Ion Channels in Plants
Since the first recordings of single potassium channel activities in the plasma membrane of guard cells more than 25 years ago, patch-clamp studies discovered a variety of ion channels in all cell types and plant species under inspection. Their properties differed in a cell type-and cell membranedependent manner. Guard cells, for which the existence of plant potassium channels was initially documented, advanced to a versatile model system for studying plant ion channel structure, function, and
... hysiology. Interestingly, one of the first identified potassium-channel genes encoding the Shaker-type channel KAT1 was shown to be highly expressed in guard cells. KAT1-type channels from Arabidopsis thaliana and its homologs from other species were found to encode the K ϩ -selective inward rectifiers that had already been recorded in early patch-clamp studies with guard cells. Within the genome era, additional Arabidopsis Shaker-type channels appeared. All nine members of the Arabidopsis Shaker family are localized at the plasma membrane, where they either operate as inward rectifiers, outward rectifiers, weak voltage-dependent channels, or electrically silent, but modulatory subunits. The vacuole membrane, in contrast, harbors a set of two-pore K ϩ channels. Just very recently, two plant anion channel families of the SLAC/SLAH and ALMT/QUAC type were identified. SLAC1/SLAH3 and QUAC1 are expressed in guard cells and mediate Slow-and Rapid-type anion currents, respectively, that are involved in volume and turgor regulation. Anion channels in guard cells and other plant cells are key targets within often complex signaling networks. Here, the present knowledge is reviewed for the plant ion channel biology. Special emphasis is drawn to the molecular mechanisms of channel regulation, in the context of model systems and in the light of evolution. http://physrev.physiology.org/ Downloaded from FIGURE 3. Phylogenetic relationship and topology models of Arabidopsis thaliana potassium channels. A: the genome of the model plant Arabidopsis encodes for 15 K ϩ -selective channels, which are subdivided into three structural classes: the class of voltage-dependent Shaker-like potassium channels (K V ), the twin pore channels (TPKs, K 2P ), and K ir -like potassium channel (KCO3). In addition, it encodes for a cationnonselective two-pore channel (TPC1). TM, transmembrane domain; P, pore region. B: despite their structural resemblance, plant Shaker-like K V channels split up into outward (left), inward (middle), and weak (right) rectifiers according to their gating properties. Typical voltage-clamp recordings from heterologously (Xenopus oocytes) expressed K V channels are shown (cf. Refs. 2, 215, 294) .