MOLECULAR APPROACHES TOWARDS AN UNDERSTANDING OF LOADING AND UNLOADING OF ASSIMILATES IN HIGHER-PLANTS

The combination of two sets of molecular tools, namely yeast expressio n cloning and the possibility of constructing transgenic plants, has a llowed analysis of the transport processes occurring at the plasma mem brane in higher plants. To date, more than 30 different plant genes fo r plasma membrane transporters of sugars and amino acids have been ide ntified, mainly by expression cloning. Furthermore, the functional exp ression of genes in Schizosaccharomyces pombe, Saccharomyces cerevisia e and Xenopus oocytes has been applied to obtain detailed information on the biochemical properties of the transporters. The expression syst ems have also allowed the purification of the proteins for structural analysis and to study structure-function relationship using mutagenesi s approaches. A number of mutants and transgenic plants defective in c ertain transport properties are available and these will help in under standing the physiology of the long-distance transport of assimilates.