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 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, 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 in the 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.