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.

Anatomical and Biochemical Changes Induced by Gluconacetobacter diazotrophicus Stand Up for Arabidopsis thaliana Seedlings From Ralstonia solanacearum Infection.

Nowadays, fertilization and pest control are carried out using chemical compounds that contaminate soil and deteriorate human health. Plant growth promoting bacteria endophytes (PGPBEs), are a well-studied group of bacteria that offers benefits to the host plant, such as phytostimulation, biofertilization, and protection against other microorganisms. The study of Gluconacetobacter diazotrophicus-which belongs to PGPBEs-aids the development of alternative strategies of an integrated approach for crop management practices. Ralstonia solanacearum is responsible for bacterial wilt disease. This phytopathogen is of great interest worldwide due to the enormous economic losses it causes. In this study the action of G. diazotrophicus as a growth promoting bacterium in Arabidopsis thaliana seedlings is analyzed, evaluating the antagonistic mechanisms of this beneficial endophytic bacterium during biotic stress produced by R. solanacearum. Effective colonization of G. diazotrophicus was determined through bacterial counting assays, evaluation of anatomical and growth parameters, and pigments quantification. Biocontrol assays were carried out with Ralstonia pseudosolanacearum GMI1000 model strain and R. solanacearum A21 a recently isolated strain. Inoculation of A. thaliana (Col 0) with G. diazotrophicus Pal 5 triggers a set of biochemical and structural changes in roots, stems, and leaves of seedlings. Discrete callose deposits as papillae were observed at specific sites of root hairs, trichomes, and leaf tissue. Upon R. pseudosolanacearum GMI1000 infection, endophyte-treated plants demonstrated being induced for defense through an augmented callose deposition at root hairs and leaves compared with the non-endophyte-treated controls. The endophytic bacterium appears to be able to prime callose response. Roots and stems cross sections showed that integrity of all tissues was preserved in endophyte-treated plants infected with R. solanacearum A21. The mechanisms of resistance elicited by the plant after inoculation with the endophyte would be greater lignification and sclerosis in tissues and reinforcement of the cell wall through the deposition of callose. As a consequence of this priming in plant defense response, viable phytopathogenic bacteria counting were considerably fewer in endophyte-inoculated plants than in not-inoculated controls. Our results indicate that G. diazotrophicus colonizes A. thaliana plants performing a protective role against the phytopathogenic bacterium R. solanacearum promoting the activation of plant defense system.

Bacterial Photosensory Proteins and Their Role in Plant-pathogen Interactions.

Light is an important environmental signal for almost all living organisms. The light perception is achieved by photoreceptor proteins. As can be observed from the great number of bacterial genomes sequenced, plant pathogenic bacteria encode for a large number of photoreceptor proteins. The physiological implications of these photoreceptors are still poorly characterized. However, recent studies revealed the participation of these photosensory proteins in the pathogenic process. Here, we summarize what is known about these proteins and their role during the virulence process, concluding that the light environment modulates the plant-pathogen interaction.