K+ channel activity in plants: genes, regulations and functions

Potassium (K(+)) is the most abundant cation in the cytosol, and plant growth requires that large amounts of K(+) are transported from the soil to the growing organs. K(+) uptake and fluxes within the plant are mediated by several families of transporters and channels. Here, we describe the different families of K(+)-selective channels that have been identified in plants, the so-called Shaker, TPK and Kir-like channels, and what is known so far on their regulations and physiological functions in the plant.

Increased functional diversity of plant K+ channels by preferential heteromerization of the shaker-like subunits AKT2 and KAT2

Assembly of plant Shaker subunits as heterotetramers, increasing channel functional diversity, has been reported. Here we focus on a new interaction, between AKT2 and KAT2 subunits. The assembly as AKT2/KAT2 heterotetramers is demonstrated by (i) a strong signal in two-hybrid tests with intracytoplasmic C-terminal regions, (ii) the effect of KAT2 on AKT2 subunit targeting in tobacco cells, (iii) the complete inhibition of AKT2 currents by co-expression with a dominant-negative KAT2 subunit in Xenopus oocytes, and reciprocally, and (iv) the appearance, upon co-expression of wild-type AKT2 and KAT2 subunits, of new channel functional properties that cannot be explained by the co-existence of two kinds of homotetrameric channels. In particular, the instantaneous current, characteristic of AKT2, displayed new functional features when compared with those of AKT2 homotetramers: activation by external acidification (instead of inhibition) and weak inhibition by calcium. Single channel current measurements in oocytes co-expressing AKT2 and KAT2 revealed a strong preference for incorporation of subunits into heteromultimers and a diversity of individual channels. In planta, these new channels, which may undergo specific regulations, are likely to be formed in guard cells and in the phloem, where they could participate in the control of membrane potential and potassium fluxes.

External K+ modulates the activity of the Arabidopsis potassium channel SKOR via an unusual mechanism

Plant outward-rectifying K+ channels mediate K+ efflux from guard cells during stomatal closure and from root cells into the xylem for root-shoot allocation of potassium (K). Intriguingly, the gating of these channels depends on the extracellular K+ concentration, although the ions carrying the current are derived from inside the cell. This K+ dependence confers a sensitivity to the extracellular K+ concentration ([K+]) that ensures that the channels mediate K+ efflux only, regardless of the [K+] prevailing outside. We investigated the mechanism of K+-dependent gating of the K+ channel SKOR of Arabidopsis by site-directed mutagenesis. Mutations affecting the intrinsic K+ dependence of gating were found to cluster in the pore and within the sixth transmembrane helix (S6), identifying an 'S6 gating domain' deep within the membrane. Mapping the SKOR sequence to the crystal structure of the voltage-dependent K+ channel KvAP from Aeropyrum pernix suggested interaction between the S6 gating domain and the base of the pore helix, a prediction supported by mutations at this site. These results offer a unique insight into the molecular basis for a physiologically important K+-sensory process in plants.

A unique voltage sensor sensitizes the potassium channel AKT2 to phosphoregulation

Among all voltage-gated K⁺ channels from the model plant Arabidopsis thaliana, the weakly rectifying K⁺ channel (K_weak channel) AKT2 displays unique gating properties. AKT2 is exceptionally regulated by phosphorylation: when nonphosphorylated AKT2 behaves as an inward-rectifying potassium channel; phosphorylation of AKT2 abolishes inward rectification by shifting its activation threshold far positive (>200 mV) so that it closes only at voltages positive of +100 mV. In its phosphorylated form, AKT2 is thus locked in the open state in the entire physiological voltage range. To understand the molecular grounds of this unique gating behavior, we generated chimeras between AKT2 and the conventional inward-rectifying channel KAT1. The transfer of the pore from KAT1 to AKT2 altered the permeation properties of the channel. However, the gating properties were unaffected, suggesting that the pore region of AKT2 is not responsible for the unique K_weak gating. Instead, a lysine residue in S4, highly conserved among all K_weak channels but absent from other plant K⁺ channels, was pinpointed in a site-directed mutagenesis approach. Substitution of the lysine by serine or aspartate abolished the “open-lock” characteristic and converted AKT2 into an inward-rectifying channel. Interestingly, phosphoregulation of the mutant AKT2-K197S appeared to be similar to that of the K_in channel KAT1: as suggested by mimicking the phosphorylated and dephosphorylated states, phosphorylation induced a shift of the activation threshold of AKT2-K197S by about +50 mV. We conclude that the lysine residue K197 sensitizes AKT2 to phosphoregulation. The phosphorylation-induced reduction of the activation energy in AKT2 is ∼6 kT larger than in the K197S mutant. It is discussed that this hypersensitive response of AKT2 to phosphorylation equips a cell with the versatility to establish a potassium gradient and to make efficient use of it.

