Integrative metabolome and transcriptome analyses reveal the mechanism by which white light increases apple phenolics

Apple is a photophilic fruit and the nutrients content such as phenolics loss is a major problem during fruit storage. Light can affect the yield, appearance and quality of apples. However, the molecular mechanism behind how white light induces phenolics accumulation to improve quality in fruits remains unclear. In this study, we discovered that white light with an intensity of 4 w/m2 can enhance the total phenolic and flavonoid content in postharvest apple fruits. To investigate the specific phenolics that undergo changes during this process, we analyzed the metabolomes of apples exposed to white light for 5 days after harvesting. We identified a total of 97 differentially accumulated metabolites (DAMs), including 15 phenolics. Isorhamnetin-O-hexos, tangereti, phloretin, cis-p-coumaroyltartaric acid and cis-p-coumaroyltartaric acid contents decreased in white light. While myricetin 3-O-galactoside, astilbin, sieboldin 2′-hydroxygenistein, syringaresinol, pinoresino, maleoyl-caffeoylquinic acid, rosmarinyl glucosid, coniferyl alcohol, p-coumaryl alcoho, and feruloylferuloyltartaric acid contents increased in white light. Additionally, RNA-seq analysis allowed us to identify 1988 differentially expressed genes (DEGs), which included three key enzymes (phenylalanine ammonia-lyase [MdPAL-1], chalcone isomerase [MdCHI-like], and 4-coumarate: -CoA ligase [Md4CL-1]) involved in the phenolic synthesis pathway, as well as a basic leucine zipper domain (bZIP) transcription factor known as MdbZIP67. Further investigations revealed that white light can regulate the transcription of MdPAL-1, Md4CL-1, and MdCHI-like genes through MdbZIP67, resulting in an increase in phenolic content during fruit storage. Here, we have identified a new method to enhance the phenolic content in apple fruits with characteristics of easy to use and low cost. And we have elucidated the mechanism by which white light affects phenolic content during the fruit's shelf life.