The battle of two ions: Ca2+ signalling against Na+ stress

AtMYB72 aggravates photosynthetic inhibition and oxidative damage in Arabidopsis thaliana leaves caused by salt stress

MYB transcription factor is one of the largest families in plants. There are more and more studies on plants responding to abiotic stress through MYB transcription factors, but the mechanism of some family members responding to salt stress is unclear. In this study, physiological and transcriptome techniques were used to analyze the effects of the R2R3-MYB transcription factor AtMYB72 on the growth and development, physiological function, and key gene response of Arabidopsis thaliana. Phenotypic observation showed that the damage of overexpression strain was more serious than that of Col-0 after salt treatment, while the mutant strain showed less salt injury symptoms. Under salt stress, the decrease of chlorophyll content, the degree of photoinhibition of photosystem II (PSII) and photosystem I (PSI) and the degree of oxidative damage of overexpressed lines were significantly higher than those of Col-0. Transcriptome data showed that the number of differentially expressed genes (DEGs) induced by salt stress in overexpressed lines was significantly higher than that in Col-0. GO enrichment analysis showed that the response of AtMYB72 to salt stress was mainly by affecting gene expression in cell wall ectoplast, photosystem I and photosystem II, and other biological processes related to photosynthesis. Compared with Col-0, the overexpression of AtMYB72 under salt stress further inhibited the synthesis of chlorophyll a (Chla) and down-regulated most of the genes related to photosynthesis, which made the photosynthetic system more sensitive to salt stress. AtMYB72 also caused the outbreak of reactive oxygen species and the accumulation of malondialdehyde under salt stress, which decreased the activity and gene expression of key enzymes in SOD, POD, and AsA-GSH cycle, thus destroying the ability of antioxidant system to maintain redox balance. AtMYB72 negatively regulates the accumulation of osmotic regulatory substances such as soluble sugar (SS) and soluble protein (SP) in A. thaliana leaves under salt stress, which enhances the sensitivity of Arabidopsis leaves to salt. To sum up, MYB72 negatively regulates the salt tolerance of A. thaliana by destroying the light energy capture, electron transport, and antioxidant capacity of Arabidopsis.

5-Aminolevulinic acid improves strawberry salt tolerance through a NO-H2O2 signaling circuit regulated by FaWRKY70 and FaWRKY40

INTRODUCTION

5-Aminolevulinic acid (ALA) is an essential biosynthetic precursor of tetrapyrrole compounds, naturally occurring in all living organisms. It has also been suggested as a new plant growth regulator. Treatment with ALA promotes strawberry Na+ homeostasis under salt stress. Regulation of this process requires the signaling molecules nitric oxide (NO) and hydrogen peroxide (H2O2), but the specific signaling cascade and transcriptional regulatory mechanism have not previously been characterized.

OBJECTIVES

Our work focused on the dissection of the NO and H2O2 signaling cascade and transcriptional regulatory mechanism by which FaWRKY70-FaWRKY40 participated in ALA-improved Na+ homeostasis and salt tolerance of strawberry.

METHODS

It was preliminarily confirmed by transcriptome and RT-qPCR that FaWRKY40 and FaWRKY70 participated in ALA-induced salt tolerance of strawberry. Two WRKY transcription factors overexpressed in woodland strawberry as well as tobacco were used to identify the gene functions in salt tolerance. Yeast one-hybrid (Y1H), β-glucuronidase (GUS), dual luciferase reporter (DLR) and electrophoretic mobility shift assays (EMSA) were used to verify the interaction with the target gene.

RESULTS

ALA induced NO and H2O2 production, which formed a signaling circuit reciprocally regulated by FaNR1 and FaRbohD expression to coordinate Na+ homeostasis. FaWRKY40 was shown to act as a positive transcription factor in this pathway: FaWRKY40 overexpression improved salt tolerance in woodland strawberry and tobacco, whereas FaWRKY40 RNA interference increased plant salt injury. FaWRKY40 bound to the promoters of FaRbohD, FaNHX1, and FaSOS1 to promote root H2O2 generation and Na+ reallocation. Conversely, FaWRKY70, a negative WRKY transcription factor, was found to increase salt sensitivity by inhibiting expression of FvWRKY40, FvNR1, and FvHKT1. ALA inhibited FaWRKY70 but increased FaWRKY40 expression, coordinating the regulation of NO-H2O2 signaling and Na+ homeostasis when strawberry was stress by salinity.

CONCLUSION

ALA inhibits NaCl-stimulated FaWRYK70 expression, relieving the transcriptional inhibition of its downstream targets. The NO-H2O2 signaling circuit can then initiate mechanisms such as Na+ exclusion, vacuolar sequestration, and removal of Na+ from the xylem sap, limiting Na+ accumulation in the leaves and promoting Na+ homeostasis and plant salt tolerance.

An Enhanced Interaction of Graft and Exogenous SA on Photosynthesis, Phytohormone, and Transcriptome Analysis in Tomato under Salinity Stress

Salt stress can adversely affect global agricultural productivity, necessitating innovative strategies to mitigate its adverse effects on plant growth and yield. This study investigated the effects of exogenous salicylic acid (SA), grafting (G), and their combined application (GSA) on various parameters in tomato plants subjected to salt stress. The analysis focused on growth characteristics, photosynthesis, osmotic stress substances, antioxidant enzyme activity, plant hormones, ion content, and transcriptome profiles. Salt stress severely inhibits the growth of tomato seedlings. However, SA, G, and GSA improved the plant height by 22.5%, 26.5%, and 40.2%; the stem diameter by 11.0%, 26.0%, and 23.7%; the shoot fresh weight by 76.3%, 113.2%, and 247.4%; the root fresh weight by 150.9%, 238.6%, and 286.0%; the shoot dry weight by 53.5%, 65.1%, and 162.8%; the root dry weight by 150.0%, 150.0%, and 166.7%, and photosynthesis by 4.0%, 16.3%, and 32.7%, with GSA presenting the most pronounced positive effect. Regarding the osmotic stress substances, the proline content increased significantly by more than 259.2% in all treatments, with the highest levels in GSA. Under salt stress, the tomato seedlings accumulated high Na+ levels; the SA, G, and GSA treatments enhanced the K+ and Ca2+ absorption while reducing the Na+ and Al3+ levels, thereby alleviating the ion toxicity. The transcriptome analysis indicated that SA, G, and GSA influenced tomato growth under salt stress by regulating specific signaling pathways, including the phytohormone and MAPK pathways, which were characterized by increased endogenous SA and decreased ABA content. The combined application of grafting and exogenous SA could be a promising strategy for enhancing plant tolerance to salt stress, offering potential solutions for sustainable agriculture in saline environments.

