Phenylpropanoid Metabolic Responses to Downy Mildew in Susceptible and Resistant Grape Cultivars

Phenylpropanoid metabolism is closely related to plant disease resistance. In order to explore the physiological regulation mechanism of phenylpropanoid metabolism in grape varieties with different resistance levels to downy mildew, we conducted an experiment using grape varieties with different levels of resistance to downy mildew. ‘Shine Muscat’, ‘Zaoxiameigui’, ‘Wuhecuibao’ and ‘Zaoheibao’ were used as experimental materials. The leaves were inoculated with Plasmopara viticola , and the physiological indicators of phenylpropanoid metabolism were measured at 0, 0.5, 1, 2, 3, 4, 5 and 6 days after inoculation. The results showed that obvious hyphae were observed on the 6th day after inoculation in ‘Wuhecuibao’, ‘Zaoheibao’, ‘Zaoxiameigui’ and ‘Shine Muscat’, and the disease indices were 87.16, 75.18, 35.56 and 2.34, respectively, indicating that ‘Wuhecuibao’ and ‘Zaoheibao’ were susceptible to downy mildew, whereas ‘Zaoxiameigui’ and ‘Shine Muscat’ were resistant. The results of correlation analysis showed that superoxide dismutase (SOD), catalase (CAT), phenylalanine ammonia‐lyase (PAL), phenolics and proanthocyanidins were significantly negatively correlated with the disease index. The results of principal component analysis showed that the most important indicators for the first principal component were cinnamate 4‐hydroxylase (C4H), phenolics, flavonoids, proanthocyanidins and reactive oxygen species. Partial least squares regression disclosed significant variations in the concentrations of PAL, malondialdehyde (MDA), chlorophyll and flavonoids within grape leaves. Upon downy mildew infection of leaves from resistant grape varieties, the elevated PAL activity was more substantial in resistant strains compared to susceptible ones, accompanied by significant increases in the total polyphenolic compounds and procyanidins. Concurrently, SOD and CAT demonstrated efficacy in neutralising reactive oxygen species, thereby reinforcing the plants' disease resistance.