Overexpression of CaWRKY27, a subgroup IIe WRKY transcription factor of Capsicum annuum, positively regulates tobacco resistance to Ralstonia solanacearum infection.

WRKY proteins are encoded by a large gene family and are linked to many biological processes across a range of plant species. The functions and underlying mechanisms of WRKY proteins have been investigated primarily in model plants such as Arabidopsis and rice. The roles of these transcription factors in non-model plants, including pepper and other Solanaceae, are poorly understood. Here, we characterize the expression and function of a subgroup IIe WRKY protein from pepper (Capsicum annuum), denoted as CaWRKY27. The protein localized to nuclei and activated the transcription of a reporter GUS gene construct driven by the 35S promoter that contained two copies of the W-box in its proximal upstream region. Inoculation of pepper cultivars with Ralstonia solanacearum induced the expression of CaWRKY27 transcript in 76a, a bacterial wilt-resistant pepper cultivar, whereas it downregulated the expression of CaWRKY27 transcript in Gui-1-3, a bacterial wilt-susceptible pepper cultivar. CaWRKY27 transcript levels were also increased by treatments with salicylic acid (SA), methyl jasmonate (MeJA) and ethephon (ETH). Transgenic tobacco plants overexpressing CaWRKY27 exhibited resistance to R. solanacearum infection compared to that of wild-type plants. This resistance was coupled with increased transcript levels in a number of marker genes, including hypersensitive response genes, and SA-, JA- and ET-associated genes. By contrast, virus-induced gene silencing (VIGS) of CaWRKY27 increased the susceptibility of pepper plants to R. solanacearum infection. These results suggest that CaWRKY27 acts as a positive regulator in tobacco resistance responses to R. solanacearum infection through modulation of SA-, JA- and ET-mediated signaling pathways.

CaWRKY58, encoding a group I WRKY transcription factor of Capsicum annuum, negatively regulates resistance to Ralstonia solanacearum infection.

WRKY transcription factors are encoded by large gene families across the plant kingdom. So far, their biological and molecular functions in nonmodel plants, including pepper (Capsicum annuum) and other Solanaceae, remain poorly understood. Here, we report on the functional characterization of a new group I WRKY protein from pepper, termed CaWRKY58. Our data indicate that CaWRKY58 can be localized to the nucleus and can activate the transcription of the reporter ß-glucuronidase (GUS) gene driven by the 35S core promoter with two copies of the W-box in its proximal upstream region. In pepper plants infected with the bacterial pathogen Ralstonia solanacearum, CaWRKY58 transcript levels showed a biphasic response, manifested in an early/transient down-regulation and late up-regulation. CaWRKY58 transcripts were suppressed by treatment with methyl jasmonate and abscisic acid. Tobacco plants overexpressing CaWRKY58 did not show any obvious morphological phenotypes, but exhibited disease symptoms of greater severity than did wild-type plants. The enhanced susceptibility of CaWRKY58-overexpressing tobacco plants correlated with the decreased expression of hypersensitive response marker genes, as well as various defence-associated genes. Consistently, CaWRKY58 pepper plants silenced by virus-induced gene silencing (VIGS) displayed enhanced resistance to the highly virulent R. solanacearum strain FJC100301, and this was correlated with enhanced transcripts of defence-related pepper genes. Our results suggest that CaWRKY58 acts as a transcriptional activator of negative regulators in the resistance of pepper to R. solanacearum infection.