Chloride-inducible transient apoplastic alkalinizations induce stomata closure by controlling abscisic acid distribution between leaf apoplast and guard cells in salt-stressed Vicia faba

Chloride salt enhances plant resistance to biotic stresses

Biotic stresses caused by bacterial and fungal pathogens damage crops; identifying treatments that enhance disease resistance provides important information for understanding plant defenses and sustainable agriculture. Salt stress affects crop yields worldwide; however, studies have focused on the toxic sodium ion, leaving the effects of the chloride ion unclear. In this study, we found that irrigation with a combination of chloride salts (MgCl2, CaCl2, and KCl) suppressed the cell death phenotype of the ceramide kinase mutant acd5. Chloride salt pre-irrigation also significantly limited the cell death caused by Pseudomonas syringae pv maculicola infection and inhibited the multiplication of this bacterial pathogen in a mechanism partially dependent on the salicylic acid pathway. Moreover, chloride salt pre-irrigation improved plant defenses against the fungal pathogen challenge, confining the lesion area caused by Botrytis cinerea infection. Furthermore, the growth of herbivorous larvae of Spodoptera exigua was retarded by feeding on chloride salt irrigated plants. Thus, our data suggest that treatment with Cl- increases broad spectrum resistance to biotic challenges.

Water for agriculture: more crop per drop

Global crop production in agriculture depends on water availability. Future scenarios predict increasing occurrence of flash floods and rapidly developing droughts accompanied by heatwaves in humid regions that rely on rain-fed agriculture. It is challenging to maintain high crop yields, even in arid and drought-prone regions that depend on irrigation. The average water demand of crops varies significantly, depending on plant species, development stage, and climate. Most crops, such as maize and wheat, require relatively more water during the vegetative phase compared to the ripening phase. In this review, we explain WUE and options to improve water use and thus crop yield. Nutrient management might represent another possibility to manipulate water uptake and use by plants. An emerging topic involves agroforest co-cultivation, where trees in the system facilitate water transfer through hydraulic lift, benefiting neighbouring crops. Other options to enhance crop yield per water use are discussed.

Evolution of the WRKY66 Gene Family and Its Mutations Generated by the CRISPR/Cas9 System Increase the Sensitivity to Salt Stress in Arabidopsis

Group Ⅲ WRKY transcription factors (TFs) play pivotal roles in responding to the diverse abiotic stress and secondary metabolism of plants. However, the evolution and function of WRKY66 remains unclear. Here, WRKY66 homologs were traced back to the origin of terrestrial plants and found to have been subjected to both motifs' gain and loss, and purifying selection. A phylogenetic analysis showed that 145 WRKY66 genes could be divided into three main clades (Clade A-C). The substitution rate tests indicated that the WRKY66 lineage was significantly different from others. A sequence analysis displayed that the WRKY66 homologs had conserved WRKY and C2HC motifs with higher proportions of crucial amino acid residues in the average abundance. The AtWRKY66 is a nuclear protein, salt- and ABA- inducible transcription activator. Simultaneously, under salt stress and ABA treatments, the superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities, as well as the seed germination rates of Atwrky66-knockdown plants generated by the clustered, regularly interspaced, short palindromic repeats/CRISPR-associated 9 (CRISPR/Cas9) system, were all lower than those of wild type (WT) plants, but the relative electrolyte leakage (REL) was higher, indicating the increased sensitivities of the knockdown plants to the salt stress and ABA treatments. Moreover, RNA-seq and qRT-PCR analyses revealed that several regulatory genes in the ABA-mediated signaling pathway involved in stress response of the knockdown plants were significantly regulated, being evidenced by the more moderate expressions of the genes. Therefore, the AtWRKY66 likely acts as a positive regulator in the salt stress response, which may be involved in an ABA-mediated signaling pathway.

Mild and severe salt stress responses are age-dependently regulated by abscisic acid in tomato

Salt stress hampers plant growth and development through both osmotic and ionic imbalances. One of the key players in modulating physiological responses towards salinity is the plant hormone abscisic acid (ABA). How plants cope with salinity largely depends on the magnitude of the soil salt content (stress severity), but also on age-related developmental processes (ontogeny). Here we studied how ABA directs salt stress responses in tomato plants for both mild and severe salt stress in leaves of different ages. We used the ABA-deficient mutant notabilis, which contains a null-mutation in the gene of a rate-limiting ABA biosynthesis enzyme 9-cis-epoxycarotenoid dioxygenase (NCED1), leading to impaired stomatal closure. We showed that both old and young leaves of notabilis plants keep a steady-state transpiration and photosynthesis rate during salt stress, probably due to their dysfunctional stomatal closure. At the whole plant level, transpiration declined similar to the wild-type, impacting final growth. Notabilis leaves were able to produce osmolytes and accumulate ions in a similar way as wild-type plants, but accumulated more proline, indicating that osmotic responses were not impaired by the NCED1 mutation. Besides NCED1, also NCED2 and NCED6 are strongly upregulated under salt stress, which could explain why the notabilis mutant did not show a lower ABA content upon salt stress, except in young leaves. This might be indicative of a salt-mediated feedback mechanism on NCED2/6 in notabilis and might explain why notabilis plants seem to perform better under salt stress compared to wild-type plants with respect to biomass and water content accumulation.

Habitat-adapted heterologous symbiont Salinispora arenicola promotes growth and alleviates salt stress in tomato crop plants

To ensure food security given the current scenario of climate change and the accompanying ecological repercussions, it is essential to search for new technologies and tools for agricultural production. Microorganism-based biostimulants are recognized as sustainable alternatives to traditional agrochemicals to enhance and protect agricultural production. Marine actinobacteria are a well-known source of novel compounds for biotechnological uses. In addition, former studies have suggested that coral symbiont actinobacteria may support co-symbiotic photosynthetic growth and tolerance and increase the probability of corals surviving abiotic stress. We have previously shown that this activity may also hold in terrestrial plants, at least for the actinobacteria Salinispora arenicola during induced heterologous symbiosis with a wild Solanaceae plant Nicotiana attenuata under in vitro conditions. Here, we further explore the heterologous symbiotic association, germination, growth promotion, and stress relieving activity of S. arenicola in tomato plants under agricultural conditions and dig into the possible associated mechanisms. Tomato plants were grown under normal and saline conditions, and germination, bacteria-root system interactions, plant growth, photosynthetic performance, and the expression of salt stress response genes were analyzed. We found an endophytic interaction between S. arenicola and tomato plants, which promotes germination and shoot and root growth under saline or non-saline conditions. Accordingly, photosynthetic and respective photoprotective performance was enhanced in line with the induced increase in photosynthetic pigments. This was further supported by the overexpression of thermal energy dissipation, which fine-tunes energy use efficiency and may prevent the formation of reactive oxygen species in the chloroplast. Furthermore, gene expression analyses suggested that a selective transport channel gene, SlHKT1,2, induced by S. arenicola may assist in relieving salt stress in tomato plants. The fine regulation of photosynthetic and photoprotective responses, as well as the inhibition of the formation of ROS molecules, seems to be related to the induced down-regulation of other salt stress response genes, such as SlDR1A-related genes or SlAOX1b. Our results demonstrate that the marine microbial symbiont S. arenicola establishes heterologous symbiosis in crop plants, promotes growth, and confers saline stress tolerance. Thus, these results open opportunities to further explore the vast array of marine microbes to enhance crop tolerance and food production under the current climate change scenario.

