The Importance of Cl- Exclusion and Vacuolar Cl- Sequestration: Revisiting the Role of Cl- Transport in Plant Salt Tolerance

Wetland plant species and biochar amendments lead to variable salinity reduction in roadway-associated soils

Salinization is an emerging threat in freshwater wetlands, with few techniques available to mitigate anthropogenic inputs such as road salts. Phytoremediation and biochar addition have each been proposed to remediate salt-affected soils generally, but interactive effects in wetland environments to improve soil conditions adjacent to roadways are not well understood. We conducted an 88-day fully factorial greenhouse experiment to quantify the effects of three plant treatments (unvegetated, Typha × glauca and Phragmites australis) and three biochar rates (0.0, 2.5, 5.0 % wt/wt) on the soil and leachate of a simulated wetland system. Both plant species significantly reduced soil Cl- content relative to unvegetated controls, while Typha also significantly reduced Cl- content of leachate and soil Na+. The difference in effects was likely due to different salt tolerance strategies: the salt-accumulating Typha contained a significantly higher volume of Na+, Cl-, and water in its tissue than Phragmites, whose greater K+:Na+ ratio and similar soil Na+ to controls indicated a salt exclusion strategy. Biochar did not influence the growth of either species but moderately increased tissue Na+ concentration in Typha. Furthermore, biochar's effects on soil and leachate salt levels varied by application rate with the medium rate moderately increasing soil Na+ and Cl- and leachate Cl-, while the highest application did not differ from controls across all metrics. Our results suggest that phytoremediation can be optimized with salt-accumulating species, whose mechanisms of salt tolerance involve the accumulation of salt ions from the surrounding environment. The consistent flooding in our study may have inhibited the influence of biochar. We recommend future studies parse the effects of water levels and redox potential on biochar's ability to influence wetland salinity. Data repository: doi.org/10.17605/OSF.IO/9QFZ7.

Abscisic acid regulates Cl- efflux via the ABI5-ZAT10-SLAH3 module in chloride-stressed Malus hupehensis

The overload of Cl- typically causes cell damage and death in plants, especially in Cl--sensitive crops. Abscisic acid (ABA) is a stress-induced phytohormone that can alleviate chloride stress by reducing Cl- accumulation; however, the mechanism is not clear. Here, we found that the application of ABA elevated Cl- efflux from roots and reduced membrane damage and cell death in chloride-stressed Malus hupehensis. MhSLAH3, a homolog of the slow anion channel from M. hupehensis, encoded a channel controlling Cl- efflux and was induced by both chloride and ABA. MhSLAH3 overexpression accelerated Cl- efflux, which enhanced the tolerance of M. hupehensis to chloride stress, and retarded chloride-induced cell death. However, the suppression of MhSLAH3 partially offset the acceleration effect of ABA on Cl- efflux. MhZAT10L was then identified as a C2H2-type transcription factor upstream of MhSLAH3, repressing MhSLAH3 transcription under chloride stress. The suppression of MhZAT10L accelerated Cl- efflux by releasing suppressed MhSLAH3, but MhZAT10L overexpression counteracted the effects of ABA on Cl- efflux. MhABI5 promoted Cl- efflux mediated by MhSLAH3 due to induction by ABA and transcriptional repression of MhZAT10L, but this function of MhABI5 was reversed by MhZAT10L overexpression. The suppression of MhABI5 diminished the positive effects of ABA on Cl- efflux and retarding cell death. Thus, ABA repressed MhZAT10L transcription by activating MhABI5, further releasing MhSLAH3 to accelerate Cl- efflux. These findings provide a new evidence of ABA regulation of Cl- efflux.

Metabolic adaptations leading to an enhanced lignification in wheat roots under salinity stress

Analysis of salinity tolerance processes in wheat has focused on salt exclusion from shoots while root phenotypes have received limited attention. Here, we consider the varying phenotypic response of four bread wheat varieties that differ in their type and degree of salt tolerance and assess their molecular responses to salinity and changes in root cell wall lignification. These varieties were Westonia introgressed with Nax1 and Nax2 root sodium transporters (HKT1;4-A and HKT1;5-A) that reduce Na+ accumulation in leaves, as well as the 'tissue tolerant' Portuguese landrace Mocho de Espiga Branca that has a mutation in the homologous gene HKT1;5-D and has high Na+ concentration in leaves. These three varieties were compared with the relatively more salt-sensitive cultivar Gladius. Through the use of root histochemical analysis, ion concentrations, as well as differential proteomics and targeted metabolomics, we provide an integrated view of the wheat root response to salinity. We show different metabolic re-arrangements in energy conversion, primary metabolic machinery and phenylpropanoid pathway leading to monolignol production in a genotype and genotype by treatment-dependent manner that alters the extent and localisation of root lignification which correlated with an improved capacity of wheat roots to cope better under salinity stress.

The translocation of a chloride channel from the Golgi to the plasma membrane helps plants adapt to salt stress

A key mechanism employed by plants to adapt to salinity stress involves maintaining ion homeostasis via the actions of ion transporters. While the function of cation transporters in maintaining ion homeostasis in plants has been extensively studied, little is known about the roles of their anion counterparts in this process. Here, we describe a mechanism of salt adaptation in plants. We characterized the chloride channel (CLC) gene AtCLCf, whose expression is regulated by WRKY transcription factor under salt stress in Arabidopsis thaliana. Loss-of-function atclcf seedlings show increased sensitivity to salt, whereas AtCLCf overexpression confers enhanced resistance to salt stress. Salt stress induces the translocation of GFP-AtCLCf fusion protein to the plasma membrane (PM). Blocking AtCLCf translocation using the exocytosis inhibitor brefeldin-A or mutating the small GTPase gene AtRABA1b/BEX5 (RAS GENES FROM RAT BRAINA1b homolog) increases salt sensitivity in plants. Electrophysiology and liposome-based assays confirm the Cl-/H+ antiport function of AtCLCf. Therefore, we have uncovered a mechanism of plant adaptation to salt stress involving the NaCl-induced translocation of AtCLCf to the PM, thus facilitating Cl- removal at the roots, and increasing the plant's salinity tolerance.

