Differential expression and regulation of K(+) channels in the maize coleoptile: molecular and biophysical analysis of cells isolated from cortex and vasculature

Speedy stomata of a C4 plant correlate with enhanced K+ channel gating

Stomata are microscopic pores at the surface of plant leaves that facilitate gaseous diffusion to support photosynthesis. The guard cells around each stoma regulate the pore aperture. Plants that carry out C4 photosynthesis are usually more resilient than C3 plants to stress, and their stomata operate over a lower dynamic range of CO2 within the leaf. What makes guard cells of C4 plants more responsive than those of C3 plants? We used gas exchange and electrophysiology, comparing stomatal kinetics of the C4 plant Gynandropsis gynandra and the phylogenetically related C3 plant Arabidopsis thaliana. We found, with varying CO2 and light, that Gynandropsis showed faster changes in stomata conductance and greater water use efficiency when compared with Arabidopsis. Electrophysiological analysis of the dominant K+ channels showed that the outward-rectifying channels, responsible for K+ loss during stomatal closing, were characterised by a greater maximum conductance and substantial negative shift in the voltage dependence of gating, indicating a reduced inhibition by extracellular K+ and enhanced capacity for K+ flux. These differences correlated with the accelerated stomata kinetics of Gynandropsis, suggesting that subtle changes in the biophysical properties of a key transporter may prove a target for future efforts to engineer C4 stomatal kinetics.

Nyctinastic movement in legumes: Developmental mechanisms, factors and biological significance

In legumes, a common phenomenon known as nyctinastic movement is observed. This movement involves the horizontal expansion of leaves during the day and relative vertical closure at night. Nyctinastic movement is driven by the pulvinus, which consists of flexor and extensor motor cells. The turgor pressure difference between these two cell types generates a driving force for the bending and deformation of the pulvinus. This review focuses on the developmental mechanisms of the pulvinus, the factors affecting nyctinastic movement, and the biological significance of this phenomenon in legumes, thus providing a reference for further research on nyctinastic movement.

Identification and Characterization of Shaker K+ Channel Gene Family in Foxtail Millet (Setaria italica) and Their Role in Stress Response

Potassium (K⁺) is one of the indispensable elements in plant growth and development. The Shaker K⁺ channel protein family is involved in plant K⁺ uptake and distribution. Foxtail millet (Setaria italica), as an important crop, has strong tolerance and adaptability to abiotic stresses. However, no systematic study focused on the Shaker K⁺ channel family in foxtail millet. Here, ten Shaker K⁺ channel genes in foxtail millet were identified and divided into five groups through phylogenetic analysis. Gene structures, chromosome locations, cis-acting regulatory elements in promoter, and post-translation modification sites of Shaker K⁺ channels were analyzed. In silico analysis of transcript level demonstrated that the expression of Shaker K⁺ channel genes was tissue or developmental stage specific. The transcription levels of Shaker K⁺ channel genes in foxtail millet under different abiotic stresses (cold, heat, NaCl, and PEG) and phytohormones (6-BA, BR, MJ, IAA, NAA, GA3, SA, and ABA) treatments at 0, 12, and 24 h were detected by qRT-PCR. The results showed that SiAKT1, SiKAT3, SiGORK, and SiSKOR were worth further research due to their significant responses after most treatments. The yeast complementation assay verified the inward K⁺ transport activities of detectable Shaker K⁺ channels. Finally, we found interactions between SiKAT2 and SiSNARE proteins. Compared to research in Arabidopsis, our results showed a difference in SYP121 related Shaker K⁺ channel regulation mechanism in foxtail millet. Our results indicate that Shaker K⁺ channels play important roles in foxtail millet and provide theoretical support for further exploring the K⁺ absorption mechanism of foxtail millet under abiotic stress.

Genome-Wide Identification and Characterization of the Shaker-Type K+ Channel Genes in Prunus persica (L.) Batsch