Alkali salt stress causes fast leaf apoplastic alkalinization together with shifts in ion and metabolite composition and transcription of key genes during the early adaptive response of Vicia faba L

The mechanisms by which plants respond to alkali salt stress are still obscure, and the relevance of alkaline pH under combined alkali salt stress. Early stress responses can indicate mechanisms leading to damage and plant resistance. The apoplast contains essential determinants for plant growth, specifically early apoplastic pH fluctuations are induced by many stressors and hypothesized to be involved in stress signalling. Hence, this study aims to identify fast responses specific to alkaline pH and alkali salt stress by exposing the root of hydroponically grown Vicia faba L. plants to 150 min of either 50 mM NaHCO₃ (pH 9) treatment or alkaline pH 9 alone. Apoplastic pH was monitored in real-time by ratiometric fluorescence microscopy simultaneously with SWIR transmission-based measurements of leaf water content (LWC). Moreover, we examined the effect of these stresses on apoplastic, symplastic and xylem ion and metabolite composition together with transcriptions of certain stress-responsive genes. Physiological and transcriptional changes were observed in response to NaHCO₃ but not to alkaline pH alone. NaHCO₃ elicited a transient reduction in LWC, followed by a transient alkalinization of the apoplast and stomatal closure. Simultaneously, organic acids and sugars accumulated. Fast upregulation of stress-responsive genes showed the significance of gene regulation for early plant adaptation to alkali salt stress.

An Insight into Abiotic Stress and Influx Tolerance Mechanisms in Plants to Cope in Saline Environments

Simple Summary

This review focuses on plant growth and development harmed by abiotic stress, primarily salt stress. Salt stress raises the intracellular osmotic pressure, leading to hazardous sodium buildup. Plants react to salt stress signals by regulating ion homeostasis, activating the osmotic stress pathway, modulating plant hormone signaling, and altering cytoskeleton dynamics and cell wall composition. Understanding the processes underlying these physiological and biochemical responses to salt stress could lead to more effective agricultural crop yield measures. In this review, researchers outline recent advances in plant salt stress control. The study of plant salt tolerance processes is essential, both theoretically and practically, to improve agricultural output, produce novel salt-tolerant cultivars, and make full use of saline soil. Based on past research, this paper discusses the adverse effects of salt stress on plants, including photosynthesis suppression, ion homeostasis disturbance, and membrane peroxidation. The authors have also covered the physiological mechanisms of salt tolerance, such as the scavenging of reactive oxygen species and osmotic adjustment. This study further identifies specific salt stress-responsive mechanisms linked to physiological systems. Based on previous studies, this article reviews the current methodologies and techniques for improving plant salt tolerance. Overall, it is hoped that the above-mentioned points will impart helpful background information for future agricultural and crop plant production.

Abstract

Salinity is significant abiotic stress that affects the majority of agricultural, irrigated, and cultivated land. It is an issue of global importance, causing many socio-economic problems. Salt stress mainly occurs due to two factors: (1) soil type and (2) irrigation water. It is a major environmental constraint, limiting crop growth, plant productivity, and agricultural yield. Soil salinity is a major problem that considerably distorts ecological habitats in arid and semi-arid regions. Excess salts in the soil affect plant nutrient uptake and osmotic balance, leading to osmotic and ionic stress. Plant adaptation or tolerance to salinity stress involves complex physiological traits, metabolic pathways, the production of enzymes, compatible solutes, metabolites, and molecular or genetic networks. Different plant species have different salt overly sensitive pathways and high-affinity K⁺ channel transporters that maintain ion homeostasis. However, little progress has been made in developing salt-tolerant crop varieties using different breeding approaches. This review highlights the interlinking of plant morpho-physiological, molecular, biochemical, and genetic approaches to produce salt-tolerant plant species. Most of the research emphasizes the significance of plant growth-promoting rhizobacteria in protecting plants from biotic and abiotic stressors. Plant growth, survival, and yield can be stabilized by utilizing this knowledge using different breeding and agronomical techniques. This information marks existing research areas and future gaps that require more attention to reveal new salt tolerance determinants in plants—in the future, creating genetically modified plants could help increase crop growth and the toleration of saline environments.

Plant responses to heterogeneous salinity: agronomic relevance and research priorities

BACKGROUND

Soil salinity, in both natural and managed environments, is highly heterogeneous, and understanding how plants respond to this spatiotemporal heterogeneity is increasingly important for sustainable agriculture in the era of global climate change. While the vast majority of research on crop response to salinity utilizes homogeneous saline conditions, a much smaller, but important, effort has been made in the past decade to understand plant molecular and physiological responses to heterogeneous salinity mainly by using split-root studies. These studies have begun to unravel how plants compensate for water/nutrient deprivation and limit salt stress by optimizing root-foraging in the most favourable parts of the soil.

SCOPE

This paper provides an overview of the patterns of salinity heterogeneity in rain-fed and irrigated systems. We then discuss results from split-root studies and the recent progress in understanding the physiological and molecular mechanisms regulating plant responses to heterogeneous root-zone salinity and nutrient conditions. We focus on mechanisms by which plants (salt/nutrient sensing, root-shoot signalling and water uptake) could optimize the use of less-saline patches within the root-zone, thereby enhancing growth under heterogeneous soil salinity conditions. Finally, we place these findings in the context of defining future research priorities, possible irrigation management and crop breeding opportunities to improve productivity from salt-affected lands.

Transcriptomic, proteomic, metabolomic, and functional genomic approaches of Brassica napus L. during salt stress

Environmental abiotic stresses limit plant growth, development, and reproduction. This study aims to reveal the response of Brassica napus to salt stress. Here, transcriptomics, metabolomics, and proteomics analysis were performed on 15 Brassica napus leave samples treated with salt at different times. Through functional enrichment analyzing the differentially expressed genes (DEGs), differential metabolites (DMs) and differentially expressed proteins (DEPs), the key factors that dominate Brassica napus response to salt stress were identified. The results showed that the two key hormones responding to salt stress were Abscisic acid (ABA) and jasmonic acid (JA). Salt stress for 24h is an important milestone. Brassica napus adjusted multiple pathways at 24h to avoid over-response to salt stress and cause energy consumption. The increased expression in BnPP2C is tangible evidence. In response to salt stress, JA and ABA work together to reduce the damage caused by salt stress in Brassica napus. The increased expression of all BnJAZs after salt stress highlighted the function of JA that cannot be ignored responding to salt stress. In addition, some metabolites, such as N-acetyl-5-hydroxytryptamine, L-Cysteine and L-(+)-Arginine, play a critical role in maintaining the balance of ROS. Proteins like catalase-3, cysteine desulfurase, HSP90 and P450_97A3 were the most critical differential proteins in response to salt stress. These findings of this study provide data support for Brassica napus breeding.

