General features of K+ transporters in plants
A large number of genes encode proteins involved in K+ transportin plants. In arabidopsis, these transport mechanisms fall intoseveral distinct categories (Mäser et al., 2001, 2002;Very and Sentenac, 2002): (a) two families of K+ channels (Shaker-typeand KCO channels; 15 genes in total); (b) Trk/HKT transporters[Na+/K+ symporter (Schachtman, 2000); one gene]; (c) KUP/HAK/KTtransporters [H+/K+ symporter (Kim et al., 1998); 13 genes];(d) K+/H+ antiporter homologs (six genes); (e) cyclic-nucleotide-gatedchannels (CNGC; 20 genes in arabidopsis; Very and Sentenac, 2002);and (e) glutamate receptors (GLRs; 20 genes; Very and Sentenac, 2002).The Shaker-type channels are further subdividedinto SKOR and GORK channels (both depolarization-activated),KAT channels and AKT channels. AKT channels contain an ankyrin-bindingmotif, which is lacking in KAT type channels (Mäser etal., 2001). An important feature of plant Shaker-like K+ channelsis that they can form heterotetrameric structures (Pilot etal., 2003), allowing plants to tune the K+ transport activityin various cells, independently in each organ/tissue, in relationto environmental conditions. A brief summary of the functionalexpression of plasma membrane K+ channels in various leaf tissuesin Arabidopsis thaliana is given in Table 1, and some detailsof the functions and intracellular location of K+ transportersin leaves of various plant species are shown in Table 2. Thediversity of K+ transport mechanisms at the plasma membraneof a ‘generalized’ leaf cell is summarized in Fig.1.
Stomatal guard cells
Two major types of K+ channels are present at the plasma membraneof guard cells: voltage-dependent K+-selective inward (KIR)and outward (KOR) rectifying channels (Pilot et al., 2001; Schroederet al., 2001; Szyroki et al., 2001; Zimmermann et al., 2001).KIR channels are activated by membrane hyperpolarization andmediate stomatal opening, whereas KOR channels are opened byvoltages more positive than Ek and mediate stomatal closure.
Recent transcription–PCR experiments with isolated guardcell protoplasts showed that in addition to KAT1, the K+ channelsAKT1, AKT2/3, AtKC1 and KAT2 (all KIRs) were expressed (Szyrokiet al., 2001), suggesting that KAT1 inward-rectifying K+ channelsmay not play as dominant a role in K+ uptake in guard cellsas previously believed (Assmann and Wang, 2001). It was alsoshown that KAT1 and KAT2 can form heteromultimeric channels(Pilot et al., 2001; Zimmermann et al., 2001), leading to moreflexibility when adapting to altered developmental and/or environmentalconditions. Several other channels of unknown voltage-dependence(AtKC1, AKT5 and AKT6) were also shown to co-localize in arabidopsisguard cells (Dietrich et al., 2001). It is thought that sucha multiple ensemble of K+ channels provides greater versatilityand much more efficient regulation of K+ homeostasis in guardcells compared with only one type of KIR channel. The only certaincandidate in the arabidopsis genome to mediate stomatal closureis GORK, a voltage-gated outwardly rectifying K+ channel ofthe guard cell membrane (Hosy et al., 2003).
In addition to specific K+-selective channels, guard cells alsopossess a wide range of non-selective cation channels (NSCC),either depolarization- or hyperpolarization-activated (Demidchiket al., 2002). These channels are likely to be involved in releaseof solutes during turgor adjustment and, to some extent, functionallycomplement GORK channels. Finally, there is strong evidencethat guard cells possess mechanosensitive (or stretch-activated,SAS) channels at the plasma membrane (Cosgrove and Hedrich, 1991).These channels are K+ permeable and change their openprobabilities as a result of volume or turgor changes.
