The RnGME gene in blackcurrant enhances ascorbic acid accumulation to improve stress tolerance

Blackcurrant ( Ribes nigrum L.), a nutrient-rich cold-climate berry, is notable for its high ascorbic acid (AsA) content. However, the mechanisms underlying AsA biosynthesis in fruit remain unexplored. This study identified RnGME , encoding GDP-D-mannose 3′,5′-epimerase, as the key gene regulating AsA biosynthesis via the L-galactose pathway in blackcurrant. Moreover, this gene also enhances plant resistance to salt and cold stress. The expression level of RnGME gene declined during fruit development and ripening, paralleling the decrease in AsA content from young fruit to mature stages. The overexpression of RnGME in tomato increased AsA and dehydroascorbate (DHA) contents, decreased GDP-D-mannose pyrophosphorylase (GMP) enzyme activity, and elevated the enzymatic activities of GME and GDP-L-galactose phosphatase (GGP). Transient overexpression and VIGS in blackcurrant fruit revealed that RnGME enhanced AsA biosynthesis, reduced the MDA content and suppressed ethylene synthesis. Under salt and cold stress, RnGME -overexpressing tomato plants displayed higher photosynthetic efficiency and superior antioxidant enzyme activities (SOD, POD, CAT), thereby protecting PSII reaction centers against oxidative damage. These findings demonstrate that RnGME acts as a key regulator to facilitate AsA biosynthesis and improve plant tolerance to abiotic stresses.