Regulation of K+ Nutrition in Plants

Mycorrhiza-mediated potassium transport in Medicago truncatula can be evaluated by using rubidium as a proxy

Arbuscular mycorrhizal (AM) fungi considerably improve plant nutrient acquisition, particularly phosphorus and nitrogen. Despite the physiological importance of potassium (K⁺) in plants, there is increasing interest in the mycorrhizal contribution to plant K⁺ nutrition. Yet, methods to track K⁺ transport are often costly and limiting evaluation opportunities. Rubidium (Rb⁺) is known to be transported through same pathways as K⁺. As such our research efforts attempt to evaluate if Rb⁺ could serve as a viable proxy for evaluating K⁺ transport in AM symbiosis. Therefore, we examined the transport of K⁺ in Medicago truncatula colonized by the AM fungus Rhizophagus irregularis isolate 09 having access to various concentrations of Rb⁺ in custom-made two-compartment systems. Plant biomass, fungal root colonization, and shoot nutrient concentrations were recorded under sufficient and limited K⁺ regimes. We report that AM plants displayed higher shoot Rb⁺ and K⁺ concentrations and a greater K⁺:Na⁺ ratio relative to non-colonized plants in both sufficient and limited K⁺ conditions. Consequently, our results indicate that Rb⁺ can be used as a proxy to assess the movement of K⁺ in AM symbiosis, and suggest the existence of a mycorrhizal uptake pathway for K⁺ nutrition in M. truncatula.

Unlocking the potential of co-applied biochar and plant growth-promoting rhizobacteria (PGPR) for sustainable agriculture under stress conditions

Sustainable food security is a major challenge in today's world, particularly in developing countries. Among many factors, environmental stressors, i.e., drought, salinity and heavy metals are major impediments in achieving sustainable food security. This calls for finding environment-friendly and cheap solutions to address these stressors. Plant growth-promoting rhizobacteria (PGPR) have long been established as an environment-friendly means to enhance agricultural productivity in normal and stressed soils and are being applied at field scale. Similarly, pyrolyzing agro-wastes into biochar with the aim to amend soils is being proposed as a cheap additive for enhancement of soil quality and crop productivity. Many pot and some field-scale experiments have confirmed the potential of biochar for sustainable increase in agricultural productivity. Recently, many studies have combined the PGPR and biochar for improving soil quality and agricultural productivity, under normal and stressed conditions, with the assumption that both of these additives complement each other. Most of these studies have reported a significant increase in agricultural productivity in co-applied treatments than sole application of PGPR or biochar. This review presents synthesis of these studies in addition to providing insights into the mechanistic basis of the interaction of the PGPR and biochar. Moreover, this review highlights the future perspectives of the research in order to realize the potential of co-application of the PGPR and biochar at field scale.

Graphical Abstract

Comparative transcriptome analysis between inbred lines and hybrids provides molecular insights into K+ content heterosis of tobacco (Nicotiana tabacum L.)

Potassium (K⁺) is essential for crop growth. Increasing the K⁺ content can often directly promote the improvement of crop yield and quality. Heterosis plays an important role in genetic improvement and leads to genetic gains. We found that the K⁺ content of tobacco showed significant heterosis, which is highly significant for cultivating tobacco varieties with high K⁺ content. However, the mechanism by which K⁺ content heterosis occurs in tobacco leaves is not clear. In this study, a comprehensive comparative transcriptome sequencing analysis of root samples from the hybrid G70 × GDH11 and its parental inbred lines G70 and GDH11 was performed to elucidate the importance of the root uptake capacity of K⁺ in the formation of heterosis. The results showed that 29.53% and 60.49% of the differentially expressed genes (DEGs) exhibited dominant and over-dominant expression patterns, respectively. These non-additive upregulated DEGs were significantly enriched in GO terms, such as metal ion transport and reaction, ion balance and homeostasis, ion channel activity, root meristem growth, and regulation of root hairs. The KEGG annotation results indicated that these genes were mainly involved in the pathways such as energy metabolism, carbohydrate formation, amino acid metabolism, and signal transduction. Further analysis showed that probable potassium transporter 17 (NtKT17) and potassium transporter 5-like (NtKT5), associated with potassium ion absorption, glutamate receptor 2.2-like and glutamate receptor 2.8-like, associated with ion channel activity, LOC107782957, protein detoxification 42-like, and probable glutamate carboxypeptidase 2, associated with root configuration, showed a significantly higher expression in the hybrids. These results indicated that the over-dominant expression pattern of DEGs played a key role in the heterosis of K⁺ content in tobacco leaves, and the overexpression of the genes related to K⁺ uptake, transport, and root development in hybrids helped to improve the K⁺ content of plants, thus showing the phenomenon of heterosis.

Overdominant expression of related genes of ion homeostasis improves K+ content advantage in hybrid tobacco leaves

BACKGROUND

Potassium(K+) plays a vital role in improving the quality of tobacco leaves. However, how to improve the potassium content of tobacco leaves has always been a difficult problem in tobacco planting. K+ content in tobacco hybrid is characterized by heterosis, which can improve the quality of tobacco leaves, but its underlying molecular genetic mechanisms remain unclear.

RESULTS

Through a two-year field experiment, G70×GDH11 with strong heterosis and K326×GDH11 with weak heterosis were screened out. Transcriptome analyses revealed that 80.89% and 57.28% of the differentially expressed genes (DEGs) in the strong and weak heterosis combinations exhibited an overdominant expression pattern, respectively. The genes that up-regulated the overdominant expression in the strong heterosis hybrids were significantly enriched in the ion homeostasis. Genes involved in K+ transport (KAT1/2, GORK, AKT2, and KEA3), activity regulation complex (CBL-CIPK5/6), and vacuole (TPKs) genes were overdominant expressed in strong heterosis hybrids, which contributed to K+ homeostasis and heterosis in tobacco leaves.

CONCLUSIONS

K+ homeostasis and accumulation in tobacco hybrids were collectively improved. The overdominant expression of K+ transport and homeostasis-related genes conducted a crucial role in the heterosis of K+ content in tobacco leaves.

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.

Season-long exposure of bilberry plants to realistic and future ozone pollution improves the nutraceutical quality of fruits

Ozone (O₃) is a phytotoxic air pollutant capable of limiting plant yield and growth, and altering the quality of edible plant products. This study aimed to investigate the effects of long-term O₃ exposure at realistic and future concentrations (applied during fruit development) not only on morphological, physiological, and biochemical plant/leaf traits of Vaccinium myrtillus but also on its fruit yield and quality. Three-year-old saplings were grown from May to July under three levels of O₃ concentration [1.0, 1.5 and 2.0 times the ambient air concentrations, denoted as AA, 1.5_AA and 2.0_AA], using a new-generation O₃ Free Air Controlled Exposure system. Ozone induced oxidative pressure and membrane denaturation as confirmed by the accumulation of anion superoxide, hydrogen peroxide (^•O₂^-: +39 and + 29%; H₂O₂: +55 and + 59% in 1.5_AA and 2.0_AA, respectively, compared with AA), and malondialdehyde by-product (1.4- and 2.5-fold higher than AA, in 1.5_AA and 2.0_AA, respectively). The observed oxidative burst likely affected several cellular structures interested by photosynthetic processes (e.g., decrease of the maximum rate of carboxylation: -30%). This constraint likely induced a decline in plant vitality and a different partitioning of biomass allocation between above and below organs. An accelerated maturation of bilberries due to O₃ was reported, suggesting that plants grown under harsher environmental conditions suffered from metabolic changes associated with early ripening. Increasing O₃ concentrations might be responsible for an alteration of the ratio between oxidation and reduction processes mechanisms that was followed by a loss of integrity of membranes, so limiting the availability of energy/resources, triggering enzymatic oxidation of phenols to red/purple pigments, and promoting fruit maturation. To the best of our knowledge, this is the first research showing that long-term O₃ exposure during bilberry fruit development influenced not only several plant/leaf traits, but also fruit nutraceutical quality at the time of harvest.

