Role of Methyl Jasmonate on Flavonoid Pathway of UV-B-Irradiated Apple Fruit

Methyl jasmonate (MeJA) is a growth regulator that is involved in plant defense and development. Studies have demonstrated its role in pigmentation, particularly in synergy with UV-B in plant tissues. However, its role in pigment metabolism remains incompletely understood. To explore the metabolic synergistic effects, we evaluated pigments and gene expression in mature green apples. MeJA alone had no effect on pigment accumulation, while UV-B increased flavonols up to S2 stage and anthocyanins up to S3 stage. With UV-B > 2 W m–2, MeJA doubled anthocyanin accumulation compared to UV-B alone but had no synergistic effect on flavonols. MeJA selectively upregulated structural genes and transcription factors involved in the late biosynthetic pathway, transport, and the MYB-bHLH-WD40 complex, inducing anthocyanin hyperaccumulation without affecting early flavonoid biosynthetic genes or MdFLS, which controls flavonol biosynthesis. These results suggest that MeJA relates to downstream gene expression in flavonoid biosynthesis with alternative regulation of the anthocyanin pathway in UV-B-irradiated apples.