Sugar transporters in higher plants - A diversity of roles and complex regulation

Sugar-transport proteins play a crucial role in the cell-to-cell and long-d istance distribution of sugars throughout the plant, In the past decade, ge nes encoding sugar transporters (or carriers) have been identified, functio nally expressed in heterologous systems, and studied with respect to their spatial and temporal expression. Higher plants possess two distinct familie s of sugar carriers: the disaccharide transporters that primarily catalyse sucrose transport and the monosaccharide transporters that mediate the tran sport of a variable range of monosaccharides. The tissue and cellular expre ssion pattern of the respective genes indicates their specific and sometime s unique physiological tasks, Some play a purely nutritional role and suppl y sugars to cells for growth and development, whereas others are involved i n generating osmotic gradients required to drive mass flow or movement, Int riguingly, some carriers might be involved in signalling, Various levels of control regulate these sugar transporters during plant development and whe n the normal environment is perturbed. This article focuses on members of t he monosaccharide transporter and disaccharide transporter families, provid ing details about their structure, function and regulation, The tissue and cellular distribution of these sugar transporters suggests that they have i nteresting physiological roles.