Systematic Identification of Long Noncoding RNAs Expressed During Light-induced Anthocyanin Accumulation in Apple Fruit

Anthocyanin pigments contribute to the red color of apple (Malus × domestica) fruit and have a major influence on their ornamental, dietary and market value. In this study, we investigated the potential role of long noncoding RNAs (lncRNAs) in anthocyanin biosynthesis. RNA-seq analysis of apple peels from the 'Red Fuji' cultivar during light-induced rapid anthocyanin accumulation revealed 5297 putative lncRNAs. Differential expression analysis further showed that lncRNAs were induced during light treatment and were involved in photosynthesis. Using the miRNA-lncRNA-mRNA network and endogenous target mimic (eTM) analysis, we predicted that two differentially expressed lncRNAs, MLNC3.2 and MLNC4.6, were potential eTMs for miRNA156a and promoted the expression of the SPL2-like and SPL33 transcription factors. Transient expression in apple fruit and stable transformation of apple callus showed that overexpression of the eTMs and SPLs promoted anthocyanin accumulation, with the opposite results in eTM and SPL-silenced fruit. Silencing or overexpressing of miR156a also affected the expression of the identified eTMs and SPLs. These results indicated that MLNC3.2 and MLNC4.6 function as eTMs for miR156a and prevent cleavage of SPL2-like and SPL33 by miR156a during light-induced anthocyanin biosynthesis. Our study provides fundamental insights into lncRNA involvement in the anthocyanin biosynthetic pathway in apple fruit.