Potassium (K+) gradients serve as a mobile energy source in plant vascular tissues

Communication between the Plasma Membrane and Tonoplast Is an Emergent Property of Ion Transport

Serial membranes that operate through a common compartment, the cytosol for transport at the plasma membrane and tonoplast, are intrinsically connected and communicate through this pool of solutes.

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.

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.

Synchronization of developmental, molecular and metabolic aspects of source-sink interactions

Plants have evolved a multitude of strategies to adjust their growth according to external and internal signals. Interconnected metabolic and phytohormonal signalling networks allow adaption to changing environmental and developmental conditions and ensure the survival of species in fluctuating environments. In agricultural ecosystems, many of these adaptive responses are not required or may even limit crop yield, as they prevent plants from realizing their fullest potential. By lifting source and sink activities to their maximum, massive yield increases can be foreseen, potentially closing the future yield gap resulting from an increasing world population and the transition to a carbon-neutral economy. To do so, a better understanding of the interplay between metabolic and developmental processes is required. In the past, these processes have been tackled independently from each other, but coordinated efforts are required to understand the fine mechanics of source-sink relations and thus optimize crop yield. Here, we describe approaches to design high-yielding crop plants utilizing strategies derived from current metabolic concepts and our understanding of the molecular processes determining sink development.

Calcium-Regulated Phosphorylation Systems Controlling Uptake and Balance of Plant Nutrients

Essential elements taken up from the soil and distributed throughout the whole plant play diverse roles in different tissues. Cations and anions contribute to maintenance of intracellular osmolarity and the formation of membrane potential, while nitrate, ammonium, and sulfate are incorporated into amino acids and other organic compounds. In contrast to these ion species, calcium concentrations are usually kept low in the cytosol and calcium displays unique behavior as a cytosolic signaling molecule. Various environmental stresses stimulate increases in the cytosolic calcium concentration, leading to activation of calcium-regulated protein kinases and downstream signaling pathways. In this review, we summarize the stress responsive regulation of nutrient uptake and balancing by two types of calcium-regulated phosphorylation systems: CPK and CBL-CIPK. CPK is a family of protein kinases activated by calcium. CBL is a group of calcium sensor proteins that interact with CIPK kinases, which phosphorylate their downstream targets. In Arabidopsis, quite a few ion transport systems are regulated by CPKs or CBL-CIPK complexes, including channels/transporters that mediate transport of potassium (KAT1, KAT2, GORK, AKT1, AKT2, HAK5, SPIK), sodium (SOS1), ammonium (AMT1;1, AMT1;2), nitrate and chloride (SLAC1, SLAH2, SLAH3, NRT1.1, NRT2.4, NRT2.5), and proton (AHA2, V-ATPase). CPKs and CBL-CIPKs also play a role in C/N nutrient response and in acquisition of magnesium and iron. This functional regulation by calcium-dependent phosphorylation systems ensures the growth of plants and enables them to acquire tolerance against various environmental stresses. Calcium serves as the key factor for the regulation of membrane transport systems.

Ecological stoichiometry of the epiphyte community in a subtropical forest canopy

Epiphytes in tree canopies make a considerable contribution to the species diversity, aboveground biomass, and nutrient pools in forest ecosystems. However, the nutrient status of epiphytes and their possible adaptations to nutrient deficiencies in the forest canopy remain unclear. Therefore, we analyzed the stoichiometry of five macroelements (C, N, P, K, and Ca) in four taxonomic groups (lichens, bryophytes, ferns, and spermatophytes) to investigate this issue in a subtropical montane moist evergreen broad-leaved forest in Southwest China. We found that the interspecific variations in element concentrations and mass ratios were generally greater than the intraspecific variations. And there were significant stoichiometric differences among functional groups. Allometric relationships between N and P across the epiphyte community indicated that P might be in greater demand than N with an increase in nutrients. Although canopy nutrients were deficient, most epiphytes could still maintain high N and P concentrations and low N:P ratios. Moreover, ferns and spermatophytes allocated more limited nutrients to leaves than to stems and roots. To alleviate frequent drought stress in the forest canopy, vascular epiphytes maintained several times higher K concentrations in their leaves than in the tissues of lichens and bryophytes. Our results suggest that epiphytes may have evolved specific nutrient characteristics and adaptations, so that they can distribute in heterogeneous canopy habitats and maintain the stability of nutrient metabolism.

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.

Genome-Wide Association Study of 13 Traits in Maize Seedlings under Low Phosphorus Stress

Low P stress is a global issue for grain production. Significant phenotypic differences were detected among 13 traits in 356 maize lines under P-sufficient and P-deficient conditions. Significant single nucleotide polymorphisms (SNPs) and low-P stress-responsive genes were identified for 13 maize root traits based on a genome-wide association study. Hap5, harboring 12 favorable SNPs, could enhance strong root systems and P absorption under low-P stress. Phosphorus is an essential macronutrient required for normal plant growth and development. Determining the genetic basis of root traits will enhance our understanding of maize's (Zea mays L.) tolerance to low-P stress. Here, we identified significant phenotypic differences for 13 traits in maize seedlings subjected to P-sufficient and P-deficient conditions. Six extremely sensitive and seven low-P stress tolerant inbreds were selected from 356 inbred lines of maize. No significant differences were observed between temperate and tropical-subtropical groups with respect to trait ratios associated with the adaptation to low-P stress. The broad-sense heritability of these traits ranged from relatively moderate (0.59) to high (0.90). Through genome-wide association mapping with 541,575 informative single nucleotide polymorphisms (SNPs), 551, 1140 and 1157 significant SNPs were detected for the 13 traits in 2012, 2016 and both years combined, respectively, along with 23 shared candidate genes, seven of which overlapped with reported quantitative trait loci and genes for low-P stress. Five haplotypes located in candidate gene GRMZM2G009544 were identified; among these, Hap5, harboring 12 favorable SNP alleles, showed significantly greater values for the root traits studied than the other four haplotypes under both experimental conditions. The candidate genes and favorable haplotypes and alleles identified here provide promising resources for genetic studies and molecular breeding for improving tolerance to abiotic stress in maize.