Rab7 GTPase-Mediated stress signaling enhances salinity tolerance in AlRabring7 tobacco transgenics by modulating physio-biochemical parameters

The RING-type E3 ligases play a significant role in stress signaling, primarily through post-translational regulation. Ubiquitination is a crucial post-translational modification that regulates the turnover and activity of proteins. The overexpression of AlRabring7, RING-HC E3 Ub ligase in tobacco provides insights into the regulation of salinity and ABA signaling in transgenic tobacco. The seed germination potential of AlRabring7 transgenics was higher than WT, with NaCl and ABA treatments. The transgenics showed improved morpho-physio-biochemical parameters in response to salinity and ABA treatments. The photosynthetic pigments, soluble sugars, reducing sugars and proline increased in transgenics in response to NaCl and ABA treatments. The decreased ROS accumulation in transgenics on NaCl and ABA treatments can be co-related to improved activity of enzymatic and non-enzymatic antioxidants. The potential of transgenics to maintain ABA levels with ABA treatment, highlights the active participation of ABA feedback loop mechanism. Interestingly, the ability of AlRabring7 transgenics to upregulate Rab7 protein, suggests its role in facilitating vacuolar transport. Furthermore, the improved potassium accumulation and reduced sodium content indicate an efficient ion regulation mechanism in transgenic plants facilitating higher stomatal opening. The expression of downstream ion transporter (NbNHX1 and NbVHA1), ABA signaling (NbABI2 and NbABI5) and vesicle trafficking (NbMON1) responsive genes were upregulated with stress. The present study, reports that AlRabring7 participates in maintaining vacuolar transport, ion balance, ROS homeostasis, stomatal regulation through activation of Rab7 protein and regulation of downstream stress-responsive during stress. This emphasizes the potential of AlRabring7 gene for improved performance and resilience in challenging environments.

Gamma-aminobutyric acid interactions with phytohormones and its role in modulating abiotic and biotic stress in plants

Gamma-aminobutyric acid (GABA), a ubiquitous non-protein 4-carbon amino acid present in both prokaryotic and eukaryotic organisms. It is conventionally recognized as a neurotransmitter in mammals and plays a crucial role in plants. The context of this review centers on the impact of GABA in mitigating abiotic stresses induced by climate change, such as drought, salinity, heat, and heavy metal exposure. Beyond its neurotransmitter role, GABA emerges as a key player in diverse metabolic processes, safeguarding plants against multifaceted abiotic as well as biotic challenges. This comprehensive exploration delves into the GABA biosynthetic pathway, its transport mechanisms, and its intricate interplay with various abiotic stresses. The discussion extends to the nuanced relationship between GABA and phytohormones during abiotic stress acclimation, offering insights into the strategic development of mitigation strategies against these stresses. The delineation of GABA's crosstalk with phytohormones underscores its pivotal role in formulating crucial strategies for abiotic stress alleviation in plants.

Progress on Salt Tolerance in Brassica napus

In China, saline-alkali lands constitute 5.01% of the total land area, having a significant impact on both domestic and international food production. Rapeseed (Brassica napus L.), as one of the most important oilseed crops in China, has garnered considerable attention due to its potential adaptability to saline conditions. Breeding and improving salt-tolerant varieties is a key strategy for the effective utilization of saline lands. Hence, it is important to conduct comprehensive research into the adaptability and salt tolerance mechanisms of Brassica napus in saline environments as well as to breed novel salt-tolerant varieties. This review summarizes the molecular mechanism of salt tolerance, physiological and phenotypic indexes, research strategies for the screening of salt-tolerant germplasm resources, and genetic engineering tools for salt stress in Brassica napus. It also introduces various agronomic strategies for applying exogenous substances to alleviate salt stress and provide technological tools and research directions for future research on salt tolerance in Brassica napus.

Pumpkin CmoDREB2A enhances salt tolerance of grafted cucumber through interaction with CmoNAC1 to regulate H2O2 and ABA signaling and K+/Na+ homeostasis

Pumpkin CmoNAC1 enhances salt tolerance in grafted cucumbers. However, the potential interactions with other proteins that may co-regulate salt tolerance alongside CmoNAC1 have yet to be explored. In this study, we identified pumpkin CmoDREB2A as a pivotal transcription factor that interacts synergistically with CmoNAC1 in the co-regulation of salt tolerance. Both transcription factors were observed to bind to each other's promoters, forming a positive regulatory loop of their transcription. Knockout of CmoDREB2A in the root resulted in reduced salt tolerance in grafted cucumbers, whereas overexpression demonstrated the opposite effect. Multiple assays in our study provided evidence of the protein interaction between CmoDREB2A and CmoNAC1. Exploiting this interaction, CmoDREB2A facilitated the binding of CmoNAC1 to the promoters of CmoRBOHD1, CmoNCED6, CmoAKT1;2, and CmoHKT1;1, inducing H2O2 and ABA synthesis and increasing the K+/Na+ ratio in grafted cucumbers under salt stress. Additionally, CmoNAC1 also promoted the binding of CmoDREB2A to CmoHAK5;1/CmoHAK5;2 promoters, further contributing to the K+/Na+ homeostasis. In summary, these findings reveal a crucial mechanism of CmoNAC1 and CmoDREB2A forming a complex enhancing salt tolerance in grafted cucumbers.

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.