Shaker-type K+ channels are critical for plant K+ acquisition and translocation that play key roles during plant growth and development. However, molecular mechanisms towards K+ channels are extremely rare in fruit trees, especially in peach. In this study, we identified 7 putative shaker-type K+ channel genes from peach, which were unevenly distributed on 5 chromosomes. The peach shaker K+ channel proteins were classified into 5 subfamilies, I-V, and were tightly clustered with pear homologs in the phylogenetic tree. Various cis-acting regulatory elements were detected in the promoter region of the shaker-type K+ channel genes, including phytohormone-responsive, abiotic stress-responsive, and development regulatory elements. The peach shaker K+ channel genes were expressed differentially in distinct tissues, and PpSPIK was specifically expressed in the full-bloom flowers; PpKAT1 and PpGORK were predominantly expressed in the leaves, while PpAKT1, PpKC1, and PpSKOR were majorly expressed in the roots. The peach shaker K+ channel genes were differentially regulated by abiotic stresses in that K+ deficiency, and ABA treatment mainly increased the shaker K+ channel gene expression throughout the whole seedling, whereas NaCl and PEG treatment reduced the shaker K+ channel gene expression, especially in the roots. Moreover, electrophysiological analysis demonstrated that PpSKOR is a typical voltage-dependent outwardly rectifying K+ channel in peach. This study lays a molecular basis for further functional studies of the shaker-type K+ channel genes in peach and provides a theoretical foundation for K+ nutrition and balance research in fruit trees.

Recent Advances in Genome-wide Analyses of Plant Potassium Transporter Families

Plants require potassium (K⁺) as a macronutrient to support numerous physiological processes. Understanding how this nutrient is transported, stored, and utilized within plants is crucial for breeding crops with high K⁺ use efficiency. As K⁺ is not metabolized, cross-membrane transport becomes a rate-limiting step for efficient distribution and utilization in plants. Several K⁺ transporter families, such as KUP/HAK/KT and KEA transporters and Shaker-like and TPK channels, play dominant roles in plant K⁺ transport processes. In this review, we provide a comprehensive contemporary overview of our knowledge about these K⁺ transporter families in angiosperms, with a major focus on the genome-wide identification of K⁺ transporter families, subcellular localization, spatial expression, function and regulation. We also expanded the genome-wide search for the K⁺ transporter genes and examined their tissue-specific expression in Camelina sativa, a polyploid oil-seed crop with a potential to adapt to marginal lands for biofuel purposes and contribution to sustainable agriculture. In addition, we present new insights and emphasis on the study of K⁺ transporters in polyploids in an effort to generate crops with high K⁺ Utilization Efficiency (KUE).

Genome-Wide Investigation and Expression Analysis of K+-Transport-Related Gene Families in Chinese Cabbage (Brassica rapa ssp. pekinensis)

Potassium (K⁺) transport and channel systems play vital roles in plant growth, development and responses to various stresses. In this study, 44 putative K⁺-transport-related genes (18K⁺ uptake permease (KUP)/high-affinity K⁺ (HAK)/K⁺ transporter (KT) family genes and 26 channel genes, including 18 Shaker family genes and 8K⁺ channel outward (KCO) family genes) were identified in the genome of Chinese cabbage (Brassica rapa ssp. pekinensis). To clarify the molecular evolution of each family in Chinese cabbage, phylogenetic analysis and assessments of the gene structures, conserved motifs, chromosomal locations, gene duplications, expression patterns and cis-acting elements of the 44 putative K⁺-transport-related genes were performed. The phylogenetic analysis showed that these genes could be classified into five clades [KUP/HAK/KTs, KCOs, K_out, K_in (KAT) and K_in (AKT)] and that the members of a given clade shared conserved exon-intron distributions and motif compositions. These K⁺-transport-related genes were unevenly distributed over all ten chromosomes, including four duplicated gene pairs that implied an expansion of K⁺-transport-related genes in Chinese cabbage. Analyses of Illumina RNA-seq data for these 44K⁺-transport-related genes indicated tissue-/organ-specific expression patterns. In addition, an overall evaluation showed that the expression levels of KUP/HAK/KT genes were significantly higher than those of potassium channel genes in six tissues. Promoter cis-acting element analysis revealed that these 44K⁺-transport-related genes may be associated with responses to 10 abiotic stresses, primarily light, methyl jasmonate (MeJA) and abscisic acid (ABA). Our results provide a systematic and comprehensive overview of K⁺-transport-related gene families in Chinese cabbage and establish a foundation for further research on K⁺ absorption and transport functions.