A procedure for localisation and electrophysiological characterisation of ion channels heterologously expressed in a plant context

BACKGROUND

In silico analyses based on sequence similarities with animal channels have identified a large number of plant genes likely to encode ion channels. The attempts made to characterise such putative plant channels at the functional level have most often relied on electrophysiological analyses in classical expression systems, such as Xenopus oocytes or mammalian cells. In a number of cases, these expression systems have failed so far to provide functional data and one can speculate that using a plant expression system instead of an animal one might provide a more efficient way towards functional characterisation of plant channels, and a more realistic context to investigate regulation of plant channels.

RESULTS

With the aim of developing a plant expression system readily amenable to electrophysiological analyses, we optimised experimental conditions for preparation and transformation of tobacco mesophyll protoplasts and engineered expression plasmids, that were designed to allow subcellular localisation and functional characterisation of ion channels eventually in presence of their putative (possibly over-expressed) regulatory partners. Two inward K+ channels from the Shaker family were functionally expressed in this system: not only the compliant KAT1 but also the recalcitrant AKT1 channel, which remains electrically silent when expressed in Xenopus oocytes or in mammalian cells.

CONCLUSION

The level of endogenous currents in control protoplasts seems compatible with the use of the described experimental procedures for the characterisation of plant ion channels, by studying for instance their subcellular localisation, functional properties, structure-function relationships, interacting partners and regulation, very likely in a more realistic context than the classically used animal systems.

Inward rectification of the AKT2 channel abolished by voltage-dependent phosphorylation

The Arabidopsis K(+) channel AKT2 possesses the remarkable property that its voltage threshold for activation can be either within the physiological range (gating mode 1), or shifted towards considerably more positive voltages (gating mode 2). Gating mode 1 AKT2 channels behave as delayed K(+)-selective inward rectifiers; while gating mode 2 AKT2 channels are K(+)-selective 'open leaks' in the physiological range of membrane potential. In the present study we have investigated modulation of AKT2 current by effectors of phosphatases/kinases in COS cells and Xenopus oocytes. These experiments show that (i) dephosphorylation can result in AKT2 channel silencing; and (ii) phosphorylation by protein kinase A (PKA) favors both recruitment of silenced AKT2 channels and transition from gating mode 1 to gating mode 2. Interestingly, phosphorylation of AKT2 by PKA in COS cells and Xenopus oocytes is favored by hyperpolarization. Two PKA phosphorylation sites (S210 and S329) were pinpointed in the region of the pore inner mouth. The role of these phosphorylation sites in the switch between the two gating modes was assessed by electrophysiological characterization of mutant channels. The molecular aspects of AKT2 regulation by phosphorylation, and the possible physiological meaning of such regulation in the plant context, are discussed.

