Uneven HAK/KUP/KT Protein Diversity Among Angiosperms: Species Distribution and Perspectives

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

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

Potassium transporter OsHAK17 may contribute to saline-alkaline tolerant mechanisms in rice (Oryza sativa)

Rice production is seriously affected by saline-alkaline stress worldwide. To elucidate the saline-alkaline tolerance mechanisms in a novel tolerant rice variety, Shwe Nang Gyi (SNG), we investigated ion accumulation in SNG and Koshihikari (KSH), which is a saline-alkaline sensitive rice variety, and the candidates for saline-alkaline inducible genes in SNG using RNA-seq. SNG had superior ion accumulation capacity, such as K and Zn, compared to KSH. In contrast, SNG accumulated the same level of Na content in its leaf blades as KSH despite the higher dry weight of the SNG leaf blades. We further found that the expression of numerous genes, including several K+ transporter/high-affinity K+ transporter/K+ uptake protein/K+ transporter (HAK/KUP/KT) family members, were upregulated in SNG, and that OsHAK17 and OsHAK21 expression levels in the roots were significantly higher in SNG than in KSH. Moreover, yeast complementation analysis revealed that OsHAK17 was involved in K+ uptake under high-Na conditions. These results suggested that SNG has an effective K+ acquisition system supported by OsHAK17 functioning in saline-alkaline environments.

Potassium transporter OsHAK9 regulates seed germination under salt stress by preventing gibberellin degradation through mediating OsGA2ox7 in rice

Soil salinity has a major impact on rice seed germination, severely limiting rice production. Herein, a rice germination defective mutant under salt stress (gdss) was identified by using chemical mutagenesis. The GDSS gene was detected via MutMap and shown to encode potassium transporter OsHAK9. Phenotypic analysis of complementation and mutant lines demonstrated that OsHAK9 was an essential regulator responsible for seed germination under salt stress. OsHAK9 is highly expressed in germinating seed embryos. Ion contents and non-invasive micro-test technology results showed that OsHAK9 restricted K+ efflux in salt-exposed germinating seeds for the balance of K+/Na+. Disruption of OsHAK9 significantly reduced gibberellin 4 (GA4) levels, and the germination defective phenotype of oshak9a was partly rescued by exogenous GA3 treatment under salt stress. RNA sequencing (RNA-seq) and real-time quantitative polymerase chain reaction analysis demonstrated that the disruption of OsHAK9 improved the GA-deactivated gene OsGA2ox7 expression in germinating seeds under salt stress, and the expression of OsGA2ox7 was significantly inhibited by salt stress. Null mutants of OsGA2ox7 created using clustered, regularly interspaced, short palindromic repeat (CRISPR)/CRISPR-associated nuclease 9 approach displayed a dramatically increased seed germination ability under salt stress. Overall, our results highlight that OsHAK9 regulates seed germination performance under salt stress involving preventing GA degradation by mediating OsGA2ox7, which provides a novel clue about the relationship between GA and OsHAKs in rice.

Genome-wide identification and expression analysis of the HAK/KUP/KT gene family in Moso bamboo

The K+ uptake permease/high-affinity K+/K+ transporter (KUP/HAK/KT) family is the most prominent group of potassium (K+) transporters, playing a key role in K+ uptake, transport, plant growth and development, and stress tolerance. However, the presence and functions of the KUP/HAK/KT family in Moso bamboo (Phyllostachys edulis (Carriere) J. Houzeau), the fastest-growing plant, have not been studied. In this study, we identified 41 KUP/HAK/KT genes (PeHAKs) distributed across 18 chromosomal scaffolds of the Moso bamboo genome. PeHAK is a typical membrane protein with a conserved structural domain and motifs. Phylogenetic tree analysis classified PeHAKs into four distinct clusters, while collinearity analysis revealed gene duplications resulting from purifying selection, including both tandem and segmental duplications. Enrichment analysis of promoter cis-acting elements suggested their plausible role in abiotic stress response and hormone induction. Transcriptomic data and STEM analyses indicated that PeHAKs were involved in tissue and organ development, rapid growth, and responded to different abiotic stress conditions. Subcellular localization analysis demonstrated that PeHAKs are predominantly expressed at the cell membrane. In-situ PCR experiments confirmed that PeHAK was mainly expressed in the lateral root primordia. Furthermore, the involvement of PeHAKs in potassium ion transport was confirmed by studying the potassium ion transport properties of a yeast mutant. Additionally, through homology modeling, we revealed the structural properties of HAK as a transmembrane protein associated with potassium ion transport. This research provides a solid basis for understanding the classification, characterization, and functional analysis of the PeHAK family in Moso bamboo.

Relevance of the SlCIPK23 kinase in Na+ uptake and root morphology in K+-starved tomato plants

The beneficial effects of Na+ as a substitute for K+ have been well-documented at the physiological level. However, the transport systems and regulatory mechanisms that allow Na+ acquisition under K+ deficiency remain poorly understood in the majority of land plants. In tomato, SlCIPK23 kinase was involved in Na+ accumulation in K+-starved plants, in addition to activating the LKT1 K+ channel and the K+ transporter SlHAK5. We used the central role of SlCIPK23 in K+ and Na+ acquisition to study which molecular entities mediate Na+ uptake with knockout tomato mutants and expression in heterologous systems. Two main pathways for Na+ uptake were deduced in tomato plants: an NH4+-sensitive pathway dependent on SlCIPK23, and a second one sensitive to Ba2+, Ca2+, La3+, and Li+. The addition of Na+ (10 mM) to lkt1, slhak5, or slcipk23 mutant KO lines produced interesting changes in root morphology. In particular, the roots of slcipk23 plants were longer and lighter than those of the WT under K+-deficient conditions and this effect was reversed by the addition of 10 mM Na+. These results provide a stimulating perspective for the study of the beneficial effects of Na+ in crops.

