Energization of Transport Processes in Plants. Roles of the Plasma Membrane H+-ATPase

Abstract

Much has been learned about the energization of nutrient transport since Hoagland in 1944 gave his famous series of lectures on plant nutrition. Already at that time it was speculated that energy for transport of solutes into cells was provided by compounds containing energy-rich phosphate bonds (Hoagland, 1944). We now know that ATP-consuming proton pumps drive nutrient transport at several entry points in the plant body. In this Update, we will focus on those entry points within the plant body where nutrient transport is intense, and we will discuss their energization and regulation by proton pumps. Overview of main transport barriers in the plant body and the energization of cellular nutrient uptake by plasma membrane H+-ATPase. Once inside a plant cell, a given solute can diffuse from cell to cell via cellular bridges called plasmodesmata, forming a cellular continuum, the symplast. Plasmodesmal transport by diffusion is not very effective, and for long-distance transport the nutrient in question might have to leave the symplast to enter a new from the apoplast in a neighboring cell or in another part of the plant. Here again, uptake needs to be energized and occurs through specialized transport proteins. Specialized cells throughout the plant body serve as transport interfaces between symplast and apoplast, and intense transport occurs across the plasma membrane of these cells. Cells with large fluxes of solutes across the plasma membrane share a number of important characteristics in common. With regard to structure, cells specialized for transport are often characterized by (1) exposing a large surface area toward the uptake interface, e.g. the plasma membrane exhibits many protrusions (e.g. epidermal cells) or invaginations (e.g. transfer cells); and (2) having a great number of mitochondria, the role of which is to supply ATP for active transport. Biophysically, transport competent cells are characteristic by exhibiting (1) a large membrane potential difference between the internal and external face of the membrane, typically ranging from −150 mV to −200 mV, negative on the inside; and (2) an acidic exterior, where apoplastic pH is typically between pH 4 and 5. Thus, across the plasma membrane of these cells, we observe a gradient of electric charge and chemical matter, which is termed the electrochemical gradient (Fisher, 2000). Although uphill transport of solutes into cells is an energy-consuming process, it is rarely or never driven directly by metabolic energy, i.e. ATP hydrolysis. Rather, nutrient uptake systems are energized indirectly. Thus, ATP is primarily consumed by pumps, which export H+ in order to generate an electrochemical proton gradient across the plasma membrane. This electrochemical gradient of protons in turn energizes nutrient uptake by channel proteins and carriers. Accordingly, transport-competent cells have at the molecular level (1) a large number of channel proteins and carriers (symporters, if nutrients get cotransported with H+ in the same direction; antiporters, if nutrients and H+ are cotransported in opposite directions; and uniporters, if nutrients are transported as such without being accompanied by H+) and (2) large amounts of the plasma membrane H+-ATPase, a proton pump. The proton pump exports H+ from the cytoplasm into the apoplast at the expense of ATP. It is this pump that is responsible for formation of the trans-plasma membrane electrochemical gradient (Palmgren, 2001; Arango et al., 2002). Transfer cells are considered to be the most specialized cell type for membrane transport, and, therefore, it is not surprising that H+-ATPase is abundant in the various transfer cell types of these cells (Bouche-Pillon et al., 1994; Harrington et al., 1997; Schikora and Schmidt, 2002). The major ion pumps in plants and fungi are plasma membrane H+-ATPases. Similar pumps are not found in animals, in which the equivalent enzyme is the Na+/K+-ATPase, which in turn is absent from plants. However, both types of pumps are evolutionarily related and belong to the superfamily of P-type ATPases (Axelsen and Palmgren, 1998; Kuhlbrandt, 2004). P-type pumps are characterized by forming a phosphorylated reaction-cycle intermediate during catalysis. In this way, plasma membrane H+-ATPases differ from all other proton pumps in the plant cell, including the vacuolar membrane H+-ATPase, another major proton pump which energizes the vacuolar membrane. The vacuolar membrane H+-ATPase has more than 10 different subunits, whereas, in contrast, the functional unit of plasma membrane H+-ATPase is a monomer, although it might be organized in the membrane as a dimer or in oligomers. The yeast Saccharomyces cerevisiae is equipped with two plasma membrane H+-ATPase genes, of which one, PMA1, is essential for growth (Serrano et al., 1986). In plants, there are many more isoforms. The genomes of the dicot Arabidopsis and the monocot rice (Oryza sativa) have been sequenced, resulting in identificat