RcMYB8 enhances salt and drought tolerance in rose (Rosa chinensis) by modulating RcPR5/1 and RcP5CS1

Plant Myeloblastosis (MYB) proteins function crucially roles upon variegated abiotic stresses. Nonetheless, their effects and mechanisms in rose (Rosa chinensis) are not fully clarified. In this study, we characterized the effects of rose RcMYB8 under salt and drought tolerances. For induction of the RcMYB8 expression, NaCl and drought stress treatment were adopted. Rose plants overexpressing RcMYB8 displayed enhanced tolerance to salinity and drought stress, while silencing RcMYB8 resulted in decreased tolerance, as evidenced by lowered intra-leaf electrolyte leakage and callose deposition, as well as photosynthetic sustainment under stressed conditions. Here, we further show that RcMYB8 binds similarly to the promoters of RcPR5/1 and RcP5C51 in vivo and in vitro. Inhibiting RcP5CS1 by virus-induced gene silencing led to decreased drought tolerance through the reactive oxygen species (ROS) homeostatic regulation. RcP5CS1-silenced plants showed an increase in ion leakage and reduce of proline content, together with the content of malondialdehyde (MDA) increased, lowered activities of Catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD). Our study highlights the transcriptional modulator role of RcMYB8 in drought and salinity tolerances, which bridges RcPR5/1 and RcP5CS1 by promoting ROS scavenging. Besides, it is probably applicable to the rose plant engineering for enhancing their abiotic stress tolerances.

The Role of Chloride Channels in Plant Responses to NaCl

Chloride (Cl-) is considered a crucial nutrient for plant growth, but it can be a challenge under saline conditions. Excessive accumulation of Cl- in leaves can cause toxicity. Chloride channels (CLCs) are expressed in the inner membranes of plant cells and function as essential Cl- exchangers or channels. In response to salt stress in plants, CLCs play a crucial role, and CLC proteins assist in maintaining the intracellular Cl- homeostasis by sequestering Cl- into vacuoles. Sodium chloride (NaCl) is the primary substance responsible for causing salt-induced phytotoxicity. However, research on plant responses to Cl- stress is comparatively rare, in contrast to that emphasizing Na+. This review provides a comprehensive overview of the plant response and tolerance to Cl- stress, specifically focusing on comparative analysis of CLC protein structures in different species. Additionally, to further gain insights into the underlying mechanisms, the study summarizes the identified CLC genes that respond to salt stress. This review provides a comprehensive overview of the response of CLCs in terrestrial plants to salt stress and their biological functions, aiming to gain further insights into the mechanisms underlying the response of CLCs in plants to salt stress.

Biomass composite with exogenous organic acid addition supports the growth of sweet sorghum (Sorghum bicolor 'Dochna') by reducing salinity and increasing nutrient levels in coastal saline-alkaline soil

Introduction

In coastal saline lands, organic matter is scarce and saline stress is high. Exploring the promotion effect of intervention with organic acid from biological materials on soil improvement and thus forage output and determining the related mechanism are beneficial to the potential cultivation and resourceful, high-value utilization of coastal mudflats as back-up arable land.

Method

Three exogenous organic acids [humic acid (H), fulvic acid (F), and citric acid (C)] were combined with four kinds of biomass materials [cottonseed hull (CH), cow manure (CM), grass charcoal (GC), and pine needle (PN)] and applied to about 0.3% of medium-salt mudflat soil. The salinity and nutrient dynamics of the soil and the growth and physiological differences of sweet sorghum at the seedling, elongation, and heading stages were observed under different treatments to screen for efficient combinations and analyze the intrinsic causes and influencing mechanisms.

Results

The soil salinity, nutrient dynamics, and forage grass biological yield during sweet sorghum cultivation in saline soils differed significantly (p < 0.05) depending on the type of organic acid-biomass composite applied. Citric acid-pine needle composite substantially reduced the soil salinity and increased the soil nutrient content at the seedling stage and improved the root vigor and photosynthesis of sweet sorghum by increasing its stress tolerance, allowing plant morphological restructuring for a high biological yield. The improvement effect of fulvic acid-pine needle or fulvic acid-cow manure composite was manifested at the elongation and heading stages.

Discussion

Citric acid-pine needle composite promoted the growth of saline sweet sorghum seedlings, and the effect of fulvic acid-pine needle composite lasted until the middle and late stages.

Salinity-Induced Cytosolic Alkaline Shifts in Arabidopsis Roots Require the SOS Pathway

Plants have evolved elaborate mechanisms to sense, respond to and overcome the detrimental effects of high soil salinity. The role of calcium transients in salinity stress signaling is well established, but the physiological significance of concurrent salinity-induced changes in cytosolic pH remains largely undefined. Here, we analyzed the response of Arabidopsis roots expressing the genetically encoded ratiometric pH-sensor pHGFP fused to marker proteins for the recruitment of the sensor to the cytosolic side of the tonoplast (pHGFP-VTI11) and the plasma membrane (pHGFP-LTI6b). Salinity elicited a rapid alkalinization of cytosolic pH (pHcyt) in the meristematic and elongation zone of wild-type roots. The pH-shift near the plasma membrane preceded that at the tonoplast. In pH-maps transversal to the root axis, the epidermis and cortex had cells with a more alkaline pHcyt relative to cells in the stele in control conditions. Conversely, seedlings treated with 100 mM NaCl exhibited an increased pHcyt in cells of the vasculature relative to the external layers of the root, and this response occurred in both reporter lines. These pHcyt changes were substantially reduced in mutant roots lacking a functional SOS3/CBL4 protein, suggesting that the operation of the SOS pathway mediated the dynamics of pHcyt in response to salinity.

