Evolution and Structural Characteristics of Plant Voltage-Gated K+ Channels

Increased tolerance to low K+, and to cationic stress of Arabidopsis plants by expressing the F130S mutant version of the K+ transporter AtHAK5

Potassium (K+) selectivity of high-affinity K+ uptake systems is crucial for plant growth under low K+ and in the presence of inhibitors of K+ uptake that are toxic to plants such as Na+ or Cs+. Here, we express a mutated version of the Arabidopsis AtHAK5 high-affinity K+ transporter consisting on a change of phenylalanine 130 to serine (F130S) in athak5 akt1 double mutant plants. F130S-expressing plants show better growth, increased K+ uptake from low external concentrations and higher K+ contents when grown at low K+ (10 μM) and when grown at low K+ in the presence of Na+ (15 mM) or Cs+ (1 μM). In addition, these plants accumulate less Na+ and Cs+, resulting in lower Na+/K+ and Cs+/K+ ratios, which are important determinants of plant tolerance to salt stress and to Cs+-polluted soils. Structure analysis of AtHAK5 suggest that the F130 residue approaches the intracellular gate of the K+ tunnel of AtHAK5, affecting somehow its ionic selectivity. Modification of transport systems has a large potential to face challenges of future agriculture such as sustainable production under abiotic stress conditions imposed by climate change.

A charged existence: A century of transmembrane ion transport in plants

If the past century marked the birth of membrane transport as a focus for research in plants, the past 50 years has seen the field mature from arcane interest to a central pillar of plant physiology. Ion transport across plant membranes accounts for roughly 30% of the metabolic energy consumed by a plant cell, and it underpins virtually every aspect of plant biology, from mineral nutrition, cell expansion, and development to auxin polarity, fertilization, plant pathogen defense, and senescence. The means to quantify ion flux through individual transporters, even single channel proteins, became widely available as voltage clamp methods expanded from giant algal cells to the fungus Neurospora crassa in the 1970s and the cells of angiosperms in the 1980s. Here, I touch briefly on some key aspects of the development of modern electrophysiology with a focus on the guard cells of stomata, now without dispute the premier plant cell model for ion transport and its regulation. Guard cells have proven to be a crucible for many technical and conceptual developments that have since emerged into the mainstream of plant science. Their study continues to provide fundamental insights and carries much importance for the global challenges that face us today.

Mapping the contribution of the C-linker domain to gating polarity in CNBD channels

Ion channels of the cyclic nucleotide-binding domain (CNBD) family play a crucial role in the regulation of key biological processes, such as photoreception and pacemaking activity in the heart. These channels exhibit high sequence and structural similarity but differ greatly in their functional responses to membrane potential. The CNBD family includes hyperpolarization-activated ion channels and depolarization-activated ether-à-go-go channels. Structural and functional studies show that the differences in the coupling interface between these two subfamilies' voltage-sensing domain and pore domain may underlie their differential response to membrane polarity. However, other structural components may also contribute to defining the polarity differences in activation. Here, we focus on the role of the C-terminal domain, which interacts with elements in both the pore and voltage-sensing domains. By generating a series of chimeras involving the C-terminal domain derived from distant members of the CNBD family, we find that the nature of the C-termini profoundly influences the gating polarity of these ion channels. Scanning mutagenesis of the C-linker region, a helix-turn-helix motif connecting the pore helix to the CNBD, reveals that residues at the intersubunit interface between the C-linkers are crucial for hyperpolarization-dependent activation. These findings highlight the unique and unexpected role of the intersubunit interface of the C-linker region in regulating the gating polarity of voltage-gated ion channels.

Analysis of Germin-like protein genes family in Vitis vinifera (VvGLPs) using various in silico approaches

Abstract Germin-like proteins (GLPs) play an important role against various stresses. Vitis vinifera L. genome contains 7 GLPs; many of them are functionally unexplored. However, the computational analysis may provide important new insight into their function. Currently, physicochemical properties, subcellular localization, domain architectures, 3D structures, N-glycosylation & phosphorylation sites, and phylogeney of the VvGLPs were investigated using the latest computational tools. Their functions were predicted using the Search tool for the retrieval of interacting genes/proteins (STRING) and Blast2Go servers. Most of the VvGLPs were extracellular (43%) in nature but also showed periplasmic (29%), plasma membrane (14%), and mitochondrial- or chloroplast-specific (14%) expression. The functional analysis predicted unique enzymatic activities for these proteins including terpene synthase, isoprenoid synthase, lipoxygenase, phosphate permease, receptor kinase, and hydrolases generally mediated by Mn+ cation. VvGLPs showed similarity in the overall structure, shape, and position of the cupin domain. Functionally, VvGLPs control and regulate the production of secondary metabolites to cope with various stresses. Phylogenetically VvGLP1, -3, -4, -5, and VvGLP7 showed greater similarity due to duplication while VvGLP2 and VvGLP6 revealed a distant relationship. Promoter analysis revealed the presence of diverse cis-regulatory elements among which CAAT box, MYB, MYC, unnamed-4 were common to all of them. The analysis will help to utilize VvGLPs and their promoters in future food programs by developing resistant cultivars against various biotic (Erysiphe necator and in Powdery Mildew etc.) and abiotic (Salt, drought, heat, dehydration, etc.) stresses.

