Mechanisms of cytosolic calcium elevation in plants: the role of ion channels, calcium extrusion systems and NADPH oxidase-mediated 'ROS-Ca2+ Hub'

Increasing Ca2+ accumulation in salt glands under salt stress increases stronger selective secretion of Na+ in Plumbago auriculata tetraploids

Under salt stress, recretohalophyte Plumbago auriculata tetraploids enhance salt tolerance by increasing selective secretion of Na+ compared with that in diploids, although the mechanism is unclear. Using non-invasive micro-test technology, the effect of salt gland Ca2+ content on Na+ and K+ secretion were investigated in diploid and tetraploid P. auriculata under salt stress. Salt gland Ca2+ content and secretion rates of Na+ and K+ were higher in tetraploids than in diploids under salt stress. Addition of exogenous Ca2+ increased the Ca2+ content of the salt gland in diploids and is accompanied by an increase in the rate of Na+ and K+ secretion. With addition of a Ca2+ channel inhibitor, diploid salt glands retained large amounts of Ca2+, leading to higher Ca2+ content and Na+ secretion rate than those of tetraploids. Inhibiting H2O2 generation and H+-ATPase activity altered Na+ and K+ secretion rates in diploids and tetraploids under salt stress, indicating involvement in regulating Na+ and K+ secretion. Our results indicate that the increased Na+ secretion rate of salt gland in tetraploids under salt stress was associated with elevated Ca2+ content in salt gland.

NADPH oxidase-mediated reactive oxygen species, antioxidant isozymes, and redox homeostasis regulate salt sensitivity in maize genotypes

The aim of the study is to examine the relationship between oxidative bursts, their regulation with ion homeostasis, and NADPH oxidase (NOX) in different salt-sensitive maize genotypes. For this, in the first study, four differently salt-sensitive maize genotypes (BIL214 × BIL218 as tolerant, BHM-5 as sensitive, and BHM-7 and BHM-9 as moderate-tolerant) were selected on the basis of phenotype, histochemical detection of reactive oxygen species (ROS), malondialdehyde (MDA) content, and specific and in-gel activity of NOX. In the next experiment, these genotypes were further examined in 200 mM NaCl solution in half-strength Hoagland media for nine days to study salt-induced changes in NOX activity, ROS accumulation, ion and redox homeostasis, the activity of antioxidants and their isozyme responses, and to find out potential relationships among the traits. Methylglyoxal (MG) and glyoxalse enzymes (Gly I and II) were also evaluated. Fully expanded leaf samplings were collected at 0 (control), 3, 6, 9-day, and after 7 days of recovery to assay different parameters. Na+/K+, NOX, ROS, and MDA contents increased significantly with the progression of stress duration in all maize genotypes, with a significantly higher value in BHM-5 as compared to tolerant and moderate-tolerant genotypes. A continual induction of Cu/Zn-SOD was observed in BIL214 × BIL218 due to salt stress. Substantial decreases in CAT2 and CAT3 isozymes in BHM-5 might be critical for the highest H2O2 burst in that sensitive genotype under salt stress. The highest intensified POD isozymes were visualized in BHM-5, BHM-7, and BHM-9, whereas BIL214 × BIL218 showed a continual induction of POD isozymes, although GPX activity decreased in all the genotypes at 9 days. Under salt stress, the tolerant genotype BIL214 × BIL218 showed superior ASA- and GSH-redox homeostasis by keeping GR and MDHAR activity high. This genotype also had a stronger MG detoxification system by having higher glyoxalase activity. Correlation, comparative heatmap, and PCA analyses revealed positive correlations among Na+/K+, NOX, O2•-, H2O2, MG, proline, GR, GST, and Gly I activities. Importantly, the relationship depends on the salt sensitivity of the genotypes. The reduced CAT activity as well as redox homeostasis were critical to the survival of the sensitive genotype.

Long small RNA76113 targets CYCLIC NUCLEOTIDE-GATED ION CHANNEL 5 to repress disease resistance in rice

Small RNAs are widely involved in plant immune responses. However, the role of long small RNAs (25 to 40 nt) in monocot plant disease resistance is largely unknown. Here, we identified a long small RNA (lsiR76113) from rice (Oryza sativa) that is downregulated by Magnaporthe oryzae infection and targets a gene encoding CYCLIC NUCLEOTIDE-GATED CHANNEL 5 (CNGC5). The cngc5 mutant lines were more susceptible to M. oryzae than the wild type, while knocking down lsiR76113 in transgenic rice plants promoted pathogen resistance. A protoplast transient expression assay showed that OsCNGC5 promotes Ca2+ influx. These results demonstrate that OsCNGC5 enhances rice resistance to rice blast by increasing the cytosolic Ca2+ concentration. Importantly, exogenous Ca2+ application enhanced rice M. oryzae resistance by affecting reactive oxygen species (ROS) production. Moreover, cngc5 mutants attenuated the PAMP-triggered immunity response, including chitin-induced and flg22-induced ROS bursts and protein phosphorylation in the mitogen-activated protein kinase cascade, indicating that OsCNGC5 is essential for PAMP-induced calcium signaling in rice. Taken together, these results suggest that lsiR76113-mediated regulation of Ca2+ influx is important for PTI responses and disease resistance in rice.

CPK12 and Ca2+-mediated hypoxia signaling

Hypoxia triggers reactive oxygen species (ROS)-induced elevation in cytoplasmic calcium (Ca2+) in the plant cells. Calcium-dependent protein kinase 12 (CPK12) acts as a sensor to recognize the Ca2+ signature and is activated by autophosphorylation. Then, the CPK12 moves into the nucleus with the help of phosphatidic acid (PA) and phosphorylates ERF-VII family proteins that activate hypoxia signaling and response. The study provides a novel mechanism of hypoxia signaling in plants. Moreover, the mechanism of hypoxia-specific Ca2+ signature generation remains elusive.

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

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

Toxic effects of lead on plants: integrating multi-omics with bioinformatics to develop Pb-tolerant crops

MAIN CONCLUSION

Lead disrupts plant metabolic homeostasis and key structural elements. Utilizing modern biotechnology tools, it's feasible to develop Pb-tolerant varieties by discovering biological players regulating plant metabolic pathways under stress. Lead (Pb) has been used for a variety of purposes since antiquity despite its toxic nature. After arsenic, lead is the most hazardous heavy metal without any known beneficial role in the biological system. It is a crucial inorganic pollutant that affects plant biochemical and morpho-physiological attributes. Lead toxicity harms plants throughout their life cycle and the extent of damage depends on the concentration and duration of exposure. Higher levels of lead exposure disrupt numerous key metabolic activities of plants including oxygen-evolving complex, organelles integrity, photosystem II connectivity, and electron transport chain. This review summarizes the detrimental effects of lead toxicity on seed germination, crop growth, and yield, oxidative and ultra-structural alterations, as well as nutrient absorption, transport, and assimilation. Further, it discusses the Pb-induced toxic modulation of stomatal conductance, photosynthesis, respiration, metabolic-enzymatic activity, osmolytes accumulation, and antioxidant activity. It is a comprehensive review that reports on omics-based studies along with morpho-physiological and biochemical modifications caused by lead stress. With advances in DNA sequencing technologies, genomics and transcriptomics are gradually becoming popular for studying Pb stress effects in plants. Proteomics and metabolomics are still underrated and there is a scarcity of published data, and this review highlights both their technical and research gaps. Besides, there is also a discussion on how the integration of omics with bioinformatics and the use of the latest biotechnological tools can aid in developing Pb-tolerant crops. The review concludes with core challenges and research directions that need to be addressed soon.

