Potassium transporters in plants--involvement in K+ acquisition, redistribution and homeostasis

Exploring the influence of symbiosis between arbuscular mycorrhizal fungi and beans on potassium uptake and the activity of AKT and HKT genes

In plants, potassium (K+) serves multiple functions, despite being scarce due to strong soil adsorption. This study examined how the presence of arbuscular mycorrhiza fungi (AMF) like Rhizophagus irregularis and Funneliformis mosseae influenced the absorption and transport of K+ in bean roots through symbiotic interactions. In a symbiotic relationship, AMF had the potential to enhance potassium absorption and storage in various tissues of bean seedlings. Under symbiotic conditions, the concentration of potassium in stem tissues was observed to increase almost four times more than control conditions. The genome of beans was shown to contain a total of nineteen PvAKT genes and two PvHKT genes. Based on phylogeny analysis, PvAKT family members and their corresponding orthologs were categorized into four distinct groups. Subfamily 3 of the PvAKT phylogeny tree exhibited distinct variations from other subfamilies in terms of gene structure, conserved domains, and potential phosphorylation sites. The presence of cis-regulatory element related to ABA responsiveness in the upstream region led to the division of PvAKT and PvHKT genes into two specific groups. Gene expression analysis disclosed that PvAKT and PvHKT genes are induced by AMF and have tissue specific expression. PvAKT6 and PvAKT11 genes and both PvHKT genes showed differential expression in root and shoot tissues, while PvAKT3 gene increased expression in both root and shoot tissues. The results suggest that AMF had a significant impact on increasing the solubility of K+ and ultimately enhancing the function of K+ transporters.

OsPIP2;4 aquaporin water channel primarily expressed in roots of rice mediates both water and nonselective Na+ and K+ conductance

Aquaporin (AQP)-dependent water transport across membranes is indispensable in plants. Recent evidence shows that several AQPs, including plasma membrane intrinsic proteins (PIPs), facilitate the electrogenic transport of ions as well as water transport and are referred to as ion-conducting aquaporins (icAQPs). The present study attempted to identify icAQPs that exhibit cation transport activity among PIPs from rice. Electrophysiological experiments on 11 OsPIPs using Xenopus laevis oocytes revealed that OsPIP2;4 mediated the electrogenic transport of alkali monovalent cations with the selectivity sequence of Na+ ≈ K+ > Rb+ > Cs+ > Li+, suggesting non-selective cation conductance for Na+ and K+. Transcripts of OsPIP2;4 were abundant in the elongation and mature zones of roots with similar expression levels between the root stelar and remaining outer parts in the cultivar Nipponbare. Immunostaining using sections of the crown roots of Nipponbare plants revealed the expression of OsPIP2;4 in the exodermis and sclerenchyma of the surface region and in the endodermis and pericycle of the stelar region. The present results provide novel insights into OsPIP2;4-dependent non-selective Na+ and K+ transport and its physiological roles in rice.

Impact of the potassium transporter TaHAK18 on wheat growth and potassium uptake under stressful K+ conditions

Potassium (K), an indispensable nutrient for plant growth and development, plays a crucial role in plant stress resistance. Within the K+ regulatory network in plants, the HAK/KUP/KT gene family comprises a dominant group of K+ transport proteins responsible for K+ uptake and transport. This study functionally characterized the wheat gene TaHAK18, which encodes a putative K+ transporter. Plasma membrane-localized TaHAK18 was significantly upregulated under low-K+ conditions and showed tissue-specific expression, being most abundant in leaves. A functional analysis in yeast demonstrated that TaHAK18 complements K+-uptake deficiencies, confirming its role in K+ transport. Arabidopsis plants overexpressing TaHAK18 experienced enhanced growth under both low- and normal-K+ conditions, with greater fresh weight, lateral root formation, and primary root length. Barley stripe mosaic virus-mediated gene silencing in wheat revealed that TaHAK18 is instrumental for K+ accumulation and plant growth under low-K+ stress. TaHAK18 has the capacity to enhance the growth and the accumulation of K+ in transgenic rice plants. These results indicated that TaHAK18 is a key regulator of K+ uptake and homeostasis in wheat, with potential implications for improving plant tolerance to low-K+ stress.

Radiometric evaluation of Ouratea miersii (Planch.) Engl. From the Municipal Natural Park of Niterói, Brazilian Atlantic Rainforest remnant

Concern about radiation exposure to non-human biota and the environment has increased since the 1990s, resulting in several publications by UNSCEAR, IAEA, ICRP, among other agencies, which highlighted significant gaps in knowledge regarding non-human biota and resulted in the establishment of Reference Animals and Plants (RAPs). In Brazil, the Atlantic Forest biome has vast biodiversity, but there is a lack of information on the radiometric profile of plant species growing there. Thus, the objective of this study was to evaluate the radiometric profile of Ouratea miersii (Malpighiales), endemic to the Atlantic Rainforest, collected in the State of Rio de Janeiro, city of Niterói, at the Municipal Natural Park (PARNIT), which covers a region of protected Atlantic Rainforest. The specimen was completely collected (root, stem, leaf and soil including organic matter around the root), and each sample analyzed individually by gamma spectrometry using a HPGe detector (Canberra). The calculated activity concentrations (AC) indicated the presence of 40K, 226Ra and 228Ra, with significant AC of 40K in the soil (3901 Bq·kg-1). The 40K soil-plant transfer was 11.6%, being limited by some physiological mechanism and/or osmotic saturation, while the transfer factor for 226Ra was 45.3%. The dose rate in non-human biota due to AC's of 226Ra and 228Ra in soil was estimated by modeling with the ERICA Tool, which indicated the absence of adverse effect on non-human biota due to exposure. The radiological hazard index values (Raeq, ADR, AEDR, and ELCR) obtained from the soil AC collected at PARNIT were significantly higher than those from other locations in Brazil, and about three times the value reported by UNSCEAR (2000) due to a ⁴⁰K anomaly in the soil. For this reason, a more extensive environmental monitoring was necessary in the PARNIT region.

Differences in growth, ionomic and antioxidative enzymes system responded to neutral and alkali salt exposure in halophyte Haloxylon ammodendron seedlings

Soil salinity and alkalinity severely suppress plant growth and crop yields. This study compared the effects of neutral and alkaline salt exposure, both individually and mixed, on metal content and morphophysiological responses in halophyte Haloxylon ammodendron. Our results showed that alkaline salt exposure more considerably inhibited the growth and photosynthesis of H. ammodendron than neutral salt exposure. Under neutral salt conditions, Na accumulated significantly, while K and Fe absorption was hindered. In contrast, under alkaline salt stress, Na accumulation was more pronounced, leading to a greater inhibition of K absorption. Additionally, Ca accumulation was promoted, while the transport of Fe, Mg, and Cu from root to shoot was suppressed. Alkaline salt stress also induced more severe osmotic stress, triggering a stronger accumulation of soluble sugars to counteract it. Furthermore, seedlings under alkaline stress showed higher levels of REL, H2O2, and MDA, but lower activities of SOD, POD, CAT, and APX, indicating increased oxidative damage. These findings suggest that H. ammodendron can adapt well to neutral salt stress through efficient antioxidant enzyme systems and osmotic stress regulation. In contrast, alkaline stress severely inhibits the absorption and transport of mineral elements and disrupts the balance of antioxidant enzymes. Besides, the deleterious effects of neutral-alkaline salt mixed stress were significantly less than those of alkaline stress alone, indicating a reciprocal enhancement between neutral and alkaline salt stress was occurred.