Characterization of HAK protein family in Casuarina equisetifolia and the positive regulatory role of CeqHAK6 and CeqHAK11 genes in response to salt tolerance

The potassium transporter group of the HAK/KUP/KT (high-affinity K⁺)/KUP (K⁺ uptake)/KT (K⁺ transporter) family plays a crucial role in plant growth and development as well as in environmental adaptation such as tolerance to salt stress. HAK/KUP/KT genes and their functions have been characterized for a number of plant species, but they remain unknown for Casuarina equisetifolia, an important tree species for coastal protection in southern China and many other countries. In this study, 25 HAK genes were identified in the C. equisetifolia genome. Their gene structure, conserved motif, phylogeny, and expression were comprehensively and systematically analyzed to understand their functions. All HAK genes were relatively conserved and could be divided into four clusters. The expression level of two particular genes, CeqHAK11 and CeqHAK6, increased significantly with the duration of salt treatment. To further elucidated their function in response to salt stress, subcellular localization, and their functional analysis were developed. Results revealed that CeqHAK11 and CeqHAK6 were localized on the plasma membrane, which mainly mediated high-affinity K⁺ uptake. Overexpression of CeqHAK6 or CeqHAK11 in Arabidopsis showed higher germination and survival rates and longer root length than wild-type (WT) under salt stress, suggesting that both genes improve tolerance to salt stress. Moreover, CeqHAK6 and CeqHAK11 improved their ability to tolerate salt stress by increasing the K⁺/Na⁺ ratio and antioxidant enzyme activities (CAT, POD, and SOD), and decreasing reactive oxygen species (ROS) accumulation. Consequently, CeqHAK6 and CeqHAK11 were verified as potassium transport proteins and could be applied for further molecular breeding for salt tolerance in C. equisetifolia or other crops to increasing salt tolerance.

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.

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.

Isolation and identification of plant growth-promoting rhizobacteria from tall fescue rhizosphere and their functions under salt stress

Soil salinity has become one of the major factors that threaten tall fescue growth and turf quality. Plants recruit diverse microorganisms in the rhizosphere to cope with salinity stress. In this study, 15 plant growth-promoting rhizobacteria (PGPR) were isolated from the salt-treated rhizosphere of tall fescue and were annotated to 10 genera, including Agrobacterium, Fictibacillus, Rhizobium, Bhargavaea, Microbacterium, Paenarthrobacter, Pseudarthrobacter, Bacillus, Halomonas, and Paracoccus. All strains could produce indole-3-acetic acid (IAA). Additionally, eight strains exhibited the ability to solubilize phosphate and potassium. Most strains could grow on the medium containing 600 mM NaCl, such as Bacillus zanthoxyli and Bacillus altitudinis. Furthermore, Bacillus zanthoxyli and Bacillus altitudinis were inoculated with tall fescue seeds and seedlings to determine their growth-promoting effect. The results showed that Bacillus altitudinis and mixed culture significantly increased the germination rate of tall fescue seeds. Bacillus zanthoxyli can significantly increase the tillers number and leaf width of seedlings under salt conditions. Through the synergistic effect of FaSOS1, FaHKT1, and FaHAK1 genes, Bacillus zanthoxyli helps to expel the excess Na⁺ from aboveground parts and absorb more K⁺ in roots to maintain ion homeostasis in tall fescue. Unexpectedly, we found that Bacillus altitudinis displayed an inapparent growth-promoting effect on seedlings under salt stress. Interestingly, the mixed culture of the two strains was also able to alleviate, to some extent, the effects of salt stress on tall fescue. This study provides a preliminary understanding of tall fescue rhizobacteria and highlights the role of Bacillus zanthoxyli in tall fescue growth and salt tolerance.

Genome-wide identification and multiple abiotic stress transcript profiling of potassium transport gene homologs in Sorghum bicolor

Potassium (K⁺) is the most abundant cation that plays a crucial role in various cellular processes in plants. Plants have developed an efficient mechanism for the acquisition of K⁺ when grown in K⁺ deficient or saline soils. A total of 47 K⁺ transport gene homologs (27 HAKs, 4 HKTs, 2 KEAs, 9 AKTs, 2 KATs, 2 TPCs, and 1 VDPC) have been identified in Sorghum bicolor. Of 47 homologs, 33 were identified as K⁺ transporters and the remaining 14 as K⁺ channels. Chromosome 2 has been found as the hotspot of K⁺ transporters with 9 genes. Phylogenetic analysis revealed the conservation of sorghum K⁺ transport genes akin to Oryza sativa. Analysis of regulatory elements indicates the key roles that K⁺ transport genes play under different biotic and abiotic stress conditions. Digital expression data of different developmental stages disclosed that expressions were higher in milk, flowering, and tillering stages. Expression levels of the genes SbHAK27 and SbKEA2 were higher during milk, SbHAK17, SbHAK11, SbHAK18, and SbHAK7 during flowering, SbHAK18, SbHAK10, and 23 other gene expressions were elevated during tillering inferring the important role that K⁺ transport genes play during plant growth and development. Differential transcript expression was observed in different tissues like root, stem, and leaf under abiotic stresses such as salt, drought, heat, and cold stresses. Collectively, the in-depth genome-wide analysis and differential transcript profiling of K⁺ transport genes elucidate their role in ion homeostasis and stress tolerance mechanisms.

Transcriptome-wide identification and expression analysis of the KT/HAK/KUP family in Salicornia europaea L. under varied NaCl and KCl treatments

Background

The KT/HAK/KUP (KUP) transporters play important roles in potassium (K⁺) uptake and translocation, regulation of osmotic potential, salt tolerance, root morphogenesis and plant development. However, the KUP family has not been systematically studied in the typical halophyte Salicornia europaea L., and the specific expression patterns of SeKUPs under NaCl condition and K⁺ deficiency are unknown.

Methods

In this study, SeKUPs were screened from PacBio transcriptome data of Salicornia europaea L. using bioinformatics. The identification, phylogenetic analysis and prediction of conserved motifs of SeKUPs were extensively explored. Moreover, the expression levels of 24 selected SeKUPs were assayed by real-time quantitative polymerase chain reaction (RT-qPCR).

Results

In this study, a total of 24 putative SeKUPs were identified in S. europaea. Nineteen SeKUPs with the fixed domain EA[ML]FADL were used to construct the phylogenetic tree, and they were divided into four clusters (clusters I-IV). MEME analysis identified 10 motifs in S. europaea, and the motif analysis suggested that 19 of the identified SeKUPs had at least four K⁺ transporter motifs existed in all SeKUPs (with the exception of SeKUP-2). The RT-qPCR analysis showed that the expression levels of most SeKUPs were significantly up-regulated in S. europaea when they were exposed to K⁺ deficiency and high salinity, implying that these SeKUPs may play a key role in the absorption and transport of K⁺ and Na⁺ in S. europaea.

Discussions

Our results laid the foundation for revealing the salt tolerance mechanism of SeKUPs, and provided key candidate genes for further studies on the function of KUP family in S. europaea.