Apical-root apoplastic acidification affects cell wall extensibility in wheat under salinity stress

Plant salt tolerance is associated with a high rate of root growth. Although root growth is governed by cell wall and apoplastic pH, the relationship between these factors in the root elongation zone under salinity stress remains unclear. Herein, we assess apoplastic pH, pH- and expansin-dependent cell wall extensibility, and expansin expression in the root elongation zone of salt-sensitive (Yongliang-15) and -tolerant (JS-7) cultivars under salinity stress. A six-day 80 mM NaCl treatment significantly reduced apical root apoplastic pH in both cultivars. Using a pH-dependent cell wall extensibility experiment, we found that, under 0 mM NaCl treatment, the optimal pH for cell wall loosening was 6.0 in the salinity-tolerant cultivar and 4.6 in the salinity-sensitive cultivar. Under 80 mM treatment, a pH of 5.0 mitigated the cell wall stiffness caused by salinity stress in the salinity-tolerant cultivar but promoted cell wall stiffening in the salinity-sensitive cultivar. Salinity stress altered expansin expression and differentially affecting cell wall extensibility under pH 5.0 and 6.0. TaEXPA8 might be relative to cell wall loosening at pH 5.0, whereas TaEXPA5 relative to cell wall loosening at pH 6.0. These results elucidate the relationship between expansins and cell wall extensibility in the root elongation zone, with important implications for enhancing plant growth under salinity stress.

pH biosensing in the plant apoplast-a focus on root cell elongation

The pH parameter of soil plays a key role for plant nutrition as it is affecting the availability of minerals and consequently determines plant growth. Although the mechanisms by which root perceive the external pH is still unknown, the impact of external pH on tissue growth has been widely studied especially in hypocotyl and root. Thanks to technological development of cell imaging and fluorescent sensors, we can now monitor pH in real time with at subcellular definition. In this focus, fluorescent dye-based, as well as genetically-encoded pH indicators are discussed especially with respect to their ability to monitor acidic pH in the context of primary root. The notion of apoplastic subdomains is discussed and suggestions are made to develop fluorescent indicators for pH values below 5.0.

Biostimulants for the Regulation of Reactive Oxygen Species Metabolism in Plants under Abiotic Stress

Global food security for a growing population with finite resources is often challenged by multiple, simultaneously occurring on-farm abiotic stresses (i.e., drought, salinity, low and high temperature, waterlogging, metal toxicity, etc.) due to climatic uncertainties and variability. Breeding for multiple stress tolerance is a long-term solution, though developing multiple-stress-tolerant crop varieties is still a challenge. Generation of reactive oxygen species in plant cells is a common response under diverse multiple abiotic stresses which play dual role of signaling molecules or damaging agents depending on concentration. Thus, a delicate balance of reactive oxygen species generation under stress may improve crop health, which depends on the natural antioxidant defense system of the plants. Biostimulants represent a promising type of environment-friendly formulation based on natural products that are frequently used exogenously to enhance abiotic stress tolerance. In this review, we illustrate the potential of diverse biostimulants on the activity of the antioxidant defense system of major crop plants under stress conditions and their other roles in the management of abiotic stresses. Biostimulants have the potential to overcome oxidative stress, though their wider applicability is tightly regulated by dose, crop growth stage, variety and type of biostimulants. However, these limitations can be overcome with the understanding of biostimulants’ interaction with ROS signaling and the antioxidant defense system of the plants.

Regulation of Reactive Oxygen Species and Antioxidant Defense in Plants under Salinity

The generation of oxygen radicals and their derivatives, known as reactive oxygen species, (ROS) is a part of the signaling process in higher plants at lower concentrations, but at higher concentrations, those ROS cause oxidative stress. Salinity-induced osmotic stress and ionic stress trigger the overproduction of ROS and, ultimately, result in oxidative damage to cell organelles and membrane components, and at severe levels, they cause cell and plant death. The antioxidant defense system protects the plant from salt-induced oxidative damage by detoxifying the ROS and also by maintaining the balance of ROS generation under salt stress. Different plant hormones and genes are also associated with the signaling and antioxidant defense system to protect plants when they are exposed to salt stress. Salt-induced ROS overgeneration is one of the major reasons for hampering the morpho-physiological and biochemical activities of plants which can be largely restored through enhancing the antioxidant defense system that detoxifies ROS. In this review, we discuss the salt-induced generation of ROS, oxidative stress and antioxidant defense of plants under salinity.

MPK3/6-induced degradation of ARR1/10/12 promotes salt tolerance in Arabidopsis

Cytokinins are phytohormones that regulate plant development, growth, and responses to stress. In particular, cytokinin has been reported to negatively regulate plant adaptation to high salinity; however, the molecular mechanisms that counteract cytokinin signaling and enable salt tolerance are not fully understood. Here, we provide evidence that salt stress induces the degradation of the cytokinin signaling components Arabidopsis (Arabidopisis thaliana) response regulator 1 (ARR1), ARR10 and ARR12. Furthermore, the stress-activated mitogen-activated protein kinase 3 (MPK3) and MPK6 interact with and phosphorylate ARR1/10/12 to promote their degradation in response to salt stress. As expected, salt tolerance is decreased in the mpk3/6 double mutant, but enhanced upon ectopic MPK3/MPK6 activation in an MKK5^DD line. Importantly, salt hypersensitivity phenotypes of the mpk3/6 line were significantly alleviated by mutation of ARR1/12. The above results indicate that MPK3/6 enhance salt tolerance in part via their negative regulation of ARR1/10/12 protein stability. Thus, our work reveals a new molecular mechanism underlying salt-induced stress adaptation and the inhibition of plant growth, via enhanced degradation of cytokinin signaling components.

Under salt stress guard cells rewire ion transport and abscisic acid signaling

Soil salinity is an increasingly global problem which hampers plant growth and crop yield. Plant productivity depends on optimal water-use efficiency and photosynthetic capacity balanced by stomatal conductance. Whether and how stomatal behavior contributes to salt sensitivity or tolerance is currently unknown. This work identifies guard cell-specific signaling networks exerted by a salt-sensitive and salt-tolerant plant under ionic and osmotic stress conditions accompanied by increasing NaCl loads. We challenged soil-grown Arabidopsis thaliana and Thellungiella salsuginea plants with short- and long-term salinity stress and monitored genome-wide gene expression and signals of guard cells that determine their function. Arabidopsis plants suffered from both salt regimes and showed reduced stomatal conductance while Thellungiella displayed no obvious stress symptoms. The salt-dependent gene expression changes of guard cells supported the ability of the halophyte to maintain high potassium to sodium ratios and to attenuate the abscisic acid (ABA) signaling pathway which the glycophyte kept activated despite fading ABA concentrations. Our study shows that salinity stress and even the different tolerances are manifested on a single cell level. Halophytic guard cells are less sensitive than glycophytic guard cells, providing opportunities to manipulate stomatal behavior and improve plant productivity.