Enlightening the Pathway of Phytoremediation: Ecophysiology and X-ray Fluorescence Visualization of Two Chilean Hardwoods Exposed to Excess Copper

In the present climate emergency due to global warming, we are urged to move away from fossil fuels and pursue a speedy conversion to renewable energy systems. Consequently, copper (Cu) will remain in high demand because it is a highly efficient conductor used in clean energy systems to generate power from solar, hydro, thermal and wind energy across the world. Chile is the global leader in copper production, but this position has resulted in Chile having several hundred tailing deposits. We grew two Chilean native hardwood species, quillay (Quillaja saponaria Molina) and espino (Vachellia caven (Molina) Seigler & Ebinger, under three increasing Cu levels (0, 50, and 100 µM) for 6 months in a greenhouse setting. We measured growth, photosynthetic performance and elemental contents of leaves and roots to further evaluate their potential for phytoremediation. Growth of quillay was unaffected by Cu treatment but growth of espino was enhanced, as was its photosynthetic performance, indicating that espino may have an unusually high requirement for copper. Excess Cu was mostly restricted to the roots of both species, where X-ray fluorescence (XRF) mapping indicated some tendency for Cu to accumulate in tissues outside the periderm. Calcium oxalate crystals were prominently visible in XRF images of both species. Nickel (but not Cu) showed a concurrent distribution pattern with these crystals.

The role of NADPH oxidases in regulating leaf gas exchange and ion homeostasis in Arabidopsis plants under cadmium stress

NADPH oxidase, an enzyme associated with the plasma membrane, constitutes one of the main sources of reactive oxygen species (ROS) which regulate different developmental and adaptive responses in plants. In this work, the involvement of NADPH oxidases in the regulation of photosynthesis and cell ionic homeostasis in response to short cadmium exposure was compared between wild type (WT) and three RBOHs (Respiratory Burst Oxidase Homologues) Arabidopsis mutants (AtrbohC, AtrbohD, and AtrbohF). Plants were grown under hydroponic conditions and supplemented with 50 µM CdCl₂ for 24 h. Cadmium treatment differentially affected photosynthesis, stomatal conductance, transpiration, and antioxidative responses in WT and Atrbohs mutants. The loss of function of RBOH isoforms resulted in higher Cd²⁺ influx, mainly in the elongation zone of roots, which was more evident in AtrbohD and AtrbohF mutants. In the mature zone, the highest Cd²⁺ influx was observed in rbohC mutant. The lack of functional RBOH isoforms also resulted in altered patterns of net K⁺ transport across cellular membranes, both in the root epidermis and leaf mesophyll. The analysis of expression of metal transporters by qPCR demonstrated that a loss of functional RBOH isoforms has altered transcript levels for metal NRAMP3, NRAMP6 and IRT1 and the K⁺ transporters outward-rectifying K⁺ efflux GORK channel, while RBOHD specifically regulated transcripts for high-affinity K⁺ transporters KUP8 and HAK5, and IRT1 and RBOHD and F regulated the transcription factors TGA3 and TGA10. It is concluded that RBOH-dependent H₂O₂ regulation of ion homeostasis and Cd is a highly complex process involving multilevel regulation from transpirational water flow to transcriptional and posttranslational modifications of K/metals transporters.

HKT1;1 and HKT1;2 Na+ Transporters from Solanum galapagense Play Different Roles in the Plant Na+ Distribution under Salinity

Salt tolerance is a target trait in plant science and tomato breeding programs. Wild tomato accessions have been often explored for this purpose. Since shoot Na⁺/K⁺ is a key component of salt tolerance, RNAi-mediated knockdown isogenic lines obtained for Solanum galapagense alleles encoding both class I Na⁺ transporters HKT1;1 and HKT1;2 were used to investigate the silencing effects on the Na and K contents of the xylem sap, and source and sink organs of the scion, and their contribution to salt tolerance in all 16 rootstock/scion combinations of non-silenced and silenced lines, under two salinity treatments. The results show that SgHKT1;1 is operating differently from SgHKT1;2 regarding Na circulation in the tomato vascular system under salinity. A model was built to show that using silenced SgHKT1;1 line as rootstock would improve salt tolerance and fruit quality of varieties carrying the wild type SgHKT1;2 allele. Moreover, this increasing effect on both yield and fruit soluble solids content of silencing SgHKT1;1 could explain that a low expressing HKT1;1 variant was fixed in S. lycopersicum during domestication, and the paradox of increasing agronomic salt tolerance through silencing the HKT1;1 allele from S. galapagense, a salt adapted species.

Ion transporters and their regulatory signal transduction mechanisms for salinity tolerance in plants

Soil salinity is one of the most serious threats to plant growth and productivity. Due to global climate change, burgeoning population and shrinking arable land, there is an urgent need to develop crops with minimum reduction in yield when cultivated in salt-affected areas. Salinity stress imposes osmotic stress as well as ion toxicity, which impairs major plant processes such as photosynthesis, cellular metabolism, and plant nutrition. One of the major effects of salinity stress in plants includes the disturbance of ion homeostasis in various tissues. In the present study, we aimed to review the regulation of uptake, transport, storage, efflux, influx, and accumulation of various ions in plants under salinity stress. We have summarized major research advancements towards understanding the ion homeostasis at both cellular and whole-plant level under salinity stress. We have also discussed various factors regulating the function of ion transporters and channels in maintaining ion homeostasis and ionic interactions under salt stress, including plant antioxidative defense, osmo-protection, and osmoregulation. We further elaborated on stress perception at extracellular and intracellular levels, which triggers downstream intracellular-signaling cascade, including secondary messenger molecules generation. Various signaling and signal transduction mechanisms under salinity stress and their role in improving ion homeostasis in plants are also discussed. Taken together, the present review focuses on recent advancements in understanding the regulation and function of different ion channels and transporters under salt stress, which may pave the way for crop improvement.

The phosphoinositide PI(3,5)P2 inhibits the activity of plant NHX proton/potassium antiporters: Advantages of a novel electrophysiological approach

In the present work, we discuss the way in which the parallel application of the patch-clamp technique and the 2′,7′-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) fluorescence detection for recording luminal proton changes allows the functional characterization of nonelectrogenic potassium/proton vacuolar antiporters of the NHX (Na+/H+ exchanger) family. Moreover, we review the functional role of the tonoplast-specific phosphoinositide PI(3,5)P2, able to simultaneously inhibit the activity of NHXs and CLC-a transporters, whose coordinated action can play an important role in the water balance of plant cells.

MicroRNA Mediated Plant Responses to Nutrient Stress

To complete their life cycles, plants require several minerals that are found in soil. Plant growth and development can be affected by nutrient shortages or high nutrient availability. Several adaptations and evolutionary changes have enabled plants to cope with inappropriate growth conditions and low or high nutrient levels. MicroRNAs (miRNAs) have been recognized for transcript cleavage and translational reduction, and can be used for post-transcriptional regulation. Aside from regulating plant growth and development, miRNAs play a crucial role in regulating plant’s adaptations to adverse environmental conditions. Additionally, miRNAs are involved in plants’ sensory functions, nutrient uptake, long-distance root transport, and physiological functions related to nutrients. It may be possible to develop crops that can be cultivated in soils that are either deficient in nutrients or have extreme nutrient supplies by understanding how plant miRNAs are associated with nutrient stress. In this review, an overview is presented regarding recent advances in the understanding of plants’ responses to nitrogen, phosphorus, potassium, sulfur, copper, iron, boron, magnesium, manganese, zinc, and calcium deficiencies via miRNA regulation. We conclude with future research directions emphasizing the modification of crops for improving future food security.

