SUGAR SENSING AND ALPHA-AMYLASE GENE REPRESSION IN RICE EMBRYOS

We used a transient expression system to study the mechanism by which carbohydrates repress a lice (Oryza sativa L.) alpha-amylase (EC 3.2.1 .1) gene. Exogenously fed metabolizable carbohydrates are able to elic it repression of the alpha-amylase gene RAmy3D in the rice embryo, and our results indicate that repression is also triggered efficiently by endogenous carbohydrates. Glucose analogs that are taken up by plant cells but not phosphorylated by hexokinase are unable to repress the a lpha-amylase gene studied, while 2-deoxyglucose, which is phosphorylab le but not further metabolized, downregulates RAmy3D promoter activity , indicating a role for hexokinase in the sugar-sensing mechanism trig gering repression of the RAmy3D gene. We tested two different hexokina se inhibitors, mannoheptulose and glucosamine, but only the latter was able to relieve RAmy3D promoter activity from repression by endogenou s carbohydrates. This correlates with the higher ability of glucosamin e to inhibit the activity of rice hexokinases in vitro. The glucosamin e-mediated relief of RAmy3D promoter activity from repression by endog enous carbohydrates does not correlate with a reduced rate of carbohyd rate utilization.