A Broadly Distributed Rhizobacterium, Roseateles chitinivorans P500, Promotes Growth and Systemic Resistance via Jasmonic Acid-Dependent Oxylipin Signaling in Grasses

Harnessing root-associated microbiomes to promote beneficial microbial compositions could offer a sustainable strategy to increase crop resilience. Major challenges impeding this strategy are the lack of understanding of which native members of the microbiome benefit the host and molecular signaling events underlying these benefits. In this study, we isolated a strain of Roseateles chitinivorans, RcP500, corresponding to the most abundant bacterial taxon in the switchgrass root microbiome. Inoculation of roots with RcP500 promoted growth and induced systemic resistance (ISR) to Bipolaris leaf spot of switchgrass and closely related Panicum hallii. R. chitinivorans is also highly abundant in rhizosphere and root microbiomes of maize and rice and enhanced growth of these two plant species. Furthermore, RcP500 elicited ISR in maize against anthracnose leaf blight and southern corn leaf blight. Bioassays and root metabolite profiling in maize wild-type (WT) and jasmonic acid (JA)-deficient opr7opr8 mutant plants revealed the requirement of JA-dependent processes in RcP500-elicited synthesis of the JA precursor, 12-OPDA (cis-(+)-12-oxo-phytodienoic acid) and an α-ketol, 9,10-KODA (9-hydroxy-10-oxo-12(Z)-octadecadienoic acid), two oxylipins previously implicated in ISR signaling. Xylem sap transfusion of RcP500 colonized plants to naïve receiver plants corroborated the role of JA in promoting these signaling intermediates. While root JA synthesis was downregulated upon RcP500 colonization, gibberellic acid was induced, suggesting a potential mechanism behind simultaneous growth promotion and ISR triggered by this bacterium. Overall, this study identified a novel rhizobacterium with a broad host range that promotes growth and systemic resistance across multiple plant species in a JA-dependent, ketol-driven manner.