Muscadinia rotundifolia 'Noble' defense response to Plasmopara viticola inoculation by inducing phytohormone-mediated stilbene accumulation.

Downy mildew (DM), one of the most devastating grape diseases worldwide, is caused by the biotrophic oomycete Plasmopara viticola (Pv). In general, grapevine responds to Pv infection with the accumulation of phytoalexins as part of the innate immune system, and diverse phytoalexins are induced on grapevines with different DM-resistance levels in response to Pv invasion. However, the regulation of phytoalexin biosynthesis during grapevine against Pv is still unclear. Herein, we detected stilbenes by UPLC-ESI-MS/MS and found that resveratrol was accumulated to higher level and earlier in the DM-immune Muscadinia rotundifolia 'Noble' than that in the DM-susceptible Vitis vinifera 'Thompson Seedless' after Pv inoculation. Additionally, a considerable amount of pterostilbene and ε-viniferin was found in 'Noble', while a little was detected in 'Thompson Seedless'. Resveratrol was glycosylated into piceid both in 'Noble' and 'Thompson Seedless' after Pv inoculation. The qPCR analysis of gene expression indicated that the resveratrol-synthesis gene (STS) was induced by Pv inoculation earlier in 'Noble' than that in 'Thompson Seedless', while the pterostilbene-synthesis gene (ROMT) was induced in 'Noble' but not in 'Thompson Seedless' at all. The piceid-synthesis gene (GT) was generally up-regulated in both cultivars. Sequence analysis of STS, ROMT, and GT promoters revealed that they contained cis-regulatory elements responsive to phytohormones and pathogens. Following Pv inoculation, the level of SA, MeJA, and ABA was found to be consistently higher in 'Noble' than those in 'Thompson Seedless'. The results of exogenous hormone elicitation further demonstrated that the accumulation of stilbenes was regulated by phytohormones. The earlier and higher accumulation of phytohormones and consequent induction of stilbene synthesis may play an important role in grapevine defense against downy mildew disease.