GhCLCc-1, a Chloride Channel Gene from Upland Cotton, Positively Regulates Salt Tolerance by Modulating the Accumulation of Chloride Ions

The ionic toxicity induced by salinization has adverse effects on the growth and development of crops. However, researches on ionic toxicity and salt tolerance in plants have focused primarily on cations such as sodium ions (Na+), with very limited studies on chloride ions (Cl-). Here, we cloned the homologous genes of Arabidopsis thaliana AtCLCc, GhCLCc-1A/D, from upland cotton (Gossypium hirsutum), which were significantly induced by NaCl or KCl treatments. Subcellular localization showed that GhCLCc-1A/D were both localized to the tonoplast. Complementation of Arabidopsis atclcc mutant with GhCLCc-1 rescued its salt-sensitive phenotype. In addition, the silencing of the GhCLCc-1 gene led to an increased accumulation of Cl- in the roots, stems, and leaves of cotton seedlings under salt treatments, resulting in compromised salt tolerance. And ectopic expression of the GhCLCc-1 gene in Arabidopsis reduced the accumulation of Cl- in transgenic lines under salt treatments, thereby enhancing salt tolerance. These findings elucidate that GhCLCc-1 positively regulates salt tolerance by modulating Cl- accumulation and could be a potential target gene for improving salt tolerance in plants.

Ascophyllum nodosum SWE enhances root anatomy, but not POD activity in both a salt-tolerant and salt-sensitive soybean ( Glycine max) variety exposed to salt stress

There is growing evidence that seaweed extracts (SWE) may be a solution for mitigating the negative effects of salt stress on crop yield and quality, as they introduce bioactive ingredients able to regulate the expression of growth-inducing and stress-responsive genes. We demonstrate that SWE slightly ameliorated the negative physical growth effects of salt stress, especially in the root anatomy of the salt-sensitive (Clark) variety. The SWE did not stimulate or enhance peroxidase (POD) activity in either the salt-sensitive (Clark) or salt-tolerant variety (Manokin). However, a complete assessment of other antioxidant enzymes (SOD, CAT, APX) involved in the ROS detoxification process is further required.

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.

The Morphological Parameters and Cytosolic pH of Cells of Root Zones in Tobacco Plants (Nicotiana tabacum L.): Nonlinear Effects of NaCl Concentrations

Salinity impacts important processes in plants, reducing their yield. The effect of salinity on the cytosolic pH (pHcyt) has been little studied. In this research, we employed transgenic tobacco plants expressing the pH sensor Pt-GFP to investigate the alterations in pHcyt in cells across various root zones. Furthermore, we examined a wide spectrum of NaCl concentrations (ranging from 0 to 150 mM) and assessed morphological parameters and plant development. Our findings revealed a pattern of cytosolic acidification in cells across all root zones at lower NaCl concentrations (50, 100 mM). Interestingly, at 150 mM NaCl, pHcyt levels either increased or returned to normal, indicating a nonlinear effect of salinity on pHcyt. Most studied parameters related to development and morphology exhibited an inhibitory influence in response to NaCl. Notably, a nonlinear relationship was observed in the cell length within the elongation and differentiation zones. While cell elongation occurred at 50 and 100 mM NaCl, it was not evident at 150 mM NaCl. This suggests a complex interplay between stimulating and inhibitory effects of salinity, contributing to the nonlinear relationship observed between pHcyt, cell length, and NaCl concentration.

Functional Characterisation of the Transcription Factor GsWRKY23 Gene from Glycine soja in Overexpressed Soybean Composite Plants and Arabidopsis under Salt Stress

WRKY proteins are a superfamily of transcription factors (TFs) that play multiple roles in plants' growth, development, and environmental stress response. In this study, a novel WRKY gene called GsWRKY23 that is specifically upregulated in salt-tolerant Glycine soja accession BB52 seedlings was identified by transcriptomic analysis under salt stress. How the physiological functions and mechanisms of the GsWRKY23 gene affect salt tolerance was investigated using transformations of soybean hairy roots and Arabidopsis, including wild-type (WT) and atwrky23-mutant plants. The results showed that GsWRKY23 in the roots, stems, and leaves of BB52, along with its promoter in the cotyledons and root tips of GsWRKY23pro::GUS Arabidopsis seedlings, displayed enhanced induction under salt stress. GsWRKY23 localises to the nucleus and shows transcriptional activation ability in yeast cells. Compared to GsWRKY23-RNAi wild soybean hairy-root composite plants under salt stress, obvious improvements, such as superior growth appearance, plant height and fresh weight (FW), and leaf chlorophyll and relative water content (RWC), were displayed by GsWRKY23-overexpressing (OE) composite plants. Moreover, their relative electrolytic leakage (REL) values and malondialdehyde (MDA) contents in the roots and leaves declined significantly. Most of the contents of Na+ and Cl- in the roots, stems, and leaves of GsWRKY23-OE plants decreased significantly, while the content of K+ in the roots increased, and the content of NO3- displayed no obvious change. Ultimately, the Na+/K+ ratios of roots, stems, and leaves, along with the Cl-/NO3- ratios of roots and stems, decreased significantly. In the transgenic WT-GsWRKY23 and atwrky23-GsWRKY23 Arabidopsis seedlings, the salt-induced reduction in seed germination rate and seedling growth was markedly ameliorated; plant FW, leaf chlorophyll content, and RWC increased, and the REL value and MDA content in shoots decreased significantly. In addition, the accumulation of Na+ and Cl- decreased, and the K+ and NO3- levels increased markedly to maintain lower Na+/K+ and Cl-/NO3- ratios in the roots and shoots. Taken together, these results highlight the role of GsWRKY23 in regulating ionic homeostasis in NaCl-stressed overexpressed soybean composite plants and Arabidopsis seedlings to maintain lower Na+/K+ and Cl-/NO3- ratios in the roots and shoots, thus conferring improved salt tolerance.

Salt stress responses and alleviation strategies in legumes: a review of the current knowledge

Salinity is one of the most significant environmental factors limiting legumes development and productivity. Salt stress disturbs all developmental stages of legumes and affects their hormonal regulation, photosynthesis and biological nitrogen fixation, causing nutritional imbalance, plant growth inhibition and yield losses. At the molecular level, salt stress exposure involves large number of factors that are implicated in stress perception, transduction, and regulation of salt responsive genes' expression through the intervention of transcription factors. Along with the complex gene network, epigenetic regulation mediated by non-coding RNAs, and DNA methylation events are also involved in legumes' response to salinity. Different alleviation strategies can increase salt tolerance in legume plants. The most promising ones are Plant Growth Promoting Rhizobia, Arbuscular Mycorrhizal Fungi, seed and plant's priming. Genetic manipulation offers an effective approach for improving salt tolerance. In this review, we present a detailed overview of the adverse effect of salt stress on legumes and their molecular responses. We also provide an overview of various ameliorative strategies that have been implemented to mitigate/overcome the harmful effects of salt stress on legumes.

