Metabolite transport in apoplastic and symplastic phloem loaders

Abstract

If leaves are able to fix more CO2 during the light period than they need for their metabolism, the surplus of carbon is exported to support sink tissues. Long distance transport of metabolites from the source leaves to the sink tissues takes place in the phloem. The production and export of assimilates involves a cooperation of various cell types and requires several transfer steps across membranes. To understand these complex interactions, we have analyzed the concentrations of metabolites in the subcellular compartments of mesophyll cells and in the phloem sap by the methods of "non-aqueous fractionation" and "laser-aphid-stylet-technique". Metabolites can enter the phloem via apoplastic or symplastic loading steps, depending on the number of plasmodesmata between the phloem and the surrounding cells. In apoplastic phloem loaders sucrose is the only form of carbohydrate which is exported, whereas the export of amino acids is not specific. All the amino acids found in the cytosol of mesophyll cells also occur in the phloem sap. To compare symplastic phloem loaders with apoplastic phloem loaders we analyzed several members of the Scrophulariaceae. The main carbon transport form of Asarina is sucrose and in Alonsoa raffinose and stachyose (ca. 600 mM). Independent of the kind of the translocated sugar, we identified full length cDNAs of sucrose transporters in the leaves of both plant species. SUT mRNA could also be detected in pure phloem sap. The apoplastic and symplastic phloem loading mechanism will be discussed.