High-affinity Potassium Transporter Research Articles

Functional analysis on the role of HvHKT1.4 in barley (Hordeum vulgare L.) salinity tolerance

High-affinity potassium transporters (HKTs) are well known proteins that govern the partitioning of Na+ between roots and shoots. Six HvHKTs were identified in barley and designated as HvHKT1.1, HvHKT1.3, HvHKT1.4, HvHKT1.5, HvHKT2.1 and HvHKT2.2 according to their similarity to previously reported OsHKTs. Among these HvHKTs, HvHKT1.4 was highly up-regulated under salinity stress in both leaves and roots of Golden Promise. Subcellular localization analysis showed that HvHKT1.4 is a plasma-membrane–localized protein. The knockout mutants of HvHKT1.4 showed greater salinity sensitivity and higher Na+ concentration in leaves than wild-type plants. Haplotype analysis of HvHKT1.4 in 344 barley accessions showed 15 single nucleotide substitutions in the CDS region, belonging to five haplotypes. Significant differences in mean salinity damage scores, leaf Na+ contents and Na+/K+ were found between Hap5 and other haplotypes with Hap5 showing better salinity tolerance. The results indicated that HvHKT1.4 can be an effective target in improving salinity tolerance through ion homeostasis.

Revealing mechanistic basis of ameliorating detrimental effects of cadmium in cherry tomatoes by exogenous application of melatonin and brassinosteroids

Increased anthropogenic activities over the last decades have led to a gradual increase in cadmium content in the soil, which, due to its high mobility in soil, makes Cd accumulation in plants a serious threat to the health of animals and humans. Plant hormones including melatonin (Mel) and brassinosteroids (BR) are known to provide tolerance against various abiotic stresses. In this work, the role of combined and separate exogenous application of Mel and BR on Cd stress in cherry tomato plants was examined. Cd stress significantly reduced tomato growth by inducing oxidative stress and reduced K+ uptake in roots and shoots. Combined application of Mel and BR reduced detrimental effects of Cd in tomato by (i) reducing Cd accumulation in the shoot; (ii) increasing the activities of different antioxidants (SOD, CAT, APX, GR); (iii) triggering higher expression of genes relating to Cd vacuolar sequestration (Na+/H+ EXCHANGER, SlNHX1; NATURAL RESISTANCE-ASSOCIATED MACROPHAGE PROTEIN 6, SlNRAMP6), and Cd transport and detoxification (HEAVY-METAL-ASSOCIATED 3, SlHMA3; PLANT CADMIUM RESISTANT 2, SlPCR2); and (iv) improving plant K+ homeostasis and contents in root and shoot. The latter trait was associated with the reduced gene expression of K+-permeable outward rectifying channel (SlGORK3), and transcriptional upregulation of high affinity potassium transporter 5 (SIHAK5) under Cd stress. A separate application of Mel and BR showed tissue-specific regulation of tomato growth and Cd tolerance by regulating antioxidant activities, K+ uptake, Cd uptake, and translocation from root to shoot and their endogenous contents. Melatonin per se was more effective in improving Cd tolerance in shoot while beneficial BR effects were more pronounced in roots, and their combined application was effective in both tissues. Taken together, reported results show tissue-specific regulation of Cd tolerance by Mel and BR in cherry tomato plants and demonstrate the efficiency of combined Mel + BR treatment as a practical tool to reduce Cd accumulation and mitigate its negative effects on plant growth.

Analysis of Ion Transport Properties of Glycine max HKT Transporters and Identifying a Regulation of GmHKT1;1 by the Non-Functional GmHKT1;4.