Manifold roles of potassium in mediating drought tolerance in plants and its underlying mechanisms

Drought stress (DS) is a major devastating factor affecting plant growth and development worldwide. Potassium (K) is considered a vigorous moiety and stress alleviator, which crop cultivars need for better yield. It is also helpful in alleviating the DS-induced negative consequences by regulating various morphological, physiological, biochemical, and molecular mechanisms in plants. Particularly, the K application improves plant tolerance against DS by improving plant growth parameters, photosynthetic pigments, cell turgor pressure, osmotic pressure, nutritional balance, compatible solutes, and the plant's antioxidant defense system. Apart from its role as a constituent of the plant structure, biochemical processes such as protein synthesis, carbohydrate metabolism, and enzyme activation are also regulated by K. However, the exact K-mediated molecular mechanisms of DS tolerance are still unclear and require more investigation. The present review aims to provide insight into the role of K in regulating various morphological and physico-chemical aspects under DS. It also emphasizes the crosstalk of K with other nutrients and phytohormones, as well as molecular mechanisms for K homeostasis under DS. We have also shed light on genomics analysis to discover K transporter's novel genes in different plant species.

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.

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.

A putative Na+/H+ antiporter BpSOS1 contributes to salt tolerance in birch

White birch (Betula platyphylla Suk.) is an important pioneer tree which plays a critical role in maintaining ecosystem stability and forest regeneration. The growth of birch is dramatically inhibited by salt stress, especially the root inhibition. Salt Overly Sensitive 1 (SOS1) is the only extensively characterized Na+ efflux transporter in multiple plant species. The salt-hypersensitive mutant, sos1, display significant inhibition of root growth by NaCl. However, the role of SOS1 in birch responses to salt stress remains unclear. Here, we characterized a putative Na+/H+ antiporter BpSOS1 in birch and generated the loss-of-function mutants of the birch BpSOS1 by CRISPR/Cas9 approach. The bpsos1 mutant exhibit exceptional increased salt sensitivity which links to excessive Na+ accumulation in root, stem and old leaves. We observed a dramatic reduction of K+ contents in leaves of the bpsos1 mutant plants under salt stress. Furthermore, the Na+/K+ ratio of roots and leaves is significant higher in the bpsos1 mutants than the wild-type plants under salt stress. The ability of Na+ efflux in the root meristem zone is found to be impaired which might result the imbalance of Na+ and K+ in the bpsos1 mutants. Our findings indicate that the Na+/H+ exchanger BpSOS1 plays a critical role in birch salt tolerance by maintaining Na+ homeostasis and provide evidence for molecular breeding to improve salt tolerance in birch and other trees.

Dynamics of Actin Filaments Play an Important Role in Root Hair Growth under Low Potassium Stress in Arabidopsis thaliana

Potassium (K) is an essential nutrient for the growth and development of plants. Root hairs are the main parts of plants that absorb K+. The regulation of plant root hair growth in response to a wide range of environmental stresses is crucially associated with the dynamics of actin filaments, and the thick actin bundles at the apical and sub-apical regions are essential for terminating the rapid elongation of root hair cells. However, the dynamics and roles of actin filaments in root hair growth in plants' response to low K+ stress are not fully understood. Here, we revealed that root hairs grow faster and longer under low K+ stress than the control conditions. Compared to control conditions, the actin filaments in the sub-apex of fast-growing wild-type root hairs were longer and more parallel under low K+ stress, which correlates with an increased root hair growth rate under low K+ stress; the finer actin filaments in the sub-apex of the early fully grown Col-0 root hairs under low K+ stress, which is associated with low K+ stress-induced root hair growth time. Further, Arabidopsis thaliana actin bundling protein Villin1 (VLN1) and Villin4 (VLN4) was inhibited and induced under low K+ stress, respectively. Low K+ stress-inhibited VLN1 led to decreased bundling rate and thick bundle formation in the early fully grown phase. Low K+ stress-induced VLN4 functioned in keeping long filaments in the fast-growing phase. Furthermore, the analysis of genetics pointed out the involvement of VLN1 and VLN4 in the growth of root hairs under the stress of low potassium levels in plants. Our results provide a basis for the dynamics of actin filaments and their molecular regulation mechanisms in root hair growth in response to low K+ stress.

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.

Positive Regulatory Roles of Manihot esculenta HAK5 under K+ Deficiency or High Salt Stress

HAK/KUP/KT family members have been identified as playing key roles in K+ uptake and salt tolerance in numerous higher plants. However, their functions in cassava (Manihot esculenta Cantz) remain unknown. In this study, a gene encoding for a high-affinity potassium transporter (MeHAK5) was isolated from cassava and its function was investigated. Subcellular localization analysis showed that MeHAK5 is a plasma membrane-localized transporter. RT-PCR and RT-qPCR indicated that MeHAK5 is predominantly expressed in cassava roots, where it is upregulated by low potassium or high salt; in particular, its highest expression levels separately increased by 2.2 and 2.9 times after 50 µM KCl and 150 mM NaCl treatments. When heterologously expressed in yeast, MeHAK5 mediated K+ uptake within the cells of the yeast strain CY162 and rescued the salt-sensitive phenotype of AXT3K yeast. MeHAK5 overexpression in transgenic Arabidopsis plants exhibited improved growth and increased shoot K+ content under low potassium conditions. Under salt stress, MeHAK5 transgenic Arabidopsis plants accumulated more K+ in the shoots and roots and had reduced Na+ content in the shoots. As a result, MeHAK5 transgenic Arabidopsis demonstrated a more salt-tolerant phenotype. These results suggest that MeHAK5 functions as a high-affinity K+ transporter under K+ starvation conditions, improving K+/Na+ homeostasis and thereby functioning as a positive regulator of salt stress tolerance in transgenic Arabidopsis. Therefore, MeHAK5 may be a suitable candidate gene for improving K+ utilization efficiency and salt tolerance.

