Blue light sensing of Xanthomonas axonopodis pv. citri during citrus canker

IVANA KRAISELBURD; ANALÍA I. ALET,; LUCAS D. DAURELIO; MARÍA LAURA TONDO; SILVANA PETROCELLI; JESICA MONZÓN; OSCAR A. RUIZ; ABA LOSI; ELENA G. ORELLANO

Tucumán

Congreso; VII Congreso Argentino de Microbiología General SAMIGE del Bicentenario; 2011

Sociedad Argentina de Microbiología general

Light is a major signal that regulates plant physiology. Recent studies demonstrated that an appropriate light environment is required to establish efficient resistance responses in several plant-pathogen interactions. On the other hand, the participation of light in the regulation of bacterial virulence was found recently in non-photosynthetic bacteria. Data obtained from bacterial genome sequences revealed the presence of photosensory proteins of the BLUF (blue-light sensing using FAD), LOV (light, oxygen, voltage) and phytochrome families. The light-sensing capacity of these proteins is mediated by their association with specific light-absorbing molecules or chromophores, such as flavin mononucleotide (FMN) in the case of LOV proteins. The LOV photoreceptors are related to the plant phototropins which are involved in phototropic bending, light-induced stomatal opening and light-directed chloroplast movement. Xanthomonas axonopodis pv. citri (Xac) is a gram negative bacterium responsible for citrus canker. This disease affects all citrus cultivars damaging fruits, leaves and stems. The in silico analysis of Xac genome sequence showed the presence of the gene fixL, which encodes a LOV protein that contains an N-terminal LOV domain associated to a C-terminal histidin kinase and a response regulator domains (hybrid HK-RR). The LOV domain contains the highly conserved motif GNNCRFLQ that includes the reactive cysteine essential for the photochemical cycle initiated with the absorption of blue/UVA light. To study whether environmental light modulates the virulence of Xac, we have constructed a knockout mutant by replacing the fixL gene with an antibiotic resistance cassette. First, we showed that the absence of this gene did not affect the bacterial viability and division. Then we analyzed several physiological features of the bacteria, particularly those related with pathogen´s ability to colonize its host plant. We found significant differences in physiological features like motility, biofilm formation, exopolysaccharide production and bacterial adhesion between both strains which indicate that the LOV protein has a role in the regulation of these processes. We also observed that the adhesion of the mutant strain was significantly diminished compared to the wild type strain. This response showed a strong dependence on the presence of light during bacterial growth. Altogether these results suggest a light dependent regulation of Xac virulence during the host colonization.