AtKC1, a conditionally targeted Shaker-type subunit, regulates the activity of plant K+ channels

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.

Calcium signalling components underlying NPK homeostasis: potential avenues for exploration

Plants require the major macronutrients, nitrogen (N), phosphorus (P) and potassium (K) for normal growth and development. Their deficiency in soil directly affects vital cellular processes, particularly root growth and architecture. Their perception, uptake and assimilation are regulated by complex signalling pathways. To overcome nutrient deficiencies, plants have developed certain response mechanisms that determine developmental and physiological adaptations. The signal transduction pathways underlying these responses involve a complex interplay of components such as nutrient transporters, transcription factors and others. In addition to their involvement in cross-talk with intracellular calcium signalling pathways, these components are also engaged in NPK sensing and homeostasis. The NPK sensing and homeostatic mechanisms hold the key to identify and understand the crucial players in nutrient regulatory networks in plants under both abiotic and biotic stresses. In this review, we discuss calcium signalling components/pathways underlying plant responses to NPK sensing, with a focus on the sensors, transporters and transcription factors involved in their respective signalling and homeostasis.

Molecular membrane separation: plants inspire new technologies

Plants draw up their surrounding soil solution to gain water and nutrients required for growth, development and reproduction. Obtaining adequate water and nutrients involves taking up both desired and undesired elements from the soil solution and separating resources from waste. Desirable and undesirable elements in the soil solution can share similar chemical properties, such as size and charge. Plants use membrane separation mechanisms to distinguish between different molecules that have similar chemical properties. Membrane separation enables distribution or retention of resources and efflux or compartmentation of waste. Plants use specialised membrane separation mechanisms to adapt to challenging soil solution compositions and distinguish between resources and waste. Coordination and regulation of these mechanisms between different tissues, cell types and subcellular membranes supports plant nutrition, environmental stress tolerance and energy management. This review considers membrane separation mechanisms in plants that contribute to specialised separation processes and highlights mechanisms of interest for engineering plants with enhanced performance in challenging conditions and for inspiring the development of novel industrial membrane separation technologies. Knowledge gained from studying plant membrane separation mechanisms can be applied to developing precision separation technologies. Separation technologies are needed for harvesting resources from industrial wastes and transitioning to a circular green economy.

Unfolding molecular switches for salt stress resilience in soybean: recent advances and prospects for salt-tolerant smart plant production

The increasing sodium salts (NaCl, NaHCO3, NaSO4 etc.) in agricultural soil is a serious global concern for sustainable agricultural production and food security. Soybean is an important food crop, and their cultivation is severely challenged by high salt concentration in soils. Classical transgenic and innovative breeding technologies are immediately needed to engineer salt tolerant soybean plants. Additionally, unfolding the molecular switches and the key components of the soybean salt tolerance network are crucial for soybean salt tolerance improvement. Here we review our understandings of the core salt stress response mechanism in soybean. Recent findings described that salt stress sensing, signalling, ionic homeostasis (Na⁺/K⁺) and osmotic stress adjustment might be important in regulating the soybean salinity stress response. We also evaluated the importance of antiporters and transporters such as Arabidopsis K⁺ Transporter 1 (AKT1) potassium channel and the impact of epigenetic modification on soybean salt tolerance. We also review key phytohormones, and osmo-protectants and their role in salt tolerance in soybean. In addition, we discuss the progress of omics technologies for identifying salt stress responsive molecular switches and their targeted engineering for salt tolerance in soybean. This review summarizes recent progress in soybean salt stress functional genomics and way forward for molecular breeding for developing salt-tolerant soybean plant.

Multiple ALMT subunits combine to form functional anion channels: A case study for rice ALMT7

The Aluminum Activated Malate Transporter (ALMT) family members are anion channels that play important roles in organic acid transport, stress resistance, growth, development, fertilization and GABA responses. The rice malate permeable OsALMT7 influences panicle development and grain yield. A truncated OsALMT7 mutant, panicle apical abortion1 (paab1) lacking at least 2 transmembrane helices, mediates reduced malate efflux resulting in yield reducing. Here, we further investigated the contribution of OsALMT7 transmembrane helices to channel activity, using heterologous expression in Xenopus laevis oocytes. We further found that OsALMT7 formed as a homomer by co-expressing OsALMT7 and paab1 proteins in oocytes and detecting the physical interaction between two OsALMT7, and between OsALMT7 and paab1 mutant protein. Further study proved that not just OsALMT7, mutants of TaALMT1 inhibit wild-type TaALMT1 channel, indicating that ALMTs might perform channel function as homomers. Our discovery brings a light for ion channel structure and homomultimer regulation understanding for ALMT anion channels and potential for crop grain yield and stress response improvement in the context of the essential role of ALMTs in these plant processes.

Structural basis for the activity regulation of a potassium channel AKT1 from Arabidopsis

The voltage-gated potassium channel AKT1 is responsible for primary K⁺ uptake in Arabidopsis roots. AKT1 is functionally activated through phosphorylation and negatively regulated by a potassium channel α-subunit AtKC1. However, the molecular basis for the modulation mechanism remains unclear. Here we report the structures of AKT1, phosphorylated-AKT1, a constitutively-active variant, and AKT1-AtKC1 complex. AKT1 is assembled in 2-fold symmetry at the cytoplasmic domain. Such organization appears to sterically hinder the reorientation of C-linkers during ion permeation. Phosphorylated-AKT1 adopts an alternate 4-fold symmetric conformation at cytoplasmic domain, which indicates conformational changes associated with symmetry switch during channel activation. To corroborate this finding, we perform structure-guided mutagenesis to disrupt the dimeric interface and identify a constitutively-active variant Asp379Ala mediates K⁺ permeation independently of phosphorylation. This variant predominantly adopts a 4-fold symmetric conformation. Furthermore, the AKT1-AtKC1 complex assembles in 2-fold symmetry. Together, our work reveals structural insight into the regulatory mechanism for AKT1.

Arabidopsis thaliana potassium channel AKT1 is responsible for primary K + uptake from soil, which is functionally activated through phosphorylation and negatively regulated by an α-subunit AtKC1. Here, the authors report the structures of AKT1 at different states, revealing a 2- fold to 4-fold symmetry switch at cytoplasmic domain associated with AKT1 activity regulation.

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.

Genomic & structural diversity and functional role of potassium (K+) transport proteins in plants

Potassium (K⁺) is an essential macronutrient for plant growth and productivity. It is the most abundant cation in plants and is involved in various cellular processes. Variable K⁺ availability is sensed by plant roots, consequently K⁺ transport proteins are activated to optimize K⁺ uptake. In addition to K⁺ uptake and translocation these proteins are involved in other important physiological processes like transmembrane voltage regulation, polar auxin transport, maintenance of Na⁺/K⁺ ratio and stomata movement during abiotic stress responses. K⁺ transport proteins display tremendous genomic and structural diversity in plants. Their key structural features, such as transmembrane domains, N-terminal domains, C-terminal domains and loops determine their ability of K⁺ uptake and transport and thus, provide functional diversity. Most K⁺ transporters are regulated at transcriptional and post-translational levels. Genetic manipulation of key K⁺ transporters/channels could be a prominent strategy for improving K⁺ utilization efficiency (KUE) in plants. This review discusses the genomic and structural diversity of various K⁺ transport proteins in plants. Also, an update on the function of K⁺ transport proteins and their regulatory mechanism in response to variable K⁺ availability, in improving KUE, biotic and abiotic stresses is provided.