Deciphering the role of mechanosensitive channels in plant root biology: perception, signaling, and adaptive responses

MAIN CONCLUSION

Mechanosensitive channels are integral membrane proteins that rapidly translate extrinsic or intrinsic mechanical tensions into biological responses. They can serve as potential candidates for developing smart-resilient crops with efficient root systems. Mechanosensitive (MS) calcium channels are molecular switches for mechanoperception and signal transduction in all living organisms. Although tremendous progress has been made in understanding mechanoperception and signal transduction in bacteria and animals, this remains largely unknown in plants. However, identification and validation of MS channels such as Mid1-complementing activity channels (MCAs), mechanosensitive-like channels (MSLs), and Piezo channels (PIEZO) has been the most significant discovery in plant mechanobiology, providing novel insights into plant mechanoperception. This review summarizes recent advances in root mechanobiology, focusing on MS channels and their related signaling players, such as calcium ions (Ca2+), reactive oxygen species (ROS), and phytohormones. Despite significant advances in understanding the role of Ca2+ signaling in root biology, little is known about the involvement of MS channel-driven Ca2+ and ROS signaling. Additionally, the hotspots connecting the upstream and downstream signaling of MS channels remain unclear. In light of this, we discuss the present knowledge of MS channels in root biology and their role in root developmental and adaptive traits. We also provide a model highlighting upstream (cell wall sensors) and downstream signaling players, viz., Ca2+, ROS, and hormones, connected with MS channels. Furthermore, we highlighted the importance of emerging signaling molecules, such as nitric oxide (NO), hydrogen sulfide (H2S), and neurotransmitters (NTs), and their association with root mechanoperception. Finally, we conclude with future directions and knowledge gaps that warrant further research to decipher the complexity of root mechanosensing.

Hydrogen peroxide reduces root cadmium uptake but facilitates root-to-shoot cadmium translocation in rice through modulating cadmium transporters

Cadmium (Cd) contamination in agricultural soils has become a serious worldwide environmental problem threatening crop production and human health. Hydrogen peroxide (H₂O₂) is a critical second messenger in plant response to Cd exposure. However, its role in Cd accumulation in various organs of plants and the mechanistic basis of this regulation remains to be elucidated. In this study, we used electrophysiological and molecular approaches to understand how H₂O₂ regulates Cd uptake and translocation in rice plants. Our results showed that the pretreatment of H₂O₂ significantly reduced Cd uptake by rice roots, which was associated with the downregulation of OsNRAMP1 and OsNRAMP5. On the other hand, H₂O₂ promoted the root-to-shoot translocation of Cd, which might be attributed to the upregulation of OsHMA2 critical for Cd²⁺ phloem loading and the downregulation of OsHMA3 involved in the vacuolar compartmentalization of Cd²⁺, leading to the increased Cd accumulation in rice shoots. Furthermore, such regulatory effects of H₂O₂ on Cd uptake and translocation were notably amplified by the elevated level of exogenous calcium (Ca). Collectively, our results suggest that H₂O₂ can inhibit Cd uptake but increase root to shoot translocation through modulating the transcriptional levels of genes encoding Cd transporters, furthermore, application of Ca can amplify this effect. These findings will broaden our understanding of the regulatory mechanisms of Cd transport in rice plants and provide theoretical foundation for breeding rice for low Cd accumulation.

Beryllium contamination and its risk management in terrestrial and aquatic environmental settings

Beryllium (Be) is a relatively rare element and occurs naturally in the Earth's crust, in coal, and in various minerals. Beryllium is used as an alloy with other metals in aerospace, electronics and mechanical industries. The major emission sources to the atmosphere are the combustion of coal and fossil fuels and the incineration of municipal solid waste. In soils and natural waters, the majority of Be is sorbed to soil particles and sediments. The majority of contamination occurs through atmospheric deposition of Be on aboveground plant parts. Beryllium and its compounds are toxic to humans and are grouped as carcinogens. The general public is exposed to Be through inhalation of air and the consumption of Be-contaminated food and drinking water. Immobilization of Be in soil and groundwater using organic and inorganic amendments reduces the bioavailability and mobility of Be, thereby limiting the transfer into the food chain. Mobilization of Be in soil using chelating agents facilitates their removal through soil washing and plant uptake. This review provides an overview of the current understanding of the sources, geochemistry, health hazards, remediation practices, and current regulatory mandates of Be contamination in complex environmental settings, including soil and aquatic ecosystems.

Phosphate-deprivation and damage signalling by extracellular ATP

Phosphate deprivation compromises plant productivity and modulates immunity. DAMP signalling by extracellular ATP (eATP) could be compromised under phosphate deprivation by the lowered production of cytosolic ATP and the need to salvage eATP as a nutritional phosphate source. Phosphate-starved roots of Arabidopsis can still sense eATP, indicating robustness in receptor function. However, the resultant cytosolic free Ca²⁺ signature is impaired, indicating modulation of downstream components. This perspective on DAMP signalling by extracellular ATP (eATP) addresses the salvage of eATP under phosphate deprivation and its promotion of immunity, how Ca²⁺ signals are generated and how the Ca²⁺ signalling pathway could be overcome to allow beneficial fungal root colonization to fulfill phosphate demands. Safe passage for an endophytic fungus allowing root colonization could be achieved by its down-regulation of the Ca²⁺ channels that act downstream of the eATP receptors and by also preventing ROS accumulation, thus further impairing DAMP signalling.

Melatonin as a regulator of plant ionic homeostasis: implications for abiotic stress tolerance

Melatonin is a highly conserved and ubiquitous molecule that operates upstream of a broad array of receptors in animal systems. Since melatonin was discovered in plants in 1995, hundreds of papers have been published revealing its role in plant growth, development, and adaptive responses to the environment. This paper summarizes the current state of knowledge of melatonin's involvement in regulating plant ion homeostasis and abiotic stress tolerance. The major topics covered here are: (i) melatonin's control of H+-ATPase activity and its implication for plant adaptive responses to various abiotic stresses; (ii) regulation of the reactive oxygen species (ROS)-Ca2+ hub by melatonin and its role in stress signaling; and (iii) melatonin's regulation of ionic homeostasis via hormonal cross-talk. We also show that the properties of the melatonin molecule allow its direct scavenging of ROS, thus preventing negative effects of ROS-induced activation of ion channels. The above 'desensitization' may play a critical role in preventing stress-induced K+ loss from the cytosol as well as maintaining basic levels of cytosolic Ca2+ required for optimal cell operation. Future studies should focus on revealing the molecular identity of transporters that could be directly regulated by melatonin and providing a bioinformatic analysis of evolutionary aspects of melatonin sensing and signaling.

