Polyamines prevent NaCl-induced K+ efflux from pea mesophyll by blocking non-selective cation channels

Outward-rectifying potassium channels GORK and SKOR function in regulation of root growth under salt stress in Arabidopsis thaliana

Plants often face high salinity as a significant environmental challenge with roots being the first responders to this stress. Maintaining K+/Na+ ratio within plant cells is crucial for survival, as the intracellular K+ level decreases and the intracellular Na+ level increases under saline conditions. However, knowledge about the molecular regulatory mechanisms of K+ loss in response to salt stress through outward-rectifying K+ channels in plants is largely unknown. In this study, we found that the Arabidopsis double mutant gorkskor, in which the GORK and SKOR genes are disrupted, showed an improved primary root growth under salt stress compared to wild-type (WT) and the gork and skor single-mutant plants. No significant differences in the sensitivity to mannitol stress between the WT and gorkskor mutant were observed. Accumulation of ROS induced by salt stress was reduced in the gorkskor roots. The gorkskor mutant seedlings had significantly higher K+ content, lower Na+ content, and a greater resultant K+/Na+ ratio than the WT under salt stress. Moreover, salt-stress-induced elevation of cytosolic free Ca2+ concentration was reduced in the gorkskor roots. Taken together, these results suggest that Arabidopsis Shaker-type outward-rectifying K+ channels GORK and SKOR may redundantly function in regulation of primary root growth under salt stress and are involved in not only the late-stage response (e.g. K+ leakage) but also the early response including ROS production and [Ca2+]cyt elevation.

Impacts of salinity stress on crop plants: improving salt tolerance through genetic and molecular dissection

Improper use of water resources in irrigation that contain a significant amount of salts, faulty agronomic practices such as improper fertilization, climate change etc. are gradually increasing soil salinity of arable lands across the globe. It is one of the major abiotic factors that inhibits overall plant growth through ionic imbalance, osmotic stress, oxidative stress, and reduced nutrient uptake. Plants have evolved with several adaptation strategies at morphological and molecular levels to withstand salinity stress. Among various approaches, harnessing the crop genetic variability across different genepools and developing salinity tolerant crop plants offer the most sustainable way of salt stress mitigation. Some important major genetic determinants controlling salinity tolerance have been uncovered using classical genetic approaches. However, its complex inheritance pattern makes breeding for salinity tolerance challenging. Subsequently, advances in sequence based breeding approaches and functional genomics have greatly assisted in underpinning novel genetic variants controlling salinity tolerance in plants at the whole genome level. This current review aims to shed light on physiological, biochemical, and molecular responses under salt stress, defense mechanisms of plants, underlying genetics of salt tolerance through bi-parental QTL mapping and Genome Wide Association Studies, and implication of Genomic Selection to breed salt tolerant lines.

Interactions of Polyamines and Phytohormones in Plant Response to Abiotic Stress

Numerous environmental conditions negatively affect plant production. Abiotic stresses, such as salinity, drought, temperature, and heavy metals, cause damage at the physiological, biochemical, and molecular level, and limit plant growth, development, and survival. Studies have indicated that small amine compounds, polyamines (PAs), play a key role in plant tolerance to various abiotic stresses. Pharmacological and molecular studies, as well as research using genetic and transgenic approaches, have revealed the favorable effects of PAs on growth, ion homeostasis, water maintenance, photosynthesis, reactive oxygen species (ROS) accumulation, and antioxidant systems in many plant species under abiotic stress. PAs display a multitrack action: regulating the expression of stress response genes and the activity of ion channels; improving the stability of membranes, DNA, and other biomolecules; and interacting with signaling molecules and plant hormones. In recent years the number of reports indicating crosstalk between PAs and phytohormones in plant response to abiotic stresses has increased. Interestingly, some plant hormones, previously known as plant growth regulators, can also participate in plant response to abiotic stresses. Therefore, the main goal of this review is to summarize the most significant results that represent the interactions between PAs and plant hormones, such as abscisic acid, brassinosteroids, ethylene, jasmonates, and gibberellins, in plants under abiotic stress. The future perspectives for research focusing on the crosstalk between PAs and plant hormones were also discussed.

Spermine-Salicylic Acid Interplay Restrains Salt Toxicity in Wheat (Triticum aestivum L.)

Spermine (SPM) and salicylic acid (SA) are plant growth regulators, eliciting specific responses against salt toxicity. In this study, the potential role of 30 mgL^-1 SPM and/or 100 mgL^-1 SA in preventing salt damage was investigated. Wheat plants were grown under non-saline or saline conditions (6.0 and 12.0 dS m^-1) with and without SA and/or SPM foliar applications. Exogenously applied SA and/or SPM alleviated the inhibition of plant growth and productivity under saline conditions by increasing Calvin cycle enzyme activity. Foliage applications also improved ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase activities, which effectively scavenged hydrogen peroxide and superoxide radicals in stressed plants. Furthermore, foliar treatments increased antioxidants such as ascorbate and glutathione, which effectively detoxified reactive oxygen species (ROS). Exogenous applications also increased N, P, and K⁺ acquisition, roots' ATP content, and H⁺-pump activity, accompanied by significantly lower Na⁺ accumulation in stressed plants. Under saline environments, exogenous SA and/or SPM applications raised endogenous SA and SPM levels. Co-application of SA and SPM gave the best response. The newly discovered data suggest that the increased activities of Calvin cycle enzymes, root H⁺-pump, and antioxidant defense machinery in treated plants are a mechanism for salt tolerance. Therefore, combining the use of SA and SPM can be a superior method for reducing salt toxicity in sustainable agricultural systems.

Plant Growth Stimulators Improve Two Wheat Cultivars Salt-Tolerance: Insights into Their Physiological and Nutritional Responses

Spermine (SPM) and salicylic acid (SA), plant growth stimulators, are involved in various biological processes and responses to environmental cues in plants. However, the function of their combined treatment on wheat salt tolerance is unclear. In this study, wheat (Triticum aestivum L. cvs. Shandawel 1 and Sids 14) plants were grown under non-saline and saline (6.0 and 12.0 dS m^-1) conditions and were foliar sprayed with 100 mgL^-1 SA and/or 30 mgL^-1 SPM. Exogenously applied SA and/or SPM relieved the adverse effects caused by salt stress and significantly improved wheat growth and production by inducing higher photosynthetic pigment (chlorophyll a, chlorophyll b, carotenoids) content, nutrient (N, P, K⁺, Ca²⁺, Mg²⁺, Fe, Zn, Cu) acquisition, ionic (K⁺/Na⁺, Ca²⁺/Na⁺, Mg²⁺/Na⁺) homeostatics, osmolyte (soluble sugars, free amino acids, proline, glycinebetaine) accumulation, protein content, along with significantly lower Na⁺ accumulation and chlorophyll a/b ratio. The best response was registered with SA and SPM combined treatment, especially in Shandawel 1. This study highlighted the recovery impact of SA and SPM combined treatment on salinity-damaged wheat plants. The newly discovered data demonstrate that this treatment significantly improved the photosynthetic pigment content, mineral homeostasis, and osmoprotector solutes buildup in salinity-damaged wheat plants. Therefore, it can be a better strategy for ameliorating salt toxicity in sustainable agricultural systems.

