Potassium transport and signaling in higher plants

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.

Biosynthesized gold nanoparticles maintained nitrogen metabolism, nitric oxide synthesis, ions balance, and stabilizes the defense systems to improve salt stress tolerance in wheat

Green synthesis of nanoparticles (NPs) is competent in inducing physiological responses in plants for combating the abiotic stresses. Considering this, salt stress is one of the most alarming conditions that exerts complex and polygenic impacts on morph-physiological functioning of plants; resulting in reduced crop productivity and yield. Therefore, understanding the salt responses and tolerance mechanisms are important for sustaining crop productivity. In the current study, we have examined the effects of biosynthesized gold nanoparticles (AuNPs) on wheat (Triticum aestivum) plants under salt stress. Green-synthesized AuNPs were found beneficial in modulating the K⁺/Na⁺ ratio, chlorophyll concentration, defense systems, nitrogen assimilation, stomatal dynamics and growth traits under salt stress condition. Furthermore, the excessive accumulation of oxidative stress markers including reactive oxygen/nitrogen species was controlled in response of AuNPs treatment under salt stress. Overall, modulation of these traits commanded to induce salt stress tolerance in wheat plants.

Morphophysiological and transcriptome analysis reveal that reprogramming of metabolism, phytohormones and root development pathways governs the potassium (K+) deficiency response in two contrasting chickpea cultivars

Potassium (K⁺) is an essential macronutrient for plant growth and development. K⁺ deficiency hampers important plant processes, such as enzyme activation, protein synthesis, photosynthesis and stomata movement. Molecular mechanism of K⁺ deficiency tolerance has been partly understood in model plants Arabidopsis, but its knowledge in legume crop chickpea is missing. Here, morphophysiological analysis revealed that among five high yielding desi chickpea cultivars, PUSA362 shows stunted plant growth, reduced primary root growth and low K⁺ content under K⁺ deficiency. In contrast, PUSA372 had negligible effect on these parameters suggesting that PUSA362 is K⁺ deficiency sensitive and PUSA372 is a K⁺ deficiency tolerant chickpea cultivar. RNA-seq based transcriptome analysis under K⁺ deficiency revealed a total of 820 differential expressed genes (DEG's) in PUSA362 and 682 DEGs in PUSA372. These DEGs belongs to different functional categories, such as plant metabolism, signal transduction components, transcription factors, ion/nutrient transporters, phytohormone biosynthesis and signalling, and root growth and development. RNA-seq expression of randomly selected 16 DEGs was validated by RT-qPCR. Out of 16 genes, 13 showed expression pattern similar to RNA-seq expression, that verified the RNA-seq expression data. Total 258 and 159 genes were exclusively up-regulated, and 386 and 347 genes were down-regulated, respectively in PUSA362 and PUSA372. 14 DEGs showed contrasting expression pattern as they were up-regulated in PUSA362 and down-regulated in PUSA372. These include somatic embryogenesis receptor-like kinase 1, thaumatin-like protein, ferric reduction oxidase 2 and transcription factor bHLH93. Nine genes which were down-regulated in PUSA362 found to be up-regulated in PUSA372, including glutathione S-transferase like, putative calmodulin-like 19, high affinity nitrate transporter 2.4 and ERF17-like protein. Some important carbohydrate metabolism related genes, like fructose-1,6-bisphosphatase and sucrose synthase, and root growth related Expansin gene were exclusively down-regulated, while an ethylene biosynthesis gene 1-aminocyclopropane-1-carboxylate oxidase 1 (ACO1) was up-regulated in PUSA362. Interplay of these and several other genes related to hormones (auxin, cytokinin, GA etc.), signal transduction components (like CBLs and CIPKs), ion transporters and transcription factors might underlie the contrasting response of two chickpea cultivars to K⁺ deficiency. In future, some of these key genes will be utilized in genetic engineering and breeding programs for developing chickpea cultivars with improved K⁺ use efficiency (KUE) and K⁺ deficiency tolerance traits.

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.

How do rice seedlings of landrace Pokkali survive in saline fields after transplantation? Physiology, biochemistry, and photosynthesis

Rice, one of the most important staple food crops in the world, is highly sensitive to soil salinity at the seedling stage. The ultimate yield of this crop is a function of the number of seedlings surviving after transplantation in saline water. Oryza sativa cv. IR64 is a high-yielding salinity-sensitive variety, while Pokkali is a landrace traditionally cultivated by the local farmers in the coastal regions in India. However, the machinery responsible for the seedling-stage tolerance in Pokkali is not understood. To bridge this gap, we subjected young seedlings of these contrasting genotypes to salinity and performed detailed investigations about their growth parameters, ion homeostasis, biochemical composition, and photosynthetic parameters after every 24 h of salinity for three days. Taken together, all the physiological and biochemical indicators, such as proline accumulation, K⁺/Na⁺ ratio, lipid peroxidation, and electrolyte leakage, clearly revealed significant differences between IR64 and Pokkali under salinity, establishing their contrasting nature at this stage. In response to salinity, the F_v/F_m ratio (maximum quantum efficiency of Photosystem II as inferred from Chl a fluorescence) and the energy conserved for the electron transport after the reduction of Q_A (the primary electron acceptor of PSII), to Q_A^-, and reduction of the end electron acceptor molecules towards the PSI (Photosystem I) electron acceptor side was higher in Pokkali than IR64 plants. These observations reflect a direct contribution of photosynthesis towards seedling-stage salinity tolerance in rice. These findings will help to breed high-yielding crops for salinity prone agricultural lands.

Interplay between gasotransmitters and potassium is a K+ey factor during plant response to abiotic stress

Carbon monoxide (CO), nitric oxide (NO) and hydrogen sulfide (H₂S) are gasotransmitters known for their roles in plant response to (a)biotic stresses. The crosstalk between these gasotransmitters and potassium ions (K⁺) has received considerable attention in recent years, particularly due to the dual role of K⁺ as an essential mineral nutrient and a promoter of plant tolerance to abiotic stress. This review brings together what it is known about the interplay among NO, CO, H₂S and K⁺ in plants with focus on the response to high salinity. Some findings obtained for plants under water deficit and metal stress are also presented and discussed since both abiotic stresses share similarities with salt stress. The molecular targets of the gasotransmitters NO, CO and H₂S in root and guard cells that drive plant tolerance to salt stress are highlighted as well.

