Physiological roles of nonselective cation channels in plants: from salt stress to signalling and development

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

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

De-novo Domestication for Improving Salt Tolerance in Crops

Global agriculture production is under serious threat from rapidly increasing population and adverse climate changes. Food security is currently a huge challenge to feed 10 billion people by 2050. Crop domestication through conventional approaches is not good enough to meet the food demands and unable to fast-track the crop yields. Also, intensive breeding and rigorous selection of superior traits causes genetic erosion and eliminates stress-responsive genes, which makes crops more prone to abiotic stresses. Salt stress is one of the most prevailing abiotic stresses that poses severe damages to crop yield around the globe. Recent innovations in state-of-the-art genomics and transcriptomics technologies have paved the way to develop salinity tolerant crops. De novo domestication is one of the promising strategies to produce superior new crop genotypes through exploiting the genetic diversity of crop wild relatives (CWRs). Next-generation sequencing (NGS) technologies open new avenues to identifying the unique salt-tolerant genes from the CWRs. It has also led to the assembly of highly annotated crop pan-genomes to snapshot the full landscape of genetic diversity and recapture the huge gene repertoire of a species. The identification of novel genes alongside the emergence of cutting-edge genome editing tools for targeted manipulation renders de novo domestication a way forward for developing salt-tolerance crops. However, some risk associated with gene-edited crops causes hurdles for its adoption worldwide. Halophytes-led breeding for salinity tolerance provides an alternative strategy to identify extremely salt tolerant varieties that can be used to develop new crops to mitigate salinity stress.

Tissue-specificity of ROS-induced K+ and Ca2+ fluxes in succulent stems of the perennial halophyte Sarcocornia quinqueflora in the context of salinity stress tolerance

The ability of halophytes to thrive under saline conditions implies efficient ROS detoxification and signalling. In this work, the causal relationship between key membrane transport processes involved in maintaining plant ionic homeostasis and oxidative stress tolerance was investigated in a succulent perennial halophyte Sarcocornia quinqueflora. The flux responses to oxidative stresses induced by either hydroxyl radicals (OH^•) or hydrogen peroxide (H₂O₂) were governed largely by (1) the type of ROS applied; (2) the tissue-specific origin and function (parenchymatic or chlorenchymatic); and (3) the tissue location in respect to the suberized endodermal barrier. The latter implied significant differences in responses between outer (water storage-WS; palisade tissue-Pa) and inner (internal photosynthetic layer-IP; stele parenchyma-SP) stem tissues. The ability of the cell to retain K⁺ under OH^• stress varied between different tissues and was ranked in the following descending order: WS>Pa>IP>SP. OH^• always led to Ca²⁺ influx in all stem tissues, while treatment with H₂O₂ induced tissue-specific Ca²⁺ "signatures". The inner/outer K⁺ ratio was the highest (~2.6) under the optimum NaCl dosage (200 mM) in comparison to non-saline (~0.4) and severe (800 mM; ~0.7) conditions, implying that a higher K⁺ concentration in the inner tissues is important for optimum growth. The overall results demonstrate a clear link between plant anatomical structure and ability of its tissues to maintain ionic homeostasis, via modulating their ROS sensitivity.

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.

ANNEXIN1 mediates calcium-dependent systemic defense in Arabidopsis plants upon herbivory and wounding

Cellular calcium (Ca) transients are endogenous signals involved in local and systemic signaling and defense activation upon environmental stress, including wounding and herbivory. Still, not all Ca²⁺ channels contributing to the signaling have been identified, nor are their modes of action fully known. Plant annexins are proteins capable of binding to anionic phospholipids and can exhibit Ca channel-like activity. Arabidopsis ANNEXIN1 (ANN1) is suggested to contribute to Ca transport. Here, we report that wounding and simulated-herbivory-induced cytosolic free Ca elevation was impaired in systemic leaves in ann1 loss-of-function plants. We provide evidence for a role of ANN1 in local and systemic defense of plants attacked by herbivorous Spodoptera littoralis larvae. Bioassays identified ANN1 as a positive defense regulator. Spodoptera littoralis feeding on ann1 gained significantly more weight than larvae feeding on wild-type, whereas those feeding on ANN1-overexpressing lines gained less weight. Herbivory and wounding both induced defense-related responses on treated leaves, such as jasmonate accumulation and defense gene expression. These responses remained local and were strongly reduced in systemic leaves in ann1 plants. Our results indicate that ANN1 plays an important role in activation of systemic rather than local defense in plants attacked by herbivorous insects.

The Ca2+-CaM Signaling Pathway Mediates Potassium Uptake by Regulating Reactive Oxygen Species Homeostasis in Tobacco Roots Under Low-K+ Stress

Potassium (K⁺) deficiency severely threatens crop growth and productivity. Calcium (Ca²⁺) signaling and its sensors play a central role in the response to low-K⁺ stress. Calmodulin (CaM) is an important Ca²⁺ sensor. However, the mechanism by which Ca²⁺ signaling and CaM mediate the response of roots to low-K⁺ stress remains unclear. In this study, we found that the K⁺ concentration significantly decreased in both shoots and roots treated with Ca²⁺ channel blockers, a Ca²⁺ chelator, and CaM antagonists. Under low-K⁺ stress, reactive oxygen species (ROS) accumulated, and the activity of antioxidant enzymes, NAD kinase (NADK), and NADP phosphatase (NADPase) decreased. This indicates that antioxidant enzymes, NADK, and NADPase might be downstream target proteins in the Ca²⁺-CaM signaling pathway, which facilitates K⁺ uptake in plant roots by mediating ROS homeostasis under low-K⁺ stress. Moreover, the expression of NtCNGC3, NtCNGC10, K⁺ channel genes, and transporter genes was significantly downregulated in blocker-treated, chelator-treated, and antagonist-treated plant roots in the low K⁺ treatment, suggesting that the Ca²⁺-CaM signaling pathway may mediate K⁺ uptake by regulating the expression of these genes. Overall, this study shows that the Ca²⁺-CaM signaling pathway promotes K⁺ absorption by regulating ROS homeostasis and the expression of K⁺ uptake-related genes in plant roots under low-K⁺ stress.

Zinc toxicity in plants: a review

This review highlights the most recent updated information available about Zn phytotoxicity at physiological, biochemical and molecular levels, uptake mechanisms as well as excess Zn homeostasis in plants. Zinc (Zn) is a natural component of soil in terrestrial environments and is a vital element for plant growth, as it performs imperative functions in numerous metabolic pathways. However, potentially noxious levels of Zn in soils can result in various alterations in plants like reduced growth, photosynthetic and respiratory rate, imbalanced mineral nutrition and enhanced generation of reactive oxygen species. Zn enters into soils through various sources, such as weathering of rocks, forest fires, volcanoes, mining and smelting activities, manure, sewage sludge and phosphatic fertilizers. The rising alarm in environmental facet, as well as, the narrow gap between Zn essentiality and toxicity in plants has drawn the attention of the scientific community to its effects on plants and crucial role in agricultural sustainability. Hence, this review focuses on the most recent updates about various physiological and biochemical functions perturbed by high levels of Zn, its mechanisms of uptake and transport as well as molecular aspects of surplus Zn homeostasis in plants. Moreover, this review attempts to understand the mechanisms of Zn toxicity in plants and to present novel perspectives intended to drive future investigations on the topic. The findings will further throw light on various mechanisms adopted by plants to cope with Zn stress which will be of great significance to breeders for enhancing tolerance to Zn contamination.

