CaSK23, a Putative GSK3/SHAGGY-Like Kinase of , Acts as a Negative Regulator of Pepper's Response to Attack.

GSK3-like kinases have been mainly implicated in the brassinosteroids (BR) pathway and, therefore, in plant growth, development, and responses to abiotic stresses; however, their roles in plant immunity remain poorly understood. Herein, we present evidence that CaSK23, a putative GSK3/SHAGGY-like kinase in pepper, acts as a negative regulator in pepper's response to () inoculation (RSI). Data from quantitative RT-PCR (qRT-PCR) showed that the constitutively-expressed in pepper leaves was down-regulated by RSI, as well as by exogenously-applied salicylic acid (SA) or methyl jasomonate (MeJA). Silencing of by virus-induced gene silencing (VIGS) decreased the susceptibility of pepper plants to RSI, coupled with up-regulation of the tested genes encoding SA-, JA-, and ethylene (ET)-dependent pathogenesis-related (PR) proteins. In contrast, ectopic overexpression (OE) of conferred a compromised resistance of tobacco plants to RSI, accompanied by down-regulation of the tested immunity-associated SA-, JA-, and ET-dependent PR genes. In addition, transient overexpression of in pepper plants consistently led to down-regulation of the tested SA-, JA-, and ET-dependent PR genes. We speculate that CaSK23 acts as a negative regulator in pepper immunity and its constitutive expression represses pepper immunity in the absence of pathogens. On the other hand, its decreased expression derepresses immunity when pepper plants are attacked by pathogens.