Identification of Shaker K+ channel family members in Rosaceae and a functional exploration of PbrKAT1

PbrKAT1, which is inhibited by external Na⁺ in Xenopus laevis oocytes, is characterized as encoding a typical inward rectifying channel that is mainly expressed in guard cells. Potassium (K⁺) is the most abundant cation in plant cells necessary for plant growth and development. The uptake and transport of K⁺ are mainly completed through transporters and channels, and the Shaker family genes are the most studied K⁺ channels in plants. However, there is far less information about this family in Rosaceae species. We performed a genome-wide analysis and identified Shaker K⁺ channel gene family members in Rosaceae. We cloned and characterized a Shaker K⁺ channel KAT1 from pear (Pyrus × bretschneideri). In total, 36 Shaker K⁺ channel genes were identified from Rosaceae species and were classified into five subgroups based on structural characteristics and a phylogenetic analysis. Whole-genome and dispersed duplications were the primary forces underlying Shaker K⁺ channel gene family expansion in Rosaceae, and purifying selection played a key role in the evolution of Shaker K⁺ channel genes. β-Glucuronidase and qRT-PCR assays revealed that PbrKAT1 was mainly expressed in leaves, especially in guard cells. PbrKAT1 displayed a typical inward-rectifying current when expressed in Xenopus laevis oocytes. The activity of PbrKAT1 was inhibited by external sodium ions, possibly playing an important role in the regulation of salt tolerance in pear. These results provide valuable information on evolution, expression and functions of the Shaker K⁺ channel gene family in plants.

Potassium in the Grape (Vitis vinifera L.) Berry: Transport and Function

K+ is the most abundant cation in the grape berry. Here we focus on the most recent information in the long distance transport and partitioning of K+ within the grapevine and postulate on the potential role of K+ in berry sugar accumulation, berry water relations, cellular growth, disease resistance, abiotic stress tolerance and mitigating senescence. By integrating information from several different plant systems we have been able to generate new hypotheses on the integral functions of this predominant cation and to improve our understanding of how these functions contribute to grape berry growth and ripening. Valuable contributions to the study of K+ in membrane stabilization, turgor maintenance and phloem transport have allowed us to propose a mechanistic model for the role of this cation in grape berry development.

Regulation of potassium transport in plants under hostile conditions: implications for abiotic and biotic stress tolerance

Intracellular potassium homeostasis is a prerequisite for the optimal operation of plant metabolic machinery and plant's overall performance. It is controlled by K(+) uptake, efflux and intracellular and long-distance relocation, mediated by a large number of K(+) -selective and non-selective channels and transporters located at both plasma and vacuolar membranes. All abiotic and biotic stresses result in a significant disturbance to intracellular potassium homeostasis. In this work, we discuss molecular mechanisms and messengers mediating potassium transport and homeostasis focusing on four major environmental stresses: salinity, drought, flooding and biotic factors. We argue that cytosolic K(+) content may be considered as one of the 'master switches' enabling plant transition from the normal metabolism to 'hibernated state' during first hours after the stress exposure and then to a recovery phase. We show that all these stresses trigger substantial disturbance to K(+) homeostasis and provoke a feedback control on K(+) channels and transporters expression and post-translational regulation of their activity, optimizing K(+) absorption and usage, and, at the extreme end, assisting the programmed cell death. We discuss specific modes of regulation of the activity of K(+) channels and transporters by membrane voltage, intracellular Ca(2+) , reactive oxygen species, polyamines, phytohormones and gasotransmitters, and link this regulation with plant-adaptive responses to hostile environments.

Molecular biology of K+ transport across the plant cell membrane: what do we learn from comparison between plant species?

Cloning and characterizations of plant K(+) transport systems aside from Arabidopsis have been increasing over the past decade, favored by the availability of more and more plant genome sequences. Information now available enables the comparison of some of these systems between species. In this review, we focus on three families of plant K(+) transport systems that are active at the plasma membrane: the Shaker K(+) channel family, comprised of voltage-gated channels that dominate the plasma membrane conductance to K(+) in most environmental conditions, and two families of transporters, the HAK/KUP/KT K(+) transporter family, which includes some high-affinity transporters, and the HKT K(+) and/or Na(+) transporter family, in which K(+)-permeable members seem to be present in monocots only. The three families are briefly described, giving insights into the structure of their members and on functional properties and their roles in Arabidopsis or rice. The structure of the three families is then compared between plant species through phylogenic analyses. Within clusters of ortologues/paralogues, similarities and differences in terms of expression pattern, functional properties and, when known, regulatory interacting partners, are highlighted. The question of the physiological significance of highlighted differences is also addressed.

