The serotonin biosynthesis enzyme T5H5 and transcription factor bZIP23 modulate light-mediated anthocyanin accumulation in apple.

Serotonin (5-hydroxytryptamine, 5-HT), an indole derivative, has a variety of physiological and developmental functions in plants. Anthocyanins play a crucial role in fruit coloring. Although light controls melatonin biosynthesis in plants, how serotonin, a melatonin precursor, contributes to anthocyanin accumulation at the molecular level remains unknown. In this study, we investigated the potential function of the 5-HT synthetase MdT5H5 in inhibiting anthocyanin accumulation in response to light. In apple fruits, light increased MdT5H5 expression and 5-HT accumulation. Anthocyanin accumulation was reduced in light-induced calli overexpressing MdT5H5, which was also supported by the observation that anthocyanins accumulate in apple fruits when MdT5H5 is transiently silenced. In addition, we identified MdbZIP23 as a repressor of MdT5H5, which promotes anthocyanin accumulation in apple fruits. These results suggest that MdbZIP23 negatively regulates MdT5H5-mediated anthocyanin accumulation, providing insight into the molecular mechanisms of photo-induced anthocyanin accumulation in apple fruits.