RESUMO
BACKGROUND: Bacillus subtilis is well known for promoting plant growth and reducing abiotic and biotic stresses. Mutant gene-defective models can be created to understand important traits associated with rhizosphere fitness. This study aimed to analyze the role of exopolymeric genes in modulating tomato rhizosphere microbiome assembly under a gradient of soil microbiome diversities using the B. subtilis wild-type strain UD1022 and its corresponding mutant strain UD1022eps-TasA, which is defective in exopolysaccharide (EPS) and TasA protein production. RESULTS: qPCR revealed that the B. subtilis UD1022eps-TasA- strain has a diminished capacity to colonize tomato roots in soils with diluted microbial diversity. The analysis of bacterial ß-diversity revealed significant differences in bacterial and fungal community structures following inoculation with either the wild-type or mutant B. subtilis strains. The Verrucomicrobiota, Patescibacteria, and Nitrospirota phyla were more enriched with the wild-type strain inoculation than with the mutant inoculation. Co-occurrence analysis revealed that when the mutant was inoculated in tomato, the rhizosphere microbial community exhibited a lower level of modularity, fewer nodes, and fewer communities compared to communities inoculated with wild-type B. subtilis. CONCLUSION: This study advances our understanding of the EPS and TasA genes, which are not only important for root colonization but also play a significant role in shaping rhizosphere microbiome assembly. Future research should concentrate on specific microbiome genetic traits and their implications for rhizosphere colonization, coupled with rhizosphere microbiome modulation. These efforts will be crucial for optimizing PGPR-based approaches in agriculture.
RESUMO
We aimed to unravel the events which favor the seed-rhizobacterium Bacillus amyloliquefaciens strain ALB629 (hereafter ALB629) interaction and which may interfere with the rhizobacterium colonization and growth on the spermosphere of common bean. Seed exudates from common bean were tested in vitro for ALB629 biofilm formation and bacterial growth. Furthermore, the performance of ALB629 on plant-related variables under drought stress was checked. Seed exudates (1 and 5% v/v) increased ALB629 biofilm formation. Additionally, the colony forming units for ALB629 increased both in culture and on the bean seed surface. The bean seed exudates up-regulated biofilm operons in ALB629 TasA and EpsD by ca. two and sixfold, respectively. The high-performance liquid chromatography (HPLC)-coupled with MS showed that malic acid is present as a major organic acid component in the seed exudates. Seeds treated with ALB629 and amended with malic acid resulted in seedlings with a higher bacterial concentration, induced plant drought tolerance, and promoted plant growth. We showed that seed exudates promote growth of ALB629 and malic acid was identified as a major organic acid component in the bean seed exudates. Our results also show that supplementation of ALB629 induced drought tolerance and growth in plants. The research pertaining to the biological significance of seed exudates in plant-microbe interaction is unexplored field and our work shows the importance of seed exudates in priming both growth and tolerance against abiotic stress.