Organization and expression of the gene coding for the potassium transport system AKT1 of Arabidopsis thaliana

Research Progress on Plant Shaker K+ Channels

Plant growth and development are driven by intricate processes, with the cell membrane serving as a crucial interface between cells and their external environment. Maintaining balance and signal transduction across the cell membrane is essential for cellular stability and a host of life processes. Ion channels play a critical role in regulating intracellular ion concentrations and potentials. Among these, K+ channels on plant cell membranes are of paramount importance. The research of Shaker K+ channels has become a paradigm in the study of plant ion channels. This study offers a comprehensive overview of advancements in Shaker K+ channels, including insights into protein structure, function, regulatory mechanisms, and research techniques. Investigating Shaker K+ channels has enhanced our understanding of the regulatory mechanisms governing ion absorption and transport in plant cells. This knowledge offers invaluable guidance for enhancing crop yields and improving resistance to environmental stressors. Moreover, an extensive review of research methodologies in Shaker K+ channel studies provides essential reference solutions for researchers, promoting further advancements in ion channel research.

Identification of Shaker Potassium Channel Family Members in Gossypium hirsutum L. and Characterization of GhKAT1aD

K⁺ channels of the Shaker family have been shown to play crucial roles in K⁺ uptake and transport. Cotton (Gossypium hirsutum) is an important cash crop. In this study, the 24 Shaker family genes were identified in cotton. Phylogenetic analysis suggests that they were assigned to five clusters. Additionally, their chromosomal location, conserved motifs and gene structure were analyzed. The promoter of cotton Shaker K⁺ channel genes comprises drought-, low-temperature-, phytohormone-response elements, etc. As indicated by qRT-PCR (quantitative real-time PCR), cotton Shaker K⁺ channel genes responded to low K⁺ and NaCl, and especially dehydration stress, at the transcript level. Moreover, one of the Shaker K⁺ channel genes, GhKAT1aD, was characterized. This gene is localized in the plasma membrane and is predicted to contain six transmembrane segments. It restored the growth of the yeast mutant strain defective in K⁺ uptake, and silencing GhKAT1a via VIGS (virus-induced gene silencing) resulted in more severe symptoms of K⁺ deficiency in cotton leaves as well as a lower net K⁺ uptake rate. The results of this study showed the overall picture of the cotton Shaker K⁺ channel family regarding bioinformatics as well as the function of one of its members, which provide clues for future investigations of cotton K⁺ transport and molecular insights for breeding K⁺-efficient cotton varieties.

ZMK1 Is Involved in K+ Uptake and Regulated by Protein Kinase ZmCIPK23 in Zea mays

Potassium (K+) is one of essential mineral elements for plant growth and development. K+ channels, especially AKT1-like channels, play crucial roles in K+ uptake in plant roots. Maize is one of important crops; however, the K+ uptake mechanism in maize is little known. Here, we report the physiological functions of K+ channel ZMK1 in K+ uptake and homeostasis in maize. ZMK1 is a homolog of Arabidopsis AKT1 channel in maize, and mainly expressed in maize root. Yeast complementation experiments and electrophysiological characterization in Xenopus oocytes indicated that ZMK1 could mediate K+ uptake. ZMK1 rescued the low-K+-sensitive phenotype of akt1 mutant and enhanced K+ uptake in Arabidopsis. Overexpression of ZMK1 also significantly increased K+ uptake activity in maize, but led to an oversensitive phenotype. Similar to AKT1 regulation, the protein kinase ZmCIPK23 interacted with ZMK1 and phosphorylated the cytosolic region of ZMK1, activating ZMK1-mediated K+ uptake. ZmCIPK23 could also complement the low-K+-sensitive phenotype of Arabidopsis cipk23/lks1 mutant. These findings demonstrate that ZMK1 together with ZmCIPK23 plays important roles in K+ uptake and homeostasis in maize.

