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

X anthomonas 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 XAC 3733, which encodes a transcriptional activator of the NtrC family, not linked to a gene encoding a sensor protein, thus could be considered as an ‘orphan’ regulator whose function is poorly understood in X anthomonas spp. Here we show that mutation of XAC 3733 (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 PA 2567 from P seudomonas aeruginosa , which encodes a phosphodiesterase active in the degradation of cyclic diguanosine monophosphate ( c‐di‐GMP ). A knockout of xbmR led to a substantial downregulation of fliA that encodes a σ 28 transcription factor, as well as fliC and XAC 0350 which are potential member of the σ 28 regulon. XAC 0350 encodes an HD‐GYP domain c‐ 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 .