Abstract

PAC is widely applied as a gibberellin biosynthesis inhibitor to control plant growth, yet its coordinated effects on grape seed development (GSD) remain largely unresolved. Here, we demonstrate that PAC imposes a strong developmental constraint on GSD by simultaneously disrupting hormonal regulation, carbon metabolism, and cell wall dynamics. PAC application markedly reduced seed and berry growth, matching a pronounced depletion of primary metabolites, including proteins, soluble sugars, starch, and total carbohydrates, indicating restricted carbon investment during seed development. In contrast, PAC elicited a substantial metabolic reprogramming toward stress‑associated pathways, characterized by strong accumulation of proline, phenolic compounds, and flavonoids. Detailed cell wall analyses revealed that PAC significantly diminished both low‑ and high‑molecular‑weight xyloglucans as well as esterified and non‑esterified pectins, reflecting impaired polysaccharide turnover and reduced wall plasticity. Consistently, qRT‑PCR validation showed coordinated down‑regulation of key genes governing cell wall modification ( VvPME32 , VvXTH23 , VvGLU15 ), carbohydrate metabolism ( VvSUS2 , VvAPL2 , VvSS2 ), and gibberellin biosynthesis ( VvGA20ox , VvGA3ox ), providing a molecular basis for the observed structural and metabolic alterations. Correlation analysis ( p  < 0.05) uncovered a clear antagonism between growth‑related traits (seed/berry size, soluble carbohydrates) and stress‑responsive metabolites (proline, phenolics, flavonoids), while revealing strong positive correlations of VvPME32 and VvXTH23 with esterified pectins, VvEXPA1 and VvPER42 with total phenolics and flavonoids, and VvSUS2/VvAPL2/VvSS2 with starch and total carbohydrates. Conversely, VvGA20ox and VvGA3ox were negatively correlated with proline and non‑esterified pectins, positioning cell wall polysaccharides as central integrators of hormonal and metabolic control during seed develop