The EIN3/EIL-ERF9-HAK5 transcriptional cascade positively regulates high-affinity K+ uptake in Gossypium hirsutum

High-affinity K+ (HAK) transporters play essential roles in facilitating root K+ uptake in higher plants. Our previous studies revealed that GhHAK5a, a member of the HAK family, is crucial for K+ uptake in upland cotton. Nevertheless, the precise regulatory mechanism governing the expression of GhHAK5a remains unclear. The yeast one-hybrid screening was performed to identify the transcription factors responsible for regulating GhHAK5a, and ethylene response factor 9 (GhERF9) was identified as a potential candidate. Subsequent dual-luciferase and electrophoretic mobility shift assays confirmed that GhERF9 binds directly to the GhHAK5a promoter, thereby activating its expression. Silencing of GhERF9 decreased the expression of GhHAK5a and exacerbated K+ deficiency symptoms in leaves, also decreased K+ uptake rate and K+ content in roots. Additionally, it was observed that the application of ethephon (an ethylene-releasing reagent) resulted in a significant upregulation of GhERF9 and GhHAK5a, accompanied by an increased rate of K+ uptake. Expectedly, GhEIN3b and GhEIL3c, the two key components involved in ethylene signaling, bind directly to the GhERF9 promoter. These findings provide valuable insights into the molecular mechanisms underlying the expression of GhHAK5a and ethylene-mediated K+ uptake and suggest a potential strategy to genetically enhance cotton K+ uptake by exploiting the EIN3/EILs-ERF9-HAK5 module.

Ion Transport and Inhibitor Binding by Human NHE1: Insights from Molecular Dynamics Simulations and Free Energy Calculations

The human Na+/H+ exchanger (NHE1) plays a crucial role in maintaining intracellular pH by regulating the electroneutral exchange of a single intracellular H+ for one extracellular Na+ across the plasma membrane. Understanding the molecular mechanisms governing ion transport and the binding of inhibitors is of importance in the development of anticancer therapeutics targeting NHE1. In this context, we performed molecular dynamics (MD) simulations based on the recent cryo-electron microscopy (cryo-EM) structures of outward- and inward-facing conformations of NHE1. These simulations allowed us to explore the dynamics of the protein, examine the ion-translocation pore, and confirm that Asp267 is the ion-binding residue. Our free energy calculations did not show a significant difference between Na+ and K+ binding at the ion-binding site. Consequently, Na+ over K+ selectivity cannot be solely explained by differences in ion binding. Our MD simulations involving NHE1 inhibitors (cariporide and amiloride analogues) maintained stable interactions with Asp267 and Glu346. Our study highlights the importance of the salt bridge between the positively charged acylguanidine moiety and Asp267, which appears to play a role in the competitive inhibitory mechanism for this class of inhibitors. Our computational study provides a detailed mechanistic interpretation of experimental data and serves the basis of future structure-based inhibitor design.

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

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

The role of NaHS pretreatment in improving salt stress resistance in foxtail millet seedlings: physiological and molecular mechanisms

Salt stress is a prevailing abiotic stress in nature, with soil salinization becoming a pressing issue worldwide. High soil salinity severely hampers plant growth and leads to reduced crop yields. Hydrogen sulfide (H2S), a gas signal molecule, is known to be synthesized in plants exposed to abiotic stress, contributing to enhanced plant stress resistance. To investigate the impact of sodium hydrosulfide hydrate (NaHS, a H2S donor) on millet's response to salt stress, millet seedlings were subjected to pretreatment with 200 μM NaHS, followed by 100 mM NaCl stress under soil culture conditions. The growth, osmotic adjustment substances, antioxidant characteristics, membrane damage, and expression levels of related genes in millet seedlings were detected and analyzed. The results showed that NaHS pretreatment alleviated the inhibition of salt stress on the growth of foxtail millet seedlings, increased the proline content and antioxidant enzyme activities, as well as the expression levels of SiASR4, SiRPLK35 and SiHAK23 genes under salt stress. These findings demonstrated that NaHS pretreatment can enhance salt tolerance in foxtail millet seedlings by regulating the content of osmotic adjustment substances and antioxidant enzyme activity, reducing electrolyte permeability, and activating the expression of salt-resistant genes.

Type-B response regulator OsRR22 forms a transcriptional activation complex with OsSLR1 to modulate OsHKT2;1 expression in rice

Soil salinity severely limits crop yields and quality. Plants have evolved several strategies to mitigate the adverse effects of salinity, including redistribution and compartmentalization of toxic ions using ion-specific transporters. However, the mechanisms underlying the regulation of these ion transporters have not been fully elucidated. Loss-of-function mutants of OsHKT2;1, which is involved in sodium uptake, exhibit strong salt stress-resistant phenotypes. In this study, OsHKT2;1 was identified as a transcriptional target of the type-B response regulator OsRR22. Loss-of-function osrr22 mutants showed resilience to salt stress, and OsRR22-overexpression plants were sensitive to salt stress. OsRR22 was found to activate the expression of OsHKT2;1 by directly binding to the promoter region of OsHKT2;1 via a consensus cis-element of type-B response regulators. Moreover, rice DELLA protein OsSLR1 directly interacted with OsRR22 and functioned as a transcriptional co-activator. This study has uncovered a novel transcriptional regulatory mechanism by which a type-B response regulator controls sodium transport under salinity stress.

The K+ transporter NPF7.3/NRT1.5 and the proton pump AHA2 contribute to K+ transport in Arabidopsis thaliana under K+ and NO3- deficiency

Nitrate (NO3 -) and potassium (K+) are distributed in plants via short and long-distance transport. These two pathways jointly regulate NO3 - and K+ levels in all higher plants. The Arabidopsis thaliana transporter NPF7.3/NRT1.5 is responsible for loading NO3 - and K+ from root pericycle cells into the xylem vessels, facilitating the long-distance transport of NO3 - and K+ to shoots. In this study, we demonstrate a protein-protein interaction of NPF7.3/NRT1.5 with the proton pump AHA2 in the plasma membrane by split ubiquitin and bimolecular complementation assays, and we show that a conserved glycine residue in a transmembrane domain of NPF7.3/NRT1.5 is crucial for the interaction. We demonstrate that AHA2 together with NRT1.5 affects the K+ level in shoots, modulates the root architecture, and alters extracellular pH and the plasma membrane potential. We hypothesize that NRT1.5 and AHA2 interaction plays a role in maintaining the pH gradient and membrane potential across the root pericycle cell plasma membrane during K+ and/or NO3 - transport.

Modulation of potassium transport to increase abiotic stress tolerance in plants

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

Sailing in complex nutrient signaling networks: Where I am, where to go, and how to go?

To ensure survival and promote growth, sessile plants have developed intricate internal signaling networks tailored in diverse cells and organs with both shared and specialized functions that respond to various internal and external cues. A fascinating question arises: how can a plant cell or organ diagnose the spatial and temporal information it is experiencing to know "where I am," and then is able to make the accurate specific responses to decide "where to go" and "how to go," despite the absence of neuronal systems found in mammals. Drawing inspiration from recent comprehensive investigations into diverse nutrient signaling pathways in plants, this review focuses on the interactive nutrient signaling networks mediated by various nutrient sensors and transducers. We assess and illustrate examples of how cells and organs exhibit specific responses to changing spatial and temporal information within these interactive plant nutrient networks. In addition, we elucidate the underlying mechanisms by which plants employ posttranslational modification codes to integrate different upstream nutrient signals, thereby conferring response specificities to the signaling hub proteins. Furthermore, we discuss recent breakthrough studies that demonstrate the potential of modulating nutrient sensing and signaling as promising strategies to enhance crop yield, even with reduced fertilizer application.

