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

Identification and Characterization of Shaker Potassium Channel Gene Family and Response to Salt and Chilling Stress in Rice

Shaker potassium channel proteins are a class of voltage-gated ion channels responsible for K+ uptake and translocation, playing a crucial role in plant growth and salt tolerance. In this study, bioinformatic analysis was performed to identify the members within the Shaker gene family. Moreover, the expression patterns of rice Shaker(OsShaker) K+ channel genes were analyzed in different tissues and salt treatment by RT-qPCR. The results revealed that there were eight OsShaker K+ channel genes distributed on chromosomes 1, 2, 5, 6 and 7 in rice, and their promoters contained a variety of cis-regulatory elements, including hormone-responsive, light-responsive, and stress-responsive elements, etc. Most of the OsShaker K+ channel genes were expressed in all tissues of rice, but at different levels in different tissues. In addition, the expression of OsShaker K+ channel genes differed in the timing, organization and intensity of response to salt and chilling stress. In conclusion, our findings provide a reference for the understanding of OsShaker K+ channel genes, as well as their potential functions in response to salt and chilling stress in rice.

Genome-wide identification of kiwifruit K+ channel Shaker family members and their response to low-K+ stress

BACKGROUND

'Hongyang' kiwifruit (Actinidia chinensis cv 'Hongyang') is a high-quality variety of A. chinensis with the advantages of high yield, early ripening, and high stress tolerance. Studies have confirmed that the Shaker K+ genes family is involved in plant uptake and distribution of potassium (K+).

RESULTS

Twenty-eight Shaker genes were identified and analyzed from the 'Hongyang' kiwifruit (A. chinensis cv 'Hongyang') genome. Subcellular localization results showed that more than one-third of the AcShaker genes were on the cell membrane. Phylogenetic analysis indicated that the AcShaker genes were divided into six subfamilies (I-VI). Conservative model, gene structure, and structural domain analyses showed that AcShaker genes of the same subfamily have similar sequence features and structure. The promoter cis-elements of the AcShaker genes were classified into hormone-associated cis-elements and environmentally stress-associated cis-elements. The results of chromosomal localization and duplicated gene analysis demonstrated that AcShaker genes were distributed on 18 chromosomes, and segmental duplication was the prime mode of gene duplication in the AcShaker family. GO enrichment analysis manifested that the ion-conducting pathway of the AcShaker family plays a crucial role in regulating plant growth and development and adversity stress. Compared with the transcriptome data of the control group, all AcShaker genes were expressed under low-K+stress, and the expression differences of three genes (AcShaker15, AcShaker17, and AcShaker22) were highly significant. The qRT-PCR results showed a high correlation with the transcriptome data, which indicated that these three differentially expressed genes could regulate low-K+ stress and reduce K+ damage in kiwifruit plants, thus improving the resistance to low-K+ stress. Comparison between the salt stress and control transcriptomic data revealed that the expression of AcShaker11 and AcShaker18 genes was significantly different and lower under salt stress, indicating that both genes could be involved in salt stress resistance in kiwifruit.

CONCLUSION

The results showed that 28 AcShaker genes were identified and all expressed under low K+ stress, among which AcShaker22 was differentially and significantly upregulated. The AcShaker22 gene can be used as a candidate gene to cultivate new varieties of kiwifruit resistant to low K+ and provide a reference for exploring more properties and functions of the AcShaker genes.

Genome-wide transcriptome and gene family analysis reveal candidate genes associated with potassium uptake of maize colonized by arbuscular mycorrhizal fungi

BACKGROUND

Potassium (K) is an essential nutrient for plant growth and development. Maize (Zea mays) is a widely planted crops in the world and requires a huge amount of K fertilizer. Arbuscular mycorrhizal fungi (AMF) are closely related to the K uptake of maize. Genetic improvement of maize K utilization efficiency will require elucidating the molecular mechanisms of maize K uptake through the mycorrhizal pathway. Here, we employed transcriptome and gene family analysis to elucidate the mechanism influencing the K uptake and utilization efficiency of mycorrhizal maize.

METHODS AND RESULTS

The transcriptomes of maize were studied with and without AMF inoculation and under different K conditions. AM symbiosis increased the K concentration and dry weight of maize plants. RNA sequencing revealed that genes associated with the activity of the apoplast and nutrient reservoir were significantly enriched in mycorrhizal roots under low-K conditions but not under high-K conditions. Weighted gene correlation network analysis revealed that three modules were strongly correlated with K content. Twenty-one hub genes enriched in pathways associated with glycerophospholipid metabolism, glycerolipid metabolism, starch and sucrose metabolism, and anthocyanin biosynthesis were further identified. In general, these hub genes were upregulated in AMF-colonized roots under low-K conditions. Additionally, the members of 14 gene families associated with K obtain were identified (ARF: 38, ILK: 4, RBOH: 12, RUPO: 20, MAPKK: 89, CBL: 14, CIPK: 44, CPK: 40, PIN: 10, MYB: 174, NPF: 79, KT: 19, HAK/HKT/KUP: 38, and CPA: 8) from maize. The transcript levels of these genes showed that 92 genes (ARF:6, CBL:5, CIPK:13, CPK:2, HAK/HKT/KUP:7, PIN:2, MYB:26, NPF:16, RBOH:1, MAPKK:12 and RUPO:2) were upregulated with AM symbiosis under low-K conditions.

CONCLUSIONS

This study indicated that AMF increase the resistance of maize to low-K stress by regulating K uptake at the gene transcription level. Our findings provide a genome-level resource for the functional assignment of genes regulated by K treatment and AM symbiosis in K uptake-related gene families in maize. This may contribute to elucidate the molecular mechanisms of maize response to low K stress with AMF inoculation, and provided a theoretical basis for AMF application in the crop field.

Identification of Shaker Potassium Channel Family Members and Functional Characterization of SsKAT1.1 in Stenotaphrum secundatum Suggest That SsKAT1.1 Contributes to Cold Resistance

Stenotaphrum secundatum is an excellent shade-tolerant warm-season turfgrass. Its poor cold resistance severely limits its promotion and application in temperate regions. Mining cold resistance genes is highly important for the cultivation of cold-resistant Stenotaphrum secundatum. Although there have been many reports on the role of the Shaker potassium channel family under abiotic stress, such as drought and salt stress, there is still a lack of research on their role in cold resistance. In this study, the transcriptome database of Stenotaphrum secundatum was aligned with the whole genome of Setaria italica, and eight members of the Shaker potassium channel family in Stenotaphrum secundatum were identified and named SsKAT1.1, SsKAT1.2, SsKAT2.1, SsKAT2.2, SsAKT1.1, SsAKT2.1, SsAKT2.2, and SsKOR1. The KAT3-like gene, KOR2 homologous gene, and part of the AKT-type weakly inwardly rectifying channel have not been identified in the Stenotaphrum secundatum transcriptome database. A bioinformatics analysis revealed that the potassium channels of Stenotaphrum secundatum are highly conserved in terms of protein structure but have more homologous members in the same group than those of other species. Among the three species of Oryza sativa, Arabidopsis thaliana, and Setaria italica, the potassium channel of Stenotaphrum secundatum is more closely related to the potassium channel of Setaria italica, which is consistent with the taxonomic results of these species belonging to Paniceae. Subcellular location experiments demonstrate that SsKAT1.1 is a plasma membrane protein. The expression of SsKAT1.1 reversed the growth defect of the potassium absorption-deficient yeast strain R5421 under a low potassium supply, indicating that SsKAT1.1 is a functional potassium channel. The transformation of SsKAT1.1 into the cold-sensitive yeast strain INVSC1 increased the cold resistance of the yeast, indicating that SsKAT1.1 confers cold resistance. The transformation of SsKAT1.1 into the salt-sensitive yeast strain G19 increased the resistance of yeast to salt, indicating that SsKAT1.1 is involved in salt tolerance. These results suggest that the manipulation of SsKAT1.1 will improve the cold and salt stress resistance of Stenotaphrum secundatum.