Foliar element distributions in Guadua bamboo, a major forest dominant in southwestern Amazonia

Silica is the best-known component filling the spaces that form phytoliths in many plants, but phytoliths may also contain other elements. We used scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDX) to map element distributions in the leaves of Guadua bamboo, which is a successful competitor in southwestern Amazonian forests. We emphasize immobile elements that can be mineralized (silicon and calcium) as well as potassium, an abundant mobile nutrient with many vital functions. We discovered high silicon (Si) content with little or no calcium (Ca) or potassium (K) in bulliform cells, bilobate shaped short cells and stomata, all of which can form phytoliths, and moderately high Si content in the bundle sheet, prickle tips and papillae. K often surrounded Si-loaded cells, Si and K had overlapping distributions in the intercostal areas near vein margins, and Ca showed abundant spotted distribution in the intercostal areas. The dark inside content of the costal zones of some samples in light microscopy showed no mineralization but prominently contained sulfur. Adjacent fusoid cells showed different Si, K and Ca combinations, which may suggest potentially variable functions of these cells. Widespread Si deposition strengthens Guadua bamboo leaves and may help it to outcompete tree species during periods of drought.

Potassium (K+) Starvation-Induced Oxidative Stress Triggers a General Boost of Antioxidant and NADPH-Generating Systems in the Halophyte Cakile maritima

Potassium (K+) is an essential macro-element for plant growth and development given its implication in major processes such as photosynthesis, osmoregulation, protein synthesis, and enzyme function. Using 30-day-old Cakile maritima plants as halophyte model grown under K+ deprivation for 15 days, it was analyzed at the biochemical level to determine the metabolism of reactive oxygen species (ROS), key photorespiratory enzymes, and the main NADPH-generating systems. K+ starvation-induced oxidative stress was noticed by high malondialdehyde (MDA) content associated with an increase of superoxide radical (O2•−) in leaves from K+-deficient plants. K+ shortage led to an overall increase in the activity of hydroxypyruvate reductase (HPR) and glycolate oxidase (GOX), as well as of antioxidant enzymes catalase (CAT), those of the ascorbate-glutathione cycle, peroxidase (POX), and superoxide dismutase (SOD), and the main enzymes involved in the NADPH generation in both leaves and roots. Especially remarkable was the induction of up to seven CuZn-SOD isozymes in leaves due to K+ deficiency. As a whole, data show that the K+ starvation has associated oxidative stress that boosts a biochemical response leading to a general increase of the antioxidant and NADPH-generating systems that allow the survival of the halophyte Cakile maritima.

Homologous Cloning of Potassium Channel Genes From the Superior Apple Rootstock Line 12-2, Which is Tolerant to Apple Replant Disease

Potassium channels are important ion channels that are responsible for the absorption of potassium in the plant nutrient uptake system. In this study, we used homologous molecular cloning to obtain 8 K⁺ channel genes from the superior apple rootstock line 12-2 (self-named): MsAKT1-1, MsKAT3-2, MsKAT1-3, MsK2P3-4, MsK2P3-5, MsK2P5-6, MsK2P3-7, and MsK2P3-8. Their lengths varied from 942 bp (MsK2P5-6) to 2625 bp (MsAKT1-1), and the number of encoded amino acids varied from 314 (MsK2P5-6) to 874 (MsAKT1-1). Subcellular localization predictions showed that MsAKT1-1, MsKAT3-2, and MsKAT1-3 were localized on the plasma membrane, and MsK2P3-4, MsK2P3-5, MsK2P5-6, MsK2P3-7, and MsK2P3-8 were localized on the vacuole and plasma membrane. The 8 K⁺ channel proteins contained α helices, extended strands, β turns, and random coils. MsKAT1-3 had four transmembrane structures, MsKAT3-2 had six, and the other six K⁺ channel genes had five. Protein structure domain analysis showed that MsAKT1-1 contained nine protein domains, followed by MsKAT3-2 with four, MsKAT1-3 with three, and the other five two-pore domain K⁺ channel proteins with two. Semi-quantitative RT-PCR detection of the K⁺ channel genes showed that their expression levels were high in roots. qRT-PCR analysis showed that the relative expression levels of the 8 genes changed after exposure to ARD stress. The above results provide a theoretical basis for further research on the functions of potassium channel genes in 12-2 and a scientific basis for the breeding of ARD-resistant rootstock.

Vicia–Micronucleus Test Application for Saline Irrigation Water Risk Assessment

In view of climate change, increasing soil salinity is expected worldwide. It is therefore important to improve prediction ability of plant salinity effects. For this purpose, brackish/saline irrigation water from two areas in central and coastal Tunisia was sampled. The water samples were classified as C3 (EC: 2.01–2.24 dS m⁻¹) and C4 (EC: 3.46–7.00 dS m⁻¹), indicating that the water was questionable and not suitable for irrigation, respectively. The water samples were tested for their genotoxic potential and growth effects on Vicia faba seedlings. Results showed a decrease in mitotic index (MI) and, consequently, growth parameters concomitant to the appearance of micronucleus (MCN) and chromosome aberrations when the water salinity increased. Salt ion concentration had striking influence on genome stability and growth parameters. Pearson correlation underlined the negative connection between most ions in the water inappropriate for irrigation (C4) and MI as well as growth parameters. MI was strongly influenced by Mg²⁺, Na⁺, Cl⁻, and to a less degree Ca²⁺, K⁺, and SO₄²⁻. Growth parameters were moderately to weakly affected by K⁺ and Ca²⁺, respectively. Re-garding MCN, a very strong positive correlation was found for MCN and K⁺. Despite its short-term application, the Vicia-MCN Test showed a real ability to predict toxicity induced by salt ions confirming that is has a relevant role in hazard identification and risk assessment of salinity effects.

Impact of microplastics on growth, photosynthesis and essential elements in Cucurbita pepo L

In this study, Cucurbita pepo L., one of the most cultivated, consumed and economically important crop worldwide, was used as model plant to test the toxic effects of the four most abundant microplastics identified in contaminated soils, i.e. polypropylene (PP), polyethylene (PE), polyvinylchloride (PVC), and polyethyleneterephthalate (PET). Cucurbita plants were grown in pots with increasing concentrations of the microplastics, then plant biometry, photosynthetic parameters and ionome of treated vs. untreated samples were compared to evaluate the toxicity of each plastic. All the pollutants impaired root and, especially, shoot growth. Specific and concentration-dependant effects of the different microplastics were found, including reduction in leaf size, chlorophyll content and photosynthetic efficiency, as well as changes in the micro- and macro-elemental profile. Among all the microplastics, PVC was identified as the most toxic and PE as the less toxic material. PVC decreased the dimensions of the leaf lamina, the values of the photosynthetic performance index and the plant iron concentration to a higher extent in respect to the other treatments. Microplastic toxicity exerted on the growth of C. pepo raises concerns about possible yield and economic loss, as well as for risks of a possible transfer into the food chain.

Potassium in plants: Growth regulation, signaling, and environmental stress tolerance

Potassium (K) is an essential element for the growth and development of plants; however, its scarcity or excessive level leads to distortion of numerous functions in plants. It takes part in the control of various significant functions in plant advancement. Because of the importance index, K is regarded second after nitrogen for whole plant growth. Approximately, higher than 60 enzymes are reliant on K for activation within the plant system, in which K plays a vital function as a regulator. Potassium provides assistance in plants against abiotic stress conditions in the environment. With this background, the present paper reviews the physiological functions of K in plants like stomatal regulation, photosynthesis and water uptake. The article also focuses upon the uptake and transport mechanisms of K along with its role in detoxification of reactive oxygen species and in conferring tolerance to plants against abiotic stresses. It also highlights the research progress made in the direction of K mediated signaling cascades.