Plant K(in) and K(out) channels: approaching the trait of opposite rectification by analyzing more than 250 KAT1-SKOR chimeras

Members of the Shaker-like plant K(+) channel family share a common structure, but are highly diverse in their function: they behave as either hyperpolarization-activated inward-rectifying (K(in)) channels, or leak-like (K(weak)) channels, or depolarization-activated outward-rectifying (K(out)) channels. Here we created 256 chimeras between the K(in) channel KAT1 and the K(out) channel SKOR. The chimeras were screened in a potassium-uptake deficient yeast strain to identify those, which mediate potassium inward currents, i.e., which are functionally equivalent to KAT1. This strategy allowed us to identify three chimeras which differ from KAT1 in three parts of the polypeptide: the cytosolic N-terminus, the cytosolic C-terminus, and the putative voltage-sensor S4. Additionally, mutations in the K(out) channel SKOR were generated in order to localize molecular entities underlying its depolarization activation. The triple mutant SKOR-D312N-M313L-I314G, carrying amino-acid changes in the S6 segment, was identified as a channel which did not display any rectification in the tested voltage-range.

Regulation by external K+ in a maize inward shaker channel targets transport activity in the high concentration range

An inward Shaker K(+) channel identified in Zea mays (maize), ZmK2.1, displays strong regulation by external K(+) when expressed in Xenopus laevis (African clawed frog) oocytes or COS cells. ZmK2.1 is specifically activated by K(+) with an apparent K(m) close to 15 mM independent of the membrane hyperpolarization level. In the absence of K(+), ZmK2.1 appears to enter a nonconducting state. Thus, whatever the membrane potential, this maize channel cannot mediate K(+) influx in the submillimolar concentration range, unlike its relatives in Arabidopsis thaliana. Its expression is restricted to the shoots, the strongest signal (RT-PCR) being associated with vascular/bundle sheath strands. Based on sequence and gene structure, the closest relatives of ZmK2.1 in Arabidopsis are K(+) Arabidopsis Transporter 1 (KAT1) (expressed in guard cells) and KAT2 (expressed in guard cells and leaf phloem). Patch-clamp analyses of guard cell protoplasts reveal a higher functional diversity of K(+) channels in maize than in Arabidopsis. Channels endowed with regulation by external K(+) similar to that of ZmK2.1 (channel activity regulated by external K(+) with a K(m) close to 15 mM, regulation independent of external Ca(2+)) constitute a major component of the maize guard cell inward K(+) channel population. The presence of such channels in maize might reflect physiological traits of C4 and/or monocotyledonous plants.

Assembly of plant Shaker-like K(out) channels requires two distinct sites of the channel alpha-subunit

SKOR and GORK are outward-rectifying plant potassium channels from Arabidopsis thaliana. They belong to the Shaker superfamily of voltage-dependent K(+) channels. Channels of this class are composed of four alpha-subunits and subunit assembly is a prerequisite for channel function. In this study the assembly mechanism of SKOR was investigated using the yeast two-hybrid system and functional assays in Xenopus oocytes and in yeast. We demonstrate that SKOR and GORK physically interact and assemble into heteromeric K(out) channels. Deletion mutants and chimeric proteins generated from SKOR and the K(in) channel alpha-subunit KAT1 revealed that the cytoplasmic C-terminus of SKOR determines channel assembly. Two domains that are crucial for channel assembly were identified: i), a proximal interacting region comprising a putative cyclic nucleotide-binding domain together with 33 amino acids just upstream of this domain, and ii), a distal interacting region showing some resemblance to the K(T) domain of KAT1. Both regions contributed differently to channel assembly. Whereas the proximal interacting region was found to be active on its own, the distal interacting region required an intact proximal interacting region to be active. K(out) alpha-subunits did not assemble with K(in) alpha-subunits because of the absence of interaction between their assembly sites.

Hebeloma cylindrosporum- a model species to study ectomycorrhizal symbiosis from gene to ecosystem