A Soft Evaporation and Ionization Technique for Mass Spectrometric Analysis and Bio-Imaging of Metal Ions in Plants Based on Metal-Iodide Cluster Ionization

Protonation/deprotonation is the well-recognized mass spectrometric mechanism in matrix-assisted laser desorption ionization of organic molecules but not for metal ions with different oxidation states. We describe herein a soft evaporation and ionization technique for metal ions based on iodination/de-iodination in metal-iodide cluster ionization (MICI). It is not only able to determine identities and oxidation states of metal ions but also reveal spatial distributions and isotope ratios in response to physiological or environmental changes. A long chain alcohol 1-tetradecanol with no functional groups that can absorb laser irradiation was used to cover and prevent samples from direct laser ablation. Upon the irradiation of the third harmonic Nd³⁺:YAG (355 nm, 3 ns), iohexol containing three covalently bonded iodine atoms instantly generates negative iodide ions that can quantitatively form clusters with at least 14 essential metal ions present in plants. The detection limits vary with different metal ions down to low fmol. MICI eliminates the atomization process that obscures metal charges in inductively coupled plasma mass spectrometry. Because only metal ions can be iodinated with iohexol, interferences from the abundant organic molecules of plants that are confronted by secondary ion mass spectrometry (SIMS) are also greatly decreased.

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.

Benefits provided by four ectomycorrhizal fungi to Pinus taeda under different external potassium availabilities

Ectomycorrhizal fungi contribute to the nutrition of many woody plants, including those in the Pinaceae family. Loblolly pine (Pinus taeda L.), a native species of the Southeastern USA, can be colonized by multiple species of ectomycorrhizal fungi. The role of these symbionts in P. taeda potassium (K⁺) nutrition has not been previously investigated. Here, we assessed the contribution of four ectomycorrhizal fungi, Hebeloma cylindrosporum, Paxillus ammoniavirescens, Laccaria bicolor, and Suillus cothurnatus, in P. taeda K⁺ acquisition under different external K⁺ availabilities. Using a custom-made two-compartment system, P. taeda seedlings were inoculated with one of the four fungi, or kept non-colonized, and grown under K⁺-limited or -sufficient conditions for 8 weeks. Only the fungi had access to separate compartments in which rubidium, an analog tracer for K⁺, was supplied before harvest. Resulting effects of the fungi were recorded, including root colonization, biomass, and nutrient concentrations. We also analyzed the fungal performance in axenic conditions under varying supply of K⁺ and sodium. Our study revealed that these four ectomycorrhizal fungi are differentially affected by external K⁺ and sodium variations, that they are not able to provide similar benefits to the host P. taeda in our growing conditions, and that rubidium may be used with some limitations to estimate K⁺ transport from ectomycorrhizal fungi to colonized plants.

A starting guide to root ecology: strengthening ecological concepts and standardising root classification, sampling, processing and trait measurements

In the context of a recent massive increase in research on plant root functions and their impact on the environment, root ecologists currently face many important challenges to keep on generating cutting-edge, meaningful and integrated knowledge. Consideration of the below-ground components in plant and ecosystem studies has been consistently called for in recent decades, but methodology is disparate and sometimes inappropriate. This handbook, based on the collective effort of a large team of experts, will improve trait comparisons across studies and integration of information across databases by providing standardised methods and controlled vocabularies. It is meant to be used not only as starting point by students and scientists who desire working on below-ground ecosystems, but also by experts for consolidating and broadening their views on multiple aspects of root ecology. Beyond the classical compilation of measurement protocols, we have synthesised recommendations from the literature to provide key background knowledge useful for: (1) defining below-ground plant entities and giving keys for their meaningful dissection, classification and naming beyond the classical fine-root vs coarse-root approach; (2) considering the specificity of root research to produce sound laboratory and field data; (3) describing typical, but overlooked steps for studying roots (e.g. root handling, cleaning and storage); and (4) gathering metadata necessary for the interpretation of results and their reuse. Most importantly, all root traits have been introduced with some degree of ecological context that will be a foundation for understanding their ecological meaning, their typical use and uncertainties, and some methodological and conceptual perspectives for future research. Considering all of this, we urge readers not to solely extract protocol recommendations for trait measurements from this work, but to take a moment to read and reflect on the extensive information contained in this broader guide to root ecology, including sections I-VII and the many introductions to each section and root trait description. Finally, it is critical to understand that a major aim of this guide is to help break down barriers between the many subdisciplines of root ecology and ecophysiology, broaden researchers' views on the multiple aspects of root study and create favourable conditions for the inception of comprehensive experiments on the role of roots in plant and ecosystem functioning.

Ca2+-CBL-CIPK: a modulator system for efficient nutrient acquisition

Calcium (Ca²⁺) is a universal second messenger essential for the growth and development of plants in normal and stress situations. In plants, the proteins, CBL (calcineurin B-like) and CIPK (CBL-interacting protein kinase), form one of the important Ca²⁺ decoding complexes to decipher Ca²⁺ signals elicited by environmental challenges. Multiple interactors distinguish CBL and CIPK protein family members to form a signaling network for regulated perception and transduction of environmental signals, e.g., signals generated under nutrient stress conditions. Conservation of equilibrium in response to varying soil nutrient status is an important aspect for plant vigor and yield. Signaling processes have been reported to observe nutrient fluctuations as a signal responsible for regulated nutrient transport adaptation. Recent studies have identified downstream targets of CBL-CIPK modules as ion channels or transporters and their association in signaling nutrient disposal including potassium, nitrate, ammonium, magnesium, zinc, boron, and iron. Ca²⁺-CBL-CIPK pathway modulates ion transporters/channels and hence maintains a homeostasis of several important plant nutrients in the cytosol and sub-cellular compartments. In this article, we summarize recent literature to discuss the role of the Ca²⁺-CBL-CIPK pathway in cellular osmoregulation and homeostasis on exposure to nutrient excess or deprived soils. This further establishes a link between taking up the nutrient in the roots and its distribution and homeostasis during the generation of signal for the development and survival of plants.

Characterization of transcription factor MYB59 and expression profiling in response to low K+ and NO3- in indica rice (Oryza sativa L.)

Background

Nitrogen and potassium are crucial supplements for plant development and growth. Plants can detect potassium and nitrate ions in soils and in like way, they modify root-to-shoot transport of these ions to adjust the conveyance among roots and shoots. Transcription factor MYB59 plays essential roles in numerous physiological processes inclusive of hormone response, abiotic stress tolerance, plant development, and metabolic regulation. In this study, we retrieved 56 MYB59 proteins from different plant species. Multiple sequence alignment, phylogenetic tree, conserved motif, chromosomal localization, and cis-regulatory elements of the retrieved sequences were analyzed. Gene structure, protein 3D structure, and DNA binding of OsMYB59 indica were also predicted. Finally, we characterized OsMYB59 and its function under low K⁺/NO₃⁻ conditions in Oryza sativa subsp. indica.

Results

Data analysis showed that MYB59s from various groups separated in terms of conserved functional domains and gene structure, where members of genus Oryza clustered together. Plants showed reduced height and yellowish appearance when grown on K⁺ and NO₃⁻ deficient medium. Quantitative real-time PCR uncovered that the OsMYB59 reacted to abiotic stresses where its expression was increased in BRRI dhan56 but decreased in other varieties on K⁺ deficient medium. In addition, OsMYB59 transcript level increased on NO₃⁻ deficient medium.

