Transcriptional activation of flavan-3-ols biosynthesis in grape berries by UV irradiation depending on developmental stage

The accumulation of flavan-3-ols in response to ultraviolet (UV) irradiation was investigated in grape berries with emphasis on the expression of three structural genes (VvANR, VvLAR1 and VvLAR2), and three regulatory genes (VvMYB5a, VvMYB5b and VvMYBPA1), and as well as the contents of flavan-3-ols. UV-A irradiation showed a promoting effect on the transcription of three structural genes at 3-week (flavan-3-ol accumulation period), 7-week (the end of flavan-3-ol accumulation) and 11-week (the beginning of anthocyanin synthesis) periods of berry development. UV-B irradiation also up-regulated all or part of the structural genes, but the activation effect of UV-C irradiation appeared only in the 7-week and 11-week grapes. The developmental stage-dependent activation by the three types of UV was also initiated for three regulatory genes, but the transcriptional up-regulation of the structural genes by UV irradiation was not entirely regulated by these transcription factors. The increase in the content of 2,3-trans-flavan-3-ols or 2,3-cis-flavan-3-ols by UV irradiation paralleled overall with the expression up-regulation of their corresponding structural genes in the 3-week and the 7-week grapes, but not in the 11-week grapes, indicating that the overexpression of structural genes by UV radiation does not translate into a higher content of flavan 3-ols at mature stage.