Abstract
Ralstonia solanacearum, the causal agent of bacterial wilt, can naturally infect a wide range of host plants. The type III secretion system (T3SS) is a major virulence determinant in this bacterium. Studies have shown that plant-derived compounds are able to inhibit or induce the T3SS in some plant pathogenic bacteria, though no specific T3SS inhibitor or inducer has yet been identified in R. solanacearum. In this study, a total of 50 different compounds were screened and almost half of them (22 of 50) significantly inhibited or induced the T3SS expression of R. solanacearum. Based on the strong induction activity on T3SS, the T3SS inducer oleanolic acid (OA) was chosen for further study. We found that OA induced the expression of T3SS through the HrpG-HrpB pathway. Some type III effector genes were induced in T3SS inducing medium supplemented with OA. In addition, OA targeted only the T3SS and did not affect other virulence determinants. Finally, we observed that induction of T3SS by OA accelerated disease progress on tobacco. Overall our results suggest that plant-derived compounds are an abundant source of R. solanacearum T3SS regulators, which could prove useful as tools to interrogate the regulation of this key virulence pathway.
Highlights
Ralstonia solanacearum is a Gram-negative soil-borne bacterial plant pathogen that infects more than 200 plant species from over 50 families, including agriculturally and economically important crops such as tobacco, potato, eggplant, and tomato, in tropical and subtropical regions of the world (Genin, 2010)
This reporter strain was cultured in a PS medium, which had a stronger induction on popA expression than hrp-inducing minimal medium (Yoshimochi et al, 2009), supplemented with dimethyl sulfoxide (DMSO) or 100 μM of each compound
A total of 50 plant-derived compounds were tested for their activity on inhibiting or inducing the R. solanacearum T3SS expression
Summary
Ralstonia solanacearum is a Gram-negative soil-borne bacterial plant pathogen that infects more than 200 plant species from over 50 families, including agriculturally and economically important crops such as tobacco, potato, eggplant, and tomato, in tropical and subtropical regions of the world (Genin, 2010). Like many other plant and animal pathogenic bacteria, R. solanacearum depends on the type III secretion system (T3SS) to invade its hosts. This specialized needle-like delivery machine, encoded by a cluster of about 20 hypersensitive response and pathogenicity (hrp) genes, can inject effector proteins directly into host cells suppressing plant innate immunity or activating effector-triggered immunity (Cunnac et al, 2004; Jones and Dangl, 2006). The initial activation of hrp genes is triggered upon recognition of a cell wall component by the outer membrane receptor PrhA, which transfers the plant cell interaction dependent signals through a complex signal cascade
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