Molecular biology of K+ transport across the plant cell membrane: what do we learn from comparison between plant species?

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

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

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

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

Calcium Sensor SlCBL4 Associates with SlCIPK24 Protein Kinase and Mediates Salt Tolerance in Solanum lycopersicum

Soil salinity is one of the major environmental stresses that restrict the growth and development of tomato (Solanum lycopersicum L.) worldwide. In Arabidopsis, the calcium signaling pathway mediated by calcineurin B-like protein 4 (CBL4) and CBL-interacting protein kinase 24 (CIPK24) plays a critical role in salt stress response. In this study, we identified and isolated two tomato genes similar to the Arabidopsis genes, designated as SlCBL4 and SlCIPK24, respectively. Bimolecular fluorescence complementation (BiFC) and pull-down assays indicated that SlCBL4 can physically interact with SlCIPK24 at the plasma membrane of plant cells in a Ca2+-dependent manner. Overexpression of SlCBL4 or superactive SlCIPK24 mutant (SlCIPK24M) conferred salt tolerance to transgenic tomato (cv. Moneymaker) plants. In particular, the SlCIPK24M-overexpression lines displayed dramatically enhanced tolerance to high salinity. It is notable that the transgenic plants retained higher contents of Na+ and K+ in the roots compared to the wild-type tomato under salt stress. Taken together, our findings clearly suggest that SlCBL4 and SlCIPK24 are functional orthologs of the Arabidopsis counterpart genes, which can be used or engineered to produce salt-tolerant tomato plants.

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

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

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

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

Adjustment of K+ Fluxes and Grapevine Defense in the Face of Climate Change

Grapevine is one of the most economically important fruit crops due to the high value of its fruit and its importance in winemaking. The current decrease in grape berry quality and production can be seen as the consequence of various abiotic constraints imposed by climate changes. Specifically, produced wines have become too sweet, with a stronger impression of alcohol and fewer aromatic qualities. Potassium is known to play a major role in grapevine growth, as well as grape composition and wine quality. Importantly, potassium ions (K+) are involved in the initiation and maintenance of the berry loading process during ripening. Moreover, K+ has also been implicated in various defense mechanisms against abiotic stress. The first part of this review discusses the main negative consequences of the current climate, how they disturb the quality of grape berries at harvest and thus ultimately compromise the potential to obtain a great wine. In the second part, the essential electrical and osmotic functions of K+, which are intimately dependent on K+ transport systems, membrane energization, and cell K+ homeostasis, are presented. This knowledge will help to select crops that are better adapted to adverse environmental conditions.

Differential regulation of drought stress by biological membrane transporters and channels

Stress arising due to abiotic factors affects the plant's growth and productivity. Among several existing abiotic stressors like cold, drought, heat, salinity, heavy metal, etc., drought condition tends to affect the plant's growth by inducing two-point effect, i.e., it disturbs the water balance as well as induces toxicity by disturbing the ion homeostasis, thus hindering the growth and productivity of plants, and to survive under this condition, plants have evolved several transportation systems that are involved in regulating the drought stress. The role of membrane transporters has gained interest since genetic engineering came into existence, and they were found to be the important modulators for tolerance, avoidance, ion movements, stomatal movements, etc. Here in this comprehensive review, we have discussed the role of transporters (ABA, protein, carbohydrates, etc.) and channels that aids in withstanding the drought stress as well as the regulatory role of transporters involved in osmotic adjustments arising due to drought stress. This review also provides a gist of hydraulic conductivity by roots that are involved in regulating the drought stress.

Potassium physiology from Archean to Holocene: A higher-plant perspective

In this paper, we discuss biological potassium acquisition and utilization processes over an evolutionary timescale, with emphasis on modern vascular plants. The quintessential osmotic and electrical functions of the K⁺ ion are shown to be intimately tied to K⁺-transport systems and membrane energization. Several prominent themes in plant K⁺-transport physiology are explored in greater detail, including: (1) channel mediated K⁺ acquisition by roots at low external [K⁺]; (2) K⁺ loading of root xylem elements by active transport; (3) variations on the theme of K⁺ efflux from root cells to the extracellular environment; (4) the veracity and utility of the "affinity" concept in relation to transport systems. We close with a discussion of the importance of plant-potassium relations to our human world, and current trends in potassium nutrition from farm to table.

Potential Networks of Nitrogen-Phosphorus-Potassium Channels and Transporters in Arabidopsis Roots at a Single Cell Resolution

Nitrogen (N), phosphorus (P), and potassium (K) are three major macronutrients essential for plant life. These nutrients are acquired and transported by several large families of transporters expressed in plant roots. However, it remains largely unknown how these transporters are distributed in different cell-types that work together to transfer the nutrients from the soil to different layers of root cells and eventually reach vasculature for massive flow. Using the single cell transcriptomics data from Arabidopsis roots, we profiled the transcriptional patterns of putative nutrient transporters in different root cell-types. Such analyses identified a number of uncharacterized NPK transporters expressed in the root epidermis to mediate NPK uptake and distribution to the adjacent cells. Some transport genes showed cortex- and endodermis-specific expression to direct the nutrient flow toward the vasculature. For long-distance transport, a variety of transporters were shown to express and potentially function in the xylem and phloem. In the context of subcellular distribution of mineral nutrients, the NPK transporters at subcellular compartments were often found to show ubiquitous expression patterns, which suggests function in house-keeping processes. Overall, these single cell transcriptomic analyses provide working models of nutrient transport from the epidermis across the cortex to the vasculature, which can be further tested experimentally in the future.

