CHX14 is a plasma membrane K-efflux transporter that regulates K(+) redistribution in Arabidopsis thaliana

Genomic basis determining root system architecture in maize

KEY MESSAGE

A total of 389 and 344 QTLs were identified by GWAS and QTL mapping explaining accumulatively 32.2-65.0% and 23.7-63.4% of phenotypic variation for 14 shoot-borne root traits using more than 1300 individuals across multiple field trails. Efficient nutrient and water acquisition from soils depends on the root system architecture (RSA). However, the genetic determinants underlying RSA in maize remain largely unexplored. In this study, we conducted a comprehensive genetic analysis for 14 shoot-borne root traits using 513 inbred lines and 800 individuals from four recombinant inbred line (RIL) populations at the mature stage across multiple field trails. Our analysis revealed substantial phenotypic variation for these 14 root traits, with a total of 389 and 344 QTLs identified through genome-wide association analysis (GWAS) and linkage analysis, respectively. These QTLs collectively explained 32.2-65.0% and 23.7-63.4% of the trait variation within each population. Several a priori candidate genes involved in auxin and cytokinin signaling pathways, such as IAA26, ARF2, LBD37 and CKX3, were found to co-localize with these loci. In addition, a total of 69 transcription factors (TFs) from 27 TF families (MYB, NAC, bZIP, bHLH and WRKY) were found for shoot-borne root traits. A total of 19 genes including PIN3, LBD15, IAA32, IAA38 and ARR12 and 19 GWAS signals were overlapped with selective sweeps. Further, significant additive effects were found for root traits, and pyramiding the favorable alleles could enhance maize root development. These findings could contribute to understand the genetic basis of root development and evolution, and provided an important genetic resource for the genetic improvement of root traits in maize.

Types of Membrane Transporters and the Mechanisms of Interaction between Them and Reactive Oxygen Species in Plants

Membrane transporters are proteins that mediate the entry and exit of substances through the plasma membrane and organellar membranes and are capable of recognizing and binding to specific substances, thereby facilitating substance transport. Membrane transporters are divided into different types, e.g., ion transporters, sugar transporters, amino acid transporters, and aquaporins, based on the substances they transport. These membrane transporters inhibit reactive oxygen species (ROS) generation through ion regulation, sugar and amino acid transport, hormone induction, and other mechanisms. They can also promote enzymatic and nonenzymatic reactions in plants, activate antioxidant enzyme activity, and promote ROS scavenging. Moreover, membrane transporters can transport plant growth regulators, solute proteins, redox potential regulators, and other substances involved in ROS metabolism through corresponding metabolic pathways, ultimately achieving ROS homeostasis in plants. In turn, ROS, as signaling molecules, can affect the activity of membrane transporters under abiotic stress through collaboration with ions and involvement in hormone metabolic pathways. The research described in this review provides a theoretical basis for improving plant stress resistance, promoting plant growth and development, and breeding high-quality plant varieties.

Genome-wide characterization of the CPA gene family in potato and a preliminary functional analysis of its role in NaCl tolerance

BACKGROUND

The cation/proton antiporter (CPA) superfamily plays a crucial role in regulating ion homeostasis and pH in plant cells, contributing to stress resistance. However, in potato (Solanum tuberosum L.), systematic identification and analysis of CPA genes are lacking.

RESULTS

A total of 33 StCPA members were identified and classified into StNHX (n = 7), StKEA (n = 6), and StCHX (n = 20) subfamilies. StCHX owned the highest number of conserved motifs, followed by StKEA and StNHX. The StNHX and StKEA subfamilies owned more exons than StCHX. NaCl stress induced the differentially expression of 19 genes in roots or leaves, among which StCHX14 and StCHX16 were specifically induced in leaves, while StCHX2 and StCHX19 were specifically expressed in the roots. A total of 11 strongly responded genes were further verified by qPCR. Six CPA family members, StNHX1, StNHX2, StNHX3, StNHX5, StNHX6 and StCHX19, were proved to transport Na+ through yeast complementation experiments.

CONCLUSIONS

This study provides comprehensive insights into StCPAs and their response to NaCl stress, facilitating further functional characterization.

