Genetic determinants of Brevundimonas-mediated attenuation of plant pattern-triggered immunity

Root-associated commensal bacteria from diverse taxonomic groups can attenuate plant pattern-triggered immunity (PTI), yet the bacterial mechanisms underlying immune attenuation remain poorly understood. Brevundimonas sp. MF374 is a root-associated Alphaproteobacterium previously identified as a strong attenuator of flg22-triggered immunity. Here, to identify the genetic determinants underlying this phenotype, we conducted transposon mutagenesis and screened 4,032 MF374 mutants for defects in immune attenuation. Deletion and complementation of candidate loci identified tonB1 (BLR19_RS15045) as the strongest determinant of immune attenuation, with ppiD (BLR19_RS02880) making a weaker contribution. A panel of GUS reporter assays designed to distinguish among different mechanisms of attenuation showed that intact MF374 cells directly reduced the immune-eliciting activity of flg22 in a tonB1-dependent manner. TonB1 also contributed to attenuation of responses triggered by a distinct flg22 sequence and the endogenous plant elicitor AtPEP1. Together, these findings implicate TonB-dependent transport in attenuation of flg22-induced immunity by MF374 and reveal a previously uncharacterized mechanism of immune attenuation by a root-associated commensal.