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⁺.

Membrane voltage as a dynamic platform for spatiotemporal signaling, physiological, and developmental regulation

Membrane voltage arises from the transport of ions through ion-translocating ATPases, ion-coupled transport of solutes, and ion channels, and is an integral part of the bioenergetic "currency" of the membrane. The dynamics of membrane voltage-so-called action, systemic, and variation potentials-have also led to a recognition of their contributions to signal transduction, both within cells and across tissues. Here, we review the origins of our understanding of membrane voltage and its place as a central element in regulating transport and signal transmission. We stress the importance of understanding voltage as a common intermediate that acts both as a driving force for transport-an electrical "substrate"-and as a product of charge flux across the membrane, thereby interconnecting all charge-carrying transport across the membrane. The voltage interconnection is vital to signaling via second messengers that rely on ion flux, including cytosolic free Ca2+, H+, and the synthesis of reactive oxygen species generated by integral membrane, respiratory burst oxidases. These characteristics inform on the ways in which long-distance voltage signals and voltage oscillations give rise to unique gene expression patterns and influence physiological, developmental, and adaptive responses such as systemic acquired resistance to pathogens and to insect herbivory.

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

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

HKT sodium and potassium transporters in Arabidopsis thaliana and related halophyte species

High salinity induces osmotic stress and often leads to sodium ion-specific toxicity, with inhibitory effects on physiological, biochemical and developmental pathways. To cope with increased Na⁺ in soil water, plants restrict influx, compartmentalize ions into vacuoles, export excess Na⁺ from the cell, and distribute ions between the aerial and root organs. In this review, we discuss our current understanding of how high-affinity K⁺ transporters (HKT) contribute to salinity tolerance, focusing on HKT1-like family members primarily involved in long-distance transport, and in the recent research in the model plant Arabidopsis and its halophytic counterparts of the Eutrema genus. Functional characterization of the salt overly sensitive (SOS) pathway and HKT1-type transporters in these species indicate that they utilize similar approaches to deal with salinity, regardless of their tolerance.

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

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

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

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

SAUR proteins and PP2C.D phosphatases regulate H+-ATPases and K+ channels to control stomatal movements

Activation of plasma membrane (PM) H+-ATPase activity is crucial in guard cells to promote light-stimulated stomatal opening, and in growing organs to promote cell expansion. In growing organs, SMALL AUXIN UP RNA (SAUR) proteins inhibit the PP2C.D2, PP2C.D5, and PP2C.D6 (PP2C.D2/5/6) phosphatases, thereby preventing dephosphorylation of the penultimate phosphothreonine of PM H+-ATPases and trapping them in the activated state to promote cell expansion. To elucidate whether SAUR-PP2C.D regulatory modules also affect reversible cell expansion, we examined stomatal apertures and conductances of Arabidopsis thaliana plants with altered SAUR or PP2C.D activity. Here, we report that the pp2c.d2/5/6 triple knockout mutant plants and plant lines overexpressing SAUR fusion proteins exhibit enhanced stomatal apertures and conductances. Reciprocally, saur56 saur60 double mutants, lacking two SAUR genes normally expressed in guard cells, displayed reduced apertures and conductances, as did plants overexpressing PP2C.D5. Although altered PM H+-ATPase activity contributes to these stomatal phenotypes, voltage clamp analysis showed significant changes also in K+ channel gating in lines with altered SAUR and PP2C.D function. Together, our findings demonstrate that SAUR and PP2C.D proteins act antagonistically to facilitate stomatal movements through a concerted targeting of both ATP-dependent H+ pumping and channel-mediated K+ transport.

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

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

Comparative secretome analysis between salinity-tolerant and control Chlamydomonas reinhardtii strains

Secretome analysis of a salt-tolerant and control Chlamydomonas reinhardtii revealed 514 differentially expressed proteins. Membrane transport and trafficking, signal transduction and channel proteins were up-regulated in the ST secretome. Salinity is a major abiotic stress that limits crop production worldwide. Multiple adverse effects have been reported in many living organisms exposed to high-saline concentrations. Chlamydomonas reinhardtii is known for secreting proteins in response to many environmental stresses. A salinity-tolerant (ST) strain of Chlamydomonas has been developed, whose cells were able to grow at 300 mM NaCl. The current study analyzed the secretomes of ST grown in TAP medium supplemented with 300 mM NaCl and the laboratory strain CC-503 grown in TAP medium without NaCl supplement. In total, 514 secreted proteins were identified of which 203 were up-regulated and 110 were down-regulated. Bioinformatic analysis predicted 168 proteins to be secreted or in the conventional secretory pathway. Out of these, 70 were up-regulated, while 51 proteins were down-regulated. Proteins involved in membrane transport and trafficking, signal transduction and channel proteins were altered in their expression in the ST secretome, suggesting the response of saline stress acts toward not only the intracellular pool of proteins but also the extracellular proteins. This also suggested that the secreted proteins might have roles in the extracellular space. Signal peptide (SP) prediction revealed that almost 40% of the predicted secreted proteins contained a signal peptide; however, a high proportion of proteins lacked an SP, suggesting that these proteins might be secreted through an unconventional protein secretion pathway.