Membrane Proteins in Plant Salinity Stress Perception, Sensing, and Response

Plants have several mechanisms to endure salinity stress. The degree of salt tolerance varies significantly among different terrestrial crops. Proteins at the plant's cell wall and membrane mediate different physiological roles owing to their critical positioning between two distinct environments. A specific membrane protein is responsible for a single type of activity, such as a specific group of ion transport or a similar group of small molecule binding to exert multiple cellular effects. During salinity stress in plants, membrane protein functions: ion homeostasis, signal transduction, redox homeostasis, and solute transport are essential for stress perception, signaling, and recovery. Therefore, comprehensive knowledge about plant membrane proteins is essential to modulate crop salinity tolerance. This review gives a detailed overview of the membrane proteins involved in plant salinity stress highlighting the recent findings. Also, it discusses the role of solute transporters, accessory polypeptides, and proteins in salinity tolerance. Finally, some aspects of membrane proteins are discussed with potential applications to developing salt tolerance in crops.

Transcription elongation factor AtSPT4-2 positively modulates salt tolerance in Arabidopsis thaliana

BACKGROUND

Salt stress significantly influences plant growth and reduces crop yield. It is highly anticipated to develop salt-tolerant crops with salt tolerance genes and transgenic technology. Hence, it is critical to identify salt tolerance genes that can be used to improve crop salt tolerance.

RESULTS

We report that the transcription elongation factor suppressor of Ty 4-2 (SPT4-2) is a positive modulator of salt tolerance in Arabidopsis thaliana. AtSPT4-2 expression is induced by salt stress. Knockout mutants of AtSPT4-2 display a salt-sensitive phenotype, whereas AtSPT4-2 overexpression lines exhibit enhanced salt tolerance. Comparative transcriptomic analyses revealed that AtSPT4-2 may orchestrate the expression of genes associated with salt tolerance, including stress-responsive markers, protein kinases and phosphatases, salt-responsive transcription factors and those maintaining ion homeostasis, suggesting that AtSPT4-2 improves salt tolerance mainly by maintaining ion homeostasis and enhancing stress tolerance.

CONCLUSIONS

AtSPT4-2 positively modulates salt tolerance by maintaining ion homeostasis and regulating stress-responsive genes and serves as a candidate for the improvement of crop salt tolerance.

Functional and biotechnological cues of potassium homeostasis for stress tolerance and plant development

Potassium (K+) is indispensable for the regulation of a plethora of functions like plant metabolism, growth, development, and abiotic stress responses. K+ is associated with protein synthesis and entangled in the activation of scores of enzymes, stomatal regulation, and photosynthesis. It has multiple transporters and channels that assist in the uptake, efflux, transport within the cell as well as from soil to different tissues, and the grain filling sites. While it is implicated in ion homeostasis during salt stress, it acts as a modulator of stomatal movements during water deficit conditions. K+ is reported to abate the effects of chilling and photooxidative stresses. K+ has been found to ameliorate effectively the co-occurrence of drought and high-temperature stresses. Nutrient deficiency of K+ makes leaves necrotic, leads to diminished photosynthesis, and decreased assimilate utilization highlighting the role it plays in photosynthesis. Notably, K+ is associated with the detoxification of reactive oxygen species (ROS) when plants are exposed to diverse abiotic stress conditions. It is irrefutable now that K+ reduces the activity of NADPH oxidases and at the same time maintains electron transport activity, which helps in mitigating the oxidative stress. K+ as a macronutrient in plant growth, the role of K+ during abiotic stress and the protein phosphatases involved in K+ transport have been reviewed. This review presents a holistic view of the biological functions of K+, its uptake, translocation, signaling, and the critical roles it plays under abiotic stress conditions, plant growth, and development that are being unraveled in recent times.