Research Progress on the Mechanism of Salt Tolerance in Maize: A Classic Field That Needs New Efforts

Maize is the most important cereal crop globally. However, in recent years, maize production faced numerous challenges from environmental factors due to the changing climate. Salt stress is among the major environmental factors that negatively impact crop productivity worldwide. To cope with salt stress, plants developed various strategies, such as producing osmolytes, increasing antioxidant enzyme activity, maintaining reactive oxygen species homeostasis, and regulating ion transport. This review provides an overview of the intricate relationships between salt stress and several plant defense mechanisms, including osmolytes, antioxidant enzymes, reactive oxygen species, plant hormones, and ions (Na⁺, K⁺, Cl^-), which are critical for salt tolerance in maize. It addresses the regulatory strategies and key factors involved in salt tolerance, aiming to foster a comprehensive understanding of the salt tolerance regulatory networks in maize. These new insights will also pave the way for further investigations into the significance of these regulations in elucidating how maize coordinates its defense system to resist salt stress.

MhCLC-c1, a Cl channel c homolog from Malus hupehensis, alleviates NaCl-induced cell death by inhibiting intracellular Cl- accumulation

BACKGROUND

Overaccumulation of chloride (Cl) when plants suffer NaCl causes cell damage and death, and is regulated by Cl^- channel protein (CLC). Apple roots are very sensitive to Cl^-, but information associated with CLC is limited in apple crop that widely cultivated in the world.

RESULTS

We identified 9 CLCs from the apple genome and divided them into two subclasses. Among them, MdCLC-c1 promoter contained the largest number of cis-acting elements associated with NaCl stress, and only the MdCLC-c1, MdCLC-d, and MdCLC-g were predicted that may be Cl^- antiporters or channels. Expression analysis of MdCLCs homologs in the roots of Malus hupehensis showed that most of the MhCLCs expression were response to NaCl stress, especially MhCLC-c1 expression was upregulated continuously and rapidly expressed during NaCl treatment. Therefore, we isolated MhCLC-c1 and observed it was a plasma membrane-localized protein. The MhCLC-c1 suppression significantly increased sensitivity, reactive oxygen species content, and cell death of apple calli; while MhCLC-c1 overexpression decreased sensitivity, reactive oxygen species content, and cell death of apple calli and Arabidopsis by inhibiting intracellular Cl^- accumulation under NaCl stress.

CONCLUSIONS

The study selected and isolated a CLC-c gene MhCLC-c1 from Malus hupehensis based on identification of CLCs gene family in apple, and their homologs MhCLCs expression patterns during NaCl treatments, revealing that MhCLC-c1 alleviates NaCl-induced cell death by inhibiting intracellular Cl^- accumulation. Our findings confer the comprehensive and in-depth upstanding of the mechanism that plants resist salt stress, and might also confer genetic improvement of salt tolerance in horticultural crops and the development and utilization of saline-alkali land.

Twenty years of mining salt tolerance genes in soybean

Current combined challenges of rising food demand, climate change and farmland degradation exert enormous pressure on agricultural production. Worldwide soil salinization, in particular, necessitates the development of salt-tolerant crops. Soybean, being a globally important produce, has its genetic resources increasingly examined to facilitate crop improvement based on functional genomics. In response to the multifaceted physiological challenge that salt stress imposes, soybean has evolved an array of defences against salinity. These include maintaining cell homeostasis by ion transportation, osmoregulation, and restoring oxidative balance. Other adaptations include cell wall alterations, transcriptomic reprogramming, and efficient signal transduction for detecting and responding to salt stress. Here, we reviewed functionally verified genes that underly different salt tolerance mechanisms employed by soybean in the past two decades, and discussed the strategy in selecting salt tolerance genes for crop improvement. Future studies could adopt an integrated multi-omic approach in characterizing soybean salt tolerance adaptations and put our existing knowledge into practice via omic-assisted breeding and gene editing. This review serves as a guide and inspiration for crop developers in enhancing soybean tolerance against abiotic stresses, thereby fulfilling the role of science in solving real-life problems.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-023-01383-3.

Interplay between membrane proteins and membrane protein-lipid pertaining to plant salinity stress

High salinity in agricultural lands is one of the predominant issues limiting agricultural yields. Plants have developed several mechanisms to withstand salinity stress, but the mechanisms are not effective enough for most crops to prevent and persist the salinity stress. Plant salt tolerance pathways involve membrane proteins that have a crucial role in sensing and mitigating salinity stress. Due to a strategic location interfacing two distinct cellular environments, membrane proteins can be considered checkpoints to the salt tolerance pathways in plants. Related membrane proteins functions include ion homeostasis, osmosensing or ion sensing, signal transduction, redox homeostasis, and small molecule transport. Therefore, modulating plant membrane proteins' function, expression, and distribution can improve plant salt tolerance. This review discusses the membrane protein-protein and protein-lipid interactions related to plant salinity stress. It will also highlight the finding of membrane protein-lipid interactions from the context of recent structural evidence. Finally, the importance of membrane protein-protein and protein-lipid interaction is discussed, and a future perspective on studying the membrane protein-protein and protein-lipid interactions to develop strategies for improving salinity tolerance is proposed.

Morphological, physiological, and molecular scion traits are determinant for salt-stress tolerance of grafted citrus plants

Introduction

Citrus productivity has been decreasing in the last decade in the Mediterranean basin as a consequence of climate change and the high levels of salinity found in the aquifers. Citrus varieties are cultivated grafted onto a rootstock, which has been reported as responsible for plant tolerance to adverse situations. However, other important factors for stress tolerance relying in the scion have been less studied. The aim of this study was to evaluate the effect of the grafted scion on citrus tolerance to salt stress.

Methods

Four different citrus rootstock/scion combinations were subjected to salt stress for 30 days, using Carrizo citrange (CC) or Citrus macrophylla (CM) as rootstocks, and Navelina orange (NA) or Oronules mandarin (OR) as scions. CM-OR was the most tolerant combination, whereas CC-NA was the most sensitive one.