Functional analysis of TmHKT1;4-A2 promoter through deletion analysis provides new insight into the regulatory mechanism underlying abiotic stress adaptation

Bioinformatic, molecular, and biochemical analysis were performed to get more insight into the regulatory mechanism by which TmHKT1;4-A2 is regulated. HKT transporters from different plant species have been shown to play important role in plant response to salt. In previous work, TmHKT1;4-A2 gene from Triticum monococcum has been characterized as a major gene for Nax1 QTL (Tounsi et al. Plant Cell Physiol 57:2047-2057, 2016). So far, little is known about its regulatory mechanism. In this study, the promoter region of TmHKT1;4-A2 (1400 bp) was isolated and considered as the full-length promoter (PA2-1400). In silico analysis revealed the presence of important cis-acting elements related to abiotic stresses and phytohormones. Interestingly, our real-time RT-PCR analysis provided evidence that TmHKT1;4-A2 is regulated not only by salt stress but also by osmotic, heavy metal, oxidative, and hormones stresses. In transgenic Arabidopsis plants, TmHKT1;4-A2 is strongly active in vascular tissues of roots and leaves. Through 5'-end deletion analysis, we showed that PA2-1400 promoter is able to drive strong GUS activity under normal conditions and in response to different stresses compared to PA2-824 and PA2-366 promoters. These findings provide new information on the regulatory mechanism of TmHKT1;4-A2 and shed more light on its role under different stresses.

Targeting salt stress coping mechanisms for stress tolerance in Brassica: A research perspective

Brassica genus comprises numerous cultivated brassica species with various economic importance. Salt stress is an overwhelming problem causing serious losses in Brassica species (e.g. B. napus, B. rapa, B. oleracea, B. juncea) growth and grain yield production by inducing ionic and ROS toxicity. Given that a significant variation exists in salt tolerance level in Brassica genus, Brassica species exhibited numerous salt tolerance mechanisms which were either overlooked or given less importance to improve and understand innate salt stress tolerance mechanism in Brassica species. In this review, we tried to highlight the importance and recent findings relating to some overlooked and potential mechanisms such as role of neurotransmitters, and role of cytosolic Ca²⁺ and ROS as signaling elements to enhance salt stress tolerance. Studies revealed that salt tolerant brassica species retained more K⁺ in leaf mesophyll which confers overall salinity tolerance in salt tolerance brassica species. Neurotransmitter such as melatonin, dopamiane and eATP regulates K⁺ and Ca²⁺ permeable ion channels and plays a very crucial role in ionic homeostasis under salinity stress in brassica. At the end, the numerous possible salt stress agronomic strategies were also discussed to mitigate the severity of the salt stress in Brassica species.

Potassium and phosphorus transport and signaling in plants

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

How to Grow a Tree: Plant Voltage-Dependent Cation Channels in the Spotlight of Evolution

Phylogenetic analysis can be a powerful tool for generating hypotheses regarding the evolution of physiological processes. Here, we provide an updated view of the evolution of the main cation channels in plant electrical signalling: the Shaker family of voltage-gated potassium channels and the two-pore cation (K⁺) channel (TPC1) family. Strikingly, the TPC1 family followed the same conservative evolutionary path as one particular subfamily of Shaker channels (K_out) and remained highly invariant after terrestrialisation, suggesting that electrical signalling was, and remains, key to survival on land. We note that phylogenetic analyses can have pitfalls, which may lead to erroneous conclusions. To avoid these in the future, we suggest guidelines for analyses of ion channel evolution in plants.

Looking for Root Hairs to Overcome Poor Soils

Breeding new cultivars allowing reduced fertilization and irrigation is a major challenge. International efforts towards this goal focus on noninvasive methodologies, platforms for high-throughput phenotyping of large plant populations, and quantitative description of root traits as predictors of crop performance in environments with limited water and nutrient availability. However, these high-throughput analyses ignore one crucial component of the root system: root hairs (RHs). Here, we review current knowledge on RH functions, mainly in the context of plant hydromineral nutrition, and take stock of quantitative genetics data pointing at correlations between RH traits and plant biomass production and yield components.

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.

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

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

Potassium: A key modulator for cell homeostasis

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

The Potassium Transporter OsHAK5 Alters Rice Architecture via ATP-Dependent Transmembrane Auxin Fluxes

Plant HAK/KUP/KT family members function as plasma membrane (PM) H⁺/K⁺ symporters and may modulate chemiosmotically-driven polar auxin transport (PAT). Here, we show that inactivation of OsHAK5, a rice K⁺ transporter gene, decreased rootward and shootward PAT, tiller number, and the length of both lateral roots and root hairs, while OsHAK5 overexpression increased PAT, tiller number, and root hair length, irrespective of the K⁺ supply. Inhibitors of ATP-binding-cassette type-B transporters, NPA and BUM, abolished the OsHAK5-overexpression effect on PAT. The mechanistic basis of these changes included the OsHAK5-mediated decrease of transmembrane potential (depolarization), increase of extracellular pH, and increase of PM-ATPase activity. These findings highlight the dual roles of OsHAK5 in altering cellular chemiosmotic gradients (generated continuously by PM H⁺-ATPase) and regulating ATP-dependent auxin transport. Both functions may underlie the prominent effect of OsHAK5 on rice architecture, which may be exploited in the future to increase crop yield via genetic manipulations.