Potassium transporter TRH1/KUP4 contributes to distinct auxin-mediated root system architecture responses

In plants, auxin transport and development are tightly coupled, just as hormone and growth responses are intimately linked in multicellular systems. Here we provide insights into uncoupling this tight control by specifically targeting the expression of TINY ROOT HAIR 1 (TRH1), a member of plant high-affinity potassium (K+)/K+ uptake/K+ transporter (HAK/KUP/KT) transporters that facilitate K+ uptake by co-transporting protons, in Arabidopsis root cell files. Use of this system pinpointed specific root developmental responses to acropetal versus basipetal auxin transport. Loss of TRH1 function shows TRHs and defective root gravitropism, associated with auxin imbalance in the root apex. Cell file-specific expression of TRH1 in the central cylinder rescued trh1 root agravitropism, whereas positional TRH1 expression in peripheral cell layers, including epidermis and cortex, restored trh1 defects. Applying a system-level approach, the role of RAP2.11 and ROOT HAIR DEFECTIVE-LIKE 5 transcription factors (TFs) in root hair development was verified. Furthermore, ERF53 and WRKY51 TFs were overrepresented upon restoration of root gravitropism supporting involvement in gravitropic control. Auxin has a central role in shaping root system architecture by regulating multiple developmental processes. We reveal that TRH1 jointly modulates intracellular ionic gradients and cell-to-cell polar auxin transport to drive root epidermal cell differentiation and gravitropic response. Our results indicate the developmental importance of HAK/KUP/KT proton-coupled K+ transporters.

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

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

Recent Advances in Genome-wide Analyses of Plant Potassium Transporter Families

Plants require potassium (K⁺) as a macronutrient to support numerous physiological processes. Understanding how this nutrient is transported, stored, and utilized within plants is crucial for breeding crops with high K⁺ use efficiency. As K⁺ is not metabolized, cross-membrane transport becomes a rate-limiting step for efficient distribution and utilization in plants. Several K⁺ transporter families, such as KUP/HAK/KT and KEA transporters and Shaker-like and TPK channels, play dominant roles in plant K⁺ transport processes. In this review, we provide a comprehensive contemporary overview of our knowledge about these K⁺ transporter families in angiosperms, with a major focus on the genome-wide identification of K⁺ transporter families, subcellular localization, spatial expression, function and regulation. We also expanded the genome-wide search for the K⁺ transporter genes and examined their tissue-specific expression in Camelina sativa, a polyploid oil-seed crop with a potential to adapt to marginal lands for biofuel purposes and contribution to sustainable agriculture. In addition, we present new insights and emphasis on the study of K⁺ transporters in polyploids in an effort to generate crops with high K⁺ Utilization Efficiency (KUE).

Role of potassium transporter KUP8 in plant responses to heavy metals

Heavy metal concentrations, which have been increasing over the last 200 years, affect soil quality and crop yields. These elements are difficult to eliminate from soils and may constitute a human health hazard by entering the food chain. Recently, we obtained a selection of mutants with different degrees of tolerance to a mixture of heavy metals (HMmix) in order to gain a deeper insight into the underlying mechanism regulating plant responses to these elements. In this study, we characterized the mutant obtained Atkup8 (in this work, Atkup8-2), which showed one of the most resistant phenotypes, as determined by seedling root length. Atkup8-2 is affected in the potassium transporter KUP8, a member of the high-affinity K⁺ uptake family KUP/HAK/KT. Atkup8-2 mutants, which are less affected as measured by seedling root length under HMmix conditions, showed a resistant phenotype with respect to WT seedlings which, despite their delayed growth, are able to develop true leaves at levels similar to those under control conditions. Adult Atkup8-2 plants had a higher fresh weight than WT plants, a resistant phenotype under HMmix stress conditions and lower levels of oxidative damage. KUP8 did not appear to be involved in heavy metal or macro- and micro-nutrient uptake and translocation from roots to leaves, as total concentrations of these elements were similar in both Atkup8-2 and WT plants. However, alterations in cellular K⁺ homeostasis in this mutant cannot be ruled out.

The Potassium Transporter Hak1 in Candida Albicans, Regulation and Physiological Effects at Limiting Potassium and under Acidic Conditions

The three families of yeast plasma membrane potassium influx transporters are represented in Candida albicans: Trk, Acu, and Hak proteins. Hak transporters work as K⁺-H⁺ symporters, and the genes coding for Hak proteins are transcriptionally activated under potassium limitation. This work shows that C. albicans mutant cells lacking CaHAK1 display a severe growth impairment at limiting potassium concentrations under acidic conditions. This is the consequence of a defective capacity to transport K⁺, as indicated by potassium absorption experiments and by the kinetics parameters of Rb⁺ (K⁺) transport. Moreover, hak1- cells are more sensitive to the toxic cation lithium. All these phenotypes became much less robust or even disappeared at alkaline growth conditions. Finally, transcriptional studies demonstrate that the hak1- mutant, in comparison with HAK1+ cells, activates the expression of the K⁺/Na⁺ ATPase coded by CaACU1 in the presence of Na⁺ or in the absence of K⁺.

Identification and characterization of HAK/KUP/KT potassium transporter gene family in barley and their expression under abiotic stress

BACKGROUND

HAK/KUP/KT (High-affinity K⁺ transporters/K⁺ uptake permeases/K⁺ transporters) is the largest potassium transporter family in plants, and plays pivotal roles in K⁺ uptake and transport, as well as biotic and abiotic stress responses. However, our understanding of the gene family in barley (Hordeum vulgare L.) is quite limited.

RESULTS

In the present study, we identified 27 barley HAK/KUP/KT genes (hereafter called HvHAKs) through a genome-wide analysis. These HvHAKs were unevenly distributed on seven chromosomes, and could be phylogenetically classified into four clusters. All HvHAK protein sequences possessed the conserved motifs and domains. However, the substantial difference existed among HAK members in cis-acting elements and tissue expression patterns. Wheat had the most orthologous genes to barley HAKs, followed by Brachypodium distachyon, rice and maize. In addition, six barley HAK genes were selected to investigate their expression profiling in response to three abiotic stresses by qRT-PCR, and their expression levels were all up-regulated under salt, hyperosmotic and potassium deficiency treatments.

CONCLUSION

Twenty seven HAK genes (HvHAKs) were identified in barley, and they differ in tissue expression patterns and responses to salt stress, drought stress and potassium deficiency.

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

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

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.