Regulation of pollen tube growth by cellular pH and ions

Tip growth of the pollen tube is a model system for the study of cell polarity establishment in flowering plants. The tip growth of the pollen tube displays an oscillating pattern corresponding to cellular ion and pH dynamics. Therefore, cellular pH and ions play an important role in pollen growth and development. In this review, we summarized the current advances in understanding the function of cellular pH and ions in regulating pollen tube growth. We analyzed the physiological roles and underlying mechanisms of cellular pH and ions, including Ca²⁺, K⁺, and Cl^-, in regulating pollen tube growth. We further examined the function of Ca²⁺ in regulating cytoskeletons, small G proteins, and cell wall development in relation to pollen tube growth. We also examined the regulatory roles of cellular pH in pollen tube growth as well as pH regulation of ion flow, cell wall development, auxin signaling, and cytoskeleton function in pollen. In addition, we assessed the regulation of pollen tube growth by proton pumps and the maintenance of pH homeostasis in the trans-Golgi network by ion transporters. The interplay of ion homeostasis and pH dynamics was also assessed. We discussed the unanswered questions regarding pollen tube growth that need to be addressed in the future.

Genomic & structural diversity and functional role of potassium (K+) transport proteins in plants

Potassium (K⁺) is an essential macronutrient for plant growth and productivity. It is the most abundant cation in plants and is involved in various cellular processes. Variable K⁺ availability is sensed by plant roots, consequently K⁺ transport proteins are activated to optimize K⁺ uptake. In addition to K⁺ uptake and translocation these proteins are involved in other important physiological processes like transmembrane voltage regulation, polar auxin transport, maintenance of Na⁺/K⁺ ratio and stomata movement during abiotic stress responses. K⁺ transport proteins display tremendous genomic and structural diversity in plants. Their key structural features, such as transmembrane domains, N-terminal domains, C-terminal domains and loops determine their ability of K⁺ uptake and transport and thus, provide functional diversity. Most K⁺ transporters are regulated at transcriptional and post-translational levels. Genetic manipulation of key K⁺ transporters/channels could be a prominent strategy for improving K⁺ utilization efficiency (KUE) in plants. This review discusses the genomic and structural diversity of various K⁺ transport proteins in plants. Also, an update on the function of K⁺ transport proteins and their regulatory mechanism in response to variable K⁺ availability, in improving KUE, biotic and abiotic stresses is provided.

Regulation of Cytosolic pH: The Contributions of Plant Plasma Membrane H+-ATPases and Multiple Transporters

Cytosolic pH homeostasis is a precondition for the normal growth and stress responses in plants, and H+ flux across the plasma membrane is essential for cytoplasmic pH control. Hence, this review focuses on seven types of proteins that possess direct H+ transport activity, namely, H+-ATPase, NHX, CHX, AMT, NRT, PHT, and KT/HAK/KUP, to summarize their plasma-membrane-located family members, the effect of corresponding gene knockout and/or overexpression on cytosolic pH, the H+ transport pathway, and their functional regulation by the extracellular/cytosolic pH. In general, H+-ATPases mediate H+ extrusion, whereas most members of other six proteins mediate H+ influx, thus contributing to cytosolic pH homeostasis by directly modulating H+ flux across the plasma membrane. The fact that some AMTs/NRTs mediate H+-coupled substrate influx, whereas other intra-family members facilitate H+-uncoupled substrate transport, demonstrates that not all plasma membrane transporters possess H+-coupled substrate transport mechanisms, and using the transport mechanism of a protein to represent the case of the entire family is not suitable. The transport activity of these proteins is regulated by extracellular and/or cytosolic pH, with different structural bases for H+ transfer among these seven types of proteins. Notably, intra-family members possess distinct pH regulatory characterization and underlying residues for H+ transfer. This review is anticipated to facilitate the understanding of the molecular basis for cytosolic pH homeostasis. Despite this progress, the strategy of their cooperation for cytosolic pH homeostasis needs further investigation.

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.