Tobacco leaf tissue rapidly detoxifies direct salt loads without activation of calcium and SOS signaling

Salt stress is a major abiotic stress, responsible for declining agricultural productivity. Roots are regarded as hubs for salt detoxification, however, leaf salt concentrations may exceed those of roots. How mature leaves manage acute sodium chloride (NaCl) stress is mostly unknown. To analyze the mechanisms for NaCl redistribution in leaves, salt was infiltrated into intact tobacco leaves. It initiated pronounced osmotically-driven leaf movements. Leaf downward movement caused by hydro-passive turgor loss reached a maximum within 2 h. Salt-driven cellular water release was accompanied by a transient change in membrane depolarization but not an increase in cytosolic calcium ion (Ca²⁺ ) level. Nonetheless, only half an hour later, the leaves had completely regained turgor. This recovery phase was characterized by an increase in mesophyll cell plasma membrane hydrogen ion (H⁺ ) pumping, a salt uptake-dependent cytosolic alkalization, and a return of the apoplast osmolality to pre-stress levels. Although, transcript numbers of abscisic acid- and Salt Overly Sensitive pathway elements remained unchanged, salt adaptation depended on the vacuolar H⁺ /Na⁺ -exchanger NHX1. Altogether, tobacco leaves can detoxify sodium ions (Na⁺ ) rapidly even under massive salt loads, based on pre-established posttranslational settings and NHX1 cation/H⁺ antiport activity. Unlike roots, signaling and processing of salt stress in tobacco leaves does not depend on Ca²⁺ signaling.

Cytosolic Sodium Influx in Mesophyll Protoplasts of Arabidopsis thaliana, wt, sos1:1 and nhx1 Differs and Induces Different Calcium Changes

The sodium influx into the cytosol of mesophyll protoplasts from Arabidopsis thaliana cv. Columbia, wild type, was compared with the influx into sos1-1 and nhx1 genotypes, which lack the Na⁺/H⁺ antiporter in the plasma membrane and tonoplast, respectively. Changes in cytosolic sodium and calcium concentrations upon a 100 mM NaCl addition were detected by use of epifluorescence microscopy and the sodium-specific fluorescent dye SBFI, AM, and calcium sensitive Fura 2, AM, respectively. There was a smaller and mainly transient influx of Na⁺ in the cytosol of the wild type compared with the sos1-1 and nhx1 genotypes, in which the influx lasted for a longer time. Sodium chloride addition to the protoplasts' medium induced a significant increase in cytosolic calcium concentration in the wild type at 1.0 mM external calcium, and to a lesser extent in nhx1, however, it was negligible in the sos1-1 genotype. LiCl inhibited the cytosolic calcium elevation in the wild type. The results suggest that the salt-induced calcium elevation in the cytosol of mesophyll cells depends on an influx from both internal and external stores and occurs in the presence of an intact Na⁺/H⁺ antiporter at the plasma membrane. The Arabidopsis SOS1 more effectively regulates sodium homeostasis than NHX1.

A Ca2+-sensor switch for tolerance to elevated salt stress in Arabidopsis

Excessive Na⁺ in soils inhibits plant growth. Here, we report that Na⁺ stress triggers primary calcium signals specifically in a cell group within the root differentiation zone, thus forming a "sodium-sensing niche" in Arabidopsis. The amplitude of this primary calcium signal and the speed of the resulting Ca²⁺ wave dose-dependently increase with rising Na⁺ concentrations, thus providing quantitative information about the stress intensity encountered. We also delineate a Ca²⁺-sensing mechanism that measures the stress intensity in order to mount appropriate salt detoxification responses. This is mediated by a Ca²⁺-sensor-switch mechanism, in which the sensors SOS3/CBL4 and CBL8 are activated by distinct Ca²⁺-signal amplitudes. Although the SOS3/CBL4-SOS2/CIPK24-SOS1 axis confers basal salt tolerance, the CBL8-SOS2/CIPK24-SOS1 module becomes additionally activated only in response to severe salt stress. Thus, Ca²⁺-mediated translation of Na⁺ stress intensity into SOS1 Na⁺/H⁺ antiporter activity facilitates fine tuning of the sodium extrusion capacity for optimized salt-stress tolerance.

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

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

Physiological integration between Bermudagrass ramets improves overall salt resistance under heterogeneous salt stress

Connected ramets of colonal plants often suffer from different environmental conditions such as light, nutrient, and stress. Colonal Bermudagrass (Cynodon dactylon [L.] Pers.) can form interconnected ramets and this connection facilitates the tolerance to abiotic stress, which is a kind of physiological integration. However, how bermudagrass responds to heterogeneously distributed salt stress needs to be further elucidated. Here, we demonstrated that severance of stolons aggravated the damage of salt-stressed ramets, displaying higher relative electrolytic leakage (EL), lower content of chlorophyll, higher accumulation of Na⁺ , and serious oxidative damages. This finding implied the positive effects of the physiological integration of bermudagrass on salt tolerance. The unstressed ramets connected with the stressed one were mildly injured, implying the supporting and sacrifice function of the unstressed ramets. Physiological integration did not mediate the translocation of Na⁺ among ramets, but induced a higher expression of salt overly sensitive (SOS) genes in the stressed ramets, consequently reducing the accumulation of Na⁺ in leaves and roots. In addition, physiological integration upregulated the genes expression and enzymes activity of catalase (CAT) and peroxidase (POD) in both stressed and unstressed ramets. This granted a stronger antioxidant ability of the whole clonal plants under salt stress. Enhanced Na⁺ transfer and increased reactive oxygen species (ROS) scavenging are mechanisms that likely contribute to the physiological integration leading to the salt tolerance of bermudagrass.

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.

The Efficiency of Different Priming Agents for Improving Germination and Early Seedling Growth of Local Tunisian Barley under Salinity Stress

The current work aimed to investigate the effect of seed priming with different agents (CaCl₂, KCl, and KNO₃) on germination and seedling establishment in seeds of the barley species of both Hordeum vulgare (L. Manel) and Hordeum maritimum germinated with three salt concentrations (0, 100, and 200 mM NaCl). The results showed that under unprimed conditions, salt stress significantly reduced the final germination rate, the mean daily germination, and the seedling length and dry weight. It led to a decrease in the essential nutrient content (iron, calcium, magnesium, and potassium) against an increase in sodium level in both of the barley species. Moreover, this environmental constraint provoked a membrane injury caused by a considerable increase in electrolyte leakage and the malondialdehyde content (MDA). Data analysis proved that seed priming with CaCl₂, KCl, and KNO₃ was an effective method for alleviating barley seed germination caused by salt stress to varying degrees. Different priming treatments clearly stimulated germination parameters and the essential nutrient concentration, in addition to increasing the seedling growth rate. The application of seed priming reduced the accumulation of sodium ions and mitigated the oxidative stress of seeds caused by salt. This mitigation was traduced by the maintenance of low levels of MDA and electrolyte leakage. We conclude that the priming agents can be classed into three ranges based on their efficacy on the different parameters analyzed; CaCl₂ was placed in the first range, followed closely by KNO₃, while the least effective was KCl, which placed in the third range.