Conclusions

Our results can help to explain the biological functions of indica rice MYB59 protein and gave a theoretical premise to additionally describe its biological roles in response to abiotic stresses particularly drought.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-021-00248-6.

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).

MYB77 regulates high-affinity potassium uptake by promoting expression of HAK5

In Arabidopsis, the high-affinity K⁺ transporter HAK5 is the major pathway for root K⁺ uptake when below 100 µM; HAK5 responds to Low-K⁺ (LK) stress by strongly and rapidly increasing its expression during K⁺ -deficiency. Therefore, positive regulators of HAK5 expression have the potential to improve K⁺ uptake under LK. Here, we show that mutants of the transcription factor MYB77 share a LK-induced leaf chlorosis phenotype, lower K⁺ content, and lower Rb⁺ uptake of the hak5 mutant, but not the shorter root growth, and that overexpression of MYB77 enhanced K⁺ uptake and improved tolerance to LK stress. Furthermore, we demonstrated that MYB77 positively regulates the expression of HAK5, by binding to the HAK5 promoter and enhances high-affinity K⁺ uptake of roots. As such, our results reveal a novel pathway for enhancing HAK5 expression under LK stress, and provides a candidate for increasing the tolerance of plants to LK.

The CBL1/9-CIPK23-AKT1 complex is essential for low potassium response in cassava

Cassava is a food crop and an important energy crop worldwide. However, its yield and quality are easily affected by low K+ stress, and the molecular mechanism of potassium channel is unknown in cassava. Herein, we revealed that calcineurin B-like 1/9 (MeCBL1/9)-CBL-interacting protein kinase 23 (MeCIPK23)-K+ TRANSPORTER1 (MeAKT1) complex plays an important role in low potassium response in cassava. Firstly, this study verified the in vivo role of MeAKT1 in K+ uptake in yeast. Secondly, we found that MeCBL1, MeCBL9, MeCIPK23 and MeAKT1 are involved in the absorption of K+ in cassava, and MeCBL1/9-CIPK23 complex is essential for MeAKT1-mediated K+ uptake. Moreover, MeCBL1/9-MeCIPK23-MeAKT1 showed different expression in different cassava varieties contrasting in the resistance to low K+ stress. Taken together, this study provides new insights into further improvement of K+ uptake in cassava.

Potassium: A track to develop salinity tolerant plants

Salinity is one of the major constraints to plant growth and development across the globe that leads to the huge crop productivity loss. Salinity stress causes impairment in plant's metabolic and cellular processes including disruption in ionic homeostasis due to excess of sodium (Na⁺) ion influx and potassium (K⁺) efflux. This condition subsequently results in a significant reduction of the cytosolic K⁺ levels, eventually inhibiting plant growth attributes. K⁺ plays a crucial role in alleviating salinity stress by recasting key processes of plants. In addition, K⁺ acquisition and retention also serve as the perquisite trait to establish salt tolerant mechanism. In addition, an intricate network of genes and their regulatory elements are involved in coordinating salinity stress responses. Furthermore, plant growth regulators (PGRs) and other signalling molecules influence K⁺-mediated salinity tolerance in plants. Recently, nanoparticles (NPs) have also been found several implications in plants with respect to their roles in mediating K⁺ homoeostasis during salinity stress in plants. The present review describes salinity-induced adversities in plants and role of K⁺ in mitigating salinity-induced damages. The review also highlights the efficacy of PGRs and other signalling molecules in regulating K⁺ mediated salinity tolerance along with nano-technological perspective for improving K⁺ mediated salinity tolerance in plants.

Gibberellin biosynthesis inhibitor mepiquat chloride enhances root K+ uptake in cotton by modulating plasma membrane H+-ATPase

Potassium deficiency causes severe losses in yield and quality in crops. Mepiquat chloride, a plant growth regulator, can increase K+ uptake in cotton (Gossypium hirsutum), but the underlying physiological mechanisms remain unclear. In this study, we used a non-invasive micro-test technique to measure K+ and H+ fluxes in the root apex with or without inhibitors of K+ channels, K+ transporters, non-selective cation channels, and plasma membrane H+-ATPases. We found that soaking seeds in mepiquat chloride solution increased the K+ influx mediated by K+ channels and reduced the K+ efflux mediated by non-selective cation channels in cotton seedlings. Mepiquat chloride also increased negative membrane potential (Em) and the activity of plasma membrane H+-ATPases in roots, due to higher levels of gene expression and protein accumulation of plasma membrane H+-ATPases as well as phosphorylation of H+-ATPase 11 (GhAHA11). Thus, plasma membrane hyperpolarization mediated by H+-ATPases was able to stimulate the activity of K+ channels in roots treated with mepiquat chloride. In addition, reduced K+ efflux under mepiquat chloride treatment was associated with reduced accumulation of H2O2 in roots. Our results provide important insights into the mechanisms of mepiquat chloride-induced K+ uptake in cotton and hence have the potential to help in improving K nutrition for enhancing cotton production.

Potassium dependency of enzymes in plant primary metabolism

Potassium is a macroelement essential to many aspects of plant life, such as photosynthesis, phloem transport or cellular electrochemistry. Many enzymes in animals or microbes are known to be stimulated or activated by potassium (K⁺ ions). Several plant enzymes are also strictly K⁺-dependent, and this can be critical when plants are under K deficiency and thus intracellular K⁺ concentration is low. Although metabolic effects of low K conditions have been documented, there is presently no review focusing on roles of K⁺ for enzyme catalysis or activation in plants. In this mini-review, we compile the current knowledge on K⁺-requirement of plant enzymes and take advantage of structural data to present biochemical roles of K⁺. This information is instrumental to explain direct effects of low K⁺ content on metabolism and this is illustrated with recent metabolomics data.

A potassium-sensing niche in Arabidopsis roots orchestrates signaling and adaptation responses to maintain nutrient homeostasis

Organismal homeostasis of the essential ion K⁺ requires sensing of its availability, efficient uptake, and defined distribution. Understanding plant K⁺ nutrition is essential to advance sustainable agriculture, but the mechanisms underlying K⁺ sensing and the orchestration of downstream responses have remained largely elusive. Here, we report where plants sense K⁺ deprivation and how this translates into spatially defined ROS signals to govern specific downstream responses. We define the organ-scale K⁺ pattern of roots and identify a postmeristematic K⁺-sensing niche (KSN) where rapid K⁺ decline and Ca²⁺ signals coincide. Moreover, we outline a bifurcating low-K⁺-signaling axis of CIF peptide-activated SGN3-LKS4/SGN1 receptor complexes that convey low-K⁺-triggered phosphorylation of the NADPH oxidases RBOHC, RBOHD, and RBOHF. The resulting ROS signals simultaneously convey HAK5 K⁺ uptake-transporter induction and accelerated Casparian strip maturation. Collectively, these mechanisms synchronize developmental differentiation and transcriptome reprogramming for maintaining K⁺ homeostasis and optimizing nutrient foraging by roots.

