A low K+ signal is required for functional high-affinity K+ uptake through HAK5 transporters

SlCIPK9 regulates pollen tube elongation in tomato plants via a K+-independent mechanism

Potassium (K+) is an essential macronutrient which contributes to osmotic- and turgor-related processes in plants. Calcineurin-B like Interacting Protein Kinases (CIPKs) play crucial roles in plants under low-K+ supply since they activate root K+ uptake transport systems such as AKT1 and AtHAK5. In Arabidopsis, AtCIPK9 is important for low-K+ tolerance since atcipk9 plants exhibited poor growth and leaf chlorosis when K+ was scarce. Part of these phenotypes could be ascribed to the activation of AtHAK5 by AtCIPK9. It has been reported that important differences exist between Arabidopsis and other plant species such as tomato with respect to the regulation of K+ uptake systems. Thus, our aim was to evaluate the contribution of SlCIPK9, the homologous protein of AtCIPK9 in tomato, to K+ nutrition. Unexpectedly, phenotyping experiments carried out with slcipk9 loss-of-function mutants revealed that SlCIPK9 did not play a clear role in tomato K+ homeostasis. By contrast, it was found that SlCIPK9 contributed to pollen tube elongation, but not to pollen germination, via a K+-independent mechanism. Therefore, our results highlight the remarkable differences that exist in Ca2+ signaling pathways between plant species and encourage the realization of more comparative studies as the one presented here.

A viral protein competitively bound to rice CIPK23 inhibits potassium absorption and facilitates virus systemic infection in rice

Potassium (K+) plays a crucial role as a macronutrient in the growth and development of plants. Studies have definitely determined the vital roles of K+ in response to pathogen invasion. Our previous investigations revealed that rice plants infected with rice grassy stunt virus (RGSV) displayed a reduction in K+ content, but the mechanism by which RGSV infection subverts K+ uptake remains unknown. In this study, we found that overexpression of RGSV P1, a specific viral protein encoded by viral RNA1, results in enhanced sensitivity to low K+ stress and exhibits a significantly lower rate of K+ influx compared to wild-type rice plants. Further investigation revealed that RGSV P1 interacts with OsCIPK23, an upstream regulator of Shaker K+ channel OsAKT1. Moreover, we found that the P1 protein recruits the OsCIPK23 to the Cajal bodies (CBs). In vivo assays demonstrated that the P1 protein competitively binds to OsCIPK23 with both OsCBL1 and OsAKT1. In the nucleus, the P1 protein enhances the binding of OsCIPK23 to OsCoilin, a homologue of the signature protein of CBs in Arabidopsis, and facilitates their trafficking through these CB structures. Genetic analysis indicates that mutant in oscipk23 suppresses RGSV systemic infection. Conversely, osakt1 mutants exhibited increased sensitivity to RGSV infection. These findings suggest that RGSV P1 hinders the absorption of K+ in rice plants by recruiting the OsCIPK23 to the CB structures. This process potentially promotes virus systemic infection but comes at the expense of inhibiting OsAKT1 activity.

Comparative physiological and transcriptome analysis between potassium-deficiency tolerant and sensitive sweetpotato genotypes in response to potassium-deficiency stress

BACKGROUND

Sweetpotato is a typical ''potassium (K+) favoring'' food crop, which root differentiation process needs a large supply of potassium fertilizer and determine the final root yield. To further understand the regulatory network of the response to low potassium stress, here we analyze physiological and biochemical characteristics, and investigated root transcriptional changes in two sweetpotato genotypes, namely, - K tolerant "Xu32" and - K susceptible"NZ1".

RESULT

We found Xu32 had the higher capability of K+ absorption than NZ1 with better growth performance, higher net photosynthetic rate and higher chlorophyll contents under low potassium stress, and identified 889 differentially expressed genes (DEGs) in Xu32, 634 DEGs in NZ1, 256 common DEGs in both Xu32 and NZ1. The Gene Ontology (GO) term in molecular function enrichment analysis revealed that the DEGs under low K+ stress are predominately involved in catalytic activity, binding, transporter activity and antioxidant activity. Moreover, the more numbers of identified DEGs in Xu32 than that in NZ1 responded to K+-deficiency belong to the process of photosynthesis, carbohydrate metabolism, ion transport, hormone signaling, stress-related and antioxidant system may result in different ability to K+-deficiency tolerance. The unique genes in Xu32 may make a great contribution to enhance low K+ tolerance, and provide useful information for the molecular regulation mechanism of K+-deficiency tolerance in sweetpotato.

CONCLUSIONS

The common and distinct expression pattern between the two sweetpotato genotypes illuminate a complex mechanism response to low potassium exist in sweetpotato. The study provides some candidate genes, which can be used in sweetpotato breeding program for improving low potassium stress tolerance.

Potassium transporter KUP9 regulates plant response to K+ deficiency and affects carbohydrate allocation in A.thaliana

Due to the essential roles of K+ in plants, its up to 10% share in plant dry matter, and its mostly low availability in soil, effective potassium management poses a significant challenge for the plant. To enable efficient uptake and allocation of K+, numerous transporters and channels have evolved. During the last two decades, efforts have been made to characterise these transport proteins in Arabidopsis thaliana using knock-out mutants. Several KT/HAK/KUP transporters have been assigned specific functions. In this work, we contribute to an understanding of the role of AtKUP9 in plant adaptation to low K+ availability. We found that in vitro, atkup9 has reduced lateral root growth under low-K conditions, and root apical meristem proliferation is reduced in lateral roots compared with the primary root. We also documented AtKUP9 expression in both roots and shoots and showed that AtKUP9 expression is modulated during plant ontogeny and as a result of K+ deprivation. Altered carbohydrate allocation was also documented in atkup9. Mutants exported more soluble saccharides from leaves under K+ rich conditions and, under K+ deficiency, atkup9 accumulated more soluble saccharides in the shoots. A possible role of AtKUP9 in these processes is discussed.