Early differentiation of Casparian strip mediated by nitric oxide is required for efficient K transport under low K conditions in Arabidopsis

The Casparian strip (CS) is a cell wall modification made of lignin that functions as an apoplastic barrier in the root endodermis to restrict nutrient and water transport between the soil and stele. CS formation is affected by nutritional conditions, and its physiological roles have been discussed. This study found that low K condition affects CS permeability, lignin deposition, and MYB36 mRNA accumulation. To understand the mechanism underlying these findings, we focused on nitric oxide (NO). NO is known to act as a signaling molecule and participates in cell wall synthesis, especially for lignin composition. However, the mechanism by which NO affects lignin deposition and corrects CS formation in the plant roots remains unclear. Through combining fluorescent observation with histological stains, we demonstrated that the root endodermal cell lignification response to low-potassium (K) conditions is mediated by NO through the MYB36-associated lignin-polymerizing pathway. Furthermore, we discovered the noteworthy ability of NO to maintain nutrient homeostasis for adaptation to low K conditions by affecting the correct apoplastic barrier formation of CS. Collectively, our results suggest that NO is required for the lignification and apoplastic barrier formation in the root endodermis during adaptation to low K conditions, which revealing the novel physiological roles of CS under low nutrient conditions and making a significant contribution to CS biology.

Xylem K+ loading modulates K+ and Cs+ absorption and distribution in Arabidopsis under K+-limited conditions

Potassium (K+) is an essential macronutrient for plant growth. The transcriptional regulation of K+ transporter genes is one of the key mechanisms by which plants respond to K+ deficiency. Among the HAK/KUP/KT transporter family, HAK5, a high-affinity K+ transporter, is essential for root K+ uptake under low external K+ conditions. HAK5 expression in the root is highly induced by low external K+ concentration. While the molecular mechanisms of HAK5 regulation have been extensively studied, it remains unclear how plants sense and coordinates K+ uptake and translocation in response to changing environmental conditions. Using skor mutants, which have a defect in root-to-shoot K+ translocation, we have been able to determine how the internal K+ status affects the expression of HAK5. In skor mutant roots, under K+ deficiency, HAK5 expression was lower than in wild-type although the K+ concentration in roots was not significantly different. These results reveal that HAK5 is not only regulated by external K+ conditions but it is also regulated by internal K+ levels, which is in agreement with recent findings. Additionally, HAK5 plays a major role in the uptake of Cs+ in roots. Therefore, studying Cs+ in roots and having more detailed information about its uptake and translocation in the plant would be valuable. Radioactive tracing experiments revealed not only a reduction in the uptake of 137Cs+ and 42K+in skor mutants compared to wild-type but also a different distribution of 137Cs+ and 42K+ in tissues. In order to gain insight into the translocation, accumulation, and repartitioning of both K+ and Cs+ in plants, long-term treatment and split root experiments were conducted with the stable isotopes 133Cs+ and 85Rb+. Finally, our findings show that the K+ distribution in plant tissues regulates root uptake of K+ and Cs+ similarly, depending on HAK5; however, the translocation and accumulation of the two elements are different.

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.

Calcium signalling components underlying NPK homeostasis: potential avenues for exploration

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

Identification of Shaker Potassium Channel Family Members in Gossypium hirsutum L. and Characterization of GhKAT1aD

K⁺ channels of the Shaker family have been shown to play crucial roles in K⁺ uptake and transport. Cotton (Gossypium hirsutum) is an important cash crop. In this study, the 24 Shaker family genes were identified in cotton. Phylogenetic analysis suggests that they were assigned to five clusters. Additionally, their chromosomal location, conserved motifs and gene structure were analyzed. The promoter of cotton Shaker K⁺ channel genes comprises drought-, low-temperature-, phytohormone-response elements, etc. As indicated by qRT-PCR (quantitative real-time PCR), cotton Shaker K⁺ channel genes responded to low K⁺ and NaCl, and especially dehydration stress, at the transcript level. Moreover, one of the Shaker K⁺ channel genes, GhKAT1aD, was characterized. This gene is localized in the plasma membrane and is predicted to contain six transmembrane segments. It restored the growth of the yeast mutant strain defective in K⁺ uptake, and silencing GhKAT1a via VIGS (virus-induced gene silencing) resulted in more severe symptoms of K⁺ deficiency in cotton leaves as well as a lower net K⁺ uptake rate. The results of this study showed the overall picture of the cotton Shaker K⁺ channel family regarding bioinformatics as well as the function of one of its members, which provide clues for future investigations of cotton K⁺ transport and molecular insights for breeding K⁺-efficient cotton varieties.

The Sweet Potato K+ Transporter IbHAK11 Regulates K+ Deficiency and High Salinity Stress Tolerance by Maintaining Positive Ion Homeostasis

The K⁺ transporter KT/HAK/KUP (K⁺ transporter/high-affinity K⁺/K⁺ uptake) family has a critical effect on K⁺ uptake and translocation in plants under different environmental conditions. However, the functional analysis of KT/HAK/KUP members in sweet potatoes is still limited. The present work reported the physiological activity of a new gene, IbHAK11, in the KT/HAK/KUP family in sweet potatoes. IbHAK11 expression increased significantly in the low K⁺-tolerant line compared with the low K⁺-sensitive line following treatment with low K⁺ concentrations. IbHAK11 upregulation promoted root growth in Arabidopsis under low K⁺ conditions. Under high saline stress, transgenic lines had superior growth and photosynthetic characteristics compared with the wild-type (WT). As for IbHAK11-overexpressing plants, activation of both the non-enzymatic and enzymatic reactive oxygen species (ROS) scavenging systems was observed. Therefore, IbHAK11-overexpressing plants had lower malondialdehyde (MDA) and ROS levels (including H₂O₂ and O^2-) compared with WT under salt-induced stress. We also found that under both low K⁺ and high salinity conditions, overexpression of IbHAK11 enhanced K⁺ translocation from the root to the shoot and decreased Na⁺ absorption in Arabidopsis. Consequently, IbHAK11 positively regulated K⁺ deficiency and high salinity stresses by regulating K⁺ translocation and Na⁺ uptake, thus maintaining K⁺/Na⁺ homeostasis in plants.

Genome-wide identification and analysis of TCP family genes in Medicago sativa reveal their critical roles in Na+/K+ homeostasis

BACKGROUND

Medicago sativa is the most important forage world widely, and is characterized by high quality and large biomass. While abiotic factors such as salt stress can negatively impact the growth and productivity of alfalfa. Maintaining Na⁺/K⁺ homeostasis in the cytoplasm helps reduce cell damage and nutritional deprivation, which increases a salt-tolerance of plant. Teosinte Branched1/ Cycloidea/ Proliferating cell factors (TCP) family genes, a group of plant-specific transcription factors (TFs), involved in regulating plant growth and development and abiotic stresses. Recent studies have shown TCPs control the Na⁺/K⁺ concentration of plants during salt stress. In order to improve alfalfa salt tolerance, it is important to identify alfalfa TCP genes and investigate if and how they regulate alfalfa Na⁺/K⁺ homeostasis.

