SINK-ASSOCIATED AND VASCULAR-ASSOCIATED SUCROSE SYNTHASE FUNCTIONS ARE ENCODED BY DIFFERENT GENE CLASSES IN POTATO

Two differentially regulated classes of sucrose synthase genes, Sus3 a nd Sus4, were identified in potato. They cannot be classified as Sus1 and Sus2 types based on sequence homology and appear to have evolved a fter the divergence of the major families of dicotyledonous plants but before the divergence of tomato and potato. The potato sucrose syntha se clones Sus3-65 and Sus4-16 share an 87% nucleotide identity in the coding regions, and both are interrupted by 13 introns, including a lo ng leader intron. Potato Sus3 genes are expressed at the highest level s in stems and roots and appear to provide the vascular function of su crose synthase. In contrast, Sus4 genes are expressed primarily in the storage and vascular tissue of tubers and appear to facilitate sink f unction. The genes are differentially regulated in root tips, with Sus 3 expressed at high levels in the cell division zone and Sus4 expresse d at high levels in the meristem and cap.