Arabidopsis Phosphomannose Isomerase 1, but Not Phosphomannose Isomerase 2, Is Essential for Ascorbic Acid Biosynthesis

We studied molecular and functional properties of Arabidopsis phosphomannose isomerase isoenzymes (PMI1 and PMI2) that catalyze reversible isomerization between D-fructose 6-phosphate and D-mannose 6-phosphate (Man-6P). The apparent K-m and V-max values for Man-6P of purified recombinant PMI1 were 41.3 +/- 4.2 mu M and 1.89 mu mol/min/mg protein, respectively, whereas those of purified recombinant PMI2 were 372 +/- 13 mu M and 22.5 mu mol/min/mg protein, respectively. Both PMI1 and PMI2 were inhibited by incubation with EDTA, Zn2+, Cd2+, and L-ascorbic acid (AsA). Arabidopsis PMI1 protein was constitutively expressed in both vegetative and reproductive organs under normal growth conditions, whereas the PMI2 protein was not expressed in any organs under light. The induction of PMI1 expression and an increase in the AsA level were observed in leaves under continuous light, whereas the induction of PMI2 expression and a decrease in the AsA level were observed under long term darkness. PMI1 showed a diurnal expression pattern in parallel with the total PMI activity and the total AsA content in leaves. Moreover, a reduction of PMI1 expression through RNA interference resulted in a substantial decrease in the total AsA content of leaves of knock down PMI1 plants, whereas the complete inhibition of PMI2 expression did not affect the total AsA levels in leaves of knock-out PMI2 plants. Consequently, this study improves our understanding of the molecular and functional properties of Arabidopsis PMI isoenzymes and provides genetic evidence of the involvement of PMI1, but not PMI2, in the biosynthesis of AsA in Arabidopsis plants.