Light regulation of potassium in plants

Essential macronutrient potassium (K) and environmental signal light regulate a number of vital plant biological processes related to growth, development, and stress response. Recent research has shown connections between the perception of light and the regulation of K in plants. Photoreceptors-mediated wavelength-specific light perception activates signaling cascades which mediate stomatal movement by altering K⁺influx/efflux via K⁺ channels in the guard cells. The quality, intensity, and duration of light affect the regulation of K nutrition and crop quality. Blue/red illumination or red combined blue light treatment increases the expression levels of K transporter genes, K uptake and accumulation, leading to increased lycopene synthesis and improved fruit color in tomato. Despite the commonalities of light and K in multiple functions, our understanding of light regulation of K and associated physiological and molecular processes is fragmentary. In this review, we take a look at the light-controlled K uptake and utilization in plants and propose working models to show potential mechanisms. We discuss major light signaling components, their possible involvement in K nutrition, stomatal movement and crop quality by linking the perception of light signal and subsequent regulation of K. We also pose some outstanding questions to guide future research. Our analysis suggests that the enhancement of K utilization efficiency by manipulation of light quality and light signaling components can be a promising strategy for K management in crop production.

Morphophysiological and transcriptome analysis reveal that reprogramming of metabolism, phytohormones and root development pathways governs the potassium (K+) deficiency response in two contrasting chickpea cultivars

Potassium (K⁺) is an essential macronutrient for plant growth and development. K⁺ deficiency hampers important plant processes, such as enzyme activation, protein synthesis, photosynthesis and stomata movement. Molecular mechanism of K⁺ deficiency tolerance has been partly understood in model plants Arabidopsis, but its knowledge in legume crop chickpea is missing. Here, morphophysiological analysis revealed that among five high yielding desi chickpea cultivars, PUSA362 shows stunted plant growth, reduced primary root growth and low K⁺ content under K⁺ deficiency. In contrast, PUSA372 had negligible effect on these parameters suggesting that PUSA362 is K⁺ deficiency sensitive and PUSA372 is a K⁺ deficiency tolerant chickpea cultivar. RNA-seq based transcriptome analysis under K⁺ deficiency revealed a total of 820 differential expressed genes (DEG's) in PUSA362 and 682 DEGs in PUSA372. These DEGs belongs to different functional categories, such as plant metabolism, signal transduction components, transcription factors, ion/nutrient transporters, phytohormone biosynthesis and signalling, and root growth and development. RNA-seq expression of randomly selected 16 DEGs was validated by RT-qPCR. Out of 16 genes, 13 showed expression pattern similar to RNA-seq expression, that verified the RNA-seq expression data. Total 258 and 159 genes were exclusively up-regulated, and 386 and 347 genes were down-regulated, respectively in PUSA362 and PUSA372. 14 DEGs showed contrasting expression pattern as they were up-regulated in PUSA362 and down-regulated in PUSA372. These include somatic embryogenesis receptor-like kinase 1, thaumatin-like protein, ferric reduction oxidase 2 and transcription factor bHLH93. Nine genes which were down-regulated in PUSA362 found to be up-regulated in PUSA372, including glutathione S-transferase like, putative calmodulin-like 19, high affinity nitrate transporter 2.4 and ERF17-like protein. Some important carbohydrate metabolism related genes, like fructose-1,6-bisphosphatase and sucrose synthase, and root growth related Expansin gene were exclusively down-regulated, while an ethylene biosynthesis gene 1-aminocyclopropane-1-carboxylate oxidase 1 (ACO1) was up-regulated in PUSA362. Interplay of these and several other genes related to hormones (auxin, cytokinin, GA etc.), signal transduction components (like CBLs and CIPKs), ion transporters and transcription factors might underlie the contrasting response of two chickpea cultivars to K⁺ deficiency. In future, some of these key genes will be utilized in genetic engineering and breeding programs for developing chickpea cultivars with improved K⁺ use efficiency (KUE) and K⁺ deficiency tolerance traits.

Genome-Wide Identification and Expression Profiling of Potassium Transport-Related Genes in Vigna radiata under Abiotic Stresses

Potassium (K+) is one of the most important cations that plays a significant role in plants and constitutes up to 10% of plants' dry weight. Plants exhibit complex systems of transporters and channels for the distribution of K+ from soil to numerous parts of plants. In this study, we have identified 39 genes encoding putative K+ transport-related genes in Vigna radiata. Chromosomal mapping of these genes indicated an uneven distribution across eight out of 11 chromosomes. Comparative phylogenetic analysis of different plant species, i.e., V. radiata, Glycine max, Cicer arietinum, Oryza sativa, and Arabidopsis thaliana, showed their strong conservation in different plant species. Evolutionary analysis of these genes suggests that gene duplication is a major route of expansion for this family in V. radiata. Comprehensive promoter analysis identified several abiotic stresses related to cis-elements in the promoter regions of these genes, suggesting their role in abiotic stress tolerance. Our additional analyses indicated that abiotic stresses adversely affected the chlorophyll concentration, carotenoids, catalase, total soluble protein concentration, and the activities of superoxide and peroxidase in V. radiata. It also disturbs the ionic balance by decreasing the uptake of K+ content and increasing the uptake of Na+. Expression analysis from high-throughput sequencing data and quantitative real-time PCR experiments revealed that several K+ transport genes were expressed in different tissues (seed, flower, and pod) and in abiotic stress-responsive manners. A highly significant variation of expression was observed for VrHKT (1.1 and 1.2), VrKAT (1 and 2) VrAKT1.1, VrAKT2, VrSKOR, VrKEA5, VrTPK3, and VrKUP/HAK/KT (4, 5, and 8.1) in response to drought, heat or salinity stress. It reflected their potential roles in plant growth, development, or stress adaptations. The present study gives an in-depth understanding of K+ transport system genes in V. radiata and will serve as a basis for a functional analysis of these genes.

An endoplasmic reticulum-localized cytochrome b 5 regulates high-affinity K+ transport in response to salt stress in rice

High-affinity K⁺ (HAK) transporter-mediated K⁺ uptake has an important role when plants are subjected to stresses. This work identifies a mechanism of HAK regulation. The affinity of HAK at the plasma membrane for K⁺ depends on the binding of a cytochrome (CYB5) protein at the endoplasmic reticulum. This improves K⁺ uptake and the ability of plants to survive under saline conditions. The HAK–CYB5 interaction not only constitutes a mechanism of HAK regulation but also reflects interorganelle communication mediated by functional protein interactions under conditions of stress.

Potassium (K⁺) is an essential element for growth and development in both animals and plants, while high levels of environmental sodium (Na⁺) represent a threat to most plants. The uptake of K⁺ from high-saline environments is an essential mechanism to maintain intracellular K⁺/Na⁺ homeostasis, which can help reduce toxicity caused by Na⁺ accumulation, thereby improving the salt tolerance of plants. However, the mechanisms and regulation of K⁺-uptake during salt stress remain poorly understood. In this study, we identified an endoplasmic reticulum–localized cytochrome b₅ (OsCYB5-2) that interacted with a high-affinity K⁺ transporter (OsHAK21) at the plasma membrane. The association of OsCYB5-2 with the OsHAK21 transporter caused an increase in transporter activity by enhancing the apparent affinity for K⁺-binding but not Na⁺-binding. Heme binding to OsCYB5-2 was essential for the regulation of OsHAK21. High salinity directly triggered the OsHAK21–OsCYB5-2 interaction, promoting OsHAK21-mediated K⁺-uptake and restricting Na⁺ entry into cells; this maintained intracellular K⁺/Na⁺ homeostasis in rice cells. Finally, overexpression of OsCYB5-2 increased OsHAK21-mediated K⁺ transport and improved salt tolerance in rice seedlings. This study revealed a posttranslational regulatory mechanism for HAK transporter activity mediated by a cytochrome b₅ and highlighted the coordinated action of two proteins to perceive Na⁺ in response to salt stress.

