Molecular characterisation and expression of L-galactono-1,4-lactone dehydrogenase and L-ascorbic acid accumulation during fruit development in Rosa roxburghii

A cDNA clone encoding L-galactono-1,4-lactone dehydrogenase (GalLDH; EC 1.3.2.3) that catalyses the terminal step of ascorbic acid (AsA) biosynthesis in higher plants, was isolated from fruits of Rosa roxburghii Tratt, a plant species known to be extremely rich in AsA. The gene (RrGalLDH) encodes a protein of 589 amino acids, and the deduced amino acid was highly homologous to GalLDH proteins from other plant species. Southern blot analysis indicated that RrGalLDH was encoded by several genes. Accumulation of RrGalLDH mRNA in R. roxburghii fruit showed a development-specific, but not ACO- or ripening-associated, pattern. The RrGalLDH gene was expressed with a low-high-low pattern during fruit development, contributing to parallel changes in AsA accumulation. Only transitory activities in ascorbate oxidase and ascorbate peroxidase were observed throughout fruit development, suggesting that AsA catabolism by these two enzymes was not dominant in fruit. Furthermore, incubating flesh tissues of R. roxburghii fruit in vitro showed that fruit were capable of AsA biosynthesis using both D-glucuronic acid and L-galactono-1,4-lactone as precursors. These data provide information revealing, at least in part, the reasons for abundant AsA accumulation in this fruit.