Pseudomonas syringae pv. tomato bacteriophytochromes downregulate bacterial motility and infectivity during plant-pathogen interaction

The foliar hemibiotrophic pathogen Pseudomonas syringae pv. tomato DC3000 (Pst) leads to consistent losses in tomato crops and this urges to multiply the researches on the physiological bases of its infectiveness. It has been already demonstrated that light perception plays a crucial role in many physiological processes, even in nonphototrophic organisms. Pst is equipped with red/far-red (R/FR) light sensing bacteriophytochromes (BphP), binding biliverdin as chromophore and mimicking the photosensing ability of host plants. Here we report the study of the effect of different light conditions on the swarming motility of mutant strains of P. syringae lacking of the photosensory Bphy1 or Bphy2 or both proteins or heme-oxygenase-1 (HO) catalysing formation of bilins, respectively, as regards to the wild type (WT). Each of the mutants shows stronger virulence than PstWT evidenced by the macroscopic damages caused in the infected leaves of tomato plants. Moreover, they rapidly move inside the infected plants, as necrotic spots in host tissues distant from the infection site appear faster than that due to the WT infection. These results indicate that bacteriophytochromes downregulate bacterial infectivity and invasiveness within the infected leaves and underscore the importance of Pst photoreceptors in responding to environmental light inputs.