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

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 is 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 (hereafter 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 its metabolism and controlling transitions between higher and lower motility states for successful host colonization. mRNA profiles of X. citri grown on minimal media XVM2m containing 0.05% (w/v) cellobiose or starch instead of fructose and sucrose relative to the media XVM2m containing glucose as a reference.

部分植物病原菌（phytopathogen）拥有一套广泛多样的酶系统，能够分解并利用宿主多糖作为碳源、能源与信号刺激物。然而，目前对于这类酶系统的功能注释，以及其产物在宿主定殖过程中调控病原菌行为的机制，尚缺乏深入认知。本研究通过RNA测序（RNA-seq）分析，全面解析模式植物病原菌野油菜黄单胞菌柑橘致病变种306（Xanthomonas citri pv. citri 306，下称X. citri 306）对纤维二糖——结构型β-1,4-葡聚糖（主要为纤维素）的组成成分——以及贮藏型α-葡聚糖的转录响应，以期更清晰地阐明这些碳水化合物的同化途径，及其感知信号对细菌行为与生理状态的影响。结构型β-1,4-葡聚糖与贮藏型α-葡聚糖（淀粉）在植物细胞内呈现空间差异化分布，因此可作为细菌宿主定殖过程中的时空参照信号。本研究结合转录组分析、基因组挖掘、基因敲除与细胞运动性实验，证实X. citri 306拥有分解与同化这类碳水化合物的分子系统；研究发现纤维二糖可上调鞭毛组装与IV型菌毛相关基因，诱导细菌进入高运动状态。与之相反，淀粉则会抑制趋化性、鞭毛组装与生物被膜分散相关基因的表达，降低细菌运动能力。综上，本研究结果表明，黄单胞菌属（Xanthomonas）细菌不仅可将结构型β-葡聚糖与贮藏型α-葡聚糖作为碳源与能源物质，还能感知这类碳水化合物，通过调整自身代谢状态、调控高低运动状态间的转换，以实现成功的宿主定殖。本数据集包含以含葡萄糖的XVM2m基础培养基为参照，将X. citri接种于以0.05%（质量体积比，w/v）纤维二糖或淀粉替代果糖与蔗糖的XVM2m基础培养基中培养后的转录组mRNA表达谱。