Sugar Sensing and Sugar-Mediated Signal I ransduction in Plants

Information concerning the sugar status of plant cells is of great importance during a11 stages of the plant life cycle. The availability of or lack of sugars triggers many metabolic and developmental responses, and it is not surprising, therefore, that sugars profoundly affect the expression of a large number of genes (for review, see Koch, 1996; Graham, 1996). Sugar sensing occurs at the level of individual cells and the responses of such cells must be integrated at the tissue, organ, and plant level. Therefore, sugar-induced signals will interact with other sensing and signaling pathways. The mechanisms used by plant cells to sense sugars and to process this information are essentially unknown, and only recently are these questions being addressed experimentally. This lack of knowledge contrasts with the situation in yeast and bacteria, in which the molecular and physiological analysis of mutants have yielded extensive information about sugar perception (Trumbly, 1992; Ronne, 1995; Saier et al., 1995).