RESULTS

Seventy-one MsTCPs including 23 non-redundant TCP genes were identified in the database of alfalfa genome (C.V XinJiangDaYe), they were classified into class I PCF (37 members) and class II: CIN (28 members) and CYC/TB1 (9 members). Their distribution on chromosome were unequally. MsTCPs belonging to PCF were expressed specifically in different organs without regularity, which belonging to CIN class were mainly expressed in mature leaves. MsTCPs belongs to CYC/TB1 clade had the highest expression level at meristem. Cis-elements in the promoter of MsTCPs were also predicted, the results indicated that most of the MsTCPs will be induced by phytohormone and stress treatments, especially by ABA-related stimulus including salinity stress. We found 20 out of 23 MsTCPs were up-regulated in 200 mM NaCl treatment, and MsTCP3/14/15/18 were significantly induced by 10 μM KCl, a K⁺ deficiency treatment. Fourteen non-redundant MsTCPs contained miR319 target site, 11 of them were upregulated in MIM319 transgenic alfalfa, and among them four (MsTCP3/4/10A/B) genes were directly degraded by miR319. MIM319 transgene alfalfa plants showed a salt sensitive phenotype, which caused by a lower content of potassium in alfalfa at least partly. The expression of potassium transported related genes showed significantly higher expression in MIM319 plants.

CONCLUSIONS

We systematically analyzes the MsTCP gene family at a genome-wide level and reported that miR319-TCPs model played a function in K⁺ up-taking and/ or transportation especially in salt stress. The study provide valuable information for future study of TCP genes in alfalfa and supplies candidate genes for salt-tolerance alfalfa molecular-assisted breeding.

Regulation of miR319b-Targeted SlTCP10 during the Tomato Response to Low-Potassium Stress

Potassium deficiency confines root growth and decreases root-to-shoot ratio, thereby limiting root K+ acquisition. This study aimed to identify the regulation network of microRNA319 involved in low-K+ stress tolerance in tomato (Solanum lycopersicum). SlmiR319b-OE roots demonstrated a smaller root system, a lower number of root hairs and lower K+ content under low-K+ stress. We identified SlTCP10 as the target of miR319b using a modified RLM-RACE procedure from some SlTCPs' predictive complementarity to miR319b. Then, SlTCP10-regulated SlJA2 (an NAC transcription factor) influenced the response to low-K+ stress. CR-SlJA2 (CRISPR-Cas9-SlJA2) lines showed the same root phenotype to SlmiR319-OE compared with WT lines. OE-SlJA2(Overexpression-SlJA2) lines showed higher root biomass, root hair number and K+ concentration in the roots under low-K+ conditions. Furthermore, SlJA2 has been reported to promote abscisic acid (ABA) biosynthesis. Therefore, SlJA2 increases low-K+ tolerance via ABA. In conclusion, enlarging root growth and K+ absorption by the expression of SlmiR319b-regulated SlTCP10, mediating SlJA2 in roots, could provide a new regulation mechanism for increasing K+ acquisition efficiency under low-K+ stress.

Molecular membrane separation: plants inspire new technologies

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

Phylogenetic, structural, functional characterisation and effect of exogenous spermidine on rice (Oryza sativa) HAK transporters under salt stress

The HAK (High-affinity K+ ) family members mediate K+ transport that confers normal plant growth and resistance against unfavourable environmental conditions. Rice (Oryza sativa L.) HAK transporters have been extensively investigated for phylogenetic analyses with other plants species with very few of them functionally characterised. But very little information is known about their evolutionary aspects, overall structural, functional characterisation, and global expression pattern of the complete HAK family members in response to salt stress. In this study, 27 rice transporters were phylogenetically clustered with different dicot and monocot family members. Subsequently, the exon-intron structural patterns, conserved motif analyses, evolutionary divergence based different substitution matrix, orthologous-paralogous relationships were studied elaborately. Structural characterisations included a comparative study of secondary and tertiary structure, post-translational modifications, correspondence analyses, normal mode analyses, K+ /Na+ binding affinities of each of the OsHAK gene members. Global expression profile under salt stress showed clade-specific expression pattern of the proteins. Additionally, five OsHAK genes were chosen for further expression analyses in root and shoot tissues of two rice varieties during short-term salinity in the presence and absence of exogenous spermidine. All the information can be used as first-hand data for dissecting the administrative role of rice HAK transporters under various abiotic stresses.

RNA sequencing-based exploration of the effects of far-red light on microRNAs involved in the shade-avoidance response of D. officinale

Dendrobium officinale (D. officinale) has remarkable medicinal functions and high economic value. The shade-avoidance response to far-red light importantly affects the D. officinale productivity. However, the regulatory mechanism of miRNAs involved in the far-red light-avoidance response is unknown. Previous studies have found that, in D. officinale, 730 nm (far-red) light can promote the accumulation of plant metabolites, increase leaf area, and accelerate stem elongation. Here, the effects of far-red light on D. officinale were analysed via RNA-seq. KEGG analysis of miRNA target genes revealed various far-red light response pathways, among which the following played central roles: the one-carbon pool by folate; ascorbate and aldarate; cutin, suberine and wax biosynthesis; and sulfur metabolism. Cytoscape analysis of DE miRNA targets showed that novel_miR_484 and novel_miR_36 were most likely involved in the effects of far-red light on the D. officinale shade avoidance. Content verification revealed that far-red light promotes the accumulation of one-carbon compounds and ascorbic acid. Combined with qPCR validation results, the results showed that miR395b, novel_miR_36, novel_miR_159, novel_miR_178, novel_miR_405, and novel_miR_435 may participate in the far-red light signalling network through target genes, regulating the D. officinale shade avoidance. These findings provide new ideas for the efficient production of D. officinale.

Potassium transporter KUP9 participates in K+ distribution in roots and leaves under low K+ stress

Potassium (K) is a major essential element in plant cells, and KUP/HAK/KT-type K+ transporters participate in the absorption of K+ into roots and in the long-distance transport to above-ground parts. In Arabidopsis thaliana, KUP9 is involved in the transport of K+ and Cs+ in roots. In this study, we investigated KUP9 function in relation to the K+ status of the plant. The expression of KUP9 was upregulated in older leaves on K+-depleted medium, compared to the expression of the other 12 KUP genes in the KUP/HAK/KT family in Arabidopsis. When grown on low K+ medium, the kup9 mutant had reduced chlorophyll content in seedlings and chlorosis in older rosette leaves. Tissue-specific expression of KUP9 determined by KUP9 promoter:GUS assay depended on the K+ status of the plants: In K+ sufficient medium, KUP9 was expressed in the leaf blade towards the leaf tip, whereas in K+ depleted medium expression was mainly found in the petioles. In accordance with this, K+ accumulated in the roots of kup9 plants. The short-term 43K+ tracer measurement showed that 43K was transferred at a lower rate in roots and shoots of kup9, compared to the wild type. These data show that KUP9 participates in the distribution of K+ in leaves and K+ absorption in roots under low K+ conditions.