Transcriptome and Metabonomic Analysis of Tamarix ramosissima Potassium (K+) Channels and Transporters in Response to NaCl Stress

Potassium ion (K+) channels and transporters are key components of plant K+ absorption and transportation and play an important role in plant growth and development. This study revealed that K+ channels and transporters are involved in the salt tolerance molecular mechanism and metabolites of the halophyte representative plant Tamarix ramosissima (T. ramosissima) in response to NaCl stress, providing a theoretical basis for the mitigation of salt stress using halophytes. Through transcriptome sequencing and metabolite detection analysis of 0 h, 48 h and 168 h by applying exogenous K+ to the roots of T. ramosissima under NaCl stress, 15 high-quality Clean Data bases were obtained, Q20 reached more than 97%, Q30 reached more than 92%, and GC content reached 44.5%, which is in line with further bioinformatics analysis. Based on the Liquid chromatography−mass spectrometry (LC-MS) analysis, the roots of T. ramosissima were exposed to exogenous potassium for 48 h and 168 h under NaCl stress, and 1510 and 1124 metabolites were identified in positive and negative ion mode, respectively. Through orthogonal projections to latent structures discriminant analysis (OPLS-DA) model analysis, its metabolomic data have excellent predictability and stability. The results of this study showed that there were 37 differentially expressed genes (DEGs) annotated as Class 2 K+ channels (Shaker-like K+ channel and TPK channel) and Class 3 K+ transporters (HAK/KUP/KT, HKT and CPAs transporter families). Among them, 29 DEGs were annotated to the gene ontology (GO) database, and the most genes were involved in the GO Biological Process. In addition, the expression levels of Unigene0014342 in the HAK/KUP/KT transporter and Unigene0088276 and Unigene0103067 in the CPAs transporter both first decreased and then increased when treated with 200 mM NaCl for 48 h and 168 h. However, when treated with 200 mM NaCl + 10 mM KCl for 48 h and 168 h, a continuous upward trend was shown. Notably, the expression level of Unigene0016813 in CPAS transporter continued to increase when treated with 200 mM NaCl and 200 mM NaCl + 10 mM KCl for 48 h and 168 h. 3 DEGs, Unigene0088276, Unigene0016813 and Unigene0103067, were dominated by the positive regulation of their related metabolites, and this correlation was significant. The results showed that these DEGs increased the absorption of K+ and the ratio of K+/Na+ under NaCl stress at 48 h and 168 h after adding exogenous potassium and enhanced the salt tolerance of T. ramosissima. Notably, the expression level of Unigene0103067 in the CPAs transporter was consistently upregulated when 200 mM NaCl + 10 mM KCl was treated for 48 h and 168 h. The positive regulatory metabolites were always dominant, which better helped T. ramosissima resist salt stress. Unigene0103067 plays an important role in enhancing the salt tolerance of T. ramosissima and reducing the toxicity of NaCl in roots. Additionally, phylogenetic tree analysis showed that Unigene0103067 and Reaumuria trigyna had the closest genetic distance in the evolutionary relationship. Finally, 9 DEGs were randomly selected for quantitative real-time PCR (qRT-PCR) verification. Their expression trends were completely consistent with the transcriptome sequencing analysis results, proving that this study’s data are accurate and reliable. This study provides resources for revealing the molecular mechanism of NaCl stress tolerance in T. ramosissima and lays a theoretical foundation for cultivating new salt-tolerant varieties.

Polyamine oxidase 3 is involved in salt tolerance at the germination stage in rice

Soil salinity inhibits seed germination and reduces seedling survival rate, resulting in significant yield reductions in crops. Here, we report the identification of a polyamine oxidase, OsPAO3, conferring salt tolerance at the germination stage in rice (Oryza sativa L.), through map-based cloning approach. OsPAO3 is up-regulated under salt stress at the germination stage and highly expressed in various organs. Overexpression of OsPAO3 increases activity of polyamine oxidases, enhancing the polyamine content in seed coleoptiles. Increased polyamine may lead to the enhance of the activity of ROS-scavenging enzymes to eliminate over-accumulated H2O2 and to reduce Na+ content in seed coleoptiles to maintain ion homeostasis and weaken Na+ damage. These changes resulted in stronger salt tolerance at the germination stage in rice. Our findings not only provide a unique gene for breeding new salt-tolerant rice cultivars but also help to elucidate the mechanism of salt tolerance in rice.

Comparative Physiology of Indica and Japonica Rice under Salinity and Drought Stress: An Intrinsic Study on Osmotic Adjustment, Oxidative Stress, Antioxidant Defense and Methylglyoxal Detoxification

Salinity and drought stress are significant environmental threats, alone or in combination. The current study was conducted to investigate the morpho-physiology, osmotic adjustment, oxidative stress, antioxidant defense and methylglyoxal detoxification of three rice genotypes from the indica (cv. BRRI dhan29 and BRRI dhan48) and japonica (cv. Koshihikari) groups. Eighteen-day-old seedlings of these genotypes were exposed to either in alone salinity (150 mM NaCl) and drought (15% PEG 6000) or in the combination of salinity and drought (150 mM NaCl + 15% PEG 6000) stress in vitro for 72 h. Compared with the control, the water status, biomass and photosynthetic pigments were decreased, where a significant increase was seen in the mortality rate, hydrogen peroxide content, electrolyte leakage, lipoxygenase activity, level of malondialdehyde and methylglyoxal, indicating increased lipid peroxidation in rice genotypes in stress conditions. The non-enzymatic and enzymatic components of the ascorbate-glutathione (AsA-GSH) pool in rice genotypes were disrupted under all stress treatments, resulting imbalance in the redox equilibrium. In contrast, compared to other rice genotypes, BRRI dhan48 revealed a lower Na+/K+ ratio, greater proline (Pro) levels, higher activity of AsA, dehydroascorbate (DHA) and GSH, lower glutathione disulfide (GSSG) and a higher ratio of AsA/DHA and GSH/GSSG, whereas enzymatic components increased monodehydroascorbate reductase, dehydroascorbate reductase, glutathione peroxidase and glyoxalase enzymes. The results showed that a stronger tolerate ability for BRRI dhan48 against stress has been connected to a lower Na+/K+ ratio, an increase in Pro content and an improved performance of the glyoxalase system and antioxidant protection for scavenging of reactive oxygen species. These data can give insight into probable responses to single or combination salinity and drought stress in rice genotypes.

Male Fertility under Environmental Stress: Do Polyamines Act as Pollen Tube Growth Protectants?

Although pollen structure and morphology evolved toward the optimization of stability and fertilization efficiency, its performance is affected by harsh environmental conditions, e.g., heat, cold, drought, pollutants, and other stressors. These phenomena are expected to increase in the coming years in relation to predicted environmental scenarios, contributing to a rapid increase in the interest of the scientific community in understanding the molecular and physiological responses implemented by male gametophyte to accomplish reproduction. Here, after a brief introduction summarizing the main events underlying pollen physiology with a focus on polyamine involvement in its development and germination, we review the main effects that environmental stresses can cause on pollen. We report the most relevant evidence in the literature underlying morphological, cytoskeletal, metabolic and signaling alterations involved in stress perception and response, focusing on the final stage of pollen life, i.e., from when it hydrates, to pollen tube growth and sperm cell transport, with these being the most sensitive to environmental changes. Finally, we hypothesize the molecular mechanisms through which polyamines, well-known molecules involved in plant development, stress response and adaptation, can exert a protective action against environmental stresses in pollen by decoding the essential steps and the intersection between polyamines and pollen tube growth mechanisms.

Electrical Signaling of Plants under Abiotic Stressors: Transmission of Stimulus-Specific Information

Plants have developed complex systems of perception and signaling to adapt to changing environmental conditions. Electrical signaling is one of the most promising candidates for the regulatory mechanisms of the systemic functional response under the local action of various stimuli. Long-distance electrical signals of plants, such as action potential (AP), variation potential (VP), and systemic potential (SP), show specificities to types of inducing stimuli. The systemic response induced by a long-distance electrical signal, representing a change in the activity of a complex of molecular-physiological processes, includes a nonspecific component and a stimulus-specific component. This review discusses possible mechanisms for transmitting information about the nature of the stimulus and the formation of a specific systemic response with the participation of electrical signals induced by various abiotic factors.

The role of ion-transporting proteins in the evolution of salt tolerance in charophyte algae

Species within the genus Chara have variable salinity tolerance. Their close evolutionary relationship with embryophytes makes their study crucial to understanding the evolution of salt tolerance and key evolutionary processes shared among the phyla. We examined salt-tolerant Chara longifolia and salt-sensitive Chara australis for mechanisms of salt tolerance and their potential role in adaptation to salt. We hypothesize that there are shared mechanisms similar to those in embryophytes, which assist in conferring salt tolerance in Chara, including a cation transporter (HKT), a Na⁺ /H⁺ antiport (NHX), a H⁺ -ATPase (AHA), and a Na⁺ -ATPase (ENA). Illumina transcriptomes were created using cultures grown in freshwater and exposed to salt stress. The presence of these candidate genes, identified by comparing with genes known from embryophytes, has been confirmed in both species of Chara, with the exception of ENA, present only in salt-tolerant C. longifolia. These transcriptomes provide evidence for the contribution of these mechanisms to differences in salt tolerance in the two species and for the independent evolution of the Na⁺ -ATPase. We also examined genes that may have played a role in important evolutionary processes, suggested by previous work on the Chara braunii genome. Among the genes examined, cellulose synthase protein (GT43) and response regulator (RRB) were confirmed in both species. Genes absent from all three Chara species were members of the GRAS family, microtubule-binding protein (TANGLED1), and auxin synthesizers (YUCCA, TAA). Results from this study shed light on the evolutionary relationship between Chara and embryophytes through confirmation of shared salt tolerance mechanisms, as well as unique mechanisms that do not occur in angiosperms.