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.

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.

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.

The efficacy of different seed priming agents for promoting sorghum germination under salt stress

Sorghum [Sorghum bicolor (L.) Moench] seed germination is sensitive to salinity, and seed priming is an effective method for alleviating the negative effects of salt stress on seed germination. However, few studies have compared the effects of different priming agents on sorghum germination under salt stress. In this study, we quantified the effects of priming with distilled water (HP), sodium chloride (NaCl), potassium chloride (KCl), calcium chloride (CaCl₂), and polyethylene glycol (PEG) on sorghum seed germination under 150 mM NaCl stress. The germination potential, germination rate, germination index, vigor index, root length, shoot length, root fresh weight, shoot fresh weight, root dry weight, and shoot dry weight were significantly reduced by salt stress. Different priming treatments alleviated the germination inhibition caused by salt stress to varying degrees, and 50 mM CaCl₂ was the most effective treatment. In addition, the mitigation effect of priming was stronger on root traits than on shoot traits. Mitigation efficacy was closely related to both the type of agent and the concentration of the solution. Principal component analysis showed that all concentrations of CaCl₂ had higher scores and were clearly distinguished from other treatments based on their positive effects on all germination traits. The effects of the other agents varied with concentration. The priming treatments were divided into three categories based on their priming efficacy, and the 50, 100, and 150 mM CaCl₂ treatments were placed in the first category. The 150 mM KCl, 10% PEG, HP, 150 mM NaCl, 30% PEG, and 50 mM KCl treatments were placed in the second category, and the 100 mM NaCl, 100 mM KCl, 20% PEG, and 50 mM NaCl treatments were least effective and were placed in the third category. Choosing appropriate priming agents and methods for future research and applications can ensure that crop seeds germinate healthily under saline conditions.

Distinct Roles of N-Terminal Fatty Acid Acylation of the Salinity-Sensor Protein SOS3

The Salt-Overly-Sensitive (SOS) pathway controls the net uptake of sodium by roots and the xylematic transfer to shoots in vascular plants. SOS3/CBL4 is a core component of the SOS pathway that senses calcium signaling of salinity stress to activate and recruit the protein kinase SOS2/CIPK24 to the plasma membrane to trigger sodium efflux by the Na/H exchanger SOS1/NHX7. However, despite the well-established function of SOS3 at the plasma membrane, SOS3 displays a nucleo-cytoplasmic distribution whose physiological meaning is not understood. Here, we show that the N-terminal part of SOS3 encodes structural information for dual acylation with myristic and palmitic fatty acids, each of which commands a different location and function of SOS3. N-myristoylation at glycine-2 is essential for plasma membrane association and recruiting SOS2 to activate SOS1, whereas S-acylation at cysteine-3 redirects SOS3 toward the nucleus. Moreover, a poly-lysine track in positions 7-11 that is unique to SOS3 among other Arabidopsis CBLs appears to be essential for the correct positioning of the SOS2-SOS3 complex at the plasma membrane for the activation of SOS1. The nuclear-localized SOS3 protein had limited bearing on the salt tolerance of Arabidopsis. These results are evidence of a novel S-acylation dependent nuclear trafficking mechanism that contrasts with alternative subcellular targeting of other CBLs by S-acylation.

Cross-talk between gama-aminobutyric acid and calcium ion regulates lipid biosynthesis in Monoraphidium sp. QLY-1 in response to combined treatment of fulvic acid and salinity stress

In this study, the cross-talk between gama-aminobutyric acid (GABA) and calcium ion (Ca²⁺) signalling in the regulation of lipid production and cell growth in microalgae under fulvic acid and salinity stress (FA-salinity treatment) was investigated. GABA enhanced the lipid content and lipid productivity rate considerably, which were 1.27 and 1.29 times higher than those of the control, respectively. The levels of biosynthetic gene transcription, GSH, Ca²⁺ and cellular GABA were promoted by GABA addition, but decreased the ROS levels. Furthermore, the application of Ca²⁺ also increased lipid synthesis by regulating ROS and GABA signalling and lipogenesis-related genes. These results indicated that cytosolic GABA and Ca²⁺ levels exert crucial cross-talk in the modulation of cell growth and lipid accumulation induced by FA-salinity treatment. Collectively, this study demonstrated the beneficial effects caused by induction of the combination of chemical compounds on lipid production and provided new insights into lipid synthesis in microalgae.

Salinity induced physiological and biochemical changes in plants: An omic approach towards salt stress tolerance

Salinity is one of the major threats to sustainable agriculture that globally decreases plant production by impairing various physiological, biochemical, and molecular function. In particular, salinity hampers germination, growth, photosynthesis, transpiration, and stomatal conductance. Salinity decreases leaf water potential and turgor pressure and generates osmotic stress. Salinity enhances reactive oxygen species (ROS) content in the plant cell as a result of ion toxicity and disturbs ion homeostasis. Thus, it imbalances nutrient uptake, disintegrates membrane, and various ultrastructure. Consequently, salinity leads to osmotic and ionic stress. Plants respond to salinity by modulating various morpho-physiological, anatomical, and biochemical traits by regulating ion homeostasis and compartmentalization, antioxidant machinery, and biosynthesis of osmoprotectants and phytohormones, i. e, auxins, abscisic acid, brassinosteroids, cytokinins, ethylene, gibberellins, salicylic acid, jasmonic acid, and polyamines. Thus, this further modulates plant osmoticum, decreases ion toxicity, and scavenges ROS. Plants upregulate various genes and proteins that participate in salinity tolerance. They also promote the production of various phytohormones and metabolites that mitigate the toxic effect of salinity. Based on recent papers, the deleterious effect of salinity on plant physiology is discussed. Furthermore, it evaluates the physiological and biochemical responses of the plant to salinity along with phytohormone response. This review paper also highlights omics (genomics, transcriptomics, proteomics, and metabolomics) approach to understand salt stress tolerance.

