Response of wild-type and high pigment-1 tomato fruit to UV-B depletion: flavonoid profiling and gene expression

The tomato high pigment-1 (hp-1) mutant is characterised by exaggerated photoresponsiveness and increased fruit pigmentation, and carries a mutation in the HP1/LeDDB1 gene, encoding the tomato homologue of the negative regulator of the light signal transduction DDB1a from Arabidopsis. Here, we investigated the molecular events underlying flavonoid accumulation in flesh and peel of wild-type and hp-1 fruits in presence or absence of UV-B light. In hp-1 peel, a twofold higher level of rutin and an earlier accumulation of flavonoids than in wild-type were observed, which correlated to the earlier activation of most flavonoid biosynthetic genes compared to wild-type. In hp-1 flesh, flavonoid content was up to 8.5-fold higher than in wild-type and correlated to the higher transcript level of flavonoid genes compared to wild-type. In both tissues, the expression of flavonoid genes was correlated with the anticipated and/or enhanced activation of the light signal transduction genes: LeCOP1LIKE, LeCOP1 and LeHY5. In wild-type, flavonoid content was severely reduced by UV-B depletion mostly in peel, whereas in hp-1 it was significantly increased in flesh. The activation of flavonoid and light signal transduction genes was UV-B dependent mostly at the mature green stage, whereas LeDDB1 expression was not regulated by UV-B.