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.

The Arabidopsis phosphatase PP2C49 negatively regulates salt tolerance through inhibition of AtHKT1;1

Type 2C protein phosphatases (PP2Cs) are the largest protein phosphatase family. PP2Cs dephosphorylate substrates for signaling in Arabidopsis, but the functions of most PP2Cs remain unknown. Here, we characterized PP2C49 (AT3G62260, a Group G PP2C), which regulates Na⁺ distribution under salt stress and is localized to the cytoplasm and nucleus. PP2C49 was highly expressed in root vascular tissues and its disruption enhanced plant tolerance to salt stress. Compared with wild type, the pp2c49 mutant contained more Na⁺ in roots but less Na⁺ in shoots and xylem sap, suggesting that PP2C49 regulates shoot Na⁺ extrusion. Reciprocal grafting revealed a root-based mechanism underlying the salt tolerance of pp2c49. Systemic Na⁺ distribution largely depends on AtHKT1;1 and loss of function of AtHKT1;1 in the pp2c49 background overrode the salt tolerance of pp2c49, resulting in salt sensitivity. Furthermore, compared with plants overexpressing PP2C49 in the wild-type background, plants overexpressing PP2C49 in the athtk1;1 mutant background were sensitive to salt, like the athtk1;1 mutants. Moreover, protein-protein interaction and two-voltage clamping assays demonstrated that PP2C49 physically interacts with AtHKT1;1 and inhibits the Na⁺ permeability of AtHKT1;1. This study reveals that PP2C49 negatively regulates AtHKT1;1 activity and thus determines systemic Na⁺ allocation during salt stress.

Modulation of Ion Transport Across Plant Membranes by Polyamines: Understanding Specific Modes of Action Under Stress

This work critically discusses the direct and indirect effects of natural polyamines and their catabolites such as reactive oxygen species and γ-aminobutyric acid on the activity of key plant ion-transporting proteins such as plasma membrane H+ and Ca2+ ATPases and K+-selective and cation channels in the plasma membrane and tonoplast, in the context of their involvement in stress responses. Docking analysis predicts a distinct binding for putrescine and longer polyamines within the pore of the vacuolar TPC1/SV channel, one of the key determinants of the cell ionic homeostasis and signaling under stress conditions, and an additional site for spermine, which overlaps with the cytosolic regulatory Ca2+-binding site. Several unresolved problems are summarized, including the correct estimates of the subcellular levels of polyamines and their catabolites, their unexplored effects on nucleotide-gated and glutamate receptor channels of cell membranes and Ca2+-permeable and K+-selective channels in the membranes of plant mitochondria and chloroplasts, and pleiotropic mechanisms of polyamines' action on H+ and Ca2+ pumps.

Protein kinases in plant responses to drought, salt, and cold stress

Protein kinases are major players in various signal transduction pathways. Understanding the molecular mechanisms behind plant responses to biotic and abiotic stresses has become critical for developing and breeding climate-resilient crops. In this review, we summarize recent progress on understanding plant drought, salt, and cold stress responses, with a focus on signal perception and transduction by different protein kinases, especially sucrose nonfermenting1 (SNF1)-related protein kinases (SnRKs), mitogen-activated protein kinase (MAPK) cascades, calcium-dependent protein kinases (CDPKs/CPKs), and receptor-like kinases (RLKs). We also discuss future challenges in these research fields.

Exogenous spermine-induced expression of SlSPMS gene improves salinity-alkalinity stress tolerance by regulating the antioxidant enzyme system and ion homeostasis in tomato

The study tested the function of exogenous spermine (Spm) in resisting salinity-alkalinity stress in tomato seedlings and found that tomato Spm synthase gene (SlSPMS) was involved in this regulation. The tomato seedlings cultivated in normal conditions or salinity-alkalinity conditions were irrigated with 100 ml one strength Hoagland nutrient solution 100 ml mixed solution (5 ml 300 mmol/L NaCl, 45 ml 300 mmol/L Na₂SO₄, 45 ml 300 mmol/L NaHCO₃, and 5 ml 300 mmol/L Na₂CO₃ (pH = 8.90)) every 2 days, respectively. The 0.5 mM Spm pretreatment improved superoxide dismutase (SOD; EC 1.15.1.1) activity, catalase (CAT; EC 1.11.1.6) activity, ascorbate peroxidase (APX; EC 1.11.1.11) activity, and glutathione reductase (GR; EC 1.6.4.2) activity and decreased endogenous hydrogen peroxide (H₂O₂) content, malondialdehyde (MDA) content, and relative electrical conductivity (REC) in tomato leaves. Na⁺ content declined and K⁺ concentration rose in tomato seedlings when pre-treated with Spm. However the results showed that under salinity-alkalinity stress, silencing of SlSPMS with virus-induced gene silencing had lower antioxidant enzyme activities and higher Na⁺ content and lower K⁺ content than normal tomato seedlings, meaning that they had low salinity-alkalinity tolerance. Exogenous Spm could not reconstruct the tolerance to salinity-alkalinity stress in SlSPMS gene-silencing tomato seedlings. Taken together, exogenous Spm could induce the expression level of SlSPMS, which regulated the antioxidant enzyme system and ion homeostasis in tomato seedlings living in salinity-alkalinity environment, thereby improving the ability of tomato seedlings to resist salinity-alkalinity stress.

The role of NO in plant response to salt stress: interactions with polyamines

Soil salinity is a major abiotic stress that limits plant growth and productivity. High concentrations of sodium chloride can cause osmotic and ionic effects. This stress minimises a plant's ability to uptake water and minerals, and increases Na+ accumulation in the cytosol, thereby disturbing metabolic processes. Prolonged plant exposure to salt stress can lead to oxidative stress and increased production of reactive oxygen species (ROS). Higher plants developed some strategies to cope with salt stress. Among these, mechanisms involving nitric oxide (NO) and polyamines (PAs) are particularly important. NO is a key signalling molecule that mediates a variety of physiological functions and defence responses against abiotic stresses in plants. Under salinity conditions, NO donors increase growth parameters, reduce Na+ toxicity, maintain ionic homeostasis, stimulate osmolyte accumulation and prevent damages caused by ROS. NO enhances salt tolerance of plants via post-translational protein modifications through S-nitrosylation of thiol groups, nitration of tyrosine residues and modulation of multiple gene expression. Several reviews have reported on the role of polyamines in modulating salt stress plant response and the capacity to enhance PA synthesis upon salt stress exposure, and it is known that NO and PAs interact under salinity. In this review, we focus on the role of NO in plant response to salt stress, paying particular attention to the interaction between NO and PAs.

Insights into the Physiological and Biochemical Impacts of Salt Stress on Plant Growth and Development

Climate change is causing soil salinization, resulting in crop losses throughout the world. The ability of plants to tolerate salt stress is determined by multiple biochemical and molecular pathways. Here we discuss physiological, biochemical, and cellular modulations in plants in response to salt stress. Knowledge of these modulations can assist in assessing salt tolerance potential and the mechanisms underlying salinity tolerance in plants. Salinity-induced cellular damage is highly correlated with generation of reactive oxygen species, ionic imbalance, osmotic damage, and reduced relative water content. Accelerated antioxidant activities and osmotic adjustment by the formation of organic and inorganic osmolytes are significant and effective salinity tolerance mechanisms for crop plants. In addition, polyamines improve salt tolerance by regulating various physiological mechanisms, including rhizogenesis, somatic embryogenesis, maintenance of cell pH, and ionic homeostasis. This research project focuses on three strategies to augment salinity tolerance capacity in agricultural crops: salinity-induced alterations in signaling pathways; signaling of phytohormones, ion channels, and biosensors; and expression of ion transporter genes in crop plants (especially in comparison to halophytes).

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.