Maternal Embryo Effect Arrest 31 (MEE31) is a moonlighting protein involved in GDP-D-mannose biosynthesis and KAT1 potassium channel regulation

Due to anthropogenic global warming, droughts are expected to increase and water availability to decrease in the coming decades. For this reason, research is increasingly focused on developing plant varieties and crop cultivars with reduced water consumption. Transpiration occurs through stomatal pores, resulting in water loss. Potassium plays a significant role in stomatal regulation. KAT1 is an inward-rectifying potassium channel that contributes to stomatal opening. Using a yeast high-throughput screening of an Arabidopsis cDNA library, MEE31 was found to physically interact with KAT1. MEE31 was initially identified in a screen for mutants with delayed embryonic development. The gene encodes a conserved phosphomannose isomerase (PMI). We report here that MEE31 interacts with and increases KAT1 activity in yeast and this interaction was also confirmed in plants. In addition, MEE31 complements the function of the yeast homologue, whereas the truncated version recovered in the screening does not, thus uncoupling the enzymatic activity from KAT1 regulation. We show that MEE31 overexpression leads to increased stomatal opening in Arabidopsis transgenic lines. Our data suggest that MEE31 is a moonlighting protein involved in both GDP-D-mannose biosynthesis and KAT1 regulation.

Modulation of potassium transport to increase abiotic stress tolerance in plants

Potassium is the major cation responsible for the maintenance of the ionic environment in plant cells. Stable potassium homeostasis is indispensable for virtually all cellular functions, and, concomitantly, viability. Plants must cope with environmental changes such as salt or drought that can alter ionic homeostasis. Potassium fluxes are required to regulate the essential process of transpiration, so a constraint on potassium transport may also affect the plant's response to heat, cold, or oxidative stress. Sequencing data and functional analyses have defined the potassium channels and transporters present in the genomes of different species, so we know most of the proteins directly participating in potassium homeostasis. The still unanswered questions are how these proteins are regulated and the nature of potential cross-talk with other signaling pathways controlling growth, development, and stress responses. As we gain knowledge regarding the molecular mechanisms underlying regulation of potassium homeostasis in plants, we can take advantage of this information to increase the efficiency of potassium transport and generate plants with enhanced tolerance to abiotic stress through genetic engineering or new breeding techniques. Here, we review current knowledge of how modifying genes related to potassium homeostasis in plants affect abiotic stress tolerance at the whole plant level.

Genomics-assisted genetics of complex region from chickpea chromosome 4 reveals two candidate genes for Ascochyta blight resistance

Ascochyta blight (AB) disease caused by the fungus Ascochyta rabiei is a major threat to global chickpea production. Molecular breeding for improved AB resistance requires the identification of robust fine-mapped QTLs/candidate genes and associated markers. Earlier, we identified three QTLs (qABR4.1, qABR4.2, and qABR4.3) for AB resistance on chickpea chromosome 4 by employing multiple quantitative trait loci sequencing strategy on an intra-specific (FLIP84-92C x PI359075) and an inter-specific (FLIP84-92C x PI599072) crosses derived recombinant inbred lines. Here, we report the identification of AB resistance providing candidate genes under the fine mapped qABR4.2 and qABR4.3 genomic region by combining genetic mapping, haplotype block inheritance, and expression analysis. The qABR4.2 region was narrowed down from 5.94Mb to ~800kb. Among 34 predicted gene models, a secreted class III peroxidase encoding gene showed higher expression in AB resistant parent after A. rabiei conidia inoculation. Under qABR4.3, we identified a frame-shift mutation in a cyclic nucleotide-gated channel CaCNGC1 gene leading to the truncated N-terminal domain in resistant accession of chickpea. This N-terminal domain of CaCNGC1 interacts with chickpea calmodulin. Thus, our analysis has revealed narrowed genomic regions and their associated polymorphic markers, CaNIP43 and CaCNGCPD1. These co-dominant markers significantly associate with AB resistance on qABR4.2 and qABR4.3 regions. Our genetic analysis revealed that the presence of resistant alleles for two major QTLs (qABR4.1 and qABR4.2) together provide AB resistance in the field while minor QTL qABR4.3 determines the degree of resistance. The identified candidate genes and their diagnostic markers will help in biotechnological and AB resistance introgression into farmers adapted local chickpea varieties.

Genome-Scale Analysis Reveals Extensive Diversification of Voltage-Gated K+ Channels in Stem Cnidarians

Ion channels are highly diverse in the cnidarian model organism Nematostella vectensis (Anthozoa), but little is known about the evolutionary origins of this channel diversity and its conservation across Cnidaria. Here, we examined the evolution of voltage-gated K+ channels in Cnidaria by comparing genomes and transcriptomes of diverse cnidarian species from Anthozoa and Medusozoa. We found an average of over 40 voltage-gated K+ channel genes per species, and a phylogenetic reconstruction of the Kv, KCNQ, and Ether-a-go-go (EAG) gene families identified 28 voltage-gated K+ channels present in the last common ancestor of Anthozoa and Medusozoa (23 Kv, 1 KCNQ, and 4 EAG). Thus, much of the diversification of these channels took place in the stem cnidarian lineage prior to the emergence of modern cnidarian classes. In contrast, the stem bilaterian lineage, from which humans evolved, contained no more than nine voltage-gated K+ channels. These results hint at a complexity to electrical signaling in all cnidarians that contrasts with the perceived anatomical simplicity of their neuromuscular systems. These data provide a foundation from which the function of these cnidarian channels can be investigated, which will undoubtedly provide important insights into cnidarian physiology.