NRT1.1 Dual-Affinity Nitrate Transport/Signalling and its Roles in Plant Abiotic Stress Resistance

NRT1.1 is the first nitrate transport protein cloned in plants and has both high- and low-affinity functions. It imports and senses nitrate, which is modulated by the phosphorylation on Thr101 (T101). Structural studies have revealed that the phosphorylation of T101 either induces dimer decoupling or increases structural flexibility within the membrane, thereby switching the NRT1.1 protein from a low- to high-affinity state. Further studies on the adaptive regulation of NRT1.1 in fluctuating nitrate conditions have shown that, at low nitrate concentrations, nitrate binding only at the high-affinity monomer initiates NRT1.1 dimer decoupling and priming of the T101 site for phosphorylation activated by CIPK23, which functions as a high-affinity nitrate transceptor. However, nitrate binding in both monomers retains the unmodified NRT1.1, maintaining the low-affinity mode. This NRT1.1-mediated nitrate signalling and transport may provide a key to improving the efficiency of plant nitrogen use. However, recent studies have revealed that NRT1.1 is extensively involved in plant tolerance of several adverse environmental conditions. In this context, we summarise the recent progress in the molecular mechanisms of NRT1.1 dual-affinity nitrate transport/signalling and focus on its expected and unexpected roles in plant abiotic stress resistance and their regulation processes.

K Deprivation Modulates the Primary Metabolites and Increases Putrescine Concentration in Brassica napus

Potassium (K) plays a crucial role in plant growth and development and is involved in different physiological and biochemical functions in plants. Brassica napus needs higher amount of nutrients like nitrogen (N), K, phosphorus (P), sulfur (S), and boron (B) than cereal crops. Previous studies in B. napus are mainly focused on the role of N and S or combined deficiencies. Hence, little is known about the response of B. napus to K deficiency. Here, a physiological, biochemical, and molecular analysis led us to investigate the response of hydroponically grown B. napus plants to K deficiency. The results showed that B. napus was highly sensitive to the lack of K. The lower uptake and translocation of K induced BnaHAK5 expression and significantly declined the growth of B. napus after 14 days of K starvation. The lower availability of K was associated with a decrease in the concentration of both S and N and modulated the genes involved in their uptake and transport. In addition, the lack of K induced an increase in Ca2+ and Mg2+ concentration which led partially to the accumulation of positive charge. Moreover, a decrease in the level of arginine as a positively charged amino acid was observed which was correlated with a substantial increase in the polyamine, putrescine (Put). Furthermore, K deficiency induced the expression of BnaNCED3 as a key gene in abscisic acid (ABA) biosynthetic pathway which was associated with an increase in the levels of ABA. Our findings provided a better understanding of the response of B. napus to K starvation and will be useful for considering the importance of K nutrition in this crop.

Under salt stress guard cells rewire ion transport and abscisic acid signaling

Soil salinity is an increasingly global problem which hampers plant growth and crop yield. Plant productivity depends on optimal water-use efficiency and photosynthetic capacity balanced by stomatal conductance. Whether and how stomatal behavior contributes to salt sensitivity or tolerance is currently unknown. This work identifies guard cell-specific signaling networks exerted by a salt-sensitive and salt-tolerant plant under ionic and osmotic stress conditions accompanied by increasing NaCl loads. We challenged soil-grown Arabidopsis thaliana and Thellungiella salsuginea plants with short- and long-term salinity stress and monitored genome-wide gene expression and signals of guard cells that determine their function. Arabidopsis plants suffered from both salt regimes and showed reduced stomatal conductance while Thellungiella displayed no obvious stress symptoms. The salt-dependent gene expression changes of guard cells supported the ability of the halophyte to maintain high potassium to sodium ratios and to attenuate the abscisic acid (ABA) signaling pathway which the glycophyte kept activated despite fading ABA concentrations. Our study shows that salinity stress and even the different tolerances are manifested on a single cell level. Halophytic guard cells are less sensitive than glycophytic guard cells, providing opportunities to manipulate stomatal behavior and improve plant productivity.

Functional divergence of Brassica napus BnaABI1 paralogs in the structurally conserved PP2CA gene subfamily of Brassicaceae

Group A PP2C (PP2CA) genes form a gene subfamily whose members play an important role in regulating many biological processes by dephosphorylation of target proteins. In this study we examined the effects of evolutionary changes responsible for functional divergence of BnaABI1 paralogs in Brassica napus against the background of the conserved PP2CA gene subfamily in Brassicaceae. We performed comprehensive phylogenetic analyses of 192 PP2CA genes in 15 species in combination with protein structure homology modeling. Fundamentally, the number of PP2CA genes remained relatively constant in these taxa, except in the Brassica genus and Camelina sativa. The expansion of this gene subfamily in these species has resulted from whole genome duplication. We demonstrated a high degree of structural conservation of the PP2CA genes, with a few minor variations between the different PP2CA groups. Furthermore, the pattern of conserved sequence motifs in the PP2CA proteins and their secondary and 3D structures revealed strong conservation of the key ion-binding sites. Syntenic analysis of triplicated regions including ABI1 paralogs revealed significant structural rearrangements of the Brassica genomes. The functional and syntenic data clearly show that triplication of BnaABI1 in B. napus has had an impact on its functions, as well as the positions of adjacent genes in the corresponding chromosomal regions. The expression profiling of BnaABI1 genes showed functional divergence, i.e. subfunctionalization, potentially leading to neofunctionalization. These differences in expression are likely due to changes in the promoters of the BnaABI1 paralogs. Our results highlight the complexity of PP2CA gene subfamily evolution in Brassicaceae.

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.

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.

Flooding or drought which one is more offensive on pepper physiology and growth?

Both extreme usage of water in agriculture i.e., drought and flooding affect physiological and growth aspects of the plant as well as gene expression undertaken in water absorption. These affect depend on the stress duration i.e., shock or gradual stress exposer. The factorial experiment based on CRD with 10 replicates was conducted to investigate the physiological and water relation as well as aquaporin expression in (Capsicum annuum L.). Drought stress was applied gradually from - 2, - 3, - 4 to - 5 MPa during 8 days but in shock stress - 5 MPa applied at one time. The gradual flooding stress adjusted with changing the aeration duration from 15 to 0 min gradually every 2 days and for the shock- flooding, peppers keep in a nutrient solution without aeration in a sealed container. Results showed that both extreme water stress had a deleterious effect on the growth and physiological parameter of pepper for a longer duration. Antioxidant, proline, fluorescence chlorophyll stimulate in the gradual period except for ABA content, which is higher in shock stress. PIP1expression showed a reverse effect in leaf and root at flooding i.e., PIP1expression raised in root while it was reduced in leaf at shock-flooding. The highest PIP1expression was observed in gradual-drought of root and gradual duration of drought and flooding stress in leaf. In the physiological aspect of plant response to stress in pepper, results showed an enhanced in proline and phenol content to help osmotic adjustment and keep water status in moderate condition. Conclusively, shocked stress first, motivated these defense systems, and then in the next step, the other adaptive mechanism like gene expression activated to help pepper face stress. On the other hand, shock stress showed down-regulation, but when the stress lasted for a longer time results in up-regulation.