Calcium channels and transporters: Roles in response to biotic and abiotic stresses

Calcium (Ca²⁺) serves as a ubiquitous second messenger by mediating various signaling pathways and responding to numerous environmental conditions in eukaryotes. Therefore, plant cells have developed complex mechanisms of Ca²⁺ communication across the membrane, receiving the message from their surroundings and transducing the information into cells and organelles. A wide range of biotic and abiotic stresses cause the increase in [Ca²⁺]_cyt as a result of the Ca²⁺ influx permitted by membrane-localized Ca²⁺ permeable cation channels such as CYCLIC NUCLEOTIDE-GATE CHANNELs (CNGCs), and voltage-dependent HYPERPOLARIZATION-ACTIVATED CALCIUM²⁺ PERMEABLE CHANNELs (HACCs), as well as GLUTAMATE RECEPTOR-LIKE RECEPTORs (GLRs) and TWO-PORE CHANNELs (TPCs). Recently, resistosomes formed by some NUCLEOTIDE-BINDING LEUCINE-RICH REPEAT RECEPTORs (NLRs) are also proposed as a new type of Ca²⁺ permeable cation channels. On the contrary, some Ca²⁺ transporting membrane proteins, mainly Ca²⁺-ATPase and Ca²⁺/H⁺ exchangers, are involved in Ca²⁺ efflux for removal of the excessive [Ca²⁺]_cyt in order to maintain the Ca²⁺ homeostasis in cells. The Ca²⁺ efflux mechanisms mediate the wide ranges of cellular activities responding to external and internal stimuli. In this review, we will summarize and discuss the recent discoveries of various membrane proteins involved in Ca²⁺ influx and efflux which play an essential role in fine-tuning the processing of information for plant responses to abiotic and biotic stresses.

Reactive Oxygen Species Distribution Involved in Stipe Gradient Elongation in the Mushroom Flammulina filiformis

The mushroom stipe raises the pileus above the substrate into a suitable position for dispersing spores. The stipe elongates at different speeds along its length, with the rate of elongation decreasing in a gradient from the top to the base. However, the molecular mechanisms underlying stipe gradient elongation are largely unknown. Here, we used the model basidiomycete mushroom Flammulina filiformis to investigate the mechanism of mushroom stipe elongation and the role of reactive oxygen species (ROS) signaling in this process. Our results show that O₂^- and H₂O₂ exhibit opposite gradient distributions in the stipe, with higher O₂^- levels in the elongation region (ER), and higher H₂O₂ levels in the stable region (SR). Moreover, NADPH-oxidase-encoding genes are up-regulated in the ER, have a function in producing O₂^-, and positively regulate stipe elongation. Genes encoding manganese superoxide dismutase (MnSOD) are up-regulated in the SR, have a function in producing H₂O_2, and negatively regulate stipe elongation. Altogether, our data demonstrate that ROS (O₂^-/H₂O₂) redistribution mediated by NADPH oxidase and MnSODs is linked to the gradient elongation of the F. filiformis stipe.

Crosstalk between Ca2+ and Other Regulators Assists Plants in Responding to Abiotic Stress

Plants have evolved many strategies for adaptation to extreme environments. Ca²⁺, acting as an important secondary messenger in plant cells, is a signaling molecule involved in plants' response and adaptation to external stress. In plant cells, almost all kinds of abiotic stresses are able to raise cytosolic Ca²⁺ levels, and the spatiotemporal distribution of this molecule in distant cells suggests that Ca²⁺ may be a universal signal regulating different kinds of abiotic stress. Ca²⁺ is used to sense and transduce various stress signals through its downstream calcium-binding proteins, thereby inducing a series of biochemical reactions to adapt to or resist various stresses. This review summarizes the roles and molecular mechanisms of cytosolic Ca²⁺ in response to abiotic stresses such as drought, high salinity, ultraviolet light, heavy metals, waterlogging, extreme temperature and wounding. Furthermore, we focused on the crosstalk between Ca²⁺ and other signaling molecules in plants suffering from extreme environmental stress.

Ca2+ participates in the regulation of microalgae triacylglycerol metabolism under heat stress

Microalgae are the largest CO₂ fixer and O₂ producer on the earth and occupy an increasingly important position in human life and production. Various environmental factors have a significant impact on the growth and metabolism of microalgae. As global warming intensifies, heat stress has become a crucial factor affecting the microalgae industry. However, till now, it has not been clear how microalgae sensed the temperature stress, transmitted stress signals and adjusted in intracellular metabolic pathways. In this study, the growth of microalgae Auxenochlorella protothecoides UTEX2341 was inhibited at 32 °C, but the single cell dry weight increased. The cell component analyses showed that both the carbohydrate and total protein content decreased significantly, while the lipid content increased by 158%. Meanwhile, the intracellular Ca²⁺ concentration increased continuously, with a maximum increase of 1.65 times. According to the transcriptome analyses, the up-regulation of Ca²⁺ influx channel protein mid1-complementing activity 1 (MCA1) gene and the down-regulation of efflux channel protein cation exchanger 1(CAX) and autoinhibited Ca²⁺-ATPase 1 (ACA1) genes in cytoplasmic membrane jointly facilitated the increase of Ca²⁺ in the cytoplasm. Coexpression network analysis indicated that the fluctuation of Ca²⁺ in the cytoplasm could activate the expression of transcription factors MYB3 and AP2-4 through calmodulin (CAM) and calcium-dependent protein kinase (CDPK), and then regulate glycerol-3-phosphate acyltransferases (GPAT) at the beginning of TAG synthesis and diacylglycerol acyltransferase (DGAT)/phospholipid: diacylglycerol acyltransferase (PDAT) in the last step of TAG synthesis. Furthermore, the addition of Ca²⁺ specific chelator BAPTA-AM inhibited the expression of GPAT, which was consistent with the decrease in microalgae lipid content. The results proved that Ca²⁺ participated in the regulation of microalgae TAG synthesis under heat stress, which provided a new view for the understanding of the microalgae lipid accumulation mechanism.

Function of Protein Kinases in Leaf Senescence of Plants

Leaf senescence is an evolutionarily acquired process and it is critical for plant fitness. During senescence, macromolecules and nutrients are disassembled and relocated to actively growing organs. Plant leaf senescence process can be triggered by developmental cues and environmental factors, proper regulation of this process is essential to improve crop yield. Protein kinases are enzymes that modify their substrates activities by changing the conformation, stability, and localization of those proteins, to play a crucial role in the leaf senescence process. Impressive progress has been made in understanding the role of different protein kinases in leaf senescence recently. This review focuses on the recent progresses in plant leaf senescence-related kinases. We summarize the current understanding of the function of kinases on senescence signal perception and transduction, to help us better understand how the orderly senescence degeneration process is regulated by kinases, and how the kinase functions in the intricate integration of environmental signals and leaf age information.

Differential Impact of Nitric Oxide and Abscisic Acid on the Cellular and Physiological Functioning of sub1A QTL Bearing Rice Genotype under Salt Stress

Hydroponic culture containing 200 mM NaCl was used to induce oxidative stress in seedlings of cultivars initially primed with 1 mM SNP and 10 µM ABA. Exogenous application of sodium nitroprusside (SNP - a nitric oxide donor) and abscisic acid (ABA) was well sensitized more in cv. Swarna Sub1 than cv. Swarna and also reflected in different cellular responses. The major effects of salinity, irrespective of the cultivar, were lowering the water relation, including relative water content and osmotic potential, and decreasing the compatible solutes like alanine, gamma-aminobutyric acid, and glycine betaine. The accumulated polyamines were reduced more in cv. Swarna with a concomitant decrease in photosynthetic reserves. NADP-malic enzyme activity, sucrose accumulation, ascorbate peroxidase, and glutathione S-transferase activities gradually declined under NaCl stress and the catabolizing enzymes like invertase (both wall and cytosolic forms) also declined. On the contrary, plants suffered from oxidative stress through superoxide, hydrogen peroxide, and their biosynthetic enzymes like NADP(H) oxidase. Moderation of Na⁺/K⁺ by both SNP and ABA were correlated with other salt sensitivities in the plants. The maximum effects of SNP and ABA were found in the recovery of antioxidation pathways, osmotic tolerance, and carbohydrate metabolism. Findings predict the efficacy of SNP and ABA either independently or cumulatively in overcoming NaCl toxicity in rice.

