Abstract
Previously, we isolated several genes that potentially affected the expression of type III secretion system (T3SS) in Ralstonia solanacearum OE1-1. Here, we focused on the rsp0316, which encodes a putative 2-nitropropane dioxygenase (hereafter designated NpdA). The deletion of npdA substantially reduced the T3SS expression and virulence in OE1-1, and the complementation with functional NpdA could completely restore its reduced T3SS expression and virulence to that of wild type. The NpdA was highly conserved among diverse R. solanacearum species and the NpdA-dependent expression of T3SS was not specific to OE1-1 strain, but not the virulence. The NpdA was important for the T3SS expression in planta, while it was not required for the bacterial growth in planta. Moreover, the NpdA was not required for the elicitation of hypersensitive response (HR) of R. solanacearum strains in tobacco leaves. The T3SS in R. solanacearum is directly controlled by the AraC-type transcriptional regulator HrpB and regulated by a complex regulation network. The NpdA affected the T3SS expression mediated with HrpB but through some novel pathway. All these results from genetic studies demonstrate that NpdA is a novel factor for the T3SS expression in diverse R. solanacearum species in medium, but specifically for the T3SS expression in strain OE1-1 in planta. And the NpdA-dependent expression of T3SS in planta plays an important role in pathogenicity of R. solanacearum OE1-1 in host plants.
Highlights
The syringe-like type III secretion system (T3SS) is conserved in many bacterial pathogens of animals, plants, and insects and plays central roles in their pathogenicity (Hueck, 1998; Galán and Wolf-Watz, 2006)
The popA expression in npdA transposon mutant was reduced to approximately 30% of that in the wild-type strain, suggesting that NpdA might be required for the T3SS expression in R. solanacearum OE1-1 (Zhang et al, 2013)
No differences in the bacterial morphology and growth profiles were observed between the npdA mutants and wild-type strain in rich or hrp-inducing media
Summary
The syringe-like type III secretion system (T3SS) is conserved in many bacterial pathogens of animals, plants, and insects and plays central roles in their pathogenicity (Hueck, 1998; Galán and Wolf-Watz, 2006). When the bacteria directly contact host plants, the expression of the T3SS was induced to a 10- to 20fold higher level than that under nutrient-poor conditions (Marenda et al, 1998; Mukaihara et al, 2004; Yoshimochi et al, 2009b; Zhang et al, 2011). R. solanacearum might dynamically activate hrpB expression through HrpG and PrhG in a cell density-dependent manner (Zhang et al, 2013). R. solanacearum uses a complex regulation network to globally regulate the expression of the T3SS and promote disease development in host plants, but this network needs to be further elucidated (Valls et al, 2006; Hikichi et al, 2007)
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