High levels of cyclic‐di‐ GMP in plant‐associated P seudomonas correlate with evasion of plant immunity

Summary The plant innate immune system employs plasma membrane‐localized receptors that specifically perceive pathogen/microbe‐associated molecular patterns ( PAMPs / MAMPs ). This induces a defence response called pattern‐triggered immunity ( PTI ) to fend off pathogen attack. Commensal bacteria are also exposed to potential immune recognition and must employ strategies to evade and/or suppress PTI to successfully colonize the plant. During plant infection, the flagellum has an ambiguous role, acting as both a virulence factor and also as a potent immunogen as a result of the recognition of its main building block, flagellin, by the plant pattern recognition receptors ( PRRs ), including FLAGELLIN SENSING 2 ( FLS 2). Therefore, strict control of flagella synthesis is especially important for plant‐associated bacteria. Here, we show that cyclic‐di‐ GMP [bis‐(3′‐5′)‐cyclic di‐guanosine monophosphate], a central regulator of bacterial lifestyle, is involved in the evasion of PTI . Elevated cyclic‐di‐ GMP levels in the pathogen P seudomonas syringae pv. tomato ( P to ) DC 3000, the opportunist P . aeruginosa   PAO 1 and the commensal P . protegens   P f‐5 inhibit flagellin synthesis and help the bacteria to evade FLS 2‐mediated signalling in N icotiana benthamiana and A rabidopsis thaliana . Despite this, high cellular cyclic‐di‐ GMP concentrations were shown to drastically reduce the virulence of P to   DC 3000 during plant infection. We propose that this is a result of reduced flagellar motility and/or additional pleiotropic effects of cyclic‐di‐ GMP signalling on bacterial behaviour.