Xanthomonas campestris pv. campestris regulates virulence mechanisms by sensing blue light

Light is an environmental stimulus to which all living organisms are exposed. Numerous studies have shown that bacteria can modulate virulence factors through photoreceptor proteins. Xanthomonas campestris pv. campestris (Xcc) is the causative agent of the systemic vascular disease black rot, which affects cruciferous crops worldwide. Typical symptoms include V-shaped yellow lesions emanating from the leaf margins and blackening of the leaf veins. In previous work, we have shown that Xcc possesses a functional bacteriophytochrome (XccBphP) that regulates its virulence in response to red and far-red light. In addition to the XccBphP protein the Xcc genome codes for a blue light photoreceptor, a Light Oxygen Voltage (LOV) domain-containing protein with a histidine kinase (HK) as the output module. Here, we show that both photoreceptors are able to sense blue light. We demonstrated that XccLOV is a functional photoreceptor by performing loss and gain of function experiments with a knock-out and a complemented strain for the lov gene. Blue light negatively affected swimming motility, whereas xanthan production was regulated by XccBphP, in a blue light independent manner. Additionally, our studies showed that blue light altered biofilm structure patterns and enhanced virulence. Overall, these results revealed that some Xcc virulence factors are blue light modulated via at least two photoreceptors.