and in planta using several different methods and their co-colonisation and co-persistence monitored. Results include the identification of two 5-strain and one 2-strain consortia which displayed plant growth-promoting features. Future practical applications of microbiome research must include experiments aimed at identifying consortia of bacteria which can be most effective as crop amendments.

Adaptive laboratory evolution reveals regulators involved in repressing biofilm development as key players in Bacillus subtilis root colonization.

Root-associated microorganisms play an important role in plant health, such as plant growth-promoting rhizobacteria (PGPR) from the Bacillus and Pseudomonas genera. Although bacterial consortia including these two genera would represent a promising avenue to efficient biofertilizer formulation, we observed that Bacillus subtilis root colonization is decreased by the presence of Pseudomonas fluorescens and Pseudomonas protegens. To determine if B. subtilis can adapt to the inhibitory effect of Pseudomonas on roots, we conducted adaptative laboratory evolution experiments with B. subtilis in mono-association or co-cultured with P. fluorescens on tomato plant roots. Evolved isolates with various colony morphology and stronger colonization capacity of both tomato plant and Arabidopsis thaliana roots emerged rapidly from the two evolution experiments. Certain evolved isolates also had better fitness on the root in the presence of other Pseudomonas species. In all independent lineages, whole-genome resequencing revealed non-synonymous mutations in genes ywcC or sinR encoding regulators involved in repressing biofilm development, suggesting their involvement in enhanced root colonization. These findings provide insights into the molecular mechanisms underlying B. subtilis adaptation to root colonization and highlight the potential of directed evolution to enhance the beneficial traits of PGPR.IMPORTANCEIn this study, we aimed to enhance the abilities of the plant-beneficial bacterium Bacillus subtilis to colonize plant roots in the presence of competing Pseudomonas bacteria. To achieve this, we conducted adaptive laboratory experiments, allowing Bacillus to evolve in a defined environment. We successfully obtained strains of Bacillus that were more effective at colonizing plant roots than the ancestor strain. To identify the genetic changes driving this improvement, we sequenced the genomes of these evolved strains. Interestingly, mutations that facilitated the formation of robust biofilms on roots were predominant. Many of these evolved Bacillus isolates also displayed the remarkable ability to outcompete Pseudomonas species. Our research sheds light on the mutational paths selected in Bacillus subtilis to thrive in root environments and offers exciting prospects for improving beneficial traits in plant growth-promoting microorganisms. Ultimately, this could pave the way for the development of more effective biofertilizers and sustainable agricultural practices.

Rhizobium determinants of rhizosphere persistence and root colonization.

Bacterial persistence in the rhizosphere and colonization of root niches are critical for the establishment of many beneficial plant-bacteria interactions including those between Rhizobium leguminosarum and its host legumes. Despite this, most studies on R. leguminosarum have focused on its symbiotic lifestyle as an endosymbiont in root nodules. Here, we use random barcode transposon sequencing to assay gene contributions of R. leguminosarum during competitive growth in the rhizosphere and colonization of various plant species. This facilitated the identification of 189 genes commonly required for growth in diverse plant rhizospheres, mutation of 111 of which also affected subsequent root colonization (rhizosphere progressive), and a further 119 genes necessary for colonization. Common determinants reveal a need to synthesize essential compounds (amino acids, ribonucleotides, and cofactors), adapt metabolic function, respond to external stimuli, and withstand various stresses (such as changes in osmolarity). Additionally, chemotaxis and flagella-mediated motility are prerequisites for root colonization. Many genes showed plant-specific dependencies highlighting significant adaptation to different plant species. This work provides a greater understanding of factors promoting rhizosphere fitness and root colonization in plant-beneficial bacteria, facilitating their exploitation for agricultural benefit.

Influences of phosphorus-modified biochar on bacterial community and diversity in rhizosphere soil.

Root-associated bacteria play a vital role in the soil ecosystem and plant productivity. Previous studies have reported the decline of bacterial community and rhizosphere soil quality in the cultivation of some medicinal plants (i.e., Pseudostellaria heterophylla). Phosphorus (P)-modified biochar has the potential to improve soil health and quality. However, its influence on the bacterial community and diversity in the rhizosphere of medicinal plants is not well understood. Therefore, this study aims to investigate the effects of P-modified biochar on the bacterial community and diversity in the rhizosphere of P. heterophylla. Soil samples were collected from the rhizosphere of 4-month P. heterophylla under control (no biochar), 3% unmodified and 3% P-modified biochar treatments, respectively. Compared with control and unmodified biochar treatment, P-modified biochar significantly increased the relative abundance of plant-beneficial bacteria (P < 0.05), particularly Firmicutes, Nitrospirae and Acidobacteria. The relative abundance of Bacillus, belonging to Firmicutes, was dramatically raised from 0.032% in control group to 1.723% in P-modified biochar-treated group (P < 0.05). These results indicate the potential enhancement of soil quality for the growth of medicinal plants. The application of biochar significantly increased bacterial richness and bacterial diversity (P < 0.05). P modification of biochar did not have significant effects on soil bacterial richness (P > 0.05), while it reduced Shannon and increased Simpson diversity index of soil bacterial communities significantly (P < 0.05). It indicates a decrease in bacterial diversity. This research provides a new perspective for understanding the role of P-modified biochar in the rhizosphere ecosystem.

Fossil-fuel-dependent scenarios could lead to a significant decline of global plant-beneficial bacteria abundance in soils by 2100.

Exploiting the potential benefits of plant-associated microbes represents a sustainable approach to enhancing crop productivity. Plant-beneficial bacteria (PBB) provide multiple benefits to plants. However, the biogeography and community structure remain largely unknown. Here we constructed a PBB database to couple microbial taxonomy with their plant-beneficial traits and analysed the global atlas of potential PBB from 4,245 soil samples. We show that the diversity of PBB peaks in low-latitude regions, following a strong latitudinal diversity gradient. The distribution of potential PBB was primarily governed by environmental filtering, which was mainly determined by local climate. Our projections showed that fossil-fuel-dependent future scenarios would lead to a significant decline of potential PBB by 2100, especially biocontrol agents (-1.03%) and stress resistance bacteria (-0.61%), which may potentially threaten global food production and (agro)ecosystem services.