Enzymatic systems for carbohydrate utilization and biosynthesis in Xanthomonas and their role in pathogenesis and tissue specificity

Xanthomonas plant pathogens can infect hundreds of agricultural plants. These bacteria exploit sophisticated molecular strategies based on multiple secretion systems and their associated virulence factors to overcome the plant defenses, including the physical barrier imposed by the plant cell walls and the innate immune system. Xanthomonads are equipped with a broad and diverse repertoire of Carbohydrate-Active enZymes (CAZymes), which besides enabling the utilization of complex plant carbohydrates as carbon and energy source, can also play pivotal roles in virulence and bacterial lifestyle in the host. CAZymes in xanthomonads are often organized in multienzymatic systems similar to the Polysaccharide Utilization Loci (PUL) from Bacteroidetes known as CUT systems (from Carbohydrate Utilization systems associated with TonB-dependent transporters). Xanthomonas bacteria are also recognized to synthesize distinct exopolysaccharides including xanthan gum and untapped exopolysaccharides associated with biofilm formation. Here, we summarize the current knowledge on the multifaceted roles of CAZymes in xanthomonads, connecting their function with pathogenicity and tissue specificity.

Xyloglucan processing machinery in Xanthomonas pathogens and its role in the transcriptional activation of virulence factors

Xyloglucans are highly substituted and recalcitrant polysaccharides found in the primary cell walls of vascular plants, acting as a barrier against pathogens. Here, we reveal that the diverse and economically relevant Xanthomonas bacteria are endowed with a xyloglucan depolymerization machinery that is linked to pathogenesis. Using the citrus canker pathogen as a model organism, we show that this system encompasses distinctive glycoside hydrolases, a modular xyloglucan acetylesterase and specific membrane transporters, demonstrating that plant-associated bacteria employ distinct molecular strategies from commensal gut bacteria to cope with xyloglucans. Notably, the sugars released by this system elicit the expression of several key virulence factors, including the type III secretion system, a membrane-embedded apparatus to deliver effector proteins into the host cells. Together, these findings shed light on the molecular mechanisms underpinning the intricate enzymatic machinery of Xanthomonas to depolymerize xyloglucans and uncover a role for this system in signaling pathways driving pathogenesis.