Flavescence dorée phytoplasma (FDp) infections seriously affect production and survival of grapevine. We analysed the changes in the flavonoid pathway occurring in two red cultivars, the highly susceptible Barbera and the less susceptible Nebbiolo, following FDp infection. A combination of metabolic and transcript analyses was used to quantify flavonoid compounds and expression of a set of genes involved in their biosynthesis. Quantification of anthocyanins, flavonols, proanthocyanidins and related biosynthetic enzymes was performed over the vegetative season, at four time points, on healthy, infected and recovered plants. A strong activation of anthocyanin accumulation was observed in infected Barbera leaves, while the response was less marked in Nebbiolo. Proanthocyanidins also accumulated mainly in infected Barbera leaves, even if basal proanthocyanidin concentration was higher in healthy and recovered Nebbiolo. Biochemical data were supported by transcript analysis: genes of the stem flavonoid pathway and of the anthocyanin and proanthocyanidin branches were expressed at a higher level in infected than in healthy plants, with a different magnitude between the two cultivars. Based on our results, we hypothesize that flavonoid accumulation is a physiological consequence of FD infection without affecting phytoplasma multiplication, although proanthocyanidin accumulation could help repel further infection by the insect vector.
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