Gene expression and metabolic variability in fungus‐resistant <scp>PIWI</scp> (‘Pilzwiderstandsfähig’) grape cultivars

BACKGROUND: Fungus-resistant PIWI (Pilzwiderstandsfähig) grape cultivars are promising for reducing pesticide inputs in viticulture, but their enological potential is still poorly characterized. We performed a pilot, single-vintage study integrating berry gene expression and phenolic composition in five Italian red PIWI cultivars (Cabernet Eidos, Cabernet Volos, Julius, Merlot Kanthus and Merlot Khorus), using Pinot Noir as a non-PIWI Vitis vinifera benchmark. RESULTS: At harvest, transcript levels of 15 genes involved in sugar/energy signalling (TOR3, SnRK1.1, SnRK1.2), flavonoid biosynthesis (LAR1, LAR2, ANR, DFR, LDOX, FLS4, FLS5) and flavonoid glycosylation (GAT1, UFGT, Va5GT, Vl3GT, UGT72), were quantified, together with technological maturity traits, skin and seed phenolic fractions, and skin flavonol profiles. The PIWI panel showed marked variety-specific transcriptional and metabolic differences. SnRK1 isoforms varied more strongly among cultivars than TOR3. Hierarchical clustering of scaled transcript profiles separated samples into two major branches, indicating coordinated genotype-dependent modulation of sugar-signalling and flavonoid/glycosylation genes. Higher expression of flavonoid biosynthetic and glycosyltransferase genes (including LAR2, FLS5, Va5GT and UGT72) was associated with higher anthocyanins, quercetin- and myricetin-type flavonol glycosides, and colour intensity. Several, but not all, PIWI cultivars combined moderate-to-high sugars with comparatively high acidity, frequent enrichment in skin anthocyanins and flavonol glycosides, and lower extractable skin tannins than Pinot Noir. CONCLUSION: These results support a cultivar-resolved molecular and compositional framework for interpreting enological variability in Italian red PIWI grapes. The data may guide breeding and cultivar-tailored winemaking strategies, while broader generalization requires validation across additional genotypes, vintages and ripening stages. © 2026 The Author(s). Journal of the Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.