The Ca2+-CaM Signaling Pathway Mediates Potassium Uptake by Regulating Reactive Oxygen Species Homeostasis in Tobacco Roots Under Low-K+ Stress

Potassium (K⁺) deficiency severely threatens crop growth and productivity. Calcium (Ca²⁺) signaling and its sensors play a central role in the response to low-K⁺ stress. Calmodulin (CaM) is an important Ca²⁺ sensor. However, the mechanism by which Ca²⁺ signaling and CaM mediate the response of roots to low-K⁺ stress remains unclear. In this study, we found that the K⁺ concentration significantly decreased in both shoots and roots treated with Ca²⁺ channel blockers, a Ca²⁺ chelator, and CaM antagonists. Under low-K⁺ stress, reactive oxygen species (ROS) accumulated, and the activity of antioxidant enzymes, NAD kinase (NADK), and NADP phosphatase (NADPase) decreased. This indicates that antioxidant enzymes, NADK, and NADPase might be downstream target proteins in the Ca²⁺-CaM signaling pathway, which facilitates K⁺ uptake in plant roots by mediating ROS homeostasis under low-K⁺ stress. Moreover, the expression of NtCNGC3, NtCNGC10, K⁺ channel genes, and transporter genes was significantly downregulated in blocker-treated, chelator-treated, and antagonist-treated plant roots in the low K⁺ treatment, suggesting that the Ca²⁺-CaM signaling pathway may mediate K⁺ uptake by regulating the expression of these genes. Overall, this study shows that the Ca²⁺-CaM signaling pathway promotes K⁺ absorption by regulating ROS homeostasis and the expression of K⁺ uptake-related genes in plant roots under low-K⁺ stress.

Involvement of salicylic acid in the response to potassium deficiency revealed by metabolomics

Potassium (K) deficiency has consequences not only on cellular ion balance and transmembrane potential but also on metabolism. In fact, several enzymes are K-dependent including enzymes in catabolism, causing an alteration in glycolysis and respiration. In addition, K deficiency is associated with the induction of specific pathways and accumulation of metabolic biomarkers, such as putrescine. However, such drastic changes are usually observed when K deficiency is established. Here, we carried out a kinetic analysis with metabolomics to elucidate early metabolic events when nutrient conditions change from K-sufficiency to K-deficiency in Arabidopsis rosettes from both wild type and mutants affected in both K absorption and low-K signalling (hak5 akt1 cipk23). Our results show that mutants have a metabolomics pattern similar to K-deficient wild-type, showing a constitutive metabolic response to low K. In addition, shifting to low K conditions induces (i) changes in sugar metabolism and (ii) an accumulation of salicylic acid metabolites before the appearance of biomarkers of K deficiency (putrescine and aconitate), and such an accumulation is more pronounced in mutants. Our results suggest that early events in the response to low K conditions involve salicylic acid metabolism.

Patterns and drivers of cryptogam and vascular plant diversity in glacier forelands

Vascular and nonvascular plants are affected by environmental factors determining their distribution and shaping their diversity and cover. Despite the cryptogam commonness in Arctic communities, previous studies have often focused on limited number of factors and their impact on only selected species of vascular plants or cryptogams. Our study aimed to investigate in detail the differences in species diversity and cover of cryptogams and vascular plants in the glacier forelands and mature tundra on Svalbard. Furthermore, we determined the biotic and abiotic factors that affected diversity, cover and distribution of cryptogam and vascular plant species. In 2017, we established 201 plots in eight locations (each including habitat type of foreland and mature tundra) and surveyed species abundance, sampled soils and environmental data. Results revealed that diversity and cover of analysed groups differed significantly between locations and habitat types, except for cryptogam cover in mature tundra in terms of location. Distance to the glacier terminus, slope, soil conductivity, nutrient content, and clay content impacted both plant groups' diversity. In contrast, distance to the glacier terminus, nutrient content and soil pH affected their cover. In addition, for cryptogam diversity and cover, foreland location and vascular plant cover were also important, while for vascular plant cover time elapsed after glacier retreat was significant. Distribution of both groups' species in forelands was associated with time elapsed after glacier retreat, soil pH, and nutrient contents. Soil texture and distance to the glacier terminus additionally influenced cryptogam distribution. The positive impact of vascular plants on cryptogam diversity and cover indicates complex relationships between these groups, even in forelands' relatively simple communities. As the cryptogam diversity in the polar areas is high but still largely unknown, future studies on species ecology and climate change impact on vegetation should consider both vascular plants and cryptogams and interactions between these groups.

Plant low-K responses are partly due to Ca prevalence and the low-K biomarker putrescine does not protect from Ca side effects but acts as a metabolic regulator

Potassium (K) deficiency is a rather common situation that impacts negatively on biomass, photosynthesis and N assimilation, making K fertilization often unavoidable. Effects of K deficiency have been investigated for several decades and recently progress has been made in identifying metabolomics signatures thereby offering potential to monitor the K status of crops in the field. However, effects of low K conditions could also be due to the antagonism with other nutrients like calcium (Ca) and the well-known biomarker of K deficiency, putrescine, could be a response to Ca/K imbalance rather than K deficiency per se. To sort this out, we carried out experiments in sunflower grown at either low or high K, at high or low Ca, with or without putrescine added to the nutrient solution. Using metabolomics and proteomics analysis, we show that a significant part of the low K response, such as lower photosynthesis and N assimilation, is due to calcium and can be suppressed by low Ca conditions. Putrescine addition tends to restore photosynthesis and N assimilation but unlike low Ca does not suppress but aggravates the impact of low K conditions on catabolism, including the typical fall-over in pyruvate kinase. We conclude that (a) the effects of K deficiency on key metabolic processes can be partly alleviated by the use of low Ca and not only by K fertilization and (b) in addition to its role as a metabolite, putrescine participates in acclimation to low K via the regulation of the content in enzymes involved in carbon primary metabolism.

