The role of hexokinase in plant sugar signal transduction and growth and development

Previous studies have revealed a central role of Arabidopsis thaliana hexok inases (AtHXK1 and AtHXK2) in the glucose repression of photosynthetic gene s and early seedling development. However, it remains unclear whether HXK c an modulate the expression of diverse sugar-regulated genes. On the basis o f the results of analyses of gene expression in HXK transgenic plants, we s uggest that three distinct glucose signal transduction pathways exist in pl ants. The first is an AtHXK1-dependent pathway in which gene expression is correlated with the AtHXK1-mediated signaling function. The second is a gly colysis-dependent pathway that is influenced by the catalytic activity of b oth AtHXK1 and the heterologous yeast Hxk2. The last is an AtHXK1-independe nt pathway in which gene expression is independent of AtHXK1. Further inves tigation of HXK transgenic Arabidopsis discloses a role of HXK in glucose-d ependent growth and senescence. In the absence of exogenous glucose, plant growth is limited to the seedling stage with restricted true leaf developme nt even after a 3-week culture on MS medium. In the presence of glucose, ho wever, over-expressing Arabidopsis or yeast HXK in plants results in the re pression of growth and true leaf development, and early senescence, while u nder-expressing AtHXK1 delays the senescence process. These studies reveal multiple glucose signal transduction pathways that control diverse genes an d processes that are intimately linked to developmental stages and environm ental conditions.