Adaptation of Plants to Salt Stress: Characterization of Na+ and K+ Transporters and Role of CBL Gene Family in Regulating Salt Stress Response

Salinity is one of the most serious factors limiting the productivity of agricultural crops, with adverse effects on germination, plant vigor, and crop yield. This salinity may be natural or induced by agricultural activities such as irrigation or the use of certain types of fertilizer. The most detrimental effect of salinity stress is the accumulation of Na+ and Cl− ions in tissues of plants exposed to soils with high NaCl concentrations. The entry of both Na+ and Cl− into the cells causes severe ion imbalance, and excess uptake might cause significant physiological disorder(s). High Na+ concentration inhibits the uptake of K+, which is an element for plant growth and development that results in lower productivity and may even lead to death. The genetic analyses revealed K+ and Na+ transport systems such as SOS1, which belong to the CBL gene family and play a key role in the transport of Na+ from the roots to the aerial parts in the Arabidopsis plant. In this review, we mainly discuss the roles of alkaline cations K+ and Na+, Ion homeostasis-transport determinants, and their regulation. Moreover, we tried to give a synthetic overview of soil salinity, its effects on plants, and tolerance mechanisms to withstand stress.

Potassium in Root Growth and Development

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

Harnessing the new emerging imaging technologies to uncover the role of Ca2+ signalling in plant nutrient homeostasis

Increasing crop yields by using ecofriendly practices is of high priority to tackle problems regarding food security and malnutrition worldwide. A sustainable crop production requires a limited use of fertilizer and the employment of plant varieties with improved ability to acquire nutrients from soil. To reach these goals, the scientific community aims to understand plant nutrients homeostasis by deciphering the nutrient sensing and signalling mechanisms of plants. Several lines of evidence about the involvement of Ca²⁺ as the signal of an impaired nutrient availability have been reported. Ca²⁺ signalling is a tightly regulated process that requires specific protein toolkits to perceive external stimuli and to induce the specific responses in the plant needed to survive. Here, we summarize both older and recent findings concerning the involvement of Ca²⁺ signalling in the homeostasis of nutrients. In this review, we present new emerging technologies, based on the use of genetically encoded Ca²⁺ sensors and advanced microscopy, which offer the chance to perform in planta analyses of Ca²⁺ dynamics at cellular resolution. The harnessing of these technologies with different genetic backgrounds and subjected to different nutritional stresses will provide important insights to the still little-known mechanisms of nutrient sensing in plants.

Roles of membrane transporters: connecting the dots from sequence to phenotype

BACKGROUND

Plant membrane transporters are involved in diverse cellular processes underpinning plant physiology, such as nutrient acquisition, hormone movement, resource allocation, exclusion or sequestration of various solutes from cells and tissues, and environmental and developmental signalling. A comprehensive characterization of transporter function is therefore key to understanding and improving plant performance.

SCOPE AND CONCLUSIONS

In this review, we focus on the complexities involved in characterizing transporter function and the impact that this has on current genomic annotations. Specific examples are provided that demonstrate why sequence homology alone cannot be relied upon to annotate and classify transporter function, and to show how even single amino acid residue variations can influence transporter activity and specificity. Misleading nomenclature of transporters is often a source of confusion in transporter characterization, especially for people new to or outside the field. Here, to aid researchers dealing with interpretation of large data sets that include transporter proteins, we provide examples of transporters that have been assigned names that misrepresent their cellular functions. Finally, we discuss the challenges in connecting transporter function at the molecular level with physiological data, and propose a solution through the creation of new databases. Further fundamental in-depth research on specific transport (and other) proteins is still required; without it, significant deficiencies in large-scale data sets and systems biology approaches will persist. Reliable characterization of transporter function requires integration of data at multiple levels, from amino acid residue sequence annotation to more in-depth biochemical, structural and physiological studies.

System Analysis of MIRNAs in Maize Internode Elongation

MicroRNAs (miRNAs), the post-transcriptional gene regulators, are known to play an important role in plant development. The identification of differentially expressed miRNAs could better help us understand the post-transcriptional regulation that occurs during maize internode elongation. Accordingly, we compared the expression of MIRNAs between fixed internode and elongation internode samples and classified six differentially expressed MIRNAs as internode elongation-responsive miRNAs including zma-MIR160c, zma-MIR164b, zma-MIR164c, zma-MIR168a, zma-MIR396f, and zma-MIR398b, which target mRNAs supported by transcriptome sequencing. Functional enrichment analysis for predictive target genes showed that these miRNAs were involved in the development of internode elongation by regulating the genes respond to hormone signaling. To further reveal how miRNA affects internode elongation by affecting target genes, the miRNA–mRNA–PPI (protein and protein interaction) network was constructed to summarize the interaction of miRNAs and these target genes. Our results indicate that miRNAs regulate internode elongation in maize by targeting genes related to cell expansion, cell wall synthesis, transcription, and regulatory factors.

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

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

Voltage-dependent gating of SV channel TPC1 confers vacuole excitability

In contrast to the plasma membrane, the vacuole membrane has not yet been associated with electrical excitation of plants. Here, we show that mesophyll vacuoles from Arabidopsis sense and control the membrane potential essentially via the K⁺-permeable TPC1 and TPK channels. Electrical stimuli elicit transient depolarization of the vacuole membrane that can last for seconds. Electrical excitability is suppressed by increased vacuolar Ca²⁺ levels. In comparison to wild type, vacuoles from the fou2 mutant, harboring TPC1 channels insensitive to luminal Ca²⁺, can be excited fully by even weak electrical stimuli. The TPC1-loss-of-function mutant tpc1-2 does not respond to electrical stimulation at all, and the loss of TPK1/TPK3-mediated K⁺ transport affects the duration of TPC1-dependent membrane depolarization. In combination with mathematical modeling, these results show that the vacuolar K⁺-conducting TPC1 and TPK1/TPK3 channels act in concert to provide for Ca²⁺- and voltage-induced electrical excitability to the central organelle of plant cells.