High-affinity potassium transporters (HKTs) play an important role in plants responding to salt stress, but the transport properties of the soybean HKT transporters at the molecular level are still unclear. Here, using Xenopus oocyte as a heterologous expression system and two-electrode voltage-clamp technique, we identified four HKT transporters, GmHKT1;1, GmHKT1;2, GmHKT1;3 and GmHKT1;4, all of which belong to type I subfamily, but have distinct ion transport properties. While GmHKT1;1, GmHKT1;2 and GmHKT1;3 function as Na+ transporters, GmHKT1;1 is less selective against K+ than the two other transporters. Astonishingly, GmHKT1;4, which lacks transmembrane segments and has no ion permeability, is significantly expressed, and its gene expression pattern is different from the other three GmHKTs under salt stress. Interestingly, GmHKT1;4 reduced the Na+/K+ currents mediated by GmHKT1;1. Further study showed that the transport ability of GmHKT1;1 regulated by GmHKT1;4 was related to the structural differences in the first intracellular domain and the fourth repeat domain. Overall, we have identified one unique GmHKT member, GmHKT1;4, which modulates the Na+ and K+ transport ability of GmHKT1;1 via direct interaction. Thus, we have revealed a new type of HKT interaction model for altering their ion transport properties.

Involvement of the Transporter CgTrk1 in Potassium Uptake, Invasive Growth, and Full Virulence in Colletotrichum gloeosporioides

Colletotrichum gloeosporioides is one of the more economically important pathogen causing anthracnose on crops and trees worldwide. As an essential mineral nutrient, potassium play a vital role for fundamental cellular processes in organisms. In this study, a high-affinity potassium transporter CgTrk1 was identified in C. gloeosporioides. Cytological examinations revealed that CgTrk1 was localized in the plasma membrane. The gene deletion mutant of CgTRK1 significantly depressed the potassium uptake. CgTrk1 is also required for vegetative growth, appressorium development, invasive growth, and full virulence. The data also indicate that CgTrk1 plays dominant roles in potassium uptake and pathogenicity rather than its homologous protein CgTrk2. These results imply that the potassium transporter CgTrk1 is involved in invasive growth and full virulence in C. gloeosporioides.

In silico approach to investigate the potential HKT gene responsive to salt stress in rice

Rice is frequently subjected to various environmental stresses, resulting in significant production losses, with drought and salinity are the leading causes of plant damage globally. This study aims to characterize and understand the function of rice high-affinity potassium transporters (HKTs) genes in response to salinity stress. Initially, the genome-wide analysis was undertaken to reveal the evolutionarily conserved function of the OsHKT in higher plants. To investigate the transcription level of OsHKT during the vegetative and reproductive stages, two microarray datasets (GSE19024 and GSE3053) were analyzed, and salt-treated samples were subsequently evaluated using real-time PCR. Differentially expressed genes (DEGs) were identified from microarray datasets (GSE41650 and GSE14403), followed by constructing a DEG network that highlighted interaction partners of the OsHKTs. Genome mining of rice revealed 9 HKT genes, namely OsHKT1;1–1;5 and OsHKT2;1–2;4. These genes exhibited a well-conserved domain structure called TrkH. Comprehensive phylogenetic and motif analyses clustered genes encoding HKT proteins into seven monophyletic groups, and the motifs were relatively conserved. Ka/Ks ratios indicated a high degree of purifying selection during evolutionary time. Gene ontology findings suggested the involvement of OsHKT in stress response. Besides, several CRE motifs in the promoter regions of OsHKT have demonstrated their potential roles in abiotic stress responses. Furthermore, we analyzed the top 250 significant DEGs from the two datasets (p-value < 0.05; fold two change ≥ 1 or ≤ − 1) to evaluate the relationship among the DEGs and HKTs. Three co-expressed OsHKT genes were discovered to be upregulated in seedlings under salinity treatment, including OsP5CS2, OsHAK1, and OsNHX2, whereas OsP5CS1 and OsHAK27 were downregulated. The transcripts of OsHKT were found to be differentially expressed in a tissue-specific manner. Analysis of microarray datasets validated by real-time PCR shows that OsHKT1;5 had a higher expression level, followed by OsHKT1;1, OsHKT1;3, and OsHKT2;1 after salinity treatment. In addition, several micro-RNA targets in rice HKT genes regulate their expression in response to stress. This study paves the way for future investigation on genes and miRNA-target interaction in plants under environmental stresses, offering potential strategies to enhance stress tolerance in crops via targeted ion transport modification.