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.

Functional characterization and physiological roles of the single Shaker outward K+ channel in Medicago truncatula

The model legume Medicago truncatula possesses a single outward Shaker K⁺ channel, whereas Arabidopsis thaliana possesses two channels of this type, named AtSKOR and AtGORK, with AtSKOR having been shown to play a major role in K⁺ secretion into the xylem sap in the root vasculature and with AtGORK being shown to mediate the efflux of K⁺ across the guard cell membrane, leading to stomatal closure. Here we show that the expression pattern of the single M. truncatula outward Shaker channel, which has been named MtGORK, includes the root vasculature, guard cells and root hairs. As shown by patch-clamp experiments on root hair protoplasts, besides the Shaker-type slowly activating outwardly rectifying K⁺ conductance encoded by MtGORK, a second K⁺ -permeable conductance, displaying fast activation and weak rectification, can be expressed by M. truncatula. A knock-out (KO) mutation resulting in an absence of MtGORK activity is shown to weakly reduce K⁺ translocation to shoots, and only in plants engaged in rhizobial symbiosis, but to strongly affect the control of stomatal aperture and transpirational water loss. In legumes, the early electrical signaling pathway triggered by Nod-factor perception is known to comprise a short transient depolarization of the root hair plasma membrane. In the absence of the functional expression of MtGORK, the rate of the membrane repolarization is found to be decreased by a factor of approximately two. This defect was without any consequence on infection thread development and nodule production in plants grown in vitro, but a decrease in nodule production was observed in plants grown in soil.

Mechanisms of Plant Responses and Adaptation to Soil Salinity

Soil salinity is a major environmental stress that restricts the growth and yield of crops. Understanding the physiological, metabolic, and biochemical responses of plants to salt stress and mining the salt tolerance-associated genetic resource in nature will be extremely important for us to cultivate salt-tolerant crops. In this review, we provide a comprehensive summary of the mechanisms of salt stress responses in plants, including salt stress-triggered physiological responses, oxidative stress, salt stress sensing and signaling pathways, organellar stress, ion homeostasis, hormonal and gene expression regulation, metabolic changes, as well as salt tolerance mechanisms in halophytes. Important questions regarding salt tolerance that need to be addressed in the future are discussed.

Understanding the role of root-related traits in salinity tolerance of quinoa accessions with contrasting epidermal bladder cell patterning

To compensate for the lack of capacity for external salt storage in the epidermal bladder cells, quinoa plants employ tissue-tolerance traits, to confer salinity stress tolerance. Our previous studies indicated that sequestration of toxic Na⁺ and Cl^- ions into epidermal bladder cells (EBCs) is an efficient mechanism conferring salinity tolerance in quinoa. However, some halophytes do not develop EBCs but still possess superior salinity tolerance. To elucidate the possible compensation mechanism(s) underlying superior salinity tolerance in the absence of the external salt storage capacity, we have selected four quinoa accessions with contrasting patterns of EBC development. Whole-plant physiological and electrophysiological characteristics were assessed after 2 days and 3 weeks of 400 mM NaCl stress. Both accessions with low EBC volume utilised Na⁺ exclusion at the root level and could maintain low Na⁺ concentration in leaves to compensate for the inability to sequester Na⁺ load in EBC. These conclusions were further confirmed by electrophysiological experiments showing higher Na⁺ efflux from roots of these varieties (measured by a non-invasive microelectrode MIFE technique) as compared to accessions with high EBC volume. Furthermore, accessions with low EBC volume had significantly higher K⁺ concentration in their leaves upon long-term salinity exposures compared to plants with high EBC sequestration ability, suggesting that the ability to maintain high K⁺ content in the leaf mesophyll was as another important compensation mechanism.

Exposure to a natural nitric oxide donor negatively affects the potential influx of rubidium in potassium-starved Arabidopsis plants

Nitric oxide (NO) and potassium (K⁺) exert a profound influence on the acclimation of plants to multiple stress conditions. A recent report indicated that exogenous addition of an NO donor causes, under conditions of adequate K⁺ supply, a detrimental effect on K⁺ status. It remains unknown whether an exogenous NO source could negatively affect the potential capture of this element when plants are faced with a K⁺ shortage. In this work we offer evidence that, under conditions of K⁺-deprivation, the addition of the naturally occurring NO donor, S-nitrosoglutathione (GSNO), diminishes the potential inward transport of the K⁺-analogue rubidium (Rb⁺) from diluted Rb⁺ concentrations in Arabidopsis thaliana. Studies with the akt1-2 mutant, lacking the AKT1 inward-rectifier K⁺-channel involved in K⁺-uptake, unveiled that the effect of GSNO on Rb⁺-influx involves a non-AKT1 component. In addition, exposure to the NO-donor led to down-regulation of transcripts coding for the AtHAK5 K⁺-transporter, a major component of the K⁺-transport machinery in K⁺-deprived plants. Moreover, studies with the hak5 mutant showed that GSNO could either stimulate Rb⁺-uptake or does not lead to a significant effect on Rb⁺-uptake relative to -K⁺ and to -K⁺ in the presence of decayed GSNO, respectively, thus indicating that the presence of AtHAK5 is required for GSNO exerting an inhibitory effect.