Transcriptome profiling at osmotic and ionic phases of salt stress response in bread wheat uncovers trait-specific candidate genes

BACKGROUND

Bread wheat is one of the most important crops for the human diet, but the increasing soil salinization is causing yield reductions worldwide. Improving salt stress tolerance in wheat requires the elucidation of the mechanistic basis of plant response to this abiotic stress factor. Although several studies have been performed to analyze wheat adaptation to salt stress, there are still some gaps to fully understand the molecular mechanisms from initial signal perception to the onset of responsive tolerance pathways. The main objective of this study is to exploit the dynamic salt stress transcriptome in underlying QTL regions to uncover candidate genes controlling salt stress tolerance in bread wheat. The massive analysis of 3'-ends sequencing protocol was used to analyze leave samples at osmotic and ionic phases. Afterward, stress-responsive genes overlapping QTL for salt stress-related traits in two mapping populations were identified.

RESULTS

Among the over-represented salt-responsive gene categories, the early up-regulation of calcium-binding and cell wall synthesis genes found in the tolerant genotype are presumably strategies to cope with the salt-related osmotic stress. On the other hand, the down-regulation of photosynthesis-related and calcium-binding genes, and the increased oxidative stress response in the susceptible genotype are linked with the greater photosynthesis inhibition at the osmotic phase. The specific up-regulation of some ABC transporters and Na+/Ca2+ exchangers in the tolerant genotype at the ionic stage indicates their involvement in mechanisms of sodium exclusion and homeostasis. Moreover, genes related to protein synthesis and breakdown were identified at both stress phases. Based on the linkage disequilibrium blocks, salt-responsive genes within QTL intervals were identified as potential components operating in pathways leading to salt stress tolerance. Furthermore, this study conferred evidence of novel regions with transcription in bread wheat.

CONCLUSION

The dynamic transcriptome analysis allowed the comparison of osmotic and ionic phases of the salt stress response and gave insights into key molecular mechanisms involved in the salt stress adaptation of contrasting bread wheat genotypes. The leveraging of the highly contiguous chromosome-level reference genome sequence assembly facilitated the QTL dissection by targeting novel candidate genes for salt tolerance.

Extracellular Ca2+ induces desensitized cytosolic Ca2+ rise sensitive to phospholipase C inhibitor which suppresses root growth with Ca2+ dependence

Calcium (Ca) is an essential element for all organisms. In animal cells, the plasma membrane-localized Ca receptor CaSR coupled to a phospholipase C (PLC)-dependent signaling cascade monitors extracellular Ca²⁺ concentrations ([Ca²⁺]_ext) and responds with increases in cytosolic calcium concentrations ([Ca²⁺]_cyt). Plant roots encounter variable soil conditions, but how they sense changes in [Ca²⁺]_ext is largely unknown. In this study, we demonstrate that increasing [Ca²⁺]_ext evokes a transient increase in [Ca²⁺] in the cytosol, mitochondria, and nuclei of Arabidopsis thaliana root cells. These increases were strongly desensitized to repeat applications of [Ca²⁺]_ext, a typical feature of receptor-mediated cellular signaling in animal and plant cells. Treatment with gadolinium (Gd³⁺), a CaSR activator in animal cells, induced concentration-dependent increases in [Ca²⁺]_cyt in roots, which showed self-desensitization and cross-desensitization to [Ca²⁺]_ext-induced increases in [Ca²⁺]_cyt (EICC). EICC was sensitive to extracellular H⁺, K⁺, Na⁺, and Mg²⁺ levels. Treatment with the PLC inhibitor neomycin suppressed EICC and Ca accumulation in roots. The inhibitory effect of neomycin on root elongation was fully rescued by increasing [Ca²⁺]_ext but not [Mg²⁺] or [K⁺] in the growth medium. These results suggest that [Ca²⁺]_ext and the movement of Ca²⁺ into the cytosol of plant roots are regulated by a receptor-mediated signaling pathway involving PLC.

Salt-tolerance diversity in diploid and polyploid cotton (Gossypium) species

The development of salt-tolerant genotypes is pivotal for the effective utilization of salinized land and to increase global crop productivity. Several cotton species comprise the most important source of textile fibers globally, and these are increasingly grown on marginal or increasingly saline agroecosystems. The allopolyploid cotton species also provide a model system for polyploid research, of relevance here because polyploidy was suggested to be associated with increased adaptation to stress. To evaluate the genetic variation of salt tolerance among cotton species, 17 diverse accessions of allopolyploid (AD-genome) and diploid (A- and D-genome) Gossypium were evaluated for a total of 29 morphological and physiological traits associated with salt tolerance. For most morphological and physiological traits, cotton accessions showed highly variable responses to 2 weeks of exposure to moderate (50 mm NaCl) and high (100 mm NaCl) hydroponic salinity treatments. Our results showed that the most salt-tolerant species were the allopolyploid Gossypium mustelinum from north-east Brazil, the D-genome diploid Gossypium klotzschianum from the Galapagos Islands, followed by the A-genome diploids of Africa and Asia. Generally, A-genome accessions outperformed D-genome cottons under salinity conditions. Allopolyploid accessions from either diploid genomic group did not show significant differences in salt tolerance, but they were more similar to one of the two progenitor lineages. Our findings demonstrate that allopolyploidy in itself need not be associated with increased salinity stress tolerance and provide information for using the secondary Gossypium gene pool to breed for improved salt tolerance.

