Catalyzes the interconversion of mannose-6-phosphate to mannose-1-phosphate, the precursor for the synthesis of GDP-mannose (PubMed:17217471, PubMed:18086684). GDP-mannose is an essential sugar nucleotide for the synthesis of D-mannose-containing cell wall polysaccharides (galactomannans and glucomannans), glycolipids, glycoproteins and the antioxidant L-ascorbate (Probable). Can complement the yeast temperature-sensitive mutant sec53-6 (PubMed:17217471). alpha-D-mannose 1-phosphate = D-mannose 6-phosphate Glucose-1,6-bisphosphate is essential for activity. Optimum pH is 7.5. Optimum temperature is 30 degrees Celsius. Nucleotide-sugar biosynthesis; GDP-alpha-D-mannose biosynthesis; alpha-D-mannose 1-phosphate from D-fructose 6-phosphate: step 2/2. Homodimer. Expressed in roots, stems, leaves, flowers and immature fruits. The thermosensitive lethality of pmm-12 results from glycosylation defects rather than ascorbic acid depletion. Overexpression of PMM increases total leaf ascorbate content. Belongs to the eukaryotic PMM family.