Engineering a K+ channel 'sensory antenna' enhances stomatal kinetics, water use efficiency and photosynthesis

Stomata of plant leaves open to enable CO₂ entry for photosynthesis and close to reduce water loss via transpiration. Compared with photosynthesis, stomata respond slowly to fluctuating light, reducing assimilation and water use efficiency. Efficiency gains are possible without a cost to photosynthesis if stomatal kinetics can be accelerated. Here we show that clustering of the GORK channel, which mediates K⁺ efflux for stomatal closure in the model plant Arabidopsis, arises from binding between the channel voltage sensors, creating an extended 'sensory antenna' for channel gating. Mutants altered in clustering affect channel gating to facilitate K⁺ flux, accelerate stomatal movements and reduce water use without a loss in biomass. Our findings identify the mechanism coupling channel clustering with gating, and they demonstrate the potential for engineering of ion channels native to the guard cell to enhance stomatal kinetics and improve water use efficiency without a cost in carbon fixation.

When Is a Potassium Channel Not a Potassium Channel?

Ever since they were first observed in Purkinje fibers of the heart, funny channels have had close connections to potassium channels. Indeed, funny channels were initially thought to produce a potassium current in the heart called I _K2. However, funny channels are completely unlike potassium channels in ways that make their contributions to the physiology of cells unique. An important difference is the greater ability for sodium to permeate funny channels. Although it does not flow through the funny channel as easily as does potassium, sodium does permeate well enough to allow for depolarization of cells following a strong hyperpolarization. This is critical for the function of funny channels in places like the heart and brain. Computational analyses using recent structures of the funny channels have provided a possible mechanism for their unusual permeation properties.

The Sweetpotato Voltage-Gated K+ Channel β Subunit, KIbB1, Positively Regulates Low-K+ and High-Salinity Tolerance by Maintaining Ion Homeostasis

Voltage-gated K⁺ channel β subunits act as a structural component of K_in channels in different species. The β subunits are not essential to the channel activity but confer different properties through binding the T1 domain or the C-terminal of α subunits. Here, we studied the physiological function of a novel gene, KIbB1, encoding a voltage-gated K⁺ channel β subunit in sweetpotato. The transcriptional level of this gene was significantly higher in the low-K⁺-tolerant line than that in the low-K⁺-sensitive line under K⁺ deficiency conditions. In Arabidopsis, KIbB1 positively regulated low-K⁺ tolerance through regulating K⁺ uptake and translocation. Under high-salinity stress, the growth conditions of transgenic lines were obviously better than wild typr (WT). Enzymatic and non-enzymatic reactive oxygen species (ROS) scavenging were activated in transgenic plants. Accordingly, the malondialdehyde (MDA) content and the accumulation of ROS such as H₂O₂ and O²⁻ were lower in transgenic lines under salt stress. It was also found that the overexpression of KIbB1 enhanced K⁺ uptake, but the translocation from root to shoot was not affected under salt stress. This demonstrates that KIbB1 acted as a positive regulator in high-salinity stress resistance through regulating Na⁺ and K⁺ uptake to maintain K⁺/Na⁺ homeostasis. These results collectively suggest that the mechanisms of KIbB1 in regulating K⁺ were somewhat different between low-K⁺ and high-salinity conditions.

The potassium channel GhAKT2bD is regulated by CBL-CIPK calcium signaling complexes and facilitates K+ allocation in cotton

Efficient allocation of the essential nutrient potassium (K⁺ ) is a central determinant of plant ion homeostasis and involves AKT2 K⁺ channels. Here, we characterize four AKT2 K⁺ channels from cotton and report that xylem and phloem expressed GhAKT2bD facilitates K⁺ allocation and that AKT2-silencing impairs plant growth and development. We uncover kinase activity-dependent activation of GhAKT2bD-mediated K⁺ uptake by AtCBL4-GhCIPK1 calcium signaling complexes in HEK293T cells. Moreover, AtCBL4-AtCIPK6 complexes known to convey activation of AtAKT2 in Arabidopsis also activate cotton GhAKT2bD in HEK293T cells. Collectively, these findings reveal an essential role for AKT2 in the source-sink allocation of K⁺ in cotton and identify GhAKT2bD as subject to complex regulation by CBL-CIPK Ca²⁺ sensor-kinase complexes.

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.

