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

Plant pathogenic bacteria, such as Pseudomonas syringae strain PtoDC3000, respond to host signals through complex signaling networks that regulate bacterial growth and virulence. The plant hormone indole-3-acetic acid (IAA), also known as auxin, promotes bacterial pathogenesis via multiple mechanisms, including through reprogramming bacterial transcription. However, the mechanisms that PtoDC3000 uses to sense and respond to auxin are not well understood. Here, we identify pmeR, which encodes a TetR-like family transcriptional repressor, as an important regulator of IAA-responsive gene expression in PtoDC3000. Using qRT-PCR and transcriptional reporter assays, we show that pmeR is induced by IAA and regulates a set of auxin-responsive genes, including itself as well as several genes known or proposed to be involved in virulence. Plant infection assays further show that the disruption of pmeR results in reduced bacterial growth in Arabidopsis thaliana. Notably, while PmeR de-represses transcription of pmeR upon IAA treatment, it does not appear to directly bind IAA. Rather, our biochemical results indicate that the auxin conjugate IAA-Lysine may serve as a ligand for PmeR. Our findings reveal a complex signaling network through which IAA modulates bacterial gene expression and emphasizes the role of PmeR in acclimating PtoDC3000 for growth in plant tissue.