In Arabidopsis, two genes (At5g17310 and At3g03250) encode proteins that share high aa sequence identity to each other (93%) and to the well-characterized potato and barley UDP-Glc PPase (>80%). Recombinant At5g17310 (UGlcPP1) expressed in E. coli utilizes only Glc-1-P and UTP to form UDP-Glc. UGlcPP1 is specific for both UTP and Glc1P since TTP, GTP, ATP or other sugar-1-phosphates are not substrates for this enzyme (414). A crystal structure of UGlcPP2 and At3g03250 has been submitted (Wesenberg, G. E., Phillips, G. N., Jr., Bitto, E., Bingman, C. A., Allard, S. T.M.). Early biochemical work established that UDP-Glc PPase is inhibited by UDP-Xyl. If this inhibition occurs in vivo, it would imply that UDP-Xyl, in addition to gene expression, regulates the UDP-glucose pool, and thus, the NDP-sugar pool available for wall synthesis. Recent analysis of rice plants where one of the two rice UDP-Glc PPase genes, ugpl, was suppressed, suggests that the production of UDP-Glc during pollen development is critical for callose deposition (417).