High-LD SNP markers exhibiting pleiotropic effects on salt tolerance at germination and seedlings stages in spring wheat

Salt tolerance at germination and seedling growth stages was investigated. GWAS revealed nine genomic regions with pleiotropic effects on salt tolerance. Salt tolerant genotypes were identified for future breeding program. With 20% of the irrigated land worldwide affected by it, salinity is a serious threat to plant development and crop production. While wheat is the most stable food source worldwide, it has been classified as moderately tolerant to salinity. In several crop plants; such as barley, maize and rice, it has been shown that salinity tolerance at seed germination and seedling establishment is under polygenic control. As yield was the ultimate goal of breeders and geneticists, less attention has been paid to understanding the genetic architecture of salt tolerance at early stages. Thus, the genetic control of salt tolerance at these stages is poorly understood relative to the late stages. In the current study, 176 genotypes of spring wheat were tested for salinity tolerance at seed germination and seedling establishment. Genome-Wide Association Study (GWAS) has been used to identify the genomic regions/genes conferring salt tolerance at seed germination and seedling establishment. Salinity stress negatively impacted all germination and seedling development parameters. A set of 137 SNPs showed significant association with the traits of interest. Across the whole genome, 33 regions showed high linkage disequilibrium (LD). These high LD regions harbored 15 SNPs with pleiotropic effect (i.e. SNPs that control more than one trait). Nine genes belonging to different functional groups were found to be associated with the pleiotropic SNPs. Noteworthy, chromosome 2B harbored the gene TraesCS2B02G135900 that acts as a potassium transporter. Remarkably, one SNP marker, reported in an early study, associated with salt tolerance was validated in this study. Our findings represent potential targets of genetic manipulation to understand and improve salinity tolerance in wheat.

Nucleotide-binding leucine-rich repeat proteins: a missing link in controlling cell fate and plant adaptation to hostile environment?

Programmed cell death is a tightly regulated genetically controlled process that leads to cell suicide and eliminates cells that are either no longer needed or damaged/harmful. Nucleotide-binding leucine-rich repeat proteins have recently emerged as a novel class of Ca2+-permeable channels that operate in plant immune responses. This viewpoint argues that the unique structure of this channel, its permeability to other cations, and specificity of its operation make it an ideal candidate to mediate cell signaling and adaptive responses not only to pathogens but also to a broad range of abiotic stress factors.

Early signalling processes in roots play a crucial role in the differential salt tolerance in contrasting Chenopodium quinoa accessions

Significant variation in epidermal bladder cell (EBC) density and salt tolerance (ST) exists amongst quinoa accessions, suggesting that salt sequestration in EBCs is not the only mechanism conferring ST in this halophyte. In order to reveal other traits that may operate in tandem with salt sequestration in EBCs and whether these additional tolerance mechanisms acted mainly at the root or shoot level, two quinoa (Chenopodium quinoa) accessions with contrasting ST and EBC densities (Q30, low ST with high EBC density versus Q68, with high ST and low EBC density) were studied. The results indicate that responses in roots, rather than in shoots, contributed to the greater ST in the accession with low EBC density. In particular, the tolerant accession had improved root plasma membrane integrity and K+ retention in the mature root zone in response to salt. Furthermore, superior ST in the tolerant Q68 was associated with faster and root-specific H2O2 accumulation and reactive oxygen species-induced K+ and Ca2+ fluxes in the root apex within 30 min after NaCl application. This was found to be associated with the constitutive up-regulation of the membrane-localized receptor kinases regulatory protein FERONIA in the tolerant accession. Taken together, this study shows that differential root signalling events upon salt exposure are essential for the halophytic quinoa; the failure to do this limits quinoa adaptation to salinity, independently of salt sequestration in EBCs.

Melatonin improves the photosynthesis in Dracocephalum kotschyi under salinity stress in a Ca2+/CaM-dependent manner

This study investigated: (1) the effects of various concentrations of melatonin (MT) and Ca2+; and (2) the impact of crosstalk between these signal molecules on photosynthesis and salt tolerance of Dracocephalum kotschyi Boiss. Results indicated that 5mM CaCl2, as well as 100μM MT were the best concentrations for increasing shoot dry weight, leaf area, SPAD index, maximum quantum efficiency of photosystem II (Fv/Fm), and decreasing malondialdehyde content under salinity stress. The impact of MT on growth and photosynthesis was closely linked to its effect on enhancing antioxidant enzyme activities in leaves. Application of p-chlorophenylalanine, as an inhibitor of MT biosynthesis, negated the impacts of MT on the aforementioned attributes. Salinity and MT boosted cytosolic Ca2+ concentration. Exogenous MT, as well as Ca2+, enhanced tolerance index, membrane stability, leaf area, the content of chlorophyll (Chl) a, Chl b, and carotenoids (Car), Fv/Fm, and stomatal conductance under salinity stress. These impacts of MT were eliminated by applying a calmodulin antagonist, a Ca2+ chelator and a Ca2+ channel blocker. These novel findings indicate that the MT-induced effects on photosynthetic parameters and salt-evoked oxidative stress were mediated through calcium/calmodulin (Ca2+/CaM) signalling.

Differential Physiological Responses to Salt Stress between Salt-Sensitive and Salt-Tolerant japonica Rice Cultivars at the Post-Germination and Seedling Stages

Soil salinity is a key source of abiotic stress in the cultivation of rice. In this study, two currently cultivated japonica rice species-Zhegeng 78 (salt-tolerant) and Zhegeng 99 (salt-sensitive)-with similar backgrounds were identified and used to investigate their differential responses to salt stress at the post-germination and seedling stages. Quantitative RT-PCR analysis demonstrated that the expression of OsSOS1, OsHAK1, and OsHAK5 at the post-germination stage, and the expression of OsHKT1,1, OsHTK2,1, and OsHAK1 at the seedling stage, were significantly higher in the salt-tolerant Zhegeng 78 compared with those of the salt-sensitive Zhegeng 99 under salt stress. The significantly lower Na⁺ net uptake rate at the post-germination and higher K⁺ net uptake rates at the post-germination and seedling stages were observed in the salt-tolerant Zhegeng 78 compared with those of the salt-sensitive Zhegeng 99 under salt stress. Significantly higher activity of peroxidase (POD) and the lower hydrogen peroxide (H₂O₂) accumulation were observed in the salt-tolerant Zhegeng 78 compared with those of salt-sensitive Zhegeng 99 under salt stress at the seeding stage. The salt-tolerant Zhegeng 78 might be valuable in future cultivation in salinity soils.

