Xanthomonas oryzae Orphan Response Regulator EmvR Is Involved in Virulence, Extracellular Polysaccharide Production and Cell Motility.

Bacteria have evolved a large number of two-component signalling systems (TCSs), which are typically composed of a histidine sensor kinase (HK) and a response regulator (RR), to sense environmental changes and modulate subsequent adaptive responses. Here, we describe the involvement of an orphan single-domain RR named EmvR in the virulence, extracellular polysaccharide (EPS) production and cell motilities of the bacterial leaf streak pathogen Xanthomonas oryzae pv. oryzicola (Xoc), which infects rice leaves mainly via stomata and wounds. Deletion of emvR in Xoc reduced virulence when using spraying inoculation but not when using infiltration inoculation. The emvR deletion mutant displayed weakened spreading and enhanced twitching. Additionally, although deletion of emvR did not significantly affect EPS production, overexpression of emvR significantly increased EPS production. Several standard assays revealed that EmvR physically interacts with PilB and represses its ATPase activity. Combining our data with previous findings that PilB provides the energy for type IV pilus (T4P) biogenesis, we conclude that EmvR plays a vital role in modulating Xoc T4P synthesis and in the early stage of Xoc infection through rice stomata. Moreover, our data reveal that EmvR can also interact with the HK of the TCS ColS_{XOCgx_4036}/ColR_{XOCgx_4037}, which positively and negatively affects Xoc spreading and twitching, respectively. We propose a 'one-to-two' TCS working model for the role of ColS_{XOCgx_4036}, ColR_{XOCgx_4037}, and EmvR in modulating Xoc motility.