WRKY6 transcription factor modulates root potassium acquisition through promoting expression of AKT1 in Arabidopsis

Potassium (K+), being an essential macronutrient in plants, plays a central role in many aspects. Root growth is highly plastic and is affected by many different abiotic stresses including nutrient deficiency. The Shaker-type K+ channel Arabidopsis (Arabidopsis thaliana) K+ Transporter 1 (AKT1) is responsible for K+ uptake under both low and high external K+ conditions. However, the upstream transcription factor of AKT1 is not clear. Here, we demonstrated that the WRKY6 transcription factor modulates root growth to low potassium (LK) stress in Arabidopsis. WRKY6 showed a quick response to LK stress and also to many other abiotic stress treatments. The two wrky6 T-DNA insertion mutants were highly sensitive to LK treatment, whose primary root lengths were much shorter, less biomass and lower K+ content in roots than those of wild-type plants, while WRKY6-overexpression lines showed opposite phenotypes. A further investigation showed that WRKY6 regulated the expression of the AKT1 gene via directly binding to the W-box elements in its promoter through EMSA and ChIP-qPCR assays. A dual luciferase reporter analysis further demonstrated that WRKY6 enhanced the transcription of AKT1. Genetic analysis further revealed that the overexpression of AKT1 greatly rescued the short root phenotype of the wrky6 mutant under LK stress, suggesting AKT1 is epistatic to WRKY6 in the control of LK response. Further transcriptome profiling suggested that WRKY6 modulates LK response through a complex regulatory network. Thus, this study unveils a transcription factor that modulates root growth under potassium deficiency conditions by affecting the potassium channel gene AKT1 expression.

PIF transcriptional regulators are required for rhythmic stomatal movements

Stomata govern the gaseous exchange between the leaf and the external atmosphere, and their function is essential for photosynthesis and the global carbon and oxygen cycles. Rhythmic stomata movements in daily dark/light cycles prevent water loss at night and allow CO2 uptake during the day. How the actors involved are transcriptionally regulated and how this might contribute to rhythmicity is largely unknown. Here, we show that morning stomata opening depends on the previous night period. The transcription factors PHYTOCHROME-INTERACTING FACTORS (PIFs) accumulate at the end of the night and directly induce the guard cell-specific K+ channel KAT1. Remarkably, PIFs and KAT1 are required for blue light-induced stomata opening. Together, our data establish a molecular framework for daily rhythmic stomatal movements under well-watered conditions, whereby PIFs are required for accumulation of KAT1 at night, which upon activation by blue light in the morning leads to the K+ intake driving stomata opening.

Investigation of two-pore K+ (TPK) channels in Triticum aestivum L. suggests their role in stress response

Two-pore K+ (TPK) channels are voltage-independent and involved in stress response in plants. Herein, we identified 12 TaTPK genes located on nine chromosomes in the Triticum aestivum genome. The majority of TaTPK genes comprised two exons. Each TaTPK channel comprised four transmembrane (TM) helices, N- and C-terminal ion-channel domains, two EF-hand domains and one 14-3-3 binding site. Additionally, highly conserved 'GYGD' motif responsible for K+ ion specificity, was found in between the TMs in both the ion-channel domains. Nine TaTPK channels were predicted to be localised at the plasma membrane, while three were vacuolar. The protein-protein and protein-chemical interactions indicated the coordinated functioning of the TaTPK channels with the other K+ transporters and their possible interaction with the Ca2+-signaling pathway. Expression studies suggested their importance in both vegetative and reproductive tissues development. Significantly modulated expression of various TaTPK genes during heat, drought, combined heat and drought and salt stresses, and after fungal infestation, depicted their function in stress responses. The miRNAs and transcription factors interaction analyses suggested their role in the hormone, light, growth and development-related, and stress-responsive signaling cascades. The current study suggested vital functions of various TaTPK genes, especially in stress response, and would provide an opportunity for their detailed characterization in future studies.

Transcriptome Profiling Reveals Molecular Responses to Salt Stress in Common Vetch (Vicia sativa L.)

Common vetch (Vicia sativa L.) is an important annual diploid leguminous forage. In the present study, transcriptomic profiling in common vetch in response to salt stress was conducted using a salt-tolerant line (460) and a salt-sensitive line (429). The common responses in common vetch and the specific responses associated with salt tolerance in 460 were analyzed. Several KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways, including plant hormone and MAPK (mitogen-activated protein kinase) signaling, galactose metabolism, and phenylpropanoid phenylpropane biosynthesis, were enriched in both lines, though some differentially expressed genes (DEGs) showed distinct expression patterns. The roots in 460 showed higher levels of lignin than in 429. α-linolenic acid metabolism, carotenoid biosynthesis, the photosynthesis-antenna pathway, and starch and sucrose metabolism pathways were specifically enriched in salt-tolerant line 460, with higher levels of accumulated soluble sugars in the leaves. In addition, higher transcript levels of genes involved in ion homeostasis and reactive oxygen species (ROS) scavenging were observed in 460 than in 429 in response to salt stress. The transcriptomic analysis in common vetch in response to salt stress provides useful clues for further investigations on salt tolerance mechanism in the future.

Gene pyramiding for boosted plant growth and broad abiotic stress tolerance

Abiotic stresses such as salinity, heat and drought seriously impair plant growth and development, causing a significant loss in crop yield and ornamental value. Biotechnology approaches manipulating specific genes prove to be effective strategies in crop trait modification. The Arabidopsis vacuolar pyrophosphatase gene AVP1, the rice SUMO E3 ligase gene OsSIZ1 and the cyanobacterium flavodoxin gene Fld have previously been implicated in regulating plant stress responses and conferring enhanced tolerance to different abiotic stresses when individually overexpressed in various plant species. We have explored the feasibility of combining multiple favourable traits brought by individual genes to acquire superior plant performance. To this end, we have simultaneously introduced AVP1, OsSIZ1 and Fld in creeping bentgrass. Transgenic (TG) plants overexpressing these three genes performed significantly better than wild type controls and the TGs expressing individual genes under both normal and various abiotic stress conditions, exhibited significantly enhanced plant growth and tolerance to drought, salinity and heat stresses as well as nitrogen and phosphate starvation, which were associated with altered physiological and biochemical characteristics and delicately fine-tuned expression of genes involved in plant stress responses. Our results suggest that AVP1, OsSIZ1 and Fld function synergistically to regulate plant development and plant stress response, leading to superior overall performance under both normal and adverse environments. The information obtained provides new insights into gene stacking as an effective approach for plant genetic engineering. A similar strategy can be extended for the use of other beneficial genes in various crop species for trait modifications, enhancing agricultural production.