Multi-Environment Quantitative Trait Loci Mapping for Grain Iron and Zinc Content Using Bi-parental Recombinant Inbred Line Mapping Population in Pearl Millet

Pearl millet is a climate-resilient, nutritious crop with low input requirements that could provide economic returns in marginal agro-ecologies. In this study, we report quantitative trait loci (QTLs) for iron (Fe) and zinc (Zn) content from three distinct production environments. We generated a genetic linkage map using 210 F₆ recombinant inbred line (RIL) population derived from the (PPMI 683 × PPMI 627) cross using genome-wide simple sequence repeats (SSRs). The molecular linkage map (seven linkage groups) of 151 loci was 3,273.1 cM length (Kosambi). The content of grain Fe in the RIL population ranged between 36 and 114 mg/Kg, and that of Zn from 20 to 106 mg/Kg across the 3 years (2014-2016) at over the three locations (Delhi, Dharwad, and Jodhpur). QTL analysis revealed a total of 22 QTLs for grain Fe and Zn, of which 14 were for Fe and eight were for Zn on three consecutive years at all locations. The observed phenotypic variance (R ²) explained by different QTLs for grain Fe and Zn content ranged from 2.85 (QGFe.E3.2014-2016_Q3) to 19.66% (QGFe.E1.2014-2016_Q3) and from 2.93 (QGZn.E3.2014-2016_Q3) to 25. 95% (QGZn.E1.2014-2016_Q1), respectively. Two constitutive expressing QTLs for both Fe and Zn co-mapped in this population, one on LG 2 and second one on LG 3. Inside the QTLs candidate genes such as Ferritin gene, Al³⁺ Transporter, K⁺ Transporters, Zn²⁺ transporters and Mg²⁺ transporters were identified using bioinformatics approaches. The identified QTLs and candidate genes could be useful in pearl millet population improvement programs, seed, restorer parents, and marker-assisted selection programs.

Cyclic voltammetry of volatile memristors in the Venus flytrap: short-term memory

Plants have sensory, short-term and long-term memory. Possible candidates for memory in plants are memristors; resistors with memory. Memristors have been found in seeds, plants, flowers and fruits. The electrostimulation of plants by bipolar periodic waves can induce electrical responses with fingerprints of volatile or non-volatile memristors. Here, we show that the electrostimulation of the Venus flytrap (Dionaea muscipula Ellis) by unipolar sinusoidal or triangular periodic electrical trains induces electrical responses in plants with fingerprints of volatile memristors. The discovery of volatile generic memristors in plants opens new directions in the modelling and understanding of electrical phenomena in the plant kingdom.

Regulation of salt-stressed sunflower (Helianthus annuus) seedling's water status by the coordinated action of Na+/K+ accumulation, nitric oxide, and aquaporin expression

Among abiotic stresses, salt stress is a major threat to crop production all over the world. Present work demonstrates the profuse accumulation of Na+ in 2-day-old, dark-grown sunflower (Helianthus annuus L.) seedlings roots in response to salt stress (NaCl). The pattern of K+ accumulation in response to salt stress is similar to that of Na+ but on relatively lower scale. Application of nitric oxide (NO) donor (DETA) scales down Na+ accumulation in salt-stressed seedlings. The impact of NO donor on K+ accumulation is, however, different in control and salt-stressed seedling roots. In control seedlings, it enhances K+ accumulation, whereas, it gets reduced in salt-stressed seedlings. Specialised channels called 'aquaporins' (AQPs) play a major role maintaining the water status and transport across plant parts under salt-stress. Thus, accumulation of plasma-membrane intrinsic proteins (PIPs) and tonoplast-intrinsic proteins (TIPs), localised on plasma-membrane and vacuolar-membrane, respectively was undertaken in 2-day-old, dark-grown seedling roots. Salt stress increased the abundance of these isoforms, whereas, NO application resulted in decreased accumulation of PIP2 and TIP1. PIP1 and TIP2 isoforms remained undetectable. Present work thus, puts forward a correlation between AQP expression and ions (Na+ and K+) homeostasis in response to salt stress and NO.

Augmenting salt tolerance in rice by regulating uptake and tissue specific accumulation of Na+ - through Ca2+ -induced alteration of biochemical events

The protective effect of Ca²⁺ against NaCl toxicity was investigated in two rice varieties with contrasting for salt tolerance to understand the mechanistic details of the antagonism to address adverse effects of salinity on agriculture. The study primarily examined the influence of Ca²⁺ on expression/activity of the effectors and regulators involved in Na⁺ translocation. Calcium reduced uptake of Na⁺ concomitant with higher tissue K⁺ /Na⁺ in seedlings, comparatively more in salt-tolerant Nona Bokra than in salt-sensitive IR-64, together with a significant increase in root PM H⁺ ATPase in the former, but not in the latter. Increased antagonism in Nona Bokra could be the result of Ca²⁺ signalling-mediated phosphorylation of PM H⁺ ATPase in roots caused by a significant Ca²⁺ -dependent increase in expression of OsCIPK24, which did not occur in IR-64. Furthermore, significant Ca²⁺ -mediated NaCl-induced increase in transcription of 14-3-3 protein in Nona Bokra, but not in IR-64, might also lead to a greater protective effect of Ca²⁺ in the former, as 14-3-3 protein is essential for activating PM H⁺ ATPase. Thus, efficient functioning of PM H⁺ ATPase could be key in determining resistance of plants to salinity, implying that identification of the Ca²⁺ -dependent kinase phosphorylating the PM H⁺ ATPase threonine residue and manipulation of its expression, together with expression of 14-3-3 proteins could be an important strategy to improve salt tolerance of crops and their cultivation in salt-affected lands.

Involvement of ethylene receptors in the salt tolerance response of Cucurbita pepo

Abiotic stresses have a negative effect on crop production, affecting both vegetative and reproductive development. Ethylene plays a relevant role in plant response to environmental stresses, but the specific contribution of ethylene biosynthesis and signalling components in the salt stress response differs between Arabidopsis and rice, the two most studied model plants. In this paper, we study the effect of three gain-of-function mutations affecting the ethylene receptors CpETR1B, CpETR1A, and CpETR2B of Cucurbita pepo on salt stress response during germination, seedling establishment, and subsequent vegetative growth of plants. The mutations all reduced ethylene sensitivity, but enhanced salt tolerance, during both germination and vegetative growth, demonstrating that the three ethylene receptors play a positive role in salt tolerance. Under salt stress, etr1b, etr1a, and etr2b germinate earlier than WT, and the root and shoot growth rates of both seedlings and plants were less affected in mutant than in WT. The enhanced salt tolerance response of the etr2b plants was associated with a reduced accumulation of Na⁺ in shoots and leaves, as well as with a higher accumulation of compatible solutes, including proline and total carbohydrates, and antioxidant compounds, such as anthocyanin. Many membrane monovalent cation transporters, including Na⁺/H⁺ and K⁺/H⁺ exchangers (NHXs), K⁺ efflux antiporters (KEAs), high-affinity K⁺ transporters (HKTs), and K⁺ uptake transporters (KUPs) were also highly upregulated by salt in etr2b in comparison with WT. In aggregate, these data indicate that the enhanced salt tolerance of the mutant is led by the induction of genes that exclude Na⁺ in photosynthetic organs, while maintaining K⁺/Na⁺ homoeostasis and osmotic adjustment. If the salt response of etr mutants occurs via the ethylene signalling pathway, our data show that ethylene is a negative regulator of salt tolerance during germination and vegetative growth. Nevertheless, the higher upregulation of genes involved in Ca²⁺ signalling (CpCRCK2A and CpCRCK2B) and ABA biosynthesis (CpNCED3A and CpNCED3B) in etr2b leaves under salt stress likely indicates that the function of ethylene receptors in salt stress response in C. pepo can be mediated by Ca²⁺ and ABA signalling pathways.