A tale of many families: calcium channels in plant immunity

Plants launch a concerted immune response to dampen potential infections upon sensing microbial pathogen and insect invasions. The transient and rapid elevation of the cytosolic calcium concentration [Ca2+]cyt is among the essential early cellular responses in plant immunity. The free Ca2+ concentration in the apoplast is far higher than that in the resting cytoplasm. Thus, the precise regulation of calcium channel activities upon infection is the key for an immediate and dynamic Ca2+ influx to trigger downstream signaling. Specific Ca2+ signatures in different branches of the plant immune system vary in timing, amplitude, duration, kinetics, and sources of Ca2+. Recent breakthroughs in the studies of diverse groups of classical calcium channels highlight the instrumental role of Ca2+ homeostasis in plant immunity and cell survival. Additionally, the identification of some immune receptors as noncanonical Ca2+-permeable channels opens a new view of how immune receptors initiate cell death and signaling. This review aims to provide an overview of different Ca2+-conducting channels in plant immunity and highlight their molecular and genetic mode-of-actions in facilitating immune signaling. We also discuss the regulatory mechanisms that control the stability and activity of these channels.

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.

The Mechanism of Metal Homeostasis in Plants: A New View on the Synergistic Regulation Pathway of Membrane Proteins, Lipids and Metal Ions

Heavy metal stress (HMS) is one of the most destructive abiotic stresses which seriously affects the growth and development of plants. Recent studies have shown significant progress in understanding the molecular mechanisms underlying plant tolerance to HMS. In general, three core signals are involved in plants' responses to HMS; these are mitogen-activated protein kinase (MAPK), calcium, and hormonal (abscisic acid) signals. In addition to these signal components, other regulatory factors, such as microRNAs and membrane proteins, also play an important role in regulating HMS responses in plants. Membrane proteins interact with the highly complex and heterogeneous lipids in the plant cell environment. The function of membrane proteins is affected by the interactions between lipids and lipid-membrane proteins. Our review findings also indicate the possibility of membrane protein-lipid-metal ion interactions in regulating metal homeostasis in plant cells. In this review, we investigated the role of membrane proteins with specific substrate recognition in regulating cell metal homeostasis. The understanding of the possible interaction networks and upstream and downstream pathways is developed. In addition, possible interactions between membrane proteins, metal ions, and lipids are discussed to provide new ideas for studying metal homeostasis in plant cells.

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.

Molecular insights into the role of plant transporters in salt stress response

Salt stress disturbs the cellular osmotic and ionic balance, which then creates a negative impact on plant growth and development. The Na⁺ and Cl^- ions can enter into plant cells through various membrane transporters, including specific and non-specific Na⁺ , K⁺ , and Ca²⁺ transporters. Therefore, it is important to understand Na⁺ and K⁺ transport mechanisms in plants along with the isolation of genes, their characterization, the structural features, and their post-translation regulation under salt stress. This review summarizes the molecular insights of plant ion transporters, including non-selective cation transporters, cyclic nucleotide-gated cation transporters, glutamate-like receptors, membrane intrinsic proteins, cation proton antiporters, and sodium proton antiporter families. Further, we discussed the K⁺ transporter families such as high-affinity K⁺ transporters, HAK/KUP/KT transporters, shaker type K⁺ transporters, and K⁺ efflux antiporters. Besides the ion transport process, we have shed light on available literature on epigenetic regulation of transport processes under salt stress. Recent advancements of salt stress sensing mechanisms and various salt sensors within signaling transduction pathways are discussed. Further, we have compiled salt-stress signaling pathways, and their crosstalk with phytohormones.

New Insights into Plant TPK Ion Channel Evolution

Potassium (K) is a crucial element of plant nutrition, involved in many physiological and molecular processes. K⁺ membrane transporters are playing a pivotal role in K⁺ transport and tissue distribution as well as in various plant stress responses and developmental processes. Two-pore K⁺-channels (TPKs) are essential to maintain plant K⁺ homeostasis and are mainly involved in potassium transport from the vacuoles to the cytosol. Besides vacuolar specialization, some TPK members display different membrane localization including plasma membrane, protein storage vacuole membrane, and probably the organelles. In this manuscript, we elucidate the evolution of the voltage-independent TPK (two-pore K⁺-channels) family, which could be represented in some species by one pore, K⁺-inward rectifier (Kir)-like channels. A comprehensive investigation of existing databases and application of modern bioinformatic tools allowed us to make a detailed phylogenetic inventory of TPK/KCO3 (KCO: potassium channel, outward rectifying) channels through many taxa and gain insight into the evolutionary origin of TPK family proteins. Our results reveal the fundamental evolutional difference between the first and second pores, traced throughout multiple taxa variations in the ion selection filter motif, presence of thansposon, and methylation site in the proximity of some KCO members and suggest virus-mediated horizontal transfer of a KCO3-like ancestor by viruses. Additionally, we suggest several interconnected hypotheses to explain the obtained results and provide a theoretical background for future experimental validation.

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.

CNG channel structure, function, and gating: a tale of conformational flexibility

Cyclic nucleotide-gated (CNG) channels are key to the signal transduction machinery of certain sensory modalities both in vertebrate and invertebrate organisms. They translate a chemical change in cyclic nucleotide concentration into an electrical signal that can spread through sensory cells. Despite CNG and voltage-gated potassium channels sharing a remarkable amino acid sequence homology and basic architectural plan, their functional properties are dramatically different. While voltage-gated potassium channels are highly selective and require membrane depolarization to open, CNG channels have low ion selectivity and are not very sensitive to voltage. In the last few years, many high-resolution structures of intact CNG channels have been released. This wealth of new structural information has provided enormous progress toward the understanding of the molecular mechanisms and driving forces underpinning CNG channel activation. In this review, we report on the current understanding and controversies surrounding the gating mechanism in CNG channels, as well as the deep intertwining existing between gating, the ion permeation process, and its modulation by membrane voltage. While the existence of this powerful coupling was recognized many decades ago, its direct structural demonstration, and ties to the CNG channel inherent pore flexibility, is a recent achievement.