Can nutrients act as signals under abiotic stress?

Plant cells are in constant communication to coordinate development processes and environmental reactions. Under stressful conditions, such communication allows the plant cells to adjust their activities and development. This is due to intercellular signaling events which involve several components. In plant development, cell-to-cell signaling is ensured by mobile signals hormones, hydrogen peroxide (H2O2), nitric oxide (NO), or hydrogen sulfide (H2S), as well as several transcription factors and small RNAs. Mineral nutrients, including macro and microelements, are determinant factors for plant growth and development and are, currently, recognized as potential signal molecules. This review aims to highlight the role of nutrients, particularly calcium, potassium, magnesium, nitrogen, phosphorus, and iron as signaling components with special attention to the mechanism of response against stress conditions.

Ion Changes and Signaling under Salt Stress in Wheat and Other Important Crops

High concentrations of sodium (Na+), chloride (Cl-), calcium (Ca2+), and sulphate (SO42-) are frequently found in saline soils. Crop plants cannot successfully develop and produce because salt stress impairs the uptake of Ca2+, potassium (K+), and water into plant cells. Different intracellular and extracellular ionic concentrations change with salinity, including those of Ca2+, K+, and protons. These cations serve as stress signaling molecules in addition to being essential for ionic homeostasis and nutrition. Maintaining an appropriate K+:Na+ ratio is one crucial plant mechanism for salt tolerance, which is a complicated trait. Another important mechanism is the ability for fast extrusion of Na+ from the cytosol. Ca2+ is established as a ubiquitous secondary messenger, which transmits various stress signals into metabolic alterations that cause adaptive responses. When plants are under stress, the cytosolic-free Ca2+ concentration can rise to 10 times or more from its resting level of 50-100 nanomolar. Reactive oxygen species (ROS) are linked to the Ca2+ alterations and are produced by stress. Depending on the type, frequency, and intensity of the stress, the cytosolic Ca2+ signals oscillate, are transient, or persist for a longer period and exhibit specific "signatures". Both the influx and efflux of Ca2+ affect the length and amplitude of the signal. According to several reports, under stress Ca2+ alterations can occur not only in the cytoplasm of the cell but also in the cell walls, nucleus, and other cell organelles and the Ca2+ waves propagate through the whole plant. Here, we will focus on how wheat and other important crops absorb Na+, K+, and Cl- when plants are under salt stress, as well as how Ca2+, K+, and pH cause intracellular signaling and homeostasis. Similar mechanisms in the model plant Arabidopsis will also be considered. Knowledge of these processes is important for understanding how plants react to salinity stress and for the development of tolerant crops.

Xylem K+ loading modulates K+ and Cs+ absorption and distribution in Arabidopsis under K+-limited conditions

Potassium (K+) is an essential macronutrient for plant growth. The transcriptional regulation of K+ transporter genes is one of the key mechanisms by which plants respond to K+ deficiency. Among the HAK/KUP/KT transporter family, HAK5, a high-affinity K+ transporter, is essential for root K+ uptake under low external K+ conditions. HAK5 expression in the root is highly induced by low external K+ concentration. While the molecular mechanisms of HAK5 regulation have been extensively studied, it remains unclear how plants sense and coordinates K+ uptake and translocation in response to changing environmental conditions. Using skor mutants, which have a defect in root-to-shoot K+ translocation, we have been able to determine how the internal K+ status affects the expression of HAK5. In skor mutant roots, under K+ deficiency, HAK5 expression was lower than in wild-type although the K+ concentration in roots was not significantly different. These results reveal that HAK5 is not only regulated by external K+ conditions but it is also regulated by internal K+ levels, which is in agreement with recent findings. Additionally, HAK5 plays a major role in the uptake of Cs+ in roots. Therefore, studying Cs+ in roots and having more detailed information about its uptake and translocation in the plant would be valuable. Radioactive tracing experiments revealed not only a reduction in the uptake of 137Cs+ and 42K+in skor mutants compared to wild-type but also a different distribution of 137Cs+ and 42K+ in tissues. In order to gain insight into the translocation, accumulation, and repartitioning of both K+ and Cs+ in plants, long-term treatment and split root experiments were conducted with the stable isotopes 133Cs+ and 85Rb+. Finally, our findings show that the K+ distribution in plant tissues regulates root uptake of K+ and Cs+ similarly, depending on HAK5; however, the translocation and accumulation of the two elements are different.

Root PRR7 Improves the Accuracy of the Shoot Circadian Clock through Nutrient Transport

The circadian clock allows plants to anticipate and adapt to periodic environmental changes. Organ- and tissue-specific properties of the circadian clock and shoot-to-root circadian signaling have been reported. While this long-distance signaling is thought to coordinate physiological functions across tissues, little is known about the feedback regulation of the root clock on the shoot clock in the hierarchical circadian network. Here, we show that the plant circadian clock conveys circadian information between shoots and roots through sucrose and K+. We also demonstrate that K+ transport from roots suppresses the variance of period length in shoots and then improves the accuracy of the shoot circadian clock. Sucrose measurements and qPCR showed that root sucrose accumulation was regulated by the circadian clock. Furthermore, root circadian clock genes, including PSEUDO-RESPONSE REGULATOR7 (PRR7), were regulated by sucrose, suggesting the involvement of sucrose from the shoot in the regulation of root clock gene expression. Therefore, we performed time-series measurements of xylem sap and micrografting experiments using prr7 mutants and showed that root PRR7 regulates K+ transport and suppresses variance of period length in the shoot. Our modeling analysis supports the idea that root-to-shoot signaling contributes to the precision of the shoot circadian clock. We performed micrografting experiments that illustrated how root PRR7 plays key roles in maintaining the accuracy of shoot circadian rhythms. We thus present a novel directional signaling pathway for circadian information from roots to shoots and propose that plants modulate physiological events in a timely manner through various timekeeping mechanisms.