Nutrient exchange in arbuscular mycorrhizal symbiosis from a thermodynamic point of view

To obtain insights into the dynamics of nutrient exchange in arbuscular mycorrhizal (AM) symbiosis, we modelled mathematically the two-membrane system at the plant-fungus interface and simulated its dynamics. In computational cell biology experiments, the full range of nutrient transport pathways was tested for their ability to exchange phosphorus (P)/carbon (C)/nitrogen (N) sources. As a result, we obtained a thermodynamically justified, independent and comprehensive model of the dynamics of the nutrient exchange at the plant-fungus contact zone. The predicted optimal transporter network coincides with the transporter set independently confirmed in wet-laboratory experiments previously, indicating that all essential transporter types have been discovered. The thermodynamic analyses suggest that phosphate is released from the fungus via proton-coupled phosphate transporters rather than anion channels. Optimal transport pathways, such as cation channels or proton-coupled symporters, shuttle nutrients together with a positive charge across the membranes. Only in exceptional cases does electroneutral transport via diffusion facilitators appear to be plausible. The thermodynamic models presented here can be generalized and adapted to other forms of mycorrhiza and open the door for future studies combining wet-laboratory experiments with computational simulations to obtain a deeper understanding of the investigated phenomena.

Characterization of the grapevine Shaker K+ channel VvK3.1 supports its function in massive potassium fluxes necessary for berry potassium loading and pulvinus-actuated leaf movements

In grapevine, climate changes lead to increased berry potassium (K⁺ ) contents that result in must with low acidity. Consequently, wines are becoming 'flat' to the taste, with poor organoleptic properties and low potential aging, resulting in significant economic loss. Precise investigation into the molecular determinants controlling berry K⁺ accumulation during its development are only now emerging. Here, we report functional characterization by electrophysiology of a new grapevine Shaker-type K⁺ channel, VvK3.1. The analysis of VvK3.1 expression patterns was performed by qPCR and in situ hybridization. We found that VvK3.1 belongs to the AKT2 channel phylogenetic branch and is a weakly rectifying channel, mediating both inward and outward K⁺ currents. We showed that VvK3.1 is highly expressed in the phloem and in a unique structure located at the two ends of the petiole, identified as a pulvinus. From the onset of fruit ripening, all data support the role of the VvK3.1 channel in the massive K⁺ fluxes from the phloem cell cytosol to the berry apoplast during berry K⁺ loading. Moreover, the high amount of VvK3.1 transcripts detected in the pulvinus strongly suggests a role for this Shaker in the swelling and shrinking of motor cells involved in paraheliotropic leaf movements.

Regulation of K+ Nutrition in Plants

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

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 K+ Nutrition in Plants

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

High- and Low-Affinity Transport in Plants From a Thermodynamic Point of View

Plants have to absorb essential nutrients from the soil and do this via specialized membrane proteins. Groundbreaking studies about half a century ago led to the identification of different nutrient uptake systems in plant roots. Historically, they have been characterized as "high-affinity" uptake systems acting at low nutrient concentrations or as "low-affinity" uptake systems acting at higher concentrations. Later this "high- and low-affinity" concept was extended by "dual-affinity" transporters. Here, in this study it is now demonstrated that the affinity concept based on enzyme kinetics does not have proper scientific grounds. Different computational cell biology scenarios show that affinity analyses, as they are often performed in wet-lab experiments, are not suited for reliably characterizing transporter proteins. The new insights provided here clearly indicate that the classification of transporters on the basis of enzyme kinetics is largely misleading, thermodynamically in no way justified and obsolete.

Sensing of Abiotic Stress and Ionic Stress Responses in Plants

Plants need to cope with complex environments throughout their life cycle. Abiotic stresses, including drought, cold, salt and heat, can cause a reduction in plant growth and loss of crop yield. Plants sensing stress signals and adapting to adverse environments are fundamental biological problems. We review the stress sensors in stress sensing and the responses, and then discuss ionic stress signaling and the responses. During ionic stress, the calcineurin B-like proteins (CBL) and CBL-interacting protein kinases (CBL-CIPK) complex is identified as a primary element of the calcium sensor for perceiving environmental signals. The CBL-CIPK system shows specificity and variety in its response to different stresses. Obtaining a deeper understanding of stress signaling and the responses will mitigate or solve crop yield crises in extreme environments with fast-growing populations.

Changes in the Elemental and Metabolite Profile of Wheat Phloem Sap during Grain Filling Indicate a Dynamic between Plant Maturity and Time of Day

The long distance transport of Fe and Zn in the phloem sap of wheat (Triticum aestivum L.) is the key route for seed supply, due to wheat having a xylem discontinuity. To date, our knowledge is limited on Fe and Zn homeostasis in the phloem sap during the reproductive and grain filling stages. With the use of aphid stylectomy to collect samples of phloem sap, we explored maturity and morning versus afternoon (within-day) changes in nutrient and metabolite profiles. Phloem exudate was collected from a wheat breeding line, SAMNYT16, at three times during the grain filling period and at both midday and mid-afternoon. There were significant changes in the concentration of Mg, K, Fe and Zn during the course of grain loading and there were also significant within-day differences for Fe and K concentrations in the phloem exudate during the early phases of grain development. We found that, for K and Fe, there was an increase of 1.1- and 1.4-fold, respectively, for samples taken prior to midday to those from mid-afternoon. There was also a significant decrease in K, Fe and Zn phloem sap concentration of 1.5-, 1.4- and 1.1-fold, respectively, from the start of peak grain loading to the end of grain loading. Of the 79 metabolites detected within samples of phloem exudate, 43 had significant maturity differences and 38 had significant within-day variability. Glutamine was found to increase by 3.3⁻5.9-fold from midday to mid-afternoon and citric acid was found to decrease by 1.6-fold from the start of grain loading to the end of grain loading. These two metabolites are of interest as they can complex metal ions and may play a role in long distance transport of metal ions. The work presented here gives further insight into the complex composition of the phloem sap and variability that can occur during the day and also with increasing maturity.