A high-affinity potassium transporter (MeHKT1) from cassava (Manihot esculenta) negatively regulates the response of transgenic Arabidopsis to salt stress

BackgroundHigh-affinity potassium transporters (HKTs) are crucial in facilitating potassium uptake by plants. Many types of HKTs confer salt tolerance to plants through regulating K+ and Na+ homeostasis under salinity stress. However, their specific functions in cassava (Manihot esculenta) remain unclear.ResultsHerein, an HKT gene (MeHKT1) was cloned from cassava, and its expression is triggered by exposure to salt stress. The expression of a plasma membrane-bound protein functions as transporter to rescue a low potassium (K+) sensitivity of yeast mutant strain, but the complementation of MeHKT1 is inhibited by NaCl treatment. Under low K+ stress, transgenic Arabidopsis with MeHKT1 exhibits improved growth due to increasing shoot K+ content. In contrast, transgenic Arabidopsis accumulates more Na+ under salt stress than wild-type (WT) plants. Nevertheless, the differences in K+ content between transgenic and WT plants are not significant. Additionally, Arabidopsis expressing MeHKT1 displayed a stronger salt-sensitive phenotype.ConclusionThese results suggest that under low K+ condition, MeHKT1 functions as a potassium transporter. In contrast, MeHKT1 mainly transports Na+ into cells under salt stress condition and negatively regulates the response of transgenic Arabidopsis to salt stress. Our results provide a reference for further research on the function of MeHKT1, and provide a basis for further application of MeHKT1 in cassava by molecular biological means.

From swamp to field: how genes from mangroves and its associates can enhance crop salinity tolerance.

Salinity stress is a critical challenge in crop production and requires innovative strategies to enhance the salt tolerance of plants. Insights from mangrove species, which are renowned for their adaptability to high-salinity environments, provides valuable genetic targets and resources for improving crops. A significant hurdle in salinity stress is the excessive uptake of sodium ions (Na+) by plant roots, causing disruptions in cellular balance, nutrient deficiencies, and hampered growth. Specific ion transporters and channels play crucial roles in maintaining a low Na+/K+ ratio in root cells which is pivotal for salt tolerance. The family of high-affinity potassium transporters, recently characterized in Avicennia officinalis, contributes to K+ homeostasis in transgenic Arabidopsis plants even under high-salt conditions. The salt overly sensitive pathway and genes related to vacuolar-type H+-ATPases hold promise for expelling cytosolic Na+ and sequestering Na+ in transgenic plants, respectively. Aquaporins contribute to mangroves' adaptation to saline environments by regulating water uptake, transpiration, and osmotic balance. Antioxidant enzymes mitigate oxidative damage, whereas genes regulating osmolytes, such as glycine betaine and proline, provide osmoprotection. Mangroves exhibit increased expression of stress-responsive transcription factors such as MYB, NAC, and CBFs under high salinity. Moreover, genes involved in various metabolic pathways, including jasmonate synthesis, triterpenoid production, and protein stability under salt stress, have been identified. This review highlights the potential of mangrove genes to enhance salt tolerance of crops. Further research is imperative to fully comprehend and apply these genes to crop breeding to improve salinity resilience.

Understanding mechanisms for differential salinity tissue tolerance between quinoa and spinach: Zooming on ROS-inducible ion channels

Soil salinity is a worldwide issue and a major threat to global food security. Salinity tolerance is a complex mechanism that is conferred by numerous molecular, physiological, and biochemical traits. Of critical importance are plant’s ability to regulate redox balance without compromising reactive oxygen species (ROS) signalling and maintain cytosolic ion homeostasis. In this study, the mechanistic basis of K+ retention ability in leaf mesophyll (an important but highly sensitive plant tissue) was compared between halophytic quinoa and glycophytic spinach. Phenotypic data showed quinoa outperformed spinach under 100 to 500 mmol L−1 NaCl salinity. The major difference behind this differential salinity sensitivity was a differential K+ uptake in leaf mesophyll. Electrophysiological and molecular experiments revealed that a superior ability of mesophyll K+ retention in quinoa was conferred by three complementary mechanisms: (i) an intrinsically lower H+-ATPase activity in quinoa (potentially as an energy saving strategy); (ii) reduced sensitivity of K+ transporters to ROS; and (iii) increased sensitivity of ROS-inducible Ca2+-permeable channels. Moreover, the sensitivity of K+-transport systems to ROS was further examined in NaCl-acclimated quinoa and spinach plants. The key factors differentiating between K+ retention in acclimated leaf mesophyll was associated with the reduced sensitivity and gene expression of K+-permeable outward rectifying channel (GORK), Arabidopsis potassium transporter 1 (AKT1), and high affinity potassium transporter 5 (HAK5) to additional NaCl and ROS stress, along with the upregulation of ROS scavenging system. Taken together, our results showed that the tissue-specific and ROS-specific regulation of K+ retention are important for conferring salinity tolerant at least in the halophyte quinoa.