Overexpression of OsHAK5 potassium transporter enhances virus resistance in rice (Oryza sativa)

Intracellular potassium (K⁺ ) transported by plants under the action of a number of transport proteins is crucial for plant survival under distinct abiotic and biotic stresses. A correlation between K⁺ status and disease incidence has been found in many studies, but the roles of K⁺ in regulating disease resistance to viral diseases remain elusive. Here, we report that HIGH-AFFINITY K⁺ TRANSPORTER 5 (OsHAK5) regulates the infection of rice grassy stunt virus (RGSV), a negative-sense single-stranded bunyavirus, in rice (Oryza sativa). We found the K⁺ content in rice plants was significantly inhibited on RGSV infection. Meanwhile, a dramatic induction of OsHAK5 transcripts was observed in RGSV-infected rice plants and in rice plants with K⁺ deficiency. Genetic analysis indicated that disruption of OsHAK5 facilitated viral pathogenicity. In contrast, overexpression of OsHAK5 enhanced resistance to RGSV infection. Our analysis of reactive oxygen species (ROS) including H₂ O₂ and O^2- , by DAB and NBT staining, respectively, indicated that RGSV infection as well as OsHAK5 overexpression increased ROS accumulation in rice leaves. The accumulation of ROS is perhaps involved in the induction of host resistance against RGSV infection in OsHAK5 transgenic overexpression rice plants. Furthermore, RGSV-encoded P3 induced OsHAK5 promoter activity, suggesting that RGSV P3 is probably an elicitor for the induction of OsHAK5 transcripts during RGSV infection. These findings indicate the crucial role of OsHAK5 in host resistance to virus infection. Our results may be exploited in the future to increase crop yield as well as improve host resistance via genetic manipulations.

Narrowing down molecular targets for improving phosphorus-use efficiency in maize (Zea mays L.)

Conventional agricultural practices rely heavily on chemical fertilizers to boost production. Among the fertilizers, phosphatic fertilizers are copiously used to ameliorate low-phosphate availability in the soil. However, phosphorus-use efficiency (PUE) for major cereals, including maize, is less than 30%; resulting in more than half of the applied phosphate being lost to the environment. Rock phosphate reserves are finite and predicted to exhaust in near future with the current rate of consumption. Thus, the dependence of modern agriculture on phosphatic fertilizers poses major food security and sustainability challenges. Strategies to optimize and improve PUE, like genetic interventions to develop high PUE cultivars, could have a major impact in this area. Here, we present the current understanding and recent advances in the biological phenomenon of phosphate uptake, translocation, and adaptive responses of plants under phosphate deficiency, with special reference to maize. Maize is one of the most important cereal crops that is cultivated globally under diverse agro-climatic conditions. It is an industrial, feed and food crop with multifarious uses and a fast-rising global demand and consumption. The interesting aspects of diversity in the root system architecture traits, the interplay between signaling pathways contributing to PUE, and an in-depth discussion on promising candidate genes for improving PUE in maize are elaborated.

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.

The Halophyte Species Solanum chilense Dun. Maintains Its Reproduction despite Sodium Accumulation in Its Floral Organs

Salinity is a growing global concern that affects the yield of crop species, including tomato (Solanum lycopersicum). Its wild relative Solanum chilense was reported to have halophyte properties. We compared salt resistance of both species during the reproductive phase, with a special focus on sodium localization in the flowers. Plants were exposed to NaCl from the seedling stage. Salinity decreased the number of inflorescences in both species but the number of flowers per inflorescence and sepal length only in S. lycopersicum. External salt supply decreased the stamen length in S. chilense, and it was associated with a decrease in pollen production and an increase in pollen viability. Although the fruit set was not affected by salinity, fruit weight and size decreased in S. lycopersicum. Concentrations and localization of Na, K, Mg, and Ca differed in reproductive structures of both species. Inflorescences and fruits of S. chilense accumulated more Na than S. lycopersicum. Sodium was mainly located in male floral organs of S. chilense but in non-reproductive floral organs in S. lycopersicum. The expression of Na transporter genes differed in flowers of both species. Overall, our results indicated that S. chilense was more salt-resistant than S. lycopersicum during the reproductive phase and that differences could be partly related to dissimilarities in element distribution and transport in flowers.

K+ Nutrition Exchange in the Serendipita-Arabidopsis Symbiosis: Study of the Fungal K+ Transporters Involved

There is mounting evidence that the root-colonizing endosymbiotic fungus Serendipita indica improves plant growth. The beneficial effects have been observed when plants are growing in optimal conditions or under nutritionally deficient soils (e.g., phosphate poor soil) or exposed to stressful environmental conditions such as drought or salinity. However, until now its role in the nutrition of other plant essential macronutrient, such as K+, has not been fully clarified. Here, we study the role of the fungus in the K+ nutrition of Arabidopsis thaliana plants, during growth under K+ limiting conditions. As a first step, we studied the high-affinity K+ uptake of the plant and fungus when growing separately and in symbiosis. In the search for putative fungal actors involved in K+ nutrition, we also have cloned and functionally characterized the K+ transporters of S. indica SiHAK1, SiTRK1, SiTRK2, and SiTOK1, among which it has been shown that SiHAK1 is the main transporter involved in the K+ uptake in the high affinity range of concentrations. In addition, a gene expression study of these transporters and other candidates that could participate in the K+ homeostasis of the fungus has been carried out. The results indicated that, contrary to what happens with P nutrition, S. indica seems not to improve neither the growth nor the plant K+ reserves during K+ starvation. Instead, this nutritionally restrictive condition favored fungal colonization, suggesting that the fungus obtains the greatest benefit in K+ supply during symbiosis.