Emerging Roles of γ Aminobutyric Acid (GABA) Gated Channels in Plant Stress Tolerance

The signaling role for γ-Aminobutyric acid (GABA) has been documented in animals for over seven decades. However, a signaling role for GABA in plants is just beginning to emerge with the discovery of putative GABA binding site/s and GABA regulation of anion channels. In this review, we explore the role of GABA in plant growth and development under abiotic stress, its interactions with other signaling molecules and the probability that there are other anion channels with important roles in stress tolerance that are gated by GABA.

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.

Fe acquisition at the crossroad of calcium and reactive oxygen species signaling

Due to its redox properties, iron is both essential and toxic. Therefore, soil iron availability variations pose a significant problem for plants. Recent evidence suggests that calcium and reactive oxygen species coordinate signaling events related to soil iron acquisition. Calcium was found to affect directly IRT1-mediated iron import through the lipid-binding protein EHB1 and to trigger a CBL-CIPK-mediated signaling influencing the activity of the key iron-acquisition transcription factor FIT. In parallel, under prolonged iron deficiency, reactive oxygen species both inhibit FIT function and depend on FIT through the function of the catalase CAT2. We discuss the role of calcium and reactive oxygen species signaling in iron acquisition, with post-translational mechanisms influencing the localization and activity of iron-acquisition regulators and effectors.

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.

RtNAC100 involved in the regulation of ROS, Na+ accumulation and induced salt-related PCD through MeJA signal pathways in recretohalophyte Reaumuria trigyna

NAM, ATAF1/2, and CUC2 (NAC) proteins regulate plant responses to salt stress. However, the molecular mechanisms by which NAC proteins regulate salt-induced programmed cell death (PCD) are unclear. We identified 56 NAC genes, 35 of which had complete open reading frames with complete NAM domain, in the R. trigyna transcriptome. Salt stress and methyl jasmonate (MeJA) mediated PCD-induced leaf senescence in R. trigyna seedlings. Salt stress accelerated endogenous JA biosynthesis, upregulating RtNAC100 expression. This promoted salt-induced leaf senescence in R. trigyna by regulating RtRbohE and RtSAG12/20 and enhancing ROS accumulation. Transgenic assays showed that RtNAC100 overexpression aggravated salt-induced PCD in transgenic lines by promoting ROS and Na⁺ accumulation, ROS-Ca²⁺ hub activation, and PCD-related gene expression. Therefore, RtNAC100 induces PCD via the MeJA signaling pathway in R. trigyna under salt stress.

Use of transcriptomic profiling to identify candidate genes involved in Polyporus umbellatus sclerotial formation affected by oxalic acid

Polyporus umbellatus is a precious medicinal fungus. Oxalic acid was observed to affect sclerotial formation and sclerotia possessed more medicinal compounds than mycelia. In this study, the transcriptome of P. umbellatus was analysed after the fungus was exposed to various concentrations of oxalic acid. The differentially expressed genes (DEGs) encoding a series of oxidases were upregulated, and reductases were downregulated, in the low-oxalic-acid (Low OA) group compared to the control (No OA) group, while the opposite phenomenon was observed in the high-oxalic-acid (High OA) group. The detection of reactive oxygen species (ROS) in P. umbellatus mycelia was performed visually, and Ca²⁺ and H₂O₂ fluxes were measured using non-invasive micro-test technology (NMT). The sclerotial biomass in the Low OA group increased by 66%, however, no sclerotia formed in the High OA group. The ROS fluorescence intensity increased significantly in the Low OA group but decreased considerably in the High OA group. Ca²⁺ and H₂O₂ influx significantly increased in the Low OA group, while H₂O₂ exhibited efflux in the High OA group. A higher level of oxidative stress formed in the Low OA group. Different concentrations of oxalic acid were determined to affect P. umbellatus sclerotial formation in different ways.

Role of jasmonic acid in plants: the molecular point of view

Recent updates in JA biosynthesis, signaling pathways and the crosstalk between JA and others phytohormones in relation with plant responses to different stresses. In plants, the roles of phytohormone jasmonic acid (JA), amino acid conjugate (e.g., JA-Ile) and their derivative emerged in last decades as crucial signaling compounds implicated in stress defense and development in plants. JA has raised a great interest, and the number of researches on JA has increased rapidly highlighting the importance of this phytohormone in plant life. First, JA was considered as a stress hormone implicated in plant response to biotic stress (pathogens and herbivores) which confers resistance to biotrophic and hemibiotrophic pathogens contrarily to salicylic acid (SA) which is implicated in plant response to necrotrophic pathogens. JA is also implicated in plant responses to abiotic stress (such as soil salinity, wounding and UV). Moreover, some researchers have recently revealed that JA controls several physiological processes like root growth, growth of reproductive organs and, finally, plant senescence. JA is also involved in the biosynthesis of various metabolites (e.g., phytoalexins and terpenoids). In plants, JA signaling pathways are well studied in few plants essentially Arabidopsis thaliana, Nicotiana benthamiana, and Oryza sativa L. confirming the crucial role of this hormone in plants. In this review, we highlight the last foundlings about JA biosynthesis, JA signaling pathways and its implication in plant maturation and response to environmental constraints.

Revealing the Role of the Calcineurin B-Like Protein-Interacting Protein Kinase 9 (CIPK9) in Rice Adaptive Responses to Salinity, Osmotic Stress, and K+ Deficiency

In plants, calcineurin B-like (CBL) proteins and their interacting protein kinases (CIPK) form functional complexes that transduce downstream signals to membrane effectors assisting in their adaptation to adverse environmental conditions. This study addresses the issue of the physiological role of CIPK9 in adaptive responses to salinity, osmotic stress, and K⁺ deficiency in rice plants. Whole-plant physiological studies revealed that Oscipk9 rice mutant lacks a functional CIPK9 gene and displayed a mildly stronger phenotype, both under saline and osmotic stress conditions. The reported difference was attributed to the ability of Oscipk9 to maintain significantly higher stomatal conductance (thus, a greater carbon gain). Oscipk9 plants contained much less K⁺ in their tissues, implying the role of CIPK9 in K⁺ acquisition and homeostasis in rice. Oscipk9 roots also showed hypersensitivity to ROS under conditions of low K⁺ availability suggesting an important role of H₂O₂ signalling as a component of plant adaptive responses to a low-K environment. The likely mechanistic basis of above physiological responses is discussed.

The Genetic Regulation of Secondary Metabolic Pathways in Response to Salinity and Drought as Abiotic Stresses

Global development has generated a plethora of unfavorable and adverse environmental factors for the living organisms in the ecosystem. Plants are sessile organisms, and they are crucial to sustain life on earth. Since plants are sessile, they face a great number of environmental challenges related to abiotic stresses, such as temperature fluctuation, drought, salinity, flood and metal contamination. Salinity and drought are considered major abiotic stresses that negatively affect the plants’ growth and production of useful content. However, plants have evolved various molecular mechanisms to increase their tolerance to these environmental stresses. There is a whole complex system of communication (cross-talk) through massive signaling cascades that are activated and modulated in response to salinity and drought. Secondary metabolites are believed to play significant roles in the plant’s response and resistance to salinity and drought stress. Until recently, attempts to unravel the biosynthetic pathways were limited mainly due to the inadequate plant genomics resources. However, recent advancements in generating high-throughput “omics” datasets, computational tools and functional genomics approach integration have aided in the elucidation of biosynthetic pathways of many plant bioactive metabolites. This review gathers comprehensive knowledge of plants’ complex system that is involved in the response and resistance to salinity and water deficit stresses as abiotic stress. Additionally, it offers clues in determining the genes involved in this complex and measures its activity. It covers basic information regarding the signaling molecules involved in salinity and drought resistance and how plant hormones regulate the cross-talking mechanism with emphasis on transcriptional activity. Moreover, it discusses many studies that illustrate the relationship between salinity and drought and secondary metabolite production. Furthermore, several transcriptome analysis research papers of medicinal plants are illustrated. The aim of this review is to be a key for any researcher that is aspiring to study the relationship between salinity and drought stresses and secondary metabolite production at the transcriptome and transcription level.

