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Book Summary:
UDP-glucuronic acid (UDP-GlcA) occupies a pivotal position in the synthesis of plant cell wall polysaccharides. Sugars derived from UDP-GlcA account for approximately 50% of cell wall biomass in dicots. To better understand the regulation of UDP-GlcA synthesis, a gene encoding an UDP-sugar pyrophosphorylase (AtUSP) was functionally cloned from Arabidopsis thaliana. AtUSP is a single copy gene in Arabidopsis and is unique to plants. It is predicted to be a cytosolic protein and is distantly related to the UDP-glucose and UDP-N-acetylglucosamine pyrophosphorylase families. AtUSP has high activity with glucuronic acid-1-P and glucose-1-P, 25% lower activity with galactose-1-P, and minimal activity with other sugar-phosphates. Activity in the pyrophosphorylase direction with UDP-GlcA was not significantly inhibited by UDP-glucose, UDP-xylose or UDP-arabinose, however pyrophosphorylysis of UDP-glucose was inhibited 75% by equal molar UDP-GlcA. Affinity for UDP-GlcA was greater than for UDP-glucose in both reaction directions. The enzyme was specific in its requirement for Mg2+ and preference for UTP. AtUSP transcript was abundant in seedlings, flowers and stems and less abundant in old leaves. Expression of the myo-inositol oxygenase and UDP-glucose dehydrogenase (UDP-Glc DH) pathways to UDP-GlcA synthesis were compared across soybean embryo development. Enzyme activity and transcript levels were determined in soybean embryos at three ages during storage product accumulation. Activity of UDP-GlcA pyrophosphorylase was 30–100 fold higher than activity of UDP-Glc DH. UDP-GlcA pyrophosphorylase transcript was abundant in all samples while UDP-Glc DH transcript was only detectable in seedling roots. Comprehensive analysis of protein, oil, starch, soluble sugars and cell wall sugars over embryo development revealed that cell wall accumulation is directly proportional to increase in dry weight. This indicates that cell wall synthesis is competing for resources with protein and oil synthesis. UDP-GlcA pyrophosphorylase activity correlated closely with cell wall polysaccharide accumulation, suggesting that the myo-inositol oxidation pathway predominates during soybean embryo development. |
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