Results and discussion

Our results support the idea that the rootstock plays an important role in salt stress tolerance, but scion is also crucial. Thus, photosynthesis and transpiration, processes regulated by abscisic acid and jasmonic acid, are determinant of plant performance. These photosynthetic parameters were not affected in plants of the salt-tolerant combination CM-OR, probably due to the lower intoxication with Cl- ions, allowing a better performance of the photosynthetic machinery under stress conditions. The different stomatal density of the two citrus scions used in this work (higher in the sensitive NA in comparison to the tolerant OR) also contributes to the different tolerance of the grafted plants to this adverse condition. Additionally, CsDTX35.1 and CsDTX35.2, genes codifying for Cl- tonoplast transporters, were exclusively overexpressed in plants of the salt-tolerant combination CM-OR, suggesting that these transporters involved in Cl- compartmentalization could be crucial for salt stress tolerance. It is concluded that to improve citrus tolerance to high salinity, it is important that scions have a versatile photosynthetic system, an adequate stomatal density, and a proper modulation of genes coding for Cl- transporters in the tonoplast.

Salinity and temperature influence removal levels of heavy metals and chloride from water by wetland plants

Stormwater with low temperatures and elevated salinity, common in areas where deicing salt is used, might affect the removal of heavy metals by plants in stormwater treatment systems such as floating treatment wetlands. This short-term study evaluated the effects of combinations of temperature (5, 15, and 25 °C) and salinity (0, 100, and 1000 mg NaCl L-1) on the removal of Cd, Cu, Pb, and Zn (1.2, 68.5, 78.4, and 559 μg L-1) and Cl- (0, 60, and 600 mg Cl- L-1) by Carex pseudocyperus, C. riparia, and Phalaris arundinacea. These species had previously been identified as suitable candidates for floating treatment wetland applications. The study found high removal capacity in all treatment combinations, especially for Pb and Cu. However, low temperatures decreased the removal of all heavy metals, and increased salinity decreased the removal of Cd and Pb but had no effect on the removal of Zn or Cu. No interactions were found between the effects of salinity and of temperature. Carex pseudocyperus best removed Cu and Pb, whereas P. arundinacea best removed Cd, Zu, and Cl-. The removal efficacy for metals was generally high, with elevated salinity and low temperatures having small impacts. The findings indicate that efficient heavy metal removal can also be expected in cold saline waters if the right plant species are used.

Tomato responses to salinity stress: From morphological traits to genetic changes

Tomato is an essential annual crop providing human food worldwide. It is estimated that by the year 2050 more than 50% of the arable land will become saline and, in this respect, in recent years, researchers have focused their attention on studying how tomato plants behave under various saline conditions. Plenty of research papers are available regarding the effects of salinity on tomato plant growth and development, that provide information on the behavior of different cultivars under various salt concentrations, or experimental protocols analyzing various parameters. This review gives a synthetic insight of the recent scientific advances relevant into the effects of salinity on the morphological, physiological, biochemical, yield, fruit quality parameters, and on gene expression of tomato plants. Notably, the works that assessed the salinity effects on tomatoes were firstly identified in Scopus, PubMed, and Web of Science databases, followed by their sifter according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline and with an emphasis on their results. The assessment of the selected studies pointed out that salinity is one of the factors significantly affecting tomato growth in all stages of plant development. Therefore, more research to find solutions to increase the tolerance of tomato plants to salinity stress is needed. Furthermore, the findings reported in this review are helpful to select, and apply appropriate cropping practices to sustain tomato market demand in a scenario of increasing salinity in arable lands due to soil water deficit, use of low-quality water in farming and intensive agronomic practices.

Membrane Proteins in Plant Salinity Stress Perception, Sensing, and Response

Plants have several mechanisms to endure salinity stress. The degree of salt tolerance varies significantly among different terrestrial crops. Proteins at the plant's cell wall and membrane mediate different physiological roles owing to their critical positioning between two distinct environments. A specific membrane protein is responsible for a single type of activity, such as a specific group of ion transport or a similar group of small molecule binding to exert multiple cellular effects. During salinity stress in plants, membrane protein functions: ion homeostasis, signal transduction, redox homeostasis, and solute transport are essential for stress perception, signaling, and recovery. Therefore, comprehensive knowledge about plant membrane proteins is essential to modulate crop salinity tolerance. This review gives a detailed overview of the membrane proteins involved in plant salinity stress highlighting the recent findings. Also, it discusses the role of solute transporters, accessory polypeptides, and proteins in salinity tolerance. Finally, some aspects of membrane proteins are discussed with potential applications to developing salt tolerance in crops.

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.

Interplay of an array of salt-responding mechanisms in Iranian borage: Evidence from physiological, biochemical, and histochemical examinations

In order to address the lacuna of data on the response of physiological and biochemical attributes and chemical compounds of glandular trichomes of Iranian borage (Echium amoenum Fisch. & C.A.Mey.) to saline water (0, 25, 50, 75, and 100 mM NaCl) an experiment was conducted on 13 genotypes. Genotypic differences and salt-induced modifications in chlorophyll concentration and fluorescence, plant growth, relative water content, proline concentration, antioxidant defense, and chemical compounds of glandular trichomes upon exposure to salt stress were observed. Chlorophyll and carotenoids concentrations and catalase (EC 1.11.1.6) and ascorbate peroxidase (EC 1.11.1.11) activities were either enhanced or remained unchanged in the presence of moderate salt concentrations (i.e. 25 and 50 mM NaCl) in a majority of the genotypes. Though, 75 and 100 mM NaCl were modestly and severely detrimental, respectively, to the majority of the genotypes. The 75 and 100 mM NaCl led to substantial increases and decreases in the Na+ and K+, respectively, resulting in notable increase in the Na+/K+. Increases in proline, total phenolic compounds, and alkaloids concentrations, essential oils, alkaloids, and phenolic compounds of the glandular trichomes were concomitant to decreases in the relative water content, leaf area, maximum quantum efficiency of photosystem II, shoot and root dry masses. This study revealed, for the first time, that Iranian borage tolerates 25 and 50 mM NaCl and antioxidative enzymes as well as secondary metabolites such as alkaloids and phenolic compounds accumulated mainly in the trichomes play key role in this regard.

Cotton proteomics: Dissecting the stress response mechanisms in cotton

The natural environment of plants comprises a complex set of biotic and abiotic stresses, and plant responses to these stresses are complex as well. Plant proteomics approaches have significantly revealed dynamic changes in plant proteome responses to stress and developmental processes. Thus, we reviewed the recent advances in cotton proteomics research under changing environmental conditions, considering the progress and challenging factors. Finally, we highlight how single-cell proteomics is revolutionizing plant research at the proteomics level. We envision that future cotton proteomics research at the single-cell level will provide a more complete understanding of cotton's response to stresses.