Exogenous application of spermine and putrescine mitigate adversities of drought stress in wheat by protecting membranes and chloroplast ultra-structure

Polyamines (PAs) are positively charged molecules known to mitigate drought stress; however, little is known about their mechanism of alleviating drought stress. We investigated the effects of PAs exogenously applied as a seed primer and as a foliar spray on the growth, membrane stability (MS), electrolyte leakage (EL), Na+ and K+ cations, reactive oxygen species (ROS), catalase (CAT; EC 1.11.1.6) and guaiacol peroxidase (GPX; EC 1.11.1.7) activity and chloroplast ultra-structure in wheat (Triticum aestivum L.; cv. Sakha-94) under drought stress. Three PA solutions, namely, putrescine, spermine and a mixture of the two (Mix), were each applied at a concentration of 100 µM. Our study demonstrated that the retardation of chlorophyll loss and elevation of Rubisco levels were involved in PA-enhanced growth under drought stress. These relationships were mainly reflected in elevated fresh weight and dry weight in response to foliar spraying with all PA solutions and seed priming with the Mix solution. The elevated growth seemed to be due to increased photosynthetic pigments, protein and Rubisco. In contrast, drought decreased growth, photosynthetic pigments, protein and Rubisco. MS was enhanced by PAs applied as a seed primer or foliar spray, as shown by clear reductions in EL %, malondialdehyde (MDA) content and the Na+/K+ ratio as well as reduced ROS markers and elevated CAT (but not GPX) activity. Further study showed that the Mix solution of PAs, applied either during seed priming or as a foliar spray, improved chloroplast ultra-structure, suggesting that improvements in Rubisco and photosynthetic pigments were involved in PA maintenance of chloroplast stability. Therefore, the present study showed that elevated CAT activity is the main mechanism through which PAs reduce ROS and MDA, thereby improving MS and protecting mesophyll cells structurally and functionally under drought stress in wheat.

Polyamines and Legumes: Joint Stories of Stress, Nitrogen Fixation and Environment

Polyamines (PAs) are natural aliphatic amines involved in many physiological processes in almost all living organisms, including responses to abiotic stresses and microbial interactions. On other hand, the family Leguminosae constitutes an economically and ecologically key botanical group for humans, being also regarded as the most important protein source for livestock. This review presents the profuse evidence that relates changes in PAs levels during responses to biotic and abiotic stresses in model and cultivable species within Leguminosae and examines the unreviewed information regarding their potential roles in the functioning of symbiotic interactions with nitrogen-fixing bacteria and arbuscular mycorrhizae in this family. As linking plant physiological behavior with "big data" available in "omics" is an essential step to improve our understanding of legumes responses to global change, we also examined integrative MultiOmics approaches available to decrypt the interface legumes-PAs-abiotic and biotic stress interactions. These approaches are expected to accelerate the identification of stress tolerant phenotypes and the design of new biotechnological strategies to increase their yield and adaptation to marginal environments, making better use of available plant genetic resources.

Spermine application alleviates salinity induced growth and photosynthetic inhibition in Solanum lycopersicum by modulating osmolyte and secondary metabolite accumulation and differentially regulating antioxidant metabolism

Influence of exogenously applied spermine (Spm) on growth and salinity stress tolerance in tomato was investigated. Salinity reduced growth, chlorophyll synthesis and mineral uptake leading to significant reduction in photosynthesis, however Spm application proved beneficial in alleviating the decline to considerable extent. Applied Spm improved nitrate reductase activity, δ-amino levulinic acid content and gas exchange parameters more apparently at 100 μM than 50 μM concentrations. Spm application enhanced the accumulation of compatible osmolytes including proline, glycine betaine and sugars leading to greater tissue water content and photosynthesis. Salinity stress induced oxidative effects were mitigated by Spm treatment reflected interms of reduced accumulation of reactive oxygen species and the activities of protease and lipoxygenase, hence leading to membrane strengthening and protection of their function. Differential influence of exogenous Spm was evident on the functioning of antioxidant system with SOD, GR and APX activities much higher in Spm treated seedlings than CAT and DHAR. Increased synthesis of GSH, AsA and tocopherol in Spm treated seedlings was obvious thereby helping in maintaining the redox homeostasis and the enzymatic antioxidant functioning. Interestingly Spm application maintained the nitric oxide levels higher than control under normal condition while as lowered its concentrations in salinity stressed seedlings depicting existence of probable interaction. Activities of polyamine metabolizing enzymes was up-regulated and the accumulation of secondary metabolites including phenols and flavonoids also increased due to Spm application. Further studies are required to understand the mechanisms clearly.

Salt hypersensitivity is associated with excessive xylem development in a thermospermine-deficient mutant of Arabidopsis thaliana

In Arabidopsis, spermine is produced in most tissues and has been implicated in stress response, while its structural isomer thermospermine is only in xylem precursor cells. Studies on acaulis5 (acl5), a mutant defective in the biosynthesis of thermospermine, have revealed that thermospermine plays a repressive role in xylem development through enhancement of mRNA translation of the SAC51 family. In contrast, the pao5 mutant defective in the degradation of thermospermine has high levels of thermospermine and shows increased salt tolerance, suggesting a role of thermospermine in salt stress response. Here we compared acl5 with a mutant of spermine synthase, spms, in terms of abiotic stress tolerance and found that acl5 was much more sensitive to sodium than the wild-type and spms. A double-mutant of acl5 and sac51-d, which suppresses the excessive xylem phenotype of acl5, recovered normal sensitivity, while a quadruple T-DNA insertion mutant of the SAC51 family, which has an increased thermospermine level but shows excessive xylem development, showed increased salt sensitivity, unlike pao5. Together with the result that the salt tolerance of both wild-type and acl5 seedlings was improved by long-term treatment with thermospermine, we suggest a correlation of the salt tolerance with reduced xylem development rather than with the thermospermine level. We further found that the mutants containing high thermospermine levels showed increased tolerance to drought and heat stress, suggesting another role of thermospermine that may be common with that of spermine and secondary to that in restricting excess xylem development associated with salt hypersensitivity.

Calcium signaling and salt tolerance are diversely entwined in plants

In plants dehydration imposed by salinity can invoke physical changes at the interface of the plasma membrane and cell wall. Changes in hydrostatic pressure activate ion channels and cause depolarization of the plasma membrane due to disturbance in ion transport. During the initial phases of salinity stress, the relatively high osmotic potential of the rhizosphere enforces the plant to use a diverse spectrum of strategies to optimize water and nutrient uptake. Signals of salt stress are recognized by specific root receptors that activate an osmosensing network. Plant response to hyperosmotic tension is closely linked to the calcium (Ca2+) channels and interacting proteins such as calmodulin. A rapid rise in cytosolic Ca2+ levels occurs within seconds of exposure to salt stress. Plants employ multiple sensors and signaling components to sense and respond to salinity stress, of which most are closely related to Ca2+ sensing and signaling. Several tolerance strategies such as osmoprotectant accumulation, antioxidant boosting, polyaminses and nitric oxide (NO) machineries are also coordinated by Ca2+ signaling. Substantial research has been done to discover the salt stress pathway and tolerance mechanism in plants, resulting in new insights into the perception of salt stress and the downstream signaling that happens in response. Nevertheless, the role of multifunctional components such as Ca2+ has not been sufficiently addressed in the context of salt stress. In this review, we elaborate that the salt tolerance signaling pathway converges with Ca2+ signaling in diverse pathways. We summarize knowledge related to different dimensions of salt stress signaling pathways in the cell by emphasizing the administrative role of Ca2+ signaling on salt perception, signaling, gene expression, ion homeostasis and adaptive responses.

Overexpression of PbrNHX2 gene, a Na+/H+ antiporter gene isolated from Pyrus betulaefolia, confers enhanced tolerance to salt stress via modulating ROS levels

Intracellular Na+/H+ antiporters (NHXs) play important roles in plant tolerance to salt stress. However, plant NHXs functioning in salt tolerance and the underlying physiological mechanisms remain poorly understood. In this report, we report the identification and functional characterization of PbrNHX2 isolated from Pyrus betulaefolia. PbrNHX2 expression levels were induced by salt, and dehydration, but was unaffected by cold. PbrNHX2 was localized in the tonoplast. Overexpression of PbrNHX2 in tobacco and Pyrus ussuriensis conferred enhanced tolerance to salt tolerance, whereas down-regulation of PbrNHX2 in Pyrus betulaefolia by virus-induced gene silencing (VIGS) resulted in elevated salt sensitivity. The transgenic lines contained lower levels of Na+, higher levels of K+, and higher K/Na ratio, whereas they were changed in an opposite way when PbrNHX2 was silenced. In addition, the transgenic plants accumulated lower levels of reactive oxygen species compared with wild type, accompanied by higher activities of three antioxidant enzymes. Taken together, the data demonstrate that PbrNHX2 plays a positive role in salt tolerance and that it holds a great potential for engineering salt tolerance in crops.