The basidiomycete Hebeloma cylindrosporum has been extensively studied with respect to mycorrhiza differentiation and metabolism and also to population dynamics. Its life cycle can be reproduced in vitro and it can be genetically transformed. Combined biochemical, cytological, genetical and molecular approaches led to the characterisation of mutant strains affected in mycorrhiza formation. These studies demonstrated the role of fungal auxin as a signal molecule in mycorrhiza formation and should allow the characterisation of essential fungal genes necessary to achieve a compatible symbiotic interaction. Random sequencing of cDNAs has identified numerous key functional genes which allowed dissection of essential nitrogen assimilation pathways. H. cylindrosporum also proved to be a remarkable model species to uncover the dynamics of natural populations of ectomycorrhizal fungi and the way in which they respond and adapt to anthropogenic disturbance of the forest ecosystem. Although studies on mycorrhiza differentiation and functioning and those on the population dynamics of H. cylindrosporum have been carried out independently, they are likely to converge in a renewed molecular ecophysiology which will envisage how ectomycorrhizal symbiosis functions under varying field conditions. Contents Summary 481 I. Introduction 482 II. Taxonomy, distribution, autecology, and host range of H. cylindrosporum 482 III. The Hebeloma cylindrosporum toolbox 483 IV. Mycorrhiza differentiation 486 V. Nutritional interactions 488 VI. Genetic diversity and dynamics of H. cylindrosporum populations in P. pinaster forest ecosystems 491 VII. Future directions 494 Acknowledgements 494 References 494.

The Arabidopsis root transcriptome by serial analysis of gene expression. Gene identification using the genome sequence

Large-scale identification of genes expressed in roots of the model plant Arabidopsis was performed by serial analysis of gene expression (SAGE), on a total of 144,083 sequenced tags, representing at least 15,964 different mRNAs. For tag to gene assignment, we developed a computational approach based on 26,620 genes annotated from the complete sequence of the genome. The procedure selected warrants the identification of the genes corresponding to the majority of the tags found experimentally, with a high level of reliability, and provides a reference database for SAGE studies in Arabidopsis. This new resource allowed us to characterize the expression of more than 3,000 genes, for which there is no expressed sequence tag (EST) or cDNA in the databases. Moreover, 85% of the tags were specific for one gene. To illustrate this advantage of SAGE for functional genomics, we show that our data allow an unambiguous analysis of most of the individual genes belonging to 12 different ion transporter multigene families. These results indicate that, compared with EST-based tag to gene assignment, the use of the annotated genome sequence greatly improves gene identification in SAGE studies. However, more than 6,000 different tags remained with no gene match, suggesting that a significant proportion of transcripts present in the roots originate from yet unknown or wrongly annotated genes. The root transcriptome characterized in this study markedly differs from those obtained in other organs, and provides a unique resource for investigating the functional specificities of the root system. As an example of the use of SAGE for transcript profiling in Arabidopsis, we report here the identification of 270 genes differentially expressed between roots of plants grown either with NO3- or NH4NO3 as N source.

Functional analysis of AtHKT1 in Arabidopsis shows that Na(+) recirculation by the phloem is crucial for salt tolerance

Two allelic recessive mutations of Arabidopsis, sas2-1 and sas2-2, were identified as inducing sodium overaccumulation in shoots. The sas2 locus was found (by positional cloning) to correspond to the AtHKT1 gene. Expression in Xenopus oocytes revealed that the sas2-1 mutation did not affect the ionic selectivity of the transporter but strongly reduced the macro scopic (whole oocyte current) transport activity. In Arabidopsis, expression of AtHKT1 was shown to be restricted to the phloem tissues in all organs. The sas2-1 mutation strongly decreased Na(+) concentration in the phloem sap. It led to Na(+) overaccumulation in every aerial organ (except the stem), but to Na(+) underaccumulation in roots. The sas2 plants displayed increased sensitivity to NaCl, with reduced growth and even death under moderate salinity. The whole set of data indicates that AtHKT1 is involved in Na(+) recirculation from shoots to roots, probably by mediating Na(+) loading into the phloem sap in shoots and unloading in roots, this recirculation removing large amounts of Na(+) from the shoot and playing a crucial role in plant tolerance to salt.