Combined NaCl and DTT diminish harmful ER-stress effects in the sorghum seedlings CSF 20 variety

Plants have developed mechanisms to avoid harmful effects of Na⁺ accumulation, such as the signaling pathway of carrier proteins Na⁺/H⁺ (NHX) and salt overly sensitive (SOS). Besides, endoplasmic reticulum (ER) could integrate plant cell response. Thus, we aimed to understand the effects of ER homeostasis impairment, and its relationship to salt stress during early stages of the Sorghum bicolor CSF 20 a salt-tolerant variety. Three days old seedlings were challenged with NaCl (0, 50, 75 and 100 mM), dithiothreitol (DTT) at 0, 2.5, 5.0 10.0 mM, and the combined NaCl and DTT treatments. Tunicamycin (TUN) was also used as a second inducer of ER stress in a quantitative PCR, to corroborate with DTT's results. There was no significant change in growth parameters under NaCl treatments. Nevertheless, seedling length, mass and Na⁺ content were decreased as DTT concentration was increased. Under combined NaCl and DTT treatments, shoot length and fresh and dry masses were maintained at control levels. On the other hand, the levels of Na⁺ were decreased, in comparison to NaCl treatment. Genes analyzed by qPCR revealed that NaCl was able to induce all of them, except for SbbZIP60, however it was induced under combined stresses. In conclusion, the results indicated that S. bicolor seedlings of CSF 20 variety were tolerant to salt and sensible to ER stress. The combination of both stresses restored the ER homeostasis promoting a decrease of Na⁺ content via the membrane transporters SbNHX1, SbSOS1, and SbPDI ER-chaperone and the ER sensor SbbZIP60.

Calcium-Regulated Phosphorylation Systems Controlling Uptake and Balance of Plant Nutrients

Essential elements taken up from the soil and distributed throughout the whole plant play diverse roles in different tissues. Cations and anions contribute to maintenance of intracellular osmolarity and the formation of membrane potential, while nitrate, ammonium, and sulfate are incorporated into amino acids and other organic compounds. In contrast to these ion species, calcium concentrations are usually kept low in the cytosol and calcium displays unique behavior as a cytosolic signaling molecule. Various environmental stresses stimulate increases in the cytosolic calcium concentration, leading to activation of calcium-regulated protein kinases and downstream signaling pathways. In this review, we summarize the stress responsive regulation of nutrient uptake and balancing by two types of calcium-regulated phosphorylation systems: CPK and CBL-CIPK. CPK is a family of protein kinases activated by calcium. CBL is a group of calcium sensor proteins that interact with CIPK kinases, which phosphorylate their downstream targets. In Arabidopsis, quite a few ion transport systems are regulated by CPKs or CBL-CIPK complexes, including channels/transporters that mediate transport of potassium (KAT1, KAT2, GORK, AKT1, AKT2, HAK5, SPIK), sodium (SOS1), ammonium (AMT1;1, AMT1;2), nitrate and chloride (SLAC1, SLAH2, SLAH3, NRT1.1, NRT2.4, NRT2.5), and proton (AHA2, V-ATPase). CPKs and CBL-CIPKs also play a role in C/N nutrient response and in acquisition of magnesium and iron. This functional regulation by calcium-dependent phosphorylation systems ensures the growth of plants and enables them to acquire tolerance against various environmental stresses. Calcium serves as the key factor for the regulation of membrane transport systems.

Silicon and Salinity: Crosstalk in Crop-Mediated Stress Tolerance Mechanisms

Salinity stress hinders the growth potential and productivity of crop plants by influencing photosynthesis, disturbing the osmotic and ionic concentrations, producing excessive oxidants and radicals, regulating endogenous phytohormonal functions, counteracting essential metabolic pathways, and manipulating the patterns of gene expression. In response, plants adopt counter mechanistic cascades of physio-biochemical and molecular signaling to overcome salinity stress; however, continued exposure can overwhelm the defense system, resulting in cell death and the collapse of essential apparatuses. Improving plant vigor and defense responses can thus increase plant stress tolerance and productivity. Alternatively, the quasi-essential element silicon (Si)-the second-most abundant element in the Earth's crust-is utilized by plants and applied exogenously to combat salinity stress and improve plant growth by enhancing physiological, metabolomic, and molecular responses. In the present review, we elucidate the potential role of Si in ameliorating salinity stress in crops and the possible mechanisms underlying Si-associated stress tolerance in plants. This review also underlines the need for future research to evaluate the role of Si in salinity stress in plants and the identification of gaps in the understanding of this process as a whole at a broader field level.

Diverse role of γ-aminobutyric acid in dynamic plant cell responses

Gamma-aminobutyric acid (GABA), a four-carbon non-protein amino acid, is found in most prokaryotic and eukaryotic organisms. Although, ample research into GABA has occurred in mammals as it is a major inhibitory neurotransmitter; in plants, a role for GABA has often been suggested as a metabolite that changes under stress rather than as a signal, as no receptor or motif for GABA binding was identified until recently and many aspects of its biological function (ranging from perception to function) remain to be answered. In this review, flexible properties of GABA in regulation of plant responses to various environmental biotic and abiotic stresses and its integration in plant growth and development either as a metabolite or a signaling molecule are discussed. We have elaborated on the role of GABA in stress adaptation (i.e., salinity, hypoxia/anoxia, drought, temperature, heavy metals, plant-insect interplay and ROS-related responses) and its contribution in non-stress-related biological pathways (i.e., involvement in plant-microbe interaction, contribution to the carbon and nitrogen metabolism and governing of signal transduction pathways). This review aims to represent the multifunctional contribution of GABA in various biological and physiological mechanisms under stress conditions; the objective is to review the current state of knowledge about GABA role beyond stress-related responses. Our effort is to place findings about GABA in an organized and broader context to highlight its shared metabolic and biologic functions in plants under variable conditions. This will provide potential modes of GABA crosstalk in dynamic plant cell responses.