Non-autonomous stomatal control by pavement cell turgor via the K+ channel subunit AtKC1

Stomata optimize land plants' photosynthetic requirements and limit water vapor loss. So far, all of the molecular and electrical components identified as regulating stomatal aperture are produced, and operate, directly within the guard cells. However, a completely autonomous function of guard cells is inconsistent with anatomical and biophysical observations hinting at mechanical contributions of epidermal origins. Here, potassium (K+) assays, membrane potential measurements, microindentation, and plasmolysis experiments provide evidence that disruption of the Arabidopsis thaliana K+ channel subunit gene AtKC1 reduces pavement cell turgor, due to decreased K+ accumulation, without affecting guard cell turgor. This results in an impaired back pressure of pavement cells onto guard cells, leading to larger stomatal apertures. Poorly rectifying membrane conductances to K+ were consistently observed in pavement cells. This plasmalemma property is likely to play an essential role in K+ shuttling within the epidermis. Functional complementation reveals that restoration of the wild-type stomatal functioning requires the expression of the transgenic AtKC1 at least in the pavement cells and trichomes. Altogether, the data suggest that AtKC1 activity contributes to the building of the back pressure that pavement cells exert onto guard cells by tuning K+ distribution throughout the leaf epidermis.

The bare necessities of plant K+ channel regulation

Potassium (K+) channels serve a wide range of functions in plants from mineral nutrition and osmotic balance to turgor generation for cell expansion and guard cell aperture control. Plant K+ channels are members of the superfamily of voltage-dependent K+ channels, or Kv channels, that include the Shaker channels first identified in fruit flies (Drosophila melanogaster). Kv channels have been studied in depth over the past half century and are the best-known of the voltage-dependent channels in plants. Like the Kv channels of animals, the plant Kv channels are regulated over timescales of milliseconds by conformational mechanisms that are commonly referred to as gating. Many aspects of gating are now well established, but these channels still hold some secrets, especially when it comes to the control of gating. How this control is achieved is especially important, as it holds substantial prospects for solutions to plant breeding with improved growth and water use efficiencies. Resolution of the structure for the KAT1 K+ channel, the first channel from plants to be crystallized, shows that many previous assumptions about how the channels function need now to be revisited. Here, I strip the plant Kv channels bare to understand how they work, how they are gated by voltage and, in some cases, by K+ itself, and how the gating of these channels can be regulated by the binding with other protein partners. Each of these features of plant Kv channels has important implications for plant physiology.

The protein kinase SlCIPK23 boosts K+ and Na+ uptake in tomato plants

Regulation of root transport systems is essential under fluctuating nutrient supply. In the case of potassium (K⁺ ), HAK/KUP/KT K⁺ transporters and voltage-gated K⁺ channels ensure root K⁺ uptake in a wide range of K⁺ concentrations. In Arabidopsis, the CIPK23/CBL1-9 complex regulates both transporter- and channel-mediated root K⁺ uptake. However, research about K⁺ homeostasis in crops is in demand due to species-specific mechanisms. In the present manuscript, we studied the contribution of the voltage-gated K⁺ channel LKT1 and the protein kinase SlCIPK23 to K⁺ uptake in tomato plants by analysing gene-edited knockout tomato mutant lines, together with two-electrode voltage-clamp experiments in Xenopus oocytes and protein-protein interaction analyses. It is shown that LKT1 is a crucial player in tomato K⁺ nutrition by contributing approximately 50% to root K⁺ uptake under K⁺ -sufficient conditions. Moreover, SlCIPK23 was responsible for approximately 100% of LKT1 and approximately 40% of the SlHAK5 K⁺ transporter activity in planta. Mg⁺² and Na⁺ compensated for K⁺ deficit in tomato roots to a large extent, and the accumulation of Na⁺ was strongly dependent on SlCIPK23 function. The role of CIPK23 in Na⁺ accumulation in tomato roots was not conserved in Arabidopsis, which expands the current set of CIPK23-like protein functions in plants.

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).

Adjustment of K+ Fluxes and Grapevine Defense in the Face of Climate Change

Grapevine is one of the most economically important fruit crops due to the high value of its fruit and its importance in winemaking. The current decrease in grape berry quality and production can be seen as the consequence of various abiotic constraints imposed by climate changes. Specifically, produced wines have become too sweet, with a stronger impression of alcohol and fewer aromatic qualities. Potassium is known to play a major role in grapevine growth, as well as grape composition and wine quality. Importantly, potassium ions (K+) are involved in the initiation and maintenance of the berry loading process during ripening. Moreover, K+ has also been implicated in various defense mechanisms against abiotic stress. The first part of this review discusses the main negative consequences of the current climate, how they disturb the quality of grape berries at harvest and thus ultimately compromise the potential to obtain a great wine. In the second part, the essential electrical and osmotic functions of K+, which are intimately dependent on K+ transport systems, membrane energization, and cell K+ homeostasis, are presented. This knowledge will help to select crops that are better adapted to adverse environmental conditions.

Potassium physiology from Archean to Holocene: A higher-plant perspective

In this paper, we discuss biological potassium acquisition and utilization processes over an evolutionary timescale, with emphasis on modern vascular plants. The quintessential osmotic and electrical functions of the K⁺ ion are shown to be intimately tied to K⁺-transport systems and membrane energization. Several prominent themes in plant K⁺-transport physiology are explored in greater detail, including: (1) channel mediated K⁺ acquisition by roots at low external [K⁺]; (2) K⁺ loading of root xylem elements by active transport; (3) variations on the theme of K⁺ efflux from root cells to the extracellular environment; (4) the veracity and utility of the "affinity" concept in relation to transport systems. We close with a discussion of the importance of plant-potassium relations to our human world, and current trends in potassium nutrition from farm to table.

Potential Networks of Nitrogen-Phosphorus-Potassium Channels and Transporters in Arabidopsis Roots at a Single Cell Resolution

Nitrogen (N), phosphorus (P), and potassium (K) are three major macronutrients essential for plant life. These nutrients are acquired and transported by several large families of transporters expressed in plant roots. However, it remains largely unknown how these transporters are distributed in different cell-types that work together to transfer the nutrients from the soil to different layers of root cells and eventually reach vasculature for massive flow. Using the single cell transcriptomics data from Arabidopsis roots, we profiled the transcriptional patterns of putative nutrient transporters in different root cell-types. Such analyses identified a number of uncharacterized NPK transporters expressed in the root epidermis to mediate NPK uptake and distribution to the adjacent cells. Some transport genes showed cortex- and endodermis-specific expression to direct the nutrient flow toward the vasculature. For long-distance transport, a variety of transporters were shown to express and potentially function in the xylem and phloem. In the context of subcellular distribution of mineral nutrients, the NPK transporters at subcellular compartments were often found to show ubiquitous expression patterns, which suggests function in house-keeping processes. Overall, these single cell transcriptomic analyses provide working models of nutrient transport from the epidermis across the cortex to the vasculature, which can be further tested experimentally in the future.