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.

Structural insights into the Oryza sativa cation transporters HKTs in salt tolerance.

The high-affinity potassium transporters (HKTs), selectively permeable to either Na+ alone or Na+/K+, play pivotal roles in maintaining plant Na+/K+ homeostasis. Although their involvement in salt tolerance is widely reported, the molecular underpinnings of Oryza sativa HKTs remain elusive. In this study, we elucidate the structures of OsHKT1;1 and OsHKT2;1, representing two distinct classes of rice HKTs. The dimeric assembled OsHKTs can be structurally divided into four domains. At the dimer interface, a half-helix or a loop in the third domain is coordinated by the C-terminal region of the opposite subunit. Additionally, we present the structures of OsHKT1;5 salt-tolerant and salt-sensitive variants, a key quantitative trait locus associated with salt tolerance. The salt-tolerant variant of OsHKT1;5 exhibits enhanced Na+ transport capability and displays a more flexible conformation. These findings shed light on the molecular basis of rice HKTs and provide insights into their role in salt tolerance.

Structures and ion transport mechanisms of plant high-affinity potassium transporters

Plant high-affinity K+ transporters (HKTs) mediate Na+ and K+ uptake, maintain Na+/K+ homeostasis, and therefore play crucial roles in plant salt tolerance. In this study, we present cryoelectron microscopy structures of HKTs from two classes, class I HKT1;1 from Arabidopsis thaliana (AtHKT1;1) and class II HKT2;1 from Triticum aestivum (TaHKT2;1), in both Na+- and K+-bound states at 2.6- to 3.0-Å resolutions. Both AtHKT1;1 and TaHKT2;1 function as homodimers. Each HKT subunit consists of four tandem domain units (D1–D4) with a repeated K+-channel-like M–P–M topology. In each subunit, D1–D4 assemble into an ion conduction pore with a pseudo-four-fold symmetry. Although both TaHKT2;1 and AtHKT1;1 have only one putative Na+ ion bound in the selectivity filter with a similar coordination pattern, the two HKTs display different K+ binding modes in the filter. TaHKT2;1 has three K+ ions bound in the selectivity filter, but AtHKT1;1 has only two K+ ions bound in the filter, which has a narrowed external entrance due to the presence of a Ser residue in the first filter motif. These structures, along with computational, mutational, and electrophysiological analyses, enable us to pinpoint key residues that are critical for the ion selectivity of HKTs. The findings provide new insights into the ion selectivity and ion transport mechanisms of plant HKTs and improve our understanding about how HKTs mediate plant salt tolerance and enhance crop growth.

Translatome and Transcriptome Analyses Reveal the Mechanism that Underlies the Enhancement of Salt Stress by the Small Peptide Ospep5 in Plants.

Salt stress significantly impedes plant growth and the crop yield. This study utilized de novo transcriptome assembly and ribosome profiling to explore mRNA translation's role in rice salt tolerance. We identified unrecognized translated open reading frames (ORFs), including 42 upstream transcripts and 86 unannotated transcripts. A noteworthy discovery was the role of a small ORF, Ospep5, in conferring salt tolerance. Overexpression of Ospep5 in plants increased salt tolerance, while its absence led to heightened sensitivity. This hypothesis was corroborated by the findings that exogenous application of the synthetic small peptide Ospep5 bolstered salt tolerance in both rice and Arabidopsis. We found that the mechanism underpinning the Ospep5-mediated salt tolerance involves the maintenance of intracellular Na+/K+ homeostasis, facilitated by upregulation of high-affinity potassium transporters (HKT) and Na+/H+ exchangers (SOS1). Furthermore, a comprehensive multiomics approach, particularly ribosome profiling, is instrumental in uncovering unannotated ORFs and elucidating their functions in plant stress responses.

