Phosphomannose isomerase involved in the synthesis of the GDP-mannose and dolichol-phosphate-mannose required for a number of critical mannosyl transfer reactions. Involved in the ascorbic acid (AsA) biosynthesis. Required during the endosperm development. D-mannose 6-phosphate = D-fructose 6-phosphate Binds 1 zinc ion per subunit. Inhibited by EDTA, Zn(2+), Cd(2+), Co(2+), p-chloromercuribenzoate and L-ascorbic acid (AsA). Optimum pH is 7.5. Optimum temperature is 52 degrees Celsius. Nucleotide-sugar biosynthesis; GDP-alpha-D-mannose biosynthesis; alpha-D-mannose 1-phosphate from D-fructose 6-phosphate: step 1/2. Constitutively expressed in both vegetative and reproductive organs under normal growth conditions (at protein level). By light (at the protein level). Down-regulated by dark (at the protein level). Down-regulated by DCMU, an exogenous photosynthesis inhibitor. Endosperm development arrested. Belongs to the mannose-6-phosphate isomerase type 1 family.