OsHAK1 controls the vegetative growth and panicle fertility of rice by its effect on potassium-mediated sugar metabolism

Plant growth and reproduction are both energy-requiring processes; the necessary energy is supplied by the products of photosynthesis. Both the vegetative growth and reproductive success of rice are compromised by the absence of a functional copy of the gene OsHAK1. Here, a comparison between wild type rice and OsHAK1 knockout mutants not only confirmed the known detrimental effect of the absence of OsHAK1 on root growth, pollen viability and fertility, but also showed that sucrose phosphate synthase activity was lowered, and the sucrose content of the leaves was markedly increased, due to a partial block on the up-loading of sucrose into the phloem. The impaired allocation of sugar to the roots and spikelets caused by the knocking out of OsHAK1 was accompanied by a down-regulation in the leaf sheaths and panicle axes of genes encoding sucrose transporters (SUT genes), which are active in the phloem, as well as in the roots and spikelets of those encoding monosaccharide transporters (MST genes), which transport hexose sugars across the plant plasma membrane. The activity of sucrose synthase, acid invertase and neutral invertase in the roots of mutant plants assayed at the tillering stage, and in their spikelets, assayed during grain-filling, was significantly lower than in the equivalent organs of wild type plants. As a result, the supply of total soluble sugar, glucose and fructose to sink organs was reduced, consistent with the effect of the mutation on root growth and panicle fertility. Compared to wild type plants, the mutants accumulated less potassium (K) throughout the plant. The conclusion was that the failure to fully supply the demand of the mutant's sink organs for assimilate was responsible for its compromised phenotype, and that the deficiency in K uptake induced by the loss of OsHAK1 functionality was responsible for the disruption of sugar metabolism.

Identification of Two Auxin-Regulated Potassium Transporters Involved in Seed Maturation

The seed is the most important plant reproductive unit responsible for the evolutionary success of flowering plants. Aside from its essential function in the sexual reproduction of plants, the seed also represents the most economically important agricultural product worldwide, providing energy, nutrients, and raw materials for human nutrition, livestock feed, and countless manufactured goods. Hence, improvements in seed quality or size are highly valuable, due to their economic potential in agriculture. Recently, the importance of indolic compounds in regulating these traits has been reported for Arabidopsis thaliana. The transcriptional and physiological mechanisms involved, however, remain largely undisclosed. Potassium transporters have been suggested as possible mediators of embryo cell size, controlling turgor pressure during seed maturation. In addition, it has been demonstrated that the expression of K⁺ transporters is effectively regulated by auxin. Here, we provide evidence for the identification of two Arabidopsis K⁺ transporters, HAK/KT12 (At1g60160) and KUP4 (At4g23640), that are likely to be implicated in determining seed size during seed maturation and, at the same time, show a differential regulation by indole-3-acetic acid and indole-3-acetamide.

Functioning of potassium and magnesium in photosynthesis, photosynthate translocation and photoprotection

Potassium (K) and magnesium (Mg) are mineral nutrients that are required in large quantities by plants. Both elements critically contribute to the process of photosynthesis and the subsequent long-distance transport of photoassimilates. If K or Mg is not present in sufficient quantities in photosynthetic tissues, complex interactions of anatomical, physiological and biochemical responses result in a reduction of photosynthetic carbon assimilation. As a consequence, excessive production of reactive oxygen species causes photo-oxidation of the photosynthetic apparatus and causes an up-regulation of photoprotective mechanisms. In this article, we review the functioning of K and Mg in processes directly or indirectly associated with photosynthesis. Focus is given to chloroplast ultrastructure, light-dependent and -independent reactions of photosynthesis and the diffusion of CO2  - a major substrate for photosynthesis - into chloroplasts. We further emphasize their contribution to phloem-loading and long-distance transport of photoassimilates and to the photoprotection of the photosynthetic apparatus.

The Role of Potassium Channels in Arabidopsis thaliana Long Distance Electrical Signalling: AKT2 Modulates Tissue Excitability While GORK Shapes Action Potentials

Fast responses to an external threat depend on the rapid transmission of signals through a plant. Action potentials (APs) are proposed as such signals. Plant APs share similarities with their animal counterparts; they are proposed to depend on the activity of voltage-gated ion channels. Nonetheless, despite their demonstrated role in (a)biotic stress responses, the identities of the associated voltage-gated channels and transporters remain undefined in higher plants. By demonstrating the role of two potassium-selective channels in Arabidopsis thaliana in AP generation and shaping, we show that the plant AP does depend on similar Kv-like transport systems to those of the animal signal. We demonstrate that the outward-rectifying potassium-selective channel GORK limits the AP amplitude and duration, while the weakly-rectifying channel AKT2 affects membrane excitability. By computational modelling of plant APs, we reveal that the GORK activity not only determines the length of an AP but also the steepness of its rise and the maximal amplitude. Thus, outward-rectifying potassium channels contribute to both the repolarisation phase and the initial depolarisation phase of the signal. Additionally, from modelling considerations we provide indications that plant APs might be accompanied by potassium waves, which prime the excitability of the green cable.

Extensive Genetic Diversity is Present within North American Switchgrass Germplasm

Switchgrass ( is a perennial native North American grass present in two ecotypes: upland, found primarily in the northern range of switchgrass habitats, and lowland, found largely in the southern reaches of switchgrass habitats. Previous studies focused on a diversity panel of primarily northern switchgrass, so to expand our knowledge of genetic diversity in a broader set of North American switchgrass, exome capture sequence data were generated for 632 additional, primarily lowland individuals. In total, over 37 million single nucleotide polymorphisms (SNPs) were identified and a set of 1.9 million high-confidence SNPs were obtained from 1169 individuals from 140 populations (67 upland, 65 lowland, 8 admixed) were used in downstream analyses of genetic diversity and population structure. Seven separate population groups were identified with moderate genetic differentiation [mean fixation index (Fst) estimate of 0.06] between the lowland and the upland populations. Ecotype-specific and population-specific SNPs were identified for use in germplasm evaluations. Relative to rice ( L.), maize ( L.), soybean [ (L.) Merr.], and Gaertn., analyses of nucleotide diversity revealed a high degree of genetic diversity (0.0135) across all individuals, consistent with the outcrossing mode of reproduction and the polyploidy of switchgrass. This study supports the hypothesis that repeated glaciation events, ploidy barriers, and restricted gene flow caused by flowering time differences have resulted in distinct gene pools across ecotypes and geographic regions. These data provide a resource to associate alleles with traits of interest for forage, restoration, and biofuel feedstock efforts in switchgrass.