Identification and characterisation of monovalent cation/proton antiporters (CPAs) in Phyllostachys edulis and the functional analysis of PheNHX2 in Arabidopsis thaliana

Plant monovalent cation/proton antiporters (CPAs), types of transmembrane transporters, play important roles in resistance to salt stress. In this study, 37 CPA genes from moso bamboo (Phyllostachys edulis) were identified and characterised. The expression profiles of 10 CPA1 genes (PheNHXs) of moso bamboo were detected by qRT-PCR, which showed that they were specifically expressed in six tissues. In addition, the expression of 10 PheNHXs in leaves and roots changed significantly under 150/200 mM NaCl and 100 μM ABA treatments. In particular, the expression of PheNHX2 in leaves and roots was significantly upregulated under NaCl treatment, thus, we cloned PheNHX2 and analysed its function. Subcellular localisation analysis showed that PheNHX2 was located on the vacuolar membrane. Overexpression of PheNHX2 reduced seed germination and root growth of Arabidopsis thaliana under salt stress, as well as severely affecting cellular Na⁺ and K⁺ content, which in turn reduced the salt tolerance of transgenic Arabidopsis. Measurements of physiological indicators, including chlorophyll content, malondialdehyde content, peroxidase and catalase enzyme activities and relative electrical conductivity, all supported this conclusion. Under salt stress, PheNHX2 also inhibited the expression of some stress-related and ion transport-related genes in transgenic Arabidopsis. Overall, these results indicate that overexpression of PheNHX2 reduces the salt tolerance of transgenic Arabidopsis. This investigation establishes a foundation for subsequent functional studies of moso bamboo CPA genes, and it provides a deeper understanding of PheNHX2 regulation in relation to the salt tolerance of moso bamboo.

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.

Genome-wide identification, characterization, and expression analysis of the monovalent cation-proton antiporter superfamily in maize, and functional analysis of its role in salt tolerance

Na⁺, K⁺ and pH homeostasis are important for plant life and they are controlled by the monovalent cation proton antiporter (CPA) superfamily. The roles of ZmCPAs in salt tolerance are not fully elucidated. In this study, we identified 35 ZmCPAs comprising 13 Na⁺/H⁺ exchangers (ZmNHXs), 16 cation/H⁺ exchanger (ZmCHXs), and 6 K⁺ efflux antiporters (ZmKEAs). All ZmCPAs have transmembrane domains and most of them were localized to plasma membrane or tonoplast. ZmCHXs were specifically highly expressed in anthers, while ZmNHXs and ZmKEAs showed high expression in various tissues. ZmNHX5 and ZmKEA2 were up-regulated in maize seedlings under both NaCl and KCl stresses. Yeast complementation experiments revealed the roles of ZmNHX5, ZmKEA2 in NaCl tolerance. Analysis of the maize mutants further validated the salt tolerance functions of ZmNHX5 and ZmKEA2. Our study highlights comprehensive information of ZmCPAs and provides new gene targets for salt tolerance maize breeding.

The soybean plasma membrane-localized cation/H+ exchanger GmCHX20a plays a negative role under salt stress

Cation/H+ -exchanger (CHX) perform diverse functions in plants, including being a part of the protective mechanisms to cope with salt stress. GmCHX1 has been previously identified as the causal gene in a major salt-tolerance quantitative trait locus (QTL) in soybean, but little is known about another close paralog, GmCHX20a, found in the same QTL. In this study, GmCHX20a was characterized along with GmCHX1. The expression patterns of the two genes and the direction of Na+ flux directed by overexpression of these two transporters are different, suggesting that they are functionally distinct. The ectopic expression of GmCHX20a led to an increase in salt sensitivity and osmotic tolerance, which was consistent with its role in increasing Na+ uptake into the root. Although this seems counter-intuitive, it may in fact be part of the mechanism by which soybean could counter act the effects of osmotic stress, which is commonly manifested in the initial stage of salinity stress. On the other hand, GmCHX1 from salt-tolerant soybean was shown to protect plants via Na+ exclusion under salt stress. Taken together these results suggest that GmCHX20a and GmCHX1 might work complementally through a concerted effort to address both osmotic stress and ionic stress as a result of elevated salinity.

