Glucose and mannose regulate the expression of a major sucrose synthase gene in Arabidopsis via hexokinase-dependent mechanisms

Sucrose synthase (SuSy) is an important enzyme involved in sucrose synthesis/breakdown in all plants. Sus1, a major SuSy gene in Arabidopsis thaliana, was upregulated by sucrose, glucose and D-mannose, but not 3-O-methylglucose, when those compounds were fed to excised leaves. Mannose was more effective than glucose or sucrose in the induction of Sus1, with strong effects observed at a concentration as low as 20 mM. When fed to the excised leaves, N-acetyl-glucosamine, an inhibitor of hexokinase (HXK) enzymatic activity, decreased sucrose- and glucose-dependent, but not mannose-dependent, upregulation of Sus1. The sucrose/glucose-dependent Sus1 expression was strongly induced in transgenic Arabidopsis HXK-overexpressing (OE) plants, whereas mannose-dependent Sus1 expression markedly decreased in OE, but not in HXK-“antisense”, Arabidopsis plants. Feeding with sucrose resulted in a marked increase of glucose content in leaves, suggesting that it is glucose rather than sucrose that serves as a signal in upregulating Sus1 expression in sucrose-fed plants. The data suggest that Sus1 is regulated by a HXK-dependent pathway, with glucose and mannose effects differentially sensed/transmitted via the HXK step.