The Integration of Electrical Signals Originating in the Root of Vascular Plants

Plants have developed different signaling systems allowing for the integration of environmental cues to coordinate molecular processes associated to both early development and the physiology of the adult plant. Research on systemic signaling in plants has traditionally focused on the role of phytohormones as long-distance signaling molecules, and more recently the importance of peptides and miRNAs in building up this communication process has also been described. However, it is well-known that plants have the ability to generate different types of long-range electrical signals in response to different stimuli such as light, temperature variations, wounding, salt stress, or gravitropic stimulation. Presently, it is unclear whether short or long-distance electrical communication in plants is linked to nutrient uptake. This review deals with aspects of sensory input in plant roots and the propagation of discrete signals to the plant body. We discuss the physiological role of electrical signaling in nutrient uptake and how nutrient variations may become an electrical signal propagating along the plant.

Plant ion channels and transporters in herbivory-induced signalling

In contrast to many biotic stresses that plants face, feeding by herbivores produces unique mechanical and chemical signatures. Plants have evolved effective systems to recognise these mechanical stimuli and chemical elicitors at the plasma membrane (PM), where this recognition generates ion fluxes, including an influx of Ca2+ that elicits cellular Ca2+ signalling, production of reactive oxygen species (ROS), and variation in transmembrane potential. These signalling events also function in propagation of long-distance signals (Ca2+ waves, ROS waves, and electrical signals), which contribute to rapid, systemic induction of defence responses. Recent studies have identified several candidate channels or transporters that likely produce these ion fluxes at the PM. Here, we describe the important roles of these channels/transporters in transduction or transmission of herbivory-induced early signalling events, long-distance signals, and jasmonic acid and green leaf volatile signalling in plants.

The potassium battery: a mobile energy source for transport processes in plant vascular tissues

Contents 1049 I. 1049 II. 1050 III. 1050 IV. 1050 V. 1051 VI. 1051 VII. 1052 VIII. 1052 1053 References 1053 SUMMARY: Plant roots absorb potassium ions from the soil and transport them in the xylem via the transpiration stream to the shoots. There, in source tissues where sufficient chemical energy (ATP) is available, K⁺ is loaded into the phloem and then transported with the phloem stream to other parts of the plant; in part, transport is also back to the roots. This, at first sight, futile cycling of K⁺ has been uncovered to be part of a sophisticated mechanism that (1) enables the shoot to communicate its nutrient demand to the root, (2) contributes to the K⁺ nutrition of transport phloem tissues and (3) transports energy stored in the K⁺ gradient between phloem cytosol and the apoplast. This potassium battery can be tapped by opening AKT2-like potassium channels and then enables the ATP-independent energization of other transport processes, such as the reloading of sucrose. Insights into these mechanisms have only been possible by combining wet-lab and dry-lab experiments by means of computational cell biology modeling and simulations.

Interaction between calcium and potassium modulates elongation rate in cotton fiber cells

Calcium (Ca2+) is necessary for fiber cell development in cotton (Gossypium hirsutum), both as a cell wall structural component and for environmental signaling responses. It is also known that potassium (K+) plays a critical role in cotton fiber cell elongation. However, it is unclear whether Ca2+ integrates its activities with K+ to regulate fiber elongation. Here, we report the novel discovery that Ca2+ deficiency, when integrated with K+ signaling, promotes fiber elongation. Using inductively coupled plasma-mass spectrometry (ICP-MS), we determined dynamic profiles of the ionome in ovules and fibers at different developmental stages, and found that a high accumulation of macro-elements, but not Ca2+, was associated with longer fibers. Using an in vitro ovule culture system, we found that under Ca2+-deficient conditions, sufficient K+ (52 mM) rapidly induced ovule and fiber browning, while reduced K+ (2 or 27 mM) not only suppressed tissue browning but also altered fiber elongation. Reduced K+ also enhanced reactive oxygen species scavenging ability and maintained abscisic acid and jasmonic acid levels, which in turn compensated for Ca2+ deficiency. Ca2+ deficiency combined with reduced K+ (0 mM Ca2+ and 27 mM K+) produced longer fibers in cultured ovules, due to cell wall loosening by phytosulfokine (PSK), expansin (EXP), and xyloglucan endotransglycosylase/hydrolase (XTH), and an increase of the K+ content of fiber cells. Using transgenic cotton, we showed that the CBL-INTERACTING PROTEIN KINASE 6 (GhCIPK6) gene mediates the uptake of K+ under Ca2+-deficient conditions. This study establishes a new link between Ca2+, K+, and fiber elongation.

Age-dependent regulation of ERF-VII transcription factor activity in Arabidopsis thaliana

The Group VII Ethylene Responsive Factors (ERFs-VII) RAP2.2 and RAP2.12 have been mainly characterized with regard to their contribution as activators of fermentation in plants. However, transcriptional changes measured in conditions that stabilize these transcription factors exceed the mere activation of this biochemical pathway, implying additional roles performed by the ERF-VIIs in other processes. We evaluated gene expression in transgenic Arabidopsis lines expressing a stabilized form of RAP2.12, or hampered in ERF-VII activity, and identified genes affected by this transcriptional regulator and its homologs, including some involved in oxidative stress response, which are not universally induced under anaerobic conditions. The contribution of the ERF-VIIs in regulating this set of genes in response to chemically induced or submergence-stimulated mitochondria malfunctioning was found to depend on the plant developmental stage. A similar age-dependent mechanism also restrained ERF-VII activity upon the core-hypoxic genes, independently of the N-end rule pathway, which is accounted for the control of the anaerobic response. To conclude, this study shed new light on a dual role of ERF-VII proteins under submergence: as positive regulators of the hypoxic response and as repressors of oxidative-stress related genes, depending on the developmental stage at which plants are challenged by stress conditions.

Potassium in the Grape (Vitis vinifera L.) Berry: Transport and Function

K+ is the most abundant cation in the grape berry. Here we focus on the most recent information in the long distance transport and partitioning of K+ within the grapevine and postulate on the potential role of K+ in berry sugar accumulation, berry water relations, cellular growth, disease resistance, abiotic stress tolerance and mitigating senescence. By integrating information from several different plant systems we have been able to generate new hypotheses on the integral functions of this predominant cation and to improve our understanding of how these functions contribute to grape berry growth and ripening. Valuable contributions to the study of K+ in membrane stabilization, turgor maintenance and phloem transport have allowed us to propose a mechanistic model for the role of this cation in grape berry development.