Phylogenetic Diversity and Physiological Roles of Plant Monovalent Cation/H+ Antiporters

The processes of plant nutrition, stress tolerance, plant growth, and development are strongly dependent on transport of mineral nutrients across cellular membranes. Plant membrane transporters are key components of these processes. Among various membrane transport proteins, the monovalent cation proton antiporter (CPA) superfamily mediates a broad range of physiological and developmental processes such as ion and pH homeostasis, development of reproductive organs, chloroplast operation, and plant adaptation to drought and salt stresses. CPA family includes plasma membrane-bound Na⁺/H⁺ exchanger (NhaP) and intracellular Na⁺/H⁺ exchanger NHE (NHX), K⁺ efflux antiporter (KEA), and cation/H⁺ exchanger (CHX) family proteins. In this review, we have completed the phylogenetic inventory of CPA transporters and undertaken a comprehensive evolutionary analysis of their development. Compared with previous studies, we have significantly extended the range of plant species, including green and red algae and Acrogymnospermae into phylogenetic analysis. Our data suggest that the multiplication and complexation of CPA isoforms during evolution is related to land colonisation by higher plants and associated with an increase of different tissue types and development of reproductive organs. The new data extended the number of clades for all groups of CPAs, including those for NhaP/SOS, NHE/NHX, KEA, and CHX. We also critically evaluate the latest findings on the biological role, physiological functions and regulation of CPA transporters in relation to their structure and phylogenetic position. In addition, the role of CPA members in plant tolerance to various abiotic stresses is summarized, and the future priority directions for CPA studies in plants are discussed.

Diverse Physiological Functions of Cation Proton Antiporters across Bacteria and Plant Cells

Membrane intrinsic transport systems play an important role in maintaining ion and pH homeostasis and forming the proton motive force in the cytoplasm and cell organelles. In most organisms, cation/proton antiporters (CPAs) mediate the exchange of K⁺, Na⁺ and Ca²⁺ for H⁺ across the membrane in response to a variety of environmental stimuli. The tertiary structure of the ion selective filter and the regulatory domains of Escherichia coli CPAs have been determined and a molecular mechanism of cation exchange has been proposed. Due to symbiogenesis, CPAs localized in mitochondria and chloroplasts of eukaryotic cells resemble prokaryotic CPAs. CPAs primarily contribute to keeping cytoplasmic Na⁺ concentrations low and controlling pH, which promotes the detoxification of electrophiles and formation of proton motive force across the membrane. CPAs in cyanobacteria and chloroplasts are regulators of photosynthesis and are essential for adaptation to high light or osmotic stress. CPAs in organellar membranes and in the plasma membrane also participate in various intracellular signal transduction pathways. This review discusses recent advances in our understanding of the role of CPAs in cyanobacteria and plant cells.

Genetic Loci Associated with Early Salt Stress Responses of Roots

Salinity is a devastating abiotic stress accounting for major crop losses yearly. Plant roots can strikingly grow away from high-salt patches. This response is termed halotropism and occurs through auxin redistribution in roots in response to a salt gradient. Here, a natural variation screen for the early and NaCl-specific halotropic response of 333 Arabidopsis accessions revealed quantitative differences in the first 24 h. These data were successfully used to identify genetic components associated with the response through Genome-Wide Association Study (GWAS). Follow-up characterization of knockout mutants in Col-0 background confirmed the role of transcription factor WRKY25, cation-proton exchanger CHX13, and a gene of unknown function DOB1 (Double Bending 1) in halotropism. In chx13 and dob1 mutants, ion accumulation and shoot biomass under salt stress were also affected. Thus, our GWAS has identified genetic components contributing to main root halotropism that provide insight into the genetic architecture underlying plant salt responses.

Salt-Enhanced Reproductive Development of Suaeda salsa L. Coincided With Ion Transporter Gene Upregulation in Flowers and Increased Pollen K+ Content

Halophytes are adapted to saline environments and demonstrate optimal reproductive growth under high salinity. To gain insight into the salt tolerance mechanism and effects of salinity in the halophyte Suaeda salsa, the number of flowers and seeds, seed size, anther development, ion content, and flower transcript profiles, as well as the relative expression levels of genes involved in ion transport, were analyzed in S. salsa plants treated with 0 or 200 mM NaCl. The seed size, flower number, seed number per leaf axil, and anther fertility were all significantly increased by 200 mM NaCl treatment. The Na+ and Cl- contents in the leaves, stems, and pollen of NaCl-treated plants were all markedly higher, and the K+ content in the leaves and stems was significantly lower, than those in untreated control plants. By contrast, the K+ content in pollen grains did not decrease, but rather increased, upon NaCl treatment. Genes related to Na+, K+ and, Cl- transport, such as SOS1, KEA, AKT1, NHX1, and CHX, showed increased expression in the flowers of NaCl-treated plants. These results suggest that ionic homeostasis in reproductive organs, especially in pollen grains under salt-treated conditions, involves increased expression of ion transport-related genes.