Phylogenetically diverse endophytic bacteria from desert plants induce transcriptional changes of tissue-specific ion transporters and salinity stress in Arabidopsis thaliana

Salinity severely hampers crop productivity worldwide and plant growth promoting bacteria could serve as a sustainable solution to improve plant growth under salt stress. However, the molecular mechanisms underlying salt stress tolerance promotion by beneficial bacteria remain unclear. In this work, six bacterial isolates from four different desert plant species were screened for their biochemical plant growth promoting traits and salinity stress tolerance promotion of the unknown host plant Arabidopsis thaliana. Five of the isolates induced variable root phenotypes but could all increase plant shoot and root weight under salinity stress. Inoculation of Arabidopsis with five isolates under salinity stress resulted in tissue-specific transcriptional changes of ion transporters and reduced Na⁺/K⁺ shoot ratios. The work provides first insights into the possible mechanisms and the commonality by which phylogenetically diverse bacteria from different desert plants induce salinity stress tolerance in Arabidopsis. The bacterial isolates provide new tools for studying abiotic stress tolerance mechanisms in plants and a promising agricultural solution for increasing crop yields in semi-arid regions.

Cesium Inhibits Plant Growth through Jasmonate Signaling in Arabidopsis thaliana

It has been suggested that cesium is absorbed from the soil through potassium uptake machineries in plants; however, not much is known about perception mechanism and downstream response. Here, we report that the jasmonate pathway is required in plant response to cesium. Jasmonate biosynthesis mutant aos and jasmonate-insensitive mutant coi1-16 show clear resistance to root growth inhibition caused by cesium. However, the potassium and cesium contents in these mutants are comparable to wild-type plants, indicating that jasmonate biosynthesis and signaling are not involved in cesium uptake, but involved in cesium perception. Cesium induces expression of a high-affinity potassium transporter gene HAK5 and reduces potassium content in the plant body, suggesting a competitive nature of potassium and cesium uptake in plants. It has also been found that cesium-induced HAK5 expression is antagonized by exogenous application of methyl-jasmonate. Taken together, it has been indicated that cesium inhibits plant growth via induction of the jasmonate pathway and likely modifies potassium uptake machineries.

Ectopic expression of the K+ channel β subunits from Puccinellia tenuiflora (KPutB1) and rice (KOB1) alters K+ homeostasis of yeast and Arabidopsis

In this study, we cloned a cDNA for the K+ channel β subunit from the halophyte Puccinellia tenuiflora and named it KPutB1. KPutB1 was preferentially expressed in the roots and was transiently induced by K+-starvation, salt stress, or the combination of both stresses. By yeast two-hybrid assay, we demonstrated that KPutB1 interacts with PutAKT1, α subunit of an AKT1-type K+ channel of P. tenuiflora. The functional relevance of this interaction on K+-nutrition was investigated by co-expression experiments in yeast under various ionic conditions, and K+ channel α and β subunit homologues from rice (OsAKT1 and KOB1, respectively) were included for comparison. Yeast co-expressing PutAKT1 and the β subunits (KPutB1 and KOB1) had better growth and higher K+-uptake ability than yeast expressing PutAKT1 alone. In contrast, yeast co-expressing the β subunits (KPutB1 and KOB1) with OsAKT1 had slower growth and lower K+ uptake than yeast expressing OsAKT1 alone. Arabidopsis plants over-expressing the K+ channel β subunit of P. tenuiflora or rice showed increased shoot K+ content and decreased root Na+ content under control, 75 mM NaCl, and K+-starvation stress conditions. These results suggest that ectopic expression of the K+ channel β subunit could alter K+ and Na+ homeostasis in plants.

Potassium and sodium transport in non-animal cells: the Trk/Ktr/HKT transporter family

Bacterial Trk and Ktr, fungal Trk and plant HKT form a family of membrane transporters permeable to K(+) and/or Na(+) and characterized by a common structure probably derived from an ancestral K(+) channel subunit. This transporter family, specific of non-animal cells, displays a large diversity in terms of ionic permeability, affinity and energetic coupling (H(+)-K(+) or Na(+)-K(+) symport, K(+) or Na(+) uniport), which might reflect a high need for adaptation in organisms living in fluctuating or dilute environments. Trk/Ktr/HKT transporters are involved in diverse functions, from K(+) or Na(+) uptake to membrane potential control, adaptation to osmotic or salt stress, or Na(+) recirculation from shoots to roots in plants. Structural analyses of bacterial Ktr point to multimeric structures physically interacting with regulatory subunits. Elucidation of Trk/Ktr/HKT protein structures along with characterization of mutated transporters could highlight functional and evolutionary relationships between ion channels and transporters displaying channel-like features.

