Abstract: UDP-glucose pyrophosphorylases(UGPases,EC 2.7.7.9) are key enzymes in plant carbohydrate metabolism and cell wall biosynthesis,catalyzing the reversible production of glucose-1-phosphate and UTP from UDP-glucose and pyrophosphate.The rice(Oryza sativa L.) genome contains two homologous UGPase genes,Ugp1 and Ugp2.The Ugp1 gene is globally expressed throughout the plant and is critical for plant growth and pollen development.In the present study,possible effects of sugars on Ugp1 expression were tested with excised rice leaves.The Ugp1 was up-regulated by feeding sucrose,glucose and fructose to excised rice leaves.The sucrose effect on Ugp1 was mimicked by exposure of dark-adapted leaves to light.The sugar signaling pathway for Ugp1 expression was dependent on hexokinase,as N-acetyl-glucosamine(NAG,a specific HXK inhibitor) was effective in inhibiting sugar-mediated up-regulation of Ugp1.However,NAG could only partially inhibit Ugp1 expression induced by glucose together with fructose,suggesting an unknown signaling pathway was also critical for regulation of Ugp1 expression.Using two-dimensional gel electrophoresis in combination with western blot analysis,we also demonstrated that sucrose-mediated up-regulation of Ugp1 had no effect on multiple rice UGPase isoforms.