Multi-omics strategies uncover the molecular mechanisms of nitrogen, phosphorus and potassium deficiency responses in Brassica napus

BACKGROUND

Nitrogen (N), phosphorus (P) and potassium (K) are critical macronutrients in crops, such that deficiency in any of N, P or K has substantial effects on crop growth. However, the specific commonalities of plant responses to different macronutrient deficiencies remain largely unknown.

METHODS

Here, we assessed the phenotypic and physiological performances along with whole transcriptome and metabolomic profiles of rapeseed seedlings exposed to N, P and K deficiency stresses.

RESULTS

Quantities of reactive oxygen species were significantly increased by all macronutrient deficiencies. N and K deficiencies resulted in more severe root development responses than P deficiency, as well as greater chlorophyll content reduction in leaves (associated with disrupted chloroplast structure). Transcriptome and metabolome analyses validated the macronutrient-specific responses, with more pronounced effects of N and P deficiencies on mRNAs, microRNAs (miRNAs), circular RNAs (circRNAs) and metabolites relative to K deficiency. Tissue-specific responses also occurred, with greater effects of macronutrient deficiencies on roots compared with shoots. We further uncovered a set of common responders with simultaneous roles in all three macronutrient deficiencies, including 112 mRNAs and 10 miRNAs involved in hormonal signaling, ion transport and oxidative stress in the root, and 33 mRNAs and 6 miRNAs with roles in abiotic stress response and photosynthesis in the shoot. 27 and seven common miRNA-mRNA pairs with role in miRNA-mediated regulation of oxidoreduction processes and ion transmembrane transport were identified in all three macronutrient deficiencies. No circRNA was responsive to three macronutrient deficiency stresses, but two common circRNAs were identified for two macronutrient deficiencies. Combined analysis of circRNAs, miRNAs and mRNAs suggested that two circRNAs act as decoys for miR156 and participate in oxidoreduction processes and transmembrane transport in both N- and P-deprived roots. Simultaneously, dramatic alterations of metabolites also occurred. Associations of RNAs with metabolites were observed, and suggested potential positive regulatory roles for tricarboxylic acids, azoles, carbohydrates, sterols and auxins, and negative regulatory roles for aromatic and aspartate amino acids, glucosamine-containing compounds, cinnamic acid, and nicotianamine in plant adaptation to macronutrient deficiency.

CONCLUSIONS

Our findings revealed strategies to rescue rapeseed from macronutrient deficiency stress, including reducing the expression of non-essential genes and activating or enhancing the expression of anti-stress genes, aided by plant hormones, ion transporters and stress responders. The common responders to different macronutrient deficiencies identified could be targeted to enhance nutrient use efficiency in rapeseed.

Selenite affected photosynthesis of Oryza sativa L. exposed to antimonite: Electron transfer, carbon fixation, pigment synthesis via a combined analysis of physiology and transcriptome

Selenium (Se) is a microelement that can counteract (a)biotic stresses in plants. Excess antimony (Sb) will inhibit plant photosynthesis, which can be alleviated by appropriate doses of Se but the associated mechanisms at the molecular levels have not been fully explored. Here, a rice variety (Yongyou 9) was exposed to selenite [Se(IV), 0.2 and 0.8 mg L-1] alone or combined with antimonite [Sb(III), 5 and 10 mg L-1]. When compared to the 10 mg L-1 Sb treatment alone, addition of Se in a dose-dependent manner 1) reduced the heat dissipation efficiency resulting from the inhibited donors, Sb concentrations in shoots and roots, leaf concentrations of fructose, H2O2 and O2•-; 2) enhanced heat dissipation efficiency resulting from the inhibited accepters value, concentrations of Chl a, sucrose and starch, and the enzyme activity of adenosine diphosphate glucose pyrophosphorylase, sucrose phosphate synthase, and sucrose synthase; but 3) did not alter gas exchange parameters, concentrations of Chl b and total Chl, enzyme activity of soluble acid invertase, and values of maximum P700 signal, photochemical efficiency of PSI and electron transport rate of PSI. Se alleviated the damage caused by Sb to the oxygen-evolving complex and promoted the transfer of electrons from QA to QB. When compared to the 10 mg L-1 Sb treatment alone, addition of Se 1) up-regulated genes correlated to synthesis pathways of Chl, carotenoid, sucrose and glucose; 2) disturbed signal transduction pathway of abscisic acid; and 3) upregulated gene expression correlated to photosynthetic complexes (OsFd1, OsFER1 and OsFER2).

Yeast Heterologous Expression Systems for the Study of Plant Membrane Proteins

Researchers are often interested in proteins that are present in cells in small ratios compared to the total amount of proteins. These proteins include transcription factors, hormones and specific membrane proteins. However, sufficient amounts of well-purified protein preparations are required for functional and structural studies of these proteins, including the creation of artificial proteoliposomes and the growth of protein 2D and 3D crystals. This aim can be achieved by the expression of the target protein in a heterologous system. This review describes the applications of yeast heterologous expression systems in studies of plant membrane proteins. An initial brief description introduces the widely used heterologous expression systems of the baker's yeast Saccharomyces cerevisiae and the methylotrophic yeast Pichia pastoris. S. cerevisiae is further considered a convenient model system for functional studies of heterologously expressed proteins, while P. pastoris has the advantage of using these yeast cells as factories for producing large quantities of proteins of interest. The application of both expression systems is described for functional and structural studies of membrane proteins from plants, namely, K⁺- and Na⁺-transporters, various ATPases and anion transporters, and other transport proteins.

Functional and biotechnological cues of potassium homeostasis for stress tolerance and plant development

Potassium (K+) is indispensable for the regulation of a plethora of functions like plant metabolism, growth, development, and abiotic stress responses. K+ is associated with protein synthesis and entangled in the activation of scores of enzymes, stomatal regulation, and photosynthesis. It has multiple transporters and channels that assist in the uptake, efflux, transport within the cell as well as from soil to different tissues, and the grain filling sites. While it is implicated in ion homeostasis during salt stress, it acts as a modulator of stomatal movements during water deficit conditions. K+ is reported to abate the effects of chilling and photooxidative stresses. K+ has been found to ameliorate effectively the co-occurrence of drought and high-temperature stresses. Nutrient deficiency of K+ makes leaves necrotic, leads to diminished photosynthesis, and decreased assimilate utilization highlighting the role it plays in photosynthesis. Notably, K+ is associated with the detoxification of reactive oxygen species (ROS) when plants are exposed to diverse abiotic stress conditions. It is irrefutable now that K+ reduces the activity of NADPH oxidases and at the same time maintains electron transport activity, which helps in mitigating the oxidative stress. K+ as a macronutrient in plant growth, the role of K+ during abiotic stress and the protein phosphatases involved in K+ transport have been reviewed. This review presents a holistic view of the biological functions of K+, its uptake, translocation, signaling, and the critical roles it plays under abiotic stress conditions, plant growth, and development that are being unraveled in recent times.