Salinity and crop yield

Thirty crop species provide 90% of our food, most of which display severe yield losses under moderate salinity. Securing and augmenting agricultural yield in times of global warming and population increase is urgent and should, aside from ameliorating saline soils, include attempts to increase crop plant salt tolerance. This short review provides an overview of the processes that limit growth and yield in saline conditions. Yield is reduced if soil salinity surpasses crop-specific thresholds, with cotton, barley and sugar beet being highly tolerant, while sweet potato, wheat and maize display high sensitivity. Apart from Na⁺ , also Cl^- , Mg²⁺ , SO₄ ^2- or HCO₃ ^- contribute to salt toxicity. The inhibition of biochemical or physiological processes cause imbalance in metabolism and cell signalling and enhance the production of reactive oxygen species interfering with cell redox and energy state. Plant development and root patterning is disturbed, and this response depends on redox and reactive oxygen species signalling, calcium and plant hormones. The interlink of the physiological understanding of tolerance processes from molecular processes as well as the agronomical techniques for stabilizing growth and yield and their interlinks might help improving our crops for future demand and will provide improvement for cultivating crops in saline environment.

Sensing of Abiotic Stress and Ionic Stress Responses in Plants

Plants need to cope with complex environments throughout their life cycle. Abiotic stresses, including drought, cold, salt and heat, can cause a reduction in plant growth and loss of crop yield. Plants sensing stress signals and adapting to adverse environments are fundamental biological problems. We review the stress sensors in stress sensing and the responses, and then discuss ionic stress signaling and the responses. During ionic stress, the calcineurin B-like proteins (CBL) and CBL-interacting protein kinases (CBL-CIPK) complex is identified as a primary element of the calcium sensor for perceiving environmental signals. The CBL-CIPK system shows specificity and variety in its response to different stresses. Obtaining a deeper understanding of stress signaling and the responses will mitigate or solve crop yield crises in extreme environments with fast-growing populations.

Overexpression of a Novel ROP Gene from the Banana (MaROP5g) Confers Increased Salt Stress Tolerance

Rho-like GTPases from plants (ROPs) are plant-specific molecular switches that are crucial for plant survival when subjected to abiotic stress. We identified and characterized 17 novel ROP proteins from Musa acuminata (MaROPs) using genomic techniques. The identified MaROPs fell into three of the four previously described ROP groups (Groups II⁻IV), with MaROPs in each group having similar genetic structures and conserved motifs. Our transcriptomic analysis showed that the two banana genotypes tested, Fen Jiao and BaXi Jiao, had similar responses to abiotic stress: Six genes (MaROP-3b, -5a, -5c, -5f, -5g, and -6) were highly expressed in response to cold, salt, and drought stress conditions in both genotypes. Of these, MaROP5g was most highly expressed in response to salt stress. Co-localization experiments showed that the MaROP5g protein was localized at the plasma membrane. When subjected to salt stress, transgenic Arabidopsis thaliana overexpressing MaROP5g had longer primary roots and increased survival rates compared to wild-type A. thaliana. The increased salt tolerance conferred by MaROP5g might be related to reduced membrane injury and the increased cytosolic K⁺/Na⁺ ratio and Ca²⁺ concentration in the transgenic plants as compared to wild-type. The increased expression of salt overly sensitive (SOS)-pathway genes and calcium-signaling pathway genes in MaROP5g-overexpressing A. thaliana reflected the enhanced tolerance to salt stress by the transgenic lines in comparison to wild-type. Collectively, our results suggested that abiotic stress tolerance in banana plants might be regulated by multiple MaROPs, and that MaROP5g might enhance salt tolerance by increasing root length, improving membrane injury and ion distribution.

Calcium Signaling during Salt Stress and in the Regulation of Ion Homeostasis

Soil composition largely defines the living conditions of plants and represents one of their most relevant, dynamic and complex environmental cues. The effective concentrations of many either tolerated or essential ions and compounds in the soil usually differ from the optimum that would be most suitable for plants. In this regard, salinity - caused by excess of NaCl - represents a widespread adverse growth condition but also shortage of ions like K+, NO3- and Fe2+ restrains plant growth. During the past years many components and mechanisms that function in the sensing and establishment of ion homeostasis have been identified and characterized. Here, we reflect on recent insights that extended our understanding of components and mechanisms, which govern and fine-tune plant salt stress tolerance and ion homeostasis. We put special emphasis on mechanisms that allow for interconnection of the salt overly sensitivity pathway with plant development and discuss newly emerging functions of Ca2+ signaling in salinity tolerance. Moreover, we review and discuss accumulating evidence for a central and unifying role of Ca2+ signaling and Ca2+ dependent protein phosphorylation in regulating sensing, uptake, transport and storage processes of various ions. Finally, based on this cross-field inventory, we deduce emerging concepts and arising questions for future research.

Both NaCl and H2O2 Long-Term Stresses Affect Basal Cytosolic Ca2+ Levels but Only NaCl Alters Cytosolic Ca2+ Signatures in Arabidopsis

Salinity is one of the formidable environmental factors that affect plant growth and development and constrain agricultural productivity. Experimentally imposed short-term NaCl treatment triggers a transient increase in cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) via Ca²⁺ influx across the plasma membrane. Salinity stress, as well as other stresses, induces the production of reactive oxygen species (ROS), such as H₂O₂. It is well established that short-term H₂O₂ treatment also triggers a transient increase in [Ca²⁺]_i. However, whether and how long-term NaCl and H₂O₂ treatments affect the basal levels of [Ca²⁺]_i as well as plant responses to additional NaCl and H₂O₂ stresses remain poorly understood. Using an aequorin-based Ca²⁺ imaging assay, we found that the long-term treatment of Arabidopsis seedlings with both moderate NaCl and H₂O₂ in the growth media reduced the basal [Ca²⁺]_i levels. Interestingly, we found that the long-term treatment with NaCl, but not H₂O_2, affected the responses of plants to additional NaCl stress, and remarkably the roots displayed enhanced responses while the leaves showed reduced responses. These findings suggest that plants adapt to the long-term NaCl stress, while H₂O₂ might be an integrator of many stresses.