Potassium and phosphorus transport and signaling in plants

Nitrogen (N), potassium (K), and phosphorus (P) are essential macronutrients for plant growth and development, and their availability affects crop yield. Compared with N, the relatively low availability of K and P in soils limits crop production and thus threatens food security and agricultural sustainability. Improvement of plant nutrient utilization efficiency provides a potential route to overcome the effects of K and P deficiencies. Investigation of the molecular mechanisms underlying how plants sense, absorb, transport, and use K and P is an important prerequisite to improve crop nutrient utilization efficiency. In this review, we summarize current understanding of K and P transport and signaling in plants, mainly taking Arabidopsis thaliana and rice (Oryza sativa) as examples. We also discuss the mechanisms coordinating transport of N and K, as well as P and N.

How to Grow a Tree: Plant Voltage-Dependent Cation Channels in the Spotlight of Evolution

Phylogenetic analysis can be a powerful tool for generating hypotheses regarding the evolution of physiological processes. Here, we provide an updated view of the evolution of the main cation channels in plant electrical signalling: the Shaker family of voltage-gated potassium channels and the two-pore cation (K⁺) channel (TPC1) family. Strikingly, the TPC1 family followed the same conservative evolutionary path as one particular subfamily of Shaker channels (K_out) and remained highly invariant after terrestrialisation, suggesting that electrical signalling was, and remains, key to survival on land. We note that phylogenetic analyses can have pitfalls, which may lead to erroneous conclusions. To avoid these in the future, we suggest guidelines for analyses of ion channel evolution in plants.

Identification of Shaker K+ channel family members in sweetpotato and functional exploration of IbAKT1

The Shaker K⁺ channel family plays a vital role in potassium absorption and stress resistance in plants. However little information on the genes family is available about sweetpotato. In the present study, eleven sweetpotato Shaker K⁺ channel genes were identified and classified into five groups based on phylogenetic relationships, conserved motifs, and gene structure analyses. Based on synteny analysis, four duplicated gene pairs were identified, derived from both ancient and recent duplication, whereas only one resulted from tandem duplication events. Different expression pattern of Shaker K⁺ channel genes in roots of Xu32 and NZ1 resulted in different K⁺ deficiency tolerances, suggesting there is different mechanism of K⁺ uptake in sweetpotato cultivars with different K⁺-tolerance levels. Quantitative real-time PCR analysis revealed that the shaker K⁺ channel genes responded to drought and high salt stresses. Higher K⁺ influx under normal condition and lower K⁺ efflux under K⁺ deficiency stress were observed in IbAKT1 overexpressing transgenic roots than in adventitious roots, which indicated that IbAKT1 may play an important role in the regulation of K⁺ deficiency tolerance in sweetpotato. This is the first genome-wide analysis of Shaker K⁺ channel genes and the first functional analysis of IbAKT1 in sweetpotato. Our results provide valuable information on the gene structure, evolution, expression and functions of the Shaker K⁺ channel gene family in sweetpotato.

Functional characterization and physiological roles of the single Shaker outward K+ channel in Medicago truncatula

The model legume Medicago truncatula possesses a single outward Shaker K⁺ channel, whereas Arabidopsis thaliana possesses two channels of this type, named AtSKOR and AtGORK, with AtSKOR having been shown to play a major role in K⁺ secretion into the xylem sap in the root vasculature and with AtGORK being shown to mediate the efflux of K⁺ across the guard cell membrane, leading to stomatal closure. Here we show that the expression pattern of the single M. truncatula outward Shaker channel, which has been named MtGORK, includes the root vasculature, guard cells and root hairs. As shown by patch-clamp experiments on root hair protoplasts, besides the Shaker-type slowly activating outwardly rectifying K⁺ conductance encoded by MtGORK, a second K⁺ -permeable conductance, displaying fast activation and weak rectification, can be expressed by M. truncatula. A knock-out (KO) mutation resulting in an absence of MtGORK activity is shown to weakly reduce K⁺ translocation to shoots, and only in plants engaged in rhizobial symbiosis, but to strongly affect the control of stomatal aperture and transpirational water loss. In legumes, the early electrical signaling pathway triggered by Nod-factor perception is known to comprise a short transient depolarization of the root hair plasma membrane. In the absence of the functional expression of MtGORK, the rate of the membrane repolarization is found to be decreased by a factor of approximately two. This defect was without any consequence on infection thread development and nodule production in plants grown in vitro, but a decrease in nodule production was observed in plants grown in soil.

Tale of tail current

The largest biomass of channel proteins is located in unicellular organisms and bacteria that have no organs. However, orchestrated bidirectional ionic currents across the cell membrane via the channels are important for the functioning of organs of organisms, and equally concern both fauna or flora. Several ion channels are activated in the course of action potentials. One of the hallmarks of voltage-dependent channels is a 'tail current' - deactivation as observed after prior and sufficient activation predominantly at more depolarized potentials e.g. for Kv while upon hyperpolarization for HCN α subunits. Tail current also reflects the timing of channel closure that is initiated upon termination of stimuli. Finally, deactivation of currents during repolarization could be a selective estimate for given channel as in case of HERG, if dedicated long and more depolarized 'tail pulse' is used. Since from a holding potential of e.g. -70 mV are often a family of outward K⁺ currents comprising I_A and I_K are simultaneously activated in native cells.

A repertoire of cationic and anionic conductances at the plasma membrane of Medicago truncatula root hairs

Root hairs, as lateral extensions of epidermal cells, provide large absorptive surfaces to the root and are major actors in plant hydromineral nutrition. In contact with the soil they also constitute a site of interactions between the plant and rhizospheric microorganisms. In legumes, initiation of symbiotic interactions with N₂ -fixing rhizobia is often triggered at the root hair cell membrane in response to nodulation factors secreted by rhizobia, and involves early signaling events with changes in H⁺ , Ca²⁺ , K⁺ and Cl^- fluxes inducing transient depolarization of the cell membrane. Here, we aimed to build a functional repertoire of the major root hair conductances to cations and anions in the sequenced legume model Medicago truncatula. Five root hair conductances were characterized through patch-clamp experiments on enzymatically recovered root hair protoplasts. These conductances displayed varying properties of voltage dependence, kinetics and ion selectivity. They consisted of hyperpolarization- and depolarization-activated conductances for K⁺ , cations or Cl^- . Among these, one weakly outwardly rectifying cationic conductance and one hyperpolarization-activated slowly inactivating anionic conductance were not known as active in root hairs. All five conductances were detected in apical regions of young growing root hairs using membrane spheroplasts obtained by laser-assisted cell-wall microdissection. Combined with recent root hair transcriptomes of M. truncatula, this functional repertoire of conductances is expected to help the identification of candidate genes for reverse genetics studies to investigate the possible role of each conductance in root hair growth and interaction with the biotic and abiotic environment.