Stable potassium isotopes (41K/39K) track transcellular and paracellular potassium transport in biological systems

As the most abundant cation in archaeal, bacterial, and eukaryotic cells, potassium (K⁺) is an essential element for life. While much is known about the machinery of transcellular and paracellular K transport–channels, pumps, co-transporters, and tight-junction proteins—many quantitative aspects of K homeostasis in biological systems remain poorly constrained. Here we present measurements of the stable isotope ratios of potassium (⁴¹K/³⁹K) in three biological systems (algae, fish, and mammals). When considered in the context of our current understanding of plausible mechanisms of K isotope fractionation and K⁺ transport in these biological systems, our results provide evidence that the fractionation of K isotopes depends on transport pathway and transmembrane transport machinery. Specifically, we find that passive transport of K⁺ down its electrochemical potential through channels and pores in tight-junctions at favors ³⁹K, a result which we attribute to a kinetic isotope effect associated with dehydration and/or size selectivity at the channel/pore entrance. In contrast, we find that transport of K⁺ against its electrochemical gradient via pumps and co-transporters is associated with less/no isotopic fractionation, a result that we attribute to small equilibrium isotope effects that are expressed in pumps/co-transporters due to their slower turnover rate and the relatively long residence time of K⁺ in the ion pocket. These results indicate that stable K isotopes may be able to provide quantitative constraints on transporter-specific K⁺ fluxes (e.g., the fraction of K efflux from a tissue by channels vs. co-transporters) and how these fluxes change in different physiological states. In addition, precise determination of K isotope effects associated with K⁺ transport via channels, pumps, and co-transporters may provide unique constraints on the mechanisms of K transport that could be tested with steered molecular dynamic simulations.

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.

Research Advances on Molecular Mechanism of Salt Tolerance in Suaeda

Plant growth and development are inevitably affected by various environmental factors. High salinity is the main factor leading to the reduction of cultivated land area, which seriously affects the growth and yield of plants. The genus Suaeda is a kind of euhalophyte herb, with seedlings that grow rapidly in moderately saline environments and can even survive in conditions of extreme salinity. Its fresh branches can be used as vegetables and the seed oil is rich in unsaturated fatty acids, which has important economic value and usually grows in a saline environment. This paper reviews the progress of research in recent years into the salt tolerance of several Suaeda species (for example, S. salsa, S. japonica, S. glauca, S. corniculata), focusing on ion regulation and compartmentation, osmotic regulation of organic solutes, antioxidant regulation, plant hormones, photosynthetic systems, and omics (transcriptomics, proteomics, and metabolomics). It helps us to understand the salt tolerance mechanism of the genus Suaeda, and provides a theoretical foundation for effectively improving crop resistance to salt stress environments.

Transcriptome and Metabonomic Analysis of Tamarix ramosissima Potassium (K+) Channels and Transporters in Response to NaCl Stress

Potassium ion (K+) channels and transporters are key components of plant K+ absorption and transportation and play an important role in plant growth and development. This study revealed that K+ channels and transporters are involved in the salt tolerance molecular mechanism and metabolites of the halophyte representative plant Tamarix ramosissima (T. ramosissima) in response to NaCl stress, providing a theoretical basis for the mitigation of salt stress using halophytes. Through transcriptome sequencing and metabolite detection analysis of 0 h, 48 h and 168 h by applying exogenous K+ to the roots of T. ramosissima under NaCl stress, 15 high-quality Clean Data bases were obtained, Q20 reached more than 97%, Q30 reached more than 92%, and GC content reached 44.5%, which is in line with further bioinformatics analysis. Based on the Liquid chromatography−mass spectrometry (LC-MS) analysis, the roots of T. ramosissima were exposed to exogenous potassium for 48 h and 168 h under NaCl stress, and 1510 and 1124 metabolites were identified in positive and negative ion mode, respectively. Through orthogonal projections to latent structures discriminant analysis (OPLS-DA) model analysis, its metabolomic data have excellent predictability and stability. The results of this study showed that there were 37 differentially expressed genes (DEGs) annotated as Class 2 K+ channels (Shaker-like K+ channel and TPK channel) and Class 3 K+ transporters (HAK/KUP/KT, HKT and CPAs transporter families). Among them, 29 DEGs were annotated to the gene ontology (GO) database, and the most genes were involved in the GO Biological Process. In addition, the expression levels of Unigene0014342 in the HAK/KUP/KT transporter and Unigene0088276 and Unigene0103067 in the CPAs transporter both first decreased and then increased when treated with 200 mM NaCl for 48 h and 168 h. However, when treated with 200 mM NaCl + 10 mM KCl for 48 h and 168 h, a continuous upward trend was shown. Notably, the expression level of Unigene0016813 in CPAS transporter continued to increase when treated with 200 mM NaCl and 200 mM NaCl + 10 mM KCl for 48 h and 168 h. 3 DEGs, Unigene0088276, Unigene0016813 and Unigene0103067, were dominated by the positive regulation of their related metabolites, and this correlation was significant. The results showed that these DEGs increased the absorption of K+ and the ratio of K+/Na+ under NaCl stress at 48 h and 168 h after adding exogenous potassium and enhanced the salt tolerance of T. ramosissima. Notably, the expression level of Unigene0103067 in the CPAs transporter was consistently upregulated when 200 mM NaCl + 10 mM KCl was treated for 48 h and 168 h. The positive regulatory metabolites were always dominant, which better helped T. ramosissima resist salt stress. Unigene0103067 plays an important role in enhancing the salt tolerance of T. ramosissima and reducing the toxicity of NaCl in roots. Additionally, phylogenetic tree analysis showed that Unigene0103067 and Reaumuria trigyna had the closest genetic distance in the evolutionary relationship. Finally, 9 DEGs were randomly selected for quantitative real-time PCR (qRT-PCR) verification. Their expression trends were completely consistent with the transcriptome sequencing analysis results, proving that this study’s data are accurate and reliable. This study provides resources for revealing the molecular mechanism of NaCl stress tolerance in T. ramosissima and lays a theoretical foundation for cultivating new salt-tolerant varieties.

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.

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.

Ca2+ signals in plant immunity

Calcium ions function as a key second messenger ion in eukaryotes. Spatially and temporally defined cytoplasmic Ca²⁺ signals are shaped through the concerted activity of ion channels, exchangers, and pumps in response to diverse stimuli; these signals are then decoded through the activity of Ca²⁺ -binding sensor proteins. In plants, Ca²⁺ signaling is central to both pattern- and effector-triggered immunity, with the generation of characteristic cytoplasmic Ca²⁺ elevations in response to potential pathogens being common to both. However, despite their importance, and a long history of scientific interest, the transport proteins that shape Ca²⁺ signals and their integration remain poorly characterized. Here, we discuss recent work that has both shed light on and deepened the mysteries of Ca²⁺ signaling in plant immunity.

