Ion and water channels are proteins embedded in membranes that facilitate the passive transport (non-energised flow) of solutes and water down their respective energy gradients. Movement of a solute or water through these transport proteins is not coupled to movement of any other solute, or requires ATP. The proteins that facilitate passive transport are diverse; some are speciﬁc for particular ions and allow high transport rates per protein molecule (ion channels), some are speciﬁc for water (water channels or aquaporins) and some are speciﬁc for neutral solutes and may have slower transport rates per protein molecule.
Some plant and animal membranes have much higher permeability to water than can be explained by diffusion rates through a lipid bilayer. Furthermore, the activation energy for diffusion of water across a plant membrane is lower than would be expected across a lipid bilayer, where water has to overcome the high-energy barrier of partitioning into a very hydrophobic oily layer. Some reports put the activation energy for water flow across membranes as low as the value for free diffusion of water. In other words, water enters the membrane about as readily as it diffuses through a solution. This suggests that water is moving across the membrane through a pathway other than the lipid, perhaps some kind of water pore or water channel. Since the discovery of water channel proteins in animal cell membranes, molecular biologists discovered that similar proteins exist in plants.
The approach has been to inject genetic material from plants into Xenopus oocytes (a particular type of frog’s egg). Either cDNA arising from screens of cDNA libraries can be injected into the Xenopus nucleus or poly (A)+-RNA injected into the oocyte cytoplasm where it is translated. The Xenopus oocyte is particularly useful because it is large, enabling observations of cell response to foreign proteins. It is one of several expression systems along with Chara (giant algal cells) and yeast cells. Plant water transport proteins expressed in the oocyte plasma membrane result in physiological changes; for example, the oocyte swells rapidly when the external osmotic pressure in the bathing medium is lowered (Figure 3.71a). Provided that the increase in water permeability is not a consequence of some other change or a side effect of other types of transport, it can be concluded that the protein catalyses transport of water across membranes. In plants, this protein could be located in the tonoplast, plasma membrane or endo-membranes. The ﬁrst plant aquaporin gTIP (tonoplast integral protein) that was discovered occurs in the tonoplast and probably accounts for its high water permeability.
Mercuric chloride inhibits water channels in Xenopus occytes (Figure 3.71b), and also in plant cells (Figure 3.71c). Inhibition is reversed by applying mercaptoethanol to block the action of HgCl2. Under mercury inhibition, the activation energy of water flow increases markedly indicating that water flow is now restricted to diffusional flow across the lipid bilayer, that is, aquaporins are blocked.
Why are there water channels in membranes when the lipid itself is already somewhat permeable to water? There are several rationales for the presence of water channels in plant membranes. One is that specialised transport proteins permit control of water flow. That is, a water channel protein may be turned on and off, for example by phosphorylation, while water permeability of the lipid is essentially constant. In animal cells, speciﬁcally kidney, water channels are controlled by the antidiuretic hormone. It remains to be seen if plant hormones could also influence the function of water channels.
A second rationale for the presence of water channels is to balance water flow and prevent bottlenecks. In roots, water channels are most abundant in the endodermis and inner stele where water flow across membranes is rapid.
In summary, aquaporins play a central role in regulating plant water relations. Water diffusion across cell membranes is facilitated by aquaporins that provide plants with the means to rapidly and reversibly modify water permeability. This is done by changing aquaporin density and activity in the membrane, including posttranslational modifications and protein interaction that act on their trafficking and gating (reviewed by Chaumont and Tyerman 2014). At the whole organ level aquaporins modify water conductance and gradients at key “gatekeeper” cell layers that impact on whole plant water flow and plant water potential. In this way they may act in concert with stomatal regulation. Variations in root and leaf hydraulic conductivity can be accounted for by aquaporins, along with variation in vein anatomy.
This section has shown how the flow of water and ions across membranes are linked. The function of membrane transport in regulating nutrient supply is covered in Chapter 4.