Citric acid inhibits Cd uptake by improving the preferential transport of Mn and triggering the defense response of amino acids in grains

Citric acid (CA) can regulate the balance of anions and cations in plants, and improve their resistance to heavy metals. It is not clear if foliar application with CA has any effect on migration of Cd and Mn in rice plant. In this work, a low-Cd-accumulating indica rice line (P7) and a high-Cd-accumulating line (HZ) were used to investigate the influence of CA on the transport of Cd and Mn as well as amino acid metabolism in grains. Content of Cd in grains and other organs increased with the increase of Cd content (0.1-2.4 mg kg^-1) in soil, while decreased with the foliar application with CA. With the increase of Cd content in rice grains, the content of most amino acids in HZ, P7, HZ+CA and P7 + CA showed an obvious decline trend. Foliar application with CA efficiently raised the Mn:Cd ratio in grains and nodes of both HZ and P7. Meanwhile, the expression levels of OsNramp2, 3 and 5 in panicles were efficiently enhanced by CA application when plants grew in soil with Cd content of 0.6-2.4 mg kg^-1. The increasing effect of CA on the content of 4 amino acids (i.e., Glu, Phe, Thr and Ala) in grains was related to varieties and Cd pollution. These results indicate that foliar application with CA can regulate the transport of Cd and Mn in the opposite directions in tissues and inhibit Cd accumulation in grains by enhancing expression of OsNRAMP 2, 3 or 5 and triggering the defense response of some amino acids in Cd-contaminated environment.

Effect of NaCl road salt on the ionic composition of soils and Aesculus hippocastanum L. foliage and leaf damage intensity

We investigated the accumulation of sodium chloride in roadside soils and common horse chestnut Aesculus hippocastanum L. under urban conditions to evaluate changes in soil and leaf ionic content and their relationship with foliar damage, considering the visual assessment of trees of the same health status. A total of 15 field sites were assessed in late June 2016. The analysis included soil granulometric composition, pH, electrical conductivity, and the content of Cl⁻, Na⁺, K⁺, Ca²⁺, and Mg²⁺ ions in soil and foliage samples. The results showed increased salinity and alkalization of roadside soils together with the decreased magnesium content. Foliage samples manifested significantly higher concentrations of Na⁺ and Cl⁻. A wide range of Cl⁻ content was noted in leaves (2.0–11.8% d.w.) regardless of their damage index. On the contrary, leaf damage was strongly correlated with increasing Na⁺ concentrations and decreasing K⁺ and Mg²⁺. A severe imbalance of nutrients, and therefore poor urban tree vitality, can be attributed to the excessive accumulation of de-icing salt. However, further research would be needed to clarify the discrepancy between the extent of leaf damage and chloride content.

Populus euphratica annexin1 facilitates cadmium enrichment in transgenic Arabidopsis

Phytoremediation offers a great potential for affordable remediation of heavy metal (HM)-polluted soil and water. Screening and identifying candidate genes related to HM uptake and transport is prerequisite for improvement of phytoremediation by genetic engineering. Using the cadmium (Cd)-hypersensitive Populus euphratica, an annexin encoding gene facilitating Cd enrichment was identified in this study. With a 12 h exposure to CdCl₂ (50-100 μM), P. euphratica cells down-regulated transcripts of annexin1 (PeANN1). PeANN1 was homologue to Arabidopsis annexin1 (AtANN1) and localized mainly to the plasma membrane (PM) and cytosol. Compared with wild type and Atann1 mutant, PeANN1 overexpression in Arabidopsis resulted in a more pronounced decline in survival rate and root length after a long-term Cd stress (10 d, 50 μM), due to a higher cadmium accumulation in roots. PeANN1-transgenic roots exhibited enhanced influx conductance of Cd²⁺ under cadmium shock (30 min, 50 μM) and short-term stress (12 h, 50 μM). Noteworthy, the PeANN1-facilitated Cd²⁺ influx was significantly inhibited by a calcium-permeable channel (CaPC) inhibitor (GdCl₃) but was promoted by 1 mM H₂O₂, indicating that Cd²⁺ entered root cells via radical-activated CaPCs in the PM. Therefore, PeANN1 can serve as a candidate gene for improvement of phytoremediation by genetic engineering.

Antioxidant Enzymatic Activity and Osmotic Adjustment as Components of the Drought Tolerance Mechanism in Carex duriuscula

Drought stress is a major environmental constraint for plant growth. Climate-change-driven increases in ambient temperatures resulted in reduced or unevenly distributed rainfalls, leading to increased soil drought. Carex duriuscula C. A. Mey is a typical drought-tolerant sedge, but few reports have examined the mechanisms conferring its tolerant traits. In the present study, the drought responses of C. duriuscula were assessed by quantifying activity of antioxidant enzymes in its leaf and root tissues and evaluating the relative contribution of organic and inorganic osmolyte in plant osmotic adjustment, linking it with the patterns of the ion acquisition by roots. Two levels of stress-mild (MD) and severe (SD) drought treatments-were used, followed by re-watering. Drought stress caused reduction in a relative water content and chlorophyll content of leaves; this was accompanied by an increase in the hydrogen peroxide (H2O2) and superoxide (O2-) contents in leaves and roots. Under MD stress, the activities of catalase (CAT), peroxidase (POD), and glutathione peroxidase (GPX) increased in leaves, whereas, in roots, only CAT and POD activities increased. SD stress led to an increase in the activities of CAT, POD, superoxide dismutase (SOD), and GPX in both tissues. The levels of proline, soluble sugars, and soluble proteins in the leaves also increased. Under both MD and SD stress conditions, C. duriuscula increased K+, Na+, and Cl- uptake by plant roots, which resulted in an increased K+, Na+, and Cl- concentrations in leaves and roots. This reliance on inorganic osmolytes enables a cost-efficient osmotic adjustment in C. duriuscula. Overall, this study revealed that C. duriuscula was able to survive arid environments due to an efficient operation of its ROS-scavenging systems and osmotic adjustment mechanisms.

The role of PQL genes in response to salinity tolerance in Arabidopsis and barley

While soil salinity is a global problem, how salt enters plant root cells from the soil solution remains underexplored. Non-selective cation channels (NSCCs) are suggested to be the major pathway for the entry of sodium ions (Na+), yet their genetic constituents remain unknown. Yeast PQ loop (PQL) proteins were previously proposed to encode NSCCs, but the role of PQLs in plants is unknown. The hypothesis tested in this research is that PQL proteins constitute NSCCs mediating some of the Na+ influx into the root, contributing to ion accumulation and the inhibition of growth in saline conditions. We identified plant PQL homologues, and studied the role of one clade of PQL genes in Arabidopsis and barley. Using heterologous expression of AtPQL1a and HvPQL1 in HEK293 cells allowed us to resolve sizable inwardly directed currents permeable to monovalent cations such as Na+, K+, or Li+ upon membrane hyperpolarization. We observed that GFP-tagged PQL proteins localized to intracellular membrane structures, both when transiently over-expressed in tobacco leaf epidermis and in stable Arabidopsis transformants. Expression of AtPQL1a, AtPQL1b, and AtPQL1c was increased by salt stress in the shoot tissue compared to non-stressed plants. Mutant lines with altered expression of AtPQL1a, AtPQL1b, and AtPQL1c developed larger rosettes in saline conditions, while altered levels of AtPQL1a severely reduced development of lateral roots in all conditions. This study provides the first step toward understanding the function of PQL proteins in plants and the role of NSCC in salinity tolerance.

