EVIDENCE FOR AN ESSENTIAL ROLE OF THE SUCROSE TRANSPORTER IN PHLOEM LOADING AND ASSIMILATE PARTITIONING

Sucrose is the principal transport form of assimilates in most plants. In many species, translocation of assimilates from the mesophyll into the phloem for long distance transport is assumed to be carrier media ted. A putative sucrose proton cotransporter cDNA has been isolated fr om potato and shown to be expressed mainly in the phloem of mature exp orting leaves. To study the in vivo role and function of the protein, potato plants were transformed with an antisense construct of the sucr ose transporter cDNA under control of the CaMV 35S promoter. Upon matu ration of the leaves, five transformants that expressed reduced levels of sucrose transporter mRNA developed local bleaching and curling of leaves. These leaves contained >20-fold higher concentrations of solub le carbohydrates and showed a 5-fold increase in starch content as com pared with wild type plants, as expected from a block in export. Trans genic plants with a reduced amount of sucrose carrier mRNA show a dram atic reduction in root development and tuber yield. Maximal photosynth etic activity was reduced at least in the strongly affected transforma nts. The effects observed in the antisense plants strongly support an apoplastic model for phloem loading, in which the sucrose transporter located at the phloem plasma membrane represents the primary route for sugar uptake into the long distance distribution network.