Generation of nucleotide sugars for biomass formation in plants

Nucleotide sugars are activated forms of monosaccharides and serve as glycosyl donors for glycosyltransferases in the synthesis of cell wall polysaccharides. Since they affect the amounts and architecture of cell wall polysaccharides in plants, the levels of nucleotide sugars, as well as the levels of the glycosyltransferases, at least partially, regulate the production of plant cellulosic biomass. Nucleotide sugars are principally generated through de novo pathways, in which various UDP- and GDP-sugars are produced through sequential interconversions from starting substrates such as UDP-glucose. The salvage pathway is an alternative route to synthesize nucleotide sugars. In the salvage pathway, free monosaccharides released during the metabolism of polysaccharides and glycoconjugates are first phosphorylated by monosaccharide kinases, and then converted to nucleotide sugars by nucleotide sugar pyrophosphorylases in the presence of the respective nucleotide triphosphates as co-substrates. In this review, we focus on the recent progress in our understanding of the mechanism for the generation of nucleotide sugars through the salvage pathway for free monosaccharides in higher plants.