Nano-enabled agriculture: How do nanoparticles cross barriers in plants?

Nano-enabled agriculture is a topic of intense research interest. However, our knowledge of how nanoparticles enter plants, plant cells, and organelles is still insufficient. Here, we discuss the barriers that limit the efficient delivery of nanoparticles at the whole-plant and single-cell levels. Some commonly overlooked factors, such as light conditions and surface tension of applied nano-formulations, are discussed. Knowledge gaps regarding plant cell uptake of nanoparticles, such as the effect of electrochemical gradients across organelle membranes on nanoparticle delivery, are analyzed and discussed. The importance of controlling factors such as size, charge, stability, and dispersibility when properly designing nanomaterials for plants is outlined. We mainly focus on understanding how nanoparticles travel across barriers in plants and plant cells and the major factors that limit the efficient delivery of nanoparticles, promoting a better understanding of nanoparticle-plant interactions. We also provide suggestions on the design of nanomaterials for nano-enabled agriculture.

Combined transcriptomic and metabolomic analysis reveals a role for adenosine triphosphate-binding cassette transporters and cell wall remodeling in response to salt stress in strawberry

Strawberry (Fragaria × ananassa Duch) are sensitive to salt stress, and breeding salt-tolerant strawberry cultivars is the primary method to develop resistance to increased soil salinization. However, the underlying molecular mechanisms mediating the response of strawberry to salinity stress remain largely unknown. This study evaluated the salinity tolerance of 24 strawberry varieties, and transcriptomic and metabolomic analysis were performed of 'Sweet Charlie' (salt-tolerant) and 'Benihoppe' (salt-sensitive) to explore salt tolerance mechanisms in strawberry. Compared with the control, we identified 3412 differentially expressed genes (DEGs) and 209 differentially accumulated metabolites (DAMs) in 'Benihoppe,' and 5102 DEGs and 230 DAMs in 'Sweet Charlie.' DEGs Gene Ontology (GO) enrichment analyses indicated that the DEGs in 'Benihoppe' were enriched for ion homeostasis related terms, while in 'Sweet Charlie,' terms related to cell wall remodeling were over-represented. DEGs related to ion homeostasis and cell wall remodeling exhibited differential expression patterns in 'Benihoppe' and 'Sweet Charlie.' In 'Benihoppe,' 21 ion homeostasis-related DEGs and 32 cell wall remodeling-related DEGs were upregulated, while 23 ion homeostasis-related DEGs and 138 cell wall remodeling-related DEGs were downregulated. In 'Sweet Charlie,' 72 ion homeostasis-related DEGs and 275 cell wall remodeling-related DEGs were upregulated, while 11 ion homeostasis-related DEGs and 20 cell wall remodeling-related DEGs were downregulated. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses showed only four KEGG enriched pathways were shared between 'Benihoppe' and 'Sweet Charlie,' including flavonoid biosynthesis, phenylalanine metabolism, phenylpropanoid biosynthesis and ubiquinone, and other terpenoid-quinone biosynthesis. Integrating the results of transcriptomic and metabolomics analyses showed that adenosine triphosphate-binding cassette (ABC) transporters and flavonoid pathway genes might play important roles in the salt stress response in strawberry, and DAMs and DEGs related to ABC transporter and flavonoid pathways were differentially expressed or accumulated. The results of this study reveal that cell wall remodeling and ABC transporters contribute to the response to salt stress in strawberry, and that related genes showed differential expression patterns in varieties with different salt tolerances. These findings provide new insights into the underlying molecular mechanism of strawberry response to salt stress and suggest potential targets for the breeding of salt-tolerant strawberry varieties.

Comparing Essentiality of SOS1-Mediated Na+ Exclusion in Salinity Tolerance between Cultivated and Wild Rice Species

Soil salinity is a major constraint that affects plant growth and development. Rice is a staple food for more than half of the human population but is extremely sensitive to salinity. Among the several known mechanisms, the ability of the plant to exclude cytosolic Na+ is strongly correlated with salinity stress tolerance in different plant species. This exclusion is mediated by the plasma membrane (PM) Na+/H+ antiporter encoded by Salt Overly Sensitive (SOS1) gene and driven by a PM H+-ATPase generated proton gradient. However, it is not clear to what extent this mechanism is operational in wild and cultivated rice species, given the unique rice root anatomy and the existence of the bypass flow for Na+. As wild rice species provide a rich source of genetic diversity for possible introgression of abiotic stress tolerance, we investigated physiological and molecular basis of salinity stress tolerance in Oryza species by using two contrasting pairs of cultivated (Oryza sativa) and wild rice species (Oryza alta and Oryza punctata). Accordingly, dose- and age-dependent Na+ and H+ fluxes were measured using a non-invasive ion selective vibrating microelectrode (the MIFE technique) to measure potential activity of SOS1-encoded Na+/H+ antiporter genes. Consistent with GUS staining data reported in the literature, rice accessions had (~4-6-fold) greater net Na+ efflux in the root elongation zone (EZ) compared to the mature root zone (MZ). Pharmacological experiments showed that Na+ efflux in root EZ is suppressed by more than 90% by amiloride, indicating the possible involvement of Na+/H+ exchanger activity in root EZ. Within each group (cultivated vs. wild) the magnitude of amiloride-sensitive Na+ efflux was higher in tolerant genotypes; however, the activity of Na+/H+ exchanger was 2-3-fold higher in the cultivated rice compared with their wild counterparts. Gene expression levels of SOS1, SOS2 and SOS3 were upregulated under 24 h salinity treatment in all the tested genotypes, with the highest level of SOS1 transcript detected in salt-tolerant wild rice genotype O. alta (~5-6-fold increased transcript level) followed by another wild rice, O. punctata. There was no significant difference in SOS1 expression observed for cultivated rice (IR1-tolerant and IR29-sensitive) under both 0 and 24 h salinity exposure. Our findings suggest that salt-tolerant cultivated rice relies on the cytosolic Na+ exclusion mechanism to deal with salt stress to a greater extent than wild rice, but its operation seems to be regulated at a post-translational rather than transcriptional level.