Genome-wide characterization and expression analysis of the HAK gene family in response to abiotic stresses in Medicago

The high-affinity K⁺ transporter (HAK) family plays a vital role in K⁺ uptake and transport as well as in salt and drought stress responses. In the present study, we identified 22 HAK genes in each Medicago truncatula and Medicago sativa genome. Phylogenetic analysis suggested that these HAK proteins could be divided into four clades, and the members of the same subgroup share similar gene structure and conserved motifs. Many cis-acting elements related with defense and stress were found in their promoter region. In addition, gene expression profiles analyzed with genechip and transcriptome data showed that these HAK genes exhibited distinct expression pattern in different tissues, and in response to salt and drought treatments. Furthermore, co-expression analysis showed that 6 homologous HAK hub gene pairs involved in direct network interactions. RT-qPCR verified that the expression level of six HAK gene pairs was induced by NaCl and mannitol treatment to different extents. In particular, MtHK2/7/12 from M. truncatula and MsHAK2/6/7 from M. sativa were highly induced. The expression level of MsHAK1/2/11 determined by RT-qPCR showed significantly positive correlation with transcriptome data. In conclusion, our study shows that HAK genes play a key role in response to various abiotic stresses in Medicago, and the highly inducible candidate HAK genes could be used for further functional studies and molecular breeding in Medicago.

Grapevine (Vitis vinifera) responses to salt stress and alkali stress: transcriptional and metabolic profiling

BACKGROUND

Soil salinization and alkalization are widespread environmental problems that limit grapevine (Vitis vinifera L.) growth and yield. However, little is known about the response of grapevine to alkali stress. This study investigated the differences in physiological characteristics, chloroplast structure, transcriptome, and metabolome in grapevine plants under salt stress and alkali stress.

RESULTS

We found that grapevine plants under salt stress and alkali stress showed leaf chlorosis, a decline in photosynthetic capacity, a decrease in chlorophyll content and Rubisco activity, an imbalance of Na⁺ and K⁺, and damaged chloroplast ultrastructure. Fv/Fm decreased under salt stress and alkali stress. NPQ increased under salt stress whereas decreased under alkali stress. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment showed the differentially expressed genes (DEGs) induced by salt stress and alkali stress were involved in different biological processes and have varied molecular functions. The expression of stress genes involved in the ABA and MAPK signaling pathways was markedly altered by salt stress and alkali stress. The genes encoding ion transporter (AKT1, HKT1, NHX1, NHX2, TPC1A, TPC1B) were up-regulated under salt stress and alkali stress. Down-regulation in the expression of numerous genes in the 'Porphyrin and chlorophyll metabolism', 'Photosynthesis-antenna proteins', and 'Photosynthesis' pathways were observed under alkali stress. Many genes in the 'Carbon fixation in photosynthetic organisms' pathway in salt stress and alkali stress were down-regulated. Metabolome showed that 431 and 378 differentially accumulated metabolites (DAMs) were identified in salt stress and alkali stress, respectively. L-Glutamic acid and 5-Aminolevulinate involved in chlorophyll synthesis decreased under salt stress and alkali stress. The abundance of 19 DAMs under salt stress related to photosynthesis decreased. The abundance of 16 organic acids in salt stress and 22 in alkali stress increased respectively.

CONCLUSIONS

Our findings suggested that alkali stress had more adverse effects on grapevine leaves, chloroplast structure, ion balance, and photosynthesis than salt stress. Transcriptional and metabolic profiling showed that there were significant differences in the effects of salt stress and alkali stress on the expression of key genes and the abundance of pivotal metabolites in grapevine plants.

Is Oxalic Acid Secretion A Detoxification Strategy for Rice Exposed to Tl(I) or Tl(III)?

Thallium (Tl) is a highly toxic element with two species, Tl(I) and Tl(III). We discovered the Tl uptake in rice exposed to Tl(III) hydroponic treatment was significantly lower than that to Tl(I) treatment, but the content of oxalic acid secreted from roots in Tl(III) treatment was higher than that in Tl(I). The physiological and molecular mechanisms underlying the difference between the two Tl species were studied using a hydroponic system. The results showed the reduction of oxalic acid content had no effect on the amount of Tl on the root surface, indicating oxalic acid might not immobilize Tl to affect the Tl uptake. Therefore, the secretion of oxalic acid from roots may not be a strategy for detoxifying Tl in rice. Notably, Tl(III) increased the expression of Oryza sativa H⁺-ATPase genes OsAs and the activity of H⁺-ATPase, and decreased potassium transport gene expression of OsKAT1.1 and OsHKT2;4, which indicated that the difference in Tl uptake of rice between the two Tl species mainly cause by the potassium transport system rather than oxalic acid.

FveARF2 negatively regulates fruit ripening and quality in strawberry

Auxin response factors (ARFs) are transcription factors that play important roles in plants. ARF2 is a member of the ARF family and participates in many plant growth and developmental processes. However, the role of ARF2 in strawberry fruit quality remains unclear. In this study, FveARF2 was isolated from the woodland strawberry 'Ruegen' using reverse transcription-polymerase chain reaction (RT-PCR), which showed that FveARF2 expression levels were higher in the stem than in other organs of the 'Ruegen' strawberry. Moreover, FaARF2 was higher in the white fruit stage of cultivated strawberry fruit than in other stage. Subcellular localization analysis showed that FveARF2 is located in the nucleus, while transcriptional activation assays showed that FveARF2 inhibited transcription in yeast. Silencing FveARF2 in cultivated strawberry fruit revealed earlier coloration and higher soluble solid, sugar, and anthocyanin content in the transgenic fruit than in the control fruit, overexpression of FveARF2 in strawberry fruit delayed ripening and lower soluble solid, sugar, and anthocyanin content compared to the control fruit. Gene expression analysis indicated that the transcription levels of the fruit ripening genes FaSUT1, FaOMT, and FaCHS increased in FveARF2-RNAi fruit and decreased in FveARF2-OE fruit, when compared with the control. Furthermore, yeast one-hybrid (Y1H) and GUS activity experiments showed that FveARF2 can directly bind to the AuxRE (TGTCTC) element in the FaSUT1, FaOMT, and FaCHS promoters in vitro and in vivo. Potassium ion supplementation improved the quality of strawberry fruit, while silencing FveARF2 increased potassium ion content in transgenic fruit. The Y1H and GUS activity experiments also confirmed that FveARF2 could directly bind to the promoter of FveKT12, a potassium transporter gene, and inhibited its expression. Taken together, we found that FveARF2 can negatively regulate strawberry fruit ripening and quality, which provides new insight for further study of the molecular mechanism of strawberry fruit ripening.

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.