Genome-wide characterization and expression analysis of HAK K+ transport family in Ipomoea

The potassium transporter high-affinity K⁺ transporter/K⁺ uptake permease/K⁺ transporter (HAK/KUP/KT) family plays a vital role in potassium uptake, and potassium ion (K⁺)-mediated environmental stress. In the present study, we identified 22 IbHAK/KUP/KT (HAK) genes in sweet potato [Ipomoea batata (L.) Lam] and the same number of HAK genes from sweet potato wild relative Ipomoea trifida. Phylogeny analysis indicated that the HAKs can be divided into five clades. Chromosomal distribution and genome synteny analyses revealed two tandem-duplicated gene pairs IbHAK16/17 and IbHAK17/18 on chromosomes 13 and eight segmental-duplicated gene pairs on chromosomes 1, 3, 5, 8, 10, 12, 14 among the IbHAK gene family. Eleven orthologous HAK gene pairs between I. batata and I. trifida were involved in the duplication of genomic blocks based on comparative genomic analysis. The Ka/Ks ratios of these IbHAK genes ranged from 0.02 to 0.55(< 1), further indicated that purifying selection was the primary force driving the evolution of HAKs in Ipomoea. A heat map based on RNA-seq data showed that 13 HAKs in Xushu32 (a K⁺-tolerant sweet potato genotype) and 10 HAKs in Ningzi1 (a K⁺-sensitive sweet potato genotype) in response to K⁺ deficiency stress. Quantitative real-time PCR (qRT-PCR) analysis revealed IbHAK2, -3, -8, -10, -11, -18, -19, and -21 were induced in both Xushu32 and Ningzi1 under low K⁺ stress. Compared with other IbHAK genes, IbHAK8 showed more strongly upregulation after exposure to drought and salt stress. Furthermore, co-expression analysis showed that only IbHAK8 of 22 IbHAK genes involved in network interactions with 30 genes related to abiotic and biotic stresses. Taken together, these results are helpful for further functional studies on IbHAK and molecular breeding of sweet potato.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-020-02552-3.

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

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

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

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

HAK/KUP/KT family potassium transporter genes are involved in potassium deficiency and stress responses in tea plants (Camellia sinensis L.): expression and functional analysis

BACKGROUND

Tea plant is one of the most important non-alcoholic beverage crops worldwide. While potassium (K+) is an essential macronutrient and greatly affects the growth and development of plants, the molecular mechanism underlying K+ uptake and transport in tea plant root, especially under limited-K+ conditions, is still poorly understood. In plants, HAK/KUP/KT family members play a crucial role in K+ acquisition and translocation, growth and development, and response to stresses. Nevertheless, the biological functions of these genes in tea plant are still in mystery, especially their roles in K+ uptake and stress responses.

RESULTS

In this study, a total of 21 non-redundant HAK/KUP/KT genes (designated as CsHAKs) were identified in tea plant. Phylogenetic and structural analysis classified the CsHAKs into four clusters (I, II, III, IV), containing 4, 8, 4 and 5 genes, respectively. Three major categories of cis-acting elements were found in the promoter regions of CsHAKs. Tissue-specific expression analysis indicated extremely low expression levels in various tissues of cluster I CsHAKs with the exception of a high root expression of CsHAK4 and CsHAK5, a constitutive expression of clusters II and III CsHAKs, and a moderate cluster IV CsHAKs expression. Remarkably, the transcript levels of CsHAKs in roots were significantly induced or suppressed after exposure to K+ deficiency, salt and drought stresses, and phytohormones treatments. Also notably, CsHAK7 was highly expressed in all tissues and was further induced under various stress conditions. Therefore, functional characterization of CsHAK7 was performed, and the results demostrated that CsHAK7 locates on plasma membrane and plays a key role in K+ transport in yeast. Taken together, the results provide promising candidate CsHAKs for further functional studies and contribute to the molecular breeding for new tea plants varieties with highly efficient utilization of K+.

CONCLUSION

This study demonstrated the first genome-wide analysis of CsHAK family genes of tea plant and provides a foundation for understanding the classification and functions of the CsHAKs in tea plants.

Halophytic Hordeum brevisubulatum HbHAK1 Facilitates Potassium Retention and Contributes to Salt Tolerance

Potassium retention under saline conditions has emerged as an important determinant for salt tolerance in plants. Halophytic Hordeum brevisubulatum evolves better strategies to retain K⁺ to improve high-salt tolerance. Hence, uncovering K⁺-efficient uptake under salt stress is vital for understanding K⁺ homeostasis. HAK/KUP/KT transporters play important roles in promoting K⁺ uptake during multiple stresses. Here, we obtained nine salt-induced HAK/KUP/KT members in H. brevisubulatum with different expression patterns compared with H. vulgare through transcriptomic analysis. One member HbHAK1 showed high-affinity K⁺ transporter activity in athak5 to cope with low-K⁺ or salt stresses. The expression of HbHAK1 in yeast Cy162 strains exhibited strong activities in K⁺ uptake under extremely low external K⁺ conditions and reducing Na⁺ toxicity to maintain the survival of yeast cells under high-salt-stress. Comparing with the sequence of barley HvHAK1, we found that C170 and R342 in a conserved domain played pivotal roles in K⁺ selectivity under extremely low-K⁺ conditions (10 μM) and that A13 was responsible for the salt tolerance. Our findings revealed the mechanism of HbHAK1 for K⁺ accumulation and the significant natural adaptive sites for HAK1 activity, highlighting the potential value for crops to promote K⁺-uptake under stresses.

Emerging crosstalk between two signaling pathways coordinates K+ and Na+ homeostasis in the halophyte Hordeum brevisubulatum

K+/Na+ homeostasis is the primary core response for plant to tolerate salinity. Halophytes have evolved novel regulatory mechanisms to maintain a suitable K+/Na+ ratio during long-term adaptation. The wild halophyte Hordeum brevisubulatum can adopt efficient strategies to achieve synergistic levels of K+ and Na+ under high salt stress. However, little is known about its molecular mechanism. Our previous study indicated that HbCIPK2 contributed to prevention of Na+ accumulation and K+ reduction. Here, we further identified the HbCIPK2-interacting proteins including upstream Ca2+ sensors, HbCBL1, HbCBL4, and HbCBL10, and downstream phosphorylated targets, the voltage-gated K+ channel HbVGKC1 and SOS1-like transporter HbSOS1L. HbCBL1 combined with HbCIPK2 could activate HbVGKC1 to absorb K+, while the HbCBL4/10-HbCIPK2 complex modulated HbSOS1L to exclude Na+. This discovery suggested that crosstalk between the sodium response and the potassium uptake signaling pathways indeed exists for HbCIPK2 as the signal hub, and paved the way for understanding the novel mechanism of K+/Na+ homeostasis which has evolved in the halophytic grass.

SlHAK20: a new player in plant salt tolerance

The majority of crops remain sensitive to salt stress despite the steady increase in salt concentration in agricultural soil. In this issue of The EMBO Journal, Wang et al (2020) screen hundreds of tomato accessions to identify SlHAK20 as a gene accounting for quantitative differences in salt tolerance between accessions. SlHAK20 is a potassium transporter belonging to a poorly studied clade in a large family of transporters, and its mutation induces salt susceptibility both in tomato and rice.

