Estudos das bases moleculares dos sistemas de degradação e utilização de carboidratos do fitopatógeno Xanthomonas citri pv. citri

In nature, microorganisms from different ecological niches established elaborated and diversified strategies for the degradation of complex polysaccharides. These systems have been of great interest both by their biological importance, for example, in carbon recycling or host-pathogen interactions, as by their biotechnological applications, such as in advanced biofuel production, animal nutrition, and healthcare. Phytopathogenic bacteria from the "Xanthomonas" genus, which can significantly impact the productivity of several crops, show an extensive repertoire of carbohydrate-active enzymes (CAZymes). However, the molecular strategies used by these bacteria to depolymerize and uptake plant polysaccharides, as the main physiological changes triggered by these carbohydrates during the process of infection, are still partially understood. Therefore, this thesis aimed to unravel the different polysaccharides utilization systems of the bacteria X. citri pv. citri strain 306, the causal agent of citrus canker, and shed light on the global expression response of this bacteria in the presence of carbohydrates from the plant cell wall or storage organelles. Our RNA-seq results combined with genome mining, enrichment analysis and literature curation revealed three "Polysaccharide Utilization Loci" (PUL) for starch, xyloglucan and galactomannan utilization, besides indicating a putative role of enzymes from families GH115 and GH2 in xylan depolymerization, which has not yet been reported in the literature for this bacterial genus. Our results also evidence that X. citri can distinguish the glycosidic bonds and decorations of each carbohydrate, as revealed by the differences in the genic expression profiles in the presence of cellobiose and starch or xylan and arabinoxylan. It was also observed that each plant carbohydrate can act as a spatial or temporal marker, modulating the infection strategies of X. citri. In the xyloglucan utilization system, for example, the products of degradation of this polysaccharide were able to activate key virulence factors: the type III secretion system (T3SS) and type III effectors (T3E). Moreover, this work also brings the structural and biochemical characterization of the endo-?-1,4-mannanase XagD, providing insights into its putative role in biofilm remodeling in X. citri and clues for the composition of the unknown exopolysaccharide Xag, besides revealing new structural features of the underexplored GH113 family. In conclusion, using the combination of different computational and experimental approaches, this work expands the current knowledge of how complex carbohydrates can be depolymerized, assimilated and used as signals in the modulation of phytopathogen metabolism, highlighting the importance of complex carbohydrate utilization systems, in particular CAZymes, during the infection process (AU)