Roles of Higher Plant K+ Channels

Abstract

Living organisms maintain a cellular solute composition very different from that of the externa1 environment. This implicitly requires the transport of solutes across the cell membrane, and ion channels are integral membrane proteins that play indispensable roles in such transport. In the past dozen years, radical advances have aided in our understanding of ion channel function and regulation in higher plants. Nowhere are these advances more striking than with respect to K+ channels, where the synergistic application of electrophysiological, cell biological, physiological, and molecular techniques has demonstrated an array of channel types playing diverse but defined roles in plant physiology.The major function of K+ channels in animal cells is that of membrane voltage control and short-term repolarization of the membrane. Although K+ channels in plants share similar roles in the regulation of the membrane voltage, early research on guard cells led to the model that shows that plant K+ channels in addition provide important pathways for long-term physiological K+ uptake and release. An extensive range of recent studies suggests diverse longterm transport functions of plant K+ channels, including participation in osmotically driven movements, solute loading into the xylem, cation nutrition, and, by virtue of the presence of K+ channels at endomembranes, intracellular solute redistribution and cytosolic volume control. Most plant K+ channels remain activated for long periods of time, which is critica1 for this proposed long-term transport function of K+ channels in plants. Because higher plant K+ channels are proposed to play a role in regulating both the influx and the efflux of K+ from cells, activity of these channels may impinge upon aspects of turgor and water relations of a11 plant cells. In this Update we focus on important principles of plant K+ channel function and on the proposed physiological roles of specific plant K + channel types in the plasma membrane and tonoplast (Fig. 1A) of different plant cells.

Signalling drought in guard cells

“…In guard cells, as in animal cells, turgor is also regulated by the membrane channels and pumps. Studies have shown that the inward (I Kin ) and outward K + channels (I Kout ) responsible for K + fluxes across the plasma membrane are among the most important channels in turgor regulation in guard cells (Assmann 1993;Blatt & Grabov 1997;Maathuis et al 1997). Moreover, as in animal cells, osmotic stress can regulate the activity of these membrane channels and thereby control the cell volume (or turgor) (Liu & Luan 1998).…”

Citrate-Permeable Channels in the Plasma Membrane of Cluster Roots from White Lupin

“…10B). It is noteworthy that the time-dependent K 1 inwardly rectifying channels, often reported in root cells (Maathuis et al, 1997), were not observed in any protoplast tested (n 5 7; Fig. 10A).…”

Membrane depolarization induces K+ efflux from subapical maize root segments

“…To date, most of the information on the biophysical characteristics and physiological roles of ORCs has been obtained by means of electrophysiological studies conducted on highly simplified systems such as protoplasts, membrane fragments or heterologous expression systems (Maathuis et al , 1997;Ward, 1997;Miller & Zhou, 2000). It is clear that the results obtained by these attractive approaches could acquire more physiological significance if combined with, or compared with, results obtained with techniques that induce as little physiological perturbation as possible (Tester, 1997).…”

“…The involvement of potassium in fundamental processes of plant biology is strictly related to the evolution of different K + transport systems in the cell plasma membrane (Maathuis et al , 1997;Chrispeels et al , 1999). Several studies have revealed that the movement of K + across the plasma membrane involves a multitude of transport proteins, including carriers and channels (Chrispeels et al , 1999).…”

“…Two classes of K + channels have been identified and characterized in plant cells: the inward rectifying K + channels (IRCs) and the outward rectifying K + channels (ORCs). These types of transporters are different protein entities, characterized by specific kinetic profiles, pharmacological sensitivity and opposite voltage dependence, being activated by either hyperpolarization ( IRCs) or depolarization (ORCs) of the plasma membrane ( Maathuis et al , 1997). The activity of IRCs and ORCs significantly contributes to several physiological processes, including: K + homeostasis in the cells; regulation of membrane voltage; K + nutrition; loading and unloading of the xylem; osmoregulation of the cells; stomatal and leaf movements (for an extensive review see Maathuis et al , 1997).…”