Essential protein during embryogenesis (PubMed:15266054). Catalyzes a reaction of the Smirnoff-Wheeler pathway, the major route to ascorbate biosynthesis in plants. Plays an essential role in plant growth and development and cell-wall architecture. Provides GDP-mannose, used for cell wall carbohydrate biosynthesis, protein N-glycosylation, as well as for the biosynthesis of the antioxidant ascorbate. alpha-D-mannose 1-phosphate + GTP + H(+) = diphosphate + GDP-alpha-D-mannose Nucleotide-sugar biosynthesis; GDP-alpha-D-mannose biosynthesis; GDP-alpha-D-mannose from alpha-D-mannose 1-phosphate (GTP route): step 1/1. Interacts in vitro with CSN5A and CSN5B, but in planta with CSN5B only, targeting CYT1 for degradation in the dark by the 26S proteasome. The N-terminus (1-40) is necessary for interaction with CNS5B. Embryo defective arrested at the cotyledon stage (PubMed:15266054). Deficient in N-glycosylation, altered in cytokinesis and cell wall architecture during embryogenesis. Mutants vtc1 and hsn1 with reduced enzyme activity show reduced root length and leaf cell size, reduced accumulation of ascorbate, increased sensitivity to ozone, UV-B and oxidative stresses, increased levels of abscisic acid (ABA), salicylic acid (SA) and resistance to virulent pathogens and root growth inhibition in the presence of NH(4)(+). Belongs to the transferase hexapeptide repeat family.