PmeR, a TetR-like transcriptional regulator, is involved in both auxin signaling and virulence in the plant pathogen Pseudomonas syringae strain PtoDC3000.

The plant hormone auxin plays a role in many plant-microbe interactions, including promoting pathogen growth and disease symptom development in plants infected by the foliar pathogen Pseudomonas syringae.Auxins (e.g.Indole-3-acetic acid, IAA) promote virulence of P. syringae strain PtoDC3000 on Arabidopsis and tomato, either when applied exogenously or when endogenous IAA levels are elevated (Fig. 1; 1-4).IAA promotes PtoDC3000 pathogenesis via two mechanisms: 1) suppression of salicylic acidmediated host defenses and 2) as a microbial signal that regulates bacterial gene expression (Fig. 2; 4, 5).We are investigating the mechanisms PtoDC3000 uses to sense and respond to IAA and have identified pmeR, which encodes a TetR-like transcriptional repressor, to be an important regulator of IAA responsive gene expression.Using qRT-PCR and transcriptional reporters, we show that pmeR is induced by IAA and regulates expression of itself, as well as several other IAA-responsive genes (Fig. 3).Plant infection assays show disruption of pmeR results in reduced bacterial growth in Arabidopsis, suggesting that the ability to sense IAA is important for virulence (Fig. 4).While PmeR de-represses transcription of pmeR upon IAA treatment, it does not appear to directly bind IAA.Rather, our biochemical studies suggest that the auxin conjugate IAA-Lysine is a ligand for PmeR (Fig. 5).Our findings reveal a complex signaling network through which IAA modulates bacterial gene expression and emphasize the role of PmeR in acclimating PtoDC3000 for growth in plant tissue (Fig. 6). Some remaining questions and future directions IAA promotes PtoDC3000 pathogenesis via two different mechanismsPmeR, a TetR-like transcriptional regulator, is involved in auxin signaling and virulence in Pseudomonas syringae strain PtoDC3000.