Stomatal conductance of tomato leaves is regulated by both abscisic acid and leaf water potential under combined water and salt stress

Stomatal conductance (g_s ) affects water use efficiency (WUE) through coordinating photosynthesis and transpiration and is regulated by chemical and/or hydraulic signals. However, the regulation mechanism of g_s of tomato leaves has not been fully explored under combined water and salt stress. Here, we set up four salt treatments and two water treatments in a climate greenhouse and measured stomatal morphologies and conductance and other photosynthesis parameters. Water and salt stress reduced stomatal length (SL), width, perimeter, area (a_max ), density (SD), and the maximum stomatal conductance (g_smax ). Water and salt stress had a separate weakening effect on net photosynthetic rate (A) and transpiration rate but interactively reduced g_s . The contents of abscisic acid (ABA) and Na⁺ in tomato leaves increased with the NaCl concentration, while leaf water potential (Ψ_l ) and chlorophyll content decreased. Under full irrigation, g_smax was coordinated by SD and a_max , and g_s by ABA content under salt stress. Under water and salt combined stress, g_smax was affected by a_max , and g_s was coordinated with ABA and Ψ_l . The decrease of A was caused by both a reduction of chlorophyll content and g_s under water and salt stress. Intrinsic WUE did not reduce under full irrigation or mild to moderate salt stress but decreased under a combination of water and severe salt stress, indicating that the leaves of the tested tomato cultivar performed better under moderate salt stress. Collectively, these results can provide useful insights for the efficient management of water and salt to adapt to drought and high salt environments.

The response mechanism to salt stress in Arabidopsis transgenic lines over-expressing of GmG6PD

Glucose-6-phosphate dehydrogenase (G6PD or G6PDH) plays an important role in response to salt stress in plants. However, much less is known about G6PD proteins in soybean (Glycine max L.). Here, we found that a soybean cytosolic G6PD gene, GmG6PD7, was induced by NaCl. We generated Arabidopsis transgenic lines overexpressing GmG6PD7. The seed germination rate and primary root length of Arabidopsis thaliana over-expressing GmG6PD7 under NaCl treatment were enhanced. Salt stress induced an obvious increase of the total and cytosolic G6PD activity and the marked decrease of ROS levels in the transgenic plants. At the same time, over-expressing GmG6PD7 in Arabidopsis affected the glutathione and NADPH level and activated ROS scavengers, suggesting that GmG6PD7 contributes to increase salinity tolerance by decreasing ROS accumulation. What's more, we found GmG6PD7 overexpression led to the up-regulation of abscisic acid (ABA) degradation gene and the down-regulation of ABA synthesis and ABA-responsive genes, which finally reduced ABA content to improve seed germination rate under salinity stress. It was noteworthy that GmG6PD7 can rescue the seed and root phenotype of Arabidopsis cytosolic G6PD mutant (Atg6pd5 and Atg6pd6) under salt stress, suggesting cytosolic G6PD may have a conserved function in soybean and Arabidopsis.

Membrane voltage as a dynamic platform for spatiotemporal signaling, physiological, and developmental regulation

Membrane voltage arises from the transport of ions through ion-translocating ATPases, ion-coupled transport of solutes, and ion channels, and is an integral part of the bioenergetic "currency" of the membrane. The dynamics of membrane voltage-so-called action, systemic, and variation potentials-have also led to a recognition of their contributions to signal transduction, both within cells and across tissues. Here, we review the origins of our understanding of membrane voltage and its place as a central element in regulating transport and signal transmission. We stress the importance of understanding voltage as a common intermediate that acts both as a driving force for transport-an electrical "substrate"-and as a product of charge flux across the membrane, thereby interconnecting all charge-carrying transport across the membrane. The voltage interconnection is vital to signaling via second messengers that rely on ion flux, including cytosolic free Ca2+, H+, and the synthesis of reactive oxygen species generated by integral membrane, respiratory burst oxidases. These characteristics inform on the ways in which long-distance voltage signals and voltage oscillations give rise to unique gene expression patterns and influence physiological, developmental, and adaptive responses such as systemic acquired resistance to pathogens and to insect herbivory.

Leaf apoplastic alkalization promotes transcription of the ABA-synthesizing enzyme Vp14 and stomatal closure in Zea mays

The chloride component of NaCl salinity causes the leaf apoplast to transiently alkalinize. This transition in pH reduces stomatal aperture. However, whether this apoplastic pH (pHapo) transient initiates stomatal closure by interacting with other chloride stress-induced responses or whether the pH transient alone initiates stomatal closure is unknown. To clarify the problem, the transient alkalinization of the leaf apoplast was mimicked in intact maize (Zea mays L.) by infiltrating near-neutral pH buffers into the leaf apoplast. Effects of the pHapo transient could thus be investigated independently from other chloride stress-derived effects. Microscopy-based ratiometric live pHapo imaging was used to monitor pHapoin planta. LC-MS/MS and real-time quantitative reverse transcription-PCR leaf analyses showed that the artificially induced pHapo transient led to an increase in the concentrations of the stomata-regulating plant hormone abscisic acid (ABA) and in transcripts of the key ABA-synthesizing gene ZmVp14 in the leaf. Since stomatal aperture and stomatal conductance decreased according to pHapo, we conclude that the pHapo transient alone initiates stomatal closure. Therefore, the functionality does not depend on interactions with other compounds induced by chloride stress. Overall, our data indicate that the pH of the leaf apoplast links chloride salinity with the control of stomatal aperture via effects exerted on the transcription of ABA.

Acclimatisation of guard cell metabolism to long-term salinity

Stomatal movements are enabled by changes in guard cell turgor facilitated via transient accumulation of inorganic and organic ions imported from the apoplast or biosynthesized within guard cells. Under salinity, excess salt ions accumulate within plant tissues resulting in osmotic and ionic stress. To elucidate whether (a) Na⁺ and Cl^- concentrations increase in guard cells in response to long-term NaCl exposure and how (b) guard cell metabolism acclimates to the anticipated stress, we profiled the ions and primary metabolites of leaves, the apoplast and isolated guard cells at darkness and during light, that is, closed and fully opened stomata. In contrast to leaves, the primary metabolism of guard cell preparations remained predominantly unaffected by increased salt ion concentrations. Orchestrated reductions of stomatal aperture and guard cell osmolyte synthesis were found, but unlike in leaves, no increases of stress responsive metabolites or compatible solutes occurred. Diverging regulation of guard cell metabolism might be a prerequisite to facilitate the constant adjustment of turgor that affects aperture. Moreover, the photoperiod-dependent sucrose accumulation in the apoplast and guard cells changed to a permanently replete condition under NaCl, indicating that stress-related photosynthate accumulation in leaves contributes to the permanent closing response of stomata under stress.

