The role of aquaporins in root water uptake

Abstract

The capacity of roots to take up water is determined in part by the resistance of living tissues to radial water flow. Both the apoplastic and cell-to-cell paths mediate water transport in these tissues but the contribution of cell membranes to the latter path has long been difficult to estimate. Aquaporins are water channel proteins that are expressed in various membrane compartments of plant cells, including the plasma and vacuolar membranes. Plant aquaporins are encoded by a large multigene family, with 35 members in Arabidopsis thaliana, and many of these aquaporins show a cell-specific expression pattern in the root. Mercury acts as an efficient blocker of most aquaporins and has been used to demonstrate the significant contribution of water channels to overall root water transport. Aquaporin-rich membranes may be needed to facilitate intense water flow across root tissues and may represent critical points where an efficient and spatially restricted control of water uptake can be exerted. Roots, in particular, show a remarkable capacity to alter their water permeability over the short term (i.e. in a few hours to less than 2-3 d) in response to many stimuli, such as day/night cycles, nutrient deficiency or stress. Recent data suggest that these rapid changes can be mostly accounted for by changes in cell membrane permeability and are mediated by aquaporins. Although the processes that allow perception of environmental changes by root cells and subsequent aquaporin regulation are nearly unknown, the study of root aquaporins provides an interesting model to understand the regulation of water transport in plants and sheds light on the basic mechanisms of water uptake by roots.

Fig. 1. Tissue organization in roots of soybean (Glycine max ‘Labrador’) (A) and maize (Zea mays) (B) plantlets. Soybean and maize plants were cultured in hydroponic conditions, for 18 and 10 d, respectively, and a cross‐section of the principal root was taken 55 and 25 mm from the apex, respectively. The photographs illustrate the typical organization of roots in concentric cell layers, with differences between the two plant (dicot and monocot) species. Note the large size of xylem vessels in maize roots. ep, Epidermis; cp, cortical parenchyma; ed, endodermis; xy, xylem; ph, phloem.

Fig. 2. Schematic representation of the three paths involved in water transport across living tissues.

Fig. 3. Effects of mercury on pressure‐induced water transport in excised tomato roots. Roots from detopped tomato plants were inserted in a chamber under constant pressure and sap flux was measured continuously. HgCl₂ (0·5mm) and β‐mercaptoethanol (ME; 60 mm) were injected in the chamber at the times indicated. The sap fluxes recorded in an HgCl₂‐treated root system (closed circles) and a corresponding untreated control root system (open circles) are shown. The figure clearly shows that in this experiment HgCl₂ reduced by >70 % the intensity of sap flux i.e. root L_p, and that this effect was partially reversed upon ME application. Reproduced with permission from Maggio and Joly (1995).

Fig. 4. Diurnal fluctuations of overall L_p and aquaporin expression in the roots of Lotus japonicus plants. A, Roots were excised from plants grown in sand and their L_p was measured in a pressure chamber at the times indicated, over a 15‐min time period. B, RNA was extracted from plants grown in aeroponic conditions at the times indicated and Northern blot analyses were performed using a full‐length PIP1;2 cDNA probe from Arabidopsis thaliana. This probe probably cross‐hybridized with several PIP1 homologues in L. japonicus. Adapted with permission of the authors from Henzler et al. (1999).