Spread the news: systemic dissemination and local impact of Ca²⁺ signals along the phloem pathway

We explored the idea of whether electropotential waves (EPWs) primarily act as vehicles for systemic spread of Ca(2+) signals. EPW-associated Ca(2+) influx may trigger generation and amplification of countless long-distance signals along the phloem pathway given the fact that gating of Ca(2+)-permeable channels is a universal response to biotic and abiotic challenges. Despite fundamental differences, both action and variation potentials are associated with a sudden Ca(2+) influx. Both EPWs probably disperse in the lateral direction, which could be of essential functional significance. A vast set of Ca(2+)-permeable channels, some of which have been localized, is required for Ca(2+)-modulated events in sieve elements. There, Ca(2+)-permeable channels are clustered and create so-called Ca(2+) hotspots, which play a pivotal role in sieve element occlusion. Occlusion mechanisms play a central part in the interaction between plants and phytopathogens (e.g. aphids or phytoplasmas) and in transient re-organization of the vascular symplasm. It is argued that Ca(2+)-triggered systemic signalling occurs in partly overlapping waves. The forefront of EPWs may be accompanied by a burst of free Ca(2+) ions and Ca(2+)-binding proteins in the sieve tube sap, with a far-reaching impact on target cells. Lateral dispersion of EPWs may induce diverse Ca(2+) influx and handling patterns (Ca(2+) signatures) in various cell types lining the sieve tubes. As a result, a variety of cascades may trigger the fabrication of signals such as phytohormones, proteins, or RNA species released into the sap stream after product-related lag times. Moreover, transient reorganization of the vascular symplasm could modify cascades in disjunct vascular cells.

Light-induced modification of plant plasma membrane ion transport

Light is not only the driving force for electron and ion transport in the thylakoid membrane, but also regulates ion transport in various other membranes of plant cells. Light-dependent changes in ion transport at the plasma membrane and associated membrane potential changes have been studied intensively over the last century. These studies, with various species and cell types, revealed that apart from regulation by chloroplasts, plasma membrane transport can be controlled by phytochromes, phototropins or channel rhodopsins. In this review, we compare light-dependent plasma membrane responses of unicellular algae (Eremosphaera and Chlamydomonas), with those of a multicellular alga (Chara), liverworts (Conocephalum), mosses (Physcomitrella) and several angiosperm cell types. Light-dependent plasma membrane responses of Eremosphaera and Chara are characterised by the dominant role of K(+) channels during membrane potential changes. In most other species, the Ca(2+)-dependent activation of plasma membrane anion channels represents a general light-triggered event. Cell type-specific responses are likely to have evolved by modification of this general response or through the development of additional light-dependent signalling pathways. Future research to elucidate these light-activated signalling chains is likely to benefit from the recent identification of S-type anion channel genes and proteins capable of regulating these channels.

Potassium channels in barley: cloning, functional characterization and expression analyses in relation to leaf growth and development

It is not known how the uptake and retention of the key osmolyte K(+) in cells are mediated in growing leaf tissue. In the present study on the growing leaf 3 of barley, we have cloned the full-length coding sequence of three genes which encode putative K(+) channels (HvAKT1, HvAKT2, HvKCO1/HvTPK1), and of one gene which encodes a putative K(+) transporter (HvHAK4). The functionality of the gene products of HvAKT1 and HvAKT2 was tested through expression in Xenopus laevis oocytes. Both are inward-rectifying K(+) channels which are inhibited by Cs(+). Function of HvAKT1 in oocytes requires co-expression of a calcineurin-interacting protein kinase (AtCIPK23) and a calcineurin B-like protein (AtCBL9) from Arabidopsis, showing cross-species complementation of function. In planta, HvAKT1 is expressed primarily in roots, but is also expressed in leaf tissue. HvAKT2 is expressed particularly in leaf tissue, and HvHAK4 is expressed particularly in growing leaf tissue. Within leaves, HvAKT1 and HvAKT2 are expressed predominantly in mesophyll. Expression of genes changes little in response to low external K(+) or salinity, despite major changes in K(+) concentrations and osmolality of cells. Possible contributions of HvAKT1, HvAKT2, HvKCO1 and HvHAK4 to regulation of K(+) relations of growing barley leaf cells are discussed.