Expression of the AKT1-type K(+) channel gene from Puccinellia tenuiflora, PutAKT1, enhances salt tolerance in Arabidopsis

Potassium channels are important for many physiological functions in plants, one of which is to regulate plant adaptation to stress conditions. In this study, a K(+) channel PutAKT1 cDNA was isolated from the salt-tolerant plant Puccinellia tenuiflora. A phylogenetic analysis showed that PutAKT1 belongs to the AKT1-subfamily in the Shaker K(+) channel family. PutAKT1 was localized in the plasma membrane and it was preferentially expressed in the roots. The expression of PutAKT1 was induced by K(+)-starvation stress in the roots and was not down-regulated by the presence of excess Na(+). Arabidopsis plants over-expressing PutAKT1 showed enhanced salt tolerance compared to wild-type plants as shown by their shoot phenotype and dry weight. Expression of PutAKT1 increased the K(+) content of Arabidopsis under normal, K(+)-starvation, and NaCl-stress conditions. Arabidopsis expressing PutAKT1 also showed a decrease in Na(+) accumulation both in the shoot and in the root. These results suggest that PutAKT1 is involved in mediating K(+) uptake (i) both in low- and in high-affinity K(+) uptake range, and (ii) unlike its homologs in rice, even under salt-stress condition.

A grapevine Shaker inward K(+) channel activated by the calcineurin B-like calcium sensor 1-protein kinase CIPK23 network is expressed in grape berries under drought stress conditions

Grapevine (Vitis vinifera), the genome sequence of which has recently been reported, is considered as a model species to study fleshy fruit development and acid fruit physiology. Grape berry acidity is quantitatively and qualitatively affected upon increased K(+) accumulation, resulting in deleterious effects on fruit (and wine) quality. Aiming at identifying molecular determinants of K(+) transport in grapevine, we have identified a K(+) channel, named VvK1.1, from the Shaker family. In silico analyses indicated that VvK1.1 is the grapevine counterpart of the Arabidopsis AKT1 channel, known to dominate the plasma membrane inward conductance to K(+) in root periphery cells, and to play a major role in K(+) uptake from the soil solution. VvK1.1 shares common functional properties with AKT1, such as inward rectification (resulting from voltage sensitivity) or regulation by calcineurin B-like (CBL)-interacting protein kinase (CIPK) and Ca(2+)-sensing CBL partners (shown upon heterologous expression in Xenopus oocytes). It also displays distinctive features such as activation at much more negative membrane voltages or expression strongly sensitive to drought stress and ABA (upregulation in aerial parts, downregulation in roots). In roots, VvK1.1 is mainly expressed in cortical cells, like AKT1. In aerial parts, VvK1.1 transcripts were detected in most organs, with expression levels being the highest in the berries. VvK1.1 expression in the berry is localized in the phloem vasculature and pip teguments, and displays strong upregulation upon drought stress, by about 10-fold.VvK1.1 could thus play a major role in K(+) loading into berry tissues, especially upon drought stress.

Heteromeric AtKC1{middle dot}AKT1 channels in Arabidopsis roots facilitate growth under K+-limiting conditions

Plant growth and development is driven by osmotic processes. Potassium represents the major osmotically active cation in plants cells. The uptake of this inorganic osmolyte from the soil in Arabidopsis involves a root K(+) uptake module consisting of the two K(+) channel alpha-subunits, AKT1 and AtKC1. AKT1-mediated potassium absorption from K(+)-depleted soil was shown to depend on the calcium-sensing proteins CBL1/9 and their interacting kinase CIPK23. Here we show that upon activation by the CBL.CIPK complex in low external potassium homomeric AKT1 channels open at voltages positive of E(K), a condition resulting in cellular K(+) leakage. Although at submillimolar external potassium an intrinsic K(+) sensor reduces AKT1 channel cord conductance, loss of cytosolic potassium is not completely abolished under these conditions. Depending on channel activity and the actual potassium gradients, this channel-mediated K(+) loss results in impaired plant growth in the atkc1 mutant. Incorporation of the AtKC1 subunit into the channel complex, however, modulates the properties of the K(+) uptake module to prevent K(+) loss. Upon assembly of AKT1 and AtKC1, the activation threshold of the root inward rectifier voltage gate is shifted negative by approximately -70 mV. Additionally, the channel conductance gains a hypersensitive K(+) dependence. Together, these two processes appear to represent a safety strategy preventing K(+) loss through the uptake channels under physiological conditions. Similar growth retardation phenotypes of akt1 and atkc1 loss-of-function mutants in response to limiting K(+) supply further support such functional interdependence of AKT1 and AtKC1. Taken together, these findings suggest an essential role of AtKC1-like subunits for root K(+) uptake and K(+) homeostasis when plants experience conditions of K(+) limitation.

