POTATO SUCROSE TRANSPORTER EXPRESSION IN MINOR VEINS INDICATES A ROLEIN PHLOEM LOADING

The major transport form of assimilates in most plants is sucrose. Tra nslocation from the mesophyll into the phloem for long-distance transp ort is assumed to be carrier mediated in many species. A sucrose trans porter cDNA was isolated from potato by complementation of a yeast str ain that is unable to grow on sucrose because of the absence of an end ogenous sucrose uptake system and the lack of a secreted invertase. Th e deduced amino acid sequence of the potato sucrose transporter gene S tSUT1 is highly hydrophobic and is 68% identical to the spinach sucros e transporter SoSUT1 (pS21). In yeast, the sensitivity of sucrose tran sport to protonophores and to an increase in pH is consistent with an active proton cotransport mechanism. Substrate specificity and inhibit ion by protein modifiers are similar to results obtained for sucrose t ransport into protoplasts and plasma membrane vesicles and for the spi nach transporter, with the exception of a reduction in maltose affinit y. RNA gel blot analysis shows that the StSUT1 gene is highly expresse d in mature leaves, whereas stem and sink tissues, such as developing leaves, show only low expression. RNA in situ hybridization studies sh ow that the transporter gene is expressed specifically in the phloem. Both the properties and the expression pattern are consistent with a f unction of the sucrose transporter protein in phloem loading.