A novel BLUF photoreceptor modulates the Xanthomonas citri subsp. citri-host plant interaction.

Plant-pathogen interaction is influenced by multiple environmental factors, including temperature and light. Recent works have shown that light modulates not only the defense response of plants but also the pathogens virulence. Xanthomonas citri subsp. citri (Xcc) is the bacterium responsible for citrus canker, an important plant disease worldwide. The Xcc genome presents four genes encoding putative photoreceptors: one bacteriophytochrome and three blue light photoreceptors, one LOV and two BLUFs (bluf1: XAC2120 and bluf2: XAC3278). The presence of two BLUFs proteins is an outstanding feature of Xcc. In this work we show that the bluf2 gene is functional. The mutant strain, XccΔbluf2, was constructed demonstrating that BLUF2 regulates swimming-type motility, adhesion to leaves, exopolysaccharide production and biofilm formation, features involved in the Xcc virulence processes. An important aspect during the plant-pathogen interaction is the oxidative response of the host and the consequent reaction of the pathogen. We observed that ROS detoxification is regulated by Xcc bluf2 gene. The phenotypes of disease in orange plants produced by WT and XccΔbluf2 strains were evaluated, observing different phenotypes. Altogether, these results show that BLUF2 negatively regulates virulence during citrus canker. This work constitutes the first report on BLUF-like receptors in plant pathogenic bacteria.

Light modulates important physiological features of Ralstonia pseudosolanacearum during the colonization of tomato plants.

Ralstonia pseudosolanacearum GMI1000 (Rpso GMI1000) is a soil-borne vascular phytopathogen that infects host plants through the root system causing wilting disease in a wide range of agro-economic interest crops, producing economical losses. Several features contribute to the full bacterial virulence. In this work we study the participation of light, an important environmental factor, in the regulation of the physiological attributes and infectivity of Rpso GMI1000. In silico analysis of the Rpso genome revealed the presence of a Rsp0254 gene, which encodes a putative blue light LOV-type photoreceptor. We constructed a mutant strain of Rpso lacking the LOV protein and found that the loss of this protein and light, influenced characteristics involved in the pathogenicity process such as motility, adhesion and the biofilms development, which allows the successful host plant colonization, rendering bacterial wilt. This protein could be involved in the adaptive responses to environmental changes. We demonstrated that light sensing and the LOV protein, would be used as a location signal in the host plant, to regulate the expression of several virulence factors, in a time and tissue dependent way. Consequently, bacteria could use an external signal and Rpsolov gene to know their location within plant tissue during the colonization process.