The long noncoding RNA MdLNC499 bridges MdWRKY1 and MdERF109 function to regulate early-stage light-induced anthocyanin accumulation in apple fruit

Abstract

Anthocyanin pigments contribute to plant coloration and are valuable sources of antioxidants in the human diet as components of fruits and vegetables. Their production is known to be induced by light in apple fruit (Malus domestica); however, the underlying molecular mechanism responsible for early-stage light-induced anthocyanin biosynthesis remains unclear. Here, we identified an ethylene response factor (ERF) protein, ERF109, involved in light-induced anthocyanin biosynthesis and found that it promotes coloration by directly binding to anthocyanin-related gene promoters. Promoter::β-glucuronidase reporter analysis and Hi-C sequencing showed that a long noncoding RNA, MdLNC499, located nearby MdERF109, induces the expression of MdERF109. A W-box cis-element in the MdLNC499 promoter was found to be regulated by a transcription factor, MdWRKY1. Transient expression in apple fruit and stable transformation of apple calli allowed us to reconstruct a MdWRKY1-MdLNC499-MdERF109 transcriptional cascade in which MdWRKY1 is activated by light to increase the transcription of MdLNC499, which in turn induces MdERF109. The MdERF109 protein induces the expression of anthocyanin-related genes and the accumulation of anthocyanins in the early stages of apple coloration. Our results provide a platform for better understanding the various regulatory mechanisms involved in light-induced apple fruit coloration.