Degradation of phenolic compounds by Xanthomonas citri subsp. citri: virulence, adaptation and environmental implications

IVANA KRAISELBURD; SOFÍA BELTRAMINO; ASSIS RENATA; MOREIRA LEANDRO; ELENA G. ORELLANO

Córdoba

Congreso; XI Congreso Argentino de Microbiología General (SAMIGE); 2015

SAMIGE

Plants are constantly exposed to pathogens and they possess a complex set of defense responses to prevent the invasion and spreading of pathogenic microorganisms. Virulent pathogens modulate host processes in order to delay or counteract these defense responses. The production of phenolic compounds at the site of invasion plays an important role in plant innate defenseas they can cause deleterious effects on the bacterial cell structure. Xanthomonas citri subsp. citri (Xcc) is the bacterium responsible for citrus canker, a disease that affects all commercial varieties of Citrus. The Xcc genome is completely sequencedand it includes several genes related to the degradation of phenolic compounds. In this work we present a study of the catabolic pathways of Xcc, potentially involved in the degradation of phenolic compounds. First we performed an in silico analysis of Xcc genome sequence using the Kyoto Encyclopedia of Genes and Genomes database, where we identified 98 genes associated with transport and degradation of phenolic compounds. A bioinformatic compilation and evaluation of these data allowed us tofind the complete degradation route for vanillate, 4-hydroxybenzaldehyde, 4-hydroxybenzoate and 3,4-dihydroxybenzoate. Wethus postulated that Xcc is able to detoxify these compounds and, as many of the pathways generate intermediates of the tricarboxylic acid cycle, they may act as an alternative source of carbon for the bacterial energy metabolism. In order todemonstrate the functionality of these routes and to evaluate the ability of Xcc to use these compounds as a carbon source, bacterial cells were grown in a minimal medium supplemented with 2 mM vanillate or 3,4-dihydroxybenzoate. Controls were performed in minimal medium supplemented with glucose. Bacterial populations were evaluated by the optical density at 600 nm and by analyzing the number of colony forming units in plates containing rich medium. Moreover, we performed a transcriptional evaluation of Xcc genes encoding central enzymes of these catabolic pathways by RT-PCR. Finally, to evaluate the role of the degradation of phenolic compounds in the bacterial virulence, modified strains of Xcc were generated, deficient in the genes encoding dioxigenase enzymes potentially involved in the degradation of these compounds. The presence and functionality of the above mentioned catabolic pathways is probably associated to the bacterial ability to detoxify the phenolic compounds released during the plant defense response, favoring the successful colonization of the host tissue. Gaining knowledge of these adaptive pathways is crucial to propose methods to combat pathogens. Moreover, as the degradation of phenolic compounds is carried out only by a reduced number of microorganisms expressing specific enzymes, the ability of Xcc to use thesecompounds as a carbon source represents an opportunity for the development of biotechnological strategies for the degradationof xenobiotics.