The superior growth of Kluyveromyces marxianus at very low potassium concentrations is enabled by the high-affinity potassium transporter Hak1.

The non-conventional yeast Kluyveromyces marxianus has recently emerged as a promising candidate for many food, environment, and biotechnology applications. This yeast is thermotolerant and has robust growth under many adverse conditions. Here, we show that its ability to grow under potassium-limiting conditions is much better than that of Saccharomyces cerevisiae, suggesting a very efficient and high-affinity potassium uptake system(s) in this species. The K. marxianus genome contains two genes for putative potassium transporters: KmHAK1 and KmTRK1. To characterize the products of the two genes, we constructed single and double knock-out mutants in K. marxianus and also expressed both genes in an S. cerevisiae mutant, that lacks potassium importers. Our results in K. marxianus and S. cerevisiae revealed that both genes encode efficient high-affinity potassium transporters, contributing to potassium homeostasis and maintaining plasma-membrane potential and cytosolic pH. In K. marxianus, the presence of HAK1 supports growth at low K+ much better than that of TRK1, probably because the substrate affinity of KmHak1 is about 10-fold higher than that of KmTrk1, and its expression is induced ~80-fold upon potassium starvation. KmHak1 is crucial for salt stress survival in both K. marxianus and S. cerevisiae. In co-expression experiments with ScTrk1 and ScTrk2, its robustness contributes to an increased tolerance of S. cerevisiae cells to sodium and lithium salt stress.

Effect of salinity stress on growth, chlorophyll, antioxidant enzymes and nutrient content in Azolla spp.

Azolla is an aquatic fern that has a symbiotic association with nitrogen-fixing cyanobacteria. It is mainly used as a biofertilizer in rice; however, its potential under salt-affected rice cultivated area was compromised. Therefore, the present study was undertaken to understand the effect of salinity stress on morpho-physiological, biochemical characteristics, photosynthetic efficacy, nutrient and High Affinity Potassium Transporter (HKT) genes in Azolla. The results indicated that out of 102, 8 Azolla (A. microphylla, BLCC 5, BLCC 18, BLCC 28, Pa Car WTY, R 18, R 54 and R 59) were found tolerant to 80 mM NaCl. The best species for salt tolerant (80 mM NaCl) was A. microphylla, whereas the least-tolerant was A. rubra. Fresh biomass production, frond length and width in A. microphylla were significantly (p < 0.05) higher in A. microphylla than A. rubra in both 40 and 80 mM NaCl. Moreover, chlorophyll a/b ratio, carotenoids and chlorophyll fluorescence (CHF)-derived FO, Fm, Fv/Fm and root architecture (root length, average root diameter, root volume, projectile and surface area) were higher in A. microphylla than A. rubra under 40 and 80 mM NaCl. Contents of Na+ and Ca2+ increased in both A. microphylla and A. rubra, which can interfere with the uptake of essential macronutrients; however, these were accumulated comparatively less in A. microphylla than A. rubra, whereas a reverse trend was observed in cellular accumulation of K+ content. A. microphylla had higher superoxide dismutase (SOD), ascorbate peroxidase (APX), and proline activities in 40 and 80 mM NaCl than A. rubra. For the first time, twenty six HKT primers were designed as a molecular marker to identify salt-tolerant Azolla. Out of these, three HKT primers (Req 6, Aeq14, and Aeq16) were amplified in A. microphylla under NaCl stress, while their amplifications were not observed in A. rubra (salt susceptible). In A. microphylla, the expression of the Req 6 (HKT) gene were more under NaCl stress. Moreover, further research is needed to discover and validate the biochemical and molecular processes that confer salinity tolerance in Azolla plants.