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.

Recent Advances in the Physiology of Ion Channels in Plants

Our knowledge of plant ion channels was significantly enhanced by the first application of the patch-clamp technique to isolated guard cell protoplasts over 35 years ago. Since then, research has demonstrated the importance of ion channels in the control of gas exchange in guard cells, their role in nutrient uptake in roots, and the participation of calcium-permeable cation channels in the regulation of cell signaling affected by the intracellular concentrations of this second messenger. In recent years, through the employment of reverse genetics, mutant proteins, and heterologous expression systems, research on ion channels has identified mechanisms that modify their activity through protein-protein interactions or that result in activation and/or deactivation of ion channels through posttranslational modifications. Additional and confirmatory information on ion channel functioning has been derived from the crystallization and molecular modeling of plant proteins that, together with functional analyses, have helped to increase our knowledge of the functioning of these important membrane proteins that may eventually help to improve crop yield. Here, an update on the advances obtained in plant ion channel function during the last few years is presented.

Genome-Wide Analysis of Potassium Channel Genes in Rice: Expression of the OsAKT and OsKAT Genes under Salt Stress

Potassium (K+), as a vital element, is involved in regulating important cellular processes such as enzyme activity, cell turgor, and nutrient movement in plant cells, which affects plant growth and production. Potassium channels are involved in the transport and release of potassium in plant cells. In the current study, three OsKAT genes and two OsAKT genes, along with 11 nonredundant putative potassium channel genes in the rice genome, were characterized based on their physiochemical properties, protein structure, evolution, duplication, in silico gene expression, and protein-protein interactions. In addition, the expression patterns of OsAKTs and OsKATs were studied in root and shoot tissues under salt stress using real-time PCR in three rice cultivars. K+ channel genes were found to have diverse functions and structures, and OsKATs showed high genetic divergence from other K+ channel genes. Furthermore, the Ka/Ks ratios of duplicated gene pairs from the K+ channel gene family in rice suggested that these genes underwent purifying selection. Among the studied K+ channel proteins, OsKAT1 and OsAKT1 were identified as proteins with high potential N-glycosylation and phosphorylation sites, and LEU, VAL, SER, PRO, HIS, GLY, LYS, TYR, CYC, and ARG amino acids were predicted as the binding residues in the ligand-binding sites of K+ channel proteins. Regarding the coexpression network and KEGG ontology results, several metabolic pathways, including sugar metabolism, purine metabolism, carbon metabolism, glycerophospholipid metabolism, monoterpenoid biosynthesis, and folate biosynthesis, were recognized in the coexpression network of K+ channel proteins. Based on the available RNA-seq data, the K+ channel genes showed differential expression levels in rice tissues in response to biotic and abiotic stresses. In addition, the real-time PCR results revealed that OsAKTs and OsKATs are induced by salt stress in root and shoot tissues of rice cultivars, and OsKAT1 was identified as a key gene involved in the rice response to salt stress. In the present study, we found that the repression of OsAKTs, OsKAT2, and OsKAT2 in roots was related to salinity tolerance in rice. Our findings provide valuable insights for further structural and functional assays of K+ channel genes in rice.

Insights into the mechanisms of transport and regulation of the arabidopsis high-affinity K+ transporter HAK51

The high-affinity K+ transporter HAK5 from Arabidopsis (Arabidopsis thaliana) is essential for K+ acquisition and plant growth at low micromolar K+ concentrations. Despite its functional relevance in plant nutrition, information about functional domains of HAK5 is scarce. Its activity is enhanced by phosphorylation via the AtCIPK23/AtCBL1-9 complex. Based on the recently published three-dimensionalstructure of the bacterial ortholog KimA from Bacillus subtilis, we have modeled AtHAK5 and, by a mutational approach, identified residues G67, Y70, G71, D72, D201, and E312 as essential for transporter function. According to the structural model, residues D72, D201, and E312 may bind K+, whereas residues G67, Y70, and G71 may shape the selective filter for K+, which resembles that of K+shaker-like channels. In addition, we show that phosphorylation of residue S35 by AtCIPK23 is required for reaching maximal transport activity. Serial deletions of the AtHAK5 C-terminus disclosed the presence of an autoinhibitory domain located between residues 571 and 633 together with an AtCIPK23-dependent activation domain downstream of position 633. Presumably, autoinhibition of AtHAK5 is counteracted by phosphorylation of S35 by AtCIPK23. Our results provide a molecular model for K+ transport and describe CIPK-CBL-mediated regulation of plant HAK transporters.