Phenolic compound abundance in Pak choi leaves is controlled by salinity and dependent on pH of the leaf apoplast

Onset of salinity induces the pH of the leaf apoplast of Pak choi transiently to increase over a period of 2 to 3 hr. This pH event causes protein abundances in leaves to increase. Among them are enzymes that are key for the phenylpropanoid pathway. To answer the questions whether this short-term salt stress also influences contents of the underlying phenylpropanoids and for clarifying as to whether the apoplastic pH transient plays a role for such a putative effect, Pak choi plants were treated with 37.5 mM CaCl2 against a non-stressed control. A third experimental group, where the leaf apoplast of plants treated with 37.5 mM CaCl2, was clamped in the acidic range by means of infiltration of 5 mM citric acid/sodium citrate (pH 3.6), enabled validation of pH-dependent effects. Microscopy-based live cell imaging was used to quantify leaf apoplastic pH in planta. Phenolics were quantified shortly after the formation of the leaf apoplastic pH transient by means of HPLC-DAD-ESI-MS. Results showed that different phenolic compounds were modulated at 150 and 200 min after the onset of chloride salinity. A pH-independent reduction in phenolic acid abundance as well as an accumulation of phenolic acid:malate conjugates was quantified after 200 min of salt stress. However, at 150 min after the onset of salt stress, flavonoids were significantly reduced by salinity in a pH-dependent manner. These results provided a strong indication that the pH of the apoplast is a relevant component for the short-term metabolic response to chloride salinity.

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

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

Early Changes in Nitrate Uptake and Assimilation Under Drought in Relation to Transpiration

Soil drying combined with nitrogen (N) deficiency poses a grave threat to agricultural crop production. The rate at which nitrate (NO3 -) is taken up depends partly on the uptake and transpiration of water. Rapid changes in nitrate assimilation, in contrast to other N forms, may serve as a component of the plant stress response to drought because nitrate assimilation may lead to changes in xylem pH. The modulation of xylem sap pH may be relevant for stomata regulation via the delivery of abscisic acid (ABA) to guard cells. In several factorial experiments, we investigated the interactions between nitrate and water availability on nitrate fate in the plant, as well as their possible implications for the early drought-stress response. We monitored the short-term response (2-6 days) of nitrate in biomass, transport to shoot and reduction in Pisum sativum, Hordeum vulgare, Vicia faba, and Nicotiana tabacum and correlated this with sap pH and transpiration rates (TRs). Cultivation on inorganic substrate ensured control over nutrient and water supply and prevented nodulation in legume species. NO3 - content in biomass decreased in most of the species under drought indicating significant decline in NO3 - uptake. Hordeum vulgare had the highest NO3 - concentrations in all organs even under drought and low NO3 - treatment. This species can likely respond much better to the combined adverse effects of low NO3 - and water scarcity. Nitrate reductase activity (NRA) was reduced in both roots and leaves of water deficient (WD) plants in all species except H. vulgare, presumably due to its high NO3 - contents. Further, transient reduction in NO3 - availability had no effect on sap pH. Therefore, it seems unlikely that NRA shifts from shoot root leading to the supposed alkalization of sap. We also did not observe any interactive effects of NO3 - and water deficiency on transpiration. Hence, as long as leaf NO3 - content remains stable, NO3 - availability in soil is not linked to short-term modulation of transpiration.

Physiological Response of Populus balsamifera and Salix eriocephala to Salinity and Hydraulic Fracturing Wastewater: Potential for Phytoremediation Applications

Natural and anthropogenic soil degradation is resulting in a substantial rise in the extension of saline and industrially-polluted soils. Phytoremediation offers an environmentally and economically advantageous solution to soil contamination. Three growth trials were conducted to assess the stress tolerance of native Canadian genotypes of Populus balsamifera L., Salix eriocephala Michx., and one hybrid willow (S. discolor × S. dasyclados) to salinity and hydraulic fracturing (fracking) wastewater. Thirty-three genotypes were grown in NaCl or fracking wastewater solutions between 0 and 7 mS⁻¹ over a period of 3–4 months. P. balsamifera was observed to be relatively salt-intolerant compared to S. eriocephala and hybrid willow, which is likely caused by an inability of P. balsamifera to restrict Na⁺ translocation. Photosynthesis and transpiration decreased with salinity treatments, and severe reductions occurred with exposure to fracking solutions. Raffinose and stachyose content was tripled in leaf and root tissues. In willows, Na⁺ was primarily confined to root tissues, Cl⁻ accumulated up to 5% dry weight in leaves, and K⁺ was translocated from roots to leaves. Willow genotypes CAM-2 and STL-2 displayed the greatest maintenance of growth and resistance to necrotic symptoms in all trials, suggesting that these genotypes may be useful for practical application and further field study.

The jasmonic acid pathway, rather than abscisic acid, may partly explain contrasting stomatal responses in two strawberry cultivars under osmotic stress

Drought is a major threat in agriculture and horticulture, including commercial strawberry production. Here, we compare hormonal regulation of a first-line drought stress response, namely stomatal closure, in two Fragaria x ananassa cultivars, known to differ in their drought stress phenotype. We show that the observed difference in xylem abscisic acid accumulation cannot explain the different stomatal responses under osmotic stress. Foliar abscisic acid accumulation cannot fully account for the stomatal behavior in one of both cultivars either. An indirect effect of abscisic acid on stomatal conductance via an impact on leaf hydraulic conductance, possibly mediated via aquaporins, as is recently proposed in literature, was not observed here. Next, we show that these two cultivars respond differently to jasmonic acid and one of its precursors. This difference in sensitivity of the jasmonates pathway between both cultivars may partly explain the different stomatal response. This study contributes to the understanding of the regulation of an important drought stress response in an economically important crop prone to water deficit stress.

Mechanisms of Plant Responses and Adaptation to Soil Salinity

Soil salinity is a major environmental stress that restricts the growth and yield of crops. Understanding the physiological, metabolic, and biochemical responses of plants to salt stress and mining the salt tolerance-associated genetic resource in nature will be extremely important for us to cultivate salt-tolerant crops. In this review, we provide a comprehensive summary of the mechanisms of salt stress responses in plants, including salt stress-triggered physiological responses, oxidative stress, salt stress sensing and signaling pathways, organellar stress, ion homeostasis, hormonal and gene expression regulation, metabolic changes, as well as salt tolerance mechanisms in halophytes. Important questions regarding salt tolerance that need to be addressed in the future are discussed.

