Bacterially produced spermidine induces plant systemic susceptibility to pathogens

Plant root-associated microbes promote plant growth, in part by the induction of systemic resistance (ISR) to foliar pathogens. In an attempt to find novel growth-promoting and ISR inducing strains, we previously identified strains of root-associated Pseudomonas spp. that promote plant growth but unexpectedly induced systemic susceptibility (ISS) to foliar pathogens. Here we demonstrate that the ISS-inducing phenotype is common among root-associated Pseudomonas spp. and we identified the underlying genetic and molecular basis of ISS. Using comparative genomics we identified a single P. fluorescens locus containing a novel periplasmic spermidine biosynthesis gene speE2 that is unique to ISS strains. We generated a clean deletion of the speE2 gene in two ISS strains and found that speE2 is necessary for the ISS phenotype. Spermidine but not spermine is sufficient to phenocopy ISS strains. The ISS locus is present in diverse bacteria and has previously been implicated in pathogenesis in animals. Collectively these data show that a single bacterially derived molecule can modulate systemic plant immunity.