Regulation of K+ Nutrition in Plants

Modern agriculture relies on mineral fertilization. Unlike other major macronutrients, potassium (K+) is not incorporated into organic matter but remains as soluble ion in the cell sap contributing up to 10% of the dry organic matter. Consequently, K+ constitutes a chief osmoticum to drive cellular expansion and organ movements, such as stomata aperture. Moreover, K+ transport is critical for the control of cytoplasmic and luminal pH in endosomes, regulation of membrane potential, and enzyme activity. Not surprisingly, plants have evolved a large ensemble of K+ transporters with defined functions in nutrient uptake by roots, storage in vacuoles, and ion translocation between tissues and organs. This review describes critical transport proteins governing K+ nutrition, their regulation, and coordinated activity, and summarizes our current understanding of signaling pathways activated by K+ starvation.

The Complex Fine-Tuning of K⁺ Fluxes in Plants in Relation to Osmotic and Ionic Abiotic Stresses

As the main cation in plant cells, potassium plays an essential role in adaptive responses, especially through its involvement in osmotic pressure and membrane potential adjustments. K⁺ homeostasis must, therefore, be finely controlled. As a result of different abiotic stresses, especially those resulting from global warming, K⁺ fluxes and plant distribution of this ion are disturbed. The hormone abscisic acid (ABA) is a key player in responses to these climate stresses. It triggers signaling cascades that ultimately lead to modulation of the activities of K⁺ channels and transporters. After a brief overview of transcriptional changes induced by abiotic stresses, this review deals with the post-translational molecular mechanisms in different plant organs, in Arabidopsis and species of agronomical interest, triggering changes in K⁺ uptake from the soil, K⁺ transport and accumulation throughout the plant, and stomatal regulation. These modifications involve phosphorylation/dephosphorylation mechanisms, modifications of targeting, and interactions with regulatory partner proteins. Interestingly, many signaling pathways are common to K⁺ and Cl^-/NO3^- counter-ion transport systems. These cross-talks are also addressed.

Regulation of K+ Nutrition in Plants

Modern agriculture relies on mineral fertilization. Unlike other major macronutrients, potassium (K⁺) is not incorporated into organic matter but remains as soluble ion in the cell sap contributing up to 10% of the dry organic matter. Consequently, K⁺ constitutes a chief osmoticum to drive cellular expansion and organ movements, such as stomata aperture. Moreover, K⁺ transport is critical for the control of cytoplasmic and luminal pH in endosomes, regulation of membrane potential, and enzyme activity. Not surprisingly, plants have evolved a large ensemble of K⁺ transporters with defined functions in nutrient uptake by roots, storage in vacuoles, and ion translocation between tissues and organs. This review describes critical transport proteins governing K⁺ nutrition, their regulation, and coordinated activity, and summarizes our current understanding of signaling pathways activated by K⁺ starvation.

Cesium Inhibits Plant Growth Primarily Through Reduction of Potassium Influx and Accumulation in Arabidopsis

Cesium (Cs+) is known to compete with the macronutrient potassium (K+) inside and outside of plants and to inhibit plant growth at high concentrations. However, the detailed molecular mechanisms of how Cs+ exerts its deleterious effects on K+ accumulation in plants are not fully elucidated. Here, we show that mutation in a member of the major K+ channel AKT1-KC1 complex renders Arabidopsis thaliana hypersensitive to Cs+. Higher severity of the phenotype and K+ loss were observed for these mutants in response to Cs+ than to K+ deficiency. Electrophysiological analysis demonstrated that Cs+, but not sodium, rubidium or ammonium, specifically inhibited K+ influx through the AKT1-KC1 complex. In contrast, Cs+ did not inhibit K+ efflux through the homomeric AKT1 channel that occurs in the absence of KC1, leading to a vast loss of K+. Our observation suggests that reduced K+ accumulation due to blockage/competition in AKT1 and other K+ transporters/channels by Cs+ plays a major role in plant growth retardation. This report describes the mechanical role of Cs+ in K+ accumulation, and in turn in plant performance, providing actual evidence at the plant level for what has long been believed, i.e. K+ channels are, therefore AKT1 is, 'blocked' by Cs+.

The K+ channel KZM2 is involved in stomatal movement by modulating inward K+ currents in maize guard cells

Stomata are the major gates in plant leaf that allow water and gas exchange, which is essential for plant transpiration and photosynthesis. Stomatal movement is mainly controlled by the ion channels and transporters in guard cells. In Arabidopsis, the inward Shaker K⁺ channels, such as KAT1 and KAT2, are responsible for stomatal opening. However, the characterization of inward K⁺ channels in maize guard cells is limited. In the present study, we identified two KAT1-like Shaker K⁺ channels, KZM2 and KZM3, which were highly expressed in maize guard cells. Subcellular analysis indicated that KZM2 and KZM3 can localize at the plasma membrane. Electrophysiological characterization in HEK293 cells revealed that both KZM2 and KZM3 were inward K⁺ (K_in ) channels, but showing distinct channel kinetics. When expressed in Xenopus oocytes, only KZM3, but not KZM2, can mediate inward K⁺ currents. However, KZM2 can interact with KZM3 forming heteromeric K_in channel. In oocytes, KZM2 inhibited KZM3 channel conductance and negatively shifted the voltage dependence of KZM3. The activation of KZM2-KZM3 heteromeric channel became slower than the KZM3 channel. Patch-clamping results showed that the inward K⁺ currents of maize guard cells were significantly increased in the KZM2 RNAi lines. In addition, the RNAi lines exhibited faster stomatal opening after light exposure. In conclusion, the presented results demonstrate that KZM2 functions as a negative regulator to modulate the K_in channels in maize guard cells. KZM2 and KZM3 may form heteromeric K_in channel and control stomatal opening in maize.

Transport and homeostasis of potassium and phosphate: limiting factors for sustainable crop production

Potassium (K) and phosphate (Pi) are both macronutrients essential for plant growth and crop production, but the unrenewable resources of phosphorus rock and potash have become limiting factors for food security. One critical measure to help solve this problem is to improve nutrient use efficiency (NUE) in plants by understanding and engineering genetic networks for ion uptake, translocation, and storage. Plants have evolved multiple systems to adapt to various nutrient conditions for growth and production. Within the NUE networks, transport proteins and their regulators are the primary players for maintaining nutrient homeostasis and could be utilized to engineer high NUE traits in crop plants. A large number of publications have detailed K+ and Pi transport proteins in plants over the past three decades. Meanwhile, the discovery and validation of their regulatory mechanisms are fast-track topics for research. Here, we provide an overview of K+ and Pi transport proteins and their regulatory mechanisms, which participate in the uptake, translocation, storage, and recycling of these nutrients in plants.