As well as their voltage-dependence, K+ channels in guard cellsare regulated by several other factors, the most obvious ofwhich is pH. Both apoplastic (Pilot et al., 2001; Roelfsema and Hedrich, 2002)and cytosolic (Dietrich et al., 2001) acidificationlead to the activation of inward K+ currents in guard cells.The effect is voltage-dependent (Roelfsema and Hedrich, 2002).Most other second messengers such as cytosolic Ca2+, IP3, GTP,G-proteins, polyamines, reactive oxygen species (ROS) and phosphorylationevents also exert direct control over K+ channel activity (Assmann and Shimazaki, 1999;Blatt, 2000; Schroeder et al., 2001; Kohleret al., 2003). It has also been shown that guard cells may utilizevoltage-dependent K+ channels as targets of the osmosensingpathway by regulating channel opening probability by the osmogradientacross the plasma membrane of guard cells (Liu and Luan, 1998).
Molecular studies suggested that both AKT1 and AKT2/3 genesare expressed in arabidopsis leaf mesophyll (Dennison et al.,2001; Cherel et al., 2002). A specific feature of AKT2/3 channelsis their weak dependence on the membrane potential, sensitivityto ATP and an inverted pH regulation (Marten et al., 1999).Being only weakly controlled by the membrane potential, AKT2/3channels are able to conduct both inward and outward currents.
Electrophysiologically, K+-permeable channels at the plasmamembrane of mesophyll cells were characterized as KIRs (Kourie and Goldsmith, 1992;Karley et al., 2000a), and KORs (Spaldinget al., 1992; Blom-Zandstra et al., 1997; Pineros and Kochian, 2003).These KOR channels are regulated by Ca2+ and G-proteins(Krol and Trebacz, 2000) and may play a role in stabilizingcell membrane potential (Pineros and Kochian, 2003). Also, Ca2+-sensitive,depolarization-activated NSCCs were found at the plasma membranein arabidopsis mesophyll cells (Spalding et al., 1992).
In addition to KIR channels there is a need for active K+ transportersin mesophyll cells, since environmental fluctuations, such aslight/dark transitions, may leave Em positive to Ek (Shabala and Newman, 1999),making the function of KIR channels impossible.Both HAK/KT/KUP and HKT type transporters are present at theplasma membrane of leaf mesophyll cells (Table 2; Leigh, 2001;Golldack et al., 2002; Su et al., 2002).
Only a few electrophysiological studies have specifically targetedK+ transporters from epidermal cells other than stomata. Twotime-dependent Ca2+-regulated K+-selective channels (resemblingguard cell KIR and KOR, respectively) were found in subsidiarycells of maize (Majore et al., 2002). Time-dependent inwardK+ currents were also reported for barley epidermis (Karleyet al., 2000a) and there is evidence that NSCCs may also bepresent in leaf epidermal cells (Elzenga and Van Volkenburgh, 1994;Majore et al., 2002). At the molecular level, expressionof AKT2 genes (Cherel et al., 2002) and the HAK/KT/KUP K+ transporters(Su et al., 2002) have been attributed to epidermal cells.
Phloem loading with assimilates is accompanied by a significantincrease in symplastic K+ concentration, required to maintainelectrical neutrality during vectorial H+ transfer. The mostabundant K+ channels in the phloem tissue are AKT3 (Marten etal., 1999; Cherel et al., 2002) and their homologues (Golldacket al., 2003). The unique characteristics of this channel (itsweak voltage dependence, inhibition by physiological concentrationsof external Ca2+ and by extracellular acidification, and theability to be open in the entire physiological voltage range;Marten et al., 1999), allow AKT3 to mediate both K+ influx andefflux, determining the diverse roles of AKT3 in the phloem.Another major type of K+ channel detected in minor veins isKAT2 (Pilot et al., 2001), involved in K+ loading into the phloemsap.
There is both electrophysiological and molecular evidence for
the presence of high affinity K+
transporters, in addition to
channels, in the leaf vascular system. McHAKs transcripts showed
signals in the leaf vascular bundles (Su et al., 2002
) and the
HKT1 transporter was localized in cell layers bordering the
vascular tissue in leaves (Schachtman, 2000