Genome-wide association study dissects the genetic bases of salt tolerance in maize seedlings

Excess salinity is a natural stress that causes crop yield losses worldwide. The genetic bases of maize salt tolerance remain largely unknown. Here we investigated the survival rates of 445 maize natural accessions after salt treatments. A skewed distribution of the salt-tolerant phenotypes was observed in this population. Genome-wide association studies (GWAS) revealed 57 loci significantly associated with salt tolerance. Forty-nine candidate genes were detected from these loci. About 10% of these genes were co-localized with loci from QTL mapping. Forty four percent of the candidate genes were involved in stress responses, ABA signaling, stomata division, DNA binding/transcription regulation and auxin signaling, suggesting that they are key genetic mechanisms of maize salt tolerance. Transgenic studies showed that two genes, the salt-tolerance-associated-gene 4 (SAG4, GRMZM2G077295) and SAG6 (GRMZM2G106056), which encode a protein transport protein and the double-strand break repair protein MRE11, respectively, had positive roles in plant salt tolerance, and their salt-tolerant haplotypes were revealed. The genes we identified in this study provide a list of candidate targets for further study of maize salt tolerance, and of genetic markers and materials that may be used for breeding salt-tolerance in maize.

TaCIPK10 interacts with and phosphorylates TaNH2 to activate wheat defense responses to stripe rust

Calcineurin B-like interacting protein kinase (CIPKs) has been shown to be required for biotic stress tolerance of plants in plant-pathogen interactions. However, the roles of CIPKs in immune signalling of cereal crops and an in-depth knowledge of substrates of CIPKs in response to biotic stress are under debate. In this study, we identified and cloned a CIPK homologue gene TaCIPK10 from wheat. TaCIPK10 was rapidly induced by Puccinia striiformis f. sp. tritici (Pst) inoculation and salicylic acid (SA) treatment. In vitro phosphorylation assay demonstrated that the kinase activity of TaCIPK10 is regulated by Ca²⁺ and TaCBL4. Knockdown TaCIPK10 significantly reduced wheat resistance to Pst, whereas TaCIPK10 overexpression resulted in enhanced wheat resistance to Pst by the induction of defense response in different aspects, including hypersensitive cell death, ROS accumulation and pathogenesis-relative genes expression. Moreover, TaCIPK10 physically interacted with and phosphorylated TaNH2, which was homologous to AtNPR3/4. Silencing of TaNH2 in wheat resulted in enhanced susceptibility to the avirulent Pst race, CYR23, indicating its positive role in wheat resistance. Our results demonstrate that TaCIPK10 positively regulate wheat resistance to Pst as molecular links between of Ca²⁺ and downstream components of defense response and TaCIPK10 interacts with and phosphorylates TaNH2 to regulate wheat resistance to Pst.

Mesophyll cells' ability to maintain potassium is correlated with drought tolerance in tea (Camellia sinensis)

Tea plant is an important economic crop and is vulnerable to drought. A good understanding of tea drought tolerance mechanisms is required for breeding robust drought tolerant tea varieties. Previous studies showed mesophyll cells' ability to maintain K⁺ is associated with its stress tolerance. Here, in this study, 12 tea varieties were used to investigate the role of mesophyll K⁺ retention ability towards tea drought stress tolerance. A strong and negative correlation (R² = 0.8239, P < 0.001) was found between PEG (mimic drought stress)-induced K⁺ efflux from tea mesophyll cells and overall drought tolerance in 12 tea varieties. In agreement with this, a significantly higher retained leaf K⁺ content was found in drought tolerant than the sensitive tea varieties. Furthermore, exogenous applied K⁺ (5 mM) significantly alleviated drought-induced symptom in tea plants, further supporting our finding that mesophyll K⁺ retention is an important component for drought tolerance mechanisms in tea plants. Moreover, pharmacological experiments showed that the contribution of K⁺ outward rectifying channels and non-selective cation channels in controlling PEG-induced K⁺ efflux from mesophylls cells are varied between drought tolerant and sensitive tea varieties.

Plant Salinity Stress: Many Unanswered Questions Remain

Salinity is a major threat to modern agriculture causing inhibition and impairment of crop growth and development. Here, we not only review recent advances in salinity stress research in plants but also revisit some basic perennial questions that still remain unanswered. In this review, we analyze the physiological, biochemical, and molecular aspects of Na+ and Cl- uptake, sequestration, and transport associated with salinity. We discuss the role and importance of symplastic versus apoplastic pathways for ion uptake and critically evaluate the role of different types of membrane transporters in Na+ and Cl- uptake and intercellular and intracellular ion distribution. Our incomplete knowledge regarding possible mechanisms of salinity sensing by plants is evaluated. Furthermore, a critical evaluation of the mechanisms of ion toxicity leads us to believe that, in contrast to currently held ideas, toxicity only plays a minor role in the cytosol and may be more prevalent in the vacuole. Lastly, the multiple roles of K+ in plant salinity stress are discussed.

Soil drenching of paclobutrazol: An efficient way to improve quinoa performance under salinity

Salinity extent and severity is rising because of poor management practices on agricultural lands, possibility lies to grow salt-tolerant crops with better management techniques. Therefore, a highly nutritive salt-tolerant crop quinoa with immense potential to contribute for future food security was selected for this investigation. Soil drenching of paclobutrazol (PBZ; 20 mg l^-1 ) was used to understand the ionic relations, gaseous exchange characteristics, oxidative defense system and yield under saline conditions (400 mM NaCl) including normal (0 mM NaCl) and no PBZ (0 mg l^-1 ) as controls. The results revealed that salinity stress reduced the growth and yield of quinoa through perturbing ionic homeostasis with the consequences of overproduction of reactive oxygen species (ROS), oxidative damages and reduced photosynthesis. PBZ improved the quinoa performance through regulation of ionic homeostasis by decreasing Na⁺ , Cl^- , while improving K⁺ , Mg²⁺ and Ca²⁺ concentration. It also enhanced the antioxidative system including ascorbic acid, phenylalanine ammonia-lyase, polyphenol oxidase and glutathione peroxidase, which scavenged the ROS (H₂ O₂ and O₂ ^•- ) and lowered the oxidative damages (malondialdehyde level) under salinity in roots and more specifically in leaf tissues. The photosynthetic rate and stomatal conductance consequently improved (16 and 21%, respectively) in salt-stressed quinoa PBZ-treated compared to the non-treated ones and contributed to the improvement of panicle length (33%), 100-grain weight (8%) and grain yield (38%). Therefore, PBZ can be opted as a shotgun approach to improve quinoa performance and other crops under high saline conditions.

Reproductive Physiology of Halophytes: Current Standing

Background: Halophytes possess efficient salt-tolerance mechanisms and can complete their life cycles in naturally saline soils with NaCl contents exceeding 200 mM. While a significant progress have been made in recent decades elucidating underlying salt-tolerance mechanisms, these studies have been mostly confined to the vegetative growth stage. At the same time, the capacity to generate high-quality seeds and to survive early developmental stages under saline conditions, are both critically important for plants. Halophytes perform well in both regards, whereas non-halophytes cannot normally complete their life cycles under saline conditions. Scope: Research into the effects of salinity on plant reproductive biology has gained momentum in recent years. However, it remains unclear whether the reproductive biology of halophytes differs from that of non-halophytes, and whether their reproductive processes benefit, like their vegetative growth, from the presence of salt in the rhizosphere. Here, we summarize current knowledge of the mechanisms underlying the superior reproductive biology of halophytes, focusing on critical aspects including control of flowering time, changes in plant hormonal status and their impact on anther and pollen development and viability, plant carbohydrate status and seed formation, mechanisms behind the early germination of halophyte seeds, and the role of seed polymorphism. Conclusion: Salt has beneficial effects on halophyte reproductive growth that include late flowering, increased flower numbers and pollen vitality, and high seed yield. This improved performance is due to optimal nutrition during vegetative growth, alterations in plant hormonal status, and regulation of flowering genes. In addition, the seeds of halophytes harvested under saline conditions show higher salt tolerance than those obtained under non-saline condition, largely due to increased osmolyte accumulation, more optimal hormonal composition (e.g., high gibberellic acid and low abcisic acid content) and, in some species, seed dimorphism. In the near future, identifying key genes involved in halophyte reproductive physiology and using them to transform crops could be a promising approach to developing saline agriculture.