Effects of top excision on the potassium accumulation and expression of potassium channel genes in tobacco

The effects of the removal of the shoot apex of tobacco on the relative transcript levels of potassium channel genes, determined by real-time PCR, and on the relationship between the expression of genes encoding potassium channels and potassium concentration, were studied. The results from the study indicated that comparatively more assimilates of photosynthesis were allocated to the apex in control plants than in both decapitated and IAA-treated decapitated plants. By contrast, dry matter in the upper leaves, roots, and stems in both decapitated and IAA-treated plants was significantly increased relative to control plants. The potassium level in whole plants decreased post-decapitation compared with control plants, and so did the potassium concentration in middle and upper leaves, stem, and roots. Expression of NKT1, NtKC1, NTORK1, and NKT2 was inhibited by decapitation in tobacco leaves with a gradual reduction after decapitation, but was induced in roots. The relative expression of NKT1, NTORK1, and NKT2 in tobacco leaves was higher than that in roots, whereas the expression of NtKC1 was higher in roots. The levels of inhibition and induction of NKT1, NtKC1, NTORK1, and NKT2 in leaves and roots, respectively, associated with decapitation were reduced by the application of IAA on the cut surface of the decapitated stem. Further results showed that the level of endogenous auxin IAA in decapitated plants, which dropped in leaves and increased in roots by 140.7% at 14 d compared with the control plant, might be attributed to the change in the expression of potassium channel genes. The results suggest that there is a reciprocal relationship among endogenous auxin IAA, expression of potassium channel genes and potassium accumulation. They further imply that the endogenous IAA probably plays a role in regulating the expression of potassium channel genes, and that variations in expression of these genes affected the accumulation and distribution of potassium in tobacco.

Properties of shaker-type potassium channels in higher plants

Potassium (K(+)), the most abundant cation in biological organisms, plays a crucial role in the survival and development of plant cells, modulation of basic mechanisms such as enzyme activity, electrical membrane potentials, plant turgor and cellular homeostasis. Due to the absence of a Na(+)/K(+) exchanger, which widely exists in animal cells, K(+) channels and some type of K(+) transporters function as K(+) uptake systems in plants. Plant voltage-dependent K(+) channels, which display striking topological and functional similarities with the voltage-dependent six-transmembrane segment animal Shaker-type K(+) channels, have been found to play an important role in the plasma membrane of a variety of tissues and organs in higher plants. Outward-rectifying, inward-rectifying and weakly-rectifying K(+) channels have been identified and play a crucial role in K(+) homeostasis in plant cells. To adapt to the environmental conditions, plants must take advantage of the large variety of Shaker-type K(+) channels naturally present in the plant kingdom. This review summarizes the extensive data on the structure, function, membrane topogenesis, heteromerization, expression, localization, physiological roles and modulation of Shaker-type K(+) channels from various plant species. The accumulated results also help in understanding the similarities and differences in the properties of Shaker-type K(+) channels in plants in comparison to those of Shaker channels in animals and bacteria.

Ion channels meet auxin action

The regulation of cell division and elongation in plants is accomplished by the action of different phytohormones. Auxin as one of these growth regulators is known to stimulate cell elongation growth in the aerial parts of the plant. Here, auxin enhances cell enlargement by increasing the extensibility of the cell wall and by facilitating the uptake of osmolytes such as potassium ions into the cell. Starting in the late 1990s, the auxin regulation of ion channels mediating K+ import into the cell has been studied in great detail. In this article we will focus on the molecular mechanisms underlying the modulation of K+ transport by auxin and present a model to explain how the regulation of K+ channels is involved in auxin-induced cell elongation growth.