A ras-related small GTP-binding protein, RabE1c, regulates stomatal movements and drought stress responses by mediating the interaction with ABA receptors

Drought represents a leading constraint over crop productivity worldwide. The plant response to this stress is centered on the behavior of the cell membrane, where the transduction of abscisic acid (ABA) signaling occurs. Here, the Ras-related small GTP-binding protein RabE1c has been shown able to bind to an ABA receptor in the Arabidopsis thaliana plasma membrane, thereby positively regulating ABA signaling. RabE1c is highly induced by drought stress and expressed abundantly in guard cells. In the loss-of-function rabe1c mutant, both stomatal closure and the whole plant drought stress response showed a reduced sensitivity to ABA treatment, demonstrating that RabE1c is involved in the control over transpirative water loss through the stomata. Impairment of RabE1c's function suppressed the accumulation of the ABA receptor PYL4. The over-expression of RabE1c in A. thaliana enhanced the plants' ability to tolerate drought, and a similar phenotypic effect was achieved by constitutively expressing the gene in Chinese cabbage (Brassica rapassp. pekinensis). The leading conclusion was that RabE1c promotes the degradation of PYL4, suggesting a possible genetic strategy to engineer crop plants to better withstand drought stress.

Diurnal variation of transitory starch metabolism is regulated by plastid proteins WXR1/WXR3 in Arabidopsis young seedlings

Transitory starch is the portion of starch that is synthesized during the day in the chloroplast and usually used for plant growth overnight. Here, we report altered metabolism of transitory starch in the wxr1/wxr3 (weak auxin response 1/3) mutants of Arabidopsis. WXR1/WXR3 were previously reported to regulate root growth of young seedlings and affect the auxin response mediated by auxin polar transport in Arabidopsis. In this study the wxr1/wxr3 mutants accumulated transitory starch in cotyledon, young leaf, and hypocotyl at the end of night. WXR1/WXR3 expression showed diurnal variation. Grafting experiments indicated that the WXRs in root were necessary for proper starch metabolism and plant growth. We also found that photosynthesis was inhibited and the transcription level of DIN1/DIN6 (Dark-Inducible 1/6) was reduced in wxr1/wxr3. The mutants also showed a defect in the ionic equilibrium of Na+ and K+, consistent with our bioinformatics data that genes related to ionic equilibrium were misregulated in wxr1. Loss of function of WXR1 also resulted in abnormal trafficking of membrane lipids and proteins. This study reveals that the plastid proteins WXR1/WXR3 play important roles in promoting transitory starch degradation for plant growth over night, possibly through regulating ionic equilibrium in the root.

Transcriptome Analysis Unravels Key Factors Involved in Response to Potassium Deficiency and Feedback Regulation of K+ Uptake in Cotton Roots

To properly understand cotton responses to potassium (K+) deficiency and how its shoot feedback regulates K+ uptake and root growth, we analyzed the changes in root transcriptome induced by low K+ (0.03 mM K+, lasting three days) in self-grafts of a K+ inefficient cotton variety (CCRI41/CCRI41, scion/rootstock) and its reciprocal grafts with a K+ efficient variety (SCRC22/CCRI41). Compared with CCRI41/CCRI41, the SCRC22 scion enhanced the K+ uptake and root growth of CCRI41 rootstock. A total of 1968 and 2539 differently expressed genes (DEGs) were identified in the roots of CCRI41/CCRI41 and SCRC22/CCRI41 in response to K+ deficiency, respectively. The overlapped and similarly (both up- or both down-) regulated DEGs in the two grafts were considered the basic response to K+ deficiency in cotton roots, whereas the DEGs only found in SCRC22/CCRI41 (1954) and those oppositely (one up- and the other down-) regulated in the two grafts might be the key factors involved in the feedback regulation of K+ uptake and root growth. The expression level of four putative K+ transporter genes (three GhHAK5s and one GhKUP3) increased in both grafts under low K+, which could enable plants to cope with K+ deficiency. In addition, two ethylene response factors (ERFs), GhERF15 and GhESE3, both down-regulated in the roots of CCRI41/CCRI41 and SCRC22/CCRI41, may negatively regulate K+ uptake in cotton roots due to higher net K+ uptake rate in their virus-induced gene silencing (VIGS) plants. In terms of feedback regulation of K+ uptake and root growth, several up-regulated DEGs related to Ca2+ binding and CIPK (CBL-interacting protein kinases), one up-regulated GhKUP3 and several up-regulated GhNRT2.1s probably play important roles. In conclusion, these results provide a deeper insight into the molecular mechanisms involved in basic response to low K+ stress in cotton roots and feedback regulation of K+ uptake, and present several low K+ tolerance-associated genes that need to be further identified and characterized.

Effects of Supplemental Lighting on Potassium Transport and Fruit Coloring of Tomatoes Grown in Hydroponics

Supplemental blue/red lighting accelerated fruit coloring and promoted lycopene synthesis in tomato fruits. Potassium (K) is the most enriched cation in tomato fruits, and its fertigation improved tomato yield and fruit color. However, the effects of supplemental lighting on K uptake and transport by tomatoes and whether supplemental lighting accelerates fruit coloring through enhancing K uptake and transport are still unclear. We investigated the effects of supplemental light-emitting diode (LED) lighting (SL; 100% red, 100% blue; 75% red combined 25% blue) on K uptake in roots and transport in the fruits as well as the fruit coloring of tomatoes (Micro-Tom) grown in an experimental greenhouse in hydroponics. The use of red SL or red combined blue SL enhanced K uptake and K accumulation as well as carotenoid (phytoene, lycopene, γ-carotene, and β-carotene) content in fruits by increasing photosynthesis, plant growth, and fruit weight. The genes related to ethylene signaling were upregulated by red SL. Quantitative real-time PCR (qRT-PCR) results showed that K transporter genes (SlHAKs) are differentially expressed during fruit development and ripening. The highest-expressed gene was SlHAK10 when fruit reached breaker and ripening. SlHAK3 and SlHAK19 were highly expressed at breaker, and SlHAK18 was highly expressed at ripening. These might be related to the formation of tomato fruit ripening and quality. SlHAK4, SlHAK6, SlHAK8, and SlHAK9 were significantly downregulated with fruit ripening and induced by low K. The expression level of SlHAK6, SlHAK10, SlHAK15, and SlHAK19 were significantly increased by blue SL or red combined blue SL during breaker and ripening. Blue SL or red combined blue SL increased content of phytoene, β-carotene, α-carotene, and γ-carotene and accelerated fruit coloring by enhancing K uptake in roots and transport in fruits during fruit ripening. This was consistent with the expression level of SlHAK6, SlHAK10, SlHAK15, and SlHAK19 during fruit development and ripening. The key genes of photoreceptors, light signaling transcript factors as well as abscisic acid (ABA) transduction induced by blue SL or red combined blue SL were consistent with the upregulated genes of SlHAK6, SlHAK10, SlHAK15, and SlHAK19 under blue SL and red combined blue SL. The K transport in tomato fruits might be mediated by light signaling and ABA signaling transduction. These results provide valuable information for fruit quality control and the light regulating mechanism of K transport and fruit coloring in tomatoes.