Nutrient cycling is an important mechanism for homeostasis in plant cells

Homeostasis in living cells refers to the steady state of internal, physical, and chemical conditions. It is sustained by self-regulation of the dynamic cellular system. To gain insight into the homeostatic mechanisms that maintain cytosolic nutrient concentrations in plant cells within a homeostatic range, we performed computational cell biology experiments. We mathematically modeled membrane transporter systems and simulated their dynamics. Detailed analyses of 'what-if' scenarios demonstrated that a single transporter type for a nutrient, irrespective of whether it is a channel or a cotransporter, is not sufficient to calibrate a desired cytosolic concentration. A cell cannot flexibly react to different external conditions. Rather, at least two different transporter types for the same nutrient, which are energized differently, are required. The gain of flexibility in adjusting a cytosolic concentration was accompanied by the establishment of energy-consuming cycles at the membrane, suggesting that these putatively "futile" cycles are not as futile as they appear. Accounting for the complex interplay of transporter networks at the cellular level may help design strategies for increasing nutrient use efficiency of crop plants.

The rectification control and physiological relevance of potassium channel OsAKT2

AKT2 potassium (K+) channels are members of the plant Shaker family which mediate dual-directional K+ transport with weak voltage-dependency. Here we show that OsAKT2 of rice (Oryza sativa) functions mainly as an inward rectifier with strong voltage-dependency and acutely suppressed outward activity. This is attributed to the presence of a unique K191 residue in the S4 domain. The typical bi-directional leak-like property was restored by a single K191R mutation, indicating that this functional distinction is an intrinsic characteristic of OsAKT2. Furthermore, the opposite R195K mutation of AtAKT2 changed the channel to an inward-rectifier similar to OsAKT2. OsAKT2 was modulated by OsCBL1/OsCIPK23, evoking the outward activity and diminishing the inward current. The physiological relevance in relation to the rectification diversity of OsAKT2 was addressed by functional assembly in the Arabidopsis (Arabidopsis thaliana) akt2 mutant. Overexpression (OE) of OsAKT2 complemented the K+ deficiency in the phloem sap and leaves of the mutant plants but did not significantly contribute to the transport of sugars. However, the expression of OsAKT2-K191R overcame both the shortage of phloem K+ and sucrose of the akt2 mutant, which was comparable to the effects of the OE of AtAKT2, while the expression of the inward mutation AtAKT2-R195K resembled the effects of OsAKT2. Additionally, OE of OsAKT2 ameliorated the salt tolerance of Arabidopsis.

Molecular insights into the role of plant transporters in salt stress response

Salt stress disturbs the cellular osmotic and ionic balance, which then creates a negative impact on plant growth and development. The Na⁺ and Cl^- ions can enter into plant cells through various membrane transporters, including specific and non-specific Na⁺ , K⁺ , and Ca²⁺ transporters. Therefore, it is important to understand Na⁺ and K⁺ transport mechanisms in plants along with the isolation of genes, their characterization, the structural features, and their post-translation regulation under salt stress. This review summarizes the molecular insights of plant ion transporters, including non-selective cation transporters, cyclic nucleotide-gated cation transporters, glutamate-like receptors, membrane intrinsic proteins, cation proton antiporters, and sodium proton antiporter families. Further, we discussed the K⁺ transporter families such as high-affinity K⁺ transporters, HAK/KUP/KT transporters, shaker type K⁺ transporters, and K⁺ efflux antiporters. Besides the ion transport process, we have shed light on available literature on epigenetic regulation of transport processes under salt stress. Recent advancements of salt stress sensing mechanisms and various salt sensors within signaling transduction pathways are discussed. Further, we have compiled salt-stress signaling pathways, and their crosstalk with phytohormones.

Essential Role of Potassium in Apple and Its Implications for Management of Orchard Fertilization

K (K) is of paramount importance for apple (Malus × domestica Borkh.), not only for tree growth and development but also for the size and quality of fruit yield. The apple plant's demand for K varies, along with the progression of phenological phases, during the growing season. The K demand peaks during ripening of fruits featuring relatively high concentration of K comparable to that of the leaves. The mainstream method of apple tree K fertilization is through application of the fertilizer to the soils to improve K uptake by the roots. The bioavailability of K depends on assorted various factors, including pH, interaction with other nutrients in soil solution, temperature, and humidity. An important role in making the K from soil available for uptake by plants is played by plant growth-promoting microorganisms (PGPM), and the specific role of the PGPM is discussed. Advantages of fertigation (the combination of irrigation and fertilization) as an approach include allowing to balance application rate of K fertilizer against its variable demand by plants during the growing season. Excess K in the soil leads to competitive inhibition of calcium uptake by plants. The K-dependent deficiency of Ca leads to its predominant channeling to the leaves and hence to its decline in fruits. Consequently, the apple fruits affected by the K/Ca imbalance frequently develop physiological disorders in storage. This emphasizes the importance of the balanced K application, especially during the last months of the growing season, depending on the crop load and the actual K demand. The potential use of modern approaches to automated crop load estimation through machine vision for adjustment of K fertilization is underlined.

A Comparison of the Effect of Lead (Pb) on the Slow Vacuolar (SV) and Fast Vacuolar (FV) Channels in Red Beet (Beta vulgaris L.) Taproot Vacuoles

Little is known about the effect of lead on the activity of the vacuolar K+ channels. Here, the patch-clamp technique was used to compare the impact of lead (PbCl2) on the slow-activating (SV) and fast-activating (FV) vacuolar channels. It was revealed that, under symmetrical 100-mM K+, the macroscopic currents of the SV channels exhibited a typical slow activation and a strong outward rectification of the steady-state currents, while the macroscopic currents of the FV channels displayed instantaneous currents, which, at the positive potentials, were about three-fold greater compared to the one at the negative potentials. When PbCl2 was added to the bath solution at a final concentration of 100 µM, it decreased the macroscopic outward currents of both channels but did not change the inward currents. The single-channel recordings demonstrated that cytosolic lead causes this macroscopic effect by a decrease of the single-channel conductance and decreases the channel open probability. We propose that cytosolic lead reduces the current flowing through the SV and FV channels, which causes a decrease of the K+ fluxes from the cytosol to the vacuole. This finding may, at least in part, explain the mechanism by which cytosolic Pb2+ reduces the growth of plant cells.

The Landscape of Alternative Splicing Regulating Potassium Use Efficiency in Nicotiana tabacum

Alternative splicing (AS) occurs extensively in eukaryotes as an essential mechanism for regulating transcriptome complexity and diversity, but the AS landscape regulating potassium (K) use efficiency in plants is unclear. In this study, we performed high-throughput transcriptome sequencing of roots and shoots from allopolyploid Nicotiana tabacum under K⁺ deficiency. Preliminary physiological analysis showed that root system architecture was dramatically changed due to potassium deficiency and that IAA content was significantly reduced in root and shoot. AS analysis showed that a total of 28,179 genes exhibited 54,457 AS events, and 1,510 and 1,732 differentially alternatively spliced (DAS) events were identified in shoots and roots under low K⁺ stress. Nevertheless, only 120 DAS events occurred in both shoots and roots, implying that most DAS events were tissue-specific. Both in shoot and the root, the proportion of DAS genes in differentially expressed (DE) genes equaled that in non-DE genes, which indicated that AS might play a unique regulatory role in response to low potassium. Gene ontology analysis further indicated that transcription regulation and AS modulation worked independently in response to low K⁺ stress in tobacco, as their target biological processes were different. Totally 45 DAS transcription factors (TFs) were found, which were involved in 18 TF families. Five Auxin response factor (ARF) TFs were significantly DAS in root, suggesting that response to auxin was probably subject to AS regulation in the tobacco root. Our study shows that AS variation occurs extensively and has a particular regulatory mechanism under K⁺ deficiency in tobacco. The study also links changes in root system architecture with the changes in AS of ARF TFs, which implied the functional significance of these AS events for root growth and architecture.