Commandeering Channel Voltage Sensors for Secretion, Cell Turgor, and Volume Control

Control of cell volume and osmolarity is central to cellular homeostasis in all eukaryotes. It lies at the heart of the century-old problem of how plants regulate turgor, mineral and water transport. Plants use strongly electrogenic H⁺-ATPases, and the substantial membrane voltages they foster, to drive solute accumulation and generate turgor pressure for cell expansion. Vesicle traffic adds membrane surface and contributes to wall remodelling as the cell grows. Although a balance between vesicle traffic and ion transport is essential for cell turgor and volume control, the mechanisms coordinating these processes have remained obscure. Recent discoveries have now uncovered interactions between conserved subsets of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins that drive the final steps in secretory vesicle traffic and ion channels that mediate in inorganic solute uptake. These findings establish the core of molecular links, previously unanticipated, that coordinate cellular homeostasis and cell expansion.

PIP Water Transport and Its pH Dependence Are Regulated by Tetramer Stoichiometry

Many plasma membrane channels form oligomeric assemblies, and heterooligomerization has been described as a distinctive feature of some protein families. In the particular case of plant plasma membrane aquaporins (PIPs), PIP1 and PIP2 monomers interact to form heterotetramers. However, the biological properties of the different heterotetrameric configurations formed by PIP1 and PIP2 subunits have not been addressed yet. Upon coexpression of tandem PIP2-PIP1 dimers in Xenopus oocytes, we can address, for the first time to our knowledge, the functional properties of single heterotetrameric species having 2:2 stoichiometry. We have also coexpressed PIP2-PIP1 dimers with PIP1 and PIP2 monomers to experimentally investigate the localization and biological activity of each tetrameric assembly. Our results show that PIP2-PIP1 heterotetramers can assemble with 3:1, 1:3, or 2:2 stoichiometry, depending on PIP1 and PIP2 relative expression in the cell. All PIP2-PIP1 heterotetrameric species localize at the plasma membrane and present the same water transport capacity. Furthermore, the contribution of any heterotetrameric assembly to the total water transport through the plasma membrane doubles the contribution of PIP2 homotetramers. Our results also indicate that plasma membrane water transport can be modulated by the coexistence of different tetrameric species and by intracellular pH. Moreover, all the tetrameric species present similar cooperativity behavior for proton sensing. These findings throw light on the functional properties of PIP tetramers, showing that they have flexible stoichiometry dependent on the quantity of PIP1 and PIP2 molecules available. This represents, to our knowledge, a novel regulatory mechanism to adjust water transport across the plasma membrane.

Single Mutations in the Transmembrane Domains of Maize Plasma Membrane Aquaporins Affect the Activity of Monomers within a Heterotetramer

Aquaporins are channels facilitating the diffusion of water and/or small uncharged solutes across biological membranes. They assemble as homotetramers but some of them also form heterotetramers, especially in plants. In Zea mays, aquaporins belonging to the plasma membrane intrinsic protein (PIP) subfamily are clustered into two groups, PIP1 and PIP2, which exhibit different water-channel activities when expressed in Xenopus oocytes. When PIP1 and PIP2 isoforms are co-expressed, they physically interact to modulate their subcellular localization and channel activity. Here, we demonstrated by affinity chromatography purification that, when co-expressed in Xenopus oocytes, the maize PIP1;2 and PIP2;5 isoforms assemble as homo- and heterodimers within heterotetramers. We built the 3D structure of such heterotetramers by comparative modeling on the basis of the spinach SoPIP2;1 X-ray structure and identified amino acid residues in the transmembrane domains which putatively interact at the interfaces between monomers. Their roles in the water-channel activity, subcellular localization, protein abundance, and physical interaction were investigated by mutagenesis. We highlighted single-residue substitutions that either inactivated PIP2;5 or activated PIP1;2 without affecting their interaction. Interestingly, the Phe220Ala mutation in the transmembrane domain 5 of PIP1;2 activated its water-channel activity and, at the same time, inactivated PIP2;5 within a heterotetramer. Altogether, these data contribute to a better understanding of the interaction mechanisms between PIP isoforms and the role of heterotetramerization on their water-channel activity.

Overexpression of the rice AKT1 potassium channel affects potassium nutrition and rice drought tolerance

Potassium (K(+)) is the most important cationic nutrient for all living organisms and has roles in most aspects of plant physiology. To assess the impact of one of the main K(+) uptake components, the K(+) inward rectifying channel AKT1, we characterized both loss of function and overexpression of OsAKT1 in rice. In many conditions, AKT1 expression correlated with K(+) uptake and tissue K(+) levels. No salinity-related growth phenotype was observed for either loss or gain of function mutants. However, a correlation between AKT1 expression and root Na(+) when the external Na/K ratio was high suggests that there may be a role for AKT1 in Na(+) uptake in such conditions. In contrast to findings with Arabidopsis thaliana, we did not detect any change in growth of AKT1 loss of function mutants in the presence of NH4 (+) Nevertheless, NH4 (+)-dependent inhibition was detected during K(+) uptake assays in loss of function and wild type plants, depending on pre-growth conditions. The most prominent result of OsAKT1 overexpression was a reduction in sensitivity to osmotic/drought stress in transgenic plants: the data suggest that AKT1 overexpression improved rice osmotic and drought stress tolerance by increasing tissue levels of K(+), especially in the root.

Physiology of intracellular potassium channels: A unifying role as mediators of counterion fluxes?

Plasma membrane potassium channels importantly contribute to maintain ion homeostasis across the cell membrane. The view is emerging that also those residing in intracellular membranes play pivotal roles for the coordination of correct cell function. In this review we critically discuss our current understanding of the nature and physiological tasks of potassium channels in organelle membranes in both animal and plant cells, with a special emphasis on their function in the regulation of photosynthesis and mitochondrial respiration. In addition, the emerging role of potassium channels in the nuclear membranes in regulating transcription will be discussed. The possible functions of endoplasmic reticulum-, lysosome- and plant vacuolar membrane-located channels are also referred to. Altogether, experimental evidence obtained with distinct channels in different membrane systems points to a possible unifying function of most intracellular potassium channels in counterbalancing the movement of other ions including protons and calcium and modulating membrane potential, thereby fine-tuning crucial cellular processes. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-7, 2016', edited by Prof. Paolo Bernardi.

Ion Channels in Plant Bioenergetic Organelles, Chloroplasts and Mitochondria: From Molecular Identification to Function

Recent technical advances in electrophysiological measurements, organelle-targeted fluorescence imaging, and organelle proteomics have pushed the research of ion transport a step forward in the case of the plant bioenergetic organelles, chloroplasts and mitochondria, leading to the molecular identification and functional characterization of several ion transport systems in recent years. Here we focus on channels that mediate relatively high-rate ion and water flux and summarize the current knowledge in this field, focusing on targeting mechanisms, proteomics, electrophysiology, and physiological function. In addition, since chloroplasts evolved from a cyanobacterial ancestor, we give an overview of the information available about cyanobacterial ion channels and discuss the evolutionary origin of chloroplast channels. The recent molecular identification of some of these ion channels allowed their physiological functions to be studied using genetically modified Arabidopsis plants and cyanobacteria. The view is emerging that alteration of chloroplast and mitochondrial ion homeostasis leads to organelle dysfunction, which in turn significantly affects the energy metabolism of the whole organism. Clear-cut identification of genes encoding for channels in these organelles, however, remains a major challenge in this rapidly developing field. Multiple strategies including bioinformatics, cell biology, electrophysiology, use of organelle-targeted ion-sensitive probes, genetics, and identification of signals eliciting specific ion fluxes across organelle membranes should provide a better understanding of the physiological role of organellar channels and their contribution to signaling pathways in plants in the future.