Arabidopsis IAR4 Modulates Primary Root Growth Under Salt Stress Through ROS-Mediated Modulation of Auxin Distribution

High salinity is one of the major environmental stresses that plants encounter. Roots are the initial and direct organs to perceive the signal. However, how plant roots perceive and respond to salinity at the molecular and physiological levels is still poorly understood. Here, we report that IAA-CONJUGATE-RESISTANT 4 (IAR4) plays a key role in primary root growth under salt stress conditions. Mutation of IAR4 led to increased sensitivity to salt stress conditions, with strongly inhibited primary root growth and reduced survival rate in two iar4 mutant alleles. iar4 mutants accumulated greater Na⁺ and exhibited a greater Na⁺/K⁺ ratio under NaCl treatment. In addition, more reactive oxygen species (ROS) accumulated in the iar4 mutants due to reduced ROS scavenging. NaCl treatment greatly suppressed the expression levels of ProPIN1:PIN1-GFP, ProPIN2:PIN2-GFP, ProPIN3:PIN3-GFP, and ProDR5:GFP, and suppressed root meristem activity in iar4. GSH or auxin treatment greatly recovered the PIN expression, auxin distribution and primary root growth in the iar4 mutants, suggesting ROS is a vital mediator between salt stress and auxin response. Our data support a model in which IAR4 integrates ROS and auxin pathways to modulate primary root growth under salinity stress conditions, by regulation of PIN-mediated auxin transport.

The role of ion disequilibrium in induction of root cell death and autophagy by environmental stresses

Environmental stresses such as salinity, drought, oxidants, heavy metals, hypoxia, extreme temperatures and others can induce autophagy and necrosis-type programmed cell death (PCD) in plant roots. These reactions are accompanied by the generation of reactive oxygen species (ROS) and ion disequilibrium, which is induced by electrolyte/K+ leakage through ROS-activated ion channels, such as the outwardly-rectifying K+ channel GORK and non-selective cation channels. Here, we discuss mechanisms of the stress-induced ion disequilibrium and relate it with ROS generation and onset of morphological, biochemical and genetic symptoms of autophagy and PCD in roots. Based on our own data and that in the literature, we propose a hypothesis on the induction of autophagy and PCD in roots by loss of cytosolic K+. To support this, we present data showing that in conditions of salt stress-induced autophagy, gork1-1 plants lacking root K+ efflux channel have fewer autophagosomes compared with the wild type. Overall, literature analyses and presented data strongly suggest that stress-induced root autophagy and PCD are controlled by the level of cytosolic potassium and ROS.

Methods Related to Polyamine Control of Cation Transport Across Plant Membranes

Polyamines (PAs) are unique polycationic metabolites, which modulate plants' growth, development, and stress responses. As polycations, PAs interfere with cationic transport systems as ion channels and ionotropic pumps. Here, we describe the application of two techniques, MIFE to study the effects of PAs on cation fluxes in vivo and conventional patch-clamp to evaluate the PA blockage of ion currents in isolated plant vacuoles. Preparation of vacuoles for patch-clamp assays is described and solutions and voltage protocols are given, which allow separate recordings of major vacuolar channel currents and quantify their blockage by PAs.

Anionic Cerium Oxide Nanoparticles Protect Plant Photosynthesis from Abiotic Stress by Scavenging Reactive Oxygen Species

Plant abiotic stress leads to accumulation of reactive oxygen species (ROS) and a consequent decrease in photosynthetic performance. We demonstrate that a plant nanobionics approach of localizing negatively charged, sub-11 nm, spherical cerium oxide nanoparticles (nanoceria) inside chloroplasts in vivo augments ROS scavenging and photosynthesis of Arabidopsis thaliana plants under excess light (2000 μmol m^-2 s^-1, 1.5 h), heat (35 °C, 2.5 h), and dark chilling (4 °C, 5 days). Poly(acrylic acid) nanoceria (PNC) with a hydrodynamic diameter (10.3 nm)-lower than the maximum plant cell wall porosity-and negative ζ-potential (-16.9 mV) exhibit significantly higher colocalization (46%) with chloroplasts in leaf mesophyll cells than aminated nanoceria (ANC) (27%) of similar size (12.6 nm) but positive charge (9.7 mV). Nanoceria are transported into chloroplasts via nonendocytic pathways, influenced by the electrochemical gradient of the plasma membrane potential. PNC with a low Ce³⁺/Ce⁴⁺ ratio (35.0%) reduce leaf ROS levels by 52%, including hydrogen peroxide, superoxide anion, and hydroxyl radicals. For the latter ROS, there is no known plant enzyme scavenger. Plants embedded with these PNC that were exposed to abiotic stress exhibit an increase up to 19% in quantum yield of photosystem II, 67% in carbon assimilation rates, and 61% in Rubisco carboxylation rates relative to plants without nanoparticles. In contrast, PNC with high Ce³⁺/Ce⁴⁺ ratio (60.8%) increase overall leaf ROS levels and do not protect photosynthesis from oxidative damage during abiotic stress. This study demonstrates that anionic, spherical, sub-11 nm PNC with low Ce³⁺/Ce⁴⁺ ratio can act as a tool to study the impact of oxidative stress on plant photosynthesis and to protect plants from abiotic stress.

Spermine Regulates Pollen Tube Growth by Modulating Ca2+-Dependent Actin Organization and Cell Wall Structure

Proper growth of the pollen tube depends on an elaborate mechanism that integrates several molecular and cytological sub-processes and ensures a cell shape adapted to the transport of gametes. This growth mechanism is controlled by several molecules among which cytoplasmic and apoplastic polyamines. Spermine (Spm) has been correlated with various physiological processes in pollen, including structuring of the cell wall and modulation of protein (mainly cytoskeletal) assembly. In this work, the effects of Spm on the growth of pear pollen tubes were analyzed. When exogenous Spm (100 μM) was supplied to germinating pollen, it temporarily blocked tube growth, followed by the induction of apical swelling. This reshaping of the pollen tube was maintained also after growth recovery, leading to a 30-40% increase of tube diameter. Apical swelling was also accompanied by a transient increase in cytosolic calcium concentration and alteration of pH values, which were the likely cause for major reorganization of actin filaments and cytoplasmic organelle movement. Morphological alterations of the apical and subapical region also involved changes in the deposition of pectin, cellulose, and callose in the cell wall. Thus, results point to the involvement of Spm in cell wall construction as well as cytoskeleton organization during pear pollen tube growth.

In vivo inhibition of polyamine oxidase by a spermine analogue, MDL-72527, in tomato exposed to sublethal and lethal salt stress

The spermine analogue N1,N4-bis-(2,3-butadienyl)-1,4-butanediamine (MDL-72527), an effective inhibitor of polyamine oxidases (PAOs), triggers a systemic response in tomato (Solanum lycopersicum L.) exposed to sublethal (100mM) and lethal (250mM) NaCl concentrations. The accumulation of free polyamines (PAs), the terminal oxidation of PAs by diamine oxidases (DAOs) and PAOs, and the production of H2O2 by PA oxidases depends on the intensity of salt stress. Spermidine and spermine content increased significantly under sublethal salt concentrations, but remained low under lethal salt stress. Along with increased expression of the selected SlDAO1 and SlPAO1 genes in the leaves and roots, respectively, DAO and PAO activities and their product, H2O2, increased and initiated cell death by irreversible loss of electrolytes at 250mM NaCl. MDL-72527 significantly increased spermine, spermidine and/or putrescine contents as a result of reduced activity of PA oxidases; furthermore, it inhibited H2O2 and NO production during salt treatment. These results indicate that PAO contributed to H2O2 and NO production under salt stress, and the terminal activities of DAO and PAO play a role in cell death induction at 250mM NaCl. However, the inhibition of PAO by MDL-72527 does not increase the salt tolerance of plants, since electrolyte leakage increased significantly in the presence of the inhibitor.