Physiological and Proteomic Changes in the Apoplast Accompany Leaf Senescence in Arabidopsis

The apoplast, i.e. the cellular compartment external to the plasma membrane, undergoes important changes during senescence. Apoplastic fluid volume increases quite significantly in senescing leaves, thereby diluting its contents. Its pH elevates by about 0.8 units, similar to the apoplast alkalization in response to abiotic stresses. The levels of 159 proteins decrease, whereas 24 proteins increase in relative abundance in the apoplast of senescing leaves. Around half of the apoplastic proteins of non-senescent leaves contain a N-terminal signal peptide for secretion, while all the identified senescence-associated apoplastic proteins contain the signal peptide. Several of the apoplastic proteins that accumulate during senescence also accumulate in stress responses, suggesting that the apoplast may constitute a compartment where developmental and stress-related programs overlap. Other senescence-related apoplastic proteins are involved in cell wall modifications, proteolysis, carbohydrate, ROS and amino acid metabolism, signaling, lipid transport, etc. The most abundant senescence-associated apoplastic proteins, PR2 and PR5 (e.g. pathogenesis related proteins PR2 and PR5) are related to leaf aging rather than to the chloroplast degradation program, as their levels increase only in leaves undergoing developmental senescence, but not in dark-induced senescent leaves. Changes in the apoplastic space may be relevant for signaling and molecular trafficking underlying senescence.

Strengthened antioxidant capacity improves photosynthesis by regulating stomatal aperture and ribulose-1,5-bisphosphate carboxylase/oxygenase activity

ABA is important for plant growth and development; however, it also inhibits photosynthesis by regulating the stomatal aperture and ribulose-1,5-bisphosphate carboxylase/oxygenase activity. Noteworthy, this negative effect can be alleviated by antioxidants including ascorbic acid (AsA) and catalase (CAT), but the underlying mechanism remains unclear. Two rice cultivars, Zhefu802 (recurrent parent) and its near-isogenic line, fgl were selected and planted in a greenhouse with 30/24 °C (day/night) under natural sunlight conditions. Compared to fgl, Zhefu802 had significantly lower net photosynthetic rate (PN) and stomatal conductance (Cond) as well as significantly higher ABA and H₂O₂ contents. However, AsA and CAT increased PN, Cond, and stomatal aperture, which decreased H₂O₂ and malondialdehyde (MDA) levels. In this process, AsA and CAT significantly increased the ribulose-1,5-bisphosphate carboxylase activity, while they strongly decreased the ribulose-1,5-bisphosphate oxygenase activity, and finally caused an obvious decrease in the ratio of photorespiration (Pr) to PN. Additionally, AsA and CAT significantly increased the expression levels of RbcS and RbcL genes of leaves, while H₂O₂ significantly decreased them, especially the RbcS gene. In summary, the removal of H₂O₂ by AsA and CAT can improve the leaf photosynthesis by alleviating the inhibition on the stomatal conductance and ribulose-1,5-bisphosphate carboxylase capacity caused by ABA.

Shoot chloride translocation as a determinant for NaCl tolerance in Vicia faba L

Faba bean (Vicia faba L.) is sensitive to salinity. While toxic effects of sodium (Na⁺) are well studied, toxicity aspects of chloride (Cl^-) and the underlying tolerance mechanisms to Cl^- are not well understood. For this reason, shoot Cl^- translocation and its effect as potential determinant for tolerance was tested. Diverse V. faba varieties were grown hydroponically and stressed with 100 mM NaCl until necrotic leaf spots appeared. At this point, biomass formation, oxidative damage of membranes as well as Na⁺, Cl^- and potassium concentrations were measured. The V. faba varieties contrasted in the length of the period they could withstand the NaCl stress treatment. More tolerant varieties survived longer without evolving necrosis and were less affected by inhibitory effects on photosynthesis. The concentration of Cl^- at the time point of developing leaf necrosis was in the same range irrespective of the variety, while that of Na⁺ varied. This indicates that Cl^- concentrations, and not Na⁺ concentrations are critical for the formation of salt necrosis in faba bean. Tolerant varieties profited from lower Cl^- translocation to leaves. Therefore, photosynthesis was less affected in those varieties with lower Cl^-. This mechanism is a new trait of interest for salt tolerance in V. faba.

Friend or Foe? Chloride Patterning in Halophytes

In this opinion article, we challenge the traditional view that breeding for reduced Cl^- uptake would benefit plant salinity tolerance. A negative correlation between shoot Cl^- concentration and plant biomass does not hold for halophytes - naturally salt tolerant species. We argue that, under physiologically relevant conditions, Cl^- uptake requires plants to invest metabolic energy, and that the poor selectivity of Cl^--transporting proteins may explain the reported negative correlation between Cl^- accumulation and crop salinity tolerance. We propose a new paradigm: salinity tolerance could be achieved by improving the selectivity of some of the broadly selective anion-transporting proteins (e.g., for NO₃^->Cl^-), alongside tight control of Cl^- uptake, rather than targeting traits mediating its efflux from the root.

Dawn regulates guard cell proteins in Arabidopsis thaliana that function in ATP production from fatty acid beta-oxidation

Based on the nature of the proteins that are altered in abundance, we conclude that guard cells switch their energy source from fatty acid metabolism to chloroplast activity, at the onset of dawn. During stomatal opening at dawn, evidence was recently presented for a breakdown and liquidation of stored triacylglycerols in guard cells to supply ATP for use in stomatal opening. However, proteome changes that happen in the guard cells during dawn were until now poorly understood. Bad accessibility to pure and intact guard cell samples can be considered as the primary reason behind this lack of knowledge. To overcome these technical constraints, epidermal guard cell samples with ruptured pavement cells were isolated at 1 h pre-dawn, 15 min post-dawn and 1 h post-dawn from Arabidopsis thaliana. Proteomic changes were analysed by ultra-performance-liquid-chromatography-mass-spectrometry. With 994 confidently identified proteins, we present the first analysis of the A. thaliana guard cell proteome that is not influenced by side effects of guard cell protoplasting. Data are available via ProteomeXchange with identifier PXD009918. By elucidating the identities of enzymes that change in abundance by the transition from dark to light, we corroborate the hypothesis that respiratory ATP production for stomatal opening results from fatty acid beta-oxidation. Moreover, we identified many proteins that were never reported in the context of guard cell biology. Among them are proteins that might play a role in signalling or circadian rhythm.

Stomata in a saline world

Salt stress results in a dramatic increase in ABA biosynthesis, H₂O₂ accumulation, and reduced K⁺ availability in the shoot. Each of these factors leads to stomata closure, so reducing CO₂ assimilation and imposing yield penalties. However, halophytes, naturally salt tolerant plant species, flourish under saline conditions that would cause massive yield penalties in glycophytic crops. Is there anything special about the stomata of halophytes, why is guard cell function in these salt tolerant species not affected by the above factors? This opinion paper addresses these questions by providing a comprehensive assessment of the molecular identity and operational modes of major plasma membrane transporters that mediate stomata movements.

How do Plants Keep their Functional Integrity?

Unlike animals, plants possess a non-strict and sometimes very fuzzy morphology. Mutual proportions of plant parts can vary to a much greater extent than in animals, changing according to the environmental conditions and the plant needs of nutrients, water and light. Despite the existence of this fundamental difference between plants and animals, it passes almost non-reflected in most studies on plants. In this review we make a preliminary attempt to gather together the mechanisms by which plants preserve their integrity, not loosing at the same time the physiological (and morphological) flexibility which allows them adapting to the different environments they can populate.

