Plant structural and storage glucans trigger distinct transcriptional responses that modulate the motility of Xanthomonas pathogens.

Abstract

Some phytopathogens are outfitted with a broad and diverse repertoire of enzymatic systems that enable the breakdown and utilization of host polysaccharides as a source of carbon, energy, and stimuli. However, the functional assignment of these enzymatic systems and the influence of their products on modulating pathogen behavior during host colonization are yet poorly comprehended. In this study, we performed RNA-seq analyses to provide a comprehensive, genome-wide view of the transcriptional response of the model phytopathogen Xanthomonas citri pv. citri 306 (known as X. citri 306) to cellobiose, a component of structural β-1,4-glucans (majorly cellulose), and storage α-glucans, seeking to better understand how they are assimilated and the impacts of their sensing on bacterial behavior and physiology. Structural β-1,4-glucans and storage α-glucans (starch) are spatially discretized in the plant cell, therefore representing spatiotemporal references for the bacterium during host colonization. Combining transcriptional and genome mining analyses with gene knockout and cell motility assays, we show that X. citri 306 harbors molecular systems for the breakdown and assimilation of these carbohydrates, revealing that cellobiose upregulates genes related to flagellum assembly and type IV pili, inducing a higher motility state. In contrast, starch suppresses genes related to chemotaxis, flagellum assembly, and biofilm dispersion, decreasing motility. Taken together, these results unravel that, besides using structural β-glucans and storage α-glucans as sources of carbon and energy, Xanthomonas bacteria also sense them, adapting their metabolism and controlling transitions between higher and lower motility states for successful host colonization. IMPORTANCE Pathogenic Xanthomonas bacteria can affect a variety of economically relevant crops causing losses in productivity, limiting commercialization and requiring phytosanitary measures. These plant pathogens exhibit high level of host and tissue specificity through multiple molecular strategies including several secretion systems, effector proteins, and a broad repertoire of carbohydrate-active enzymes (CAZymes). Many of these CAZymes act on the plant cell wall and storage carbohydrates, such as cellulose and starch, releasing products used as nutrients and modulators of transcriptional responses to support host colonization by mechanisms yet poorly understood. Here, we reveal that structural and storage β-glucans from the plant cell function as spatial markers, providing distinct chemical stimuli that modulate the transition between higher and lower motility states in Xanthomonas citri, a key virulence trait for many bacterial pathogens.