Exploring OsHAK Transporters for the Salt Responsiveness in Rice

Environmental stresses are the major constraint in crop growth and productivity, particularly in rice. Improving rice against major abiotic stressors such as salinity is one of the major thrust areas. In this direction, the trait of sodium exclusion was the primary focus among the rice breeders to enhance salt tolerance. Along similar lines, increased uptake of potassium, the counter ion of Na+, through high-affinity potassium transporters (HAK) assumes significance in the context of maintaining a favorable Na/K ratio, a low Na/K is an indicator of salt tolerance. The OsHAK family in rice includes 27 members and an insight into the evolution, structure, and expression through in silico approaches was attempted. Phylogenetic analysis exhibited four distinct groupings of the OsHAK family, and functional motif analysis revealed a characteristic consensus sequence among the OsHAK transporters. Relative gene expression analysis based on published microarray data put forth the differential regulation of OsHAK3 X2, OsHAK8, OsHAK14(X2), OsHAK15 and OsHAK26 between salt-tolerant landrace Pokkali and susceptible cultivar IR29 and their tissue-specific expression profiles predicted. This study narrowed down a set of putative OsHAKs from the larger family for their role in enhancing cellular potassium under salt stress thus paving the way for crop improvement.

Identification of the High-Affinity Potassium Transporter Gene Family (HKT) in Brassica U-Triangle Species and Its Potential Roles in Abiotic Stress in Brassica napus L.

Members of the high-affinity potassium transporter (HKT) protein family regulate the uptake and homeostasis of sodium and potassium ions, but little research describes their roles in response to abiotic stresses in rapeseed (Brassica napus L.). In this study, we identified and characterized a total of 36 HKT genes from the species comprising the triangle of U model (U-triangle species): B. rapa, B. nigra, B. oleracea, B. juncea, B. napus, and B. carinata. We analyzed the phylogenetic relationships, gene structures, motif compositions, and chromosomal distributions of the HKT family members of rapeseed. Based on their phylogenetic relationships and assemblage of functional domains, we classified the HKT members into four subgroups, HKT1;1 to HKT1;4. Analysis of the nonsynonymous substitutions (Ka), synonymous substitutions (Ks), and the Ka/Ks ratios of HKT gene pairs suggested that these genes have experienced strong purifying selective pressure after duplication, with their evolutionary relationships supporting the U-triangle theory. Furthermore, the expression profiles of BnaHKT genes varies among potassium, phytohormone and heavy-metal treatment. Their repression provides resistance to heavy-metal stress, possibly by limiting uptake. Our results systematically reveal the characteristics of HKT family proteins and their encoding genes in six Brassica species and lay a foundation for further exploration of the role of HKT family genes in heavy-metal tolerance.

Zinc finger transcription factor MtZPT2-2 negatively regulates salt tolerance in Medicago truncatula.

Zinc finger proteins (ZFPs) are transcription factors involved in multiple cellular functions. We identified a C2H2 type ZFP (MtZPT2-2) in Medicago truncatula and demonstrated that it localizes to the nucleus and inhibits the transcription of 2 genes encoding high-affinity potassium transporters (MtHKT1;1 and MtHKT1;2). MtZPT2-2 transcripts were detected in stem, leaf, flower, seeds and roots, with the highest level in the xylem and phloem of roots and stems. MtZPT2-2 transcription in leaves was reduced after salt stress. Compared with the wild-type (WT), transgenic lines overexpressing MtZPT2-2 had decreased salt tolerance, while MtZPT2-2-knockout mutants showed increased salt tolerance. MtHKT1;1 and MtHKT1;2 transcripts and Na+ accumulation in shoots and roots, as well as in the xylem of all genotypes of plants, were increased after salt treatment, with higher levels of MtHKT1;1 and MtHKT1;2 transcripts and Na+ accumulation in MtZPT2-2-knockout mutants and lower levels in MtZPT2-2-overexpressing lines compared with the WT. K+ levels showed no significant difference among plant genotypes under salt stress. Moreover, MtZPT2-2 was demonstrated to bind with the promoter of MtHKT1;1 and MtHKT1;2 to inhibit their expression. Antioxidant enzyme activities and the gene transcript levels were accordingly upregulated in response to salt, with higher levels in MtZPT2-2-knockout mutants and lower levels in MtZPT2-2-overexpressing lines compared with WT. The results suggest that MtZPT2-2 regulates salt tolerance negatively through downregulating MtHKT1;1 and MtHKT1;2 expression directly to reduce Na+ unloading from the xylem and regulates antioxidant defense indirectly.