Improving Performance of Salt-Grown Crops by Exogenous Application of Plant Growth Regulators

Soil salinity is one of the major abiotic stresses restricting plant growth and development. Application of plant growth regulators (PGRs) is a possible practical means for minimizing salinity-induced yield losses, and can be used in addition to or as an alternative to crop breeding for enhancing salinity tolerance. The PGRs auxin, cytokinin, nitric oxide, brassinosteroid, gibberellin, salicylic acid, abscisic acid, jasmonate, and ethylene have been advocated for practical use to improve crop performance and yield under saline conditions. This review summarizes the current knowledge of the effectiveness of various PGRs in ameliorating the detrimental effects of salinity on plant growth and development, and elucidates the physiological and genetic mechanisms underlying this process by linking PGRs with their downstream targets and signal transduction pathways. It is shown that, while each of these PGRs possesses an ability to alter plant ionic and redox homeostasis, the complexity of interactions between various PGRs and their involvement in numerous signaling pathways makes it difficult to establish an unequivocal causal link between PGRs and their downstream effectors mediating plants' adaptation to salinity. The beneficial effects of PGRs are also strongly dependent on genotype, the timing of application, and the concentration used. The action spectrum of PGRs is also strongly dependent on salinity levels. Taken together, this results in a rather narrow "window" in which the beneficial effects of PGR are observed, hence limiting their practical application (especially under field conditions). It is concluded that, in the light of the above complexity, and also in the context of the cost-benefit analysis, crop breeding for salinity tolerance remains a more reliable avenue for minimizing the impact of salinity on plant growth and yield. Further progress in the field requires more studies on the underlying cell-based mechanisms of interaction between PGRs and membrane transporters mediating plant ion homeostasis.

Hypoxia-induced increase in GABA content is essential for restoration of membrane potential and preventing ROS-induced disturbance to ion homeostasis

When plants are exposed to hypoxic conditions, the level of γ-aminobutyric acid (GABA) in plant tissues increases by several orders of magnitude. The physiological rationale behind this elevation remains largely unanswered. By combining genetic and electrophysiological approach, in this work we show that hypoxia-induced increase in GABA content is essential for restoration of membrane potential and preventing ROS-induced disturbance to cytosolic K⁺ homeostasis and Ca²⁺ signaling. We show that reduced O₂ availability affects H⁺-ATPase pumping activity, leading to membrane depolarization and K⁺ loss via outward-rectifying GORK channels. Hypoxia stress also results in H₂O₂ accumulation in the cell that activates ROS-inducible Ca²⁺ uptake channels and triggers self-amplifying "ROS-Ca hub," further exacerbating K⁺ loss via non-selective cation channels that results in the loss of the cell's viability. Hypoxia-induced elevation in the GABA level may restore membrane potential by pH-dependent regulation of H⁺-ATPase and/or by generating more energy through the activation of the GABA shunt pathway and TCA cycle. Elevated GABA can also provide better control of the ROS-Ca²⁺ hub by transcriptional control of RBOH genes thus preventing over-excessive H₂O₂ accumulation. Finally, GABA can operate as a ligand directly controlling the open probability and conductance of K⁺ efflux GORK channels, thus enabling plants adaptation to hypoxic conditions.

Electrical signalling on Bt and non-Bt cotton plants under stress by Aphis gossypii

Plants have developed various mechanisms to respond specifically to each biotrophic attack. It has been shown that the electrical signals emitted by plants are associated with herbivory stress responses and can lead to the activation of multiple defences. Bt cotton is a genetically modified pest-resistant plant that produces an insecticide from Bacillus thuringiensis (Bt) to control Lepidopteran species. Surprisingly, there is no study-yet, that characterizes the signalling mechanisms in transgenic cotton plants attacked by non-target insects, such as aphids. In this study, we characterized the production of electrical signals on Bt and non-Bt cotton plants infested with Aphis gossypii and, in addition, we characterized the dispersal behaviour of aphids to correlate this behaviour to plant signalling responses. Electrical signalling of the plants was recorded with an extracellular measurement technique. Impressively, our results showed that both Bt and non-Bt cotton varieties, when attacked by A. gossypii, emitted potential variation-type electrical signals and clearly showed the presence of distinct responses regarding their perception and the behaviour of aphids, with evidence of delay, in terms of signal amount, and almost twice the amount of Cry1F protein was observed on Bt cotton plants at the highest density of insects/plant. We present in our article some hypotheses that are based on plant physiology and insect behaviour to explain the responses found on Bt cotton plants under aphid stress.

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.

Review: Microtubules monitor calcium and reactive oxygen species signatures in signal transduction

Signal transductions require calcium (Ca²⁺) or reactive oxygen species (ROS) signatures, which act as chemical and electrical signals in response to various biotic and abiotic stresses. Calcium as an ion or second messenger affects the membrane potential and microtubules (MTs) dynamicity, while MTs can modulate auto-propagating waves of calcium and ROS signatures in collaboration with ion channels depending on the stimulus type. Thus, in the current review, we highlight advances in research focused on the relationship between dynamic MTs and calcium and ROS signatures in short-distance transmission. The challenges of Ca²⁺-MTs-ROS crosstalk in cold sensing are addressed, which could suggest the prioritization of ROS or Ca²⁺ in signalling.

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.

Genome-wide analysis of the apple CaCA superfamily reveals that MdCAX proteins are involved in the abiotic stress response as calcium transporters

BACKGROUND

Calcium (Ca²⁺) plays an important role in plant growth and development, and the maintenance of calcium homeostasis is necessary for the survival of all plant species. Ca²⁺/H⁺ exchangers (CAXs) are a subgroup of the CaCA (Ca²⁺/cation antiporter) superfamily. In general, CAX proteins mediate cytosolic Ca²⁺ entry into vacuoles to prevent excessive accumulation of Ca²⁺ in the cytosol. The CaCA superfamily has been identified and characterised in many plant species; however, characterisation of the CaCA superfamily and functional study of apple CAX proteins have yet to be conducted in apple (Malus × domestica Borkh.).