Benefits provided by four ectomycorrhizal fungi to Pinus taeda under different external potassium availabilities

Ectomycorrhizal fungi contribute to the nutrition of many woody plants, including those in the Pinaceae family. Loblolly pine (Pinus taeda L.), a native species of the Southeastern USA, can be colonized by multiple species of ectomycorrhizal fungi. The role of these symbionts in P. taeda potassium (K⁺) nutrition has not been previously investigated. Here, we assessed the contribution of four ectomycorrhizal fungi, Hebeloma cylindrosporum, Paxillus ammoniavirescens, Laccaria bicolor, and Suillus cothurnatus, in P. taeda K⁺ acquisition under different external K⁺ availabilities. Using a custom-made two-compartment system, P. taeda seedlings were inoculated with one of the four fungi, or kept non-colonized, and grown under K⁺-limited or -sufficient conditions for 8 weeks. Only the fungi had access to separate compartments in which rubidium, an analog tracer for K⁺, was supplied before harvest. Resulting effects of the fungi were recorded, including root colonization, biomass, and nutrient concentrations. We also analyzed the fungal performance in axenic conditions under varying supply of K⁺ and sodium. Our study revealed that these four ectomycorrhizal fungi are differentially affected by external K⁺ and sodium variations, that they are not able to provide similar benefits to the host P. taeda in our growing conditions, and that rubidium may be used with some limitations to estimate K⁺ transport from ectomycorrhizal fungi to colonized plants.

The Arabidopsis protein NPF6.2/NRT1.4 is a plasma membrane nitrate transporter and a target of protein kinase CIPK23

Nitrate and potassium nutrition is tightly coordinated in vascular plants. Physiological and molecular genetics studies have demonstrated that several NPF/NRT1 nitrate transporters have a significant impact on both uptake and the root-shoot partition of these nutrients. However, how these traits are biochemically connected remain controversial since some NPF proteins, e.g. NPF7.3/NRT1.5, have been suggested to mediate K⁺/H⁺ exchange instead of nitrate fluxes. Here we show that NPF6.2/NRT1.4, a protein that gates nitrate accumulation at the leaf petiole of Arabidopsis thaliana, also affects the root/shoot distribution of potassium. We demonstrate that NPF6.2/NRT1.4 is a plasma membrane nitrate transporter phosphorylated at threonine-98 by the CIPK23 protein kinase that is a regulatory hub for nitrogen and potassium nutrition. Heterologous expression of NPF6.2/NRT1.4 and NPF7.3/NRT1.5 in yeast mutants with altered potassium uptake and efflux systems showed no evidence of nitrate-dependent potassium transport by these proteins.

Disruption of AtHAK/KT/KUP9 enhances plant cesium accumulation under low potassium supply

Understanding the molecular mechanisms that underlie cesium (Cs⁺ ) transport in plants is important to limit the entry of its radioisotopes from contaminated areas into the food chain. The potentially toxic element Cs⁺ , which is not involved in any biological process, is chemically closed to the macronutrient potassium (K⁺ ). Among the multiple K⁺ carriers, the high-affinity K⁺ transporters family HAK/KT/KUP is thought to be relevant in mediating opportunistic Cs⁺ transport. Of the 13 KUP identified in A. thaliana, only HAK5, the major contributor to root K⁺ acquisition under low K⁺ supply, has been functionally demonstrated to be involved in Cs⁺ uptake in planta. In the present study, we showed that accumulation of Cs⁺ increased by up to 30% in two A. thaliana mutant lines lacking KUP9 and grown under low K⁺ supply. Since further experiments revealed that Cs⁺ release from contaminated plants to the external medium is proportionally lower in the two kup9 mutant alleles, we proposed that KUP9 disruption could impair Cs⁺ efflux. By contrast, K⁺ status in kup9 mutants is not affected, suggesting that KUP9 disruption does not alter substantially K⁺ transport in experimental conditions used. The putative primary role of KUP9 in plants is further discussed.

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.

Molecular basis of plasma membrane H+-ATPase function and potential application in the agricultural production

Increase of crop yield is always the desired goal, manipulation of genes in relation to plant growth is a shortcut to promote crop yield. The plasma membrane (PM) H⁺-ATPase is the plant master enzyme; the energy yielded by ATP hydrolysis pumps H⁺ out of cells, establishes the membrane potential, maintains pH homeostasis and provides the proton-motive force required for transmembrane transport of many materials. PM H⁺-ATPase is involved in root nutrient uptake, epidermal stomatal opening, phloem sucrose loading and unloading, and hypocotyl cell elongation. In this review, we summarize the recent progresses in roles of PM H⁺-ATPase in nutrient uptake and light-induced stomatal opening and discuss the pivotal role of PM H⁺-ATPase in crop yield improvement and its potential application in agricultural production by modulating the expression of PM H⁺-ATPase in crops.

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

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

Plant Tolerance to Drought Stress in the Presence of Supporting Bacteria and Fungi: An Efficient Strategy in Horticulture

Increasing temperature leads to intensive water evaporation, contributing to global warming and consequently leading to drought stress. These events are likely to trigger modifications in plant physiology and microbial functioning due to the altered availability of nutrients. Plants exposed to drought have developed different strategies to cope with stress by morphological, physiological, anatomical, and biochemical responses. First, visible changes influence plant biomass and consequently limit the yield of crops. The presented review was undertaken to discuss the impact of climate change with respect to drought stress and its impact on the performance of plants inoculated with plant growth-promoting microorganisms (PGPM). The main challenge for optimal performance of horticultural plants is the application of selected, beneficial microorganisms which actively support plants during drought stress. The most frequently described biochemical mechanisms for plant protection against drought by microorganisms are the production of phytohormones, antioxidants and xeroprotectants, and the induction of plant resistance. Rhizospheric or plant surface-colonizing (rhizoplane) and interior (endophytic) bacteria and fungi appear to be a suitable alternative for drought-stress management. Application of various biopreparations containing PGPM seems to provide hope for a relatively cheap, easy to apply and efficient way of alleviating drought stress in plants, with implications in productivity and food condition.

The CBL1/9-CIPK23-AKT1 complex is essential for low potassium response in cassava

Cassava is a food crop and an important energy crop worldwide. However, its yield and quality are easily affected by low K+ stress, and the molecular mechanism of potassium channel is unknown in cassava. Herein, we revealed that calcineurin B-like 1/9 (MeCBL1/9)-CBL-interacting protein kinase 23 (MeCIPK23)-K+ TRANSPORTER1 (MeAKT1) complex plays an important role in low potassium response in cassava. Firstly, this study verified the in vivo role of MeAKT1 in K+ uptake in yeast. Secondly, we found that MeCBL1, MeCBL9, MeCIPK23 and MeAKT1 are involved in the absorption of K+ in cassava, and MeCBL1/9-CIPK23 complex is essential for MeAKT1-mediated K+ uptake. Moreover, MeCBL1/9-MeCIPK23-MeAKT1 showed different expression in different cassava varieties contrasting in the resistance to low K+ stress. Taken together, this study provides new insights into further improvement of K+ uptake in cassava.

