The molecular details of sugar sensing and sugar-mediated signal transduction pathways are unclear but recent results suggest that hexokinase functions as an important plant sugar sensor in a way that is similar to that found in yeast. The use of mutants in Arabidopsis defective in specific signaling steps is of particular importance because these give access to the genes encoding components in the signaling pathways. In addition, the physiological analysis of such mutants may reveal the interaction of sugar-induced signaling pathways and those induced by other stimuli such as environmental or biotic stress.