Loss of salt tolerance during tomato domestication conferred by variation in a Na+ /K+ transporter

Domestication has resulted in reduced salt tolerance in tomato. To identify the genetic components causing this deficiency, we performed a genome-wide association study (GWAS) for root Na⁺ /K⁺ ratio in a population consisting of 369 tomato accessions with large natural variations. The most significant variations associated with root Na⁺ /K⁺ ratio were identified within the gene SlHAK20 encoding a member of the clade IV HAK/KUP/KT transporters. We further found that SlHAK20 transports Na⁺ and K⁺ and regulates Na⁺ and K⁺ homeostasis under salt stress conditions. A variation in the coding sequence of SlHAK20 was found to be the causative variant associated with Na⁺ /K⁺ ratio and confer salt tolerance in tomato. Knockout mutations in tomato SlHAK20 and the rice homologous genes resulted in hypersensitivity to salt stress. Together, our study uncovered a previously unknown molecular mechanism of salt tolerance responsible for the deficiency in salt tolerance in cultivated tomato varieties. Our findings provide critical information for molecular breeding to improve salt tolerance in tomato and other crops.

Grapevine Potassium Nutrition and Fruit Quality in the Context of Climate Change

Potassium (K⁺) nutrition is of relevant interest for winegrowers because it influences grapevine growth, berry composition, as well as must and wine quality. Indeed, wine quality strongly depends on berry composition at harvest. However, K⁺ content of grape berries increased steadily over the last decades, in part due to climate change. Currently, the properties and qualities of many fruits are also impacted by environment. In grapevine, this disturbs berry properties resulting in unbalanced wines with poor organoleptic quality and low acidity. This requires a better understanding of the molecular basis of K⁺ accumulation and its control along grape berry development. This mini-review summarizes our current knowledge on K⁺ nutrition in relation with fruit quality in the context of a changing environment.

Genome-wide systematic characterization of the HAK/KUP/KT gene family and its expression profile during plant growth and in response to low-K+ stress in Saccharum

BACKGROUND

Plant genomes contain a large number of HAK/KUP/KT transporters, which play important roles in potassium uptake and translocation, osmotic potential regulation, salt tolerance, root morphogenesis and plant development. Potassium deficiency in the soil of a sugarcane planting area is serious. However, the HAK/KUP/KT gene family remains to be characterized in sugarcane (Saccharum).

RESULTS

In this study, 30 HAK/KUP/KT genes were identified in Saccharum spontaneum. Phylogenetics, duplication events, gene structures and expression patterns were analyzed. Phylogenetic analysis of the HAK/KUP/KT genes from 15 representative plants showed that this gene family is divided into four groups (clades I-IV). Both ancient whole-genome duplication (WGD) and recent gene duplication contributed to the expansion of the HAK/KUP/KT gene family. Nonsynonymous to synonymous substitution ratio (Ka/Ks) analysis showed that purifying selection was the main force driving the evolution of HAK/KUP/KT genes. The divergence time of the HAK/KUP/KT gene family was estimated to range from 134.8 to 233.7 Mya based on Ks analysis, suggesting that it is an ancient gene family in plants. Gene structure analysis showed that the HAK/KUP/KT genes were accompanied by intron gain/loss in the process of evolution. RNA-seq data analysis demonstrated that the HAK/KUP/KT genes from clades II and III were mainly constitutively expressed in various tissues, while most genes from clades I and IV had no or very low expression in the tested tissues at different developmental stages. The expression of SsHAK1 and SsHAK21 was upregulated in response to low-K⁺ stress. Yeast functional complementation analysis revealed that SsHAK1 and SsHAK21 could rescue K⁺ uptake in a yeast mutant.

CONCLUSIONS

This study provided insights into the evolutionary history of HAK/KUP/KT genes. HAK7/9/18 were mainly expressed in the upper photosynthetic zone and mature zone of the stem. HAK7/9/18/25 were regulated by sunlight. SsHAK1 and SsHAK21 played important roles in mediating potassium acquisition under limited K⁺ supply. Our results provide valuable information and key candidate genes for further studies on the function of HAK/KUP/KT genes in Saccharum.

Potassium in Root Growth and Development

Potassium is an essential macronutrient that has been partly overshadowed in root science by nitrogen and phosphorus. The current boom in potassium-related studies coincides with an emerging awareness of its importance in plant growth, metabolic functions, stress tolerance, and efficient agriculture. In this review, we summarized recent progress in understanding the role of K+ in root growth, development of root system architecture, cellular functions, and specific plant responses to K+ shortage. K+ transport is crucial for its physiological role. A wide range of K+ transport proteins has developed during evolution and acquired specific functions in plants. There is evidence linking K+ transport with cell expansion, membrane trafficking, auxin homeostasis, cell signaling, and phloem transport. This places K+ among important general regulatory factors of root growth. K+ is a rather mobile element in soil, so the absence of systemic and localized root growth response has been accepted. However, recent research confirms both systemic and localized growth response in Arabidopsis thaliana and highlights K+ uptake as a crucial mechanism for plant stress response. K+-related regulatory mechanisms, K+ transporters, K+ acquisition efficiency, and phenotyping for selection of K+ efficient plants/cultivars are highlighted in this review.

Modulation of K+ translocation by AKT1 and AtHAK5 in Arabidopsis plants

Root cells take up K⁺ from the soil solution, and a fraction of the absorbed K⁺ is translocated to the shoot after being loaded into xylem vessels. K⁺ uptake and translocation are spatially separated processes. K⁺ uptake occurs in the cortex and epidermis whereas K⁺ translocation starts at the stele. Both uptake and translocation processes are expected to be linked, but the connection between them is not well characterized. Here, we studied K⁺ uptake and translocation using Rb⁺ as a tracer in wild-type Arabidopsis thaliana and in T-DNA insertion mutants in the K⁺ uptake or translocation systems. The relative amount of translocated Rb⁺ to the shoot was positively correlated with net Rb⁺ uptake rates, and the akt1 athak5 T-DNA mutant plants were more efficient in their allocation of Rb⁺ to shoots. Moreover, a mutation of SKOR and a reduced plant transpiration prevented the full upregulation of AtHAK5 gene expression and Rb⁺ uptake in K⁺ -starved plants. Lastly, Rb⁺ was found to be retrieved from root xylem vessels, with AKT1 playing a significant role in K⁺ -sufficient plants. Overall, our results suggest that K⁺ uptake and translocation are tightly coordinated via signals that regulate the expression of K⁺ transport systems.