The Arabidopsis outward K+ channel GORK is involved in regulation of stomatal movements and plant transpiration

Microscopic pores present in the epidermis of plant aerial organs, called stomata, allow gas exchanges between the inner photosynthetic tissue and the atmosphere. Regulation of stomatal aperture, preventing excess transpirational vapor loss, relies on turgor changes of two highly differentiated epidermal cells surrounding the pore, the guard cells. Increased guard cell turgor due to increased solute accumulation results in stomatal opening, whereas decreased guard cell turgor due to decreased solute accumulation results in stomatal closing. Here we provide direct evidence, based on reverse genetics approaches, that the Arabidopsis GORK Shaker gene encodes the major voltage-gated outwardly rectifying K(+) channel of the guard cell membrane. Expression of GORK dominant negative mutant polypeptides in transgenic Arabidopsis was found to strongly reduce outwardly rectifying K(+) channel activity in the guard cell membrane, and disruption of the GORK gene (T-DNA insertion knockout mutant) fully suppressed this activity. Bioassays on epidermal peels revealed that disruption of GORK activity resulted in impaired stomatal closure in response to darkness or the stress hormone abscisic acid [corrected]. Transpiration measurements on excised rosettes and intact plants (grown in hydroponic conditions or submitted to water stress) revealed that absence of GORK activity resulted in increased water consumption. The whole set of data indicates that GORK is likely to play a crucial role in adaptation to drought in fluctuating environments.

Five-group distribution of the Shaker-like K+ channel family in higher plants

In higher plants, potassium channels of the Shaker family have been shown to play crucial roles in the uptake of K(+) from the soil solution and subsequent transport of this ion at the cell, tissue, and organ levels. In the model plant Arabidopsis thaliana, this family is composed of nine members, which are the best characterized among plant channels at the protein, gene, and functional property levels. Plant Shaker channels share a common structure: a hydrophobic core composed of six transmembrane segments, a long cytoplasmic C-terminal region harboring a putative cyclic nucleotide binding domain, and a K(HA) domain. Many channels also contain an ankyrin domain between the putative cyclic nucleotide binding domain and the K(HA) domain. The analysis of 44 Shaker channels from plants revealed a five-group classification. The members of each group share high sequence and structure similarities. This grouping also correlates with the diversification of the functional properties of the proteins, as members of an individual group have roughly the same electrophysiological characteristics. Analysis of the intron positions showed that the gene structures are also quite well conserved within the five groups. A correlation linking the evolution of the sequences and the positioning of the introns was established. Finally, a moss sequence provided additional clues about the hypothetical structure of an ancestor of the present channels and suggested that the diversification of plant Shaker channels happened before the separation of monocots and dicots and after the separation of bryophytes and tracheophytes.

Regulated expression of Arabidopsis shaker K+ channel genes involved in K+ uptake and distribution in the plant

Potassium is the most abundant cation in the cytosol, where it plays a role in basal functions. Rapid uptake and distribution of K+ is therefore required for plant growth. Three members of the so-called Shaker K+ channel gene family (nine genes identified in Arabidopsis) play a role in these transports: AKT1, SKOR and AKT2. The encoded proteins are involved in K+ uptake by the root, K+ secretion into the xylem sap and K+ transport in the phloem tissues, respectively. Using the GUS reporter strategy, we have found that another Shaker channel gene, AtKC1, is expressed in epidermal and cortical cells in roots (supporting the hypothesis of a role in K+ uptake from the soil, together with AKT1), and in trichomes and hydathodes in leaves. These four genes were selected for expression studies, and two-hybrid experiments were performed for channels displaying overlapping expression patterns. The data support the hypothesis that physical interactions could occur in planta between AtKC1 and AKT1, and between AKT1 and AKT2. Potassium deficiency, salt stress and hormonal treatments (ABA, BA, 2,4-D) were found to differentially affect channel mRNA levels, each channel displaying its own regulation pattern. The most prominent effects were (1) a strong induction of AtKC1 transcript accumulation in leaves (hydathodes, trichomes and leaf epidermis) in response to NaCl treatment, suggesting a key role of the protein in adaptation to saline conditions, and (2) a strong decrease in SKOR transcript levels by hormones, supporting the hypothesis that K+ secretion into the xylem sap is under tight hormonal control.