Functions and structure of roots and their contributions to salinity tolerance in plants

Soil salinity is an increasing threat to the productivity of glycophytic crops worldwide. The root plays vital roles under various stress conditions, including salinity, as well as has diverse functions in non-stress soil environments. In this review, we focus on the essential functions of roots such as in ion homeostasis mediated by several different membrane transporters and signaling molecules under salinity stress and describe recent advances in the impacts of quantitative trait loci (QTLs) or genetic loci (and their causal genes, if applicable) on salinity tolerance. Furthermore, we introduce important literature for the development of barriers against the apoplastic flow of ions, including Na⁺, as well as for understanding the functions and components of the barrier structure under salinity 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.

Coconut genome assembly enables evolutionary analysis of palms and highlights signaling pathways involved in salt tolerance

Coconut (Cocos nucifera) is the emblematic palm of tropical coastal areas all around the globe. It provides vital resources to millions of farmers. In an effort to better understand its evolutionary history and to develop genomic tools for its improvement, a sequence draft was recently released. Here, we present a dense linkage map (8402 SNPs) aiming to assemble the large genome of coconut (2.42 Gbp, 2n = 32) into 16 pseudomolecules. As a result, 47% of the sequences (representing 77% of the genes) were assigned to 16 linkage groups and ordered. We observed segregation distortion in chromosome Cn15, which is a signature of strong selection among pollen grains, favouring the maternal allele. Comparing our results with the genome of the oil palm Elaeis guineensis allowed us to identify major events in the evolutionary history of palms. We find that coconut underwent a massive transposable element invasion in the last million years, which could be related to the fluctuations of sea level during the glaciations at Pleistocene that would have triggered a population bottleneck. Finally, to better understand the facultative halophyte trait of coconut, we conducted an RNA-seq experiment on leaves to identify key players of signaling pathways involved in salt stress response. Altogether, our findings represent a valuable resource for the coconut breeding community.

Potassium and phosphorus transport and signaling in plants

Nitrogen (N), potassium (K), and phosphorus (P) are essential macronutrients for plant growth and development, and their availability affects crop yield. Compared with N, the relatively low availability of K and P in soils limits crop production and thus threatens food security and agricultural sustainability. Improvement of plant nutrient utilization efficiency provides a potential route to overcome the effects of K and P deficiencies. Investigation of the molecular mechanisms underlying how plants sense, absorb, transport, and use K and P is an important prerequisite to improve crop nutrient utilization efficiency. In this review, we summarize current understanding of K and P transport and signaling in plants, mainly taking Arabidopsis thaliana and rice (Oryza sativa) as examples. We also discuss the mechanisms coordinating transport of N and K, as well as P and N.

Melatonin improves K+ and Na+ homeostasis in rice under salt stress by mediated nitric oxide

Rice (Oryza sativa L.) productivity is greatly affected by soil salinity and melatonin (MLT) has long been recognized as a positive molecule that can alleviate the damage caused by salt. Here, the role of nitric oxide (NO) in the regulation of salt tolerance by MLT was investigated in rice. MLT pretreatment increased the fresh and dry weight of rice seedlings under salt stress. Its beneficial effects include less relative electrolyte leakage (REL) and better K⁺/Na⁺ homeostasis. MLT increased the activity of nitric oxide synthase (NOS). The polyamines (PAs) content and the utilization of arginine were also increased, thereby increasing NO content in salt-stressed rice seedlings. Pharmacological approach showed that NO, as a necessary downstream signaling molecule, was involved in the regulation of MLT on the K⁺/Na⁺ homeostasis of rice. Under salt stress, MLT improved the H⁺-pumps activities in plasma membrane (PM) and vacuole membrane (VM) in roots, MLT also increased the ATP content of rice roots by increasing the NO content of rice. Thus, the efflux of Na⁺ and the influx of K⁺ were promoted. When endogenous NO was scavenged, the regulation of K⁺/Na⁺ homeostasis by MLT was blocked. Therefore, MLT mediated K⁺/Na⁺ homeostasis of rice under salt stress by mediating NO.

Biochemical and molecular characterisations of salt tolerance components in rice varieties tolerant and sensitive to NaCl: the relevance of Na+ exclusion in salt tolerance in the species

Soil salinisation is a major abiotic stress in agriculture, and is especially a concern for rice production because among cereal crops, rice is the most salt-sensitive. However, the production of rice must be increased substantially by the year 2050 to meet the demand of the ever growing population. Hence, understanding the biochemical events determining salt tolerance in rice is highly desirable so that the trait can be introduced in cultivars of interest through biotechnological intervention. In this context, an initial study on NaCl response in four Indica rice varieties showed a lower uptake of Na+ in the salt-tolerant Nona Bokra and Pokkali than in the salt-sensitive IR64 and IR29, indicating Na+ exclusion as a primary requirement of salt tolerance in the species. This was also supported by the following features in the salt-tolerant, but not in the -sensitive varieties: (1) highly significant NaCl-induced increase in the activity of PM-H+ATPase, (2) a high constitutive level and NaCl-induced threonine phosphorylation of PM-H+ATPase, necessary to promote its activity, (3) a high constitutive expression of 14-3-3 protein that makes PM-H+ATPase active by binding with the phosphorylated threonine at the C-terminal end, (4) a high constitutive and NaCl-induced expression of SOS1 in roots, and (5) significant NaCl-induced expression of OsCIPK 24, a SOS2 that phosphorylates SOS1. The vacuolar sequestration of Na+ in seedlings was not reflected from the expression pattern of NHX1/NHX1 in response to NaCl. NaCl-induced downregulation of expression of HKTs in roots of Nona Bokra, but upregulation in Pokkali also indicates that their role in salt tolerance in rice could be cultivar specific. The study indicates that consideration of increasing exclusion of Na+ by enhancing the efficiency of SOS1/PM-H+ATPase Na+ exclusion module could be an important aspect in attempting to increase salt tolerance in the rice varieties or cultivars of interest.

Glucosinolate Transporter1 involves in salt-induced jasmonate signaling and alleviates the repression of lateral root growth by salt in Arabidopsis

Salt stress has negative impact on plant development and growth. Jasmonic acid (JA), a phytohormone, has been shown to involve in salt-induced inhibition of primary root growth. The Arabidopsis Glucosinolate transporter1 (GTR1/NPF2.10) is characterized as a JA-Ile, a bioactive form of JA, transporter. However, whether GTR1 participates in salt responses is not clear. In this study, we confirmed that GTR1 is induced by both JA and salinity. Salt-induced JA signaling is affected in gtr1 mutant. The JA responsive genes, JAZ1, JAZ5, MYC2, LOX3, are down-regulated in gtr1 mutant. Phenotypic analyses showed that the salinity-induced lateral root growth inhibition is enhanced in gtr1 mutant, suggesting that GTR1 plays a positive role in lateral root development under salt stress. Interestingly, the expression of a Na⁺ transporter, HKT1, is upregulated in gtr1. Since HKT1 is a negative regulator for lateral root development under salt stress, we proposed that GTR1 alleviates the repression of lateral root development by salt stress by mediating JA signaling and repressing HKT1 expression. This study demonstrates that GTR1 is the molecular link between salt stress, JA signaling, and lateral root development.

