Plant Physiology and Biochemistry | Researchers from the Institute of Bast Fiber Crops (CAAS) and other institutions reveal a novel mechanism by which the transcription factor CaMYB108a negatively regulates anthocyanin and flavonoid biosynthesis in pepper.
Fruit color is a key agronomic trait determining the economic value of chili peppers, primarily governed by the accumulation of flavonoids such as anthocyanins. While the R2R3-MYB transcription factor family plays a central role in regulating anthocyanin biosynthesis, the mechanisms underlying its negative regulation in chili peppers remain unclear. Recently, a research team—including scientists from the Institute of Bast Fiber Crops and the Agricultural Genomics Institute at Shenzhen (both under the Chinese Academy of Agricultural Sciences)—identified an R2R3-MYB transcription factor, CaMYB108a, that interacts with the core light-signaling factor CaHY5. They further elucidated the molecular mechanism by which CaMYB108a negatively regulates anthocyanin and flavonoid biosynthesis in chili pepper fruit. These findings were published in the academic journal Plant Physiology and Biochemistry under the title “The transcription factor CaMYB108a negatively regulates anthocyanin and other flavonoid biosynthesis in chili pepper (Capsicum annuum L.) fruit.”
In this study, the researchers first employed a yeast two-hybrid screen using the known positive anthocyanin regulator CaHY5 as bait to identify its interacting partner, CaMYB108a. Bioinformatics analysis revealed that CaMYB108a contains an 873-bp open reading frame encoding a protein with a molecular weight of 34.14 kDa; the protein belongs to Subgroup V of the R2R3-MYB transcription factor family. Localized in the nucleus, the protein’s expression level was negatively correlated with anthocyanin content in chili pepper fruit, suggesting it acts as a negative regulator of anthocyanin biosynthesis.
To validate the function of CaMYB108a, the team used virus-induced gene silencing (VIGS) to silence the gene in chili pepper fruit. The results showed that silencing CaMYB108a caused the fruit color to shift from green to yellow and led to a significant increase in anthocyanin content. Furthermore, the expression of key structural genes involved in anthocyanin biosynthesis (such as CaCHI, CaF3’5’H, CaDFR, and CaUF3GT) was upregulated by 2.4- to 4.5-fold. Meanwhile, total flavonoid and quercetin contents increased significantly, whereas luteolin content decreased slightly, indicating that CaMYB108a differentially regulates distinct branches of the flavonoid pathway. Conversely, transient overexpression of CaMYB108a delayed fruit coloration, suppressed the expression of anthocyanin biosynthetic genes, and reduced anthocyanin and total flavonoid levels.
Further analyses—including DAP-seq, yeast one-hybrid assays, electrophoretic mobility shift assays (EMSA), and dual-luciferase reporter assays—confirmed that CaMYB108a directly binds to the promoter regions of key anthocyanin biosynthetic genes (CaCHI, CaF3’5’H, and CaDFR) and represses their transcriptional activity. Additionally, dual-luciferase assays demonstrated that co-expression of CaHY5 significantly alleviated the inhibitory effect of CaMYB108a on the CaCHI promoter, suggesting that CaHY5 antagonizes the transcriptional repression function of CaMYB108a through protein-protein interaction.
In summary, this study reveals for the first time the molecular mechanism by which CaMYB108a—an R2R3-MYB transcriptional repressor—negatively regulates anthocyanin and flavonoid biosynthesis in pepper. It functions by interacting with the light signaling factor CaHY5 to directly bind to and repress the expression of key structural genes involved in anthocyanin biosynthesis. These findings not only deepen our understanding of the regulatory network governing pepper fruit coloration but also provide valuable genetic resources and a theoretical basis for the precise improvement of fruit color and nutritional quality through molecular breeding.