Photosynthesis is not the unique useful trait for discriminating salt tolerance capacity between sensitive and tolerant quinoa varieties

Growth was not strictly linked to photosynthesis performance under salinity conditions in quinoa. Other key traits, which were varieties-specific, rather than photosynthesis explained better growth performance. Phenotyping for salinity stress tolerance in quinoa is of great interest to select traits contributing to overall salinity tolerance and to understand the response mechanisms to salinity at a whole plant level. The objective of this work was to dissect the responses of specific traits and analyse relations between these traits to better understand growth response under salinity conditions in quinoa. Growth response to salinity was mostly related to differences in basal values of biomass, being reduced the most in plants with higher basal biomass. Regarding the relationship between growth and specific traits, in Puno variety, better photosynthetic performance was related to a better maintenance of growth. Nevertheless, in the rest of the varieties other traits rather than photosynthesis could better explain growth response. In this way, the development of succulence in F-16 and Collana varieties, also the osmotic adjustment but in smaller dimensions in Pasankalla, Marisma and S-15-15 helped to maintain better growth. Besides, smaller increases of Cl^- could have caused a limited nitrate uptake reducing more growth in Vikinga. Ascorbate was considered a key trait as a noticeable fall of it was also related to higher reductions in growth in Titicaca. These results suggest that, due to the genetic variability of quinoa and the complexity of salinity tolerance, no unique and specific traits should be taken into consideration when using phenotyping for analysing salinity tolerance in quinoa.

Silicate Inhibits the Cytosolic Influx of Chloride in Protoplasts of Wheat and Affects the Chloride Transporters, TaCLC1 and TaNPF2.4/2.5

Chloride is an essential nutrient for plants, but high concentrations can be harmful. Silicon ameliorates both abiotic and biotic stresses in plants, but it is unknown if it can prevent cellular increase of chloride. Therefore, we investigated the influx of Cl⁻ ions in two wheat cultivars different in salt sensitivity, by epifluorescence microscopy and a highly Cl⁻-sensitive dye, MQAE, N-[ethoxycarbonylmethyl]-6-methoxy-quinolinium bromide, in absence and presence of potassium silicate, K₂SiO₃. The Cl⁻-influx was higher in the salt-sensitive cv. Vinjett, than in the salt-tolerant cv. S-24, and silicate pre-treatment of protoplasts inhibited the Cl⁻-influx in both cultivars, but more in the sensitive cv. Vinjett. To investigate if the Cl⁻-transporters TaCLC1 and TaNPF2.4/2.5 are affected by silicate, expression analyses by RT-qPCR were undertaken of TaCLC1 and TaNPF 2.4/2.5 transcripts in the absence and presence of 100 mM NaCl, with and without the presence of K₂SiO₃. The results show that both transporter genes were expressed in roots and shoots of wheat seedlings, but their expressions were differently affected by silicate. The TaNPF2.4/2.5 expression in leaves was markedly depressed by silicate. These findings demonstrate that less chloride accumulates in the cytosol of leaf mesophyll by Si treatment and increases salt tolerance.

Changes in Morpho-Anatomical and Eco-Physiological Responses of Viburnum tinus L. var lucidum as Modulated by Sodium Chloride and Calcium Chloride Salinization

Salinity in water and soil is among the major constraints to the cultivation of ornamental crops since it can affect their growth and aesthetic value. A greenhouse experiment was carried out to assess whether the application of two different salts (80 mM NaCl or 53.3 mM CaCl2, with a final ionic concentration of 160 mM) could differently modulate the anatomical and physiological acclimation of an important ornamental species such as Viburnum tinus L. var. lucidum. Eco-physiological analyses (e.g., leaf gas exchange and chlorophyll a fluorescence emission) were performed and leaves were subjected to light microscopy analysis to quantify functional anatomical traits through digital image analysis. Results showed that the two iso-osmotic solutions induced different structure-mediated physiological alterations in V. tinus plants. Photosynthesis was lowered by CaCl2 treatments (−58%) more than by NaCl (−37%), also due to the occurrence of photodamage apart from stomatal limitations. Neither Na+ nor Cl− exhibited toxic effects in leaf lamina structure which was reflected in the limited reduction in dry matter accumulation. Overall data were interpreted focusing on the coordination among leaf structural and functional traits suggesting that the fine control of functional anatomical traits contributes to physiological acclimation to both stressful conditions.

Adaptive Mechanisms of Halophytes and Their Potential in Improving Salinity Tolerance in Plants

Soil salinization, which is aggravated by climate change and inappropriate anthropogenic activities, has emerged as a serious environmental problem, threatening sustainable agriculture and future food security. Although there has been considerable progress in developing crop varieties by introducing salt tolerance-associated traits, most crop cultivars grown in saline soils still exhibit a decline in yield, necessitating the search for alternatives. Halophytes, with their intrinsic salt tolerance characteristics, are known to have great potential in rehabilitating salt-contaminated soils to support plant growth in saline soils by employing various strategies, including phytoremediation. In addition, the recent identification and characterization of salt tolerance-related genes encoding signaling components from halophytes, which are naturally grown under high salinity, have paved the way for the development of transgenic crops with improved salt tolerance. In this review, we aim to provide a comprehensive update on salinity-induced negative effects on soils and plants, including alterations of physicochemical properties in soils, and changes in physiological and biochemical processes and ion disparities in plants. We also review the physiological and biochemical adaptation strategies that help halophytes grow and survive in salinity-affected areas. Furthermore, we illustrate the halophyte-mediated phytoremediation process in salinity-affected areas, as well as their potential impacts on soil properties. Importantly, based on the recent findings on salt tolerance mechanisms in halophytes, we also comprehensively discuss the potential of improving salt tolerance in crop plants by introducing candidate genes related to antiporters, ion transporters, antioxidants, and defense proteins from halophytes for conserving sustainable agriculture in salinity-prone areas.

Identifying the genetic control of salinity tolerance in the bread wheat landrace Mocho de Espiga Branca

Salinity tolerance in bread wheat is frequently reported to be associated with low leaf sodium (Na+) concentrations. However, the Portuguese landrace, Mocho de Espiga Branca, accumulates significantly higher leaf Na+ but has comparable salinity tolerance to commercial bread wheat cultivars. To determine the genetic loci associated with the salinity tolerance of this landrace, an F2 mapping population was developed by crossing Mocho de Espiga Branca with the Australian cultivar Gladius. The population was phenotyped for 19 salinity tolerance subtraits using both non-destructive and destructive techniques. Genotyping was performed using genotyping-by-sequencing (GBS). Genomic regions associated with salinity tolerance were detected on chromosomes 1A, 1D, 4B and 5A for the subtraits of relative and absolute growth rate (RGR, AGR respectively), and on chromosome 2A, 2B, 4D and 5D for Na+, potassium (K+) and chloride (Cl-) accumulation. Candidate genes that encode proteins associated with salinity tolerance were identified within the loci including Na+/H+ antiporters, K+ channels, H+-ATPase, calcineurin B-like proteins (CBLs), CBL-interacting protein kinases (CIPKs), calcium dependent protein kinases (CDPKs) and calcium-transporting ATPase. This study provides a new insight into the genetic control of salinity tolerance in a Na+ accumulating bread wheat to assist with the future development of salt tolerant cultivars.