Rice grains alleviate cadmium toxicity by expending glutamate and increasing manganese in the cadmium contaminated farmland

The accumulation of cadmium (Cd) in rice grains is closely associated with the content of mineral nutrients and amino acid metabolism, but the causal link among them is unclear. Profiles of amino acids (AAs) and quantities of essential nutrients in grains from early and late rice cultivars grown at four sites with different Cd levels were analyzed in the present study. Hazard quotients (HQs) for consumers by intake of rice from late cultivars were much higher than that from early cultivars at sites with soil Cd content of 0.25, 0.61 and 0.84 mg kg^-1. Cadmium accumulation in grains resulted in a sharp reduction of total essential AAs and non-essential AAs in both early and late rice cultivars. High-Cd-accumulating (HCA) cultivars had significantly higher level of glutamate (Glu) than low-Cd-accumulating (LCA) cultivars when rice Cd content was less than 0.20 mg kg^-1. However, Glu level in grains dramatically declined with the accumulation of Cd, which subsequently leaded to the reduction of other AAs. Cadmium content was well predicted by five amino acids (i.e., Glu, Alanine, Phenylalanine, Glycine and Threonine) or four essential elements (Ca, Fe, Mn and Zn) when rice Cd was less than 0.80 mg kg^-1. Amino acids played more important roles than nutrients in Cd accumulation. When Cd content was in the range of 0.40-1.16 mg kg^-1, the Mn content in rice increased significantly with the increase of Cd content, while the Glu content dropped down synchronously. Remarkably, the ratio between Mn and Glu displayed the highest direct path coefficient on Cd accumulation than any single cation or amino acid. These results indicate that high capacity in synthesizing Glu and concentrating Mn is the determinant factor for Cd accumulation in rice grains, and abundant Glu in aleurone layer may alleviate Cd toxicity by forming Glu-Cd complex.

Genetic and Physiological Characterization of a Calcium Deficiency Phenotype in Maize

Calcium (Ca) is an essential plant nutrient, required for signaling, cell wall fortification and growth and development. Calcium deficiency (Ca-deficiency) in maize causes leaf tip rot and a so-called “bull-whipping” or “buggy-whipping” phenotype. Seedlings of the maize line B73 displayed these Ca-deficiency-like symptoms when grown in the greenhouse with excess fertilizer during the winter months, while seedlings of the Mo17 maize line did not display these symptoms under the same conditions. These differential phenotypes could be recapitulated in ‘mini-hydroponic’ systems in the laboratory in which high ammonium, but not nitrate, levels induced the symptoms in B73 but not Mo17 seedlings. Consistent with this phenotype being caused by Ca-deficiency, addition of Ca²⁺ completely relieved the symptoms. These data suggest that ammonium reduces the seedling’s ability to absorb calcium, which causes the Ca-deficiency phenotype, and that this trait varies among genotypes. A recombinant inbred line (RIL) population derived from a B73 x Mo17 cross was used to map quantitative trait loci (QTL) associated with the Ca-deficiency phenotype. QTL associated with variation in susceptibility to Ca-deficiency were detected on chromosomes 1, 2, 3, 6 which explained between 3.30–9.94% of the observed variation. Several genes predicted to bind or be activated by calcium map to these QTL on chromosome 1, 2, 6. These results describe for the first time the genetics of Ca-deficiency symptoms in maize and in plants in general.

Salt Stress Signals on Demand: Cellular Events in the Right Context

Plant stress is a real dilemma; it puzzles plant biologists and is a global problem that negatively affects people’s daily lives. Of particular interest is salinity, because it represents one of the major water-related stress types. We aimed to determine the signals that guide the cellular-related events where various adaptation mechanisms cross-talk to cope with salinity-related water stress in plants. In an attempt to unravel these mechanisms and introduce cellular events in the right context, we expansively discussed how salt-related signals are sensed, with particular emphasis on aquaporins, nonselective cation channels (NSCCs), and glycosyl inositol phosphorylceramide (GIPC). We also elaborated on the critical role Ca2⁺, H⁺, and ROS in mediating signal transduction pathways associated with the response and tolerance to salt stress. In addition, the fragmentary results from the literature were compiled to develop a harmonized, informational, and contemplative model that is intended to improve our perception of these adaptative mechanisms and set a common platform for plant biologists to identify intriguing research questions in this area.

Salt Tolerance Mechanisms of Plants

Crop loss due to soil salinization is an increasing threat to agriculture worldwide. This review provides an overview of cellular and physiological mechanisms in plant responses to salt. We place cellular responses in a time- and tissue-dependent context in order to link them to observed phases in growth rate that occur in response to stress. Recent advances in phenotyping can now functionally or genetically link cellular signaling responses, ion transport, water management, and gene expression to growth, development, and survival. Halophytes, which are naturally salt-tolerant plants, are highlighted as success stories to learn from. We emphasize that (a) filling the major knowledge gaps in salt-induced signaling pathways, (b) increasing the spatial and temporal resolution of our knowledge of salt stress responses, (c) discovering and considering crop-specific responses, and (d) including halophytes in our comparative studies are all essential in order to take our approaches to increasing crop yields in saline soils to the next level.

Engineering salinity tolerance in plants: progress and prospects

There is a need to integrate conceptual framework based on the current understanding of salt stress responses with different approaches for manipulating and improving salt tolerance in crop plants. Soil salinity exerts significant constraints on global crop production, posing a serious challenge for plant breeders and biotechnologists. The classical transgenic approach for enhancing salinity tolerance in plants revolves by boosting endogenous defence mechanisms, often via a single-gene approach, and usually involves the enhanced synthesis of compatible osmolytes, antioxidants, polyamines, maintenance of hormone homeostasis, modification of transporters and/or regulatory proteins, including transcription factors and alternative splicing events. Occasionally, genetic manipulation of regulatory proteins or phytohormone levels confers salinity tolerance, but all these may cause undesired reduction in plant growth and/or yields. In this review, we present and evaluate novel and cutting-edge approaches for engineering salt tolerance in crop plants. First, we cover recent findings regarding the importance of regulatory proteins and transporters, and how they can be used to enhance salt tolerance in crop plants. We also evaluate the importance of halobiomes as a reservoir of genes that can be used for engineering salt tolerance in glycophytic crops. Additionally, the role of microRNAs as critical post-transcriptional regulators in plant adaptive responses to salt stress is reviewed and their use for engineering salt-tolerant crop plants is critically assessed. The potentials of alternative splicing mechanisms and targeted gene-editing technologies in understanding plant salt stress responses and developing salt-tolerant crop plants are also discussed.

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.

Understanding Mechanisms of Salinity Tolerance in Barley by Proteomic and Biochemical Analysis of Near-Isogenic Lines

Salt stress is one of the major environmental factors impairing crop production. In our previous study, we identified a major QTL for salinity tolerance on chromosome 2H on barley (Hordeum vulgare L.). For further investigation of the mechanisms responsible for this QTL, two pairs of near-isogenic lines (NILs) differing in this QTL were developed. Sensitive NILs (N33 and N53) showed more severe damage after exposure to 300 mM NaCl than tolerant ones (T46 and T66). Both tolerant NILs maintained significantly lower Na⁺ content in leaves and much higher K⁺ content in the roots than sensitive lines under salt conditions, thus indicating the presence of a more optimal Na⁺/K⁺ ratio in plant tissues. Salinity stress caused significant accumulation of H₂O₂, MDA, and proline in salinity-sensitive NILs, and a greater enhancement in antioxidant enzymatic activities at one specific time or tissues in tolerant lines. One pair of NILs (N33 and T46) were used for proteomic studies using two-dimensional gel electrophoresis. A total of 53 and 51 differentially expressed proteins were identified through tandem mass spectrometry analysis in the leaves and roots, respectively. Proteins which are associated with photosynthesis, reactive oxygen species (ROS) scavenging, and ATP synthase were found to be specifically upregulated in the tolerant NIL. Proteins identified in this study can serve as a useful resource with which to explore novel candidate genes for salinity tolerance in barley.

