Matrix polysaccharide precursors in Arabidopsis cell walls are synthesized by alternate pathways with organ-specific expression patterns.

The expression pattern of the single-copy gene UDP-glucose dehydrogenase (Ugd) was analysed in transgenic Arabidopsis plants by promoter:GUS and GFP fusions, Western blots, activity assays and histochemical activity staining. The enzyme oxidizes UDP-glucose to UDP-glucuronic acid and thus directs carbohydrates irreversibly into a cell wall-specific pool of nucleotide sugars. UDP-glucuronic acid is the central intermediate in the interconversion pathway to other nucleotide sugars, including the UDP-derivatives of arabinose, xylose, apiose and galacturonic acid which account for half the biomass of a typical Arabidopsis leaf cell wall. These activated sugars are needed as substrates for the biosynthesis of matrix polysaccharide polymers. In plants up to 5 days old the Ugd gene is strongly expressed in young roots, but very little in hypocotyls. Older plants show a more uniform expression pattern with a preference for the vascular system. A complex expression pattern was observed in flowers with high activity in the stamen, stigma and nectaries. Meristems in the leaf axil of rosette and inflorescence leaves exhibit a high level of activity of the Ugd gene. Although many of the growing tissues show high activity levels of the Ugd gene, others such as the hypocotyl and the cotyledons of young seedlings do not. Instead these tissues efficiently incorporate 3H-inositol into their cell walls. This indicates the biosynthesis of UDP-glucuronic acid through an alternative pathway via the oxidation of inositol to glucuronic acid and subsequent activation to the nucleotide sugar. The data strongly suggest two alternative pathways for matrix polysaccharide precursors with spatial and developmental regulation.