Most growth and storage organs (sinks) of higher plants import assimilates in
solution by bulk flow through the phloem, driven by differences in hydrostatic
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 compounds
and potassium represent the osmotically important solutes found in phloem
contents of most species. Phloem loading and unloading events of these
assimilate species play central roles in determining phloem translocation
rates and partitioning of assimilates and water. Depending on plant species,
leaf vein order and sink type, phloem loading and unloading may follow
apoplasmic or symplasmic routes. Irrespective of the cellular pathway
followed, assimilates are transported across plasma and organellar membranes.
Aquaporins, amino-nitrogen transporters, sucrose transporters and potassium
channels have been detected in key sites along the
source–phloem–sink transport pathway. Reverse genetics has
demonstrated that sucrose/proton symporters are important in transport
events necessary for phloem loading in Solanaceousplant
species. Drawing on circumstantial evidence, we review possible functions the
remaining membrane transporters and channels may serve in driving phloem
translocation of assimilates and water from source to sink.