Expression of TaGF14b, a 14-3-3 adaptor protein gene from wheat, enhances drought and salt tolerance in transgenic tobacco

MAIN CONCLUSION

TaGF14b enhances tolerance to multiple stresses through ABA signaling pathway by altering physiological and biochemical processes, including ROS-scavenging system, stomatal closure, compatible osmolytes, and stress-related gene expressions in tobaccos. The 14-3-3 proteins are involved in plant growth, development, and in responding to abiotic stresses. However, the precise functions of 14-3-3s in responding to drought and salt stresses remained unclear, especially in wheat. In this study, a 14-3-3 gene from wheat, designated TaGF14b, was cloned and characterized. TaGF14b was upregulated by polyethylene glycol 6000, sodium chloride, hydrogen peroxide, and abscisic acid (ABA) treatments. Ectopic expression of TaGF14b in tobacco conferred enhanced tolerance to drought and salt stresses. Transgenic tobaccos had longer root, better growth status, and higher relative water content, survival rate, photosynthetic rate, and water use efficiency than control plants under drought and salt stresses. The contribution of TaGF14b to drought and salt tolerance relies on the regulations of ABA biosynthesis and ABA signaling, as well as stomatal closure and stress-related gene expressions. Moreover, TaGF14b expression could significantly enhance the reactive oxygen species (ROS) scavenging system to ameliorate oxidative damage to cells. In addition, TaGF14b increased tolerance to osmotic stress evoked by drought and salinity through modifying water conservation and compatible osmolytes in plants. In conclusion, TaGF14b enhances tolerance to multiple abiotic stresses through the ABA signaling pathway in transgenic tobaccos by altering physiological and biochemical processes.

Exposure to lower red to far-red light ratios improve tomato tolerance to salt stress

BACKGROUND

Red (R) and far-red (FR) light distinctly influence phytochrome-mediated initial tomato growth and development, and more recent evidence indicates that these spectra also modulate responses to a multitude of abiotic and biotic stresses. This research investigated whether different R: FR values affect tomato growth response and salinity tolerance. Tomato seedlings were exposed to different R: FR conditions (7.4, 1.2 and 0.8) under salinity stress (100 mM NaCl), and evaluated for their growth, biochemical changes, active reactive oxygen species (ROS) and ROS scavenging enzymes, pigments, rate of photosynthesis, and chlorophyll fluorescence.

RESULTS

The results showed that under conditions of salinity, tomato seedlings subjected to a lower R: FR value (0.8) significantly increased both their growth, proline content, chlorophyll content and net photosynthesis rate (Pn), while they decreased malondialdehyde (MDA) compared to the higher R: FR value (7.4). Under conditions of salinity, the lower R: FR value caused a decrease in both the superoxide anion (O₂^•-) and in hydrogen peroxide (H₂O₂) generation, an increase in the activities of superoxidase dismutase (SOD, EC 1.15.1.1), peroxidase (POD, EC 1.11.1.7) and catalase (CAT, EC 1.11.1.7). Tomato seedlings grown under the lower R: FR value and conditions of salinity showed a higher actual quantum yield of photosynthesis (ΦPSII), electron transport rate (ETR), and photochemical quenching (qP) than those exposed to a higher R: FR, indicating overall healthier growth. However, the salinity tolerance induced at the lower R: FR condition disappeared in the tomato phyB1 mutant.

CONLUSION

These results suggest that growing tomato with a lower R: FR value could improve seedlings' salinity tolerance, and phytochrome B1 play an very important role in this process. Therefore, different qualities of light can be used to efficiently develop abiotic stress tolerance in tomato cultivation.

Chloride: from Nutrient to Toxicant

In salinized soils in which chloride (Cl-) is the dominant salt anion, growth of plants that tolerate only low concentrations of salt (glycophytes) is disturbed by Cl- toxicity. Chlorotic discolorations precede necrotic lesions, causing yield reductions. Little is known about the effects of Cl- toxicity on these dysfunctions. A lack of understanding exists regarding (i) the molecular and physiological mechanisms that lead to Cl--induced damage and (ii) the adaptive aspects of induced tolerance to Cl- salinity. Here, mechanistic explanations for the Cl--induced stress responses are proposed and novel ideas and strategies by which glycophytic plants avoid the excessive accumulation of Cl- are reviewed. New experiments are suggested to test the proposed hypotheses. Cl- salinity constrains global food security and thus we urgently need more research into the causes and consequences of Cl- salinity.

Attenuated accumulation of jasmonates modifies stomatal responses to water deficit

To determine whether drought-induced root jasmonate [jasmonic acid (JA) and jasmonic acid-isoleucine (JA-Ile)] accumulation affected shoot responses to drying soil, near-isogenic wild-type (WT) tomato (Solanum lycopersicum cv. Castlemart) and the def-1 mutant (which fails to accumulate jasmonates during water deficit) were self- and reciprocally grafted. Rootstock hydraulic conductance was entirely rootstock dependent and significantly lower in def-1, yet def-1 scions maintained a higher leaf water potential as the soil dried due to their lower stomatal conductance (gs). Stomatal sensitivity to drying soil (the slope of gsversus soil water content) was low in def-1 self-grafts but was normalized by grafting onto WT rootstocks. Although soil drying increased 12-oxo-phytodienoic acid (OPDA; a JA precursor and putative antitranspirant) concentrations in def-1 scions, foliar JA accumulation was negligible and foliar ABA accumulation reduced compared with WT scions. A WT rootstock increased drought-induced ABA and JA accumulation in def-1 scions, but decreased OPDA accumulation. Xylem-borne jasmonates were biologically active, since supplying exogenous JA via the transpiration stream to detached leaves decreased transpiration of WT seedlings but had the opposite effect in def-1. Thus foliar accumulation of both ABA and JA at WT levels is required for both maximum (well-watered) gs and stomatal sensitivity to drying soil.

Elucidating the molecular mechanisms mediating plant salt-stress responses

Contents Summary 523 I. Introduction 523 II. Sensing salt stress 524 III. Ion homeostasis regulation 524 IV. Metabolite and cell activity responses to salt stress 527 V. Conclusions and perspectives 532 Acknowledgements 533 References 533 SUMMARY: Excess soluble salts in soil (saline soils) are harmful to most plants. Salt imposes osmotic, ionic, and secondary stresses on plants. Over the past two decades, many determinants of salt tolerance and their regulatory mechanisms have been identified and characterized using molecular genetics and genomics approaches. This review describes recent progress in deciphering the mechanisms controlling ion homeostasis, cell activity responses, and epigenetic regulation in plants under salt stress. Finally, we highlight research areas that require further research to reveal new determinants of salt tolerance in plants.

Early response to salt ions in maize (Zea mays L.)