Rice K+ uptake channel OsAKT1 is sensitive to salt stress

Potassium ions constitute the most important macronutrients taken up by plants. To unravel the mechanisms of K+ uptake and its sensitivity to salt stress in the model plant rice, we isolated and functionally characterized OsAKT1, a potassium channel homologous to the Arabidopsis root inward rectifier AKT1. OsAKT1 transcripts were predominantly found in the coleoptile and in the roots of young rice seedlings. K+ channel mRNA decreases in response to salt stress, both in the shoot and in the root of 4-day-old rice seedlings. Following expression in HEK293 cells, we were able to characterize OsAKT1 as a voltage-dependent, inward-rectifying K+ channel regulated by extracellular Ca2+ and protons. Patch-clamp studies on rice root protoplasts identified a K+ inward rectifier with similar channel properties as heterologously expressed OsAKT1. In line with the transcriptional downregulation of OsAKT1 in response to salt stress, inward K+ currents were significantly reduced in root protoplasts. Thus, OsAKT1 seems to represent the dominant salt-sensitive K+ uptake channel in rice roots.

AKT2/3 subunits render guard cell K+ channels Ca2+ sensitive

Inward-rectifying K+ channels serve as a major pathway for Ca2+-sensitive K+ influx into guard cells. Arabidopsis thaliana guard cell inward-rectifying K+ channels are assembled from multiple K+ channel subunits. Following the recent isolation and characterization of an akt2/3-1 knockout mutant, we examined whether the AKT2/3 subunit carries the Ca2+ sensitivity of the guard cell inward rectifier. Quantification of RT-PCR products showed that despite the absence of AKT2 transcripts in guard cells of the knockout plant, expression levels of the other K+ channel subunits (KAT1, KAT2, AKT1, and AtKC1) remained largely unaffected. Patch-clamp experiments with guard cell protoplasts from wild type and akt2/3-1 mutant, however, revealed pronounced differences in Ca2+ sensitivity of the K+ inward rectifier. Wild-type channels were blocked by extracellular Ca2+ in a concentration- and voltage-dependent manner. Akt2/3-1 mutants lacked the voltage-dependent Ca2+ block, characteristic for the K+ inward rectifier. To confirm the akt2/3-1 phenotype, two independent knockout mutants, akt2-1 and akt2::En-1 were tested, demonstrating that the loss of AKT2/3 indeed affects the Ca2+ dependence of guard cell inward rectifier. In contrast to AKT2 knockout plants, AKT1, AtKC1, and KAT1 loss-of-function mutants retained Ca2+ block of the guard cell inward rectifier. When expressed in HEK293 cells, AKT2 channel displayed a pronounced susceptibility toward extracellular Ca2+, while the dominant guard cell K+ channel KAT2 was Ca2+ insensitive. Thus, we conclude that the AKT2/3 subunit constitutes the Ca2+ sensitivity of the guard cell K+ uptake channel.

Molecular dissection of the contribution of negatively and positively charged residues in S2, S3, and S4 to the final membrane topology of the voltage sensor in the K+ channel, KAT1

Voltage-dependent ion channels control changes in ion permeability in response to membrane potential changes. The voltage sensor in channel proteins consists of the highly positively charged segment, S4, and the negatively charged segments, S2 and S3. The process involved in the integration of the protein into the membrane remains to be elucidated. In this study, we used in vitro translation and translocation experiments to evaluate interactions between residues in the voltage sensor of a hyperpolarization-activated potassium channel, KAT1, and their effect on the final topology in the endoplasmic reticulum (ER) membrane. A D95V mutation in S2 showed less S3-S4 integration into the membrane, whereas a D105V mutation allowed S4 to be released into the ER lumen. These results indicate that Asp(95) assists in the membrane insertion of S3-S4 and that Asp(105) helps in preventing S4 from being releasing into the ER lumen. The charge reversal mutation, R171D, in S4 rescued the D105R mutation and prevented S4 release into the ER lumen. A series of constructs containing different C-terminal truncations of S4 showed that Arg(174) was required for correct integration of S3 and S4 into the membrane. Interactions between Asp(105) and Arg(171) and between negative residues in S2 or S3 and Arg(174) may be formed transiently during membrane integration. These data clarify the role of charged residues in S2, S3, and S4 and identify posttranslational electrostatic interactions between charged residues that are required to achieve the correct voltage sensor topology in the ER membrane.

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.

