Abstract
BackgroundThe gram-negative Xanthomonas genus contains a large group of economically important plant pathogens, which cause severe diseases on many crops worldwide. The diffusible signal factor (DSF) - mediated quorum sensing (QS) system coordinates expression of virulence factors in plant pathogenic Xanthomonas spp. However, the regulatory effects of this system during the Xanthomonas- plant interactions remain unclear from both the pathogen and host aspects.ResultsIn this study, we investigated the in planta DSF- mediated QS regulon of X. citri subsp. citri (Xac), the causal agent of citrus canker. We also characterized the transcriptional responses of citrus plants to DSF-mediated Xac infection via comparing the gene expression patterns of citrus trigged by wild type Xac strain 306 with those trigged by its DSF- deficient (∆rpfF) mutant using the dual RNA-seq approach. Comparative global transcript profiles of Xac strain 306 and the ∆rpfF mutant during host infection revealed that DSF- mediated QS specifically modulates bacterial adaptation, nutrition uptake and metabolisms, stress tolerance, virulence, and signal transduction to favor host infection. The transcriptional responses of citrus to DSF-mediated Xac infection are characterized by downregulation of photosynthesis genes and plant defense related genes, suggesting photosynthetically inactive reactions and repression of defense responses. Alterations of phytohormone metabolism and signaling pathways were also triggered by DSF-mediated Xac infection to benefit the pathogen.ConclusionsCollectively, our findings provide new insight into the DSF- mediated QS regulation during plant-pathogen interactions and advance the understanding of traits used by Xanthomonas to promote infection on host plants.
Highlights
The gram-negative Xanthomonas genus contains a large group of economically important plant pathogens, which cause severe diseases on many crops worldwide
RpfB was initially thought to be involved in diffusible signal factor (DSF) biosynthesis, but it was later identified as a fatty Acyl-Coenzyme A (CoA) ligase involved in the turnover of the DSF family of signals in Xanthomonas [11]
Canker progression and symptoms in inoculated citrus plants Duncan grapefruit (Citrus paradisi Macfadyen) seedlings were inoculated with Xanthomonas citri subsp. citri (Xac) wild type strain 306 and its DSF deficient (ΔrpfF) mutant strain for the development of typical symptoms of citrus canker
Summary
The gram-negative Xanthomonas genus contains a large group of economically important plant pathogens, which cause severe diseases on many crops worldwide. Key virulence factors of Xanthomonas spp. include, among others, the type III secretion system (T3SS) and its effectors [3, 4], bacterial polysaccharides such as the xanthan extracellular polysaccharides (EPS) and lipopolysaccharides (LPS) [5], and cell wall degrading enzymes [1]. Expression of these virulence factors is regulated by different extracellular stimuli via multiple coordinated regulatory systems, including cell-to-cell communication (quorumsensing, QS) pathways, two-component systems and various transcriptional regulators [1]. RpfH is a putative membrane protein with no known role in DSF signaling [10]
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