Review: Nutrient loading of developing seeds

Interest in nutrient loading of seeds is fuelled by its central importance to plant reproductive success and human nutrition. Rates of nutrient loading, imported through the phloem, are regulated by transport and transfer processes located in sources (leaves, stems, reproductive structures), phloem pathway and seed sinks. During the early phases of seed development, most control is likely to be imposed by a low conductive pathway of differentiating phloem cells serving developing seeds. Following the onset of storage product accumulation by seeds, and, depending on nutrient species, dominance of path control gives way to regulation by processes located in sources (nitrogen, sulfur, minor minerals), phloem path (transition elements) or seed sinks (sugars and major mineral elements, such as potassium). Nutrients and accompanying water are imported into maternal seed tissues and unloaded from the conducting sieve elements into an extensive post-phloem symplasmic domain. Nutrients are released from this symplasmic domain into the seed apoplasm by poorly understood membrane transport mechanisms. As seed development progresses, increasing volumes of imported phloem water are recycled back to the parent plant by process(es) yet to be discovered. However, aquaporins concentrated in vascular and surrounding parenchyma cells of legume seed coats could provide a gated pathway of water movement in these tissues. Filial cells, abutting the maternal tissues, take up nutrients from the seed apoplasm by membrane proteins that include sucrose and amino acid/H⁺ symporters functioning in parallel with non-selective cation channels. Filial demand for nutrients, that comprise the major osmotic species, is integrated with their release and phloem import by a turgor-homeostat mechanism located in maternal seed tissues. It is speculated that turgors of maternal unloading cells are sensed by the cytoskeleton and transduced by calcium signalling cascades.

In planta AKT2 subunits constitute a pH- and Ca2+-sensitive inward rectifying K+ channel

Heterologous expression of plant genes in yeast and animal cells represents a common approach to study plant ion channels. When expressed in Xenopus oocytes and COS cells the Arabidopsis Shaker-like K+ channel, AKT2 forms a weakly voltage-dependent channel, blocked by Ca2+ and protons. Channels with these characteristics, however, were not found in AKT2-expressing Arabidopsis cell types. To understand this phenomenon, we employed Agrobacterium-mediated transient transformation to functionally characterise Arabidopsis thaliana channels in Nicotiana benthamiana mesophyll cells. In this expression system we used AtTPK4 as a control for voltage-independent A. thaliana channels. Agrobacteria harbouring GFP-tagged constructs with the coding sequences of AKT2 and AtTPK4 were infiltrated into intact tobacco leaves. With quantitative RT-PCR analyses channel transcripts of AKT2 and AtTPK4 were determined in transformed leaves. These results were confirmed by Western blots with V5 epitope-tagged AKT2 and AtTPK4 proteins, showing that the channel protein was indeed synthesised. For functional analysis of these channels, mesophyll protoplasts were isolated from infiltrated leaf sections. Patch-clamp studies revealed that AKT2 channels in mesophyll protoplasts retained Ca2+ and pH sensitivity, characteristics of the heterologously expressed protein, but displayed pronounced differences in voltage-dependence and kinetics. AKT2-transformed mesophyll cells, displayed inward-rectifying, rather than voltage-independent K+ channels, initially characterised in AKT2-expressing animal cells. In contrast, AtTPK4 showed the same electrophysiological characteristics both, in oocytes and plant cells. Our data suggest that heterologous systems do not always possess all regulatory components for functional expression of plant channels. Therefore, transient expression of plant proteins in planta provides an additional research tool for rapid biophysical analysis of plant ion channels.

Electrical signals and their physiological significance in plants

Electrical excitability and signalling, frequently associated with rapid responses to environmental stimuli, are well known in some algae and higher plants. The presence of electrical signals, such as action potentials (AP), in both animal and plant cells suggested that plant cells, too, make use of ion channels to transmit information over long distances. In the light of rapid progress in plant biology during the past decade, the assumption that electrical signals do not only trigger rapid leaf movements in 'sensitive' plants such as Mimosa pudica or Dionaea muscipula, but also physiological processes in ordinary plants proved to be correct. Summarizing recent progress in the field of electrical signalling in plants, the present review will focus on the generation and propagation of various electrical signals, their ways of transmission within the plant body and various physiological effects.

Inositol 1,4,5-trisphosphate and Ran expression during simulated and real microgravity

In order to gain further insight into the signal transduction pathway concerning gravitropism, we studied the expression profiles of mRNA in etiolated sunflower (Helianthus annuus L.) seedlings. Differential-display reverse transcriptase PCR product assayed by capillary electrophoresis revealed the small GTPase Ran, regulating nuclear import and export of proteins. Parallel analysis of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) release by a highly advanced system of metal-dye detection combined with high-performance liquid chromatography provided evidence that the second messenger Ins(1,4,5)P3 is modulated by changes of the gravity vector. Investigations by fast clinorotation and sounding rockets established a positive correlation between the Ins(1,4,5)P3 level and the expression rate of Ran mRNA during simulated and real microgravity. Since an asymmetric distribution of auxin during graviresponse is suggested to induce differential cell elongation, additional information on the perception and transduction pathways was achieved by auxin stimulation experiments. While we were able to demonstrate an auxin-dependent production of Ins(1,4,5)P3, the expression of Ran mRNA was not affected by auxin. Finally, besides the phosphoinositide system as one element of the signal transduction chain linking graviperception to graviresponse, a Ran-mediated interaction model of extracellular microgravity signal perception and intercellular transduction pathway is proposed.