Rice OsHAK16 functions in potassium uptake and translocation in shoot, maintaining potassium homeostasis and salt tolerance

OsHAK16 mediates K uptake and root-to-shoot translocation in a broad range of external K concentrations, thereby contributing to the maintenance of K homeostasis and salt tolerance in the rice shoot. The HAK/KUP/KT transporters have been widely associated with potassium (K) transport across membranes in both microbes and plants. Here, we report the physiological function of OsHAK16, a member belonging to the HAK/KUP/KT family in rice (Oryza sativa L.). Transcriptional expression of OsHAK16 was up-regulated by K deficiency or salt stress. OsHAK16 is localized at the plasma membrane. OsHAK16 knockout (KO) dramatically reduced root K net uptake rate and growth at both 0.1 mM and 1 mM K supplies, while OsHAK16 overexpression (OX) increased total K uptake and growth only at 0.1 mM K level. OsHAK16-KO decreased the rate of rubidium (Rb) uptake and translocation compared to WT at both 0.2 mM and 1 mM Rb levels. OsHAK16 disruption decreased while its overexpression increased K concentration in root slightly but in shoot remarkably. The relative distribution of total K between shoot and root decreased by 30% in OsHAK16-KO lines and increased by 30% in its OX lines compared to WT. OsHAK16-KO diminished K uptake and K/Na ratio, while OsHAK16-OX improved K uptake and translocation from root to shoot, resulting in increased sensitivity and tolerance to salt stress, respectively. Expression of OsHAK16 enhanced the growth of high salt-sensitive yeast mutant by increasing its K but no Na content. Taking all these together, we conclude that OsHAK16 plays crucial roles in maintaining K homeostasis and salt tolerance in rice shoot.

ZmHAK5 and ZmHAK1 function in K+ uptake and distribution in maize under low K+ conditions

Potassium (K+ ) is an essential macronutrient for plant growth and development. Transporters from the KT/HAK/KUP family play crucial roles in K+ homeostasis and cell growth in various plant species. However, their physiological roles in maize are still unknown. In this study, we cloned ZmHAK5 and ZmHAK1 and investigated their functions in maize (Zea mays L.). In situ hybridization showed that ZmHAK5 was mainly expressed in roots, especially in the epidermis, cortex, and vascular bundle. ZmHAK5 was characterized as a high-affinity K+ transporter. Loss of function of ZmHAK5 led to defective K+ uptake in maize, under low K+ conditions, whereas ZmHAK5-overexpressing plants showed increased K+ uptake activity and improved growth. ZmHAK1 was upregulated under low K+ stress, as revealed by RT-qPCR. ZmHAK1 mediated K+ uptake when heterologously expressed in yeast, but its transport activity was weaker than that of ZmHAK5. Overexpression of ZmHAK1 in maize significantly affected K+ distribution in shoots, leading to chlorosis in older leaves. These findings indicate that ZmHAK5 and ZmHAK1 play distinct roles in K+ homeostasis in maize, functioning in K+ uptake and K+ distribution, respectively. Genetic manipulation of ZmHAK5 may represent a feasible way to improve K+ utilization efficiency in maize.

The Potassium Transporter SlHAK10 Is Involved in Mycorrhizal Potassium Uptake

Most terrestrial plants form a root symbiosis with arbuscular mycorrhizal (AM) fungi, which receive fixed carbon from the plant and enhance the plant's uptake of mineral nutrients. AM symbiosis improves the phosphorous and nitrogen nutrition of host plants; however, little is known about the role of AM symbiosis in potassium (K+) nutrition. Here, we report that inoculation with the AM fungus Rhizophagus irregularis improved tomato (Solanum lycopersicum) plant growth and K+ acquisition and that K+ deficiency has a negative effect on root growth and AM colonization. Based on its homology to a Lotus japonicus AM-induced K+ transporter, we identified a mycorrhiza-specific tomato K+ transporter, SlHAK10 (Solanum lycopersicum High-affinity Potassium Transporter10), that was exclusively expressed in arbuscule-containing cells. SlHAK10 could restore a yeast K+ uptake-defective mutant in the low-affinity concentration range. Loss of function of SlHAK10 led to a significant decrease in mycorrhizal K+ uptake and AM colonization rate under low-K+ conditions but did not affect arbuscule development. Overexpressing SlHAK10 from the constitutive cauliflower mosaic virus 35S promoter or the AM-specific Solanum melongena Phosphate Transporter4 not only improved plant growth and K+ uptake but also increased AM colonization efficiency and soluble sugar content in roots supplied with low K+ Our results indicate that tomato plants have a SlHAK10-mediated mycorrhizal K+ uptake pathway and that improved plant K+ nutrition could increase carbohydrate accumulation in roots, which facilitates AM fungal colonization.

The Cotton High-Affinity K+ Transporter, GhHAK5a, Is Essential for Shoot Regulation of K+ Uptake in Root under Potassium Deficiency

Potassium (K) deficiency is a key limiting factor in cotton (Gossypium hirsutum) production. By grafting two contrasting cotton cultivars, CCRI41 (more susceptible to K+ deficiency) and SCRC22 (more tolerant of K+ deficiency), we established that cotton shoot plays a vital role in the regulation of root K+ uptake. To identify the genetic basis of this finding, we performed RNA sequencing (RNA-seq) of roots of CCRI41 self-grafts (CCRI41/CCRI41, scion/rootstock) and SCRC22/CCRI41 reciprocal-grafts exposed to K+ deficiency. We found that GhHAK5a, an orthologous of Arabidopsis thaliana high-affinity K+ transporter, AtHAK5, was significantly induced in the CCRI41 rootstock by the SCRC22 scion. This gene was mainly expressed in roots and was more highly induced by K+ deficiency in roots of SCRC22 than those of CCRI41. Agrobacterium-mediated virus-induced gene silencing and yeast complementary assay showed that GhHAK5a is a high-affinity K+ uptake transporter. Importantly, silencing of GhHAK5a in the CCRI41 rootstock almost completely inhibited the K+ uptake induced by SCRC22 scion in CCRI41 rootstock. We identified a key high-affinity K+ transporter, GhHAK5a in cotton, which is the essential target for shoot regulation of root K+ uptake under K+ deficiency.

Regulation of K+ Nutrition in Plants

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

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

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

The combination of K+ deficiency with other environmental stresses: What is the outcome?