Identification, characterization and expression profiling of cation-proton antiporter superfamily in Triticum aestivum L. and functional analysis of TaNHX4-B

The monovalent cation proton antiporter (CPA) superfamily comprises Na⁺/H⁺ exchanger (NHX), K⁺ efflux antiporter (KEA), and cation/H⁺ exchanger (CHX) family proteins, which play vital functions in plants. A total of 107 TaCPA proteins were identified in Triticum aestivum, and phylogenetically classified into 35 TaNHX, 24 TaKEA and 48 TaCHX proteins. These families had representatives derived from all three sub-genomes. TaKEA genes consisted of higher number of exons, followed by TaNHXs and TaCHXs. The occurrence of about 10 transmembrane regions and higher composition of helices and coils support their membrane-bound and hydrophobic nature. Diverse expression in various tissues and modulated expression under stress conditions suggested their role in development and in response to stress. Co-expression analyses revealed their complex interaction networks. Expression of TaNHX4-B.1 and TaNHX4-B.4 facilitated differential abiotic stress tolerance to Escherichia coli. Our study provides comprehensive information about CPA genes, which would be useful in their future functional characterization.

Golgi-localized cation/proton exchangers regulate ionic homeostasis and skotomorphogenesis in Arabidopsis

Multiple transporters and channels mediate cation transport across the plasma membrane and tonoplast to regulate ionic homeostasis in plant cells. However, much less is known about the molecular function of transporters that facilitate cation transport in other organelles such as Golgi. We report here that Arabidopsis KEA4, KEA5, and KEA6, members of cation/proton antiporters-2 (CPA2) superfamily were colocalized with the known Golgi marker, SYP32-mCherry. Although single kea4,5,6 mutants showed similar phenotype as the wild type under various conditions, kea4/5/6 triple mutants showed hypersensitivity to low pH, high K⁺ , and high Na⁺ and displayed growth defects in darkness, suggesting that these three KEA-type transporters function redundantly in controlling etiolated seedling growth and ion homeostasis. Detailed analysis indicated that the kea4/5/6 triple mutant exhibited cell wall biosynthesis defect during the rapid etiolated seedling growth and under high K⁺ /Na⁺ condition. The cell wall-derived pectin homogalacturonan (GalA)₃ partially suppressed the growth defects and ionic toxicity in the kea4/5/6 triple mutants when grown in the dark but not in the light conditions. Together, these data support the hypothesis that the Golgi-localized KEAs play key roles in the maintenance of ionic and pH homeostasis, thereby facilitating Golgi function in cell wall biosynthesis during rapid etiolated seedling growth and in coping with high K⁺ /Na⁺ stress.

Lateral Transport of Organic and Inorganic Solutes

Organic (e.g., sugars and amino acids) and inorganic (e.g., K⁺, Na⁺, PO₄2-, and SO₄2-) solutes are transported long-distance throughout plants. Lateral movement of these compounds between the xylem and the phloem, and vice versa, has also been reported in several plant species since the 1930s, and is believed to be important in the overall resource allocation. Studies of Arabidopsis thaliana have provided us with a better knowledge of the anatomical framework in which the lateral transport takes place, and have highlighted the role of specialized vascular and perivascular cells as an interface for solute exchanges. Important breakthroughs have also been made, mainly in Arabidopsis, in identifying some of the proteins involved in the cell-to-cell translocation of solutes, most notably a range of plasma membrane transporters that act in different cell types. Finally, in the future, state-of-art imaging techniques should help to better characterize the lateral transport of these compounds on a cellular level. This review brings the lateral transport of sugars and inorganic solutes back into focus and highlights its importance in terms of our overall understanding of plant resource allocation.