Identification of genomic regions of wheat associated with grain Fe and Zn content under drought and heat stress using genome-wide association study

Wheat is the staple food crop of global importance for its grain nutrient quality. Grain iron and zinc content of the wheat grain is an important quantitatively inherited trait that is influenced by the environmental factors such as drought and heat stress. Phenotypic evaluation of 295 advanced breeding lines from the wheat stress breeding program of IARI was carried out under timely sown irrigated (IR), restricted irrigated, and late-sown conditions at New Delhi during the cropping season of 2020-21, and grain iron (GFeC) and zinc (GZnC) contents were estimated from both control and treatments. A statistically significant increase in GFeC and GZnC was observed under stress conditions compared to that of the control. Genotyping was carried out with the SNPs from the 35K Axiom Breeder's array, and marker-trait association was identified by GWAS analysis. Of the 23 MTAs identified, seven were linked with GFeC and sixteen were linked with GZnC. In silico analysis revealed a few important transcripts involved in various plant metabolism, growth, and development activities such as auxin response factor, root UVB sensitive proteins, potassium transporter, glycosyl transferase, COBRA, and F-box-like domain. The identified MTAs can be used for molecular breeding after validation and also for rapid development of micronutrient-rich varieties of wheat to mitigate hidden hunger.

Genome-wide identification, characterization and expression analysis of HAK genes and decoding their role in responding to potassium deficiency and abiotic stress in Medicago truncatula

Background

The HAK family is the largest potassium (K⁺) transporter family, vital in K⁺ uptake, plant growth, and both plant biotic and abiotic stress responses. Although HAK family members have been characterized and functionally investigated in many species, these genes are still not studied in detail in Medicago truncatula, a good model system for studying legume genetics.

Methods

In this study, we screened the M. truncatula HAK family members (MtHAKs). Furthermore, we also conducted the identification, phylogenetic analysis, and prediction of conserved motifs of MtHAKs. Moreover, we studied the expression levels of MtHAKs under K⁺ deficiency, drought, and salt stresses using quantitative real-time PCR (qRT-PCR).

Results

We identified 20 MtHAK family members and classified them into three clusters based on phylogenetic relationships. Conserved motif analyses showed that all MtHAK proteins besides MtHAK10 contained the highly conserved K⁺ transport domain (GVVYGDLGTSPLY). qRT-PCR analysis showed that several MtHAK genes in roots were induced by abiotic stress. In particular, MtHAK15, MtHAK17, and MtHAK18 were strongly up-regulated in the M. truncatula roots under K⁺ deficiency, drought, and salt stress conditions, thereby implying that these genes are good candidates for high-affinity K⁺ uptake and therefore have essential roles in drought and salt tolerance.

Discussions

Our results not only provided the first genetic description and evolutionary relationships of the K⁺ transporter family in M. truncatula, but also the potential information responding to K⁺ deficiency and abiotic stresses, thereby laying the foundation for molecular breeding of stress-resistant legume crops in the future.

Overexpression of OsHAK5 potassium transporter enhances virus resistance in rice (Oryza sativa)

Intracellular potassium (K⁺ ) transported by plants under the action of a number of transport proteins is crucial for plant survival under distinct abiotic and biotic stresses. A correlation between K⁺ status and disease incidence has been found in many studies, but the roles of K⁺ in regulating disease resistance to viral diseases remain elusive. Here, we report that HIGH-AFFINITY K⁺ TRANSPORTER 5 (OsHAK5) regulates the infection of rice grassy stunt virus (RGSV), a negative-sense single-stranded bunyavirus, in rice (Oryza sativa). We found the K⁺ content in rice plants was significantly inhibited on RGSV infection. Meanwhile, a dramatic induction of OsHAK5 transcripts was observed in RGSV-infected rice plants and in rice plants with K⁺ deficiency. Genetic analysis indicated that disruption of OsHAK5 facilitated viral pathogenicity. In contrast, overexpression of OsHAK5 enhanced resistance to RGSV infection. Our analysis of reactive oxygen species (ROS) including H₂ O₂ and O^2- , by DAB and NBT staining, respectively, indicated that RGSV infection as well as OsHAK5 overexpression increased ROS accumulation in rice leaves. The accumulation of ROS is perhaps involved in the induction of host resistance against RGSV infection in OsHAK5 transgenic overexpression rice plants. Furthermore, RGSV-encoded P3 induced OsHAK5 promoter activity, suggesting that RGSV P3 is probably an elicitor for the induction of OsHAK5 transcripts during RGSV infection. These findings indicate the crucial role of OsHAK5 in host resistance to virus infection. Our results may be exploited in the future to increase crop yield as well as improve host resistance via genetic manipulations.

Estimation Model of Potassium Content in Cotton Leaves Based on Wavelet Decomposition Spectra and Image Combination Features

Potassium (K) is one of the most important elements influencing cotton metabolism, quality, and yield. Due to the characteristics of strong fluidity and fast redistribution of the K in plants, it leads to rapid transformation of the K lack or abundance in plant leaves; therefore, rapid and accurate estimation of potassium content in leaves (LKC, %) is a necessary prerequisite to solve the regulation of plant potassium. In this study, we concentrated on the LKC of cotton in different growth stages, an estimation model based on the combined characteristics of wavelet decomposition spectra and image was proposed, and discussed the potential of different combined features in accurate estimation of the LKC. We collected hyperspectral imaging data of 60 main-stem leaves at the budding, flowering, and boll setting stages of cotton, respectively. The original spectrum (R) is decomposed by continuous wavelet transform (CWT). The competitive adaptive reweighted sampling (CARS) and random frog (RF) algorithms combined with partial least squares regression (PLSR) model were used to determine the optimal decomposition scale and characteristic wavelengths at three growth stages. Based on the best "CWT spectra" model, the grayscale image databases were constructed, and the image features were extracted by using color moment and gray level co-occurrence matrix (GLCM). The results showed that the best decomposition scales of the three growth stages were CWT-1, 3, and 9. The best growth stage for estimating LKC in cotton was the boll setting stage, with the feature combination of "CWT-9 spectra + texture," and its determination coefficients (R ²val) and root mean squared error (RMSEval) values were 0.90 and 0.20. Compared with the single R model (R ²val = 0.66, RMSEval = 0.34), the R ²val increased by 0.24. Different from our hypothesis, the combined feature based on "CWT spectra + color + texture" cannot significantly improve the estimation accuracy of the model, it means that the performance of the estimation model established with more feature information is not correspondingly better. Moreover, the texture features contributed more to the improvement of model performance than color features did. These results provide a reference for rapid and non-destructive monitoring of the LKC in cotton.

Transcriptional and Metabolic Responses of Maize Shoots to Long-Term Potassium Deficiency

Potassium is important for plant growth and crop yield. However, the effects of potassium (K⁺) deficiency on silage maize biomass yield and how maize shoot feedback mechanisms of K⁺ deficiency regulate whole plant growth remains largely unknown. Here, the study aims to explore the maize growth, transcriptional and metabolic responses of shoots to long-term potassium deficiency. Under the K⁺ insufficiency condition, the biomass yield of silage maize decreased. The transcriptome data showed that there were 922 and 1,107 differential expression genes in DH605 and Z58, respectively. In the two varieties, 390 differently expressed overlapping genes were similarly regulated. These genes were considered the fundamental responses to K⁺ deficiency in maize shoots. Many stress-induced genes are involved in transport, primary and secondary metabolism, regulation, and other processes, which are involved in K⁺ acquisition and homeostasis. Metabolic profiles indicated that most amino acids, phenolic acids, organic acids, and alkaloids were accumulated in shoots under K⁺ deficiency conditions and part of the sugars and sugar alcohols also increased. It revealed that putrescine and putrescine derivatives were specifically accumulated under the K⁺ deficiency condition, which may play a role in the feedback regulation of shoot growth. These results confirmed the importance of K⁺ on silage maize production and provided a deeper insight into the responses to K⁺ deficiency in maize shoots.

