Transcription and regulation of proanthocyanidin biosynthesis in wine grape:A review

Proanthocyanidin，also known as condensed tannin，is a mixture of flavanol-3-ols and flavan-3,4-diols in complex forms，and is one of the important polyphenolic compounds to measure the quality of grape berries and wines. In this paper，the composition and detection of proanthocyanidins，biosynthesis and transcriptional regulation of proanthocyanidins in grapes were reviewed. Proanthocyanidins，which were mainly existed in grape skins and seeds，could contribute protection to grapes under biotic and abiotic stress. In the process of wine-making，the proanthocyanidins from the fruit were pressed and impregnated into the wine，contributing to the bitterness and astringency of wine，and the copigmentation between them and other pigments could also strengthen the stability of wine color. Proanthocyanidin biosynthesis was a branch of the phenylpropane-flavonoid metabolic pathway，which shared the upstream pathway with flavonols and anthocyanins biosynthesis，and it largely accumulated during early development of grapes. Leucoanthocyanidin reductase（LAR）and anthocyanidin reductase（ANR）were key enzymes in proanthocyanidin biosynthesis. Grape possessed two LAR enzymes，which were encoded by VviLAR1 and VviLAR2 genes respectively，both of them had differences in temporal and spatial expression. The biosynthesis of proanthocyanidin in grapes was mainly regulated by MYB and bHLH family transcription factors. Until now，eight R2R3-MYB family transcription factors（VviMYBPA1，VviMYBPA2，VviMYBPAR，VviMYB5a，VviMYB5b，VviMYB86，VdMYB14，VviMYBC2-L1）and three bHLH family transcription factors（VviCA1，VviMYC1，VvibHLH93）have been identified. Currently，most of the identified transcription factors utilize VviLAR1 and VviANR as target genes，and the transcription factors that specifically regulated the expression of VviLAR2 gene needed to be explored. In the future，attention should be paid to the response mechanism of transcription factors to various biotic and abiotic stresses，the modification ways of transcription factors by transcription and posttranscriptional regulation，and the interaction mechanism between these transcription factors and various signals and hormones. This study is expected to provide insights for the study of the regulation mechanism of wine grape proanthocyanidins，and then guide the practice of wine viticulture management in China，help the selection and breeding of high-quality wine grape varieties，and promote the production of diversified and personalized wines.