Sucrose transporters in two members of the Scrophulariaceae with differenttypes of transport sugar

In order to study differences between sugar transport in oligosaccharide-tr anslocating and sucrose-translocating species, two members of the Scrophula riaceae, Asarina barclaiana Pennell and Alonsoa meridionalis O. Kuntze, wer e analysed regarding minor-vein anatomy, sugar concentrations in leaves and phloem sap, and expression of sucrose transporters. The minor veins of Asa rina barclaiana possess mainly transfer cells and modified intermediary cel ls and those of Alonsoa meridionalis have intermediary cells and ordinary c ompanion cells. Phloem sap from these plants was collected by the laser-aph id-stylet technique. The main carbon transport forms in Asarina were sucros e and in Alonsoa raffinose and stachyose. The sum of the carbohydrate conce ntrations in the phloem sap of both species was as high as that in apoplast ic phloem loaders. In Asarina the ratio of the sucrose concentration in the phloem to that in the cytosol of source cells was about 35 and the corresp onding:ratio in Alonsoa was about two. Sucrose transporter cDNAs were isola ted from leaves of both species. By means of semi-quantitative reverse tran scription-polymerase chain reaction, sucrose transporter mRNA was detected in different organs and also in the phloem sap. This is the first time that sucrose transporters have been found in oligosaccharide-translocating spec ies and that the mRNA of these sucrose transporters has been localized dire ctly in the phloem sap. Taken together, our observations indicate that Asar ina is an apoplastic phloem loader, while the results for Alonsoa are ambig uous: some properties are typical of the symplastic phloem-loading mechanis m, but probably a sucrose transporter is involved in loading and/or retriev al of sucrose into the phloem.