Sucrose transport in higher plants

Presumably due to its physicochemical properties, sucrose represents the major transport form of photosynthetically assimilated carbohydrates in plants. Sucrose synthesized in green leaves is transported via the phloem, the long distance distribution network for assimilates in order to supply nonphotosynthetic organs with energy and carbon skeletons. At least in Solanaceae, sugar export seems to be a tightly regulated process involving a number of specific plasma membrane proteins. Significant progress in this field was made possible by the recent identification of plasma membrane sucrose transporter genes. These carriers represent important parts of the long-distance transport machinery and can serve as a starting point to obtain a complete picture of long-distance sucrose transport in plants. A combination of biochemical studies of transporter properties together with expression and localization studies allow specific functions to be assigned to the individual proteins. Furthermore, the use of transgenic plants specifically impaired in sucrose transporter expression has provided strong evidence that SUT1 transporter function is required for phloem loading. Physiological analyses of these plants demonstrate that sucrose transporters are essential components of the sucrose translocation pathway at least in potato and tobacco.