The PdeK-PdeR two-component system promotes unipolar localization of FimX and pilus extension in pv. .

Bacterial type IV pili (T4P) contribute to virulence and can be rapidly extended and retracted to mediate twitching motility. T4P biogenesis, which is normally limited to the cell poles, is regulated by extracellular stimuli and internal signals such as cyclic di-GMP (c-di-GMP). The c-di-GMP–binding protein FimX interacts with the T4P assembly complex and, when intracellular c-di-GMP concentrations are low, assumes a unipolar localization and promotes T4P biogenesis. Here, we demonstrated that FimX formed a complex with the two-component system consisting of the histidine kinase PdeK and its downstream response regulator PdeR. This complex promoted T4P assembly in the phytopathogen pv. and virulence in rice. PdeK and the c-di-GMP phosphodiesterase activity of PdeR were required for the unipolar localization of FimX, leading to T4P extension. High amounts of c-di-GMP reduced the affinity of FimX for PdeR in vitro, consistent with FimX promoting T4P extension only under conditions of low c-di-GMP. We propose that low intracellular amounts of c-di-GMP created by PdeR facilitate the recruitment of FimX to the leading pole of motile cells. Our findings indicate that the PdeK-PdeR two-component system connects environmental cues to second messenger turnover, resulting in a change in the intracellular concentration of c-di-GMP that promotes T4P biogenesis and virulence.