Comprehensive Transcriptome Analysis Uncovers Hub Long Non-coding RNAs Regulating Potassium Use Efficiency in Nicotiana tabacum

Potassium (K) is the essential element for plant growth. It is one of the critical factors that determine crop yield, quality, and especially leaf development in tobacco. However, the molecular mechanism of potassium use efficiency (KUE), especially non-coding RNA, is still unknown. In this study, tobacco seedlings were employed, and their hydro-cultivation with K treatments of low and sufficient concentrations was engaged. Physiological analysis showed that low potassium treatment could promote malondialdehyde (MDA) accumulation and antioxidant enzyme activities such as peroxidase (POD), ascorbate-peroxidase (APX). After transcriptomic analysis, a total of 10,585 LncRNA transcripts were identified, and 242 of them were significantly differently expressed under potassium starvation. Furthermore, co-expression networks were constructed and generated 78 potential regulation modules in which coding gene and LncRNAs are involved and functional jointly. By further module-trait analysis and module membership (MM) ranking, nine modules, including 616 coding RNAs and 146 LncRNAs, showed a high correlation with K treatments, and 20 hub K-responsive LncRNAs were finally predicted. Following gene ontology (GO) analysis, the results showed potassium starvation inducing the pathway of antioxidative stress which is consistent with the physiology result mentioned above. Simultaneously, a part of detected LncRNAs, such as MSTRG.6626.1, MSTRG.11330.1, and MSTRG.16041.1, were co-relating with a bench of MYB, C3H, and NFYC transcript factors in response to the stress. Overall, this research provided a set of LncRNAs that respond to K concentration from starvation and sufficient supply. Simultaneously, the regulation network and potential co-functioning genes were listed as well. This massive dataset would serve as an outstanding clue for further study in tobacco and other plant species for nutrient physiology and molecular regulation mechanism.

Ethylene-induced potassium transporter AcKUP2 gene is involved in kiwifruit postharvest ripening

BACKGROUND

Potassium (K) is important in the regulation of plant growth and development. It is the most abundant mineral element in kiwifruit, and its content increases during fruit ripening. However, how K⁺ transporter works in kiwifruit postharvest maturation is not yet clear.

RESULTS

Here, 12 K⁺ transporter KT/HAK/KUP genes, AcKUP1 ~ AcKUP12, were isolated from kiwifruit, and their phylogeny, genomic structure, chromosomal location, protein properties, conserved motifs and cis-acting elements were analysed. Transcription analysis revealed that AcKUP2 expression increased rapidly and was maintained at a high level during postharvest maturation, consistent with the trend of K content; AcKUP2 expression was induced by ethylene, suggesting that AcKUP2 might play a role in ripening. Fluorescence microscopy showed that AcKUP2 is localised in the plasma membrane. Cis-elements, including DER or ethylene response element (ERE) responsive to ethylene, were found in the AcKUP2 promoter sequence, and ethylene significantly enhanced the AcKUP2 promoter activity. Furthermore, we verified that AcERF15, an ethylene response factor, directly binds to the AcKUP2 promoter to promote its expression. Thus, AcKUP2 may be an important potassium transporter gene which involved in ethylene-regulated kiwifruit postharvest ripening.

CONCLUSIONS

Therefore, our study establishes the first genome-wide analysis of the kiwifruit KT/HAK/KUP gene family and provides valuable information for understanding the function of the KT/HAK/KUP genes in kiwifruit postharvest ripening.

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.

Potassium in plants: Growth regulation, signaling, and environmental stress tolerance

Potassium (K) is an essential element for the growth and development of plants; however, its scarcity or excessive level leads to distortion of numerous functions in plants. It takes part in the control of various significant functions in plant advancement. Because of the importance index, K is regarded second after nitrogen for whole plant growth. Approximately, higher than 60 enzymes are reliant on K for activation within the plant system, in which K plays a vital function as a regulator. Potassium provides assistance in plants against abiotic stress conditions in the environment. With this background, the present paper reviews the physiological functions of K in plants like stomatal regulation, photosynthesis and water uptake. The article also focuses upon the uptake and transport mechanisms of K along with its role in detoxification of reactive oxygen species and in conferring tolerance to plants against abiotic stresses. It also highlights the research progress made in the direction of K mediated signaling cascades.

Genome-wide analysis of potassium transport genes in Gossypium raimondii suggest a role of GrHAK/KUP/KT8, GrAKT2.1 and GrAKT1.1 in response to abiotic stress

Potassium (K⁺) is an important macro-nutrient for plants, which comprises almost 10% of plant's dry mass. It plays a crucial role in the growth of plants as well as other important processes related to metabolism and stress tolerance. Plants have a complex and well-organized potassium distribution system (channels and transporters). Cotton is the most important economic crop, which is the primary source of natural fiber. Soil deficiency in K⁺ can negatively affect yield and fiber quality of cotton. However, potassium transport system in cotton is poorly studied. Current study identified 43 Potassium Transport System (PTS) genes in Gossypium raimondii genome. Based on conserved domains, transmembrane domains, and motif structures, these genes were classified as K⁺ transporters (2 HKTs, 7 KEAs, and 16 KUP/HAK/KTs) and K⁺ channels (11 Shakers and 7 TPKs/KCO). The phylogenetic comparison of GrPTS genes from Arabidopsis thaliana, Glycine max, Oryza sativa, Medicago truncatula and Cicer arietinum revealed variations in PTS gene conservation. Evolutionary analysis predicted that most GrPTS genes were segmentally duplicated. Gene structure analysis showed that the intron/exon organization of these genes was conserved in specific-family. Chromosomal localization demonstrated a random distribution of PTS genes across all the thirteen chromosomes except chromosome six. Many stress responsive cis-regulatory elements were predicted in promoter regions of GrPTS genes. The RNA-seq data analysis followed by qRT-PCR validation demonstrated that PTS genes potentially work in groups against environmental factors. Moreover, a transporter gene (GrHAK/KUP/KT8) and two channel genes (GrAKT2.1 and GrAKT1.1) are important candidate genes for plant stress response. These results provide useful information for further functional characterization of PTS genes with the breeding aim of stress-resistant cultivars.