Molecular mechanisms and regulation of K+ transport in higher plants

Potassium (K+) plays a number of important roles in plant growth and development. Over the past few years, molecular approaches associated with electrophysiological analyses have greatly advanced our understanding of K+ transport in plants. A large number of genes encoding K+ transport systems have been identified, revealing a high level of complexity. Characterization of some transport systems is providing exciting information at the molecular level on functions such as root K+ uptake and secretion into the xylem sap, K+ transport in guard cells, or K+ influx into growing pollen tubes. In this review, we take stock of this recent molecular information. The main families of plant K+ transport systems (Shaker and KCO channels, KUP/HAK/KT and HKT transporters) are described, along with molecular data on how these systems are regulated. Finally, we discuss a few physiological questions on which molecular studies have shed new light.

Molecular mechanisms and regulation of K+ transport in higher plants

Potassium (K+) plays a number of important roles in plant growth and development. Over the past few years, molecular approaches associated with electrophysiological analyses have greatly advanced our understanding of K+ transport in plants. A large number of genes encoding K+ transport systems have been identified, revealing a high level of complexity. Characterization of some transport systems is providing exciting information at the molecular level on functions such as root K+ uptake and secretion into the xylem sap, K+ transport in guard cells, or K+ influx into growing pollen tubes. In this review, we take stock of this recent molecular information. The main families of plant K+ transport systems (Shaker and KCO channels, KUP/HAK/KT and HKT transporters) are described, along with molecular data on how these systems are regulated. Finally, we discuss a few physiological questions on which molecular studies have shed new light.

Physical and functional interaction of the Arabidopsis K(+) channel AKT2 and phosphatase AtPP2CA

The AKT2 K(+) channel is endowed with unique functional properties, being the only weak inward rectifier characterized to date in Arabidopsis. The gene is expressed widely, mainly in the phloem but also at lower levels in leaf epiderm, mesophyll, and guard cells. The AKT2 mRNA level is upregulated by abscisic acid. By screening a two-hybrid cDNA library, we isolated a protein phosphatase 2C (AtPP2CA) involved in abscisic acid signaling as a putative partner of AKT2. We further confirmed the interaction by in vitro binding studies. The expression of AtPP2CA (beta-glucuronidase reporter gene) displayed a pattern largely overlapping that of AKT2 and was upregulated by abscisic acid. Coexpression of AtPP2CA with AKT2 in COS cells and Xenopus laevis oocytes was found to induce both an inhibition of the AKT2 current and an increase of the channel inward rectification. Site-directed mutagenesis and pharmacological analysis revealed that this functional interaction involves AtPP2CA phosphatase activity. Regulation of AKT2 activity by AtPP2CA in planta could allow the control of K(+) transport and membrane polarization during stress situations.

Cation channels in the Arabidopsis plasma membrane

In vivo analyses have identified different functional types of ion channels in various plant tissues and cells. The Arabidopsis genome contains approximately 70 genes for ion channels, of which 57 might be cation-selective channels (K(+), Ca(2+) or poorly discriminating channels). Here, we describe the different families of (putative) cation channels: the Shakers, the two-P-domain and Kir K(+) channels (encoded by the KCO genes), the cyclic-nucleotide-gated channels, the glutamate receptors, and the Ca(2+) channel TPC1. We also compare molecular data with the data obtained in planta, which should lead to a better understanding of the identity of these channels and provide clues about their roles in plant nutrition and cell signalling.

A grapevine gene encoding a guard cell K(+) channel displays developmental regulation in the grapevine berry

SIRK is a K(+) channel identified in grapevine (Vitis vinifera), belonging to the so-called Shaker family. The highest sequence similarities it shares with the members of this family are found with channels of the KAT type, although SIRK displays a small ankyrin domain. This atypical feature provides a key to understand the evolution of the plant Shaker family. Expression in Xenopus laevis oocytes indicated that SIRK is an inwardly rectifying channel displaying functional properties very similar to those of KAT2. The activity of SIRK promoter region fused to the GUS reporter gene was analyzed in both grapevine and Arabidopsis. Like other KAT-like channels, SIRK is expressed in guard cells. In Arabidopsis, the construct is also expressed in xylem parenchyma. Semiquantitative reverse transcriptase-polymerase chain reaction experiments indicated that SIRK transcript was present at low levels in the berry, during the first stages of berry growth. After veraison, the period of berry development that corresponds to the inception of ripening and that is associated with large biochemical and structural modifications, such as evolution of stomata in nonfunctional lenticels and degeneration of xylem vasculature, the transcript was no longer detected. The whole set of data suggests that in the berries SIRK is expressed in guard cells and, possibly, in xylem tissues. The encoded channel polypeptide could therefore play a role in the regulation of transpiration and water fluxes in grapevine fruits.

