Aquaporins. A Molecular Entry into Plant Water Relations

Abstract

Water relations are obviously crucial to the physiology of terrestrial plants, but 25 years ago most plant biologists viewed this area of research as somewhat of an oddity. Dominated by equations and unusual physical theories about liquids flowing in pipes, water relations seemed to be a field with little seeming need for the burgeoning concepts and approaches offered by molecular and cellular biology. The discovery of aquaporins united these two “cultures” of biophysicists and molecular geneticists and piqued the interest of cell and molecular biologists in plant water relations. Early (and present day) plant biophysicists, exploiting sophisticated physical theories, formulated the general and unifying concept that water flow through plant cells and tissues can be understood as the product of motive force and conductance (3, 16). In plants, under natural conditions, water uptake by the roots and loss from the leaves are driven by ever changing forces, and plants keep a proper water balance by continuously adjusting the water conductance of their tissues. The concept of water potential unifies the description of these forces, whether hydrostatic, osmotic, matrix-derived, or gravitational in nature (3, 16). However, the in situ measurement of these forces is subject to pitfalls, as exemplified by current controversies over the mechanisms of the ascent of xylem sap. Water potential gradients can also be experimentally manipulated and in these studies the conductance of plant cells and tissues has long been viewed as a black box (16). How do cells regulate their conductance in this black box? Aquaporins facilitate the diffusion of water and small neutral solutes across plant cell membranes. The residual permeability of the lipid membrane to these molecules is also indicated. The capacity of some aquaporins to transport solutes was demonstrated using test molecules such as glycerol and urea, and the physiological significance of this property is not yet clearly understood (see text). Plant aquaporins similar to their animal counterparts are thought to form tetramers, each monomer being functionally independent. The putative structure of an aquaporin monomer with six tilted membrane-spanning domains is shown. Studies on the structure of human AQP1 suggest that the two connecting loops that contain the highly conserved Asn-Pro-Ala motifs dip into the membrane and form part of the aqueous pore. In contrast to what was initially assumed, specific membrane lipid compositions can hardly account for some of the high water permeability values (>100 μm s−1) that were initially reported for plant membranes. Thus, the discovery of aquaporins gave strong support to the idea that such high water permeability properties might be required for certain physiological processes. Most importantly, aquaporins immediately provided a solid molecular basis for the fast and reversible regulation of transmembrane water transport. Once a protein is involved, the cell has the ability to regulate its abundance (transcriptional or post-transcriptional regulation) or to modulate its activity. Gene expression of aquaporins is regulated developmentally in a cell-specific manner, via hormones, and by environmental signals as diverse as nematode infection or drought (reviewed in 5 and 15). Reversible phosphorylation controls the activity of some plant aquaporins (9). In spinach leaves, for instance, the phosphorylation level of a plasma membrane aquaporin is dependent on the apoplastic water potential, thus establishing an intriguing link between driving force and conductance (reviewed in 5). Recent work done in animals points to other mechanisms for aquaporin regulation, through pH or free calcium change or controlled protein targeting (reviewed in 19). Exquisite gating control by aquaporins as has been revealed for ion channels can thus be expected. With these and new regulatory mechanisms to be discovered in plants, the water transport properties of membranes now appear as a new and important focus in modern plant cell physiology. Tools, such as the cell pressure probe, which were developed earlier to measure water transport at the cellular level (16), are thus gaining a renewed interest. After the discovery of the first aquaporins, a second surprising finding has been their amazing diversity in plants (21). More than 30 genes encode aquaporin homologs in Arabidopsis. Based on sequence homology, plant aquaporins can be classified into three plant-specific subfamilies (5, 15), two of which appear to correspond to distinct subcellular locations. Tonoplast intrinsic proteins (TIPs) and plasma membrane intrinsic proteins (PIPs) reside mainly in the vacuolar and plasma membrane, respectively. Nodulin 26-like MIPs occur in the peribacteroid membrane of nitrogen-fixing nodules (4), at least in legumes, but their location in other plants remains to be discovered (21). Although all proteins in the PIP clade may not be in the plasma membrane (7) and al