High-affinity potassium transporter from a mangrove tree Avicennia officinalis increases salinity tolerance of Arabidopsis thaliana

Salinity reduces the growth and productivity of crop plants worldwide. Mangroves have evolved efficient ion homeostasis mechanisms to survive under their natural saline growth habitat. Information obtained from them may be utilized for increasing the salt tolerance of crop plants. We identified and characterized a high-affinity potassium transporter gene (AoHKT1) from Avicennia officinalis. The expression of AoHKT1 was induced by NaCl mainly in the leaves. Functional study by heterologous expression of AoHKT1 in Arabidopsis T-DNA insertional mutants athkt1–1 and athkt1–4 revealed that it could enhance the salt tolerance of the mutant plants. This was accompanied by an increase in K+ accumulation in the leaves. AoHKT1 was localized to the plasma membrane in Arabidopsis, and when expressed in yeast, it could complement the functions of both Na+ and K+ transporters. An attempt was made to identify the upstream regulator of AtHKT1, a close homolog of AoHKT1. Using chromatin immunoprecipitation, luciferase assay and yeast one-hybrid assays, WRKY9 was identified as the main transcription factor in the process. Furthermore, this was corroborated by the observation that AtHKT1 levels were significantly reduced in the atwrky9 seedlings. These findings revealed a part of the molecular regulatory mechanism of HKT1 induction in response to salt treatment in Arabidopsis. Our study suggests that AoHKT1 is a potential candidate for generating crop plants with increased salt tolerance.

The SbWRKY54–SbHKT2b transcriptional cascade confers cadmium stress tolerance in sorghum

In plants, high–affinity potassium transporters (HKTs) not only play a significant role in potassium transport but may also be important for the abiotic stress response. However, information regarding the response of HKTs to cadmium (Cd) stress is limited. Here, we focus on SbHKT2b, the expression of which is highly induced by Cd exposure. SbHKT2b overexpression reduced the accumulation of Cd and increased tolerance to Cd exposure, while SbHKT2b–RNA interference (RNAi) lines exhibited the opposite phenotype. In addition, the overexpression of SbHKT2b resulted in reduced reactive oxygen species (ROS) accumulation and enhanced ROS quenching capacity under Cd stress. Further analyses revealed that SbWRKY54 directly binds to the SbHKT2b promoter and upregulates its expression. Overall, the overexpression of SbWRKY54 decreased Cd and ROS accumulation in transgenic sorghum. Taken together, our results showed that SbWRKY54–SbHKT2b is an important Cd stress response pathway in sorghum.

Potassium transporter OsHAK18 mediates potassium and sodium circulation and sugar translocation in rice.

High-affinity potassium (K+) transporter (HAK)/K+ uptake permease (KUP)/K+ transporter (KT) have been identified in all genome-sequenced terrestrial plants. They play an important role in K+ acquisition and translocation and in enhancing salt tolerance. Here, we report that plasma membrane-located OsHAK18 functions in K+ and sodium (Na+) circulation and sugar translocation in rice (Oryza sativa). OsHAK18 was expressed mainly, though not exclusively, in vascular tissues and particularly in the phloem. Knockout (KO) of OsHAK18 reduced K+ concentration in phloem sap and roots but increased K+ accumulation in the shoot of both 'Nipponbare' and 'Zhonghua11' cultivars, while overexpression (OX) of OsHAK18 driven by its endogenous promoter increased K+ concentration in phloem sap and roots and promoted Na+ retrieval from the shoot to the root under salt stress. Split-root experimental analysis of rubidium (Rb+) uptake and circulation indicated that OsHAK18-OX promoted Rb+ translocation from the shoot to the root. In addition, OsHAK18-KO increased while OsHAK18-OX reduced soluble sugar content in the shoot and oppositely affected the sugar concentration in the phloem and its content in the root. Moreover, OsHAK18-OX dramatically increased grain yield and physiological K+ utilization efficiency. Our results suggest that-unlike other OsHAKs analyzed heretofore-OsHAK18 is critical for K+ and Na+ recirculation from the shoot to the root and enhances the source-to-sink translocation of photo-assimilates.