Genome-Wide Identification and Expression Profiling of Potassium Transport-Related Genes in Vigna radiata under Abiotic Stresses

Potassium (K+) is one of the most important cations that plays a significant role in plants and constitutes up to 10% of plants' dry weight. Plants exhibit complex systems of transporters and channels for the distribution of K+ from soil to numerous parts of plants. In this study, we have identified 39 genes encoding putative K+ transport-related genes in Vigna radiata. Chromosomal mapping of these genes indicated an uneven distribution across eight out of 11 chromosomes. Comparative phylogenetic analysis of different plant species, i.e., V. radiata, Glycine max, Cicer arietinum, Oryza sativa, and Arabidopsis thaliana, showed their strong conservation in different plant species. Evolutionary analysis of these genes suggests that gene duplication is a major route of expansion for this family in V. radiata. Comprehensive promoter analysis identified several abiotic stresses related to cis-elements in the promoter regions of these genes, suggesting their role in abiotic stress tolerance. Our additional analyses indicated that abiotic stresses adversely affected the chlorophyll concentration, carotenoids, catalase, total soluble protein concentration, and the activities of superoxide and peroxidase in V. radiata. It also disturbs the ionic balance by decreasing the uptake of K+ content and increasing the uptake of Na+. Expression analysis from high-throughput sequencing data and quantitative real-time PCR experiments revealed that several K+ transport genes were expressed in different tissues (seed, flower, and pod) and in abiotic stress-responsive manners. A highly significant variation of expression was observed for VrHKT (1.1 and 1.2), VrKAT (1 and 2) VrAKT1.1, VrAKT2, VrSKOR, VrKEA5, VrTPK3, and VrKUP/HAK/KT (4, 5, and 8.1) in response to drought, heat or salinity stress. It reflected their potential roles in plant growth, development, or stress adaptations. The present study gives an in-depth understanding of K+ transport system genes in V. radiata and will serve as a basis for a functional analysis of these genes.

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.

Genome-wide identification of soybean Shaker K+ channel gene family and functional characterization of GmAKT1 in transgenic Arabidopsis thaliana under salt and drought stress

Potassium is a major cationic nutrient involved in numerous physiological processes in plants. The uptake of K+ is mediated by K+ channels and transporters, and the Shaker K+ channel gene family plays an essential role in K+ uptake and stress resistance in plants. However, little is known regarding this family in soybean. In this study, 14 members of the Shaker K+ channel gene family were identified in soybean and were classified into five groups. Protein domain analysis revealed that Shaker K+ channel gene members have an ion transport domain (ion trans), a cyclic nucleotide-binding domain, ankyrin repeat domains, and a dimerization domain in the potassium ion channel. Quantitative real-time polymerase chain reaction analysis indicated that the expression of eight genes (notably GmAKT1) in soybean leaves and roots was significantly increased in response to salt and drought stress. Furthermore, the overexpression of GmAKT1 in Arabidopsis enhanced root length, K+ concentration, and fresh/dry weight ratio compared with wild-type plants subjected to salt and drought stress; this suggests that GmAKT1 improves the tolerance of soybean to abiotic stress. Our results provide important insight into the characterization of Shaker K+ channel gene family members in soybean and highlight the function of GmAKT1 in soybean plants under salt and drought stress.

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.

Genome-Wide Identification, Genomic Organization, and Characterization of Potassium Transport-Related Genes in Cajanus cajan and Their Role in Abiotic Stress

Potassium is the most important and abundant inorganic cation in plants and it can comprise up to 10% of a plant's dry weight. Plants possess complex systems of transporters and channels for the transport of K+ from soil to numerous parts of plants. Cajanus cajan is cultivated in different regions of the world as an economical source of carbohydrates, fiber, proteins, and fodder for animals. In the current study, 39 K+ transport genes were identified in C. cajan, including 25 K+ transporters (17 carrier-like K+ transporters (KUP/HAK/KTs), 2 high-affinity potassium transporters (HKTs), and 6 K+ efflux transporters (KEAs) and 14 K+ channels (9 shakers and 5 tandem-pore K+ channels (TPKs). Chromosomal mapping indicated that these genes were randomly distributed among 10 chromosomes. A comparative phylogenetic analysis including protein sequences from Glycine max, Arabidopsis thaliana, Oryza sativa, Medicago truncatula Cicer arietinum, and C. cajan suggested vital conservation of K+ transport genes. Gene structure analysis showed that the intron/exon organization of K+ transporter and channel genes is highly conserved in a family-specific manner. In the promoter region, many cis-regulatory elements were identified related to abiotic stress, suggesting their role in abiotic stress response. Abiotic stresses (salt, heat, and drought) adversely affect chlorophyll, carotenoids contents, and total soluble proteins. Furthermore, the activities of catalase, superoxide, and peroxidase were altered in C. cajan leaves under applied stresses. Expression analysis (RNA-seq data and quantitative real-time PCR) revealed that several K+ transport genes were expressed in abiotic stress-responsive manners. The present study provides an in-depth understanding of K+ transport system genes in C. cajan and serves as a basis for further characterization of these genes.

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).

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.

Plant electrical signals: A multidisciplinary challenge

Plant electrical signals, an early event in the plant-stimulus interaction, rapidly transmit information generated by the stimulus to other organs, and even the whole plant, to promote the corresponding response and trigger a regulatory cascade. In recent years, many promising state-of-the-art technologies applicable to study plant electrophysiology have emerged. Research focused on expression of genes associated with electrical signals has also proliferated. We propose that it is appropriate for plant electrical signals to be considered in the form of a "plant electrophysiological phenotype". This review synthesizes research on plant electrical signals from a novel, interdisciplinary perspective, which is needed to improve the efficient aggregation and use of plant electrical signal data and to expedite interpretation of plant electrical signals.

Isolation and Functional Determination of SKOR Potassium Channel in Purple Osier Willow, Salix purpurea

Potassium (K⁺) plays key roles in plant growth and development. However, molecular mechanism studies of K⁺ nutrition in forest plants are largely rare. In plants, SKOR gene encodes for the outward rectifying Shaker-type K⁺ channel that is responsible for the long-distance transportation of K⁺ through xylem in roots. In this study, we determined a Shaker-type K⁺ channel gene in purple osier (Salix purpurea), designated as SpuSKOR, and determined its function using a patch clamp electrophysiological system. SpuSKOR was closely clustered with poplar PtrSKOR in the phylogenetic tree. Quantitative real-time PCR (qRT-PCR) analyses demonstrated that SpuSKOR was predominantly expressed in roots, and expression decreased under K⁺ depletion conditions. Patch clamp analysis via HEK293-T cells demonstrated that the activity of the SpuSKOR channel was activated when the cell membrane voltage reached at -10 mV, and the channel activity was enhanced along with the increase of membrane voltage. Outward currents were recorded and induced in response to the decrease of external K⁺ concentration. Our results indicate that SpuSKOR is a typical voltage dependent outwardly rectifying K⁺ channel in purple osier. This study provides theoretical basis for revealing the mechanism of K⁺ transport and distribution in woody plants.

