Synergistic Enhancement of Phenolic Accumulation, Antioxidant Capacity and Fruit Quality in Marselan Grape (Vitis vinifera cv. Marselan) by Nano Zero-Valent Iron Combined with Potassium Dihydrogen Phosphat

Precision nano-fertilization offers transformative potential for sustainable improvement of grape quality, yet the underlying molecular mechanisms remain poorly understood. Here, we investigated the effects of foliar-applied nano zero-valent iron (nZVI) and potassium dihydrogen phosphate (KH2PO4), in combination, on berry quality and secondary metabolic reprogramming in Vitis vinifera cv. Marselan. The combined nZVI/KH2PO4 treatment improved photosynthetic capacity, Fe/P co-accumulation, and berry quality traits including soluble solid content, sugar–acid ratio, and phenolic and aroma metabolite profiles. Crucially, integrated transcriptomic and metabolomic profiling identified 631 differentially expressed genes and 838 differentially accumulated metabolites, converging on flavonoid biosynthesis and glutathione metabolism as the dominant regulatory axes. Correlation network analysis pinpointed five hub regulatory genes—VvHCT, VvFLS1, VvLAR1/2, VvUGT88F5, and VvODC—as central orchestrators of nanomaterial-driven metabolic reprogramming: VvHCT and VvFLS1 coordinately redirected carbon flux toward hydroxycinnamic acid conjugates and flavonol accumulation, while VvLAR1/2 governed proanthocyanidin polymerization, and VvUGT88F5 modulated glycosylation-dependent metabolite stabilization. Notably, VvODC linked polyamine metabolism to glutathione-mediated stress buffering, revealing a previously uncharacterized crosstalk between nano-iron signaling and antioxidant reprogramming. These findings establish a mechanistic framework in which nZVI and KH2PO4 synergistically remodel the secondary metabolome through discrete yet interconnected transcriptional nodes, providing molecular targets for nano-enabled precision viticulture and broader applications of engineered nanomaterials in high-value crop improvement.