The Pepper MYB Transcription Factor CaMYB306 Accelerates Fruit Coloration and Negatively Regulates Cold Resistance

The extensively studied MYB transcription factor (TF) family has significant functions in plant development and stress responses. However, little is known about the role of MYB TFs in fruit coloration and cold resistance in pepper (Capsicum annuum L.). In this study, we identified the novel MYB gene CaMYB306 in pepper. CaMYB306 expression was higher in red fruit. Knockdown of CaMYB306 inhibited carotenoid biosynthesis and decreased expression of carotenoid-related genes in pepper fruit. CaMYB306-overexpressing (CaMYB306-OE) tomato plants showed earlier coloration with increased carotenoid accumulation and decreased chlorophyll and anthocyanin contents. The expression levels of key carotenoid biosynthesis genes (SlPSY and SlPDS) and key chlorophyll catabolic enzymes genes (SlSGR and SlPAO) were increased in CaMYB306-OE fruit, whereas key anthocyanin biosynthesis genes (SlF3H and SlCHS) were down-regulated. Additionally, CaMYB306 expression was up-regulated under cold treatment. CaMYB306 played a negative role in cold response via the regulation of the reactive oxygen species system and the expression of cold-related genes. CaMYB306 suppressed the transcriptional activity of Calcineurin B-Like-Interacting Protein Kinase gene 13 (CaCIPK13), which was a positive regulator of cold resistance. These findings clarify the role of MYB TFs in fruit coloration and cold resistance in pepper.