Xanthomonas campestris attenuates virulence by sensing light through a bacteriophytochrome photoreceptor

Phytochromes constitute a major photoreceptor family found in plants, algae, fungi, and prokaryotes, including pathogens. Here, we report that Xanthomonas campestris pv. campestris ( Xcc ), the causal agent of black rot disease which affects cruciferous crops worldwide, codes for a functional bacteriophytochrome ( Xcc BphP). Xcc BphP possesses an N‐terminal PAS2‐GAF‐PHY photosensory domain triad and a C‐terminal PAS9 domain as its output module. Our results show that illumination of Xcc , prior to plant infection, attenuates its virulence in an Xcc BphP‐dependent manner. Moreover, in response to light, Xcc BphP downregulates xanthan exopolysaccharide production and biofilm formation, two known Xcc virulence factors. Furthermore, the XccbphP null mutant shows enhanced virulence, similar to that of dark‐adapted Xcc cultures. Stomatal aperture regulation and callose deposition, both well‐established plant defense mechanisms against bacterial pathogens, are overridden by the XccbphP strain. Additionally, an RNA‐Seq analysis reveals that far‐red light or Xcc BphP overexpression produces genomewide transcriptional changes, including the inhibition of several Xcc virulence systems. Our findings indicate that Xcc senses light through Xcc BphP, eliciting bacterial virulence attenuation via downregulation of bacterial virulence factors. The capacity of Xcc BphP to respond to light both in vitro and in vivo was abolished by a mutation on the conserved Cys13 residue. These results provide evidence for a novel bacteriophytochrome function affecting an infectious process. Synopsis Upon sensing light, the Xcc BphP photoreceptor elicits a transcriptional response that suppresses virulence. This report establishes light as an important environmental cue, which ultimately governs plant–microbe interactions in the pathogen Xanthomonas campestris . Xcc BphP is a bathy‐type photoreceptor that photoconverts between red‐absorbing (Pr) and far‐red‐absorbing (Pfr) states in vitro . Xcc BphP negatively regulates Xcc virulence in a light‐dependent manner. Light sensed by Xcc BphP downregulates xanthan production and biofilm formation. Far‐red light or Xcc BphP overexpression produces genomewide transcriptional changes, including downregulation of virulence systems. Graphical Abstract Upon sensing light, the Xcc BphP photoreceptor elicits a transcriptional response that suppresses virulence. This report establishes light as an important environmental cue, which ultimately governs pla