Comparing Kinetics of Xylem Ion Loading and Its Regulation in Halophytes and Glycophytes

Although control of xylem ion loading is essential to confer salinity stress tolerance, specific details behind this process remain elusive. In this work, we compared the kinetics of xylem Na+ and K+ loading between two halophytes (Atriplex lentiformis and quinoa) and two glycophyte (pea and beans) species, to understand the mechanistic basis of the above process. Halophyte plants had high initial amounts of Na+ in the leaf, even when grown in the absence of the salt stress. This was matched by 7-fold higher xylem sap Na+ concentration compared with glycophyte plants. Upon salinity exposure, the xylem sap Na+ concentration increased rapidly but transiently in halophytes, while in glycophytes this increase was much delayed. Electrophysiological experiments using the microelectrode ion flux measuring technique showed that glycophyte plants tend to re-absorb Na+ back into the stele, thus reducing xylem Na+ load at the early stages of salinity exposure. The halophyte plants, however, were capable to release Na+ even in the presence of high Na+ concentrations in the xylem. The presence of hydrogen peroxide (H2O2) [mimicking NaCl stress-induced reactive oxygen species (ROS) accumulation in the root] caused a massive Na+ and Ca2+ uptake into the root stele, while triggering a substantial K+ efflux from the cytosol into apoplast in glycophyte but not halophytes species. The peak in H2O2 production was achieved faster in halophytes (30 min vs 4 h) and was attributed to the increased transcript levels of RbohE. Pharmacological data suggested that non-selective cation channels are unlikely to play a major role in ROS-mediated xylem Na+ loading.

Energy costs of salt tolerance in crop plants

Agriculture is expanding into regions that are affected by salinity. This review considers the energetic costs of salinity tolerance in crop plants and provides a framework for a quantitative assessment of costs. Different sources of energy, and modifications of root system architecture that would maximize water vs ion uptake are addressed. Energy requirements for transport of salt (NaCl) to leaf vacuoles for osmotic adjustment could be small if there are no substantial leaks back across plasma membrane and tonoplast in root and leaf. The coupling ratio of the H⁺ -ATPase also is a critical component. One proposed leak, that of Na⁺ influx across the plasma membrane through certain aquaporin channels, might be coupled to water flow, thus conserving energy. For the tonoplast, control of two types of cation channels is required for energy efficiency. Transporters controlling the Na⁺ and Cl^- concentrations in mitochondria and chloroplasts are largely unknown and could be a major energy cost. The complexity of the system will require a sophisticated modelling approach to identify critical transporters, apoplastic barriers and root structures. This modelling approach will inform experimentation and allow a quantitative assessment of the energy costs of NaCl tolerance to guide breeding and engineering of molecular components.

Sodium Influx and Potassium Efflux Currents in Sunflower Root Cells Under High Salinity

Helianthus annuus L. is an important oilseed crop, which exhibits moderate salt tolerance and can be cultivated in areas affected by salinity. Using patch-clamp electrophysiology, we have characterized Na⁺ influx and K⁺ efflux conductances in protoplasts of salt-tolerant H. annuus L. hybrid KBSH-53 under high salinity. This work demonstrates that the plasma membrane of sunflower root cells has a classic set of ionic conductances dominated by K⁺ outwardly rectifying channels (KORs) and non-selective cation channels (NSCCs). KORs in sunflower show extreme Na⁺ sensitivity at high extracellular [Ca²⁺] that can potentially have a positive adaptive effect under salt stress (decreasing K⁺ loss). Na⁺ influx currents in sunflower roots demonstrate voltage-independent activation, lack time-dependent component, and are sensitive to Gd³⁺. Sunflower Na⁺-permeable NSCCs mediate a much weaker Na⁺ influx currents on the background of physiological levels of Ca²⁺ as compared to other species. This suggests that sunflower NSCCs have greater Ca²⁺ sensitivity. The responses of Na⁺ influx to Ca²⁺ correlates well with protection of sunflower growth by external Ca²⁺ in seedlings treated with NaCl. It can be, thus, hypothesized that NaCl tolerance in sunflower seedling roots is programmed at the ion channel level via their sensitivity to Ca²⁺ and Na⁺.

Redox-regulation of ion homeostasis in growing lily pollen tubes

The pollen tube is characterized by cytoplasm compartmentalization typical for cells with polar growth. This concept includes "ion zoning", i.e. gradient distribution of ionic currents across the plasma membrane and free inorganic ions in the cytoplasm. One of the putative mechanisms for maintaining "ion zoning" is indicated by the sensitivity of the ion transport systems to reactive oxygen species (ROS). Here we test the possibility of redox regulation of ionic gradients and membrane potential (MP) gradient in growing pollen tubes using quantitative fluorescence microscopy. ROS quencher MnTMPP and exogenic H₂O₂ cause different alterations of intracellular Ca²⁺ gradient, pH gradient and MP gradient during short-term exposure. MnTMPP significantly shifts the gradients of Ca²⁺ and MP at low concentrations while high concentration cause growth alterations (ballooned tips) and cytoplasm acidification. H₂O₂ at 0,5 and 1 mM affects ion homeostasis as well (MP, Ca²⁺, pH) but doesn't decrease viability or alter shape of the tubes. Here we present original quantitative data on the interconnection between ROS and ion transport during tip growth.

Potential role of root membrane phosphatidic acid in superior agronomic performance of silage-corn cultivated in cool climate cropping systems

The literature is replete with information describing the composition of the root lipidome in several plant species grown under various environmental conditions. However, it is unknown to what extent the root membrane lipidome vary between silage-corn genotypes, and how such variation could influence agronomic performances during field cultivation in cool climate. To address this issue, the root membrane lipidome and agronomic performance were assessed for five silage-corn genotypes (Fusion-RR, Yukon-R, A4177G3-RIB, DKC23-17RIB, DKC26-28RIB) cultivated under cool climatic conditions. Leaf area, plant height and biomass production were used as agronomic performance indicators. Varieties DKC26-28RIB and Yukon-R expressed significantly higher leaf area, plant height and biomass production compared to the other genotypes. A strong positive Spearman rank-order correlation (P = 0.001) was observed between biomass production and root phosphatidic acid (PA). The high correlation observed between PA and agronomic performance indicates PA could potentially be used as biomarker to assist in the selection of silage-corn genotypes with superior agronomic performance ideally suited for field cultivations in cool climatic conditions.

Rice vegetative organs alleviate cadmium toxicity by altering the chemical forms of cadmium and increasing the ratio of calcium to manganese

Altering Cd chemical form is one of the mechanisms to alleviate Cd toxicity in rice plant. Field experiments were carried out in this study to investigate the potential of rice vegetative organs in altering Cd into insoluble chemical forms in the natural environment. Experimental results showed that more than 80% of Cd in rice roots existed in the insoluble forms. Uppermost nodes altered Cd into insoluble form preferentially and generally had higher content of insoluble Cd than other organs. Rachises displayed a slow increasing trend in soluble Cd when total Cd in roots was less than 1.8 mg kg^-1. However, when Cd content in roots exceeded 2.8 mg kg^-1, the ratio of insoluble to soluble Cd remained stable at 85:15 in rachises and roots, and at 75:25 in uppermost nodes and flag leaves. Cd concentration in grains was greatly lower than that in vegetative organs, and closely correlated with the content of soluble Cd in rachises (r = 0.991**) as well as in uppermost nodes. Soluble Cd in the uppermost nodes displayed a much lower mobility than that in other organs. Accumulation of soluble Cd was always companied by decrease of Ca and increase of Mn in roots, uppermost nodes and rachises. A small increase of soluble Cd from 0.05 to 0.1 mg kg^-1 caused a sharp decline of Ca:Mn ratio in roots and rachises. Roots and nodes had much higher Ca:Mn ratio than rachises when soluble Cd was less than 0.5 mg kg^-1 in them. These results indicate that vegetative organs have a great potential to alter more than 75% Cd into insoluble forms and increasing Ca:Mn ratio may be another way to alleviate Cd toxicity by establishing new ionic homeostasis in rice plants.