Phosphatidic acid directly binds with rice potassium channel OsAKT2 to inhibit its activity

The plant Shaker K⁺ channel AtAKT2 has been identified as a weakly rectifying channel that can stabilize membrane potentials to promote photoassimilate phloem loading and translocation. Thus, studies on functional characterization and regulatory mechanisms of AtAKT2-like channels in crops are highly important for improving crop production. Here, we identified the rice OsAKT2 as the ortholog of Arabidopsis AtAKT2, which is primarily expressed in the shoot phloem and localized at the plasma membrane. Using an electrophysiological assay, we found that OsAKT2 operated as a weakly rectifying K⁺ channel, preventing H⁺ /sucrose-symport-induced membrane depolarization. Three critical amino acid residues (K193, N206, and S326) are essential to the phosphorylation-mediated gating change of OsAKT2, consistent with the roles of the corresponding sites in AtAKT2. Disruption of OsAKT2 results in delayed growth of rice seedlings under short-day conditions. Interestingly, the lipid second messenger phosphatidic acid (PA) inhibits OsAKT2-mediated currents (both instantaneous and time-dependent components). Lipid dot-blot assay and liposome-protein binding analysis revealed that PA directly bound with two adjacent arginine residues in the ANK domain of OsAKT2, which is essential to PA-mediated inhibition of OsAKT2. Electrophysiological and phenotypic analyses also showed the PA-mediated inhibition of AtAKT2 and the negative correlation between intrinsic PA level and Arabidopsis growth, suggesting that PA may inhibit AKT2 function to affect plant growth and development. Our results functionally characterize the Shaker K⁺ channel OsAKT2 and reveal a direct link between phospholipid signaling and plant K⁺ channel modulation.

GORK Channel: A Master Switch of Plant Metabolism?

Potassium regulates a plethora of metabolic and developmental response in plants, and upon exposure to biotic and abiotic stresses a substantial K⁺ loss occurs from plant cells. The outward-rectifying potassium efflux GORK channels are central to this stress-induced K⁺ loss from the cytosol. In the mammalian systems, signaling molecules such as gamma-aminobutyric acid, G-proteins, ATP, inositol, and protein phosphatases were shown to operate as ligands controlling many K⁺ efflux channels. Here we present the evidence that the same molecules may also regulate GORK channels in plants. This mechanism enables operation of the GORK channels as a master switch of the cell metabolism, thus adjusting intracellular K⁺ homeostasis to altered environmental conditions, to maximize plant adaptive potential.

KUP9 maintains root meristem activity by regulating K+ and auxin homeostasis in response to low K

Potassium (K) is essential for plant growth and development. Here, we show that the KUP/HAK/KT K⁺ transporter KUP9 controls primary root growth in Arabidopsis thaliana. Under low-K⁺ conditions, kup9 mutants displayed a short-root phenotype that resulted from reduced numbers of root cells. KUP9 was highly expressed in roots and specifically expressed in quiescent center (QC) cells in root tips. The QC acts to maintain root meristem activity, and low-K⁺ conditions induced QC cell division in kup9 mutants, resulting in impaired root meristem activity. The short-root phenotype and enhanced QC cell division in kup9 mutants could be rescued by exogenous auxin treatment or by specifically increasing auxin levels in QC cells, suggesting that KUP9 affects auxin homeostasis in QC cells. Further studies showed that KUP9 mainly localized to the endoplasmic reticulum (ER), where it mediated K⁺ and auxin efflux from the ER lumen to the cytoplasm in QC cells under low-K⁺ conditions. These results demonstrate that KUP9 maintains Arabidopsis root meristem activity and root growth by regulating K⁺ and auxin homeostasis in response to low-K⁺ stress.

Coordinated Transport of Nitrate, Potassium, and Sodium

Potassium (K+) and nitrogen (N) are essential nutrients, and their absorption and distribution within the plant must be coordinated for optimal growth and development. Potassium is involved in charge balance of inorganic and organic anions and macromolecules, control of membrane electrical potential, pH homeostasis and the regulation of cell osmotic pressure, whereas nitrogen is an essential component of amino acids, proteins, and nucleic acids. Nitrate (NO3 -) is often the primary nitrogen source, but it also serves as a signaling molecule to the plant. Nitrate regulates root architecture, stimulates shoot growth, delays flowering, regulates abscisic acid-independent stomata opening, and relieves seed dormancy. Plants can sense K+/NO3 - levels in soils and adjust accordingly the uptake and root-to-shoot transport to balance the distribution of these ions between organs. On the other hand, in small amounts sodium (Na+) is categorized as a "beneficial element" for plants, mainly as a "cheap" osmolyte. However, at high concentrations in the soil, Na+ can inhibit various physiological processes impairing plant growth. Hence, plants have developed specific mechanisms to transport, sense, and respond to a variety of Na+ conditions. Sodium is taken up by many K+ transporters, and a large proportion of Na+ ions accumulated in shoots appear to be loaded into the xylem by systems that show nitrate dependence. Thus, an adequate supply of mineral nutrients is paramount to reduce the noxious effects of salts and to sustain crop productivity under salt stress. In this review, we will focus on recent research unraveling the mechanisms that coordinate the K+-NO3 -; Na+-NO3 -, and K+-Na+ transports, and the regulators controlling their uptake and allocation.

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.

