Ferric-Chelate Reductase FRO3 Is Involved in Iron Homeostasis in Table Grape and Enhanced Plant Tolerance to Iron-Deficient Conditions

In plants, ferric-chelate reductase (FRO) plays a critical role in mediating extracellular iron (Fe) reduction, a process essential for cellular Fe homeostasis and abiotic stress tolerance. However, the biological functions and regulatory mechanisms of FRO proteins in fruit crops remain poorly characterized. Here, six <i>VvFRO</i> genes were identified in the table grape cultivar 'Yanhong'. Transcriptional analysis revealed that root expression of these genes was mainly induced under Fe deficiency, Fe depletion, NaCl stress, and PEG-induced drought stress, respectively, but remained unchanged by low temperature (4 °C) or heat treatment (45 °C). Among them, <i>VvFRO3</i> exhibited the highest constitutive expression, predominantly in leaves, and was significantly up-regulated under Fe deficiency, Fe depletion, or NaCl treatment. Functional complementation assays demonstrated that heterologous overexpression of <i>VvFRO3</i> in the <i>Arabidopsis thaliana fro2</i> knockout mutant rescued its growth retardation phenotype, particularly under Fe-deficient conditions. This study advances our understanding of Fe uptake, transport, and homeostasis mechanisms in perennial fruit crops.