Can NDPK Contribute to the Regulation of Ascorbate Biosynthesis in Arabidopsis?

Abstract

The Smirnoff-Wheeler pathway is the pivotal pathway for the biosynthesis of L-ascorbic acid (AsA) in Arabidopsis. Reproduction of GDP generated by phosphorylation of GDP-L-galactose in this pathway to GTP is catalyzed by nucleoside diphosphate kinase (NDPK; EC 2.7.4.6). We found the activity of NDPK to be light responsive with correlation to changing AsA levels in Arabidopsis. The expression levels of genes in the AsA pathway were analyzed using an Arabidopsis ndpk1 knock-out mutant. The mutant showed 25-30% lower AsA content compared to the wild type. The reduction in AsA level was found to correlate with NDPK activity as well as the expressions of some vital AsA biosynthetic genes in various tissues of the plants. The redox status, but not oxidized AsA, of the wild type was significantly different from that of the mutants. The mutant was not impaired in the activities of its major antioxidant enzymes. The activity of L-galactono-1,4-lactone dehydrogenase activity was reduced in the mutant but feeding with AsA precursors caused AsA level to increase. These results suggest that NDPK1 may be implicated in the overall biosynthesis of AsA in Arabidopsis.