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While bacterial diversity is beneficial for the functioning of rhizosphere microbiomes, multi-species bioinoculants often fail to promote plant growth. One potential reason for this is that competition between different species of inoculated consortia members creates conflicts for their survival and functioning. To circumvent this, we used transposon insertion mutagenesis to increase the functional diversity within Bacillus amyloliquefaciens bacterial species and tested if we could improve plant growth promotion by assembling consortia of highly clonal but phenotypically dissimilar mutants. While most insertion mutations were harmful, some significantly improved B. amyloliquefaciens plant growth promotion traits relative to the wild-type strain. Eight phenotypically distinct mutants were selected to test if their functioning could be improved by applying them as multifunctional consortia. We found that B. amyloliquefaciens consortium richness correlated positively with plant root colonization and protection from Ralstonia solanacearum phytopathogenic bacterium. Crucially, 8-mutant consortium consisting of phenotypically dissimilar mutants performed better than randomly assembled 8-mutant consortia, suggesting that improvements were likely driven by consortia multifunctionality instead of consortia richness. Together, our results suggest that increasing intra-species phenotypic diversity could be an effective way to improve probiotic consortium functioning and plant growth promotion in agricultural systems., Competing Interests: JL, CY, AJ, KY, XW, ZX, TY, XM, ZZ, YX, QS, VF, ZW No competing interests declared, (© 2023, Li, Yang et al.)

Antibiosis and resource competition are major drivers shaping the assembly, diversity and functioning of microbial communities. While it is recognized that competition is sensitive to environmental conditions, it is unclear to what extent this mediated by the availability of different carbon resources.Here, we used a model laboratory system to directly test this by exploring how carbon resource richness and identity shape resource competition and antibiosis between plant probiotic Bacillus amyloliquefaciens and phytopathogenic Ralstonia solanacearum bacteria. We found that while sugars typically promoted B. amyloliquefaciens growth, organic and amino acids increased the production of both bacillaene and macrolactin antibiotics and the direct inhibition of R. solanacearum. In contrast, when multiple different carbon resources were available, B. amyloliquefaciens could efficiently grow and produce antibiotics at the same time.Together, these results suggest that high carbon resource richness allows concurrent expression of growth‐ and antibiosis‐related traits, potentially altering bacterial competitive dynamics and plant growth promotion in microbial communities. A plain language summary is available for this article. [ABSTRACT FROM AUTHOR]

Investigation of the properties and mechanisms of the interactions of root-colonizing biocontrol bacteria and plant pathogens is necessary to optimize the biocontrol strategies. In the present study, the interaction of a biocontrol strain Bacillus amyloliquefaciens T-5 tagged with a green fluorescent protein marker and a bacterial wilt pathogen Ralstonia solanacearum QL- Rs1115 tagged with red fluorescent protein marker was studied on tomato roots using different inoculation methods. The results showed that in the co-culture experiment, the population of pathogen QL-RFP was decreased by increasing the initial inoculum concentration of biocontrol strain. In the greenhouse experiment, both strains T-5-GFP and QL-RFP colonized tomato roots (root tips, root hairs, primary roots, and root junctions) and formed a biofilm on the root surfaces as determined by dilution plating and confocal laser scanning microscopy (CLSM) techniques. However, the root colonization of pathogen strain QL-RFP was almost completely suppressed in the presence of biocontrol strain T-5-GFP when both soil and plant seedlings were treated with T-5-GFP. The results of this study revealed the effectiveness of strain B. amyloliquefaciens T-5 as a biocontrol agent against tomato wilt pathogen and the significance of inoculation method used to inoculate biocontrol strain. [ABSTRACT FROM AUTHOR]

Soilborne plant diseases seriously limit agricultural production. Due to environmental concerns, sustainable agricultural practices must involve the use of environmentally friendly alternatives, such as biocontrol microbes (BMs), to suppress various plant diseases. However, the success of BM application largely depends on the carrier formulation. An ideal carrier that can support the survival of a BM while discouraging the growth of the target pathogen is expected to enhance the performance of the BM in plant disease control. In this study, various commercial composts and peats were evaluated as carriers for Bacillus amyloliquefaciens QL-18, a BM used to control tomato bacterial wilt (TBW) caused by the soilborne bacterial pathogen Ralstonia solanacearum. Among the evaluated carriers, rapeseed cake compost (RCC) was found to be the best carrier, acting as both a substrate and food source for B. amyloliquefaciens QL-18. RCC facilitated the growth and survival of strain QL-18 by supplying adequate levels of dissolved carbon and nitrogen. Moreover, aqueous extracts of RCC alone inhibited R. solanacearum growth, suggesting that certain RCC components are toxic to the pathogen. Greenhouse and field experiments demonstrated that RCC improved the ability of B. amyloliquefaciens QL-18 to reduce the incidence of TBW and the population of the pathogen in the tomato rhizosphere. Although other carriers maintained large populations of B. amyloliquefaciens QL-18, similar to those obtained with RCC, they were not deemed suitable carriers because they constituted a poor nutrient supply for B. amyloliquefaciens QL-18 growth; thus, they had less of an impact on disease control. The principle of BM carrier selection should be adopted in the future when developing new bio-products for the effective control of soilborne plant diseases. [ABSTRACT FROM AUTHOR]

Abstract: Root colonization by Bacillus amyloliquefaciens is directly related to bacterial growth, chemotaxis, biofilm formation, and the interaction with host plant root exudates. In this study, root exudates were collected from two tomato plant varieties that supported bacterial cell division and induced the B. amyloliquefaciens T-5 chemotactic response, even at the concentration of 10μgml−1. Root exudates also induced biofilm formation, but lower than control treatment. In addition, five organic acids were identified in the root exudates and subsequently evaluated. Malic acid, citric acid, succinic acid and fumaric acid significantly induced the chemotaxis response and swarming motility. Maximal chemotactic response and swarming motility were induced by malic acid, and all the organic acid did not have a significant effect on biofilm formation. Furthermore, these organic acids promoted the B. amyloliquefaciens T-5 recruitment under gnotobiotic conditions, increasing the rhizosphere bacterial population. This data suggested that tomato root colonization by B. amyloliquefaciens T-5 was influenced by organic acids secreted by roots. This study expands our understanding of B. amyloliquefaciens T-5 colonization on tomato roots under natural conditions and reflects the significance of B. amyloliquefaciens T-5 strain as biocontrol agent which will be useful for preparing formulations for the better control of plant wilt diseases. [Copyright &y& Elsevier]