SUGAR SENSING IN HIGHER-PLANTS

Sugar repression of photosynthetic genes is likely a central control m echanism. mediating energy homeostasis in a wide range of algae and hi gher plants. It overrides light activation and is coupled to developme ntal and environmental regulations. How sugar signals are sensed and t ransduced to the nucleus remains unclear To elucidate sugar-sensing me chanisms, we monitored the effects of a variety of sugars, glucose ana logs, and metabolic intermediates on photosynthetic fusion genes in a sensitive and versatile maize protoplast transient expression system. The results show that sugars that are the substrates of hexokinase (HK ) cause repression at a low concentration (1 to 10 mM), indicating a l ow degree of specificity and the irrelevance of osmotic change. Studie s with various glucose analogs suggest that glucose transport across t he plasma membrane is necessary but not sufficient to trigger repressi on, whereas subsequent phosphorylation by HK may be required. The effe ctiveness of 2-deoxyglucose, a nonmetabolizable glucose analog, and th e ineffectiveness of various metabolic intermediates in eliciting repr ession eliminate the involvement of glycolysis and other metabolic pat hways. Replenishing intracellular phosphate and ATP diminished by hexo ses does not overcome repression. Because mannoheptulose, a specific H K inhibitor, blocks the severe repression triggered by 2-deoxyglucose and yet the phosphorylated products per se do not act as repression si gnals, we propose that HK may have dual functions and may act as a key sensor end signal transmitter of sugar repression in higher plants.