The mur4 Mutant of Arabidopsis Is Partially Defective in the de Novo Synthesis of Uridine Diphospho l-Arabinose

To obtain information on the synthesis and function of arabinosylated glycans, the mur4 mutant of Arabidopsis was characterized. This mutation leads to a 50% reduction in the monosaccharide l-arabinose in most organs and affects arabinose-containing pectic cell wall polysaccharides and arabinogalactan proteins. Feeding l-arabinose to mur4 plants restores the cell wall composition to wild-type levels, suggesting a partial defect in the de novo synthesis of UDP-l-arabinose, the activated sugar used by arabinosyltransferases. The defect was traced to the conversion of UDP-d-xylose to UDP-l-arabinose in the microsome fraction of leaf material, indicating that mur4 plants are defective in a membrane-bound UDP-d-xylose 4-epimerase.