Short Term Effect of Salt Shock on Ethylene and Polyamines Depends on Plant Salt Sensitivity

In the present manuscript the short term effect (3-24 h) of a saline shock (NaCl 100 mM) on fresh weight, water content, respiration rate, ethylene production and Na⁺, Cl^-, ACC and polyamine concentration was studied in four plant species with different salt sensitivity, pepper, lettuce, spinach, and beetroot. Higher reduction in fresh weight and water content as a consequence of saline shock was found in pepper and lettuce plants than in spinach and beetroot, the latter behaving as more salinity tolerant. In general, salinity led to rapid increases in respiration rate, ethylene production and ACC and polyamine (putrescine, spermidine, and spermine) concentrations in shoot and root. These increases were related to plant salinity sensitivity, since they were higher in the most sensitive species and vice versa. However, ethylene and respiration rates in salt stressed plants recovered similar values to controls after 24 h of treatment in salt tolerant plants, while still remaining high in the most sensitive. On the other hand, sudden increases in putrescine, spermidine, and spermine concentration were higher and occurred earlier in pepper and lettuce, the most sensitive species, than in spinach and beetroot, the less sensitive ones. These increases tended to disappear after 24 h, except in lettuce. These changes would support the conclusion that ethylene and polyamine increases could be considered as a plant response to saline shock and related to the plant species sensitivity to this stress. In addition, no competition between polyamines and ethylene biosynthesis for their common precursor was observed.

Treatment with spermidine protects chrysanthemum seedlings against salinity stress damage

Salinity-stressed plants of salinity sensitive ('Qx096') and tolerant ('Qx097') chrysanthemum cultivar were treated with a range of concentrations of spermidine (Spd). Plant performance, as indicated by various parameters associated with growth, was improved by the treatment, as was the tissue content of soluble protein and proline. The extent of both Na(+) accumulation and K(+) loss was reduced. Activity levels of the stress-related enzymes SOD, POD, APX and CAT were significantly increased and the production of malondialdehyde (MDA) decreased. The suggestion was that treatment with 1.5 mM Spd would be an effective means alleviating salinity-stress induced injury through its positive effect on photosynthetic efficiency, reactive oxygen species scavenging ability and the control of ionic balance and osmotic potential. Its protective capacity was more apparent in 'Qx096' than in 'Qx097'.

Role of Cyclic Nucleotide Gated Channels in Stress Management in Plants

Tolerance of plants to a number of biotic and abiotic stresses such as pathogen and herbivore attack, drought, salinity, cold and nutritional limitations is ensued by complex multimodule signaling pathways. The outcome of this complex signaling pathways results in adaptive responses by restoring the cellular homeostasis and thus promoting survival. Functions of many plant cation transporter and channel protein families such as glutamate receptor homologs (GLRs), cyclic nucleotide-gated ion channel (CNGC) have been implicated in providing biotic and abiotic stress tolerance. Ion homeostasis regulated by several transporters and channels is one of the crucial parameters for the optimal growth, development and survival of all living organisms. The CNGC family members are known to be involved in the uptake of cations such as Na(+), K(+) and Ca(2+) and regulate plant growth and development. Detail functional genomics approaches have given an emerging picture of CNGCs wherein these protein are believed to play crucial role in pathways related to cellular ion homeostasis, development and as a 'guard' in defense against biotic and abiotic challenges. Here, we discuss the current knowledge of role of CNGCs in mediating stress management and how they aid plants in survival under adverse conditions.

Spermine modulates the expression of two probable polyamine transporter genes and determines growth responses to cadaverine in Arabidopsis

Two genes, LAT1 and OCT1 , are likely to be involved in polyamine transport in Arabidopsis. Endogenous spermine levels modulate their expression and determine the sensitivity to cadaverine. Arabidopsis spermine (Spm) synthase (SPMS) gene-deficient mutant was previously shown to be rather resistant to the diamine cadaverine (Cad). Furthermore, a mutant deficient in polyamine oxidase 4 gene, accumulating about twofold more of Spm than wild type plants, showed increased sensitivity to Cad. It suggests that endogenous Spm content determines growth responses to Cad in Arabidopsis thaliana. Here, we showed that Arabidopsis seedlings pretreated with Spm absorbs more Cad and has shorter root growth, and that the transgenic Arabidopsis plants overexpressing the SPMS gene are hypersensitive to Cad, further supporting the above idea. The transgenic Arabidopsis overexpressing L-Amino acid Transporter 1 (LAT1) absorbed more Cad and showed increased Cad sensitivity, suggesting that LAT1 functions as a Cad importer. Recently, other research group reported that Organic Cation Transporter 1 (OCT1) is a causal gene which determines the Cad sensitivity of various Arabidopsis accessions. Furthermore, their results suggested that OCT1 is involved in Cad efflux. Thus we monitored the expression of OCT1 and LAT1 during the above experiments. Based on the results, we proposed a model in which the level of Spm content modulates the expression of OCT1 and LAT1, and determines Cad sensitivity of Arabidopsis.

Transport Across Chloroplast Membranes: Optimizing Photosynthesis for Adverse Environmental Conditions

Chloroplasts are central to solar light harvesting and photosynthesis. Optimal chloroplast functioning is vitally dependent on a very intensive traffic of metabolites and ions between the cytosol and stroma, and should be attuned for adverse environmental conditions. This is achieved by an orchestrated regulation of a variety of transport systems located at chloroplast membranes such as porines, solute channels, ion-specific cation and anion channels, and various primary and secondary active transport systems. In this review we describe the molecular nature and functional properties of the inner and outer envelope and thylakoid membrane channels and transporters. We then discuss how their orchestrated regulation affects thylakoid structure, electron transport and excitation energy transfer, proton-motive force partition, ion homeostasis, stromal pH regulation, and volume regulation. We link the activity of key cation and anion transport systems with stress-specific signaling processes in chloroplasts, and discuss how these signals interact with the signals generated in other organelles to optimize the cell performance, with a special emphasis on Ca(2+) and reactive oxygen species signaling.

Silicon-mediated changes in polyamines participate in silicon-induced salt tolerance in Sorghum bicolor L

Silicon (Si) is generally considered a beneficial element for the growth of higher plants, especially under stress conditions, but the mechanisms remain unclear. Here, we tested the hypothesis that Si improves salt tolerance through mediating important metabolism processes rather than acting as a mere mechanical barrier. Seedlings of sorghum (Sorghum bicolor L.) growing in hydroponic culture were treated with NaCl (100 mm) combined with or without Si (0.83 mm). The result showed that supplemental Si enhanced sorghum salt tolerance by decreasing Na(+) accumulation. Simultaneously, polyamine (PA) levels were increased and ethylene precursor (1-aminocyclopropane-1-carboxylic acid: ACC) concentrations were decreased. Several key PA synthesis genes were up-regulated by Si under salt stress. To further confirm the role of PA in Si-mediated salt tolerance, seedlings were exposed to spermidine (Spd) or a PA synthesis inhibitor (dicyclohexylammonium sulphate, DCHA) combined with salt and Si. Exogenous Spd showed similar effects as Si under salt stress whereas exogenous DCHA eliminated Si-enhanced salt tolerance and the beneficial effect of Si in decreasing Na(+) accumulation. These results indicate that PAs and ACC are involved in Si-induced salt tolerance in sorghum and provide evidence that Si plays an active role in mediating salt tolerance.

Metabolites and hormones are involved in the intraspecific variability of drought hardening in radiata pine

Studies of metabolic and physiological bases of plant tolerance and hardening against drought are essential to improve genetic breeding programs, especially in productive species such as Pinus radiata. The exposure to different drought cycles is a highly effective tool that improves plant conditioning, but limited information is available about the mechanisms that modulate this process. To clarify this issue, six P. radiata breeds with well-known differences in drought tolerance were analyzed after two consecutive drought cycles. Survival rate, concentration of several metabolites such as free soluble amino acids and polyamines, and main plant hormones varied between them after drought hardening, while relative growth ratio and water potential at both predawn and dawn did not. Hardening induced a strong increase in total soluble amino acids in all breeds, accumulating mainly those implicated in the glutamate metabolism (GM), especially L-proline, in the most tolerant breeds. Other amino acids from GM such as γ-aminobutyric acid (GABA) and L-arginine (Arg) were also strongly increased. GABA pathway could improve the response against drought, whereas Arg acts as precursor for the synthesis of spermidine. This polyamine showed a positive relationship with the survival capacity, probably due to its role as antioxidant under stress conditions. Finally, drought hardening also induced changes in phytohormone content, showing each breed a different profile. Although all of them accumulated indole-3-acetic acid and jasmonic acid and reduced zeatin content in needles, significant differences were observed regarding abscisic acid, salicylic acid and mainly zeatin riboside. These results confirm that hardening is not only species-dependent but also an intraspecific processes controlled through metabolite changes.

