Temperature-dependent action of pepper mildew resistance locus O 1 in inducing pathogen immunity and thermotolerance

Plant diseases tend to be more serious under conditions of high-temperature/high-humidity (HTHH) than under moderate conditions, and hence disease resistance under HTHH is an important determinant for plant survival. However, how plants cope with diseases under HTHH remains poorly understood. In this study, we used the pathosystem consisting of pepper (Capsicum annuum) and Ralstonia solanacearum (bacterial wilt) as a model to examine the functions of the protein mildew resistance locus O 1 (CaMLO1) and U-box domain-containing protein 21 (CaPUB21) under conditions of 80% humidity and either 28 degrees C or 37 degrees C. Expression profiling, loss- and gain-of-function assays involving virus-induced gene-silencing and overexpression in pepper plants, and protein-protein interaction assays were conducted, and the results showed that CaMLO1 acted negatively in pepper immunity against R. solanacearum at 28 degrees C but positively at 37 degrees C. In contrast, CaPUB21 acted positively in immunity at 28 degrees C but negatively at 37 degrees C. Importantly, CaPUB21 interacted with CaMLO1 under all of the tested conditions, but only the interaction in response to R. solanacearum at 37 degrees C or to exposure to 37 degrees C alone led to CaMLO1 degradation, thereby turning off defence responses against R. solanacearum at 37 degrees C and under high-temperature stress to conserve resources. Thus, we show that CaMLO1 and CaPUB21 interact with each other and function distinctly in pepper immunity against R. solanacearum in an environment-dependent manner. Mildew resistance locus O 1 in pepper interacts with U-box domain-containing protein 21 to provide defence against mildew that is specific to high temperatures, and also confers thermotolerance.