Regulation of potassium transport and signaling in plants

As an essential macronutrient, potassium (K⁺) plays crucial roles in diverse physiological processes during plant growth and development. The K⁺ concentration in soils is relatively low and fluctuating. Plants are able to perceive external K⁺ changes and generate chemical and physical signals in plant cells. The signals can be transducted across the plasma membrane and into the cytosol, and eventually regulates the downstream targets, particularly K⁺ channels and transporters. As a result, K⁺ homeostasis in plant cells is modulated, which facilitates plant adaptation to K⁺ deficient conditions. This minireview focuses on the latest research progress in the diverse functions of K⁺ channels and transporters as well as their regulatory mechanisms in plant response to low-K⁺ stress.

GsCHX19.3, a member of cation/H+ exchanger superfamily from wild soybean contributes to high salinity and carbonate alkaline tolerance

Cation/H+ exchangers (CHX) are characterized to be involved in plant growth, development and stress responses. Although soybean genome sequencing has been completed, the CHX family hasn't yet been systematically analyzed, especially in wild soybean. Here, through Hidden Markov Model search against Glycine soja proteome, 34 GsCHXs were identified and phylogenetically clustered into five groups. Members within each group showed high conservation in motif architecture. Interestingly, according to our previous RNA-seq data, only Group IVa members exhibited highly induced expression under carbonate alkaline stress. Among them, GsCHX19.3 displayed the greatest up-regulation in response to carbonate alkaline stress, which was further confirmed by quantitative real-time PCR analysis. We also observed the ubiquitous expression of GsCHX19.3 in different tissues and its localization on plasma membrane. Moreover, we found that GsCHX19.3 expression in AXT4K, a yeast mutant lacking four ion transporters conferred resistance to low K+ at alkali pH, as well as carbonate stress. Consistently, in Arabidopsis, GsCHX19.3 overexpression increased plant tolerance both to high salt and carbonate alkaline stresses. Furthermore, we also confirmed that GsCHX19.3 transgenic lines showed lower Na+ concentration but higher K+/Na+ values under salt-alkaline stress. Taken together, our findings indicated that GsCHX19.3 contributed to high salinity and carbonate alkaline tolerance.

Diverse functions of multidrug and toxin extrusion (MATE) transporters in citric acid efflux and metal homeostasis in Medicago truncatula

The multidrug and toxin extrusion (MATE) transporter family comprises 70 members in the Medicago truncatula genome, and they play seemingly important, yet mostly uncharacterized, physiological functions. Here, we employed bioinformatics and molecular genetics to identify and characterize MATE transporters involved in citric acid export, Al³⁺ tolerance and Fe translocation. MtMATE69 is a citric acid transporter induced by Fe-deficiency. Overexpression of MtMATE69 in hairy roots altered Fe homeostasis and hormone levels under Fe-deficient or Fe-oversupplied conditions. MtMATE66 is a plasma membrane citric acid transporter primarily expressed in root epidermal cells. The mtmate66 mutant had less root growth than the wild type under Al³⁺ stress, and seedlings were chlorotic under Fe-deficient conditions. Overexpression of MtMATE66 rendered hairy roots more tolerant to Al³⁺ toxicity. MtMATE55 is involved in seedling development and iron homeostasis, as well as hormone signaling. The mtmate55 mutant had delayed development and chlorotic leaves in mature plants. Both knock-out and overexpression mutants of MtMATE55 showed altered Fe accumulation and abnormal hormone levels compared with the wild type. We demonstrate that the zinc-finger transcription factor MtSTOP is essentially required for MtMATE66 expression and plant resistance to H⁺ and Al³⁺ toxicity. The proper expression of two previously characterized MATE flavonoid transporters MtMATE1 and MtMATE2 also depends on several transcription factors. This study reveals not only functional diversity of MATE transporters and regulatory mechanisms in legumes against H⁺ and Al³⁺ stresses, but also casts light on their role in metal nutrition and hormone signaling under various stresses.

