Transcriptome analysis of calcium-induced accumulation of anthocyanins in grape skin

Anthocyanin in grape skin has positive effects on fruit coloration, plant development and human health. Calcium is not only an essential macroelement for plant growth, but also a second messenger involved in regulating various physiological process. Calcium-containing sprays is an effective horticultural practice to improve anthocyanin accumulation during the ripening stages. However, the underlying mechanism of calcium-induced anthocyanin biosynthesis is largely unexplored, and few studies have analyzed the genome-wide gene expression in grape skin exposed to calcium treatment. In this study, Illumina RNA-Sequencing (RNA-Seq) was performed on grape skin at one week after whole-plant calcium-sprays. A total of 1894 significantly differentially expressed genes (DEGs) were affected by calcium treatment, including 1266 up-regulated and 628 down-regulated genes. Further, we found calcium triggered a large number of DEGs associated with calcium transport and signaling, anthocyanin biosynthesis and transport and regulation, sugar transport, and plant hormone biosynthesis and signaling. By analyzing the up/down-regulated DEGs, we clarified exogenous calcium may improve grape berry color in the following four pathways: (1) calcium increased total soluble sugars content, which may promote anthocyanin accumulation by activating transcription factors (TFs) related to anthocyanin biosynthesis; (2) calcium enhanced the accumulation of anthocyanin by stimulating the interaction between calmodulin and UDP-glucose : flavonoid 3-0-glucosyltransferase (UFGT); (3) Ca2+/calmodulin may modulate TFs related to anthocyanin, which will then stimulate the genes in the biosynthetic pathway; (4) calcium signal may stimulate the biosynthesis of JA (jasmonic acid) and ET (ethylene), which may enhance anthocyanin content by increasing endogenous sugar level or directly activating TFs related to anthocyanin biosynthesis. This work is the first comprehensive transcriptomic analysis of calcium-treated fruit and provides deep insight into the molecular mechanism of calcium-induced accumulation of anthocyanin.