LED white light-activated transcription factor MdHY5 inhibits ethylene biosynthesis during apple fruit ripening

Ethylene is one of the most important plant hormones involved in the climacteric fruit ripening process. Apple is a climacteric fruit, and its ripening process is regulated by ethylene. The effect of light-emitting diode (LED) light on plant growth and development has been extensively investigated; however, it remains unclear how LED light affects apple fruit ripening. Herein, we observed that the ripening of apple fruit treated with eight kinds of LED light was significantly inhibited compared with that of the dark control, with inhibition by white light being the most significant. Furthermore, the optimal light intensity for the inhibition of ethylene production was 20 μmol·m−2s−1, with an optimal light duration of 12 h per day. LED white light treatment enhanced the expression of two ELONGATED HYPOCOTYL 5 (HY5) genes, MdHY5L and MdHY5S, which encode transcription factors involved in the light signaling pathway. These two HY5s inhibited the transcription of MdACS1, a crucial rate-limiting enzyme in the ethylene synthesis pathway, by directly binding to its promoter. Collectively, these findings imply that LED white light induces the expression of MdHY5L and MdHY5S and delays the expression of the ethylene synthesis-related gene MdACS1 to prevent fruit ripening. This study preliminarily elucidates the molecular mechanism underlying LED white light-induced inhibition of ethylene synthesis in apple fruit, providing an important theoretical basis for studying how to improve the storage ability and prolong the shelf life of apple fruit.