Development of Erect Leaves in a Modern Maize Hybrid is Associated with Reduced Responsiveness to Auxin and Light of Young Seedlings In Vitro

Modern corn (Zea mays L.) varieties have been selected for their ability to maintain productivity in dense plantings. We have tested the possibility that the physiological consequence of the selection involves changes in responsiveness to light and auxin.Etiolated seedlings of two older corn hybrids 307 and 3306 elongated significantly more than seedlings of a modern corn hybrid 3394. The level of endogenous auxin and activity of PAT in 307 and 3394 were similar. Hybrid 3394 shows resistance to auxin- and light-induced responses at the seedling, cell and molecular levels. Intact 3394 plants exhibited less responsiveness to the inhibitory effect of R, FR and W, auxin, anti-auxin and inhibitors of PAT. In excised mesocotyl tissue 3394 seedlings also showed essentially low responsiveness to NAA. Cells of 3394 were insensitive to auxin- and light-induced hyperpolarization of the plasma membrane. Expression of ABP4 was much less in 3394 than in 307, and in contrast to 307, it was not upregulated by NAA, R and FR. Preliminary analysis of abp mutants suggests that ABPs may be involved in development of leaf angle in corn.Our results confirm the understanding that auxin interacts with light in the regulation of growth and development of young seedlings and suggest that in corn ABPs may be involved in growth of maize seedlings and development of leaf angle. We hypothesize that ABP4 plays an important role in the auxin- and/or light-induced growth responses. We also hypothesize that in the modern corn hybrid 3394, ABP4 is "mutated," which may result in the observed 3394 phenotypes, including upright leaves.

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.

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.

Phosphorylation of SPICK2, an AKT2 channel homologue from Samanea motor cells

SPICK2, a homologue of the weakly-inward-rectifying Shaker-like Arabidopsis K channel, AKT2, is a candidate K+-influx channel participating in light- and clock-regulated leaf movements of the legume, Samanea saman. Light and the biological clock regulate the in situ K+-influx channel activity differentially in extensor and flexor halves of the pulvinus (the S. saman leaf motor organ), and also-though differently-the transcript level of SPICK2 in the pulvinus. This disparity between the in situ channel activity versus its candidate transcript, along with the sequence analysis of SPICK2, suggest an in situ regulation of the activity of SPICK2, possibly by phosphorylation and/or by interaction with cAMP. Consistent with this (i) the activity of the voltage-dependent K+-selective fraction of the inward current in extensor and flexor cells was affected differentially in whole-cell patch-clamp assays promoting phosphorylation (using the protein phosphatase inhibitor okadaic acid); (ii) several proteins in isolated plasma membrane-enriched vesicles of the motor cells underwent phosphorylation without an added kinase in conditions similar to patch-clamp; and (iii) the SPICK2 protein was phosphorylated in vitro by the catalytic subunit of the broad-range cAMP-dependent protein kinase. All of these results are consistent with the notion that SPICK2 is the K+-influx channel, and is regulated in vivo directly by phosphorylation.

Differential expression of K+ channels between guard cells and subsidiary cells within the maize stomatal complex

Grass stomata are characterized by dumbbell-shaped guard cells forming a complex with a pair of specialized epidermal cells, the subsidiary cells. Stomatal movement is accomplished by a reversible exchange of potassium and chloride between these two cell types. To gain insight into the molecular machinery involved in K+ transport within the stomatal complex of Zea mays, we determined the spatial and temporal expression pattern of potassium channels in the maize leaf. KZM2 and ZORK were isolated and identified as new members of the plant Shaker K+ channel family. Northern blot analysis identified fully developed leaves as the predominant site of KZM2 expression. Following enzymatic digestion and separation of leaf tissue into epidermal, mesophyll, and vascular fractions, KZM2 and ZORK transcripts were localized in the epidermis. Using a collection of individually isolated guard cell or subsidiary cell protoplasts, ZORK transcripts were found in both cell types while KZM2 was exclusively expressed in the guard cell population. The previously identified K+ channel genes ZMK1 and KZM1 were expressed in subsidiary cells and guard cells, respectively, whereas ZMK2 transcripts were not detected. These data indicate that the interaction between subsidiary cells and guard cells is based on overlapping as well as differential expression of K+ channels in the two cell types of the maize stomatal complex.