Constant change: dynamic regulation of membrane transport by calcium signalling networks keeps plants in tune with their environment

Despite substantial variation and irregularities in their environment, plants must conform to spatiotemporal demands on the molecular composition of their cytosol. Cell membranes are the major interface between organisms and their environment and the basis for controlling the contents and intracellular organization of the cell. Membrane transport proteins (MTPs) govern the flow of molecules across membranes, and their activities are closely monitored and regulated by cell signalling networks. By continuously adjusting MTP activities, plants can mitigate the effects of environmental perturbations, but effective implementation of this strategy is reliant on precise coordination among transport systems that reside in distinct cell types and membranes. Here, we examine the role of calcium signalling in the coordination of membrane transport, with an emphasis on potassium transport. Potassium is an exceptionally abundant and mobile ion in plants, and plant potassium transport has been intensively studied for decades. Classic and recent studies have underscored the importance of calcium in plant environmental responses and membrane transport regulation. In reviewing recent advances in our understanding of the coding and decoding of calcium signals, we highlight established and emerging roles of calcium signalling in coordinating membrane transport among multiple subcellular locations and distinct transport systems in plants, drawing examples from the CBL-CIPK signalling network. By synthesizing classical studies and recent findings, we aim to provide timely insights on the role of calcium signalling networks in the modulation of membrane transport and its importance in plant environmental responses.

Comparison between Arabidopsis and Rice for Main Pathways of K(+) and Na(+) Uptake by Roots

K(+) is an essential macronutrient for plants. It is acquired by specific uptake systems located in roots. Although the concentrations of K(+) in the soil solution are widely variable, K(+) nutrition is secured by uptake systems that exhibit different affinities for K(+). Two main systems have been described for root K(+) uptake in several species: the high-affinity HAK5-like transporter and the inward-rectifier AKT1-like channel. Other unidentified systems may be also involved in root K(+) uptake, although they only seem to operate when K(+) is not limiting. The use of knock-out lines has allowed demonstrating their role in root K(+) uptake in Arabidopsis and rice. Plant adaptation to the different K(+) supplies relies on the finely tuned regulation of these systems. Low K(+)-induced transcriptional up-regulation of the genes encoding HAK5-like transporters occurs through a signal cascade that includes changes in the membrane potential of root cells and increases in ethylene and reactive oxygen species concentrations. Activation of AKT1 channels occurs through phosphorylation by the CIPK23/CBL1 complex. Recently, activation of the Arabidopsis HAK5 by the same complex has been reported, pointing to CIPK23/CBL as a central regulator of the plant's adaptation to low K(+). Na(+) is not an essential plant nutrient but it may be beneficial for some plants. At low concentrations, Na(+) improves growth, especially under K(+) deficiency. Thus, high-affinity Na(+) uptake systems have been described that belong to the HKT and HAK families of transporters. At high concentrations, typical of saline environments, Na(+) accumulates in plant tissues at high concentrations, producing alterations that include toxicity, water deficit and K(+) deficiency. Data concerning pathways for Na(+) uptake into roots under saline conditions are still scarce, although several possibilities have been proposed. The apoplast is a significant pathway for Na(+) uptake in rice grown under salinity conditions, but in other plant species different mechanisms involving non-selective cation channels or transporters are under discussion.

Increased phosphate transport of Arabidopsis thaliana Pht1;1 by site-directed mutagenesis of tyrosine 312 may be attributed to the disruption of homomeric interactions

Members of the Pht1 family of plant phosphate (Pi) transporters play vital roles in Pi acquisition from soil and in planta Pi translocation to maintain optimal growth and development. The study of the specificities and biochemical properties of Pht1 transporters will contribute to improving the current understanding of plant phosphorus homeostasis and use-efficiency. In this study, we show through split in vivo interaction methods and in vitro analysis of microsomal root tissues that Arabidopsis thaliana Pht1;1 and Pht1;4 form homomeric and heteromeric complexes. Transient and heterologous expression of the Pht1;1 variants, Pht1;1(Y312D), Pht1;1(Y312A) and Pht1;1(Y312F), was used to analyse the role of a putative Pi binding residue (Tyr 312) in Pht1;1 transporter oligomerization and function. The homomeric interaction among Pht1;1 proteins was disrupted by mutation of Tyr 312 to Asp, but not to Ala or Phe. In addition, the Pht1;1(Y312D) variant conferred enhanced Pi transport when expressed in yeast cells. In contrast, mutation of Tyr 312 to Ala or Phe did not affect Pht1;1 transport kinetics. Our study demonstrates that modifications to the Pht1;1 higher-order structure affects Pi transport, suggesting that oligomerization may serve as a regulatory mechanism for modulating Pi uptake.

The Arabidopsis R-SNARE VAMP721 Interacts with KAT1 and KC1 K+ Channels to Moderate K+ Current at the Plasma Membrane

SNARE (soluble N-ethylmaleimide-sensitive factor protein attachment protein receptor) proteins drive vesicle traffic, delivering membrane and cargo to target sites within the cell and at its surface. They contribute to cell homeostasis, morphogenesis, and pathogen defense. A subset of SNAREs, including the Arabidopsis thaliana SNARE SYP121, are known also to coordinate solute uptake via physical interactions with K(+) channels and to moderate their gating at the plasma membrane. Here, we identify a second subset of SNAREs that interact to control these K(+) channels, but with opposing actions on gating. We show that VAMPs (vesicle-associated membrane proteins), which target vesicles to the plasma membrane, also interact with and suppress the activities of the inward-rectifying K(+) channels KAT1 and KC1. Interactions were evident in yeast split-ubiquitin assays, they were recovered in vivo by ratiometric bimolecular fluorescence complementation, and they were sensitive to mutation of a single residue, Tyr-57, within the longin domain of VAMP721. Interaction was also recovered on exchange of the residue at this site in the homolog VAMP723, which normally localizes to the endoplasmic reticulum and otherwise did not interact. Functional analysis showed reduced channel activity and alterations in voltage sensitivity that are best explained by a physical interaction with the channel gates. These actions complement those of SYP121, a cognate SNARE partner of VAMP721, and lead us to propose that the channel interactions reflect a "hand-off" in channel control between the two SNARE proteins that is woven together with vesicle fusion.