Understanding Mechanisms of Salinity Tolerance in Barley by Proteomic and Biochemical Analysis of Near-Isogenic Lines

Salt stress is one of the major environmental factors impairing crop production. In our previous study, we identified a major QTL for salinity tolerance on chromosome 2H on barley (Hordeum vulgare L.). For further investigation of the mechanisms responsible for this QTL, two pairs of near-isogenic lines (NILs) differing in this QTL were developed. Sensitive NILs (N33 and N53) showed more severe damage after exposure to 300 mM NaCl than tolerant ones (T46 and T66). Both tolerant NILs maintained significantly lower Na⁺ content in leaves and much higher K⁺ content in the roots than sensitive lines under salt conditions, thus indicating the presence of a more optimal Na⁺/K⁺ ratio in plant tissues. Salinity stress caused significant accumulation of H₂O₂, MDA, and proline in salinity-sensitive NILs, and a greater enhancement in antioxidant enzymatic activities at one specific time or tissues in tolerant lines. One pair of NILs (N33 and T46) were used for proteomic studies using two-dimensional gel electrophoresis. A total of 53 and 51 differentially expressed proteins were identified through tandem mass spectrometry analysis in the leaves and roots, respectively. Proteins which are associated with photosynthesis, reactive oxygen species (ROS) scavenging, and ATP synthase were found to be specifically upregulated in the tolerant NIL. Proteins identified in this study can serve as a useful resource with which to explore novel candidate genes for salinity tolerance in barley.

Comparing Kinetics of Xylem Ion Loading and Its Regulation in Halophytes and Glycophytes

Although control of xylem ion loading is essential to confer salinity stress tolerance, specific details behind this process remain elusive. In this work, we compared the kinetics of xylem Na+ and K+ loading between two halophytes (Atriplex lentiformis and quinoa) and two glycophyte (pea and beans) species, to understand the mechanistic basis of the above process. Halophyte plants had high initial amounts of Na+ in the leaf, even when grown in the absence of the salt stress. This was matched by 7-fold higher xylem sap Na+ concentration compared with glycophyte plants. Upon salinity exposure, the xylem sap Na+ concentration increased rapidly but transiently in halophytes, while in glycophytes this increase was much delayed. Electrophysiological experiments using the microelectrode ion flux measuring technique showed that glycophyte plants tend to re-absorb Na+ back into the stele, thus reducing xylem Na+ load at the early stages of salinity exposure. The halophyte plants, however, were capable to release Na+ even in the presence of high Na+ concentrations in the xylem. The presence of hydrogen peroxide (H2O2) [mimicking NaCl stress-induced reactive oxygen species (ROS) accumulation in the root] caused a massive Na+ and Ca2+ uptake into the root stele, while triggering a substantial K+ efflux from the cytosol into apoplast in glycophyte but not halophytes species. The peak in H2O2 production was achieved faster in halophytes (30 min vs 4 h) and was attributed to the increased transcript levels of RbohE. Pharmacological data suggested that non-selective cation channels are unlikely to play a major role in ROS-mediated xylem Na+ loading.

Doing 'business as usual' comes with a cost: evaluating energy cost of maintaining plant intracellular K+ homeostasis under saline conditions

Salinization of agricultural lands is a major threat to agriculture. Many different factors affect and determine plant salt tolerance. Nonetheless, there is a consensus on the relevance of maintaining an optimal cytosolic potassium : sodium ion (K⁺  : Na⁺ ) ratio for salinity tolerance in plants. This ratio depends on the operation of plasma membrane and tonoplast transporters. In the present review we focus on some aspects related to the energetic cost of maintaining that K⁺  : Na⁺ ratio. One of the factors that affect the cost of the first step of K⁺ acquisition - root K⁺ uptake through High Affinity K⁺ transporter and Arabidopsis K⁺ transport system 1 transport systems - is the value of the plasma membrane potential of root cells, a parameter that may differ amongst plant species. In addition to its role in nutrition, cytosolic K⁺ also is important for signalling, and K⁺ efflux through gated outward-rectifying K⁺ and nonselective cation channels can be regarded as a switch to redirect energy towards defence reactions. In maintaining cytosolic K⁺ , the great buffer capacity of the vacuole should be considered. The possible role of high-affinity K⁺ transporters (HKT)2s in mediating K⁺ uptake under saline conditions and the importance of cycling of K⁺ throughout the plant also are discussed.

Constitutive Contribution by the Rice OsHKT1;4 Na+ Transporter to Xylem Sap Desalinization and Low Na+ Accumulation in Young Leaves Under Low as High External Na+ Conditions