Potassium content diminishes in infected cells of Medicago truncatula nodules due to the mislocation of channels MtAKT1 and MtSKOR/GORK

Rhizobia establish a symbiotic relationship with legumes that results in the formation of root nodules, where bacteria encapsulated by a membrane of plant origin (symbiosomes), convert atmospheric nitrogen into ammonia. Nodules are more sensitive to ionic stresses than the host plant itself. We hypothesize that such a high vulnerability might be due to defects in ion balance in the infected tissue. Low temperature SEM (LTSEM) and X-ray microanalysis of Medicago truncatula nodules revealed a potassium (K⁺) decrease in symbiosomes and vacuoles during the life span of infected cells. To clarify K⁺ homeostasis in the nodule, we performed phylogenetic and gene expression analyses, and confocal and electron microscopy localization of two key plant Shaker K⁺ channels, AKT1 and SKOR/GORK. Phylogenetic analyses showed that the genome of some legume species, including the Medicago genus, contained one SKOR/GORK and one AKT1 gene copy, while other species contained more than one copy of each gene. Localization studies revealed mistargeting and partial depletion of both channels from the plasma membrane of M. truncatula mature nodule-infected cells that might compromise ion transport. We propose that root nodule-infected cells have defects in K⁺ balance due to mislocation of some plant ion channels, as compared with non-infected cells. The putative consequences are discussed.

Potassium Control of Plant Functions: Ecological and Agricultural Implications

Potassium, mostly as a cation (K+), together with calcium (Ca2+) are the most abundant inorganic chemicals in plant cellular media, but they are rarely discussed. K+ is not a component of molecular or macromolecular plant structures, thus it is more difficult to link it to concrete metabolic pathways than nitrogen or phosphorus. Over the last two decades, many studies have reported on the role of K+ in several physiological functions, including controlling cellular growth and wood formation, xylem-phloem water content and movement, nutrient and metabolite transport, and stress responses. In this paper, we present an overview of contemporary findings associating K+ with various plant functions, emphasizing plant-mediated responses to environmental abiotic and biotic shifts and stresses by controlling transmembrane potentials and water, nutrient, and metabolite transport. These essential roles of K+ account for its high concentrations in the most active plant organs, such as leaves, and are consistent with the increasing number of ecological and agricultural studies that report K+ as a key element in the function and structure of terrestrial ecosystems, crop production, and global food security. We synthesized these roles from an integrated perspective, considering the metabolic and physiological functions of individual plants and their complex roles in terrestrial ecosystem functions and food security within the current context of ongoing global change. Thus, we provide a bridge between studies of K+ at the plant and ecological levels to ultimately claim that K+ should be considered at least at a level similar to N and P in terrestrial ecological studies.

Potassium: A key modulator for cell homeostasis

Potassium (K) is the most vital and abundant macro element for the overall growth of plants and its deficiency or, excess concentration results in many diseases in plants. It is involved in regulation of many crucial roles in plant development. Depending on soil-root interactions, complex soil dynamics often results in unpredictable availability of the elements. Based on the importance index, K is considered to be the second only to nitrogen for the overall growth of plants. More than 60 enzymes within the plant system depend on K for its activation, in which K act as a key regulator. K helps plants to resist several abiotic and biotic stresses in the environment. We have reviewed the research progress about K's role in plants covering various important considerations of K highlighting the effects of microbes on soil K⁺; K and its contribution to adsorbed dose in plants; the importance of K⁺ deficiency; physiological functions of K⁺ transporters and channels; and interference of abiotic stressor in the regulatory role of K. This review further highlights the scope of future research regarding K.

Genome-Wide Survey and Expression Analysis of the KT/HAK/KUP Family in Brassica napus and Its Potential Roles in the Response to K+ Deficiency

The KT/HAK/KUP (HAK) family is the largest potassium (K⁺) transporter family in plants, which plays key roles in K⁺ uptake and homeostasis, stress resistance, and root and embryo development. However, the HAK family has not yet been characterized in Brassica napus. In this study, 40 putative B. napus HAK genes (BnaHAKs) are identified and divided into four groups (Groups I–III and V) on the basis of phylogenetic analysis. Gene structure analysis revealed 10 conserved intron insertion sites across different groups. Collinearity analysis demonstrated that both allopolyploidization and small-scale duplication events contributed to the large expansion of BnaHAKs. Transcription factor (TF)-binding network construction, cis-element analysis, and microRNA prediction revealed that the expression of BnaHAKs is regulated by multiple factors. Analysis of RNA-sequencing data further revealed extensive expression profiles of the BnaHAKs in groups II, III, and V, with limited expression in group I. Compared with group I, most of the BnaHAKs in groups II, III, and V were more upregulated by hormone induction based on RNA-sequencing data. Reverse transcription-quantitative polymerase reaction analysis revealed that the expression of eight BnaHAKs of groups I and V was markedly upregulated under K⁺-deficiency treatment. Collectively, our results provide valuable information and key candidate genes for further functional studies of BnaHAKs.

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.

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

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

Characterization and Expression of KT/HAK/KUP Transporter Family Genes in Willow under Potassium Deficiency, Drought, and Salt Stresses

The K⁺ transporter/high-affinity K⁺/K⁺ uptake (KT/HAK/KUP) transporters dominate K⁺ uptake, transport, and allocation that play a pivotal role in mineral homeostasis and plant adaptation to adverse abiotic stresses. However, molecular mechanisms towards K⁺ nutrition in forest trees are extremely rare, especially in willow. In this study, we identified 22 KT/HAK/KUP transporter genes in purple osier willow (designated as SpuHAK1 to SpuHAK22) and examined their expression under K⁺ deficiency, drought, and salt stress conditions. Both transcriptomic and quantitative real-time PCR (qRT-PCR) analyses demonstrated that SpuHAKs were predominantly expressed in stems, and the expression levels of SpuHAK1, SpuHAK2, SpuHAK3, SpuHAK7, and SpuHAK8 were higher at the whole plant level, whereas SpuHAK9, SpuHAK11, SpuHAK20, and SpuHAK22 were hardly detected in tested tissues. In addition, both K⁺ deficiency and salt stress decreased the tissue K⁺ content, while drought increased the tissue K⁺ content in purple osier plant. Moreover, SpuHAK genes were differentially responsive to K⁺ deficiency, drought, and salt stresses in roots. K⁺ deficiency and salt stress mainly enhanced the expression level of responsive SpuHAK genes. Fifteen putative cis-acting regulatory elements, including the stress response, hormone response, circadian regulation, and nutrition and development, were identified in the promoter region of SpuHAK genes. Our findings provide a foundation for further functional characterization of KT/HAK/KUP transporters in forest trees and may be useful for breeding willow rootstocks that utilize potassium more efficiently.

