Untargeted metabolomics identifies a bacterial cyclic dipeptide that induces resistance to a rust fungus of beans

Genistein and daidzein are isoflavonoid phytoalexins that increase rapidly during bean hypersensitive immunity to Pseudomonas savastanoi pv. phaseolicola. To understand how genistein and daidzein affect P. savastanoi pv. phaseolicola, non-targeted metabolomic mass spectrometry was performed on the bacterium treated in vitro. The bacterium catabolized genistein and daidzein and responded by producing several different classes of compounds including auxin-like indoles and cyclic dipeptides. Non-targeted metabolomic investigation of bean leaves infiltrated with the cyclic dipeptides revealed no similarities to auxin-induced metabolic changes, but one cyclic dipeptide, cyclo-Trp-Pro (cWP), induced the accumulation of phytoalexins. This implied that cWP application might make beans more resistant to pathogens. Challenge with Uromyces appendiculatus, a rust fungal pathogen, revealed that beans pretreated with cWP had 90% reductions in disease. Arabidopsis thaliana sprayed with cWP had activated salicylic acid-mediated immune responses. Overall, these results reveal that P. savastanoi pv. phaseolicola is adapted to tolerate bean genistein and daidzein, likely sensing the compounds as host signals and producing cyclic dipeptides in response. In turn, beans respond to at least one cyclic dipeptide, cWP, by producing phytoalexins to increase resistance. cWP may be useful for protecting beans and other plants from microbial disease.