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.

Auxin activates KAT1 and KAT2, two K+-channel genes expressed in seedlings of Arabidopsis thaliana

The transcript abundance of the K+-channel gene ZMK1 (Zea mays K+ channel 1) in maize coleoptiles is controlled by the phytohormone auxin. Thus, ZMK1 is thought to function in auxin-regulated coleoptile elongation, as well as during gravitropism and phototropism. To investigate related growth phenomena in the dicotyledonous plant Arabidopsis thaliana, we screened etiolated seedlings for auxin-induced K+-channel genes. Among the members of the Shaker-like K+ channels, we thereby identified transcripts of the inward rectifiers, KAT1 (K+ transporter of Arabidopsis thaliana) and KAT2, to be upregulated by auxin. The phloem-associated KAT2 was localised in cotyledons and the apical part of etiolated seedlings. In contrast, the K+-channel gene KAT1 was expressed in the cortex and epidermis of etiolated hypocotyls, as well as in flower stalks. Furthermore, KAT1 was induced by active auxins in auxin-sensitive tissues characterised by rapid cell elongation. Applying the patch-clamp technique to protoplasts of etiolated hypocotyls, we correlated the electrical properties of K+ currents with the expression profile of K+-channel genes. In KAT1-knockout mutants, K+ currents after auxin stimulation were characterised by reduced amplitudes. Thus, this change in the electrical properties of the K+-uptake channel in hypocotyl protoplasts resulted from an auxin-induced increase of active KAT1 proteins. The loss of KAT1-channel subunits, however, did not affect the auxin-induced growth rate of hypocotyls, pointing to compensation by residual, constitutive K+ transporters. From gene expression and electrophysiological data, we suggest that auxin regulation of KAT1 is involved in elongation growth of Arabidopsis. Furthermore, a role for KAT2 in the auxin-controlled vascular patterning of leaves is discussed.

Isolation of AtSUC2 promoter-GFP-marked companion cells for patch-clamp studies and expression profiling

K+ channels control K+ homeostasis and the membrane potential in the sieve element/companion cell complexes. K+ channels from Arabidopsis phloem cells expressing green fluorescent protein (GFP) under the control of the AtSUC2 promoter were analysed using the patch-clamp technique and quantitative RT-PCR. Single green fluorescent protoplasts were selected after being isolated enzymatically from vascular strands of rosette leaves. Companion cell protoplasts, which could be recognized by their nucleus, vacuole and chloroplasts, and by their expression of the phloem-specific marker genes SUC2 and AHA3, formed the basis for a cell-specific cDNA library and expressed sequence tag (EST) collection. Although we used primers for all members of the Shaker K+ channel family, we identified only AKT2, KAT1 and KCO6 transcripts. In addition, we also detected transcripts for AtPP2CA, a protein phosphatase, that interacts with AKT2/3. In line with the presence of the K+ channel transcripts, patch-clamp experiments identified distinct K+ channel types. Time-dependent inward rectifying K+ currents were activated upon hyperpolarization and were characterized by a pronounced Ca2+-sensitivity and inhibition by protons. Whole-cell inward currents were carried by single K+-selective channels with a unitary conductance of approximately 4 pS. Outward rectifying K+ channels (approximately 19 pS), with sigmoidal activation kinetics, were elicited upon depolarization. These two dominant phloem K+ channel types provide a versatile mechanism to mediate K+ fluxes required for phloem action and potassium cycling.

The K+ channel KZM1 mediates potassium uptake into the phloem and guard cells of the C4 grass Zea mays

In search of K(+) channel genes expressed in the leaf of the C(4) plant Zea mays, we isolated the cDNA of KZM1 (for K(+) channel Zea mays 1). KZM1 showed highest similarity to the Arabidopsis K(+) channels KAT1 and KAT2, which are localized in guard cells and phloem. When expressed in Xenopus oocytes, KZM1 exhibited the characteristic features of an inward-rectifying, potassium-selective channel. In contrast to KAT1- and KAT2-type K(+) channels, however, KZM1 currents were insensitive to external pH changes. Northern blot analyses identified the leaf, nodes, and silks as sites of KZM1 expression. Following the separation of maize leaves into epidermal, mesophyll, and vascular fractions, quantitative real-time reverse transcriptase-PCR allowed us to localize KZM1 transcripts predominantly in vascular strands and the epidermis. Cell tissue separation and KZM1 localization were followed with marker genes such as the bundle sheath-specific ribulose-1,5-bisphosphate carboxylase, the phloem K(+) channel ZMK2, and the putative sucrose transporter ZmSUT1. When expressed in Xenopus oocytes, ZmSUT1 mediated proton-coupled sucrose symport. Coexpression of ZmSUT1 with the phloem K(+) channels KZM1 and ZMK2 revealed that ZMK2 is able to stabilize the membrane potential during phloem loading/unloading processes and KZM1 to mediate K(+) uptake. During leaf development, sink-source transitions, and diurnal changes, KZM1 is constitutively expressed, pointing to a housekeeping function of this channel in K(+) homeostasis of the maize leaf. Therefore, the voltage-dependent K(+)-uptake channel KZM1 seems to mediate K(+) retrieval and K(+) loading into the phloem as well as K(+)-dependent stomatal opening.