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

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

Physiological and Biochemical Response of Wild Olive (Olea europaea Subsp. europaea var. sylvestris) to Salinity

In the face of climate change, water deficit and increasing soil salinity pose an even greater challenge to olive cultivation in the Mediterranean basin. Due to its tolerance to abiotic stresses, wild olive (Olea europaea subsp. europaea var. sylvestris) presents a good candidate in breeding climate-resilient olive varieties. In this study, the early response of the native Croatian wild olive genotype (WOG) to salinity was evaluated and compared with that of well-known cultivars (cv.) Leccino and Koroneiki. Potted olive plants were exposed either to 150 mM NaCl or 300 mM mannitol for 3 weeks to distinguish between the osmotic and ionic components of salt stress. To determine the impact of the plant age on salinity, 1-, 2-, and 3-year-old WOG plants were used in the study. The growth parameters of both the cultivars and WOG of different ages decreased in response to the mannitol treatment. In contrast to cv. Leccino, the NaCl treatment did not significantly affect the growth of cv. Koroneiki or WOG of any age. The contents of Na⁺ and Cl^- were considerably higher in the salt-treated WOG, regardless of age, compared with the cultivars. However, while both treatments significantly reduced the K⁺ content of cv. Koroneiki, that nutrient was not significantly affected in either cv. Leccino or WOG. Unlike the cultivars and older WOG, the NaCl treatment caused a significant decline of photosynthetic pigments in the 1-year-old WOG. The cultivars and WOG of different ages experienced a similar drop in the chlorophyll a content under the isotonic mannitol treatment. The absence of lipid peroxidation, modulation of superoxide dismutase, and guaiacol peroxidase activity were noted in all WOG ages under both stressors. These data suggest that WOG resilience to salinity is associated with its large leaf capacity for Na⁺ and Cl^- accumulation, K⁺ retention, and its adaptable antioxidative mechanisms. The results are promising with regard to obtaining a new olive cultivar with better resilience to soil salinity.

K Deprivation Modulates the Primary Metabolites and Increases Putrescine Concentration in Brassica napus

Potassium (K) plays a crucial role in plant growth and development and is involved in different physiological and biochemical functions in plants. Brassica napus needs higher amount of nutrients like nitrogen (N), K, phosphorus (P), sulfur (S), and boron (B) than cereal crops. Previous studies in B. napus are mainly focused on the role of N and S or combined deficiencies. Hence, little is known about the response of B. napus to K deficiency. Here, a physiological, biochemical, and molecular analysis led us to investigate the response of hydroponically grown B. napus plants to K deficiency. The results showed that B. napus was highly sensitive to the lack of K. The lower uptake and translocation of K induced BnaHAK5 expression and significantly declined the growth of B. napus after 14 days of K starvation. The lower availability of K was associated with a decrease in the concentration of both S and N and modulated the genes involved in their uptake and transport. In addition, the lack of K induced an increase in Ca2+ and Mg2+ concentration which led partially to the accumulation of positive charge. Moreover, a decrease in the level of arginine as a positively charged amino acid was observed which was correlated with a substantial increase in the polyamine, putrescine (Put). Furthermore, K deficiency induced the expression of BnaNCED3 as a key gene in abscisic acid (ABA) biosynthetic pathway which was associated with an increase in the levels of ABA. Our findings provided a better understanding of the response of B. napus to K starvation and will be useful for considering the importance of K nutrition in this crop.

Linking genetic determinants with salinity tolerance and ion relationships in eggplant, tomato and pepper

The Solanaceae family includes commercially important vegetable crops characterized by their relative sensitivity to salinity. Evaluation of 8 eggplant (Solanum melongena), 7 tomato (Solanum lycopersicum), and 8 pepper (Capsicum spp.) heirloom cultivars from different geographic regions revealed significant variation in salt tolerance. Relative fruit yield under salt treatment varied from 52 to 114% for eggplant, 56 to 84% for tomato, and 52 to 99% for pepper. Cultivars from all three crops, except Habanero peppers, restricted Na transport from roots to shoots under salinity. The high salt tolerance level showed a strong association with low leaf Na concentration. Additionally, the leaf K-salinity/K-control ratio was critical in determining the salinity tolerance of a genotype. Differences in relative yield under salinity were regulated by several component traits, which was consistent with the gene expression of relevant genes. Gene expression analyses using 12 genes associated with salt tolerance showed that, for eggplant and pepper, Na⁺ exclusion was a vital component trait, while sequestration of Na⁺ into vacuoles was critical for tomato plants. The high variability for salt tolerance found in heirloom cultivars helped characterize genotypes based on component traits of salt tolerance and will enable breeders to increase the salt tolerance of Solanaceae cultivars.

Endogenous nutrients are concentrated in specific tissues in the Zea mays seedling

K, P, Cl, and Ca are distributed in tissue-specific patterns in Zea mays seedlings. These elements were mapped and analyzed using a relatively simple semi-quantitative technique, i.e., fast freezing, followed by freeze fracturing, then freeze drying, and finally scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS). In the radicle, endogenously derived (i.e., from seed) K and P transition from being homogenous in the apical meristem to tissue-specific in older regions. At 3 mm from the radicle apex, K concentration is approximately 40 mM in mid-cortex and decreases by approximately 50% at 15 mm. From 3 to 55 mm, P concentration in pericycle is approximately twice that found in adjacent regions. Ca is not detectable in younger portions of the radicle by SEM/EDS, but in older regions, it is present at 13 mM in mid-cortex. K concentration values of entire radicles analyzed with inductively coupled plasma optical emission spectrometry (ICP-OES) exceeded the SEM/EDS values. For Ca, the reverse was true. But, SEM/EDS analysis did not include several vascular tissues that contained high concentrations of K and low concentrations of Ca. The inception of lateral root primordia was accompanied by a localized decrease in Ca in cortical regions that were centrifugal to the primordium tip. A region of O-rich cells in endosperm was identified centripetal to the aleurone. These results indicate that (1) outer, mid-, and inner cortical regions, as well as the adjacent tissues, have distinct ion accumulation properties, and (2) ions are concentrated in some radicle tissues prior to development of Casparian strips.

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

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

Nutritional deficiency in scarlet eggplant limits its growth by modifying the absorption and use efficiency of macronutrients

The intensity damages caused by nutritional deficiency in growing plants can vary with nutrients. The effects caused by nutrient omission in the plant nutritional efficiency in relation to the absorption and use of the missing nutrient, and the reasons why these damages reflect in other nutrients have not yet been reported in the culture of scarlet eggplant. A better understanding of the nutritional mechanisms involved may clarify why certain nutrients cause greater limitations than other during plants growth. Thus, this study was designed with the aim of evaluating the damages caused by macronutrients deficiency in the culture of scarlet eggplant in the accumulation of these nutrients, nutritional deficiency, plants growth and in visual symptoms. The experiment was carried out in a controlled environment where plants were cultivated in a hydroponic system. Treatments consisted of supplying a complete Hoagland and Arnon solution (CS), and other nutrient solutions with individual omissions of nitrogen (-N), phosphorus (-P), potassium (-K), calcium (-Ca), magnesium (-Mg) and sulphur (-S). When a nutrient deficiency arose, nutritional analyses, growth and visual symptoms were analyzed. The omissions of N, S and K in the nutrient solution resulted in lower accumulation of all macronutrients in both the above and below ground biomass. Individual omissions resulted in nutritional imbalances with reflexes in the absorption efficiencies and use of the missing nutrient, as well as of other nutrients, revealing that the metabolism involves multiple nutritional interactions. Losses of nutritional efficiencies of macronutrients caused detrimental effects on plants growth, with reduced height, stem diameter, number of leaves, leaf area, and biomass production in above ground and below ground. From the losses in production in above ground biomass, the order of macronutrients limitation was N, S, K, Ca, Mg, and P, with reductions of 99, 96, 94, 76, 51 and 46%, respectively, in comparison to plants cultivated in CS. The most limiting nutrients were N, S, and K, seen that its deficiencies affected the metabolism of all other nutrients. This study demonstrates the importance of an adequate nutritional management of N, S, and K in the cultivation of scarlet eggplant.