The genome of the extremophile Artemia provides insight into strategies to cope with extreme environments

BACKGROUND

Brine shrimp Artemia have an unequalled ability to endure extreme salinity and complete anoxia. This study aims to elucidate its strategies to cope with these stressors.

RESULTS AND DISCUSSION

Here, we present the genome of an inbred A. franciscana Kellogg, 1906. We identified 21,828 genes of which, under high salinity, 674 genes and under anoxia, 900 genes were differentially expressed (42%, respectively 30% were annotated). Under high salinity, relevant stress genes and pathways included several Heat Shock Protein and Leaf Embryogenesis Abundant genes, as well as the trehalose metabolism. In addition, based on differential gene expression analysis, it can be hypothesized that a high oxidative stress response and endocytosis/exocytosis are potential salt management strategies, in addition to the expression of major facilitator superfamily genes responsible for transmembrane ion transport. Under anoxia, genes involved in mitochondrial function, mTOR signalling and autophagy were differentially expressed. Both high salt and anoxia enhanced degradation of erroneous proteins and protein chaperoning. Compared with other branchiopod genomes, Artemia had 0.03% contracted and 6% expanded orthogroups, in which 14% of the genes were differentially expressed under high salinity or anoxia. One phospholipase D gene family, shown to be important in plant stress response, was uniquely present in both extremophiles Artemia and the tardigrade Hypsibius dujardini, yet not differentially expressed under the described experimental conditions.

CONCLUSIONS

A relatively complete genome of Artemia was assembled, annotated and analysed, facilitating research on its extremophile features, and providing a reference sequence for crustacean research.

GsCLC-c2 from wild soybean confers chloride/salt tolerance to transgenic Arabidopsis and soybean composite plants by regulating anion homeostasis

The responses of the GsCLC-c2 gene and its promoter to NaCl stress, as well as the Cl- /salt tolerance of GsCLC-c2-transgenic Arabidopsis and overexpressed or RNAi wild soybean hairy root composite plants, were investigated. Results showed that both GsCLC-c2 and its promoter display enhanced induction under salt stress. In the transgenic Arabidopsis WT-GsCLC-c2 and atclc-c-GsCLC-c2 seedlings, the salt-induced growth reduction was markedly ameliorated; plant fresh weight, leaf area, and relative water content (RWC) increased; relative electrolytic leakage (REL), and malondialdehyde (MDA) content in shoots decreased significantly. In addition, accumulation of Cl- and K+ , especially Cl- , increased markedly in roots to minimize Cl- transport to shoots and maintain higher and lower Cl- /NO3 - ratios in roots and shoots, respectively. When compared to GsCLC-c2-RNAi wild soybean composite plants under salt stress, clear advantages, such as growth appearance, plant height, and leaf area, were displayed by GsCLC-c2-overexpressing composite plants. Moreover, their REL values in roots and leaves declined significantly. The accumulation of absorbed Cl- and Na+ in the roots increased, as the transportation to the stems and leaves decreased, the NO3 - content in roots, stems, and leaves significantly increased, and the changes in K+ contents were small, which resulted in the maintenance of a low Cl- /NO3 - ratio in all plant parts and low Na+ /K+ ratio in stems and leaves. Taken together, these results highlight the role of GsCLC-c2 in regulating anionic homeostasis in NaCl-stressed transgenic Arabidopsis and soybean composite plants to maintain lower Cl- /NO3 - ratios in shoots, thus conferring enhanced Cl- /salt tolerance.

Reduction of Nitrate Content in Baby-Leaf Lettuce and Cichorium endivia Through the Soilless Cultivation System, Electrical Conductivity and Management of Nutrient Solution

Soilless cultivation systems are efficient tools to control nitrates by managing nutrient solution (NS) salinity and nitrogen availability, however, these nitrate-lowering strategies require appropriate calibration based on species/genotype-specific responses interacting with climate and growing conditions. Three experiments were carried out on lettuce and Cichorium endivia grown in ebb-and-flow (EF) and floating (FL) systems at two levels of NS salinity (EC = 2.5 and 3.5 dS m^-1) (EC2.5, EC3.5, respectively) under autumn and early-spring (lettuce) and winter and late-spring conditions (C. endivia). Nitrogen deprivation (NS withdrawal a few days before the harvest) was tested at EC2.5, in the autumn and winter cycles. The EF-system caused an increase in salinity in the substrate where roots mainly develop so it mimicked the effect of the EC3.5 treatment. In the winter-grown lettuce, the EF-system or EC3.5 treatment was effective in reducing the nitrate level without effects on yield, with the EF baby-leaf showing an improved quality (color, dry matter, chlorophylls, carotenoid, vitamin C, phenol). In both seasons, the EF/EC3.5 treatment resulted in a decline in productivity, despite a further reduction in nitrate content and a rise in product quality occurring. This response was strictly linked to the increasing salt-stress loaded by the EC3.5/EF as highlighted by the concurrent Cl^- accumulation. In early-spring, the FL/EC3.5 combination may represent a trade-off between yield, nitrate content and product quality. In contrast, in winter-grown endive/escarole the EC3.5, EF and EC3.5/EF reduced the nitrate level with no effect on yield, product quality or Cl^- uptake, thus proving them to be more salt-tolerant than lettuce. High temperatures during the late-spring cycle promoted nitrate and Cl^- uptake, overcoming the nitrate-controlling effect of salinity charged by the EF system or EC3.5. The nitrate level decreased after 3 day-long (lettuce) or 6 day-long (C. endivia) NS withdrawal. In C. endivia and EF-grown lettuce, it provoked a decrease in yield, but a concurrent improvement in baby-leaf appearance and nutritional quality. More insights are needed to fine-tune the duration of the NS removal taking into account the soilless system used and species-specific characteristics.