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.

Exogenous sucrose influences KEA1 and KEA2 to regulate abscisic acid-mediated primary root growth in Arabidopsis

Arabidopsis K⁺-efflux antiporter (KEA)1 and KEA2 are chloroplast inner envelope membrane K⁺/H⁺ antiporters that play an important role in plastid development and seedling growth. However, the function of KEA1 and KEA2 during early seedling development is poorly understood. In this work, we found that in Arabidopsis, KEA1 and KEA2 mediated primary root growth by regulating photosynthesis and the ABA signaling pathway. Phenotypic analyses revealed that in the absence of sucrose, the primary root length of the kea1kea2 mutant was significantly shorter than that of the wild-type Columbia-0 (Col-0) plant. However, this phenotype could be remedied by the external application of sucrose. Meanwhile, HPLC-MS/MS results showed that in sucrose-free medium, ABA accumulation in the kea1kea2 mutant was considerably lower than that in Col-0. Transcriptome analysis revealed that many key genes involved in ABA signals were repressed in the kea1kea2 mutant. We concluded that KEA1 and KEA2 deficiency not only affected photosynthesis but was also involved in primary root growth likely through an ABA-dependent manner. This study confirmed the new function of KEA1 and KEA2 in affecting primary root growth.

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.

Homologous Cloning of Potassium Channel Genes From the Superior Apple Rootstock Line 12-2, Which is Tolerant to Apple Replant Disease

Potassium channels are important ion channels that are responsible for the absorption of potassium in the plant nutrient uptake system. In this study, we used homologous molecular cloning to obtain 8 K⁺ channel genes from the superior apple rootstock line 12-2 (self-named): MsAKT1-1, MsKAT3-2, MsKAT1-3, MsK2P3-4, MsK2P3-5, MsK2P5-6, MsK2P3-7, and MsK2P3-8. Their lengths varied from 942 bp (MsK2P5-6) to 2625 bp (MsAKT1-1), and the number of encoded amino acids varied from 314 (MsK2P5-6) to 874 (MsAKT1-1). Subcellular localization predictions showed that MsAKT1-1, MsKAT3-2, and MsKAT1-3 were localized on the plasma membrane, and MsK2P3-4, MsK2P3-5, MsK2P5-6, MsK2P3-7, and MsK2P3-8 were localized on the vacuole and plasma membrane. The 8 K⁺ channel proteins contained α helices, extended strands, β turns, and random coils. MsKAT1-3 had four transmembrane structures, MsKAT3-2 had six, and the other six K⁺ channel genes had five. Protein structure domain analysis showed that MsAKT1-1 contained nine protein domains, followed by MsKAT3-2 with four, MsKAT1-3 with three, and the other five two-pore domain K⁺ channel proteins with two. Semi-quantitative RT-PCR detection of the K⁺ channel genes showed that their expression levels were high in roots. qRT-PCR analysis showed that the relative expression levels of the 8 genes changed after exposure to ARD stress. The above results provide a theoretical basis for further research on the functions of potassium channel genes in 12-2 and a scientific basis for the breeding of ARD-resistant rootstock.

Genotypic Variation of Nitrogen Use Efficiency and Amino Acid Metabolism in Barley

Owing to the large genetic diversity of barley and its resilience under harsh environments, this crop is of great value for agroecological transition and the need for reduction of nitrogen (N) fertilizers inputs. In the present work, we investigated the diversity of a North African barley genotype collection in terms of growth under limiting N (LN) or ample N (HN) supply and in terms of physiological traits including amino acid content in young seedlings. We identified a Moroccan variety, Laanaceur, accumulating five times more lysine in its leaves than the others under both N nutritional regimes. Physiological characterization of the barley collection showed the genetic diversity of barley adaptation strategies to LN and highlighted a genotype x environment interaction. In all genotypes, N limitation resulted in global biomass reduction, an increase in C concentration, and a higher resource allocation to the roots, indicating that this organ undergoes important adaptive metabolic activity. The most important diversity concerned leaf nitrogen use efficiency (LNUE), root nitrogen use efficiency (RNUE), root nitrogen uptake efficiency (RNUpE), and leaf nitrogen uptake efficiency (LNUpE). Using LNUE as a target trait reflecting barley capacity to deal with N limitation, this trait was positively correlated with plant nitrogen uptake efficiency (PNUpE) and RNUpE. Based on the LNUE trait, we determined three classes showing high, moderate, or low tolerance to N limitation. The transcriptomic approach showed that signaling, ionic transport, immunity, and stress response were the major functions affected by N supply. A candidate gene encoding the HvNRT2.10 transporter was commonly up-regulated under LN in the three barley genotypes investigated. Genes encoding key enzymes required for lysine biosynthesis in plants, dihydrodipicolinate synthase (DHPS) and the catabolic enzyme, the bifunctional Lys-ketoglutarate reductase/saccharopine dehydrogenase are up-regulated in Laanaceur and likely account for a hyperaccumulation of lysine in this genotype. Our work provides key physiological markers of North African barley response to low N availability in the early developmental stages.

Effect of Wetland Restoration and Degradation on Nutrient Trade-Off of Carex schmidtii

Plant nutrient trade-off, a growth strategy, regulates nutrient stoichiometry, allocation and stoichiometric relationships, which is essential in revealing the stoichiometric mechanism of wetland plants under environmental fluctuations. Nonetheless, how wetland restoration and degradation affect nutrient trade-off of wetland plants was still unclear. In this study, field experiments were conducted to explore the dynamic of nutrient stoichiometry and nutrient limitation of Carex schmidtii under wetland restoration and degradation. Plant nutrient stoichiometry and stoichiometric relationships among natural (NW), restored (RW), and degraded (DW) tussock wetlands were examined. Results showed that nutrient stoichiometry of C. schmidtii was partly affected by wetland restoration and degradation, and growth stages. The N:P and N:K ratios indicated N-limitation for the growth of C. schmidtii. Robust stoichiometric scaling relationships were quantified between some plant nutrient concentrations and their ratios of C. schmidtii. Some N- and P-related scaling exponents are varied among NW, RW, and DW. PCA indicated that wetland restoration and degradation had significantly affected on the nutrient trade-offs of C. schmidtii (May∼August). Compared to NW, nutrient trade-off in RW was more similar to DW. Carex schmidtii had significant correlation between most nutrients and their ratios, and the SEM indicated that plant P and K concentrations had a high proportional contribution to plant C and N concentrations. Insights into these aspects are expected to contribute to a better understanding of nutrient trade-off of C. schmidtii under wetland restoration and degradation, providing invaluable information for the protection of C. schmidtii tussock wetlands.