The fungal UmSrt1 and maize ZmSUT1 sucrose transporters battle for plant sugar resources

The biotrophic fungus Ustilago maydis causes corn smut disease, inducing tumor formation in its host Zea mays. Upon infection, the fungal hyphae invaginate the plasma membrane of infected maize cells, establishing an interface where pathogen and host are separated only by their plasma membranes. At this interface the fungal and maize sucrose transporters, UmSrt1 and ZmSUT1, compete for extracellular sucrose in the corn smut/maize pathosystem. Here we biophysically characterized ZmSUT1 and UmSrt1 in Xenopus oocytes with respect to their voltage-, pH- and substrate-dependence and determined affinities toward protons and sucrose. In contrast to ZmSUT1, UmSrt1 has a high affinity for sucrose and is relatively pH- and voltage-independent. Using these quantitative parameters, we developed a mathematical model to simulate the competition for extracellular sucrose at the contact zone between the fungus and the host plant. This approach revealed that UmSrt1 exploits the apoplastic sucrose resource, which forces the plant transporter into a sucrose export mode providing the fungus with sugar from the phloem. Importantly, the high sucrose concentration in the phloem appeared disadvantageous for the ZmSUT1, preventing sucrose recovery from the apoplastic space in the fungus/plant interface.

Transport and homeostasis of potassium and phosphate: limiting factors for sustainable crop production

Potassium (K) and phosphate (Pi) are both macronutrients essential for plant growth and crop production, but the unrenewable resources of phosphorus rock and potash have become limiting factors for food security. One critical measure to help solve this problem is to improve nutrient use efficiency (NUE) in plants by understanding and engineering genetic networks for ion uptake, translocation, and storage. Plants have evolved multiple systems to adapt to various nutrient conditions for growth and production. Within the NUE networks, transport proteins and their regulators are the primary players for maintaining nutrient homeostasis and could be utilized to engineer high NUE traits in crop plants. A large number of publications have detailed K+ and Pi transport proteins in plants over the past three decades. Meanwhile, the discovery and validation of their regulatory mechanisms are fast-track topics for research. Here, we provide an overview of K+ and Pi transport proteins and their regulatory mechanisms, which participate in the uptake, translocation, storage, and recycling of these nutrients in plants.

Potassium deficiency affects the carbon-nitrogen balance in cotton leaves

Potassium (K) plays important roles in the metabolism of carbon (C) and nitrogen (N), but studies of K deficiency affecting C-N balance are lacking. This study explored the influence of K deficiency on C-N interaction in cotton leaves by conducting a field experiment with cotton cultivar DP0912 under two K rates (K0: 0 kg K₂O ha^-1 and K67: 67 kg K₂O ha^-1) and a controlled environment experiment with K-deficient solution (K1: 0 mM K⁺) and K-sufficient solution (K2: 6 mM K⁺). The results showed that leaf K content, leaf number, leaf area, boll number, reproductive dry weight and total dry weight were significant lower under K deficiency (K0 or K1). Lower total chlorophyll content and Chl a/b ratio, and decreased Pn along with lower Gs and higher Ci were measured under K deficiency, suggesting that the decrease in Pn was resulted from non-stomatal limitation. Leaf glucose, fructose, sucrose and starch contents were higher under K deficiency, because lower sucrose export was detected in phloem. Although leaf nitrate and ammonium contents significantly decreased, free amino acid content was increased by 40-63% under K deficiency, since lower amino acid export was also measured in phloem. K deficiency also induced lower soluble protein content in leaves. Leaf ATP level was significantly increased under K deficiency, indicating ATP utilization was lower, so that less energy was supplied to C and N metabolism. The ratio of soluble sugar to free amino acid and the C/N ratio markedly increased under K deficiency, and one reason was that the phloem export reduced more prominent for sucrose (54.6-78.0%) than amino acid (36.7-85.4%) under K deficiency. In addition, lower phosphoenolpyruvate carboxylase activity limited malate and citrate biosynthesis under K deficiency, causing a decrease of C flux into the amino acids, which was not beneficial for maintaining C-N balance. Sucrose phosphate synthase and nitrate reductase activities were lower under K deficiency, which would limit sucrose biosynthesis and nitrate assimilation. This was another factor altering soluble sugar to free amino acid ratio and C/N ratio in the K-deficient leaves.

The Membrane Transport System of the Guard Cell and Its Integration for Stomatal Dynamics

Stomatal guard cells are widely recognized as the premier plant cell model for membrane transport, signaling, and homeostasis. This recognition is rooted in half a century of research into ion transport across the plasma and vacuolar membranes of guard cells that drive stomatal movements and the signaling mechanisms that regulate them. Stomatal guard cells surround pores in the epidermis of plant leaves, controlling the aperture of the pore to balance CO₂ entry into the leaf for photosynthesis with water loss via transpiration. The position of guard cells in the epidermis is ideally suited for cellular and subcellular research, and their sensitivity to endogenous signals and environmental stimuli makes them a primary target for physiological studies. Stomata underpin the challenges of water availability and crop production that are expected to unfold over the next 20 to 30 years. A quantitative understanding of how ion transport is integrated and controlled is key to meeting these challenges and to engineering guard cells for improved water use efficiency and agricultural yields.

Transcriptome Analysis of Differentially Expressed Genes Induced by Low and High Potassium Levels Provides Insight into Fruit Sugar Metabolism of Pear

