Aquaporin trafficking in plant cells: an emerging membrane-protein model.

Aquaporins (AQPs) are channel proteins that facilitate the transport of water and small solutes across biological membranes. In plants, AQPs exhibit a high multiplicity of isoforms in relation to a high diversity of sub-cellular localizations, at the plasma membrane (PM) and in various intracellular compartments. Some members also exhibit a dual localization in distinct cell compartments, whereas others show polarized or domain-specific expression at the PM or tonoplast, respectively. A diversity of mechanisms controlling the routing of newly synthesized AQPs towards their destination membranes and involving diacidic motifs, phosphorylation or tetramer assembly is being uncovered. Recent approaches using single particle tracking, fluorescence correlation spectroscopy and fluorescence recovery after photobleaching have, in combination with pharmacological interference, stressed the peculiarities of AQP sub-cellular dynamics in environmentally challenging conditions. A role for clathrin and sterol-rich domains in cell surface dynamics and endocytosis of PM AQPs was uncovered. These recent advances provide deep insights into the cellular mechanisms of water transport regulation in plants. They also point to AQPs as an emerging model for studying the sub-cellular dynamics of plant membrane proteins.