Role of rice cytosolic hexokinase OsHXK7 in sugar signaling and metabolism

We characterized the function of the rice cytosolic hexokinase OsHXK7 ( O ryza s ativa H e x o k inase 7 ), 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 α‐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.