Pollen tube development and competitive ability are impaired by disruption of a Shaker K(+) channel in Arabidopsis

Sexual reproduction in plants requires elongation of the pollen tube through the transmitting tissues toward the ovary. Tube growth rate is a major determinant of pollen competitive ability. We report that a K(+) channel of the Shaker family in Arabidopsis, SPIK, plays an important role in pollen tube development. SPIK was found to be specifically expressed in pollen. When SPIK was heterologously expressed in COS cells, its product formed hyperpolarization-activated K(+) channels. Disruption (T-DNA insertion) of the SPIK coding sequence strongly affected inwardly rectifying K(+)-channel activity in the pollen-grain plasma membrane. Measurements of membrane potential in growing pollen tubes yielded data compatible with a contribution of SPIK to K(+) influx. In vitro pollen germination assays were performed, revealing that the disruption results in impaired pollen tube growth. Analysis of the transmission rate of the disrupted allele in the progeny of heterozygous plants revealed a decrease in pollen competitive ability, the probability of fertilization by mutant pollen being approximately 1.6 times lower than that by wild-type pollen. The whole set of data supports the hypothesis that functional expression of SPIK plays a role in K(+) uptake in the growing pollen tube, and thereby in tube development and pollen competitive ability.

Guard cell inward K+ channel activity in arabidopsis involves expression of the twin channel subunits KAT1 and KAT2

Stomatal opening, which controls gas exchanges between plants and the atmosphere, results from an increase in turgor of the two guard cells that surround the pore of the stoma. KAT1 was the only inward K(+) channel shown to be expressed in Arabidopsis guard cells, where it was proposed to mediate a K(+) influx that enables stomatal opening. We report that another Arabidopsis K(+) channel, KAT2, is expressed in guard cells. More than KAT1, KAT2 displays functional features resembling those of native inward K(+) channels in guard cells. Coexpression in Xenopus oocytes and two-hybrid experiments indicated that KAT1 and KAT2 can form heteromultimeric channels. The data indicate that KAT2 plays a crucial role in the stomatal opening machinery.

Biochemical characterization of the Arabidopsis K+ channels KAT1 and AKT1 expressed or co-expressed in insect cells

KAT1 and AKT1 belong to the multigenic family of the inwardly rectifying Shaker-like plant K+ channels. They were biochemically characterized after expression in insect cells using recombinant baculoviruses. The channels were solubilized from microsomal fractions prepared from infected cells (among eight different detergents only one, L-alpha-lysophosphatidylcholine, was efficient for solubilization), and purified to homogeneity using immunoaffinity (KAT1) or ion-exchange and size exclusion (AKT1) techniques. The following results were obtained with the purified polypeptides: (i) neither KAT1 nor AKT1 was found to be glycosylated; (ii) both polypeptides were mainly present as homotetrameric structures, supporting the hypothesis of a tetrameric structure for the functional channels; (iii) no heteromeric KAT1/AKT1 assembly was detected when the two polypeptides were co-expressed in insect cells. The use of the two-hybrid system in yeast also failed to detect any interaction between KAT1 and AKT1 polypeptides. Because of these negative results, the hypothesis that plant K+-channel subunits are able to co-assemble without any discrimination, previously put forward based on co-expression in Xenopus oocytes of various K+-channel subunits (including KAT1 and AKT1), has still to be supported by independent approaches. Co-localization of channel subunits within the same plant tissue/cell does not allow us to conclude that the subunits form heteromultimeric channels.