Membrane voltage as a dynamic platform for spatiotemporal signaling, physiological, and developmental regulation

Membrane voltage arises from the transport of ions through ion-translocating ATPases, ion-coupled transport of solutes, and ion channels, and is an integral part of the bioenergetic "currency" of the membrane. The dynamics of membrane voltage-so-called action, systemic, and variation potentials-have also led to a recognition of their contributions to signal transduction, both within cells and across tissues. Here, we review the origins of our understanding of membrane voltage and its place as a central element in regulating transport and signal transmission. We stress the importance of understanding voltage as a common intermediate that acts both as a driving force for transport-an electrical "substrate"-and as a product of charge flux across the membrane, thereby interconnecting all charge-carrying transport across the membrane. The voltage interconnection is vital to signaling via second messengers that rely on ion flux, including cytosolic free Ca2+, H+, and the synthesis of reactive oxygen species generated by integral membrane, respiratory burst oxidases. These characteristics inform on the ways in which long-distance voltage signals and voltage oscillations give rise to unique gene expression patterns and influence physiological, developmental, and adaptive responses such as systemic acquired resistance to pathogens and to insect herbivory.

External Cd2+ and protons activate the hyperpolarization-gated K+ channel KAT1 at the voltage sensor

The functionally diverse cyclic nucleotide binding domain (CNBD) superfamily of cation channels contains both depolarization-gated (e.g., metazoan EAG family K+ channels) and hyperpolarization-gated channels (e.g., metazoan HCN pacemaker cation channels and the plant K+ channel KAT1). In both types of CNBD channels, the S4 transmembrane helix of the voltage sensor domain (VSD) moves outward in response to depolarization. This movement opens depolarization-gated channels and closes hyperpolarization-gated channels. External divalent cations and protons prevent or slow movement of S4 by binding to a cluster of acidic charges on the S2 and S3 transmembrane domains of the VSD and therefore inhibit activation of EAG family channels. However, a similar divalent ion/proton binding pocket has not been described for hyperpolarization-gated CNBD family channels. We examined the effects of external Cd2+ and protons on Arabidopsisthaliana KAT1 expressed in Xenopus oocytes and found that these ions strongly potentiate voltage activation. Cd2+ at 300 µM depolarizes the V50 of KAT1 by 150 mV, while acidification from pH 7.0 to 4.0 depolarizes the V50 by 49 mV. Regulation of KAT1 by Cd2+ is state dependent and consistent with Cd2+ binding to an S4-down state of the VSD. Neutralization of a conserved acidic charge in the S2 helix in KAT1 (D95N) eliminates Cd2+ and pH sensitivity. Conversely, introduction of acidic residues into KAT1 at additional S2 and S3 cluster positions that are charged in EAG family channels (N99D and Q149E in KAT1) decreases Cd2+ sensitivity and increases proton potentiation. These results suggest that KAT1, and presumably other hyperpolarization-gated plant CNBD channels, can open from an S4-down VSD conformation homologous to the divalent/proton-inhibited conformation of EAG family K+ channels.

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.

Fungal Shaker-like channels beyond cellular K+ homeostasis: A role in ectomycorrhizal symbiosis between Hebeloma cylindrosporum and Pinus pinaster

Potassium (K+) acquisition, translocation and cellular homeostasis are mediated by various membrane transport systems in all organisms. We identified and described an ion channel in the ectomycorrhizal fungus Hebeloma cylindrosporum (HcSKC) that harbors features of animal voltage-dependent Shaker-like K+ channels, and investigated its role in both free-living hyphae and symbiotic conditions. RNAi lines affected in the expression of HcSKC were produced and used for in vitro mycorrhizal assays with the maritime pine as host plant, under standard or low K+ conditions. The adaptation of H. cylindrosporum to the downregulation of HcSKC was analyzed by qRT-PCR analyses for other K+-related transport proteins: the transporters HcTrk1, HcTrk2, and HcHAK, and the ion channels HcTOK1, HcTOK2.1, and HcTOK2.2. Downregulated HcSKC transformants displayed greater K+ contents at standard K+ only. In such conditions, plants inoculated with these transgenic lines were impaired in K+ nutrition. Taken together, these results support the hypothesis that the reduced expression of HcSKC modifies the pool of fungal K+ available for the plant and/or affects its symbiotic transfer to the roots. Our study reveals that the maintenance of K+ transport in H. cylindrosporum, through the regulation of HcSKC expression, is required for the K+ nutrition of the host plant.

Overexpression of HvAKT1 improves drought tolerance in barley by regulating root ion homeostasis and ROS and NO signaling

Potassium (K+) is the major cationic inorganic nutrient utilized for osmotic regulation, cell growth, and enzyme activation in plants. Inwardly rectifying K+ channel 1 (AKT1) is the primary channel for root K+ uptake in plants, but the function of HvAKT1 in barley plants under drought stress has not been fully elucidated. In this study, we conducted evolutionary bioinformatics, biotechnological, electrophysiological, and biochemical assays to explore molecular mechanisms of HvAKT1 in response to drought in barley. The expression of HvAKT1 was significantly up-regulated by drought stress in the roots of XZ5-a drought-tolerant wild barley genotype. We isolated and functionally characterized the plasma membrane-localized HvAKT1 using Agrobacterium-mediated plant transformation and Barley stripe mosaic virus-induced gene silencing of HvAKT1 in barley. Evolutionary bioinformatics indicated that the K+ selective filter in AKT1 originated from streptophyte algae and is evolutionarily conserved in land plants. Silencing of HvAKT1 resulted in significantly decreased biomass and suppressed K+ uptake in root epidermal cells under drought treatment. Disruption of HvAKT1 decreased root H+ efflux, H+-ATPase activity, and nitric oxide (NO) synthesis, but increased hydrogen peroxide (H2O2) production in the roots under drought stress. Furthermore, we observed that overexpression of HvAKT1 improves K+ uptake and increases drought resistance in barley. Our results highlight the importance of HvAKT1 for root K+ uptake and its pleiotropic effects on root H+-ATPase, and H2O2 and NO in response to drought stress, providing new insights into the genetic basis of drought tolerance and K+ nutrition in barley.