Tomato phytochromes B1 and B2 are part of the responses to the nutritional stress induced by NPK deficiency

Phytochromes are red-light photoreceptors that play an important role in regulating many responses of plants, including its nutritional control. Nutrient deficiency in plants has become a constraint for agricultural production; thus, we investigated the role of phytochromes B1 and B2 in the nutritional, physiological, and growth changes of the control genotype (WT) and both phyB1 and phyB2 tomato mutants (deficient in phyB1 and phyB2) under nutritional sufficiency and individual deficiency of N, P, and K. Under complete solution, the plants of phyB1 and phyB2 had a decreased N, P, and K accumulation compared with WT and consequently a reduced content of chlorophyll and carotenoids, and dry weight production. In the condition of N deficiency, phyB1 had decreased N absorption, pigments concentration, and plant dry weight, while increased oxidative stress of membranes (MDA content). Similarly, phyB2 also had reduced N absorption. The deficiency of phyB1 mitigated the effects of P deficiency as phyB1 mutant had improved nutritional and physiological responses, increasing plant dry weight production. In contrast, phyB2 reduced N accumulation, quantum efficiency of photosystem II (Fv/Fm), and the concentration of pigments, while it increased MDA. Under K deficiency, phyB1 displayed a reduced P accumulation, as well as the total concentration of chlorophylls and carotenoids and K use efficiency. An increased concentration of MDA was found in phyB2 plants, as well as a reduction in chlorophylls concentration and in the use efficiency of K. Together, these results indicate a new perspective on the control of phytochromes in the nutrition of tomato plants under nutritional stress.

Ca2+-CBL-CIPK: a modulator system for efficient nutrient acquisition

Calcium (Ca²⁺) is a universal second messenger essential for the growth and development of plants in normal and stress situations. In plants, the proteins, CBL (calcineurin B-like) and CIPK (CBL-interacting protein kinase), form one of the important Ca²⁺ decoding complexes to decipher Ca²⁺ signals elicited by environmental challenges. Multiple interactors distinguish CBL and CIPK protein family members to form a signaling network for regulated perception and transduction of environmental signals, e.g., signals generated under nutrient stress conditions. Conservation of equilibrium in response to varying soil nutrient status is an important aspect for plant vigor and yield. Signaling processes have been reported to observe nutrient fluctuations as a signal responsible for regulated nutrient transport adaptation. Recent studies have identified downstream targets of CBL-CIPK modules as ion channels or transporters and their association in signaling nutrient disposal including potassium, nitrate, ammonium, magnesium, zinc, boron, and iron. Ca²⁺-CBL-CIPK pathway modulates ion transporters/channels and hence maintains a homeostasis of several important plant nutrients in the cytosol and sub-cellular compartments. In this article, we summarize recent literature to discuss the role of the Ca²⁺-CBL-CIPK pathway in cellular osmoregulation and homeostasis on exposure to nutrient excess or deprived soils. This further establishes a link between taking up the nutrient in the roots and its distribution and homeostasis during the generation of signal for the development and survival of plants.

Transcriptomic and Biochemical Analysis Reveal Integrative Pathways Between Carbon and Nitrogen Metabolism in Guzmania monostachia (Bromeliaceae) Under Drought

Most epiphytes are found in low-nutrient environments with an intermittent water supply. To deal with water limitation, many bromeliads perform crassulacean acid metabolism (CAM), such as Guzmania monostachia, which shifts from C₃ to CAM and can recycle CO₂ from the respiration while stomata remain closed during daytime and nighttime (CAM-idling mode). Since the absorbing leaf trichomes can be in contact with organic (urea) and inorganic nutrients (NO₃ ^-, NH₄ ⁺) and the urea hydrolysis releases NH₄ ⁺ and CO₂, we hypothesized that urea can integrate the N and C metabolism during periods of severe drought. Under this condition, NH₄ ⁺ can be assimilated into amino acids through glutamine synthetase (GS), while the CO₂ can be pre-fixated by phosphoenolpyruvate carboxylase (PEPC). In this context, we evaluated the foliar transcriptome of G. monostachia to compare the relative gene expression of some genes involved with CAM and the N metabolism when bromeliads were submitted to 7days of drought. We also conducted a controlled experiment with an extended water deficit period (21days) in which bromeliads were cultivated in different N sources (urea, NH₄ ⁺, and NO₃ ^-). Our transcriptome results demonstrated an increment in the expression of genes related to CAM, particularly those involved in the carboxylation metabolism (PEPC1, PPCK, and NAD-MDH), the movement of malate through vacuolar membrane (ALMT9), and the decarboxylation process (PEPCK). Urea stimulated the expression of PEPC1 and ALMT9, while Urease transcripts increased under water deficit. Under this same condition, GS1 gene expression increased, indicating that the NH₄ ⁺ from urea hydrolysis can be assimilated in the cytosol. We suggest that the link between C and N metabolism occurred through the supply of carbon skeleton (2-oxoglutarate, 2-OG) by the cytosolic isocitrate dehydrogenase since the number of NADP-ICDH transcripts was also higher under drought conditions. These findings indicate that while urea hydrolysis provides NH₄ ⁺ that can be consumed by glutamine synthetase-cytosolic/glutamate synthase (GS1/GOGAT) cycle, the CO₂ can be used by CAM, maintaining photosynthetic efficiency even when most stomata remain closed 24h (CAM-idling) as in the case of a severe water deficit condition. Thus, we suggest that urea could be used by G. monostachia as a strategy to increase its survival under drought, integrating N and C metabolism.