Dose-Effect Relationship of Water Salinity Levels on Osmotic Regulators, Nutrient Uptake, and Growth of Transplanting Vetiver [Vetiveria zizanioides (L.) Nash]

Vetiver grass [Vetiveria zizanioides (L.) Nash] without seeds, suitable for growing on coastal saline land, has attracted attention because of oil extraction from its roots and industrial and agricultural use. In this study, a pot experiment with different NaCl contents was used to investigate the influence of water salinity levels on vetiver, salt tolerance, and the feasibility of transferring it to coastal saline regions. The results indicated that the fresh weight of roots and shoots increased initially and then gradually decreased with an increase in NaCl content, and the maximum was attributed to a water salinity of 0.3%. The vetiver can tolerate a maximum saline content of up to 2%. The promotion of vetiver growth under water salinity could be attributed to the acceleration of nutrient uptake-induced saline, including K, N, and Cl. The growth of vetiver was insignificantly inhibited with 0.5% water salinity (mild stress), significantly inhibited with 1.0% water salinity (moderate stress: biomass decrease), and severe inhibited with >1.5% water salinity (intense stress: biomass decrease). The salt tolerance of vetiver was due to osmotic regulation by reducing sugars under mild stress and of proline under intense stress, and Na+ sequestration in roots and the transformation of Cl- away from sensitive roots. The vetiver could be cultivated in slightly coastal saline soil (0.1-0.2% soil salinity) and even moderately saline coastal soil (0.2-0.4% soil salinity) under irrigation with low salt water during transplanting.

Reduction of Nitrate Content in Baby-Leaf Lettuce and Cichorium endivia Through the Soilless Cultivation System, Electrical Conductivity and Management of Nutrient Solution

Soilless cultivation systems are efficient tools to control nitrates by managing nutrient solution (NS) salinity and nitrogen availability, however, these nitrate-lowering strategies require appropriate calibration based on species/genotype-specific responses interacting with climate and growing conditions. Three experiments were carried out on lettuce and Cichorium endivia grown in ebb-and-flow (EF) and floating (FL) systems at two levels of NS salinity (EC = 2.5 and 3.5 dS m^-1) (EC2.5, EC3.5, respectively) under autumn and early-spring (lettuce) and winter and late-spring conditions (C. endivia). Nitrogen deprivation (NS withdrawal a few days before the harvest) was tested at EC2.5, in the autumn and winter cycles. The EF-system caused an increase in salinity in the substrate where roots mainly develop so it mimicked the effect of the EC3.5 treatment. In the winter-grown lettuce, the EF-system or EC3.5 treatment was effective in reducing the nitrate level without effects on yield, with the EF baby-leaf showing an improved quality (color, dry matter, chlorophylls, carotenoid, vitamin C, phenol). In both seasons, the EF/EC3.5 treatment resulted in a decline in productivity, despite a further reduction in nitrate content and a rise in product quality occurring. This response was strictly linked to the increasing salt-stress loaded by the EC3.5/EF as highlighted by the concurrent Cl^- accumulation. In early-spring, the FL/EC3.5 combination may represent a trade-off between yield, nitrate content and product quality. In contrast, in winter-grown endive/escarole the EC3.5, EF and EC3.5/EF reduced the nitrate level with no effect on yield, product quality or Cl^- uptake, thus proving them to be more salt-tolerant than lettuce. High temperatures during the late-spring cycle promoted nitrate and Cl^- uptake, overcoming the nitrate-controlling effect of salinity charged by the EF system or EC3.5. The nitrate level decreased after 3 day-long (lettuce) or 6 day-long (C. endivia) NS withdrawal. In C. endivia and EF-grown lettuce, it provoked a decrease in yield, but a concurrent improvement in baby-leaf appearance and nutritional quality. More insights are needed to fine-tune the duration of the NS removal taking into account the soilless system used and species-specific characteristics.

The R2R3-MYB transcription factor AtMYB49 modulates salt tolerance in Arabidopsis by modulating the cuticle formation and antioxidant defence

Salt stress activates defence responses in plants, including changes in leaf surface structure. Here, we showed that the transcriptional activation of cutin deposition and antioxidant defence by the R2R3-type MYB transcription factor AtMYB49 contributed to salt tolerance in Arabidopsis thaliana. Characterization of loss-of-function myb49 mutants, and chimeric AtMYB49-SRDX-overexpressing SRDX49 transcriptional repressor and AtMYB49-overexpressing (OX49) overexpressor plants demonstrated a positive role of AtMYB49 in salt tolerance. Transcriptome analysis revealed that many genes belonging to the category "cutin, suberin and wax biosyntheses" were markedly up-regulated and down-regulated in OX49 and SRDX49 plants, respectively, under normal and/or salt stress conditions. Some of these differentially expressed genes, including MYB41, ASFT, FACT and CYP86B1, were also shown to be the direct targets of AtMYB49 and activated by AtMYB49. Biochemical analysis indicated that AtMYB49 modulated cutin deposition in the leaves. Importantly, cuticular transpiration, chlorophyll leaching and toluidine blue-staining assays revealed a link between increased AtMYB49-mediated cutin deposition in leaves and enhanced salt tolerance. Additionally, increased AtMYB49 expression elevated Ca²⁺ level in leaves and improved antioxidant capacity by up-regulating genes encoding peroxidases and late embryogenesis abundant proteins. These results suggest that genetic manipulation of AtMYB49 may provide a novel way to improve salt tolerance in plants.

Chloride and amino acids are associated with K+-alleviated drought stress in tea (Camellia sinesis)

Drought is one of the main limiting factors affecting tea plant yield and quality. Previous studies have reported that K+ (potassium) application significantly alleviated drought-induced damage in tea plants. However, the intrinsic mechanisms underlying K+-alleviated drought stress are still obscure. In our study, two contrasting varieties, Taicha12 (drought tolerant) and Fuyun6 (drought sensitive), were used to investigate the intrinsic mechanisms behind K+-alleviated drought stress in tea plants. In the present study, we compared with the case of tea plants under drought: higher water and chlorophyll contents were found in drought-stressed tea plants with an external K+ supply, confirming the role of externally supplied K+ in mitigating drought stress. We also found that an adequate K+ supply promoted Cl- accumulation in the mesophyll of Taicha12 (drought tolerant) over that of in Fuyun6 (drought sensitive). Moreover, Gly, Cys, Lys and Arg were not detected in Fuyun6 under 'Drought' or 'Drought + K+' conditions. Results showed that an exogenous supply of Arg and Val significantly alleviated drought-induced damage in Fuyun6, suggesting their role in K+-alleviated drought stress in tea plants. Collectively, our results show that chloride and amino acids are important components associated with K+-alleviated drought stress in tea plants.