Abstract
In higher plants, phosphomannose isomerase (PMI) catalyzes the reversible isomerization of D-fructose 6-phosphate and D-mannose 6-phosphate as the first step of GDP-mannose and ascorbate (AsA) biosynthesis. In this study, we analyzed molecular properties of Arabidopsis PMI isoenzymes (PMI1 and PMI2). The induction of PMI1 expression and an increase in the AsA level were observed in leaves under continuous light, while the induction of PMI2 expression and a decrease in the AsA level were observed under long-term darkness. Moreover, a reduction of PMI1 expression through RNA interference resulted in a substantial decrease in the total AsA content of leaves of knockdown-PMI1 plants, while the complete inhibition of PMI2 expression did not affect the total AsA levels. Therefore, PMI1 is essential for biosynthesis of AsA, while PMI2 is involved in the utilization of Mannose-derived carbohydrates as an energy source in leaves under sugar-starved conditions (J. Biol. Chem. 283: 28842-28851, 2008).