Potassium (K⁺ ) is a macronutrient known for its high mobility and positive charge, which allows efficient and fast control of the electrical balance and osmotic potential in plant cells. Such features allow K⁺ to remarkably contribute to plant stress adaptation. Some agricultural lands are deficient in K⁺ , imposing a stress that reduces crop yield and makes fertilization a common practice. However, individual stress conditions in the field are rare, and crops usually face a combination of different stresses. As plant response to a stress combination cannot always be deduced from individual stress action, it is necessary to gain insights into the specific mechanisms that connect K⁺ homeostasis with other stress effects to improve plant performance in the context of climate change. Surprisingly, plant responses to environmental stresses under a K⁺ -limiting scenario are poorly understood. In the present review, we summarize current knowledge and find substantial gaps regarding specific outcomes of K⁺ deficiency in addition to other environmental stresses. In this regard, combined nutrient deficiencies of K⁺ and other macronutrients are covered in the first part of the review and interactions arising from K⁺ deficiency with salinity, drought and biotic factors in the second part. Information available so far suggests a prominent role of potassium and nitrate transport systems and their regulatory proteins in the response of plants to several stress combinations. Thus, such molecular pathways, which are located at the crossroad between K⁺ homeostasis and environmental stresses, could be considered biotechnological targets in future studies.

Regulation of K+ Nutrition in Plants

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

The Arabidopsis thaliana K+-Uptake Permease 5 (AtKUP5) Contains a Functional Cytosolic Adenylate Cyclase Essential for K+ Transport

Potassium (K⁺) is the most abundant cation in plants, and its uptake and transport are key to growth, development and responses to the environment. Here, we report that Arabidopsis thaliana K⁺ uptake permease 5 (AtKUP5) contains an adenylate cyclase (AC) catalytic center embedded in its N-terminal cytosolic domain. The purified recombinant AC domain generates cAMP in vitro; and when expressed in Escherichia coli, increases cAMP levels in vivo. Both the AC domain and full length AtKUP5 rescue an AC-deficient E. coli mutant, cyaA, and together these data provide evidence that AtKUP5 functions as an AC. Furthermore, full length AtKUP5 complements the Saccharomyces cerevisiae K⁺ transport impaired mutant, trk1 trk2, demonstrating its function as a K⁺ transporter. Surprisingly, a point mutation in the AC center that impairs AC activity, also abolishes complementation of trk1 trk2, suggesting that a functional catalytic AC domain is essential for K⁺ uptake. AtKUP5-mediated K⁺ uptake is not affected by cAMP, the catalytic product of the AC, but, interestingly, causes cytosolic cAMP accumulation. These findings are consistent with a role for AtKUP5 as K⁺ flux sensor, where the flux-dependent cAMP increases modulate downstream components essential for K⁺ homeostasis, such as cyclic nucleotide gated channels.

Foxtail millet SiHAK1 excites extreme high-affinity K+ uptake to maintain K+ homeostasis under low K+ or salt stress

This is the first evidence that SiHAK1 acts as a K⁺ transporter and is modulated by internal and external K⁺, which expands our understanding of the significant physiological roles of large HAK/KUP/KT transporters in crops. Crop genomes have shown the richness of K⁺ transporters in HAK/KUP/KT (High Affinity K⁺/K⁺ Uptake Proteins/K⁺ Transporter) family, and much progress have been achieved toward understanding the diverse roles of K⁺ uptake and translocation, and abiotic stresses resistance in this family. The HAK/KUP/KT family has increasingly been recognized to be at a pivotal status in the mediation of K⁺ translocation and long-term transport; however, our understanding of the molecular mechanisms remains limited. Foxtail millet is an ideal plant for studying long-distance potassium (K) transport because of its small diploid genome and better adaptability to arid lands. Here, we identified 29 putative HAK/KUP/KT proteins from the Setaria italica genome database. These genes were distributed in seven chromosomes of foxtail millet and divided into five clusters. SiHAK1 exhibited widespread expression in various tissues and significant up-regulation in the shoots under low K condition. SiHAK1 was localized in the cell membrane and low K elicited SiHAK1-meidated high-affinity K⁺ uptake activity in Cy162 yeast cells and Arabidopsis athak5 mutants. The transport activity of SiHAK1 was coordinately modulated by external K⁺ supply and internal K⁺ content in the cell under low K and high salt environment. Our findings reveal the K uptake mechanisms of SiHAK1 and indicated that it may be involved in the mediation of K homeostasis in S. italica under K⁺-deficiency and salt stress.

Genome-wide identification, evolution, and expression analysis of the KT/HAK/KUP family in pear

The K⁺ transporter/high-affinity K⁺/K⁺ uptake (KT/HAK/KUP) family, as one of the largest K⁺ transporter families in higher plants, plays an essential role in plant growth, mineral element absorption, salt stress tolerance, and other physiological processes. However, little is known about this family in pear (Pyrus). Here, we identified 20 K⁺ transporter genes in pear (P. bretschneideri) using genome-wide analysis. Their gene structure, chromosomal distribution, conserved motifs, phylogenetics, duplication events, and expression patterns were also examined. The results of phylogenetic analysis showed that PbrKT/HAK/KUP genes were clustered into three major groups (Groups I-III). Among the 20 PbrKT/HAK/KUP genes, 18 were mapped to nine chromosomes and two to scaffolds. Four WGD/segmental gene pairs were identified, indicating that WGD/segmental duplication may have contributed to the expansion of the KT/HAK/KUP family in pear. Among the four pairs of WGD/segmentally duplicated genes, both members of three pairs had been subjected to purifying selection, whereas the fourth pair had been subjected to positive selection. Furthermore, phenotypic experiments showed that the growth of pear seedlings was affected by potassium deficiency treatment. Expression patterns of 20 PbrKT/HAK/KUP genes in roots were further assayed with qRT-PCR. PbrHAK1 and PbrHAK12/16 were significantly expressed in response to K⁺ deficiency, suggesting that these genes are crucial for K⁺ uptake in pear, especially under the condition of K⁺ starvation. Our results provide a foundation for further study on the function of KT/HAK/KUP genes in pear.

A survey of sequences of KT-HAK-KUP transporters in green algae and basal land plants

In this data article, information is provided on sequences of KT-HAK-KUP transporters from green algae and basal land plants. A data set is offered containing sequences corresponding to the chlorophyte algae Chlamydomonas eustigma, Gonium pectorale and Coccomyxa subellipsoidea, the charophyte algae Coleochaete orbicularis and Klebsormidium flaccidum, the bryophyte Sphagnum fallax, the marchantophyte Marchantia polymorpha and the gymnosperm Pinus taeda, which have been not formerly analyzed. In addition, an analysis of similarity scores among representatives of the clusters recognized in photosynthetic green organisms (namely, chlorophyte algae, charophyte algae, basal embryophytes and higher embryophytes) is performed as well as an analysis of membrane topology for them.

