LED white light suppresses ethylene biosynthesis in apple fruit.

As a typical climacteric fruit, the apple (Malus domestica) ripening is predominantly regulated by ethylene. Light-emitting diode (LED) white light, an energy-efficient and widely promoted light source, can significantly inhibit ethylene biosynthesis during apple ripening, however, its underlying regulatory mechanism remains unclear. This study demonstrates that LED white light inhibits ethylene production by enhancing spermidine (Spd) production, which competes with ethylene for the common precursor S-adenosylmethionine (SAM). Mechanistically, LED white light induces the transcription factor ELONGATED HYPOCOTYL 5 LONG (MdHY5L) to enhance the transcription of Spd biosynthetic genes, thereby redirecting SAM flux toward Spd accumulation. Consistently, exogenous Spd application also inhibits ethylene biosynthesis, indicating that the light-induced Spd exerts a sustained inhibitory effect on ethylene production. Furthermore, MdHY5L functions upstream of light signaling component LATE ELONGATED HYPOCOTYL (MdLHY) to active its transcription. MdLHY directly binds to the promoter of ethylene biosynthesis gene MdACS1 to repress its transcription. Collectively, our findings reveal that LED white light suppresses apple fruit ripening by activating the MdHY5L-MdLHY transcriptional cascade, which precisely modulates polyamine-mediated ethylene biosynthesis. This study provides an effective strategy for prolonging the storage life of apple fruit.