Plant growth-promoting rhizobacteria and Trichoderma shift common vetch (Vicia sativa) physiology and phyllosphere bacteria toward antagonism against anthracnose caused by Colletotrichum spinaciae

Background: Plant phyllosphere microbes are important for the host plant's protection. Plant growth-promoting rhizobacteria (PGPR) and Trichoderma are common biocontrol agents (BCAs) for disease management. Pathogens and BCAs can change the rhizosphere microbial composition; however, the effect of PGPR or Trichoderma on plant phyllosphere microbes, particularly for mesocosms involving the interaction between pathogens and BCAs, is not well known. Methods: High-throughput sequencing was used to identify the phyllosphere bacterial community of common vetch interacting with Colletotrichum spinaciae, two PGPRs (Bacillus subtilis and Bacillus licheniformis), and Trichoderma longibrachiatum. We evaluated anthracnose severity, phyllosphere bacteria diversity and composition, and the relationship between the activities of plant defense enzymes and hormonal molecules in plants treated with individual and combined inoculations of PGPRs, Trichoderma, and C. spinaciae. Results: PGPR or Trichoderma alone reduced disease severity. Trichoderma reduced the salicylic acid content, PGPR increased the catalase activity in plants, and co-inoculation of PGPR and Trichoderma decreased the salicylic acid content. Inoculation of PGPR and Trichoderma individually or in combination changed the disease-associated phyllosphere bacteria, and this effect was related to plant defense enzymes and hormonal molecules. Conclusions: We suggest that the plant defense response induced by PGPR and Trichoderma results in the enrichment of a fraction of favorable chloroplastic bacteria, which facilitates plant defense against diseases.