The molecular regulation of stilbene phytoalexin biosynthesis in Vitis vinifera during grape berry development

The molecular regulation of stilbene phytoalexin biosynthesis in developing Vitis vinifera L. grape berries was investigated using a UV induction syst em. Berries were collected at 1, 5, 10 and 16 weeks post-flowering from the cultivars Shiraz, Semillon, Cabernet Sauvignon and Chardonnay and the skin s analysed for resveratrol production following irradiation with UV-C light . The rate and maximal level of resveratrol accumulation increased markedly in berries sampled from 1-5 weeks post-flowering and then dramatically dec lined in maturing berries sampled from 10-16 weeks post-flowering in all cu ltivars. In berries sampled at 1 and 5 weeks post-flowering, maximal levels of resveratrol accumulation were recorded at incubation periods of 24 and 48 h respectively whereas maximal resveratrol levels were not recorded in w eek 16 berry skins until 72 h after UV-treatment. Gene expression analysis indicated that stilbene synthase (STS) mRNA accumulated within 4-8 h of UV treatment in berries sampled at 1 and 5 weeks post-flowering, but did not i ncrease in week 16 berries until 24-48 h following UV-irradiation. Furtherm ore, the overall level of STS gene expression declined in berries sampled 1 0-16 weeks post-flowering. The results demonstrate that inducible stilbene accumulation in ripening grape berries is highly regulated at the level of STS gene transcription. This decline in inducible STS gene expression may b e a major factor contributing to the increased susceptibility of ripening g rape berries to Botrytis cinerea infection.