ENHANCEMENT OF GLDHASE ACTIVITY BY LIGHT IN MAINTAINING ASA CONTENT IN LEAVES OF BRASSICA CAMPESTRIS SSP. CHINENSIS

A full length cDNA for L-galactono-g-lactone dehydrogenase (GLDH) that catalyzes the final step in the biosynthetic pathway of L-ascorbic acid (vitamin C), were isolated from Brassica campestris ssp. chinensis (B. campestris L. ssp. chinensis Makino var. communis Tsen et Lee). The gene (BcGLDH) encodes a polypeptide of 601 amino acids and the predicted amino acid sequence was highly homologous to GLDHs from other plant species. The relationship between the GLDHase activity and AsA (ascorbic acid) content in mature leaves of B. campestris ssp. chinensis was discussed. It was found that the GLDHase activity was higher in the light than in the dark. The GLDHase activity exhibited a diurnal change in the light; the activity was low in the morning and gradually increased, reaching the maximum value in the evening. Then the level decreased during the night until dawn of the next day. Similar diurnal changes were also found in TAsA (AsA+DHAsA) content, but not in AsA content. While in the dark, the GLDHase activity showed no substantially changes, and the AsA content gradually decreased. These results suggest that GLDH, which may play an important role in maintaining AsA content in leaves of B. campestris ssp. chinensis, is regulated by light.