Natural variation in the expression of ORGANIC CATION TRANSPORTER 1 affects root length responses to cadaverine in Arabidopsis

Polyamines, including cadaverine, are organic cations that affect numerous biological processes including transcription, translation, cell signalling, and ion channel activity. They often function in biotic and abiotic stress responses in plants. Because little is known about how plants respond to cadaverine, a quantitative natural variation approach was used to identify genetic factors that contribute to this response. Here it is shown that Arabidopsis thaliana accessions have varying root length responses to exogenous cadaverine: Cape Verde Islands (Cvi) was one of the most resistant accessions tested, whereas Landsberg erecta (Ler) was one of the most sensitive. Recombinant inbred lines, near isogenic lines, and a microarray were used to show that variation in ORGANIC CATION TRANSPORTER 1 (OCT1) is at least partially responsible for this difference. OCT1 expression was higher in Cvi than in Ler, and oct1 mutants were more sensitive to cadaverine than wild-type plants. In oct1 mutants transformed with an ectopic copy of OCT1 originating from either Cvi or Ler, the expression level of the transgene, not its accession, correlated with the cadaverine response. These results suggest that decreased OCT1 expression confers cadaverine sensitivity in some accessions.

Polyamine metabolism influences antioxidant defense mechanism in foxtail millet (Setaria italica L.) cultivars with different salinity tolerance

KEY MESSAGE

Polyamines can regulate the expression of antioxidant enzymes and impart plants tolerance to abiotic stresses. A comparative analysis of polyamines, their biosynthetic enzymes at kinetic and at transcriptional level, and their role in regulating the induction of antioxidant defense enzymes under salt stress condition in two foxtail millet (Setaria italica L.) cultivars, namely Prasad, a salt-tolerant, and Lepakshi, a salt-sensitive cultivar was conducted. Salt stress resulted in elevation of free polyamines due to increase in the activity of spermidine synthase and S-adenosyl methionine decarboxylase enzymes in cultivar Prasad compared to cultivar Lepakshi under different levels of NaCl stress. These enzyme activities were further confirmed at the transcript level via qRT-PCR analysis. The cultivar Prasad showed a greater decrease in diamine oxidase and polyamine oxidase activity, which results in the accumulation of polyamine pools over cultivar Lepakshi. Generation of free radicals, such as O 2 (·-) and H2O2, was also analyzed quantitatively. A significant increase in O 2 (·-) and H2O2 in the cultivar Lepakshi compared with cultivar Prasad was recorded in overall pool sizes. Further, histochemical staining showed lesser accumulation of O 2 (·-) and of H2O2 in the leaves of cultivar Prasad than cultivar Lepakshi. Our results also suggest the ability of polyamine oxidation in regulating the induction of antioxidative defense enzymes, which involve in the elimination of toxic levels of O 2 (·-) and H2O2, such as Mn-superoxide dismutase, catalase and ascorbate peroxidase. The contribution of polyamines in modulating antioxidative defense mechanism in NaCl stress tolerance is discussed.

Salinity tolerance in soybean is modulated by natural variation in GmSALT3

The identification of genes that improve the salt tolerance of crops is essential for the effective utilization of saline soils for agriculture. Here, we use fine mapping in a soybean (Glycine max (L.) Merr.) population derived from the commercial cultivars Tiefeng 8 and 85-140 to identify GmSALT3 (salt tolerance-associated gene on chromosome 3), a dominant gene associated with limiting the accumulation of sodium ions (Na+) in shoots and a substantial enhancement in salt tolerance in soybean. GmSALT3 encodes a protein from the cation/H+ exchanger family that we localized to the endoplasmic reticulum and which is preferentially expressed in the salt-tolerant parent Tiefeng 8 within root cells associated with phloem and xylem. We identified in the salt-sensitive parent, 85-140, a 3.78-kb copia retrotransposon insertion in exon 3 of Gmsalt3 that truncates the transcript. By sequencing 31 soybean landraces and 22 wild soybean (Glycine soja) a total of nine haplotypes including two salt-tolerant haplotypes and seven salt-sensitive haplotypes were identified. By analysing the distribution of haplotypes among 172 Chinese soybean landraces and 57 wild soybean we found that haplotype 1 (H1, found in Tiefeng 8) was strongly associated with salt tolerance and is likely to be the ancestral allele. Alleles H2-H6, H8 and H9, which do not confer salinity tolerance, were acquired more recently. H1, unlike other alleles, has a wide geographical range including saline areas, which indicates it is maintained when required but its potent stress tolerance can be lost during natural selection and domestication. GmSALT3 is a gene associated with salt tolerance with great potential for soybean improvement.

Durum and bread wheat differ in their ability to retain potassium in leaf mesophyll: implications for salinity stress tolerance

Understanding the intrinsic mechanisms involved in the differential salinity tolerance between bread wheat and durum wheat is essential for breeding salt-tolerant varieties to cope with the global salinity issue threatening future food supply. In the past, higher salinity tolerance in bread wheat compared with durum wheat has been attributed to its better ability to exclude Na(+) from uptake. Here we show that another mechanism, namely more superior K(+) retention ability in the leaf mesophyll, also contributes to this difference. A strong positive correlation (R(2) > 0.41, P < 0.001) was found between NaCl-induced K(+) efflux in the leaf mesophyll and overall salinity tolerance in 48 wheat varieties. However, while the above correlation was strong in bread wheat, it was statistically insignificant in durum wheat. Consistent with these findings, a significantly higher relative leaf K(+) content was found in bread wheat than in durum wheat. In contrast to root tissues, the role of voltage-gated K(+) channels in K(+) retention in the wheat mesophyll was relatively small, and non-selective cation channels played a major role in controlling intracellular K(+) homeostasis. Moreover, a significant negative correlation between NaCl-induced mesophyll H(+) flux and mesophyll K(+) retention was found, and interpreted as a compensatory mechanism employed by sensitive varieties to regain K(+) leaked into the apoplast. It is concluded that bread wheat and durum wheat show different strategies of coping with salinity, and that targeting mechanisms conferring K(+) retention in the leaf mesophyll may be a promising way to improve the overall salinity tolerance in these species.

Bcl-2 suppresses activation of VPEs by inhibiting cytosolic Ca²⁺ level with elevated K⁺ efflux in NaCl-induced PCD in rice

Bcl-2 is one of the most important antiapoptotic members in mammals and prevents many forms of apoptosis in a variety of cell types. Our previous study revealed that overexpression of Bcl-2 significantly suppressed H2O2/NaCl-induced programmed cell death via inhibiting the transcriptional activation of OsVPE2 and OsVPE3 in transgenic rice. However, Ca(2+) and K(+) homeostasis of this process remains largely unknown. In the present study, we investigate whether nonselective cation channels (NSCC) blockers affect Bcl-2 function in rice under salt stress and how Bcl-2 affects ion homeostasis in salt stress-induced PCD. The results showed that overexpression of Bcl-2 significantly decreased transient elevations in the cytosolic Ca(2+) levels, inhibited NaCl-induced K(+) efflux but not H(+) efflux across the plasma membrane, and further suppressed the expression levels of OsVPE2 and OsVPE3, leading to the inhibition of salt-induced PCD and increase of tolerance to salt stress in transgenic rice. During the NaCl-induced PCD, the effects of a NSCC blocker La(3+) on ion homeostasis and VPEs expression in wild-type were similar to the effects of Bcl-2 overexpression in transgenic line. However, a synergistic effect of Bcl-2 and La(3+) was not obviously detectable. Our results suggested that Bcl-2 played an important role in suppression of NaCl-induced PCD by disruption of ion homeostasis, providing an insight into the mechanistic study of plant VPEs, cytosolic Ca(2+) level and K(+) efflux.