Potassium transporter TRH1 subunits assemble regulating root-hair elongation autonomously from the cell fate determination pathway

Trichoblasts of trh1 plants form root-hair initiation sites that fail to undergo tip growth resulting in a tiny root-hair phenotype. TRH1 belongs to Arabidopsis KT/KUP/HAK potassium transporter family controlling root-hair growth and gravitropism. Double mutant combinations between trh1 and root-hair mutants affecting cell fate or root-hair initiation exhibited additive phenotypes, suggesting that TRH1 acts independently and developmentally downstream of root-hair initiation. Bimolecular Fluorescence Complementation (BiFC), upon TRH1-YFP(C) and TRH1-YFP(N) co-transformation into tobacco epidermal cells, led to fluorescence emission indicative of TRH1 subunit homodimerization. Yeast two-hybrid analysis revealed two types of interactions. The hydrophilic segment between the second and the third transmembrane domain extending from residues Q105 to T141 is competent for a relatively weak interaction, whereas the region at the C-terminal beyond the last transmembrane domain, extending from amino acids R565 to A729, strongly self-interacts. These domains likely facilitate the co-assembly of TRH1 subunits forming an active K(+) transport system within cellular membrane structures. The results support the role of TRH1 acting as a convergence point between the developmental root-hair pathway and the environmental/hormonal signaling pathway to preserve auxin homeostasis ensuring plant adaptation in changing environments.

Strategies for improving potassium use efficiency in plants

Potassium is a macronutrient that is crucial for healthy plant growth. Potassium availability, however, is often limited in agricultural fields and thus crop yields and quality are reduced. Therefore, improving the efficiency of potassium uptake and transport, as well as its utilization, in plants is important for agricultural sustainability. This review summarizes the current knowledge on the molecular mechanisms involved in potassium uptake and transport in plants, and the molecular response of plants to different levels of potassium availability. Based on this information, four strategies for improving potassium use efficiency in plants are proposed; 1) increased root volume, 2) increasing efficiency of potassium uptake from the soil and translocation in planta, 3) increasing mobility of potassium in soil, and 4) molecular breeding new varieties with greater potassium efficiency through marker assisted selection which will require identification and utilization of potassium associated quantitative trait loci.

The physiology of channel-mediated K+ acquisition in roots of higher plants

K(+) channels are among the best-characterized classes of membrane protein in plants. Nevertheless, in-planta demonstrations of traits emerging from molecular characterizations have often been insufficient or lacking altogether. Such linkages are, however, critical to our basic understanding of plant nutrition and to addressing 'real-world' issues that are faced in environmental and agricultural settings. Here, we cover some of the recent advances in K(+) acquisition with particular focus on voltage-gated K(+) channel functioning and regulation in roots, and highlight where linkages to in-planta behavior have been successfully made and, conversely, where such linkages are yet to be made.

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.

K+ uptake in plant roots. The systems involved, their regulation and parallels in other organisms

Potassium (K(+)) is an essential macronutrient for plants. It is taken into the plant by the transport systems present in the plasma membranes of root epidermal and cortical cells. The identity of these systems and their regulation is beginning to be understood and the systems of K(+) transport in the model species Arabidopsis thaliana remain far better characterized than in any other plant species. Roots can activate different K(+) uptake systems to adapt to their environment, important to a sessile organism that needs to cope with a highly variable environment. The mechanisms of K(+) acquisition in the model species A. thaliana are the best characterized at the molecular level so far. According to the current model, non-selective channels are probably the main pathways for K(+) uptake at high concentrations (>10mM), while at intermediate concentrations (1mM), the inward rectifying channel AKT1 dominates K(+) uptake. Under lower concentrations of external K(+) (100μM), AKT1 channels, together with the high-affinity K(+) uptake system HAK5 contribute to K(+) acquisition, and at extremely low concentrations (<10μM) the only system capable of taking up K(+) is HAK5. Depending on the species the high-affinity system has been named HAK5 or HAK1, but in all cases it fulfills the same functions. The activation of these systems as a function of the K(+) availability is achieved by different mechanisms that include phosphorylation of AKT1 or induction of HAK5 transcription. Some of the characteristics of the systems for root K(+) uptake are shared by other organisms, whilst others are specific to plants. This indicates that some crucial properties of the ancestral of K(+) transport systems have been conserved through evolution while others have diverged among different kingdoms.

Going beyond nutrition: regulation of potassium homoeostasis as a common denominator of plant adaptive responses to environment

Partially and fully completed plant genome sequencing projects in both lower and higher plants allow drawing a comprehensive picture of the molecular and structural diversities of plant potassium transporter genes and their encoded proteins. While the early focus of the research in this field was aimed on the structure-function studies and understanding of the molecular mechanisms underlying K(+) transport, availability of Arabidopsis thaliana mutant collections in combination with micro-array techniques have significantly advanced our understanding of K(+) channel physiology, providing novel insights into the transcriptional regulation of potassium homeostasis in plants. More recently, posttranslational regulation of potassium transport systems has moved into the center stage of potassium transport research. The current review is focused on the most exciting developments in this field. By summarizing recent work on potassium transporter regulation we show that potassium transport in general, and potassium channels in particular, represent important targets and are mediators of the cellular responses during different developmental stages in a plant's life cycle. We show that regulation of intracellular K(+) homeostasis is essential to mediate plant adaptive responses to a broad range of abiotic and biotic stresses including drought, salinity, and oxidative stress. We further link post-translational regulation of K(+) channels with programmed cell death and show that K(+) plays a critical role in controlling the latter process. Thus, is appears that K(+) is not just the essential nutrient required to support optimal plant growth and yield but is also an important signaling agent mediating a wide range of plant adaptive responses to environment.

Transport, signaling, and homeostasis of potassium and sodium in plants

Potassium (K⁺) is an essential macronutrient in plants and a lack of K⁺ significantly reduces the potential for plant growth and development. By contrast, sodium (Na⁺), while beneficial to some extent, at high concentrations it disturbs and inhibits various physiological processes and plant growth. Due to their chemical similarities, some functions of K⁺ can be undertaken by Na⁺ but K⁺ homeostasis is severely affected by salt stress, on the other hand. Recent advances have highlighted the fascinating regulatory mechanisms of K⁺ and Na⁺ transport and signaling in plants. This review summarizes three major topics: (i) the transport mechanisms of K⁺ and Na⁺ from the soil to the shoot and to the cellular compartments; (ii) the mechanisms through which plants sense and respond to K⁺ and Na⁺ availability; and (iii) the components involved in maintenance of K⁺/Na⁺ homeostasis in plants under salt stress.

Molecular mechanisms involved in plant adaptation to low K(+) availability

Potassium is a major inorganic constituent of the living cell and the most abundant cation in the cytosol. It plays a role in various functions at the cell level, such as electrical neutralization of anionic charges, protein synthesis, long- and short-term control of membrane polarization, and regulation of the osmotic potential. Through the latter function, K(+) is involved at the whole-plant level in osmotically driven functions such as cell movements, regulation of stomatal aperture, or phloem transport. Thus, plant growth and development require that large amounts of K(+) are taken up from the soil and translocated to the various organs. In most ecosystems, however, soil K(+) availability is low and fluctuating, so plants have developed strategies to take up K(+) more efficiently and preserve vital functions and growth when K(+) availability is becoming limited. These strategies include increased capacity for high-affinity K(+) uptake from the soil, K(+) redistribution between the cytosolic and vacuolar pools, ensuring cytosolic homeostasis, and modification of root system development and architecture. Our knowledge about the mechanisms and signalling cascades involved in these different adaptive responses has been rapidly growing during the last decade, revealing a highly complex network of interacting processes. This review is focused on the different physiological responses induced by K(+) deprivation, their underlying molecular events, and the present knowledge and hypotheses regarding the mechanisms responsible for K(+) sensing and signalling.