RESULTS

Here, we identified 21 CaCA family proteins in apple for the first time. Phylogenetic and gene structure analysis, as well as prediction of conserved motifs, suggested that these proteins could be classified into four groups: CAX, CCX, NCL, and MHX. Expression analysis showed that the 10 MdCAX genes we cloned strongly responded to calcium and abiotic stress treatments. Collinearity analysis and characterisation of calcium transport capacity resulted in the identification of a pair of segmental duplication genes: MdCAX3L-1 and MdCAX3L-2; MdCAX3L-2 showed strong calcium transport capacity, whereas MdCAX3L-1 showed no calcium transport capacity. Yeast two-hybrid (Y2H) assays showed that these two proteins could interact with each other. The high sequence similarity (94.6%) makes them a good model for studying the crucial residues and structural basis of the calcium transport of CAX proteins. Prediction of the protein interaction network revealed several proteins that may interact with CAX proteins and play important roles in plant stress responses, such as SOS2, CXIP1, MHX, NRAMP3, and MTP8.

CONCLUSIONS

Our analysis indicated that MdCAX proteins have strong calcium transport capacity and are involved in the abiotic stress response in apple. These findings provide new insight and rich resources for future studies of MdCAX proteins in apple.

Up-regulated 2-alkenal reductase expression improves low-nitrogen tolerance in maize by alleviating oxidative stress

In plants, cellular lipid peroxidation is enhanced under low nitrogen (LN) stress; this increases the lipid-derived reactive carbonyl species (RCS) levels. The cellular toxicity of RCS can be reduced by various RCS-scavenging enzymes. However, the roles of these enzymes in alleviating oxidative stress and improving nutrient use efficiency (NUE) under nutrient stress remain unknown. Here, we overexpressed maize endogenous NADPH-dependent 2-alkenal reductase (ZmAER) in maize; it significantly increased the tolerance of transgenic plants (OX-AER) to LN stress. Under LN condition, the biomass, nitrogen accumulation, NUE, and leaf photosynthesis of the OX-AER plants were significantly higher than those of the wild-type (WT) plants. The leaf and root malondialdehyde and H₂ O₂ levels in the transgenic plants were significantly lower than those in WT. The expression of antioxidant enzyme-related genes ZmCAT3, ZmPOD5 and ZmPOD13 was significantly higher in the transgenic lines than in WT. Under LN stress, the nitrate reductase activity in the OX-AER leaves was significantly increased compared with that in the WT leaves. Furthermore, under LN stress, ZmNRT1.1 and ZmNRT2.5 expression was upregulated in the OX-AER plants compared with that in WT. Overall, up-regulated ZmAER expression could enhance maize's tolerance to LN stress by alleviating oxidative stress and improve NUE.

Synergistic consequences of salinity and potassium deficiency in quinoa: Linking with stomatal patterning, ionic relations and oxidative metabolism

Quinoa emerged as an ideal food security crop due to its exceptional nutritive profile and stress enduring potential and also deemed as model plant to study the salt-tolerance mechanisms. However to fill the research gaps of this imperative crop, the present work aimed to study the effect of potassium (K) deficiency either separately or in combination with salinity. First, we investigated the stomatal and physiological based variations in quinoa growth under salinity and K, then series of analytical tools were used with model approach to interpret the stomatal aperture (SA) and photosynthesis (Pn) changes. Results revealed that quinoa efficiently deployed antioxidants to scavenge the excessive reactive oxygen species (ROS), had high uptake and retention of K⁺, Ca²⁺, Mg²⁺ with Cl⁻ as charge balancing ion, increased stomata density (SD) and declined the SA to maintain the Pn which resulted the improved growth under salinity. Whereas, K-deficiency caused the stunted growth more severally under salinity due to disruption in ionic homeostasis, excessive ROS production elicited the oxidative damages, SD and SA reduced and ultimately declined in Pn. Our best fitted regression model explored that dependent variables like Pn and SA changed according to theirs signified explanatory variables with quantification per unit based as stomatal conductance (Gs, 51), SD (0.05), ROS (-0.79) and K⁺ (0.08), Cl⁻ (0.34) and Na^{+ (-} 0.52) respectively. Overall, moderate salinity promoted the quinoa growth, while K-deficiency particularly with salinity reduced the quinoa performance by affecting stomatal and non-stomatal factors.

Decoding Heavy Metal Stress Signalling in Plants: Towards Improved Food Security and Safety

The mining of heavy metals from the environment leads to an increase in soil pollution, leading to the uptake of heavy metals into plant tissue. The build-up of toxic metals in plant cells often leads to cellular damage and senescence. Therefore, it is of utmost importance to produce plants with improved tolerance to heavy metals for food security, as well as to limit heavy metal uptake for improved food safety purposes. To achieve this goal, our understanding of the signaling mechanisms which regulate toxic heavy metal uptake and tolerance in plants requires extensive improvement. In this review, we summarize recent literature and data on heavy metal toxicity (oral reference doses) and the impact of the metals on food safety and food security. Furthermore, we discuss some of the key events (reception, transduction, and response) in the heavy metal signaling cascades in the cell wall, plasma membrane, and cytoplasm. Our future perspectives provide an outlook of the exciting advances that will shape the plant heavy metal signaling field in the near future.

Ca2+/Calmodulin Complex Triggers CAMTA Transcriptional Machinery Under Stress in Plants: Signaling Cascade and Molecular Regulation

Calcium (Ca2+) ion is a critical ubiquitous intracellular second messenger, acting as a lead currency for several distinct signal transduction pathways. Transient perturbations in free cytosolic Ca2+ ([Ca2+]cyt) concentrations are indispensable for the translation of signals into adaptive biological responses. The transient increase in [Ca2+]cyt levels is sensed by an array of Ca2+ sensor relay proteins such as calmodulin (CaM), eventually leading to conformational changes and activation of CaM. CaM, in a Ca2+-dependent manner, regulates several transcription factors (TFs) that are implicated in various molecular, physiological, and biochemical functions in cells. CAMTA (calmodulin-binding transcription activator) is one such member of the Ca2+-loaded CaM-dependent family of TFs. The present review focuses on Ca2+ as a second messenger, its interaction with CaM, and Ca2+/CaM-mediated CAMTA transcriptional regulation in plants. The review recapitulates the molecular and physiological functions of CAMTA in model plants and various crops, confirming its probable involvement in stress signaling pathways and overall plant development. Studying Ca2+/CaM-mediated CAMTA TF will help in answering key questions concerning signaling cascades and molecular regulation under stress conditions and plant growth, thus improving our knowledge for crop improvement.

Melatonin improves rice salinity stress tolerance by NADPH oxidase-dependent control of the plasma membrane K+ transporters and K+ homeostasis

This study aimed to reveal the mechanistic basis of the melatonin-mediated amelioration of salinity stress in plants. Electrophysiological experiments revealed that melatonin decreased salt-induced K⁺ efflux (a critical determinant of plant salt tolerance) in a dose- and time-dependent manner and reduced sensitivity of the plasma membrane K⁺ -permeable channels to hydroxyl radicals. These beneficial effects of melatonin were abolished by NADPH oxidase blocker DPI. Transcriptome analyses revealed that melatonin induced 585 (448 up- and 137 down-regulated) and 59 (54 up- and 5 down-regulated) differentially expressed genes (DEGs) in the root tip and mature zone, respectively. The most noticeable changes in the root tip were melatonin-induced increase in the expression of several DEGs encoding respiratory burst NADPH oxidases (OsRBOHA and OsRBOHF), calcineurin B-like/calcineurin B-like-interacting protein kinase (OsCBL/OsCIPK), and calcium-dependent protein kinase (OsCDPK) under salt stress. Melatonin also enhanced the expression of potassium transporter genes (OsAKT1, OsHAK1, and OsHAK5). Taken together, these results indicate that melatonin improves salt tolerance in rice by enabling K⁺ retention in roots, and that the latter process is conferred by melatonin scavenging of hydroxyl radicals and a concurrent OsRBOHF-dependent ROS signalling required to activate stress-responsive genes and increase the expression of K⁺ uptake transporters in the root tip.