Cold- and menthol-evoked membrane potential changes in the moss Physcomitrella patens: influence of ion channel inhibitors and phytohormones

Microelectrode measurements carried out on leaf cells from Physcomitrella patens revealed that a sudden temperature drop and application of menthol evoked two types of different-shaped membrane potential changes. Cold stimulation evoked spike-type responses. Menthol depolarized the cell membrane with different rates. When it reached above 1 mV s^-1 , the full response was recorded. Characteristic for the full responses was also a few-minute plateau of the membrane potential recorded after depolarization. The influence of inhibitors of calcium channels (5 mM Gd³⁺ ), potassium channels (5 mM Ba²⁺ ), chloride channels (200 μM Zn²⁺ , 50 μM niflumic acid) and proton pumps (10 μM DES), an activator of calcium release from intracellular stores (Sr²⁺ ), calcium chelation (by 400 μM EGTA) and phytohormones (50 μM auxin, 50 μM abscisic acid (ABA), 500 μM salicylic acid) on cold- and menthol-evoked responses was tested. Both responses are different in respect to the ion mechanism: cold-evoked depolarizations were influenced by Ba²⁺ and DES; in turn, menthol-evoked potential changes were most effectively blocked by Zn²⁺ . Moreover, the effectiveness of menthol in generation of full responses was reduced after administration of auxin or ABA, i.e. phytohormones known for their participation in responses to cold and regulation of proton pumps. The effects of DES indicated that one of the main conditions for generation of menthol-evoked responses is inhibition of the proton pump activity. Our results indicate that perception of cold and menthol by plants proceeds in different ways due to the differences in ionic mechanism and hormone dependence of cold- and menthol-evoked responses.

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

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

Gadolinium inhibits cadmium transport by blocking non-selective cation channels in rice seedlings

Non-selective cation channels (NSCCs) play important roles in uptake of heavy metals in plants. However, little information is available concerning the contribution of NSCCs to cadmium (Cd) transport in rice seedlings. Results from the hydroponic experiment showed that the inhibition of 2.7 μM Cd on the development of rice roots was alleviated by adding 0.1 mM gadolinium (Gd) in nutrient solution, companied by reduction of Cd content by 55.3% in roots and by 45.0% in shoots. Inhibition of Gd on Cd accumulation in cytoplasm fraction (F3) was much greater than that in cell walls (F1) and organelles (F2) in roots. After increasing concentrations of Mn and Zn in nutrient solution, adding 0.1 mM Gd resulted in reductions in Cd content by 89.1%, in micronutrients by 54.9% and in macronutrients by 5.4% in roots, respectively. Cd stress resulted in significant increase of PC_2∼4 and free amino acids, but decrease of V-ATPase activity by 32.3% in roots. These results indicate that NSCCs make a great contribution to uptake of Cd in rice seedlings and opportunities for Cd to be transported by NSCCs can be efficiently reduced by blocking NSCCs and/or increasing essential microelements like Mn and Zn.

Sieve element biology provides leads for research on phytoplasma lifestyle in plant hosts

Phytoplasmas reside exclusively in sieve tubes, tubular arrays of sieve element-companion cell complexes. Hence, the cell biology of sieve elements may reveal (ultra)structural and functional conditions that are of significance for survival, propagation, colonization, and effector spread of phytoplasmas. Electron microscopic images suggest that sieve elements offer facilities for mobile and stationary stages in phytoplasma movement. Stationary stages may enable phytoplasmas to interact closely with diverse sieve element compartments. The unique, reduced sieve element outfit requires permanent support by companion cells. This notion implies a future focus on the molecular biology of companion cells to understand the sieve element-phytoplasma inter-relationship. Supply of macromolecules by companion cells is channelled via specialized symplasmic connections. Ca2+-mediated gating of symplasmic corridors is decisive for the communication within and beyond the sieve element-companion cell complex and for the dissemination of phytoplasma effectors. Thus, Ca2+ homeostasis, which affects sieve element Ca2+ signatures and induces a range of modifications, is a key issue during phytoplasma infection. The exceptional physical and chemical environment in sieve elements seems an essential, though not the only factor for phytoplasma survival.

Involvement of glutamate receptors in regulating calcium influx in rice seedlings under Cr exposure

Glutamate receptors (GLRs) are ligand-gated Ca²⁺-permeable channels that govern and modulate the dynamic influx of cytosolic Ca²⁺ in plants. The present study investigated the interaction of OsGLR3 gene expression with subcellular Ca distribution in rice seedlings exposed to chromium (Cr) solution containing Cr(III) or Cr(VI). The results displayed that the accumulation of Ca was evaluated or higher in shoots compared to roots under Cr exposure, and a similar pattern of subcellular Ca distribution was observed between rice tissues exposed to Cr(III) and Cr(VI). Real-time quantitative polymerase chain reaction (qRT-PCR) analysis revealed that eight OsGLR3 isogenes were distinctly expressed in different rice tissues at different levels of Cr exposures. This differential expressions could possible be due to the uptake variations, subcellular distribution and chemical speciation of the two Cr species. Notably, distinct expression patterns of OsGLR3 genes were found between Cr(III) and Cr(VI) exposures, suggesting that different regulation strategies are used to mediate Ca influx in rice materials under different Cr exposures. These results demonstrated a full picture of Cr-induced transcriptional alterations in OsGLR3 expression levels in rice seedlings, and provided suggestive evidence for further investigation on specific OsGLR3 genes participated in the regulation of cytosolic Ca²⁺ concentrations under Cr exposure.

Fundamental parenchyma cells are involved in Na+ and Cl- removal ability in rice leaf sheath

Salt sensitivity in rice plants is associated with the accumulated amount of Na+ and Cl- in shoots and, more significantly, in photosynthetic tissues. Therefore, salt removal ability at the leaf sheath level is an important mechanism of salt tolerance. In the present study we attempted to determine whether rice leaf sheaths excluded Cl- as well as Na+, and to identify the tissues that were involved in the removal ability of both ions. In two rice genotypes, salt-tolerant FL478 and -sensitive IR29, leaf sheaths excluded Na+ and Cl- under NaCl treatment as estimated using their sheath:blade ratios. The sheath:blade ratio of Na+ but not of Cl-, was increased by NaCl treatment. Under NaCl treatment, Na+ concentration was higher in the basal leaf sheath, whereas Cl- concentration was higher in the middle and tip parts. At the tissue level, fundamental parenchyma cells of leaf sheaths retained the highest amounts of Na and Cl when treated with high amount of NaCl. These results imply that the leaf sheath potentially functions to remove excess Na+ and Cl- from xylem vessels in different locations along the axis, with the fundamental parenchyma cells of leaf sheaths being involved in over-accumulation of both Na+ and Cl-.

