Update on sucrose transport in higher plants

Sucrose as the major transported form of fixed carbon, must be translocated from the sites of synthesis, i.e. the green tissues, to the sites of consumption and storage, i.e. the non-green cells and organs. For apoplasmic transport, carrier-mediated processes are required at the plasma membrane. Functional complementation of modified yeast strains has enabled the isolation and characterization of a large family of sucrose carriers (SUT) from a wide variety of species. In Xenopus oocytes, electrophysiological methods demonstrated that the SUTs function as proton co-transporters. Localization studies show that at least SUT1 is present at the plasma membrane of enucleated sieve elements, indicating macromolecular transport of its mRNA or protein from the companion cell to the sieve element. Inhibition of the transport activity in several transgenic plant species proves that SUT function is essential for long-distance transport. Further experiments will be required to assign specific functions to the other members of the SUT family.