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

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

Transcriptome Analysis of Pyrus betulaefolia Seedling Root Responses to Short-Term Potassium Deficiency

Potassium (K) plays a crucial role in multiple physiological and developmental processes in plants. Its deficiency is a common abiotic stress that inhibits plant growth and reduces crop productivity. A better understanding of the mechanisms involved in plant responses to low K could help to improve the efficiency of K use in plants. However, such responses remain poorly characterized in fruit tree species such as pears (Pyrus sp). We analyzed the physiological and transcriptome responses of a commonly used pear rootstock, Pyrus betulaefolia, to K-deficiency stress (0 mM). Potassium deprivation resulted in apparent changes in root morphology, with short-term low-K stress resulting in rapidly enhanced root growth. Transcriptome analyses indicated that the root transcriptome was coordinately altered within 6 h after K deprivation, a process that continued until 15 d after treatment. Potassium deprivation resulted in the enhanced expression (up to 5-fold) of a putative high-affinity K+ transporter, PbHAK5 (Pbr037826.1), suggesting the up-regulation of mechanisms associated with K+ acquisition. The enhanced root growth in response to K-deficiency stress was associated with a rapid and sustained decrease in the expression of a transcription factor, PbMYB44 (Pbr015309.1), potentially involved in mediating auxin responses, and the increased expression of multiple genes associated with regulating root growth. The concentrations of several phytohormones including indoleacetic acid (IAA), ABA, ETH, gibberellin (GA3), and jasmonic acid (JA) were higher in response to K deprivation. Furthermore, genes coding for enzymes associated with carbon metabolism such as SORBITOL DEHYDROGENASE (SDH) and SUCROSE SYNTHASE (SUS) displayed greatly enhanced expression in the roots under K deprivation, presumably indicating enhanced metabolism to meet the increased energy demands for growth and K+ acquisition. Together, these data suggest that K deprivation in P. betulaefolia results in the rapid re-programming of the transcriptome to enhance root growth and K+ acquisition. These data provide key insights into the molecular basis for understanding low-K-tolerance mechanisms in pears and in other related fruit trees and identifying potential candidates that warrant further analyses.

Potassium Efflux and Cytosol Acidification as Primary Anoxia-Induced Events in Wheat and Rice Seedlings

Both ion fluxes and changes of cytosolic pH take an active part in the signal transduction of different environmental stimuli. Here we studied the anoxia-induced alteration of cytosolic K⁺ concentration, [K⁺]_cyt, and cytosolic pH, pH_cyt, in rice and wheat, plants with different tolerances to hypoxia. The [K⁺]_cyt and pH_cyt were measured by fluorescence microscopy in single leaf mesophyll protoplasts loaded with the fluorescent potassium-binding dye PBFI-AM and the pH-sensitive probe BCECF-AM, respectively. Anoxic treatment caused an efflux of K⁺ from protoplasts of both plants after a lag-period of 300-450 s. The [K⁺]_cyt decrease was blocked by tetraethylammonium (1 mM, 30 min pre-treatment) suggesting the involvement of plasma membrane voltage-gated K⁺ channels. The protoplasts of rice (a hypoxia-tolerant plant) reacted upon anoxia with a higher amplitude of the [K⁺]_cyt drop. There was a simultaneous anoxia-dependent cytosolic acidification of protoplasts of both plants. The decrease of pH_cyt was slower in wheat (a hypoxia-sensitive plant) while in rice protoplasts it was rapid and partially reversible. Ion fluxes between the roots of intact seedlings and nutrient solutions were monitored by ion-selective electrodes and revealed significant anoxia-induced acidification and potassium leakage that were inhibited by tetraethylammonium. The K⁺ efflux from rice was more distinct and reversible upon reoxygenation when compared with wheat seedlings.

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.

Water relations of two Sicilian grapevine cultivars in response to potassium availability and drought stress

We investigated the response of two Sicilian grapevine cultivars, Catarratto and Nero d'Avola, to potassium deficiency and drought stress. Two-year-old plants grafted on 1103 Paulsen were grown in agriperlite, with or without potassium in the fertigation solution for six weeks, and subjected to moderate drought stress by suspending irrigation for one week. Potassium content of leaves, roots and xylem sap were measured with an ion-selective electrode. Changes in stomatal conductance, stem and leaf water potential and hydraulic conductance were compared between genotypes and treatments. Potassium deficiency led to significant decreases in leaf potassium content in both cultivars and under both well-watered and drought stress conditions. Potassium content in xylem sap showed no significant differences between cultivars and was correlated with stem hydraulic conductance, particularly in the drought stress treatments. Under drought stress conditions, potassium availability led to an increase in stomatal conductance, particularly in Nero d'Avola. Both cultivars showed a rather isohydric behavior under these experimental conditions, and the level of isohydry varied with potassium availability. These results can be useful for the development of optimal fertigation practices and the selection of drought tolerant varieties.

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.

Roles of plasma membrane proton ATPases AHA2 and AHA7 in normal growth of roots and root hairs in Arabidopsis thaliana

Plasma membrane H⁺ -ATPase pumps build up the electrochemical H⁺ gradients that energize most other transport processes into and out of plant cells through channel proteins and secondary active carriers. In Arabidopsis thaliana, the AUTOINHIBITED PLASMA MEMBRANE H⁺ -ATPases AHA1, AHA2 and AHA7 are predominant in root epidermal cells. In contrast to other H⁺ -ATPases, we find that AHA7 is autoinhibited by a sequence present in the extracellular loop between transmembrane segments 7 and 8. Autoinhibition of pump activity was regulated by extracellular pH, suggesting negative feedback regulation of AHA7 during establishment of an H⁺ gradient. Due to genetic redundancy, it has proven difficult to test the role of AHA2 and AHA7, and mutant phenotypes have previously only been observed under nutrient stress conditions. Here, we investigated root and root hair growth under normal conditions in single and double mutants of AHA2 and AHA7. We find that AHA2 drives root cell expansion during growth but that, unexpectedly, restriction of root hair elongation is dependent on AHA2 and AHA7, with each having different roles in this process.

High External K+ Concentrations Impair Pi Nutrition, Induce the Phosphate Starvation Response, and Reduce Arsenic Toxicity in Arabidopsis Plants

Potassium (K⁺) and phosphorous (Pi) are two of the most important nutrients required by plants and there is an interest in studying how they are acquired. Most studies have focused on the characterization of the mechanisms involved in K⁺ and Pi uptake and their distribution within the plants, as well as the regulatory mechanisms involved. Evidence is emerging which points to interactions in the nutrition of different nutrients and to the existence of crosstalk in the signaling cascades regulating their acquisition. However, the interaction between K⁺ and Pi has been scarcely studied. Here we show that high concentrations of K⁺ in the external solution inhibit Pi uptake and impair Pi nutrition in Arabidopsis plants, resulting in the induction of phosphate starvation response (PSR) and the upregulation of genes encoding root phosphate uptake systems. The high K⁺-induced PSR depends on the PHR1 and PHL1 transcription factors that are key pieces of Pi signaling in Arabidopsis. Importantly, high K⁺ reduces arsenic accumulation in plants and its toxic effects. The results presented may help to design strategies to reduce Pi deficiency as well as the accumulation of arsenic in crops.