Involvement of TAAAG elements suggests a role for Dof transcription factors in guard cell-specific gene expression

Due to their unique structure and function, guard cells have attracted much attention at the physiological level. Very little, however, is known about the molecular events involved in the determination and maintenance of guard cell specificity. The KST1 gene encodes a K+ influx channel of guard cells in potato, and was therefore chosen as a model to study regulation of guard cell-specific gene expression. Transgenic potato plants carrying a fusion between the KST1 promoter and the E. coli uidA (beta-glucuronidase) reporter gene revealed promoter activity in guard cells and in flowers. A detailed dissection of the KST1 promoter led to the discovery of two independent small TATA box-proximal regulatory units, each of which was sufficient to direct guard cell-specific gene transcription. Both fragments contain the sequence motif, 5'-TAAAG-3', which is related to known target sites for a novel class of zinc finger transcription factors, called Dof proteins. Block mutagenesis of these Dof target sites in the context of different promoter constructs dramatically reduced guard cell promoter activity. A Dof gene, StDof1, was cloned and shown to be expressed in epidermal fragments highly enriched for guard cells. In gel retardation experiments, the StDof1 protein interacted in a sequence-specific manner with a KST1 promoter fragment containing the TAAAG motif. These results provide evidence that TAAAG elements are target sites for trans-acting Dof proteins controlling guard cell-specific gene expression. Our data will add to the design of tailor-made guard cell promoters as a further tool in molecular engineering of guard cell function and, hence, control of stomatal carbon dioxide (CO2) uptake and water loss in crop plants.

Enhanced osmotolerance of a wheat mutant selected for potassium accumulation

A mutant of durum wheat was identified by screening a M4 population (sodium azide) for genotypes with enhanced capacity for potassium accumulation in leaves. The mutant (designated 422) was grown in field, controlled environment, hydroponic culture and NaCl salinized soil. Mutant 422 accumulates about 5 mg/g dry weight more K than the wild-type and is less salt sensitive, based on leaf growth and germination. During vegetative growth exists a specific tolerance of the 422 mutant to K(+) ion and a moderate tolerance to Cl(-) ion, in hydroponic culture. Under severe stress imposed by salts and mannitol, the mutant germinates better than wild type (WT). In soil containing increasing NaCl, mutant 422 had higher potassium amount than WT, but did not show augmented capacity to concentrate the ion in the leaves as salt stress increased. The capability to accumulate potassium could improve tissue hydration, because water content of 422 leaves was greater than WT and increased linearly in relation to leaf K(+) concentration.

Individual functions of the HAK and TRK potassium transporters of Schwanniomyces occidentalis

We have cloned the gene encoding the TRK transporter of the soil yeast Schwanniomyces occidentalis and obtained the HAK1 trk1 delta and the hak1 delta TRK1 mutant strains. Analyses of the transport capacities of these mutants have shown that (i) the HAK1 and the TRK1 potassium transporters are the only transporters operating at low and medium K+ concentrations (< 1 mM); (ii) the HAK1 transporter is functional at low pH but fails at high pH; and (iii) the TRK1 transporter functions at neutral and high pH and fails at low pH. At neutral pH, both transporters are functional, but HAK1 is not expressed, except at very low K+ concentrations (< 50 microM) where HAK1 is very effective. TRK1 is also involved in the control of the membrane potential.

Potassium uptake supporting plant growth in the absence of AKT1 channel activity: Inhibition by ammonium and stimulation by sodium

A transferred-DNA insertion mutant of Arabidopsis that lacks AKT1 inward-rectifying K+ channel activity in root cells was obtained previously by a reverse-genetic strategy, enabling a dissection of the K+-uptake apparatus of the root into AKT1 and non-AKT1 components. Membrane potential measurements in root cells demonstrated that the AKT1 component of the wild-type K+ permeability was between 55 and 63% when external [K+] was between 10 and 1,000 microM, and NH4+ was absent. NH4+ specifically inhibited the non-AKT1 component, apparently by competing for K+ binding sites on the transporter(s). This inhibition by NH4+ had significant consequences for akt1 plants: K+ permeability, 86Rb+ fluxes into roots, seed germination, and seedling growth rate of the mutant were each similarly inhibited by NH4+. Wild-type plants were much more resistant to NH4+. Thus, AKT1 channels conduct the K+ influx necessary for the growth of Arabidopsis embryos and seedlings in conditions that block the non-AKT1 mechanism. In contrast to the effects of NH4+, Na+ and H+ significantly stimulated the non-AKT1 portion of the K+ permeability. Stimulation of akt1 growth rate by Na+, a predicted consequence of the previous result, was observed when external [K+] was 10 microM. Collectively, these results indicate that the AKT1 channel is an important component of the K+ uptake apparatus supporting growth, even in the "high-affinity" range of K+ concentrations. In the absence of AKT1 channel activity, an NH4+-sensitive, Na+/H+-stimulated mechanism can suffice.