Distinct leaf transcriptomic response of water deficient Eucalyptus grandis submitted to potassium and sodium fertilization

While potassium fertilization increases growth yield in Brazilian eucalyptus plantations, it could also increase water requirements, making trees more vulnerable to drought. Sodium fertilization, which has been shown to promote eucalyptus growth compared to K-deficient trees, could partially mitigate this adverse effect of potassium. However, little is known about the influence of K and Na fertilization on the tree metabolic response to water deficit. The aim of the present study was thus to analyze the transcriptome of leaves sampled from Eucalyptus grandis trees subjected to 37% rainfall reduction, and fertilized with potassium (K), sodium (Na), compared to control trees (C). The multifactorial experiment was set up in a field with a throughfall exclusion system. Transcriptomic analysis was performed on leaves from two-year-old trees, and data analyzed using multifactorial statistical analysis and weighted gene co-expression network analysis (WGCNA). Significant sets of genes were seen to respond to rainfall reduction, in interaction with K or Na fertilization, or to fertilization only (regardless of the water supply regime). The genes were involved in stress signaling, primary and secondary metabolism, secondary cell wall formation and photosynthetic activity. Our focus on key genes related to cation transporters and aquaporins highlighted specific regulation of ion homeostasis, and plant adjustment to water deficit. While water availability significantly affects the transcriptomic response of eucalyptus species, this study points out that the transcriptomic response is highly dependent on the fertilization regime. Our study is based on the first large-scale field trial in a tropical region, specifically designed to study the interaction between water availability and nutrition in eucalyptus. To our knowledge, this is the first global transcriptomic analysis to compare the influence of K and Na fertilization on tree adaptive traits in water deficit conditions.

Interactive effects of salinity and nitrogen forms on plant growth, photosynthesis and osmotic adjustment in maize

To enhance crop productivity and minimize the harmful effects of various environmental stresses, such as salinity and drought, farmers often use mineral fertilizers. However, inadequate or excessive fertilization can reduce plant growth and nutritive quality and contribute to soil degradation and environmental pollution. This study investigated the effects of salinity (0, 100 or 150 mM NaCl) and nitrogen form (sole NO₃^- or NH₄⁺, or combined NO₃^-:NH₄⁺ at 25:75 or 50:50) on growth, photosynthesis, and water and ion status of a commercial variety of maize (Zea mays SY Sincero). In the absence of NaCl, the media containing ammonium only or both nitrogen forms had higher aboveground growth rates than that containing nitrate only. Indeed, the maize growth, expressed as leaf dry matter, seen on NH₄⁺ in the absence of salinity, was nearly double the biomass compared to that with NO₃^-treatment. Irrespective of N form, the presence of NaCl severely reduced leaf and roots growth; the presence of ammonium in the nutrient solution diminished these negative effects. Compared to the NH₄⁺ only and combined treatments, the leaves of plants in the NO₃^--only medium showed signs of nitrogen deficiency (general chlorosis), which was more pronounced in the lower than upper leaves, indicating that nitrate is partly replaced by chloride during root uptake. NH₄⁺ favored maize growth more than NO₃^-, especially when exposed to saline conditions, and may improve the plant's capacity to osmotically adjust to salinity by accumulating inorganic solutes.

Phosphatidic acids mediate transport of Ca2+ and H+ through plant cell membranes

Phosphatidic acids (PAs) are a key intermediate in phospholipid biosynthesis, and a central element in numerous signalling pathways. Functions of PAs are related to their fundamental role in molecular interactions within cell membranes modifying membrane bending, budding, fission and fusion. Here we tested the hypothesis that PAs are capable of direct transport of ions across bio-membranes. We have demonstrated that PAs added to the maize plasma membrane vesicles induced ionophore-like transmembrane transport of Ca2+, H+ and Mg2+. PA-induced Ca2+ fluxes increased with an increasing PAs acyl chain unsaturation. For all the PAs analysed, the effect on Ca2+ permeability increased with increasing pH (pH 8.0>pH 7.2>pH 6.0). The PA-induced Ca2+, Mg2+ and H+ permeability was also more pronounced in the endomembrane vesicles as compared with the plasma membrane vesicles. Addition of PA to protoplasts from Arabidopsis thaliana (L.) Heynh. roots constitutively expressing aequorin triggered elevation of the cytosolic Ca2+ activity, indicating that the observed PA-dependent Ca2+ transport occurs in intact plants.

Effects of Binary Mixtures and Transpiration on Accumulation of Pharmaceuticals by Spinach

Many pharmaceuticals are present in reclaimed wastewater and effluent-dominated water bodies used to irrigate edible crops. Previous research has shown that plants irrigated with reclaimed wastewater can accumulate pharmaceuticals. However, plant-driven processes that contribute to differences in accumulation among compounds are not well understood. Here, we tested the effects of exposure to mixtures on spinach accumulation and metabolism of four psychoactive pharmaceuticals found in reclaimed wastewater: carbamazepine, fluoxetine, amitriptyline, and lamotrigine. Coexposure of plants to carbamazepine and fluoxetine or amitriptyline decreased accumulation of the toxic carbamazepine metabolite 10,11-epoxycarbamazepine. Furthermore, we tested a simple transpiration-based accumulation model and found that transpiration is a strong predictor for accumulation of the studied compounds. Amitriptyline accumulated to a larger extent than predicted from transpiration alone, and we suggest the possibility that a transporter protein may be involved in its uptake. Our findings highlight the need to consider plant physiology and mixture effects in studying accumulation of polar and ionizable organic contaminants and their metabolites.

Physiological mechanisms of exogenous calcium on alleviating salinity-induced stress in rice (Oryza sativa L.)

Being more sensitive to salt stress among the cereals, growth of rice (Oryza sativa L.) has been habitually affected by salinity. Although, several practices have evolved to sustain the growth of rice under salinity, the enormous role of calcium (Ca²⁺) as a signalling molecule in salt stress mitigation is still arcane. Considering this fact, an experiment was performed aiming to explicate the mechanism of salt-induced growth inhibition in rice and its alleviation by exogenous Ca²⁺. At germination stage, 10 mM and 15 mM CaCl₂ primed rice (cv. Binadhan-10 & Binadhan-7) seeds were grown in petri dishes for 9 days under 100 mM NaCl stress. At seedling stage, 9-day-old rice seedlings grown on sand were exposed to 100 mM NaCl alone and combined with 10 mM and 15 mM CaCl₂ for 15 days. This research revealed that salinity radically slowed down growth of rice seedlings and Ca²⁺ treatment noticeably improved growth performances. At germination stage, 10 mM CaCl₂ treatment significantly increased the final germination percentage, germination rate index (in Binadhan-7), shoot, root length (89.20, 67.58% in Bindhan-10 & 84.72, 31.15% in Bindhan-7) and biomass production under salinity. Similarly, at seedling stage, 10 mM CaCl₂ supplementation in salt-stressed plants enhanced shoot length (42.17, 28.76%) and shoot dry weight (339.52, 396.20%) significantly in Binadhan-10 & Binadhan-7, respectively, but enhanced root dry weight (36.76%) only in Binadhan-10. In addition, 10 mM CaCl₂ supplementation on salt-stressed seedlings increased the chlorophyll and proline content, and oppressed the accretion of reactive oxygen species thus protecting from oxidative damage more pronouncedly in Binadhan-10 than Binadhan-7 as reflected by the elevated levels of catalase and ascorbate peroxidase activity. The 15 mM CaCl₂ somehow also enhanced some growth parameters but overall was less effective than 10 mM CaCl₂ to alleviate salt stress, and sometimes showed negative effect. Therefore, supplementary application of calcium-rich fertilizers in saline prone soils can be an effective approach to acclimatize salt stress and cultivate rice successfully.