Potassium (K) deficiency is a common abiotic stress that can inhibit the growth of fruit and thus reduce crop yields. Little research has been conducted on pear transcriptional changes under low and high K conditions. Here, we performed an experiment with 7-year-old pot-grown "Huangguan" pear trees treated with low, Control or high K levels (0, 0.4, or 0.8 g·K2O/kg soil, respectively) during fruit enlargement and mature stages. We identified 36,444 transcripts from leaves and fruit using transcriptome sequencing technology. From 105 days after full blooming (DAB) to 129 DAB, the number of differentially expressed genes (DEGs) in leaves and fruit in response to low K increased, while in response to high K, the number of DEGs in leaves and fruit decreased. We selected 17 of these DEGs for qRT-PCR analysis to confirm the RNA sequencing results. Based on GO enrichment and KEGG pathway analysis, we found that low-K treatment significantly reduced K nutrient and carbohydrate metabolism of the leaves and fruit compared with the Control treatment. During the fruit development stages, AKT1 (gene39320) played an important role on K+ transport of the leaves and fruit response to K stress. At maturity, sucrose and acid metabolic pathways were inhibited by low K. The up-regulation of the expression of three SDH and two S6PDH genes involved in sorbitol metabolism was induced by low K, promoting the fructose accumulation. Simultaneously, higher expression was found for genes encoding amylase under low K, promoting the decomposition of the starch and leading the glucose accumulation. High K could enhance leaf photosynthesis, and improve the distribution of the nutrient and carbohydrate from leaf to fruit. Sugar components of the leaves and fruit under low K were regulated by the expression of genes encoding 8 types of hormone signals and reactive oxygen species (ROS). Our data revealed the gene expression patterns of leaves and fruit in response to different K levels during the middle and late stages of fruit development as well as the molecular mechanism of improvement of fruit sugar levels by K and provided a scientific basis for improving fruit quality with supplemental K fertilizers.

The receptor-like pseudokinase MRH1 interacts with the voltage-gated potassium channel AKT2

The potassium channel AKT2 plays important roles in phloem loading and unloading. It can operate as inward-rectifying channel that allows H+-ATPase-energized K+ uptake. Moreover, through reversible post-translational modifications it can also function as an open, K+-selective channel, which taps a 'potassium battery', providing additional energy for transmembrane transport processes. Knowledge about proteins involved in the regulation of the operational mode of AKT2 is very limited. Here, we employed a large-scale yeast two-hybrid screen in combination with fluorescence tagging and null-allele mutant phenotype analysis and identified the plasma membrane localized receptor-like kinase MRH1/MDIS2 (AT4G18640) as interaction partner of AKT2. The phenotype of the mrh1-1 knockout plant mirrors that of akt2 knockout plants in energy limiting conditions. Electrophysiological analyses showed that MRH1/MDIS2 failed to exert any functional regulation on AKT2. Using structural protein modeling approaches, we instead gathered evidence that the putative kinase domain of MRH1/MDIS2 lacks essential sites that are indispensable for a functional kinase suggesting that MRH1/MDIS2 is a pseudokinase. We propose that MRH1/MDIS2 and AKT2 are likely parts of a bigger protein complex. MRH1 might help to recruit other, so far unknown partners, which post-translationally regulate AKT2. Additionally, MRH1 might be involved in the recognition of chemical signals.

Roles and Transport of Sodium and Potassium in Plants

The two alkali cations Na(+) and K(+) have similar relative abundances in the earth crust but display very different distributions in the biosphere. In all living organisms, K(+) is the major inorganic cation in the cytoplasm, where its concentration (ca. 0.1 M) is usually several times higher than that of Na(+). Accumulation of Na(+) at high concentrations in the cytoplasm results in deleterious effects on cell metabolism, e.g., on photosynthetic activity in plants. Thus, Na(+) is compartmentalized outside the cytoplasm. In plants, it can be accumulated at high concentrations in vacuoles, where it is used as osmoticum. Na(+) is not an essential element in most plants, except in some halophytes. On the other hand, it can be a beneficial element, by replacing K(+) as vacuolar osmoticum for instance. In contrast, K(+) is an essential element. It is involved in electrical neutralization of inorganic and organic anions and macromolecules, pH homeostasis, control of membrane electrical potential, and the regulation of cell osmotic pressure. Through the latter function in plants, it plays a role in turgor-driven cell and organ movements. It is also involved in the activation of enzymes, protein synthesis, cell metabolism, and photosynthesis. Thus, plant growth requires large quantities of K(+) ions that are taken up by roots from the soil solution, and then distributed throughout the plant. The availability of K(+) ions in the soil solution, slowly released by soil particles and clays, is often limiting for optimal growth in most natural ecosystems. In contrast, due to natural salinity or irrigation with poor quality water, detrimental Na(+) concentrations, toxic for all crop species, are present in many soils, representing 6 % to 10 % of the earth's land area. Three families of ion channels (Shaker, TPK/KCO, and TPC) and 3 families of transporters (HAK, HKT, and CPA) have been identified so far as contributing to K(+) and Na(+) transport across the plasmalemma and internal membranes, with high or low ionic selectivity. In the model plant Arabidopsis thaliana, these families gather at least 70 members. Coordination of the activities of these systems, at the cell and whole plant levels, ensures plant K(+) nutrition, use of Na(+) as a beneficial element, and adaptation to saline conditions.

SLAH3-type anion channel expressed in poplar secretory epithelia operates in calcium kinase CPK-autonomous manner

Extrafloral nectaries secrete a sweet sugar cocktail that lures predator insects for protection from foraging herbivores. Apart from sugars and amino acids, the nectar contains the anions chloride and nitrate. Recent studies with Populus have identified a type of nectary covered by apical bipolar epidermal cells, reminiscent of the secretory brush border epithelium in animals. Border epithelia operate transepithelial anion transport, which is required for membrane potential and/or osmotic adjustment of the secretory cells. In search of anion transporters expressed in extrafloral nectaries, we identified PttSLAH3 (Populus tremula × Populus tremuloides SLAC1 Homologue3), an anion channel of the SLAC/SLAH family. When expressed in Xenopus oocytes, PttSLAH3 displayed the features of a voltage-dependent anion channel, permeable to both nitrate and chloride. In contrast to the Arabidopsis SLAC/SLAH family members, the poplar isoform PttSLAH3 is independent of phosphorylation activation by protein kinases. To understand the basis for the autonomous activity of the poplar SLAH3, we generated and expressed chimera between kinase-independent PttSLAH3 and kinase-dependent Arabidopsis AtSLAH3. We identified the N-terminal tail and, to a lesser extent, the C-terminal tail as responsible for PttSLAH3 kinase-(in)dependent action. This feature of PttSLAH3 may provide the secretory cell with a channel probably controlling long-term nectar secretion.

