Role of membrane transport in phloem translocation of assimilates and water

Most growth and storage organs (sinks) of higher plants import assimilates in solution by bulk flow through the phloem, driven by differences in hydro static pressure. These differences in pressure, located between the ends of the interconnecting phloem path, are generated by osmotic water movement, driven in turn by membrane transport of solutes. Sucrose, amino-nitrogen co mpounds and potassium represent the osmotically important solutes found in phloem contents of most species. Phloem loading and unloading events of the se assimilate species play central roles in determining phloem translocatio n rates and partitioning of assimilates and water. Depending on plant speci es, leaf vein order and sink type, phloem loading and unloading may follow apoplasmic or symplasmic routes. Irrespective of the cellular pathway follo wed, assimilates are transported across plasma and organellar membranes. Aq uaporins, amino-nitrogen transporters, sucrose transporters and potassium c hannels have been detected in key sites along the source-phloem-sink transp ort pathway. Reverse genetics has demonstrated that sucrose/proton symporte rs are important in transport events necessary for phloem loading in Solana ceous plant species. Drawing on circumstantial evidence, we review possible functions the remaining membrane transporters and channels may serve in dr iving phloem translocation of assimilates and water from source to sink.