Introducing a hybrid artificial intelligence method for high-throughput modeling and optimizing plant tissue culture processes: the establishment of a new embryogenesis medium for chrysanthemum, as a case study

Data-driven models in a combination of optimization algorithms could be beneficial methods for predicting and optimizing in vitro culture processes. This study was aimed at modeling and optimizing a new embryogenesis medium for chrysanthemum. Three individual data-driven models, including multi-layer perceptron (MLP), adaptive neuro-fuzzy inference system (ANFIS), and support vector regression (SVR), were developed for callogenesis rate (CR), embryogenesis rate (ER), and somatic embryo number (SEN). Consequently, the best obtained results were used in the fusion process by a bagging method. For medium reformulation, effects of eight ionic macronutrients on CR, ER, and SEN and effects of four vitamins on SEN were evaluated using data fusion (DF)-non-dominated sorting genetic algorithm-II (NSGA-II) and DF-genetic algorithm (GA), respectively. Results showed that DF models with the highest R² had superb performance in comparison with all other individual models. According to DF-NSGAII, the highest ER and SEN can be obtained from the medium containing 14.27 mM NH₄⁺, 38.92 mM NO₃^-, 22.79 mM K⁺, 5.08 mM Cl^-, 3.34 mM Ca²⁺, 1.67 mM Mg²⁺, 2.17 mM SO₄^2-, and 1.44 mM H₂PO₄^-. Based on the DF-GA model, the maximum SEN can be obtained from a medium containing 0.61 μM thiamine, 5.93 μM nicotinic acid, 0.25 μM biotin, and 0.26 μM riboflavin. The efficiency of the established-optimized medium was experimentally compared to Murashige and Skoog medium (MS) for embryogenesis of five chrysanthemum cultivars, and results indicated the efficiency of optimized medium over MS medium.Key points• MLP, SVR, and ANFIS were fused by a bagging method to develop a data fusion model.• NSGA-II and GA were linked to the data fusion model for establishing and optimizing a new embryogenesis medium.• The new culture medium (HNT) had better efficiency than MS medium.

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

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

Keep Calm and Survive: Adaptation Strategies to Energy Crisis in Fruit Trees under Root Hypoxia

Plants are permanently facing challenges imposed by the environment which, in the context of the current scenario of global climate change, implies a constant process of adaptation to survive and even, in the case of crops, at least maintain yield. O₂ deficiency at the rhizosphere level, i.e., root hypoxia, is one of the factors with the greatest impact at whole-plant level. At cellular level, this O₂ deficiency provokes a disturbance in the energy metabolism which has notable consequences on the yield of plant crops. In this sense, although several physiological studies describe processes involved in plant adaptation to root hypoxia in woody fruit trees, with emphasis on the negative impacts on photosynthetic rate, there are very few studies that include -omics strategies for specifically understanding these processes in the roots of such species. Through a de novo assembly approach, a comparative transcriptome study of waterlogged Prunus spp. genotypes contrasting in their tolerance to root hypoxia was revisited in order to gain a deeper insight into the reconfiguration of pivotal pathways involved in energy metabolism. This re-analysis describes the classically altered pathways seen in the roots of woody fruit trees under hypoxia, but also routes that link them to pathways involved with nitrogen assimilation and the maintenance of cytoplasmic pH and glycolytic flow. In addition, the effects of root hypoxia on the transcription of genes related to the mitochondrial oxidative phosphorylation system, responsible for providing adenosine triphosphate (ATP) to the cell, are discussed in terms of their roles in the energy balance, reactive oxygen species (ROS) metabolism and aerenchyma formation. This review compiles key findings that help to explain the trait of tolerance to root hypoxia in woody fruit species, giving special attention to their strategies for managing the energy crisis. Finally, research challenges addressing less-explored topics in recovery and stress memory in woody fruit trees are pointed out.

Changes in Expression Level of OsHKT1;5 Alters Activity of Membrane Transporters Involved in K+ and Ca2+ Acquisition and Homeostasis in Salinized Rice Roots

In rice, the OsHKT1;5 gene has been reported to be a critical determinant of salt tolerance. This gene is harbored by the SKC1 locus, and its role was attributed to Na+ unloading from the xylem. No direct evidence, however, was provided in previous studies. Also, the reported function of SKC1 on the loading and delivery of K+ to the shoot remains to be explained. In this work, we used an electrophysiological approach to compare the kinetics of Na+ uptake by root xylem parenchyma cells using wild type (WT) and NIL(SKC1) plants. Our data showed that Na+ reabsorption was observed in WT, but not NIL(SKC1) plants, thus questioning the functional role of HKT1;5 as a transporter operating in the direct Na+ removal from the xylem. Instead, changes in the expression level of HKT1;5 altered the activity of membrane transporters involved in K+ and Ca2+ acquisition and homeostasis in the rice epidermis and stele, explaining the observed phenotype. We conclude that the role of HKT1;5 in plant salinity tolerance cannot be attributed to merely reducing Na+ concentration in the xylem sap but triggers a complex feedback regulation of activities of other transporters involved in the maintenance of plant ionic homeostasis and signaling under stress conditions.

A Novel and Potentially MultifacetedDehydroascorbate Reductase Increasing theAntioxidant Systems Is Induced by Beauvericinin Tomato

Dehydroascorbate reductases (DHARs) are important enzymes that reconvert the dehydroascorbic acid (DHA) into ascorbic acid (ASC). They are involved in the plant response to oxidative stress, such as that induced by the mycotoxin beauvericin (BEA). Tomato plants were treated with 50 µM of BEA; the main antioxidant compounds and enzymes were evaluated. DHARs were analyzed in the presence of different electron donors by native and denaturing electrophoresis as well as by western blot and mass spectrometry to identify a novel induced protein with DHAR activity. Kinetic parameters for dehydroascorbate (DHA) and glutathione (GSH) were also determined. The novel DHAR was induced after BEA treatment. It was GSH-dependent and possessed lower affinity to DHA and GSH than the classical DHARs. Interestingly, the mass spectrometry analysis of the main band appearing on sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) revealed a chloroplast sedoheptulose 1,7-bisphosphatase, a key enzyme of the Calvin cycle, and a chloroplast mRNA-binding protein, suggesting that the DHA reducing capacity could be a side activity or the novel DHAR could be part of a protein complex. These results shed new light on the ascorbate-glutathione regulation network under oxidative stress and may represent a new way to increase the plant antioxidant defense system, plant nutraceutical value, and the health benefits of plant consumption.

Roles and mechanisms of Ca2+ in regulating primary root growth of plants

Calcium (Ca2+) as a universal signal molecule plays pivotal roles in plant growth and development. It regulates root morphogenesis mainly through mediating phytohormone and stress signalings or affecting these signalings. In recent years, much progress has been made in understanding the roles of Ca2+ in primary root development. Here, we summarize recent advances in the functions and mechanisms of Ca2+ in modulating primary root growth in plants under normal and stressful conditions.