GmAKT1 is involved in K+ uptake and Na+/K+ homeostasis in Arabidopsis and soybean plants

Plant roots absorb K+ from soil via K+ channels and transporters, which are important for stress responses. In this research, GmAKT1, an AKT1-type K+ channel, was isolated and characterized. The expression of GmAKT1 was induced by K+-starvation and salinity stresses, and it was preferentially expressed in the soybean roots. And GmAKT1 was located in the plasma membrane. As an inward K+ channel, GmAKT1 participated in K+ uptake, as well as rescued the low-K+-sensitive phenotype of the yeast mutant and Arabidopsis akt1 mutant. Overexpression of GmAKT1 significantly improved the growth of plants and increased K+ concentration, leading to lower Na+/K+ ratios in transgenic Arabidopsis and chimeric soybean plants with transgenic hairy roots. In addition, GmAKT1 overexpression resulted in significant upregulation of these ion uptake-related genes, including GmSKOR, GmsSOS1, GmHKT1, and GmNHX1. Our findings suggested that GmAKT1 plays an important part in K+ uptake under low-K+ condition, and could maintain Na+/K+ homeostasis under salt stress in Arabidopsis and soybean plants.

Silicon alleviates salt stress-induced potassium deficiency by promoting potassium uptake and translocation in rice (Oryza sativa L.)

Under salt stress, plants suffer from potassium (K) deficiency caused by excess salts in growth substrate. Silicon (Si) can promote K status in many plant species under salt stress, however, the underlying mechanisms remain unclear. In this study, we assessed the effects of Si on K homeostasis in rice under salt stress and investigated the mechanisms behind using two low-Si rice mutants (lsi1 and lsi2) and their wild types (WTs). After five days' treatment with Si, plant growth was improved and salt stress-induced K deficiency was alleviated in WTs but not in mutants. Simultaneously, Si significantly enhanced K accumulation content, K uptake index and shoot K distribution rate in WTs but not in mutants. Besides, Si enhanced K concentration in xylem sap in WTs but not in mutants. Scanning ion-selected electrode technique (SIET) analysis showed net K influx rate was raised by Si addition under salt stress in WTs but not in mutants. Moreover, Si up-regulated the expression of genes responsible for K uptake (OsAKT1 and OsHAK1) and xylem loading (OsSKOR) in WTs but not in mutants. Overall, our results strongly indicate that Si can improve K uptake and translocation by up-regulating the expression of relevant genes, thereby promoting K status and alleviating salt stress in rice.

ZxNHX1 indirectly participates in controlling K+ homeostasis in the xerophyte Zygophyllum xanthoxylum

The succulent xerophyte Zygophyllum xanthoxylum (Bunge) Engl. can absorb Na+ from the soil as an osmoticum in order to resist osmotic stress. The tonoplast Na+/H+ antiporter ZxNHX1 is essential for maintaining the salt-accumulation characteristics of Z. xanthoxylum by compartmentalizing Na+ into vacuoles. Previous results revealed that the silencing of ZxNHX1 greatly decreased Na+ accumulation in Z. xanthoxylum under 50 mM NaCl due to the weakened compartmentalisation; in addition, K+ concentration also significantly reduced in ZxNHX1-RNAi lines. Yet, whether the reduction of K+ concentration was directly triggered by the silencing of ZxNHX1 remains unclear. In this study, the growth parameters and expression levels of ZxSOS1, ZxHKT1;1, ZxAKT1 and ZxSKOR were measured in wild-type and ZxNHX1-RNAi lines under control or -0.5 MPa osmotic stress. The results showed that the silencing of ZxNHX1 inhibited the plant growth, decreased Na+ concentration in leaves, reduced the transcript abundance of ZxSOS1 and dramatically increased that of ZxHKT1;1 in roots of Z. xanthoxylum under osmotic stress; whereas tissue K+ concentrations and the expression level of ZxSKOR displayed no significant variations, and the expression of ZxAKT1 were significantly reduced in ZxNHX1-RNAi lines under osmotic stress, compared with the wild type. These results suggest that in Z. xanthoxylum, ZxNHX1 can maintain the normal growth by compartmentalizing Na+ into vacuoles, and regulate the spatial distribution of Na+ indirectly by affecting the expressions of ZxSOS1 and ZxHKT1;1. Moreover, the silencing of ZxNHX1 is not the main reason that led to the reduction of K+ concentration in ZxNHX1-RNAi lines under 50 mM NaCl, and ZxNHX1 might be indirectly involved in regulating K+ homeostasis.

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.

Specificity and Plasticity of the Functional Ionome of Brassica napus and Triticum aestivum Subjected to Macronutrient Deprivation

The composition of the functional ionome was studied in Brassica napus and Triticum aestivum with respect to the response of 20 elements under macronutrient deprivation. Analysis of relative root contents showed that some nutrients, such as Fe, Ni, Cu, Na, V, and Co, were largely sequestered in roots. After 10 days of deprivation of each one of these 6 macronutrients, plant growth was similar to control plants, and this was probably the result of remobilization from roots (Mg and Ca) or old leaves (N, P, K, S). Some tissue concentrations and net nutrient uptakes into roots were either decreased or increased, revealing multiple interactions (93 in wheat, 66 in oilseed rape) that were common to both species (48) or were species specific. While some interactions have been previously described (increased uptake of Na under K deficiency; or increased uptake of Mo and Se under S deficiency), a number of new interactions were found and some key mechanisms underlying their action have been proposed from analysis of Arabidopsis mutants. For example, nitrate uptake seemed to be functionally linked to Na(influx, while the uptake of vanadium was probably mediated by sulfate transporters whose expression was stimulated during S deprivation.

Identification of QTLs associated with potassium use efficiency and underlying candidate genes by whole-genome resequencing of two parental lines in Brassica napus

Breeding crops that acquire and/or utilize potassium (K) more effectively could reduce the use of K fertilizers. Sixteen traits affecting K use efficiency (KUE) at the seedling stage were investigated in a B. napus double haploid population grown at an optimal K supply (OK) and a low K supply (LK) in a hydroponic culture system. In total, 50 and 62 QTLs associated with these traits were identified at OK and LK, respectively. A total of 25 orthologues of 23 Arabidopsis genes regulating K transport were identified in the confidence intervals of nine QTLs impacting shoot dry weight at LK, and 22 of these showed variations in coding sequences and/or exhibited significant differences in mRNA abundances in roots at LK between the two parental lines. This study provided insights to the genetic basis of KUE in B. napus, which will accelerate the breeding of K-efficient rapeseed cultivars by marker-assisted selection.