Poplar potassium transporters capable of controlling K+ homeostasis and K+-dependent xylogenesis

The cambial K+ content of poplar increases during the growth period in a K+ supply dependent manner. Upon K+ starvation or application of tetraethylammoniumchloride (TEA+), a K+ channel blocker, the average vessel lumen and expansion zone area were significantly reduced. In search for the molecular basis of potassium-dependent xylogenesis in poplar, K+ transporters homologous to those of known function in Arabidopis phloem- and xylem-physiology were isolated from a poplar wood EST library. The expression profile of three distinct K+ channel types and one K+ transporter, Populus tremula K+ uptake transporter 1 (PtKUP1), was analysed by quantitative RT-PCR. Thereby, we found P. tremula outward rectifying K+ channel (PTORK) and P. tremula K+ channel 2 (PTK2) correlated with the seasonal wood production. K+ transporter P. tremula 1 (KPT1) was predominantly found in guard cells. Following the heterologous expression in Xenopus oocytes the biophysical properties of the different channels were determined. PTORK, upon membrane de-polarization mediates potassium release. PTK2 is almost voltage independent, carrying inward K+ flux at hyperpolarized potential and K+ release upon de-polarization. PtKUP1 was expressed in a K+ uptake-deficient Escherichia coli strain, where this K+ transporter rescued K+-dependent growth. In order to link the different K+ transporters to the cambial activity and wood production, we compared the expression profiles to seasonal changes in the K+ content of the bark as well as xylem vessel diameter. Thereby, we found PTORK and PTK2 transcripts to follow the annual K+ variations in poplar branches. PtKUP1 was expressed at a low level throughout the year, suggesting a housekeeping function. From these data, we conclude that K+ channels are involved in the regulation of K+-dependent wood production.

Signal transduction in maize and Arabidopsis mesophyll protoplasts

Plant protoplasts show physiological perceptions and responses to hormones, metabolites, environmental cues, and pathogen-derived elicitors, similar to cell-autonomous responses in intact tissues and plants. The development of defined protoplast transient expression systems for high-throughput screening and systematic characterization of gene functions has greatly contributed to elucidating plant signal transduction pathways, in combination with genetic, genomic, and transgenic approaches.

VFK1, a Vicia faba K(+) channel involved in phloem unloading

In search of a K(+) channel involved in phloem transport we screened a Vicia faba cotyledon cDNA library taking advantage of a set of degenerated primers, flanking regions conserved among K(+) uptake channels. We cloned VFK1 (for Vicia faba K(+) channel 1) characterised by a structure known from the Shaker family of plant K(+) channels. When co-expressed with a KAT1 mutant in Xenopus oocytes, heteromers revealed the biophysical properties of a K(+) selective, proton-blocked channel. Northern blot analyses showed high levels of expression in cotyledons, flowers, stem and leaves. Using in situ PCR techniques we could localise the K(+) channel mRNA in the phloem. In the stem VFK1 expression levels were higher in the lower internodes. There channel transcripts increased in the light and thus under conditions of increased photosynthate allocation. VFK1 transcripts are elevated in sink leaves, and rise in source leaves during the experimental transition into sinks. Fructose- rather than sucrose- or glucose-feeding via the petiole induced VFK1 gene activity. We therefore monitored the fructose sensitivity of the sieve tube potential through cut aphid stylets. In response to an 1 h fructose treatment the sieve tube potential shift increased from 19 mV to 53 mV per 10-fold change in K(+) concentration. Under these conditions K(+) channels dominated the electrical properties of the plasma membrane. Based on the phloem localisation and expression patterns of VFK1 we conclude that this K(+) channel is involved in sugar unloading and K(+) retrieval.