EXPRESSION OF THE PMSUC1 SUCROSE CARRIER GENE FROM PLANTAGO-MAJOR L IS INDUCED DURING SEED DEVELOPMENT

A cDNA clone of the plasma membrane sucrose-Ht symporter PmSUC1 from P lantago major L. has been isolated and expressed in Saccharomyces cere visiae. The PmSUC1 protein was characterized in transgenic yeast and i n proteoliposomes with an artificial proton-motive-force (pmf) generat or. PmSUC1 catalyzes the active uptake of sucrose or maltose in the pr esence of pmf and is sensitive to uncouplers. Unlike the extremely pH- dependent PmSUC2 sucrose-H+ symporter, PmSUC1 is relatively insensitiv e to changes of the extracellular pH. In leaves and petioles of P. maj or, expression of PmSUC1 mRNA is restricted to the vascular system. Th e important new feature about PmSUC1 is that the highest mRNA levels a re found in non-vascular tissue of P. major flowers where the gene is transiently expressed during the early stages of seed development. In site hybridization experiments show that PmSUC1 is expressed only in y oung ovules; the putative physiological role of PmSUC1 is discussed.