Organic fertilizer enhances the secretion of microRNAs from tomato roots to facilitate beneficial rhizosphere microorganism expansion and suppress Ralstonia solanacearum proliferation

BACKGROUND: Key members of the rhizomicrobiome, plant root exudates, and pathogen inhibition are important for the immune system functions of disease-suppressive soils, and a "cry for help" mechanism is proposed to describe this immune response process. However, there is still a gap in the understanding of rhizomicrobiome functional genes that are regulated by plants; to date, most studies have shown that the "cry for help" process is mediated by root exudates. The cross-talk between eukaryotes and prokaryotes through microRNAs (miRNAs) represents a new route for research on host and microbe interactions. RESULTS: After six generations of plantings, the disease index of the tomato plants significantly decreased compared with that of the first-generation plants (P < 0.05) in both the organic fertilizer (OF) and chemical fertilizer (CF) groups, and the effectiveness of OF in reducing the disease index of the tomato plants was obviously greater than that of CF. Furthermore, tomato miRNAs were identified in the rhizosphere soil, and exosome-like extracellular vesicles were found to be their potential carriers. Subsequent experiments confirmed that the tomato roots secreted sly-miR159 and sly-miR319c-3p, which were both crucial miRNAs that inhibited the proliferation of Ralstonia solanacearum and that sly-miR159 promoted the growth of beneficial bacteria belonging to the Streptomyces and Bacillus genera. The active functional components of organic fertilizer included soluble macromolecular compounds (nonmicrobial components) and microbial components. Among these, the nonmicrobial components induced the roots of tomato plants to secrete key microRNAs (sly-miR159 and sly-miR319c-3p), whereas the microbial components provided beneficial microbial communities for the rhizosphere of plants, jointly promoting the inhibition of Ralstonia solanacearum. CONCLUSIONS: In this study, the role of organic manure in promoting the establishment of disease-suppressive soil for combating bacterial wilt disease in tomato plants was comprehensively investigated. Moreover, this study provides a new perspective for research on rhizosphere immunity; that is, the presence of plant-derived exosomal miRNAs in the rhizosphere could serve as a new way to explain interactions between plants and the rhizosphere microbial community. Video Abstract.