Proteins for Transport of Water and Mineral Nutrients across the Membranes of Plant Cells

Abstract

The uptake and transport of water and mineral ions are among the oldest subjects in plant physiology, and numerous studies have described these processes at the whole-plant level and at the organ level (e.g., with excised roots). Subsequent work was based on isolated membrane vesicles and electrophysiology to characterize transport processes at the level of the membrane. The recent cloning of the genes for a large number of transport proteins and the availability of knockout mutants make it possible to dissect transport processes in greater detail and to begin to understand the interactions between ion uptake processes that had often been observed at the organ level. This article summarizes recent progress in characterizing the genes and the proteins involved in transporting water and major mineral nutrients. The plasma membrane of the plant cell is a selectively permeable barrier that ensures the entry of essential ions and metabolites into the cell. Together with the vacuolar membrane (tonoplast), it permits the cytoplasm to maintain intracellular homeostasis. These membranes consist primarily of phospholipid bilayers with transmembrane proteins that permit the traversing of water, ions, and metabolites and the maintenance of a cytosolic pH that is one to three units higher than that of the cell exterior or the vacuole. Pure phospholipid bilayers are permeable to gases, such as O2 and CO2, but they are barely permeable to water and nearly impermeable to inorganic ions and other hydrophilic solutes, such as sucrose and amino acids. Proteins are required to transport protons, inorganic ions, and organic solutes across the plasma membrane and the tonoplast at rates sufficient to meet the needs of the cell. Membranes contain different types of transport proteins: ATPases or ATP-powered pumps, channel proteins, and cotransporters (for other reviews on this topic, see Lalonde et al., 1999; Sze et al., 1999, in this issue). Some of these proteins are well characterized, and their subcellular location is known. The cDNAs for others have been cloned, but functional, biochemical, and cytological data are lacking. ATPases utilize the energy released upon hydrolysis of ATP to move ions across the plasma membrane against chemical and electrical gradients. In plant cells, H +-ATPases pump protons across the plasma membrane or tonoplast to acidify the extracellular matrix or the vacuole, respectively (Sze et al., 1999). Channel proteins facilitate the diffusion of water and ions down energetically favorable gradients. These proteins form channels through which ions or water molecules pass in single file at very rapid rates—up to 106 to 107 per sec per channel. The activities of channels can be regulated: they can be opened and closed. In plant cells, both water and ion channels have been studied in detail. Cotransporters, the third class of membrane-transport proteins, can move solutes either up or down gradients at rates of 102 to 104 molecules per second. However, depending on the conditions under which they are assayed, transporters sometimes behave like channels, and vice versa, so that the distinction between them is no longer clear. Classical models assumed that ion channels and cotransporters are clearly distinct proteins. However, recent single-channel studies on animal transporters have demonstrated that cotransporters can also function as ion channels (Fairman et al., 1995; Cammack and Schwartz, 1996). Therefore, animal researchers no longer distinguish strictly between ion channels and cotransporters, and models explaining channel modes of transporters have been derived (Larsson et al., 1996; Su et al., 1996). In plant systems, analysis of the wheat HKT1 transporter has provided evidence for two modes of transport by the same protein (Rubio et al., 1995; Gassmann et al., 1996). Among the cotransporters, we distinguish uniporters, antiporters, and symporters. Uniporters, like channels, move substances down a concentration gradient, albeit much more slowly. Antiporters, also called exchangers, and symporters can move a given substance against its own concentration gradient, the energy requirement for which comes not from ATP but rather is satisfied by an electric potential and/or chemical gradient of a secondary substance. The movement of a solute against its concentration gradient is thus coupled to the movement of the secondary substance—in plants this is often a proton—down its concentration gradient. Thus, the high proton concentration in the apoplast powers the inward movement of certain ions, such as nitrate, by a symporter. Proton gradients can similarly power the movement of ions into the vacuole by the proton–sodium antiporter or the proton–calcium antiporter. The ionic compositions of the solutions on either side of both plasma membrane and tonoplast are very different. Not only can the proton concentration be up to 1000 times greater in the apoplast and vacuole than in the cytosol, but gradients of c