NPKS uptake, sensing, and signaling and miRNAs in plant nutrient stress

Sessile nature of higher plants consequently makes it highly adaptable for nutrient absorption and acquisition from soil. Plants require 17 essential elements for their growth and development which include 14 minerals (macronutrients: N, P, K, Mg, Ca, S; micronutrients: Cl, Fe, B, Mn, Zn, Cu, Ni, Mo) and 3 non-mineral (C, H, O) elements. The roots of higher plants must acquire these macronutrients and micronutrients from rhizosphere and further allocate to other plant parts for completing their life cycle. Plants evolved an intricate series of signaling and sensing cascades to maintain nutrient homeostasis and to cope with nutrient stress/availability. The specific receptors for nutrients in root, root system architecture, and internal signaling pathways help to develop plasticity in response to the nutrient starvation. Nitrogen (N), phosphorus (P), potassium (K), and sulfur (S) are essential for various metabolic processes, and their deficiency negatively effects the plant growth and yield. Genes coding for transporters and receptors for nutrients as well as some small non-coding RNAs have been implicated in nutrient uptake and signaling. This review summarizes the N, P, K, and S uptake, sensing and signaling events in nutrient stress condition especially in model plant Arabidopsis thaliana and involvement of microRNAs in nutrient deficiency. This article also provides a framework of uptake, sensing, signaling and to highlight the microRNA as an emerging major players in nutrient stress condition. Nutrient-plant-miRNA cross talk may help plant to cope up nutrient stress, and understanding their precise mechanism(s) will be necessary to develop high yielding smart crop with low nutrient input.

Electrical Wiring and Long-Distance Plant Communication

Electrical signalling over long distances is an efficient way of achieving cell-to-cell communication in living organisms. In plants, the phloem can be considered as a 'green cable' that allows the transmission of action potentials (APs) induced by stimuli such as wounding and cold. Measuring phloem potential changes and separating them from secondary responses of surrounding tissues can be achieved using living aphids as bioelectrodes. Two glutamate receptor-like genes (GLR3.3 and 3.6) were identified as being involved in the propagation of electrical activity from the damaged to undamaged leaves. However, phloem APs are initiated and propagated independently of these glutamate receptors. Here, we propose new screening approaches to obtain further information on the components required for electrical signalling in phloem cables.

Asparagine Metabolic Pathways in Arabidopsis

Inorganic nitrogen in the form of ammonium is assimilated into asparagine via multiple steps involving glutamine synthetase (GS), glutamate synthase (GOGAT), aspartate aminotransferase (AspAT) and asparagine synthetase (AS) in Arabidopsis. The asparagine amide group is liberated by the reaction catalyzed by asparaginase (ASPG) and also the amino group of asparagine is released by asparagine aminotransferase (AsnAT) for use in the biosynthesis of amino acids. Asparagine plays a primary role in nitrogen recycling, storage and transport in developing and germinating seeds, as well as in vegetative and senescence organs. A small multigene family encodes isoenzymes of each step of asparagine metabolism in Arabidopsis, except for asparagine aminotransferase encoded by a single gene. The aim of this study is to highlight the structure of the genes and encoded enzyme proteins involved in asparagine metabolic pathways; the regulation and role of different isogenes; and kinetic and physiological properties of encoded enzymes in different tissues and developmental stages.

Constant change: dynamic regulation of membrane transport by calcium signalling networks keeps plants in tune with their environment

Despite substantial variation and irregularities in their environment, plants must conform to spatiotemporal demands on the molecular composition of their cytosol. Cell membranes are the major interface between organisms and their environment and the basis for controlling the contents and intracellular organization of the cell. Membrane transport proteins (MTPs) govern the flow of molecules across membranes, and their activities are closely monitored and regulated by cell signalling networks. By continuously adjusting MTP activities, plants can mitigate the effects of environmental perturbations, but effective implementation of this strategy is reliant on precise coordination among transport systems that reside in distinct cell types and membranes. Here, we examine the role of calcium signalling in the coordination of membrane transport, with an emphasis on potassium transport. Potassium is an exceptionally abundant and mobile ion in plants, and plant potassium transport has been intensively studied for decades. Classic and recent studies have underscored the importance of calcium in plant environmental responses and membrane transport regulation. In reviewing recent advances in our understanding of the coding and decoding of calcium signals, we highlight established and emerging roles of calcium signalling in coordinating membrane transport among multiple subcellular locations and distinct transport systems in plants, drawing examples from the CBL-CIPK signalling network. By synthesizing classical studies and recent findings, we aim to provide timely insights on the role of calcium signalling networks in the modulation of membrane transport and its importance in plant environmental responses.

In situ 13CO2 pulse labelling of field-grown eucalypt trees revealed the effects of potassium nutrition and throughfall exclusion on phloem transport of photosynthetic carbon

Potassium (K) is an important limiting factor of tree growth, but little is known of the effects of K supply on the long-distance transport of photosynthetic carbon (C) in the phloem and of the interaction between K fertilization and drought. We pulse-labelled 2-year-old Eucalyptus grandis L. trees grown in a field trial combining K fertilization (+K and -K) and throughfall exclusion (+W and -W), and we estimated the velocity of C transfer by comparing time lags between the uptake of (13)CO2 and its recovery in trunk CO2 efflux recorded at different heights. We also analysed the dynamics of the labelled photosynthates recovered in the foliage and in the phloem sap (inner bark extract). The mean residence time of labelled C in the foliage was short (21-31 h). The time series of (13)C in excess in the foliage was affected by the level of fertilization, whereas the effect of throughfall exclusion was not significant. The velocity of C transfer in the trunk (0.20-0.82 m h(-1)) was twice as high in +K trees than in -K trees, with no significant effect of throughfall exclusion except for one +K -W tree labelled in the middle of the drought season that was exposed to a more pronounced water stress (midday leaf water potential of -2.2 MPa). Our results suggest that besides reductions in photosynthetic C supply and in C demand by sink organs, the lower velocity under K deficiency is due to a lower cross-sectional area of the sieve tubes, whereas an increase in phloem sap viscosity is more likely limiting phloem transport under drought. In all treatments, 10 times less (13)C was recovered in inner bark extracts at the bottom of the trunk when compared with the base of the crown, suggesting that a large part of the labelled assimilates has been exported out of the phloem and replaced by unlabelled C. This supports the 'leakage-retrieval mechanism' that may play a role in maintaining the pressure gradient between source and sink organs required to sustain high velocity of phloem transport in tall trees.