Expression profiles of the genes involved in L-ascorbic acid biosynthesis and recycling in Rosa roxburghii leaves of various ages

Vitamin C (L-ascorbic acid, AsA) serves as an antioxidant and enzyme cofactor in plants. Rosa roxburghii Tratt is a rose species that is recognized for its high AsA content. However, leaf AsA metabolism has not been explored in detail, especially at the gene expression level. In the present study, AsA accumulation and the relative expression levels of genes putatively involved in AsA biosynthesis and recycling, as well as enzyme activities in leaves of different ages were investigated. Leaf AsA content remained at its highest level during the first 50 days of development, and then markedly decreased, which correlated with the expression and activity of monodehydroascorbate reductase (MDHAR) (p<0.05). The dehydroascorbate (DHA) and T-AsA (AsA + DHA) contents increased with leaf development. Both of them reached the peak in 50-day-old leaves and then decreased rapidly. The highest expression levels of three genes, i.e. L-galactono-1,4-lactone dehydrogenase (GalLDH), D-galacturonic acid reductase (GalUR), and myo-inositol oxygenase (MIOX) involved in AsA synthesis were observed in 50-day-old leaves, although only GalUR strongly correlated with the rate of T-AsA accumulation (p<0.05). Furthermore, incubating leaf discs in vitro with its precursor candidates showed that D-galacturonic acid (D-GalUA) was the most effective precursor of AsA biosynthesis. These results suggest that Rosa roxburghii leaves accumulated high levels of AsA (9 mu mol g(-1) FW at most), especially at young to mature ages. AsA and AsA pool size accumulation in the leaves were regulated by both biosynthesis and recycling, and GalUR in the GalUA pathway and MDHAR in the recycling pathway could play important roles in this process.