Local adaptation (mostly) remains local: reassessing environmental associations of climate-related candidate SNPs in Arabidopsis halleri

Numerous landscape genomic studies have identified single-nucleotide polymorphisms (SNPs) and genes potentially involved in local adaptation. Rarely, it has been explicitly evaluated whether these environmental associations also hold true beyond the populations studied. We tested whether putatively adaptive SNPs in Arabidopsis halleri (Brassicaceae), characterized in a previous study investigating local adaptation to a highly heterogeneous environment, show the same environmental associations in an independent, geographically enlarged set of 18 populations. We analysed new SNP data of 444 plants with the same methodology (partial Mantel tests, PMTs) as in the original study and additionally with a latent factor mixed model (LFMM) approach. Of the 74 candidate SNPs, 41% (PMTs) and 51% (LFMM) were associated with environmental factors in the independent data set. However, only 5% (PMTs) and 15% (LFMM) of the associations showed the same environment-allele relationships as in the original study. In total, we found 11 genes (31%) containing the same association in the original and independent data set. These can be considered prime candidate genes for environmental adaptation at a broader geographical scale. Our results suggest that selection pressures in highly heterogeneous alpine environments vary locally and signatures of selection are likely to be population-specific. Thus, genotype-by-environment interactions underlying adaptation are more heterogeneous and complex than is often assumed, which might represent a problem when testing for adaptation at specific loci.

OsCHX14 is Involved in the K+ Homeostasis in Rice (Oryza sativa) Flowers

Previously we showed in the osjar1 mutants that the lodicule senescence which controls the closing of rice flowers was delayed. This resulted in florets staying open longer when compared with the wild type. The gene OsJAR1 is silenced in osjar1 mutants and is a key member of the jasmonic acid (JA) signaling pathway. We found that K concentrations in lodicules and flowers of osjar1-2 were significantly elevated compared with the wild type, indicating that K⁺ homeostasis may play a role in regulating the closure of rice flowers. The cation/H⁺ exchanger (CHX) family from rice was screened for potential K⁺ transporters involved as many members of this family in Arabidopsis were exclusively or preferentially expressed in flowers. Expression profiling confirmed that among 17 CHX genes in rice, OsCHX14 was the only member that showed an expression polymorphism, not only in osjar1 mutants but also in RNAi (RNA interference) lines of OsCOI1, another key member of the JA signaling pathway. This suggests that the expression of OsCHX14 is regulated by the JA signaling pathway. Green fluorescent protein (GFP)-tagged OsCHX14 protein was preferentially localized to the endoplasmic reticulum. Promoter-β-glucuronidase (GUS) analysis of transgenic rice revealed that OsCHX14 is mainly expressed in lodicules and the region close by throughout the flowering process. Characterization in yeast and Xenopus laevis oocytes verified that OsCHX14 is able to transport K⁺, Rb⁺ and Cs⁺ in vivo. Our data suggest that OsCHX14 may play an important role in K⁺ homeostasis during flowering in rice.

Ca(2+) -regulated and diurnal rhythm-regulated Na(+) /Ca(2+) exchanger AtNCL affects flowering time and auxin signalling in Arabidopsis

Calcium (Ca(2+) ) is vital for plant growth, development, hormone response and adaptation to environmental stresses, yet the mechanisms regulating plant cytosolic Ca(2+) homeostasis are not fully understood. Here, we characterize an Arabidopsis Ca(2+) -regulated Na(+) /Ca(2+) exchanger AtNCL that regulates Ca(2+) and multiple physiological processes. AtNCL was localized to the tonoplast in yeast and plant cells. AtNCL appeared to mediate sodium (Na(+) ) vacuolar sequestration and meanwhile Ca(2+) release. The EF-hand domains within AtNCL regulated Ca(2+) binding and transport of Ca(2+) and Na(+) . Plants with diminished AtNCL expression were more tolerant to high CaCl2 but more sensitive to both NaCl and auxin; heightened expression of AtNCL rendered plants more sensitive to CaCl2 but tolerant to NaCl. AtNCL expression appeared to be regulated by the diurnal rhythm and suppressed by auxin. DR5::GUS expression and root responses to auxin were altered in AtNCL mutants. The auxin-induced suppression of AtNCL was attenuated in SLR/IAA14 and ARF6/8 mutants. The mutants with altered AtNCL expression also altered flowering time and FT and CO expression; FT may mediate AtNCL-regulated flowering time change. Therefore, AtNCL is a vacuolar Ca(2+) -regulated Na(+) /Ca(2+) exchanger that regulates auxin responses and flowering time.