Multi-Omics Analysis Reveals the Changes of Monoterpenes and Anthocyanins Accumulation During Veraison in Red Muscat-Type Grape

The accumulation of anthocyanins and monoterpenes during grape berry color change at both metabolic and transcriptional levels were analyzed, to explore the spatio-temporal synthesis mechanisms of anthocyanins and monoterpenes, and provide a theoretical basis for the regulation of anthocyanin and monoterpene synthesis in table grape.

The grape berries of Ruiduhongyu were used as materials, sampling was started at 5 days before the veraison and continued until 40 days after the initial color change. The contents of total soluble solids and titratable acid in the berry samples were determined by conventional methods; The changes in monoterpene components and content in berries were determined with headspace solid-phase microextraction combined with gas chromatography-mass spectrometry (HS-SPEME-GC-MS); The content of total flavonoids and total anthocyanins were detected using a spectrophotometer; The expression changes of key genes involved in monoterpenes and anthocyanins synthesis were analyzed by transcriptome sequencing and real-time fluorescence quantitative PCR.

With the advancement of the coloration process, the main components of 25 free and bound monoterpenes in the Ruiduhongyu berries fluctuated. Most of the free monoterpenes began to be synthesized in large quantities from the 20th day of coloration and reached the highest level at the 35-40 th day of coloration. The content of bound monoterpenes reached the highest level at 30 days after coloration. The content of bound monoterpenes was higher than that of free monoterpenes. Flavonoids were synthesized and accumulated in large quantities before the fruit coloration; the synthesis of anthocyanins was initiated along with the fruit coloration and reached the highest level at 20 days after coloration, followed by a slight decrease. Based on transcriptome sequencing, a total of 5 836 differentially expressed genes were identified, and the number of differentially expressed genes varied significantly among different developmental stages. The differentially expressed genes were enriched in the pathways of phenylalanine synthesis, flavonoid synthesis, and monoterpene synthesis. Among them, 14 differentially expressed genes were related to the monoterpene synthesis pathway, and 11 were related to the anthocyanin synthesis pathway. The expression patterns of these genes were consistent with the accumulation of monoterpenes and anthocyanins. Further correlation analysis screened out 24 transcription factors that were significantly correlated with the expression of multiple genes involved in the monoterpene and anthocyanin synthesis pathways.

The synthesis of anthocyanins in red Muscat type grape berries initiates earlier than that of the aroma compound monoterpenes, and the synthesis of the two types of compounds is regulated in a spatio-temporal pattern. The accumulation of monoterpenes and anthocyanins is closely related to the expression of multiple key enzyme genes in their synthesis pathways, and their synthesis is regulated at the transcriptional level of various genes.