XbmR, a new transcription factor involved in the regulation of chemotaxis, biofilm formation and virulence in Xanthomonas citri subsp. citri

Abstract

Xanthomonas citri subsp. citri (Xcc) is the causal agent of citrus canker. Biofilm formation on citrus leaves plays an important role in epiphytic survival of Xcc. Biofilm formation is affected by transposon insertion in XAC3733, which encodes a transcriptional activator of the NtrC family, not linked to a gene encoding a sensor protein, thus be could considered as an ´orphan´ regulator whose function is poorly understood in Xanthomonas spp. Here we show that mutation of XAC3733 (named xbmR) resulted in impaired structural development of the Xcc biofilm, loss of chemotaxis and reduced virulence in grapefruit plants. All defective phenotypes were restored to wild-type levels by the introduction of PA2567 from Pseudomonas aeruginosa, which encodes a phosphodiesterase active in the degradation of cyclic diguanosine monophosphate (c-di-GMP). A knock-out of xbmR led to a substantial down regulation of fliA, which encodes a σ28 transcription factor, as well as fliC and XAC0350 which are potential member of the σ28 regulon. XAC0350 encodes an HD-GYP domain cyclic di-GMP phosphodiesterase. These findings suggest that XbmR is a key regulator of flagellar-dependent motility and chemotaxis exerting its action through a regulatory pathway that involves FliA and c-di-GMP.