Melatonin alleviates drought impact on growth and essential oil yield of lemon verbena by enhancing antioxidant responses, mineral balance, and abscisic acid content

Melatonin has recently emerged as a multifunctional biomolecule with promising aspects in plant stress tolerance. The present study examined the effects of foliar-sprayed melatonin (0, 100, and 200 μM) on growth and essential oil yield attributes of lemon verbena (Lippia citriodora) under water-shortage (mild, moderate and severe). Results revealed that melatonin minimized drought effects on lemon verbena, resulting in improved growth and essential oils yield. Drought impositions gradually and significantly reduced several growth parameters, such as plant height and biomass, whereas melatonin application revived the growth performance of lemon verbena. Melatonin protected the photosynthetic pigments and helped maintain the mineral balance at all levels of drought. Melatonin stimulated the accumulation of proline, soluble sugars and abscisic acid, which were positively correlated with a better preservation of leaf water status in drought-stressed plants. Melatonin also prevented oxidative damages by enhancing the superoxide dismutase, ascorbate peroxidase and catalase activities. Furthermore, increased levels of total phenolic compounds, chicoric acid, caffeic acid and chlorogenic acid, as well as ascorbate and total antioxidant capacity in melatonin-sprayed drought-stressed plants indicated that melatonin helped verbena plants to sustain antioxidant and medicinal properties during drought. Finally, melatonin treatments upheld the concentrations and yield of essential oils in the leaves of lemon verbena regardless of drought severities. These results provided new insights into melatonin-mediated drought tolerance in lemon verbena, and this strategy could be implemented for the successful cultivation of lemon verbena, and perhaps other medicinal plants, in drought-prone areas worldwide.

Root hairs: the villi of plants

Strikingly, evolution shaped similar tubular structures at the µm to mm scale in roots of sessile plants and in small intestines of mobile mammals to ensure an efficient transfer of essential nutrients from 'dead matter' into biota. These structures, named root hairs (RHs) in plants and villi in mammals, numerously stretch into the environment, and extremely enlarge root and intestine surfaces. They are believed to forage for nutrients, and mediate their uptake. While the conceptional understanding of plant RH function in hydromineral nutrition seems clear, experimental evidence presented in textbooks is restricted to a very limited number of reference-nutrients. Here, we make an element-by-element journey through the periodic table and link individual nutrient availabilities to the development, structure/shape and function of RHs. Based on recent developments in molecular biology and the identification of mutants differing in number, length or other shape-related characteristics of RHs in various plant species, we present comprehensive advances in (i) the physiological role of RHs for the uptake of specific nutrients, (ii) the developmental and morphological responses of RHs to element availability and (iii) RH-localized nutrient transport proteins. Our update identifies crucial roles of RHs for hydromineral nutrition, mostly under nutrient and/or water limiting conditions, and highlights the influence of certain mineral availabilities on early stages of RH development, suggesting that nutritional stimuli, as deficiencies in P, Mn or B, can even dominate over intrinsic developmental programs underlying RH differentiation.

Polyamine Metabolism in Scots Pine Embryogenic Cells under Potassium Deficiency

Polyamines (PA) have a protective role in maintaining growth and development in Scots pine during abiotic stresses. In the present study, a controlled liquid Scots pine embryogenic cell culture was used for studying the responses of PA metabolism related to potassium deficiency. The transcription level regulation of PA metabolism led to the accumulation of putrescine (Put). Arginine decarboxylase (ADC) had an increased expression trend under potassium deficiency, whereas spermidine synthase (SPDS) expression decreased. Generally, free spermidine (Spd) and spermine (Spm)/ thermospermine (t-Spm) contents were kept relatively stable, mostly by the downregulation of polyamine oxidase (PAO) expression. The low potassium contents in the culture medium decreased the potassium content of the cells, which inhibited cell mass growth, but did not affect cell viability. The reduced growth was probably caused by repressed metabolic activity and cell division, whereas there were no signs of H₂O₂-induced oxidative stress or increased cell death. The low intracellular content of K⁺ decreased the content of Na⁺. The decrease in the pH of the culture medium indicated that H⁺ ions were pumped out of the cells. Altogether, our findings emphasize the specific role(s) of Put under potassium deficiency and strict developmental regulation of PA metabolism in Scots pine.

Influence of Plant and Animal Proteins on Inflammation Markers among Adults with Chronic Kidney Disease: A Systematic Review and Meta-Analysis

Proteins, especially plant proteins, may reduce inflammation among adults with chronic kidney disease (CKD). This systematic review and meta-analysis were conducted to evaluate the effect protein types (animal or plant) have on inflammation markers (CRP, IL-6, TNF-α) among adults with varying stages of CKD. The Preferred Reporting Items for Systematic Review and Meta-analysis (PRISMA) was conducted to identify articles from inception until January 2021, utilizing six databases. Controlled trials that compared the effects of different protein types were analyzed using random-effects meta-analysis. Quality assessment and risk of bias of the included articles were assessed by using Cochrane risk of bias instrument and ROBINS-I. Out of the 10 studies that met the criteria, there was a decreasing trend in CRP levels when consuming plant proteins compared to animal proteins among non-dialysis participants. There was a statistically significant decrease when comparing animal proteins to unspecified proteins in CRP levels among dialysis participants [Hedges’ g = 2.11; 95% CI 1.12, 3.11; p ≤ 0.001], favoring unspecified proteins. Furthermore, animal proteins (eggs, red meat) showed increasing trends in CRP levels compared to whey protein isolate. Caution must be considered regarding these results as controlled, non-randomized, trials were included in the analysis, which may have contributed to high risk of bias. Future research should focus on protein types and the impact they have on kidney disease progression and inflammation markers.

The Potassium Transporter Hak1 in Candida Albicans, Regulation and Physiological Effects at Limiting Potassium and under Acidic Conditions

The three families of yeast plasma membrane potassium influx transporters are represented in Candida albicans: Trk, Acu, and Hak proteins. Hak transporters work as K⁺-H⁺ symporters, and the genes coding for Hak proteins are transcriptionally activated under potassium limitation. This work shows that C. albicans mutant cells lacking CaHAK1 display a severe growth impairment at limiting potassium concentrations under acidic conditions. This is the consequence of a defective capacity to transport K⁺, as indicated by potassium absorption experiments and by the kinetics parameters of Rb⁺ (K⁺) transport. Moreover, hak1- cells are more sensitive to the toxic cation lithium. All these phenotypes became much less robust or even disappeared at alkaline growth conditions. Finally, transcriptional studies demonstrate that the hak1- mutant, in comparison with HAK1+ cells, activates the expression of the K⁺/Na⁺ ATPase coded by CaACU1 in the presence of Na⁺ or in the absence of K⁺.

Insights into the mechanisms of transport and regulation of the arabidopsis high-affinity K+ transporter HAK51

The high-affinity K+ transporter HAK5 from Arabidopsis (Arabidopsis thaliana) is essential for K+ acquisition and plant growth at low micromolar K+ concentrations. Despite its functional relevance in plant nutrition, information about functional domains of HAK5 is scarce. Its activity is enhanced by phosphorylation via the AtCIPK23/AtCBL1-9 complex. Based on the recently published three-dimensionalstructure of the bacterial ortholog KimA from Bacillus subtilis, we have modeled AtHAK5 and, by a mutational approach, identified residues G67, Y70, G71, D72, D201, and E312 as essential for transporter function. According to the structural model, residues D72, D201, and E312 may bind K+, whereas residues G67, Y70, and G71 may shape the selective filter for K+, which resembles that of K+shaker-like channels. In addition, we show that phosphorylation of residue S35 by AtCIPK23 is required for reaching maximal transport activity. Serial deletions of the AtHAK5 C-terminus disclosed the presence of an autoinhibitory domain located between residues 571 and 633 together with an AtCIPK23-dependent activation domain downstream of position 633. Presumably, autoinhibition of AtHAK5 is counteracted by phosphorylation of S35 by AtCIPK23. Our results provide a molecular model for K+ transport and describe CIPK-CBL-mediated regulation of plant HAK transporters.