K+-Selective inward-rectifying channels and apoplastic pH in barley roots

Recent structure-function analysis of heterologously expressed K+-selective inward-rectifying channels (KIRCs) from plants has revealed that external protons can have opposite effects on different members of the same gene family. An important question is how the diverse response of KIRCs to apoplastic pH is reflected at the tissue level. Activation of KIRCs by acid external pH is well documented for guard cells, but no other tissue has yet been studied. In this paper we present, for the first time to our knowledge, in planta characterization of the effects of apoplastic pH on KIRCs in roots. Patch-clamp experiments on protoplasts derived from barley (Hordeum vulgare) roots showed that a decrease in external pH shifted the half-activation potential to more positive voltages and increased the limit conductance. The resulting enhancement of the KIRC current, together with the characteristic voltage dependence, strongly relates the KIRC of barley root cells to AKT1-type as opposed to AKT3-type channels. Measurements of cell wall pH in barley roots with fluorescent dye revealed a bulk apoplastic pH close to the pK values of KIRC activation and significant acidification of the apoplast after the addition of fusicoccin. These results indicate that channel-mediated K+ uptake may be linked to development, growth, and stress responses of root cells via the activity of H+-translocating systems.

MOLECULAR BIOLOGY OF CATION TRANSPORT IN PLANTS

This review summarizes current knowledge about genes whose products function in the transport of various cationic macronutrients (K, Ca) and micronutrients (Cu, Fe, Mn, and Zn) in plants. Such genes have been identified on the basis of function, via complementation of yeast mutants, or on the basis of sequence similarity, via database analysis, degenerate PCR, or low stringency hybridization. Not surprisingly, many of these genes belong to previously described transporter families, including those encoding Shaker-type K+ channels, P-type ATPases, and Nramp proteins. ZIP, a novel cation transporter family first identified in plants, also seems to be ubiquitous; members of this family are found in protozoa, yeast, nematodes, and humans. Emerging information on where in the plant each transporter functions and how each is controlled in response to nutrient availability may allow creation of food crops with enhanced mineral content as well as crops that bioaccumulate or exclude toxic metals.

Characterization of SKT1, an inwardly rectifying potassium channel from potato, by heterologous expression in insect cells

A cDNA encoding a novel, inwardly rectifying K+ (K+in) channel protein, SKT1, was cloned from potato (Solanum tuberosum L.). SKT1 is related to members of the AKT family of K+in channels previously identified in Arabidopsis thaliana and potato. Skt1 mRNA is most strongly expressed in leaf epidermal fragments and in roots. In electrophysiological, whole-cell, patch-clamp measurements performed on baculovirus-infected insect (Spodoptera frugiperda) cells, SKT1 was identified as a K+in channel that activates with slow kinetics by hyperpolarizing voltage pulses to more negative potentials than -60 mV. The pharmacological inhibitor Cs+, when applied externally, inhibited SKT1-mediated K+in currents half-maximally with an inhibitor concentration (IC50) of 105 microM. An almost identical high Cs+ sensitivity (IC50 = 90 microM) was found for the potato guard-cell K+in channel KST1 after expression in insect cells. SKT1 currents were reversibly activated by a shift in external pH from 6.6 to 5.5, which indicates a physiological role for pH-dependent regulation of AKT-type K+in channels. Comparative studies revealed generally higher current amplitudes for KST1-expressing cells than for SKT1-expressing insect cells, which correlated with a higher targeting efficiency of the KST1 protein to the insect cell's plasma membrane, as demonstrated by fusions to green fluorescence protein.