Modulation of Ion Transport Across Plant Membranes by Polyamines: Understanding Specific Modes of Action Under Stress

This work critically discusses the direct and indirect effects of natural polyamines and their catabolites such as reactive oxygen species and γ-aminobutyric acid on the activity of key plant ion-transporting proteins such as plasma membrane H+ and Ca2+ ATPases and K+-selective and cation channels in the plasma membrane and tonoplast, in the context of their involvement in stress responses. Docking analysis predicts a distinct binding for putrescine and longer polyamines within the pore of the vacuolar TPC1/SV channel, one of the key determinants of the cell ionic homeostasis and signaling under stress conditions, and an additional site for spermine, which overlaps with the cytosolic regulatory Ca2+-binding site. Several unresolved problems are summarized, including the correct estimates of the subcellular levels of polyamines and their catabolites, their unexplored effects on nucleotide-gated and glutamate receptor channels of cell membranes and Ca2+-permeable and K+-selective channels in the membranes of plant mitochondria and chloroplasts, and pleiotropic mechanisms of polyamines' action on H+ and Ca2+ pumps.

Exogenous application of xanthine and uric acid and nucleobase-ascorbate transporter MdNAT7 expression regulate salinity tolerance in apple

BACKGROUND

Soil salinity is a critical threat to global agriculture. In plants, the accumulation of xanthine activates xanthine dehydrogenase (XDH), which catalyses the oxidation/conversion of xanthine to uric acid to remove excess reactive oxygen species (ROS). The nucleobase-ascorbate transporter (NAT) family is also known as the nucleobase-cation symporter (NCS) or AzgA-like family. NAT is known to transport xanthine and uric acid in plants. The expression of MdNAT is influenced by salinity stress in apple.

RESULTS

In this study, we discovered that exogenous application of xanthine and uric acid enhanced the resistance of apple plants to salinity stress. In addition, MdNAT7 overexpression transgenic apple plants showed enhanced xanthine and uric acid concentrations and improved tolerance to salinity stress compared with nontransgenic plants, while opposite phenotypes were observed for MdNAT7 RNAi plants. These differences were probably due to the enhancement or impairment of ROS scavenging and ion homeostasis abilities.

CONCLUSION

Our results demonstrate that xanthine and uric acid have potential uses in salt stress alleviation, and MdNAT7 can be utilized as a candidate gene to engineer resistance to salt stress in plants.

Potassium and phosphorus transport and signaling in plants

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

Ammonium Accumulation Caused by Reduced Tonoplast V-ATPase Activity in Arabidopsis thaliana

Plant vacuoles are unique compartments that play a critical role in plant growth and development. The vacuolar H⁺-ATPase (V-ATPase), together with the vacuolar H⁺-pyrophosphatase (V-PPase), generates the proton motive force that regulates multiple cell functions and impacts all aspects of plant life. We investigated the effect of V-ATPase activity in the vacuole on plant growth and development. We used an Arabidopsisthaliana (L.) Heynh. double mutant, vha-a2 vha-a3, which lacks two tonoplast-localized isoforms of the membrane-integral V-ATPase subunit VHA-a. The mutant is viable but exhibits impaired growth and leaf chlorosis. Nitrate assimilation led to excessive ammonium accumulation in the shoot and lower nitrogen uptake, which exacerbated growth retardation of vha-a2 vha-a3. Ion homeostasis was disturbed in plants with missing VHA-a2 and VHA-a3 genes, which might be related to limited growth. The reduced growth and excessive ammonium accumulation of the double mutant was alleviated by potassium supplementation. Our results demonstrate that plants lacking the two tonoplast-localized subunits of V-ATPase can be viable, although with defective growth caused by multiple factors, which can be alleviated by adding potassium. This study provided a new insight into the relationship between V-ATPase, growth, and ammonium accumulation, and revealed the role of potassium in mitigating ammonium toxicity.

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 Identification and Expression Pattern Analysis of the HAK/KUP/KT Gene Family of Cotton in Fiber Development and Under Stresses

The potassium transporter family HAK/KUP/KT is a large group of proteins that are important in plant potassium transport and plays a crucial role in plant growth and development, especially in economic crops. Although HAK/KUP/KT genes have been identified in many species, research on these genes in cotton is still quite rare. In this study, in total, 21, 24, 45, and 44 HAK/KUP/KT genes were identified in Gossypium arboreum, Gossypium raimondii, Gossypium hirsutum, and Gossypium barbadense, respectively. Phylogenetic analysis showed that these genes were divided into four clusters. The G. hirsutum gene promoters contained diverse cis-regulatory elements, such as drought-responsive elements, low temperature-responsive elements, and other elements. The RNA-seq data and qRT-PCR results showed that HAK/KUP/KT genes had different expression patterns in fiber development. The qRT-PCR results of drought and NaCl treatment indicated that HAK/KUP/KT genes might play important roles in abiotic stress responses. These results will provide molecular insights into potassium transporter research in cotton.

Transcriptome Analysis of Pyrus betulaefolia Seedling Root Responses to Short-Term Potassium Deficiency

Potassium (K) plays a crucial role in multiple physiological and developmental processes in plants. Its deficiency is a common abiotic stress that inhibits plant growth and reduces crop productivity. A better understanding of the mechanisms involved in plant responses to low K could help to improve the efficiency of K use in plants. However, such responses remain poorly characterized in fruit tree species such as pears (Pyrus sp). We analyzed the physiological and transcriptome responses of a commonly used pear rootstock, Pyrus betulaefolia, to K-deficiency stress (0 mM). Potassium deprivation resulted in apparent changes in root morphology, with short-term low-K stress resulting in rapidly enhanced root growth. Transcriptome analyses indicated that the root transcriptome was coordinately altered within 6 h after K deprivation, a process that continued until 15 d after treatment. Potassium deprivation resulted in the enhanced expression (up to 5-fold) of a putative high-affinity K+ transporter, PbHAK5 (Pbr037826.1), suggesting the up-regulation of mechanisms associated with K+ acquisition. The enhanced root growth in response to K-deficiency stress was associated with a rapid and sustained decrease in the expression of a transcription factor, PbMYB44 (Pbr015309.1), potentially involved in mediating auxin responses, and the increased expression of multiple genes associated with regulating root growth. The concentrations of several phytohormones including indoleacetic acid (IAA), ABA, ETH, gibberellin (GA3), and jasmonic acid (JA) were higher in response to K deprivation. Furthermore, genes coding for enzymes associated with carbon metabolism such as SORBITOL DEHYDROGENASE (SDH) and SUCROSE SYNTHASE (SUS) displayed greatly enhanced expression in the roots under K deprivation, presumably indicating enhanced metabolism to meet the increased energy demands for growth and K+ acquisition. Together, these data suggest that K deprivation in P. betulaefolia results in the rapid re-programming of the transcriptome to enhance root growth and K+ acquisition. These data provide key insights into the molecular basis for understanding low-K-tolerance mechanisms in pears and in other related fruit trees and identifying potential candidates that warrant further analyses.