Diversity of voltage-gated potassium channels and cyclic nucleotide-binding domain-containing channels in eukaryotes

Voltage-gated potassium channels (K_v) and cyclic nucleotide-binding domain-containing cation channels HCN, CNG, and KCNH are the evolutionarily related families of ion channels in animals. Their homologues were found in several lineages of eukaryotes and prokaryotes; however, the actual phylogenetic and structural diversity of these ion channels remains unclear. In this work, we present a taxonomically broad investigation of evolutionary relationships and structural diversity of K_v, HCN, CNG, and KCNH and their homologues in eukaryotes focusing on channels from different protistan groups. We demonstrate that both groups of channels consist of a more significant number of lineages than it was shown before, and these lineages can be grouped in two clusters termed K_v-like channels and CNBD-channels. Moreover, we, for the first time, report the unusual two-repeat tandem K_v-like channels and CNBD-channels in several eukaryotic groups, i.e. dinoflagellates, oomycetes, and chlorarachniophytes. Our findings reveal still underappreciated phylogenetic and structural diversity of eukaryotic ion channel lineages.

Plant Cyclic Nucleotide-Gated Channels: New Insights on Their Functions and Regulation

Recent advances of plant cyclic nucleotide-gated channels give new insight into their molecular functions focusing on regulation, subunit assembly, and phosphorylation.

Genomics-assisted prediction of salt and alkali tolerances and functional marker development in apple rootstocks

Background

Saline, alkaline, and saline-alkaline stress severely affect plant growth and development. The tolerance of plants to these stressors has long been important breeding objectives, especially for woody perennials like apple. The aims of this study were to identify quantitative trait loci (QTLs) and to develop genomics-assisted prediction models for salt, alkali, and salt-alkali tolerance in apple rootstock.

Results

A total of 3258 hybrids derived from the apple rootstock cultivars ‘Baleng Crab’ (Malus robusta Rehd., tolerant) × ‘M9’ (M. pumila Mill., sensitive) were used to identify 17, 13, and two QTLs for injury indices of salt, alkali, and salt–alkali stress via bulked segregant analysis. The genotype effects of single nucleotide polymorphism (SNP) markers designed on candidate genes in each QTL interval were estimated. The genomic predicted value of an individual hybrid was calculated by adding the sum of all marker genotype effects to the mean phenotype value of the population. The prediction accuracy was 0.6569, 0.6695, and 0.5834 for injury indices of salt, alkali, and salt–alkali stress, respectively. SNP182G on MdRGLG3, which changes a leucine to an arginine at the vWFA-domain, conferred tolerance to salt, alkali, and salt-alkali stress. SNP761A on MdKCAB, affecting the Kv_beta domain that cooperated with the linked allelic variation SNP11, contributed to salt, alkali, and salt–alkali tolerance in apple rootstock.

Conclusions

The genomics-assisted prediction models can potentially be used in breeding saline, alkaline, and saline-alkaline tolerant apple rootstocks. The QTLs and the functional markers may provide insight for future studies into the genetic variation of plant abiotic stress tolerance.

HvAKT2 and HvHAK1 confer drought tolerance in barley through enhanced leaf mesophyll H+ homoeostasis

Plant K⁺ uptake typically consists low-affinity mechanisms mediated by Shaker K⁺ channels (AKT/KAT/KC) and high-affinity mechanisms regulated by HAK/KUP/KT transporters, which are extensively studied. However, the evolutionary and genetic roles of both K⁺ uptake mechanisms for drought tolerance are not fully explored in crops adapted to dryland agriculture. Here, we employed evolutionary bioinformatics, biotechnological and electrophysiological approaches to determine the role of two important K⁺ transporters HvAKT2 and HvHAK1 in drought tolerance in barley. HvAKT2 and HvHAK1 were cloned and functionally characterized using barley stripe mosaic virus-induced gene silencing (BSMV-VIGS) in drought-tolerant wild barley XZ5 and agrobacterium-mediated gene transfer in the barley cultivar Golden Promise. The hallmarks of the K⁺ selective filters of AKT2 and HAK1 are both found in homologues from strepotophyte algae, and they are evolutionarily conserved in strepotophyte algae and land plants. HvAKT2 and HvHAK1 are both localized to the plasma membrane and have high selectivity to K⁺ and Rb⁺ over other tested cations. Overexpression of HvAKT2 and HvHAK1 enhanced K⁺ uptake and H⁺ homoeostasis leading to drought tolerance in these transgenic lines. Moreover, HvAKT2- and HvHAK1-overexpressing lines showed distinct response of K⁺ , H⁺ and Ca²⁺ fluxes across plasma membrane and production of nitric oxide and hydrogen peroxide in leaves as compared to the wild type and silenced lines. High- and low-affinity K⁺ uptake mechanisms and their coordination with H⁺ homoeostasis play essential roles in drought adaptation of wild barley. These findings can potentially facilitate future breeding programs for resilient cereal crops in a changing global climate.

