Role of rice cytosolic hexokinase OsHXK7 in sugar signaling and metabolism

We characterized the function of the rice cytosolic hexokinase OsHXK7 (Oryza sativa Hexokinase7), which is highly upregulated when seeds germinate under O-2-deficient conditions. According to transient expression assays that used the promoter: luciferase fusion construct, OsHXK7 enhanced the glucose (Glc)-dependent repression of a rice a-amylase gene (RAmy3D) in the mesophyll protoplasts of maize, but its catalytically inactive mutant alleles did not. Consistently, the expression of OsHXK7, but not its catalytically inactive alleles, complemented the Arabidopsis glucose insensitive2-1 (gin2-1) mutant, thereby resulting in the wild type characteristics of Glc-dependent repression, seedling development, and plant growth. Interestingly, OsHXK7-mediated Glc-dependent repression was abolished in the O-2-deficient mesophyll protoplasts of maize. This result provides compelling evidence that OsHXK7 functions in sugar signaling via a glycolysis-dependent manner under normal conditions, but its signaling role is suppressed when O-2 is deficient. The germination of two null OsHXK7 mutants, oshxk7-1 and oshxk7-2, was affected by O-2 deficiency, but overexpression enhanced germination in rice. This result suggests the distinct role that OsHXK7 plays in sugar metabolism and efficient germination by enforcing glycolysis-mediated fermentation in O-2-deficient rice.