BraA3.AP2 regulates purple leaf by transcriptional activation and interaction of BraA7.MYB2 in Brassica rapa

A 1,647-bp insertion in promoter of BraA3.AP2 acts as enhancer to induce its expression, leading to purple leaf trait through transcriptional activation and protein interaction of BraA7.MYB2 in Chinese cabbage. Anthocyanins are natural pigments beneficial to human health, but their regulatory mechanisms in Brassica rapa are not fully elucidated. Here, we characterized a purple leaf phenotype in the non‑heading Chinese cabbage inbred line P‑KC1, where anthocyanins specifically accumulate in the adaxial epidermis of cotyledons and rosette leaves. Genetic analysis revealed that the trait is controlled by a single dominant locus. Map‑based cloning identified the underlying gene BraA3.AP2, which encodes an AP2 transcription factor. Expression analysis in Brassica rapa and Genetic transformation of Arabidopsis thaliana demonstrated that BraA3.AP2 expression positively correlates with anthocyanin accumulation in leaves. A 1,647‑bp insertion in the BraA3.AP2 promoter functions as a leaf-specific transcriptional enhancer, elevating its expression in leaf tissues. We further showed that BraA3.AP2 localizes to the nucleus, directly binds to and activates the promoter of BraA7.MYB2, a key transcriptional activator of anthocyanin biosynthesis, and physically interacts with the BraA7.MYB2 protein. Our study reveals a novel BraA3.AP2-BraA7.MYB2 regulatory module that enhances anthocyanin biosynthesis, providing new insights into the genetic control of leaf pigmentation and a potential target for breeding anthocyanin‑enriched Brassica rapa varieties.