Phosphatidic acid directly binds with rice potassium channel OsAKT2 to inhibit its activity

The plant Shaker K⁺ channel AtAKT2 has been identified as a weakly rectifying channel that can stabilize membrane potentials to promote photoassimilate phloem loading and translocation. Thus, studies on functional characterization and regulatory mechanisms of AtAKT2-like channels in crops are highly important for improving crop production. Here, we identified the rice OsAKT2 as the ortholog of Arabidopsis AtAKT2, which is primarily expressed in the shoot phloem and localized at the plasma membrane. Using an electrophysiological assay, we found that OsAKT2 operated as a weakly rectifying K⁺ channel, preventing H⁺ /sucrose-symport-induced membrane depolarization. Three critical amino acid residues (K193, N206, and S326) are essential to the phosphorylation-mediated gating change of OsAKT2, consistent with the roles of the corresponding sites in AtAKT2. Disruption of OsAKT2 results in delayed growth of rice seedlings under short-day conditions. Interestingly, the lipid second messenger phosphatidic acid (PA) inhibits OsAKT2-mediated currents (both instantaneous and time-dependent components). Lipid dot-blot assay and liposome-protein binding analysis revealed that PA directly bound with two adjacent arginine residues in the ANK domain of OsAKT2, which is essential to PA-mediated inhibition of OsAKT2. Electrophysiological and phenotypic analyses also showed the PA-mediated inhibition of AtAKT2 and the negative correlation between intrinsic PA level and Arabidopsis growth, suggesting that PA may inhibit AKT2 function to affect plant growth and development. Our results functionally characterize the Shaker K⁺ channel OsAKT2 and reveal a direct link between phospholipid signaling and plant K⁺ channel modulation.

Recognition and Activation of the Plant AKT1 Potassium Channel by the Kinase CIPK23

Plant growth largely depends on the maintenance of adequate intracellular levels of potassium (K⁺). The families of 10 Calcineurin B-Like (CBL) calcium sensors and 26 CBL-Interacting Protein Kinases (CIPKs) of Arabidopsis (Arabidopsis thaliana) decode the calcium signals elicited by environmental inputs to regulate different ion channels and transporters involved in the control of K⁺ fluxes by phosphorylation-dependent and -independent events. However, the detailed molecular mechanisms governing target specificity require investigation. Here, we show that the physical interaction between CIPK23 and the noncanonical ankyrin domain in the cytosolic side of the inward-rectifier K⁺ channel AKT1 regulates kinase docking and channel activation. Point mutations on this domain specifically alter binding to CIPK23, enhancing or impairing the ability of CIPK23 to regulate channel activity. Our data demonstrate the relevance of this protein-protein interaction that contributes to the formation of a complex between CIPK23/CBL1 and AKT1 in the membrane for the proper regulation of K⁺ transport.

Analyzing plant mechanosensitive ion channels expressed in giant E. coli spheroplasts by single-channel patch-clamp electrophysiology

Plants possess numerous ion channels that respond to a range of stimuli, including small molecules, transmembrane voltage, and mechanical force. Many in the latter category, known as mechanosensitive (MS) ion channels, open directly in response to increases in lateral membrane tension. One of the most effective techniques for characterizing ion channel properties is patch-clamp electrophysiology, in which the current through a section of membrane containing ion channels is measured. For MS channels, this technique enables the measurement of key channel properties such as tension sensitivity, conductance, and ion selectivity. These characteristics, along with the phenotypes of genetic mutants, can help reveal the physiological roles of a particular MS channel. In this protocol, we provide detailed instructions on how to study MS ion channels using single-channel patch-clamp electrophysiology in giant E. coli spheroplasts. We first present an optimized method for preparing giant spheroplasts, then describe how to measure MS channel activity using patch-clamp electrophysiology and analyze the resulting data. We also provide recommended equipment lists, setup schematics, and useful conventions.

Plant Membrane Transport Research in the Post-genomic Era

Membrane transport processes are indispensable for many aspects of plant physiology including mineral nutrition, solute storage, cell metabolism, cell signaling, osmoregulation, cell growth, and stress responses. Completion of genome sequencing in diverse plant species and the development of multiple genomic tools have marked a new era in understanding plant membrane transport at the mechanistic level. Genes coding for a galaxy of pumps, channels, and carriers that facilitate various membrane transport processes have been identified while multiple approaches are developed to dissect the physiological roles as well as to define the transport capacities of these transport systems. Furthermore, signaling networks dictating the membrane transport processes are established to fully understand the regulatory mechanisms. Here, we review recent research progress in the discovery and characterization of the components in plant membrane transport that take advantage of plant genomic resources and other experimental tools. We also provide our perspectives for future studies in the field.

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.

Exploring the fundamental role of potassium channels in novel model plants

This article comments on:

Villette J, Cuéllar T, Zimmermann SD, Verdeil JL, Gaillard I. 2019. Unique features of the grapevine VvK5.1 channel support novel functions for outward K+ channels in plants. Journal of Experimental Botany 70, 6181–6193.

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.

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.