Tetramerization of the AKT1 plant potassium channel involves its C-terminal cytoplasmic domain

All plant channels identified so far show high conservation throughout the polypeptide sequence except in the ankyrin domain which is present only in those closely related to AKT1. In this study, the architecture of the AKT1 protein has been investigated. AKT1 polypeptides expressed in the baculovirus/Sf9 cells system were found to assemble into tetramers as observed with animal Shaker-like potassium channel subunits. The AKT1 C-terminal intracytoplasmic region (downstream from the transmembrane domain) alone formed tetrameric structures when expressed in Sf9 cells, revealing a tetramerization process different from that of Shaker channels. Tests of subfragments from this sequence in the two-hybrid system detected two kinds of interaction. The first, involving two identical segments (amino acids 371-516), would form a contact between subunits, probably via their putative cyclic nucleotide-binding domains. The second interaction was found between the last 81 amino acids of the protein and a region lying between the channel hydrophobic core and the putative cyclic nucleotide-binding domain. As the interacting regions are highly conserved in all known plant potassium channels, the structural organization of AKT1 is likely to extend to these channels. The significance of this model with respect to animal cyclic nucleotide-gated channels is also discussed.

Association of plant K+(in) channels is mediated by conserved C-termini and does not affect subunit assembly

Inward rectifying potassium (K+(in)) channels play an important role in turgor regulation and ion uptake in higher plants. Here, we report a previously unrecognized feature of these proteins: K+(in) channel C-terminal polypeptides mediate channel protein interactions. Using a C-terminal fragment of potato guard cell K+(in) channel KST1 in a yeast two-hybrid screen two novel putative K+(in) channel proteins (SKT2 and SKT3) were identified by interaction of their C-termini which contained a conserved domain (K(HA)). Interactions were confirmed by Western blot-related assays utilizing K+(in) channel C-termini fused to green fluorescence protein. Although deletion of the K(HA)-domain abolished these interactions, K+(in) currents were still detectable by patch-clamp measurements of insect cells expressing these KST1 mutants, indicating that formation of a functional channel does not depend on this C-terminal domain.

Plant K+ channel alpha-subunits assemble indiscriminately

In plants a large diversity of inwardly rectifying K+ channels (K(in) channels) has been observed between tissues and species. However, only three different types of voltage-dependent plant K+ uptake channel subfamilies have been cloned so far; they relate either to KAT1, AKT1, or AtKC1. To explore the mechanisms underlying the channel diversity, we investigated the assembly of plant inwardly rectifying alpha-subunits. cRNA encoding five different K+ channel alpha-subunits of the three subfamilies (KAT1, KST1, AKT1, SKT1, and AtKC1) which were isolated from different tissues, species, and plant families (Arabidopsis thaliana and Solanum tuberosum) was reciprocally co-injected into Xenopus oocytes. We identified plant K+ channels as multimers. Moreover, using K+ channel mutants expressing different sensitivities to voltage, Cs+, Ca2+, and H+, we could prove heteromers on the basis of their altered voltage and modulator susceptibility. We discovered that, in contrast to animal K+ channel alpha-subunits, functional aggregates of plant K(in) channel alpha-subunits assembled indiscriminately. Interestingly, AKT-type channels from A. thaliana and S. tuberosum, which as homomers were electrically silent in oocytes after co-expression, mediated K+ currents. Our findings suggest that K+ channel diversity in plants results from nonselective heteromerization of different alpha-subunits, and thus depends on the spatial segregation of individual alpha-subunit pools and the degree of temporal overlap and kinetics of expression.

The baculovirus/insect cell system as an alternative to Xenopus oocytes. First characterization of the AKT1 K+ channel from Arabidopsis thaliana

Two plant (Arabidopsis thaliana) K+ transport systems, KAT1 and AKT1, have been expressed in insect cells (Sf9 cell line) using recombinant baculoviruses. Microscopic observation after immunogold staining revealed that the expressed AKT1 and KAT1 polypeptides were mainly associated with internal membranes, but that a minute fraction was targeted to the cell membrane. KAT1 was known, from earlier electrophysiological characterization in Xenopus oocytes, to be an inwardly rectifying voltage-gated channel highly selective for K+, while similar experiments had failed to characterize AKT1. Insect cells expressing KAT1 displayed an exogenous inwardly rectifying K+ conductance reminiscent of that described previously in Xenopus oocytes expressing KAT1. Under similar conditions, cells expressing AKT1 showed a disturbed cell membrane electrical stability that precluded electrophysiological analysis. Use of a baculovirus transfer vector designed so as to decrease the expression level allowed the first electrophysiological characterization of AKT1. The baculovirus system can thus be used as an alternative method when expression in Xenopus oocytes is unsuccessful for electrophysiological characterization of the ion channel of interest. The plant AKT1 protein has been shown in this way to be an inwardly rectifying voltage-gated channel highly selective for K+ ions and sensitive to cGMP.