Molecular biology of sugar transporters of the plant plasma membrane.
Plants possess highly efficient transport systems for organic substances such as monosaccharides and disaccharides. In many lower plants these transport systems can be used for heterotrophic growth in the absence of light. In higher plants which are a complex mosaic of autotrophic and heterotrophic cells, tissues and organs, they play an important role in the distribution of assimilated carbon amongst these various parts. Genes and cDNAs encoding transport proteins for mono-or disaccharides have been cloned recently. The relationship of these transporters to a large superfamily of uni-, sym-, and antiporters cloned from many other organisms is discussed. The paper describes strategies for cloning, heterologous expression (in Saccharomyces cerevisiae, Schizosaccharomyces pombe, Xenopus oocytes and heterologous plant systems), and functional characterization of individual transporters. Data on the tissue-specific expression of two monosaccharide transporters from Arabidopsis thaliana will be presented and possible glycosylation of the proteins is discussed.