KT-HAK-KUP transporters in major terrestrial photosynthetic organisms: A twenty years tale

Since their discovery, twenty years ago, KT-HAK-KUP transporters have become a keystone to understand how alkali cation fluxes are controlled in major land-dwelling photosynthetic organisms. In this review we focus on their discovery, phylogeny, and functions, as well as the regulation of its canonical member, AtHAK5. We also address issues related to structure-function studies, and the technological possibilities opened up by recent findings. Available evidence suggests that this family of transporters underwent an early divergence into major groups following the conquest of land by embryophytes. KT-HAK-KUPs are necessary to accomplish several major developmental and growth processes, as well as to ensure plant responses to environmental injuries. Although the primary function of these transporters is to mediate potassium (K⁺) fluxes, some of them can also mediate sodium (Na⁺) and cesium (Cs⁺) transport, and contribute to maintenance of K⁺ (and Na⁺) homeostasis in different plant tissues. In addition, there is evidence for a role of some members of this family in auxin movement and in adenylate cyclase activity. Recent research, focusing on the regulation of the canonical member of this family, AtHAK5, revealed the existence of a complex network that involves transcriptional and post-transcriptional phenomena which control the enhancement of AtHAK5-mediated K⁺ uptake when Arabidopsis thaliana plants are faced with low K⁺ supply. In spite of the formidable advances made since their discovery, important subjects remain to be elucidated to gain a more complete knowledge of the roles and regulation of KT-HAK-KUPs, as well as to improve their use for innovative procedures in crop breeding.

Root bacterial endophytes confer drought resistance and enhance expression and activity of a vacuolar H+ -pumping pyrophosphatase in pepper plants

It has been previously shown that the transgenic overexpression of the plant root vacuolar proton pumps H+ -ATPase (V-ATPase) and H+ -PPase (V-PPase) confer tolerance to drought. Since plant-root endophytic bacteria can also promote drought tolerance, we hypothesize that such promotion can be associated to the enhancement of the host vacuolar proton pumps expression and activity. To test this hypothesis, we selected two endophytic bacteria endowed with an array of in vitro plant growth promoting traits. Their genome sequences confirmed the presence of traits previously shown to confer drought resistance to plants, such as the synthesis of nitric oxide and of organic volatile organic compounds. We used the two strains on pepper (Capsicuum annuum L.) because of its high sensitivity to drought. Under drought conditions, both strains stimulated a larger root system and enhanced the leaves' photosynthetic activity. By testing the expression and activity of the vacuolar proton pumps, H+ -ATPase (V-ATPase) and H+ -PPase (V-PPase), we found that bacterial colonization enhanced V-PPase only. We conclude that the enhanced expression and activity of V-PPase can be favoured by the colonization of drought-tolerance-inducing bacterial endophytes.

Genome-wide association studies to identify rice salt-tolerance markers

Salinity is an ever increasing menace that affects agriculture worldwide. Crops such as rice are salt sensitive, but its degree of susceptibility varies widely between cultivars pointing to extensive genetic diversity that can be exploited to identify genes and proteins that are relevant in the response of rice to salt stress. We used a diversity panel of 306 rice accessions and collected phenotypic data after short (6 h), medium (7 d) and long (30 d) salinity treatment (50 mm NaCl). A genome-wide association study (GWAS) was subsequently performed, which identified around 1200 candidate genes from many functional categories, but this was treatment period dependent. Further analysis showed the presence of cation transporters and transcription factors with a known role in salinity tolerance and those that hitherto were not known to be involved in salt stress. Localization analysis of single nucleotide polymorphisms (SNPs) showed the presence of several hundred non-synonymous SNPs (nsSNPs) in coding regions and earmarked specific genomic regions with increased numbers of nsSNPs. It points to components of the ubiquitination pathway as important sources of genetic diversity that could underpin phenotypic variation in stress tolerance.

Genome-Wide Identification and Expression Analysis of the KUP Family under Abiotic Stress in Cassava (Manihot esculenta Crantz)

KT/HAK/KUP (KUP) family is responsible for potassium ion (K⁺) transport, which plays a vital role in the response of plants to abiotic stress by maintaining osmotic balance. However, our understanding of the functions of the KUP family in the drought-resistant crop cassava (Manihot esculenta Crantz) is limited. In the present study, 21 cassava KUP genes (MeKUPs) were identified and classified into four clusters based on phylogenetic relationships, conserved motifs, and gene structure analyses. Transcriptome analysis revealed the expression diversity of cassava KUPs in various tissues of three genotypes. Comparative transcriptome analysis showed that the activation of MeKUP genes by drought was more in roots than that in leaves of Arg7 and W14 genotypes, whereas less in roots than that in leaves of SC124 variety. These findings indicate that different cassava genotypes utilize various drought resistance mechanism mediated by KUP genes. Specific KUP genes showed broad upregulation after exposure to salt, osmotic, cold, H₂O₂, and abscisic acid (ABA) treatments. Taken together, this study provides insights into the KUP-mediated drought response of cassava at transcription levels and identifies candidate genes that may be utilized in improving crop tolerance to abiotic stress.

Production of low-Cs+ rice plants by inactivation of the K+ transporter OsHAK1 with the CRISPR-Cas system

The occurrence of radiocesium in food has raised sharp health concerns after nuclear accidents. Despite being present at low concentrations in contaminated soils (below μm), cesium (Cs⁺ ) can be taken up by crops and transported to their edible parts. This plant capacity to take up Cs⁺ from low concentrations has notably affected the production of rice (Oryza sativa L.) in Japan after the nuclear accident at Fukushima in 2011. Several strategies have been put into practice to reduce Cs⁺ content in this crop species such as contaminated soil removal or adaptation of agricultural practices, including dedicated fertilizer management, with limited impact or pernicious side-effects. Conversely, the development of biotechnological approaches aimed at reducing Cs⁺ accumulation in rice remain challenging. Here, we show that inactivation of the Cs⁺ -permeable K⁺ transporter OsHAK1 with the CRISPR-Cas system dramatically reduced Cs⁺ uptake by rice plants. Cs⁺ uptake in rice roots and in transformed yeast cells that expressed OsHAK1 displayed very similar kinetics parameters. In rice, Cs⁺ uptake is dependent on two functional properties of OsHAK1: (i) a poor capacity of this system to discriminate between Cs⁺ and K⁺ ; and (ii) a high capacity to transport Cs⁺ from very low external concentrations that is likely to involve an active transport mechanism. In an experiment with a Fukushima soil highly contaminated with ¹³⁷ Cs⁺ , plants lacking OsHAK1 function displayed strikingly reduced levels of ¹³⁷ Cs⁺ in roots and shoots. These results open stimulating perspectives to smartly produce safe food in regions contaminated by nuclear accidents.