Distinct amino acids in the C-linker domain of the Arabidopsis K+ channel KAT2 determine its subcellular localization and activity at the plasma membrane

Shaker K(+) channels form the major K(+) conductance of the plasma membrane in plants. They are composed of four subunits arranged around a central ion-conducting pore. The intracellular carboxy-terminal region of each subunit contains several regulatory elements, including a C-linker region and a cyclic nucleotide-binding domain (CNBD). The C-linker is the first domain present downstream of the sixth transmembrane segment and connects the CNBD to the transmembrane core. With the aim of identifying the role of the C-linker in the Shaker channel properties, we performed subdomain swapping between the C-linker of two Arabidopsis (Arabidopsis thaliana) Shaker subunits, K(+) channel in Arabidopsis thaliana2 (KAT2) and Arabidopsis thaliana K(+) rectifying channel1 (AtKC1). These two subunits contribute to K(+) transport in planta by forming heteromeric channels with other Shaker subunits. However, they display contrasting behavior when expressed in tobacco mesophyll protoplasts: KAT2 forms homotetrameric channels active at the plasma membrane, whereas AtKC1 is retained in the endoplasmic reticulum when expressed alone. The resulting chimeric/mutated constructs were analyzed for subcellular localization and functionally characterized. We identified two contiguous amino acids, valine-381 and serine-382, located in the C-linker carboxy-terminal end, which prevent KAT2 surface expression when mutated into the equivalent residues from AtKC1. Moreover, we demonstrated that the nine-amino acid stretch 312TVRAASEFA320 that composes the first C-linker α-helix located just below the pore is a crucial determinant of KAT2 channel activity. A KAT2 C-linker/CNBD three-dimensional model, based on animal HCN (for Hyperpolarization-activated, cyclic nucleotide-gated K(+)) channels as structure templates, has been built and used to discuss the role of the C-linker in plant Shaker inward channel structure and function.

Applications of fluorescent marker proteins in plant cell biology

Over the past decade, confocal microscopy and the ever-expanding toolchest of fluorescent protein (xFP) markers and technologies have become routine methods for the biological laboratory. A common use of xFP fluorophores is in localizing proteins and the subcellular structures with which they associate, including analyzing their distribution and dynamics and the interactions of proteins in vivo. Additionally, a number of so-called optical highlighters have proven especially useful in analyzing the kinetics of these processes in pulse-chase studies of protein relocation(s) following an experimental challenge. Here we focus on exemplary methods in transformation and live-cell imaging in plant cells, with the expectation that researchers will find these and the accompanying resources useful as a starting point in developing their own expertise.

Calcineurin B-like protein CBL10 directly interacts with AKT1 and modulates K+ homeostasis in Arabidopsis

Potassium transporters and channels play crucial roles in K+ uptake and translocation in plant cells. These roles are essential for plant growth and development. AKT1 is an important K+ channel in Arabidopsis roots that is involved in K+ uptake. It is known that AKT1 is activated by a protein kinase CIPK23 interacting with two calcineurin B-like proteins CBL1/CBL9. The present study showed that another calcineurin B-like protein (CBL10) may also regulate AKT1 activity. The CBL10-over-expressing lines showed a phenotype as sensitive as that of the akt1 mutant under low-K+ conditions. In addition, the K+ content of both CBL10-over-expressing lines and akt1 mutant plants were significantly reduced compared with wild-type plants. Moreover, CBL10 directly interacted with AKT1, as verified in yeast two-hybrid, BiFC and co-immunoprecipitation experiments. The results of electrophysiological analysis in both Xenopus oocytes and Arabidopsis root cell protoplasts demonstrated that CBL10 impairs AKT1-mediated inward K+ currents. Furthermore, the results from the yeast two-hybrid competition assay indicated that CBL10 may compete with CIPK23 for binding to AKT1 and negatively modulate AKT1 activity. The present study revealed a CBL-interacting protein kinase-independent regulatory mechanism of calcineurin B-like proteins in which CBL10 directly regulates AKT1 activity and affects ion homeostasis in plant cells.

Potassium transport in developing fleshy fruits: the grapevine inward K(+) channel VvK1.2 is activated by CIPK-CBL complexes and induced in ripening berry flesh cells

The grape berry provides a model for investigating the physiology of non-climacteric fruits. Increased K(+) accumulation in the berry has a strong negative impact on fruit acidity (and quality). In maturing berries, we identified a K(+) channel from the Shaker family, VvK1.2, and two CBL-interacting protein kinase (CIPK)/calcineurin B-like calcium sensor (CBL) pairs, VvCIPK04-VvCBL01 and VvCIPK03-VvCBL02, that may control the activity of this channel. VvCBL01 and VvCIPK04 are homologues of Arabidopsis AtCBL1 and AtCIPK23, respectively, which form a complex that controls the activity of the Shaker K(+) channel AKT1 in Arabidopsis roots. VvK1.2 remained electrically silent when expressed alone in Xenopus oocytes, but gave rise to K(+) currents when co-expressed with the pairs VvCIPK03-VvCBL02 or VvCIPK04-VvCBL01, the second pair inducing much larger currents than the first one. Other tested CIPK-CBL pairs expressed in maturing berries were found to be unable to activate VvK1.2. When activated by its CIPK-CBL partners, VvK1.2 acts as a voltage-gated inwardly rectifying K(+) channel that is activated at voltages more negative than -100 mV and is stimulated upon external acidification. This channel is specifically expressed in the berry, where it displays a very strong induction at veraison (the inception of ripening) in flesh cells, phloem tissues and perivascular cells surrounding vascular bundles. Its expression in these tissues is further greatly increased upon mild drought stress. VvK1.2 is thus likely to mediate rapid K(+) transport in the berry and to contribute to the extensive re-organization of the translocation pathways and transport mechanisms that occurs at veraison.

Energization of vacuolar transport in plant cells and its significance under stress

The plant vacuole is of prime importance in buffering environmental perturbations and in coping with abiotic stress caused by, for example, drought, salinity, cold, or UV. The large volume, the efficient integration in anterograde and retrograde vesicular trafficking, and the dynamic equipment with tonoplast transporters enable the vacuole to fulfill indispensible functions in cell biology, for example, transient and permanent storage, detoxification, recycling, pH and redox homeostasis, cell expansion, biotic defence, and cell death. This review first focuses on endomembrane dynamics and then summarizes the functions, assembly, and regulation of secretory and vacuolar proton pumps: (i) the vacuolar H(+)-ATPase (V-ATPase) which represents a multimeric complex of approximately 800 kDa, (ii) the vacuolar H(+)-pyrophosphatase, and (iii) the plasma membrane H(+)-ATPase. These primary proton pumps regulate the cytosolic pH and provide the driving force for secondary active transport. Carriers and ion channels modulate the proton motif force and catalyze uptake and vacuolar compartmentation of solutes and deposition of xenobiotics or secondary compounds such as flavonoids. ABC-type transporters directly energized by MgATP complement the transport portfolio that realizes the multiple functions in stress tolerance of plants.