Plant Aquaporin Trafficking

Abstract

Aquaporins transport water and small neutral molecules across different membranes in plant cells and thus play important roles in cellular and whole plant physiology. The high diversity of intracellular localization of aquaporin isoforms is dependent on specific trafficking machineries. ER-to-Golgi trafficking of the plasma membrane intrinsic protein (PIP) isoforms has been shown to be dependent on DxE motifs in N-terminal cytosolic region, LxxxA motif in transmembrane domain 3, phosphorylation in C-terminal cytosolic region, and heteromerization. Stress-induced downregulation of the PIPs in the early secretory pathway was uncovered. Subsets of PIPs and Nodulin 26-like intrinsic proteins (NIPs) showed polar localization in the plasma membrane (PM) in certain cell types for directional transport of water and small neutral molecules such as boric acid and silicic acid. Latest techniques to study the mobility of PIPs revealed immobile nature in the plane of the PM and constitutive cycling between the PM and the endosomes. The roles of clathrin- and microdomain-dependent endocytosis for PIPs were uncovered. When challenged by stress conditions, some PIPs and TIPs showed quick relocalization probably to adjust water status. Vacuolar trafficking of different TIPs was shown to follow multiple routes dependent or independent of Golgi apparatus. These findings greatly advanced our understanding of the trafficking machineries of plant aquaporins, as significant models of plant membrane proteins.