HKT Na⁺ transporters correspond to major salt tolerance QTLs in different plant species and are targets of great interest for breeders. In rice, the HKT family is composed of seven or eight functional genes depending on cultivars. Three rice HKT genes, OsHKT1;1, OsHKT1;4 and OsHKT1;5, are known to contribute to salt tolerance by reducing Na⁺ accumulation in shoots upon salt stress. Here, we further investigate the mechanisms by which OsHKT1;4 contributes to this process and extend this analysis to the role of this transporter in plants in presence of low Na⁺ concentrations. By analyzing transgenic rice plants expressing a GUS reporter gene construct, we observed that OsHKT1;4 is mainly expressed in xylem parenchyma in both roots and leaves. Using mutant lines expressing artificial microRNA that selectively reduced OsHKT1;4 expression, the involvement of OsHKT1;4 in retrieving Na⁺ from the xylem sap in the roots upon salt stress was evidenced. Since OsHKT1;4 was found to be also well expressed in the roots in absence of salt stress, we extended the analysis of its role when plants were subjected to non-toxic Na⁺ conditions (0.5 and 5 mM). Our finding that the transporter, expressed in Xenopus oocytes, displayed a relatively high affinity for Na⁺, just above 1 mM, provided first support to the hypothesis that OsHKT1;4 could have a physiological role at low Na⁺ concentrations. We observed that progressive desalinization of the xylem sap along its ascent to the leaf blades still occurred in plants grown at submillimolar Na⁺ concentration, and that OsHKT1;4 was involved in reducing xylem sap Na⁺ concentration in roots in these conditions too. Its contribution to tissue desalinization from roots to young mature leaf blades appeared to be rather similar in the whole range of explored external Na⁺ concentrations, from submillimolar to salt stress conditions. Our data therefore indicate that HKT transporters can be involved in controlling Na⁺ translocation from roots to shoots in a much wider range of Na⁺ concentrations than previously thought. This asks questions about the roles of such a transporter-mediated maintaining of tissue Na⁺ content gradients in non-toxic conditions.

Molecular Evolution and Expansion of the KUP Family in the Allopolyploid Cotton Species Gossypium hirsutum and Gossypium barbadense

The comprehensive analysis of gene family evolution will elucidate the origin and evolution of gene families. The K+ uptake (KUP) gene family plays important roles in K+ uptake and transport, plant growth and development, and abiotic stress responses. However, the current understanding of the KUP family in cotton is limited. In this study, 51 and 53 KUPs were identified in Gossypium barbadense and Gossypium hirsutum, respectively. These KUPs were divided into five KUP subfamilies, with subfamily 2 containing three groups. Different subfamilies had different member numbers, conserved motifs, gene structures, regulatory elements, and gene expansion and loss rates. A paleohexaploidization event caused the expansion of GhKUP and GbKUP in cotton, and duplication events in G. hirsutum and G. barbadense have happened in a common ancestor of Gossypium. Meanwhile, the KUP members of the two allopolyploid subgenomes of G. hirsutum and G. barbadense exhibited unequal gene proportions, gene structural diversity, uneven chromosomal distributions, asymmetric expansion rates, and biased gene loss rates. In addition, the KUP families of G. hirsutum and G. barbadense displayed evolutionary conservation and divergence. Taken together, these results illustrated the molecular evolution and expansion of the KUP family in allopolyploid cotton species.

Exploring the fundamental role of potassium channels in novel model plants

This article comments on:

Villette J, Cuéllar T, Zimmermann SD, Verdeil JL, Gaillard I. 2019. Unique features of the grapevine VvK5.1 channel support novel functions for outward K+ channels in plants. Journal of Experimental Botany 70, 6181–6193.

Unique features of the grapevine VvK5.1 channel support novel functions for outward K+ channels in plants

Grapevine (Vitis vinifera L.), one of the most important fruit crops, is a model plant for studying the physiology of fleshy fruits. Here, we report on the characterization of a new grapevine Shaker-type K+ channel, VvK5.1. Phylogenetic analysis revealed that VvK5.1 belongs to the SKOR-like subfamily. Our functional characterization of VvK5.1 in Xenopus oocytes confirms that it is an outwardly rectifying K+ channel that displays strict K+ selectivity. Gene expression level analyses by real-time quantitative PCR showed that VvK5.1 expression was detected in berries, roots, and flowers. In contrast to its Arabidopsis thaliana counterpart that is involved in K+ secretion in the root pericycle, allowing root to shoot K+ translocation, VvK5.1 expression territory is greatly enlarged. Using in situ hybridization we showed that VvK5.1 is expressed in the phloem and perivascular cells of berries and in flower pistil. In the root, in addition to being expressed in the root pericycle like AtSKOR, a strong expression of VvK5.1 is detected in small cells facing the xylem that are involved in lateral root formation. This fine and selective expression pattern of VvK5.1 at the early stage of lateral root primordia supports a role for outward channels to switch on cell division initiation.

Investigation of Na+ and K+ Transport in Halophytes: Functional Analysis of the HmHKT2;1 Transporter from Hordeum maritimum and Expression under Saline Conditions

Control of K+ and Na+ transport plays a central role in plant adaptation to salinity. In the halophyte Hordeum maritimum, we have characterized a transporter gene, named HmHKT2;1, whose homolog HvHKT2;1 in cultivated barley, Hordeum vulgare, was known to give rise to increased salt tolerance when overexpressed. The encoded protein is strictly identical in two H. maritimum ecotypes, from two biotopes (Tunisian sebkhas) affected by different levels of salinity. These two ecotypes were found to display distinctive responses to salt stress in terms of biomass production, Na+ contents, K+ contents and K+ absorption efficiency. Electrophysiological analysis of HmHKT2;1 in Xenopus oocytes revealed distinctive properties when compared with HvHKT2;1 and other transporters from the same group, especially a much higher affinity for both Na+ and K+, and an Na+-K+ symporter behavior in a very broad range of Na+ and K+ concentrations, due to reduced K+ blockage of the transport pathway. Domain swapping experiments identified the region including the fifth transmembrane segment and the adjacent extracellular loop as playing a major role in the determination of the affinity for Na+ and the level of K+ blockage in these HKT2;1 transporters. The analysis (quantitative reverse transcription-PCR; qRT-PCR) of HmHKT2;1 expression in the two ecotypes submitted to saline conditions revealed that the levels of HmHKT2;1 transcripts were maintained constant in the most salt-tolerant ecotype whereas they decreased in the less tolerant one. Both the unique functional properties of HmHKT2;1 and the regulation of the expression of the encoding gene could contribute to H. maritimum adaptation to salinity.