Regulation of K+ Nutrition in Plants

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

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

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

Regulation of K+ Nutrition in Plants

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

Functional Analysis of Ion Transport Properties and Salt Tolerance Mechanisms of RtHKT1 from the Recretohalophyte Reaumuria trigyna

Reaumuria trigyna is an endangered recretohalophyte and a small archaic feral shrub that is endemic to arid and semi-arid plateau regions of Inner Mongolia, China. Based on transcriptomic data, we isolated a high-affinity potassium transporter gene (RtHKT1) from R. trigyna, which encoded a plasma membrane-localized protein. RtHKT1 was rapidly up-regulated by high Na+ or low K+ and exhibited different tissue-specific expression patterns before and after stress treatment. Transgenic yeast showed tolerance to high Na+ or low K+, while transgenic Arabidopsis exhibited tolerance to high Na+ and sensitivity to high K+, or high Na+-low K+, confirming that Na+ tolerance in transgenic Arabidopsis depends on a sufficient external K+ concentration. Under external high Na+, high K+ and low K+ conditions, transgenic yeast accumulated more Na+-K+, Na+ and K+, while transgenic Arabidopsis accumulated less Na+-more K+, more Na+ and more Na+-K+, respectively, indicating that the ion transport properties of RtHKT1 depend on the external Na+-K+ environment. Salt stress induced up-regulation of some ion transporter genes (AtSOS1/AtHAK5/AtKUP5-6), as well as down-regulation of some genes (AtNHX1/AtAVP1/AtKUP9-12), revealing that multi-ion-transporter synergism maintains Na+/K+ homeostasis under salt stress in transgenic Arabidopsis. Overexpression of RtHKT1 enhanced K+ accumulation and prevented Na+ transport from roots to shoots, improved biomass accumulation and Chl content in salt-stressed transgenic Arabidopsis. The proline content and relative water content increased significantly, and some proline biosynthesis genes (AtP5CS1 and AtP5CS2) were also up-regulated in salt-stressed transgenic plants. These results suggest that RtHKT1 confers salt tolerance on transgenic Arabidopsis by maintaining Na+/K+ homeostasis and osmotic homeostasis.

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

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

Pharmacological and gene regulation properties point to the SlHAK5 K+ transporter as a system for high-affinity Cs+ uptake in tomato plants

Potassium (K⁺ ) and cesium (Cs⁺ ) are chemically similar but while K⁺ is an essential nutrient, Cs⁺ can be toxic for living organisms, plants included. Two different situations could lead to problems derived from the presence of Cs⁺ in agricultural systems: (1) presence of Cs⁺ at high concentrations that could produce toxic effects on plants, (2) presence of micromolar concentrations of radiocesium, which can be accumulated in the plant and affect animal and human health through the food chain. While K⁺ uptake has been well described in tomato plants, information on molecular mechanisms involved in Cs⁺ accumulation in this species is absent. Here, we show that in tomato plants, high concentrations of Cs⁺ produce deficiency of K⁺ but do not induce high-affinity K⁺ uptake or the gene encoding the high-affinity K⁺ transporter SlHAK5. At these concentrations, Cs⁺ uptake takes place through a Ca²⁺ -sensitive pathway, probably a non-selective cation channel. At micromolar concentrations, Cs⁺ is accumulated by a high-affinity uptake system upregulated in K⁺ -starved plants. This high-affinity Cs⁺ uptake shares features with high-affinity K⁺ uptake. It is sensitive to NH₄⁺ and insensitive to Ba²⁺ and Ca²⁺ and its presence parallels the pattern of SlHAK5 expression. Moreover, blockers of reactive oxygen species and ethylene action repress SlHAK5 and negatively regulate both high-affinity K⁺ and Cs⁺ uptake. Thus, we propose that SlHAK5 contributes to Cs⁺ uptake from micromolar concentrations in tomato plants and can constitute a pathway for radiocesium transfer from contaminated areas to the food chain.

Rice phytoglobins regulate responses under low mineral nutrients and abiotic stresses in Arabidopsis thaliana

Just like animals, plants also contain haemoglobins (known as phytoglobins in plants). Plant phytoglobins (Pgbs) have been categorized into 6 different classes, namely, Phytogb0 (Pgb0), Phytogb1 (Pgb1), Phytogb2 (Pgb2), SymPhytogb (sPgb), Leghaemoglobin (Lb), and Phytogb3 (Pgb3). Among the 6 Phytogbs, sPgb and Lb have been functionally characterized, whereas understanding of the roles of other Pgbs is still evolving. In our present study, we have explored the function of 2 rice Pgbs (OsPgb1.1 and OsPgb1.2). OsPgb1.1, OsPgb1.2, OsPgb1.3, and OsPgb1.4 displayed increased level of transcript upon salt, drought, cold, and ABA treatment. The overexpression (OX) lines of OsPgb1.2 in Arabidopsis showed a tolerant phenotype in terms of better root growth in low potassium (K+ ) conditions. The expression of the known K+ gene markers such as LOX2, HAK5, and CAX3 was much higher in the OsPgb1.2 OX as compared to wild type. Furthermore, the OsPgb1.2 OX lines showed a decrease in reactive oxygen species (ROS) production and conversely an increase in the K+ content, both in root and shoot, as compared to wild type in K+ limiting condition. Our results indicated the potential involvement of OsPgb1.2 in signalling networks triggered by the nutrient deficiency stresses.