Ionomic and transcriptomic analyses of two cotton cultivars (Gossypium hirsutum L.) provide insights into the ion balance mechanism of cotton under salt stress

Soil salinity is a major abiotic stress factor that limits cotton production worldwide. To improve salt tolerance in cotton, an in-depth understanding of ionic balance is needed. In this study, a pot experiment using three levels of soil salinity (0%, 0.2%, and 0.4%, represented as CK, SL, and SH, respectively) and two cotton genotypes (salt-tolerant genotype: L24; salt-sensitive genotype: X45) was employed to investigate how sodium chloride (NaCl) stress effects cotton growth, ion distribution, and transport, as well as to explore the related mechanism. The results showed that SL treatment mainly inhibited shoot growth, while SH treatment caused more extensive impairment to roots and shoots. The growth inhibition ratio of NaCl stress on X45 was more marked than that of L24. Under NaCl stress, the Na concentration in the roots, stems and leaves significantly increased, whereas the K, Cu, B, and Mo concentration in roots, as well as Mg and S concentrations in the leaves, significantly decreased. Under salt stress conditions, salt-tolerant cotton plants can store Na in the leaves, and as a result, a larger amount of minerals (e.g., Cu, Mo, Si, P, and B) tend to transport to the leaves. By contrast, salt-sensitive varieties tend to accumulate certain minerals (e.g., Ca, P, Mg, S, Mn, Fe, Cu, B, Mo, and Si) in the roots. Most genes related to ion transport and homeostasis were upregulated in L24, but not in X45. The expression level of GhSOS1 in X45 was higher than L24, but GhNHX1 in L24 was higher than X45. Our findings suggest that the two varieties response to salt stress differently; for X45 (salt-sensitive), the response is predominantly governed by Na+ efflux, whereas for L24 (salt-tolerant), vacuolar sequestration of Na+ is the major mechanism. The expression changes of the genes encoding the ion transporters may partially explain the genotypic difference in leaf ion accumulation under salt stress conditions.

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.

Adaptation of Plants to Salt Stress: Characterization of Na+ and K+ Transporters and Role of CBL Gene Family in Regulating Salt Stress Response

Salinity is one of the most serious factors limiting the productivity of agricultural crops, with adverse effects on germination, plant vigor, and crop yield. This salinity may be natural or induced by agricultural activities such as irrigation or the use of certain types of fertilizer. The most detrimental effect of salinity stress is the accumulation of Na+ and Cl− ions in tissues of plants exposed to soils with high NaCl concentrations. The entry of both Na+ and Cl− into the cells causes severe ion imbalance, and excess uptake might cause significant physiological disorder(s). High Na+ concentration inhibits the uptake of K+, which is an element for plant growth and development that results in lower productivity and may even lead to death. The genetic analyses revealed K+ and Na+ transport systems such as SOS1, which belong to the CBL gene family and play a key role in the transport of Na+ from the roots to the aerial parts in the Arabidopsis plant. In this review, we mainly discuss the roles of alkaline cations K+ and Na+, Ion homeostasis-transport determinants, and their regulation. Moreover, we tried to give a synthetic overview of soil salinity, its effects on plants, and tolerance mechanisms to withstand stress.

Comparative transcriptomics analysis of potassium uptake pathways mediated cesium accumulation differences and related molecular mechanisms in Brassica juncea and Vicia faba

To analyze the differences between high- and low-accumulation plants in cesium (Cs) uptake and its related mechanism, Brassica juncea (a hyperaccumulation plant for Cs) and Vicia faba (a low-accumulation plant for Cs) were selected as comparative experimental materials. The contributions to Cs uptake of a K-transporter-mediated high-affinity transport system and a K-channel-mediated low-affinity transport system in the two plants were compared and analyzed. The difference between the two plants in the mechanism of Cs uptake was further analyzed using transcription sequence technology. The results show that the transfer characteristics of Cs in the two plants had a similar distribution relationship with K. The contribution rate of the K-channel pathway to Cs uptake was 32.00% in the V. faba seedling roots, which was significantly higher than for B. juncea (9.81%) (P < 0.01); the contribution rate of the K-transporter pathway to Cs uptake of the B. juncea seedlings was 32.08%, which was significantly higher than that of the V. faba seedlings (17.13%)(P < 0.05). Other uptake pathways also mediated the uptake of Cs by roots in B. juncea and V. faba (contribution rate: 54.92-60.09% and 42.18-59.73%, respectively). The transcriptome sequencing results confirmed that Cs-induced treatment significantly inhibited the expression of the K-transporter protein and K-channel protein-related genes in the V. faba roots, but it had no significant effect on the expression of related genes in the B. juncea roots. Thus, one reason for the significant difference between the two plant in the accumulation of Cs is that Cs inhibited the expression of related transporter protein genes in the V. faba roots.

Saccharomyces cerevisiae as a Tool to Investigate Plant Potassium and Sodium Transporters

Sodium and potassium are two alkali cations abundant in the biosphere. Potassium is essential for plants and its concentration must be maintained at approximately 150 mM in the plant cell cytoplasm including under circumstances where its concentration is much lower in soil. On the other hand, sodium must be extruded from the plant or accumulated either in the vacuole or in specific plant structures. Maintaining a high intracellular K+/Na+ ratio under adverse environmental conditions or in the presence of salt is essential to maintain cellular homeostasis and to avoid toxicity. The baker's yeast, Saccharomyces cerevisiae, has been used to identify and characterize participants in potassium and sodium homeostasis in plants for many years. Its utility resides in the fact that the electric gradient across the membrane and the vacuoles is similar to plants. Most plant proteins can be expressed in yeast and are functional in this unicellular model system, which allows for productive structure-function studies for ion transporting proteins. Moreover, yeast can also be used as a high-throughput platform for the identification of genes that confer stress tolerance and for the study of protein-protein interactions. In this review, we summarize advances regarding potassium and sodium transport that have been discovered using the yeast model system, the state-of-the-art of the available techniques and the future directions and opportunities in this field.

Grafting alleviates potassium stress and improves growth in tobacco

BACKGROUND

Potassium is a nutrient element necessary for tobacco growth. Tobacco leaves with high potassium content are elastic and tough, rich in oil. And the same time, potassium can also improve the scent and aromatic value of flue-cured tobacco by regulating the synthesis of aromatic hydrocarbons in leaves.. It is an important quality indicator for flue-cured tobacco. However, the potassium concentration in tobacco leaves in most areas of China is generally lower than the global standard for high quality tobacco. Two tobacco genotypes were grafted to each other under different potassium levels to test whether potassium content and plant growth can be improved by grafting in tobacco.

RESULTS

The growth of tobacco in all treatments was inhibited under potassium starvation, and grafting significantly alleviated this potassium stress in 'Yunyan 87'. The trends in whole plant K+ uptake and K+ transfer efficiency to the leaves corresponded to the growth results of the different grafts. The nutrient depletion test results showed that the roots of 'Wufeng No.2' had higher K+ absorption potential, K+ affinity, and K+ inward flow rate. K+ enrichment circles appeared at the endoderm of the root section in the energy dispersive X-ray figure, indicating that the formation of Casparian strips may be partly responsible for the lower rate of lateral movement of K+ in the roots of 'Yunyan 87'. Gene expression analysis suggested that energy redistribution at the whole plant level might constitute one strategy for coping with potassium starvation. The feedback regulation effects between scion 'Wufeng No.2' and rootstock 'Yunyan 87' indicated that the transmission of certain signaling substances had occurred during grafting.

CONCLUSIONS

'Wufeng No.2' tobacco rootstock grafting can increase the K+ uptake and transport efficiency of 'Yunyan 87' and enhance plant growth under potassium stress. The physiological mechanism of the improved performance of grafted tobacco is related to higher K+ uptake and utilization ability, improved xylem K+ loading capacity, and up-regulated expression of genes related to energy supply systems.