Tissue-specific respiratory burst oxidase homolog-dependent H2O2 signaling to the plasma membrane H+-ATPase confers potassium uptake and salinity tolerance in Cucurbitaceae

Potassium (K+) is a critical determinant of salinity tolerance, and H2O2 has been recognized as an important signaling molecule that mediates many physiological responses. However, the details of how H2O2 signaling regulates K+ uptake in the root under salt stress remain elusive. In this study, salt-sensitive cucumber and salt-tolerant pumpkin which belong to the same family, Cucurbitaceae, were used to answer the above question. We show that higher salt tolerance in pumpkin was related to its superior ability for K+ uptake and higher H2O2 accumulation in the root apex. Transcriptome analysis showed that salinity induced 5816 (3005 up- and 2811 down-) and 4679 (3965 up- and 714 down-) differentially expressed genes (DEGs) in cucumber and pumpkin, respectively. DEGs encoding NADPH oxidase (respiratory burst oxidase homolog D; RBOHD), 14-3-3 protein (GRF12), plasma membrane H+-ATPase (AHA1), and potassium transporter (HAK5) showed higher expression in pumpkin than in cucumber under salinity stress. Treatment with the NADPH oxidase inhibitor diphenylene iodonium resulted in lower RBOHD, GRF12, AHA1, and HAK5 expression, reduced plasma membrane H+-ATPase activity, and lower K+ uptake, leading to a loss of the salinity tolerance trait in pumpkin. The opposite results were obtained when the plants were pre-treated with exogenous H2O2. Knocking out of RBOHD in pumpkin by CRISPR/Cas9 [clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9] editing of coding sequences resulted in lower root apex H2O2 and K+ content and GRF12, AHA1, and HAK5 expression, ultimately resulting in a salt-sensitive phenotype. However, ectopic expression of pumpkin RBOHD in Arabidopsis led to the opposite effect. Taken together, this study shows that RBOHD-dependent H2O2 signaling in the root apex is important for pumpkin salt tolerance and suggests a novel mechanism that confers this trait, namely RBOHD-mediated transcriptional and post-translational activation of plasma membrane H+-ATPase operating upstream of HAK5 K+ uptake transporters.

Nitric oxide and plant mineral nutrition: current knowledge

Plants under conditions of essential mineral deficiency trigger signaling mechanisms that involve common components. Among these components, nitric oxide (NO) has been identified as a key participant in responses to changes in nutrient availability. Usually, nutrient imbalances affect the levels of NO in specific plant tissues, via modification of its rate of synthesis or degradation. Changes in the level of NO affect plant morphology and/or trigger responses associated with nutrient homeostasis, mediated by its interaction with reactive oxygen species, phytohormones, and through post-translational modification of proteins. NO-related events constitute an exciting field of research to understand how plants adapt and respond to conditions of nutrient shortage. This review summarizes the current knowledge on NO as a component of the multiple processes related to plant performance under conditions of deficiency in mineral nutrients, focusing on macronutrients such as nitrogen, phosphate, potassium, and magnesium, as well as micronutrients such as iron and zinc.

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.

Complex Gene Regulation Underlying Mineral Nutrient Homeostasis in Soybean Root Response to Acidity Stress

Proton toxicity is one of the major environmental stresses limiting crop production and becomes increasingly serious because of anthropogenic activities. To understand acid tolerance mechanisms, the plant growth, mineral nutrients accumulation, and global transcriptome changes in soybean (Glycine max) in response to long-term acidity stress were investigated. Results showed that acidity stress significantly inhibited soybean root growth but exhibited slight effects on the shoot growth. Moreover, concentrations of essential mineral nutrients were significantly affected by acidity stress, mainly differing among soybean organs and mineral nutrient types. Concentrations of phosphorus (P) and molybdenum (Mo) in both leaves and roots, nitrogen (N), and potassium (K) in roots and magnesium (Mg) in leaves were significantly decreased by acidity stress, respectively. Whereas, concentrations of calcium (Ca), sulfate (S), and iron (Fe) were increased in both leaves and roots. Transcriptome analyses in soybean roots resulted in identification of 419 up-regulated and 555 down-regulated genes under acid conditions. A total of 38 differentially expressed genes (DEGs) were involved in mineral nutrients transportation. Among them, all the detected five GmPTs, four GmZIPs, two GmAMTs, and GmKUPs, together with GmIRT1, GmNramp5, GmVIT2.1, GmSKOR, GmTPK5, and GmHKT1, were significantly down-regulated by acidity stress. Moreover, the transcription of genes encoding transcription factors (e.g., GmSTOP2s) and associated with pH stat metabolic pathways was significantly up-regulated by acidity stress. Taken together, it strongly suggests that maintaining pH stat and mineral nutrient homeostasis are adaptive strategies of soybean responses to acidity stress, which might be regulated by a complex signaling network.