The nitrogen-potassium intersection: membranes, metabolism, and mechanism

Nitrogen (N) and potassium (K) are the two most abundantly acquired mineral elements by plants, and their acquisition pathways interact in complex ways. Here, we review pivotal interactions with respect to root acquisition, storage, translocation and metabolism, between the K⁺ ion and the two major N sources, ammonium (NH₄⁺ ) and nitrate (NO₃^- ). The intersections between N and K physiology are explored at a number of organizational levels, from molecular-genetic processes, to compartmentation, to whole plant physiology, and discussed in the context of both N-K cooperation and antagonism. Nutritional regulation and optimization of plant growth, yield, metabolism and water-use efficiency are also discussed.

NO3- , PO43- and SO42- deprivation reduced LKT1-mediated low-affinity K+ uptake and SKOR-mediated K+ translocation in tomato and Arabidopsis plants

Regulation of essential macronutrients acquisition by plants in response to their availability is a key process for plant adaptation to changing environments. Here we show in tomato and Arabidopsis plants that when they are subjected to NO₃^- , PO₄^3- and SO₄^2- deprivation, low-affinity K⁺ uptake and K⁺ translocation to the shoot are reduced. In parallel, these nutritional deficiencies produce reductions in the messenger levels of the genes encoding the main systems for low-affinity K⁺ uptake and K⁺ translocation, i.e. AKT1 and SKOR in Arabidopsis and LKT1 and the tomato homolog of SKOR, SlSKOR in tomato, respectively. The results suggest that the shortage of one nutrient produces a general downregulation of the acquisition of other nutrients. In the case of K⁺ nutrient, one of the mechanisms for such a response resides in the transcriptional repression of the genes encoding the systems for K⁺ uptake and translocation.

Review: Mitogen-Activated Protein Kinases in nutritional signaling in Arabidopsis

Mitogen-Activated Protein Kinase (MAPK) cascades are functional modules widespread among eukaryotic organisms. In plants, these modules are encoded by large multigenic families and are involved in many biological processes ranging from stress responses to cellular differentiation and organ development. Furthermore, MAPK pathways are involved in the perception of environmental and physiological modifications. Interestingly, some MAPKs play a role in several signaling networks and could have an integrative function for the response of plants to their environment. In this review, we describe the classification of MAPKs and highlight some of their biochemical actions. We performed an in silico analysis of MAPK gene expression in response to nutrients supporting their involvement in nutritional signaling. While several MAPKs have been identified as players in sugar, nitrogen, phosphate, iron and potassium-related signaling pathways, their biochemical functions are yet mainly unknown. The integration of these regulatory cascades in the current understanding of nutrient signaling is discussed and potential new avenues for approaches toward plants with higher nutrient use efficiencies are evoked.

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.

Inhibition of ethylene synthesis reduces salt-tolerance in tomato wild relative species Solanum chilense

Exposure to salinity induces a burst in ethylene synthesis in the wild tomato halophyte plant species Solanum chilense. In order to gain information on the role of ethylene in salt adaptation, plants of Solanum chilense (accession LA4107) and of cultivated glycophyte Solanum lycopersicum (cv. Ailsa Craig) were cultivated for 7days in nutrient solution containing 0 or 125mM NaCl in the presence or absence of the inhibitor of ethylene synthesis (aminovinylglycine (AVG) 2μM). Salt-induced ethylene synthesis in S. chilense occurred concomitantly with an increase in stomatal conductance, an efficient osmotic adjustment and the maintenance of carbon isotope discrimination value (Δ¹³C). In contrast, in S. lycopersicum, salt stress decreased stomatal conductance and Δ¹³C values while osmotic potential remained higher than in S. chilense. Inhibition of stress-induced ethylene synthesis by AVG decreased stomatal conductance and Δ¹³C in S. chilense and compromised osmotic adjustment. Solanum chilense behaved as an includer and accumulated high amounts of Na in the shoot but remained able to maintain K nutrition in the presence of NaCl. This species however did not stimulate the expression of genes coding for high-affinity K transport but genes coding for ethylene responsive factor ERF5 and JREF1 were constitutively more expressed in S. chilense than in S. lycopersicum. It is concluded that ethylene plays a key role in salt tolerance of S. chilense.

Potassium Transporter KUP7 Is Involved in K(+) Acquisition and Translocation in Arabidopsis Root under K(+)-Limited Conditions

Potassium (K(+)) is one of the essential macronutrients for plant growth and development. K(+) uptake from environment and K(+) translocation in plants are conducted by K(+) channels and transporters. In this study, we demonstrated that KT/HAK/KUP transporter KUP7 plays crucial roles in K(+) uptake and translocation in Arabidopsis root. The kup7 mutant exhibited a sensitive phenotype on low-K(+) medium, whose leaves showed chlorosis symptoms compared with wild-type plants. Loss of function of KUP7 led to a reduction of K(+) uptake rate and K(+) content in xylem sap under K(+)-deficient conditions. Thus, the K(+) content in kup7 shoot was significantly reduced under low-K(+) conditions. Localization analysis revealed that KUP7 was predominantly targeted to the plasma membrane. The complementation assay in yeast suggested that KUP7 could mediate K(+) transport. In addition, phosphorylation on S80, S719, and S721 was important for KUP7 activity. KUP7 was ubiquitously expressed in many organs/tissues, and showed a higher expression level in Arabidopsis root. Together, our data demonstrated that KUP7 is crucial for K(+) uptake in Arabidopsis root and might be also involved in K(+) transport into xylem sap, affecting K(+) translocation from root toward shoot, especially under K(+)-limited conditions.

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.

Comparison between Arabidopsis and Rice for Main Pathways of K(+) and Na(+) Uptake by Roots

K(+) is an essential macronutrient for plants. It is acquired by specific uptake systems located in roots. Although the concentrations of K(+) in the soil solution are widely variable, K(+) nutrition is secured by uptake systems that exhibit different affinities for K(+). Two main systems have been described for root K(+) uptake in several species: the high-affinity HAK5-like transporter and the inward-rectifier AKT1-like channel. Other unidentified systems may be also involved in root K(+) uptake, although they only seem to operate when K(+) is not limiting. The use of knock-out lines has allowed demonstrating their role in root K(+) uptake in Arabidopsis and rice. Plant adaptation to the different K(+) supplies relies on the finely tuned regulation of these systems. Low K(+)-induced transcriptional up-regulation of the genes encoding HAK5-like transporters occurs through a signal cascade that includes changes in the membrane potential of root cells and increases in ethylene and reactive oxygen species concentrations. Activation of AKT1 channels occurs through phosphorylation by the CIPK23/CBL1 complex. Recently, activation of the Arabidopsis HAK5 by the same complex has been reported, pointing to CIPK23/CBL as a central regulator of the plant's adaptation to low K(+). Na(+) is not an essential plant nutrient but it may be beneficial for some plants. At low concentrations, Na(+) improves growth, especially under K(+) deficiency. Thus, high-affinity Na(+) uptake systems have been described that belong to the HKT and HAK families of transporters. At high concentrations, typical of saline environments, Na(+) accumulates in plant tissues at high concentrations, producing alterations that include toxicity, water deficit and K(+) deficiency. Data concerning pathways for Na(+) uptake into roots under saline conditions are still scarce, although several possibilities have been proposed. The apoplast is a significant pathway for Na(+) uptake in rice grown under salinity conditions, but in other plant species different mechanisms involving non-selective cation channels or transporters are under discussion.

Rice potassium transporter OsHAK1 is essential for maintaining potassium-mediated growth and functions in salt tolerance over low and high potassium concentration ranges

Potassium (K) absorption and translocation in plants rely upon multiple K transporters for adapting varied K supply and saline conditions. Here, we report the expression patterns and physiological roles of OsHAK1, a member belonging to the KT/KUP/HAK gene family in rice (Oryza sativa L.). The expression of OsHAK1 is up-regulated by K deficiency or salt stress in various tissues, particularly in the root and shoot apical meristem, the epidermises and steles of root, and vascular bundles of shoot. Both oshak1 knockout mutants in comparison to their respective Dongjin or Manan wild types showed a dramatic reduction in K concentration and stunted root and shoot growth. Knockout of OsHAK1 reduced the K absorption rate of unit root surface area by ∼50-55 and ∼30%, and total K uptake by ∼80 and ∼65% at 0.05-0.1 and 1 mm K supply level, respectively. The root net high-affinity K uptake of oshak1 mutants was sensitive to salt stress but not to ammonium supply. Overexpression of OsHAK1 in rice increased K uptake and K/Na ratio. The positive relationship between K concentration and shoot biomass in the mutants suggests that OsHAK1 plays an essential role in K-mediated rice growth and salt tolerance over low and high K concentration ranges.

Calcium sensor kinase activates potassium uptake systems in gland cells of Venus flytraps

The Darwin plant Dionaea muscipula is able to grow on mineral-poor soil, because it gains essential nutrients from captured animal prey. Given that no nutrients remain in the trap when it opens after the consumption of an animal meal, we here asked the question of how Dionaea sequesters prey-derived potassium. We show that prey capture triggers expression of a K(+) uptake system in the Venus flytrap. In search of K(+) transporters endowed with adequate properties for this role, we screened a Dionaea expressed sequence tag (EST) database and identified DmKT1 and DmHAK5 as candidates. On insect and touch hormone stimulation, the number of transcripts of these transporters increased in flytraps. After cRNA injection of K(+)-transporter genes into Xenopus oocytes, however, both putative K(+) transporters remained silent. Assuming that calcium sensor kinases are regulating Arabidopsis K(+) transporter 1 (AKT1), we coexpressed the putative K(+) transporters with a large set of kinases and identified the CBL9-CIPK23 pair as the major activating complex for both transporters in Dionaea K(+) uptake. DmKT1 was found to be a K(+)-selective channel of voltage-dependent high capacity and low affinity, whereas DmHAK5 was identified as the first, to our knowledge, proton-driven, high-affinity potassium transporter with weak selectivity. When the Venus flytrap is processing its prey, the gland cell membrane potential is maintained around -120 mV, and the apoplast is acidified to pH 3. These conditions in the green stomach formed by the closed flytrap allow DmKT1 and DmHAK5 to acquire prey-derived K(+), reducing its concentration from millimolar levels down to trace levels.

High Ca(2+) reverts the repression of high-affinity K(+) uptake produced by Na(+) in Solanum lycopersycum L. (var. microtom) plants

Potassium (K(+)) is an essential nutrient for plants which is acquired by plant roots through the operation of specific transport systems. Abiotic stress conditions such as salinity impair K(+) nutrition because, in addition to other effects, high salt concentrations in the solution bathing the roots inhibit K(+) uptake systems. This detrimental effect of salinity is exacerbated when external K(+) is very low and the only system capable of mediating K(+) uptake is one with high-affinity for K(+), as that mediated by transporters of the HAK5 type. Increasing external Ca(2+) has been shown to improve K(+) nutrition under salinity and, although the specific mechanisms for this beneficial effect are largely unknown, they are beginning to be understood. The genes encoding the HAK5 transporters are induced by K(+) starvation and repressed by long-term exposure to high Na(+). This occurs in parallel with the hyperpolarization and depolarization of root cell membrane potential. In the present study it is shown in tomato plants that the presence of high Ca(2+) during the K(+) starvation period that leads to LeHAK5 induction, counteracts the repression exerted by high Na(+). High Ca(2+) reduces the Na(+)-induced plasma membrane depolarization of root cells, resorting one of the putative first steps in the low-K(+) signal cascade. This allows proper LeHAK5 expression and functional high-affinity K(+) uptake at the roots. Thus, the maintenance of HAK5-mediated K(+) nutrition under salinity by high Ca(2+) can be regarded as a specific beneficial effect of Ca(2+) contributing to salt tolerance in plants.