Abscisic acid (ABA) regulates leaf growth and transpiration rate of plants exposed to salt stress. Despite the known fact that cell dehydration is instrumental for the modulation of ABA concentrations when NaCl is high in the external environment, it was never tested as to whether sodium (Na) or chlorine (Cl) also modulate ABA concentrations. To answer this question, a hydroponic study on maize (Zea mays) was established, by exposing plants to 50mM of sodium glucosamide or glucosamine chloride. The effect of both ions on ABA was investigated in an early stage before (i) the salt ions accumulated to toxic tissue concentrations and before (ii) cells dehydrated. This allowed studying early responses to Na and Cl separately, well before plants were stressed by these ions. Gas chromatography-mass spectrometry analysis was used to quantify ABA concentrations in roots and in leaves after a period of 2h after ion application. The transcript abundance of the key regulatory enzyme of the biosynthesis of ABA in maize, the 9-cis-epoxycarotenoid dioxygenase gene viviparous 14, was quantified via real-time quantitative-reverse-transcriptase-polymerase-chain-reaction. The results reveal that Cl and Na induce the increase of leaf tissue ABA concentrations at two hours after plants were exposed to 50mM of the ions. Surprisingly, this effect was more pronounced in response to the Cl component. The increase in the guard-cell regulating ABA concentration correlated with a reduced transpiration. Mainly because of this result we suggest that the early accumulation of ABA is useful in maintaining cell turgor.

Extracellular pH imaging of a plant leaf with a polyelectrolyte multilayered nanosheet

We have developed a sheet-like pH imaging sensor based on a flexible and physically adhesive polymer thin film (referred to as a “pH sensing nanosheet”). The pH sensing nanosheet was composed of two films: one is a pH-sensitive layer-by-layer (LbL) film constructed from fluorescein-conjugated poly(acrylic acid) and poly(allylamine hydrochloride) and the other is a pH-insensitive film made from Nile red-embedded poly(d,l-lactic acid). The pH sensing nanosheet enabled the ratiometric imaging of pH changes in a leaf (500 × 500 μm²), namely the apoplastic ion milieu responding to an external NaCl stress. It was successfully mapped out that the alkalization of the leaf apoplast spread from the leaf base to the tip at 20 min after the stimulation and the pH value increased up to approximately pH 6.3 from less than pH 4.5 within 60 min when a 100 mM NaCl aqueous solution was added. The pH sensing nanosheet should be useful for energy metabolic mapping in tissue biology.

We have developed a sheet-like pH imaging sensor based on a flexible and physically adhesive polymer thin film (referred to as a “pH sensing nanosheet”).

The pH of the Apoplast: Dynamic Factor with Functional Impact Under Stress

The apoplast is an interconnected compartment with a thin water-film that alkalinizes under stress. This systemic pH increase may be a secondary effect without functional implications, arising from ion movements or proton-pump regulations. On the other hand, there are increasing indications that it is part of a mechanism to withstand stress. Regardless of this controversy, alkalinization of the apoplast has received little attention. The apoplastic pH (pH_apo) increases not only during plant-pathogen interactions but also in response to salinity or drought. Not much is known about the mechanisms that cause the leaf apoplast to alkalinize, nor whether, and if so, how functional impact is conveyed. Controversial explanations have been given, and the unusual complexity of pH_apo regulation is considered as the primary reason behind this lack of knowledge. A gathering of scattered information revealed that changes in pH_apo convey functionality by regulating stomatal aperture via the effects exerted on abscisic acid. Moreover, apoplastic alkalinization may regulate growth under stress, whereas this needs to be verified. In this review, a comprehensive survey about several physiological mechanisms that alkalize the apoplast under stress is given, and the suitability of apoplastic alkalinization as transducing element for the transmission of sensory information is discussed.

Transient alkalinization of the leaf apoplast stiffens the cell wall during onset of chloride salinity in corn leaves

During chloride salinity, the pH of the leaf apoplast (pH_apo) transiently alkalizes. There is an ongoing debate about the physiological relevance of these stress-induced pH_apo changes. Using proteomic analyses of expanding leaves of corn (Zea mays L.), we show that this transition in pH_apo conveys functionality by (i) adjusting protein abundances and (ii) affecting the rheological properties of the cell wall. pH_apo was monitored in planta via microscopy-based ratio imaging, and the leaf-proteomic response to the transient leaf apoplastic alkalinization was analyzed via ultra-high performance liquid chromatography-MS. This analysis identified 1459 proteins, of which 44 exhibited increased abundance specifically through the chloride-induced transient rise in pH_apo These elevated protein abundances did not directly arise from high tissue concentrations of Cl^- or Na⁺ but were due to changes in the pH_apo Most of these proteins functioned in growth-relevant processes and in the synthesis of cell wall-building components such as arabinose. Measurements with a linear-variable differential transducer revealed that the transient alkalinization rigidified (i.e. stiffened) the cell wall during the onset of chloride salinity. A decrease in t-coumaric and t-ferulic acids indicates that the wall stiffening arises from cross-linkage to cell wall polymers. We conclude that the pH of the apoplast represents a dynamic factor that is mechanistically coupled to cellular responses to chloride stress. By hardening the wall, the increased pH abrogates wall loosening required for cell expansion and growth. We conclude that the transient alkalinization of the leaf apoplast is related to salinity-induced growth reduction.

Effects, tolerance mechanisms and management of salt stress in grain legumes

Salt stress is an ever-present threat to crop yields, especially in countries with irrigated agriculture. Efforts to improve salt tolerance in crop plants are vital for sustainable crop production on marginal lands to ensure future food supplies. Grain legumes are a fascinating group of plants due to their high grain protein contents and ability to fix biological nitrogen. However, the accumulation of excessive salts in soil and the use of saline groundwater are threatening legume production worldwide. Salt stress disturbs photosynthesis and hormonal regulation and causes nutritional imbalance, specific ion toxicity and osmotic effects in legumes to reduce grain yield and quality. Understanding the responses of grain legumes to salt stress and the associated tolerance mechanisms, as well as assessing management options, may help in the development of strategies to improve the performance of grain legumes under salt stress. In this manuscript, we discuss the effects, tolerance mechanisms and management of salt stress in grain legumes. The principal inferences of the review are: (i) salt stress reduces seed germination (by up to more than 50%) either by inhibiting water uptake and/or the toxic effect of ions in the embryo, (ii) salt stress reduces growth (by more than 70%), mineral uptake, and yield (by 12-100%) due to ion toxicity and reduced photosynthesis, (iii) apoplastic acidification is a good indicator of salt stress tolerance, (iv) tolerance to salt stress in grain legumes may develop through excretion and/or compartmentalization of toxic ions, increased antioxidant capacity, accumulation of compatible osmolytes, and/or hormonal regulation, (v) seed priming and nutrient management may improve salt tolerance in grain legumes, (vi) plant growth promoting rhizobacteria and arbuscular mycorrhizal fungi may help to improve salt tolerance due to better plant nutrient availability, and (vii) the integration of screening, innovative breeding, and the development of transgenics and crop management strategies may enhance salt tolerance and yield in grain legumes on salt-affected soils.

Chloride: not simply a 'cheap osmoticum', but a beneficial plant macronutrient

At macronutrient levels, chloride has positive effects on plant growth, which are distinct from its function in photosynthesis..

Orchestrating rapid long-distance signaling in plants with Ca2+ , ROS and electrical signals

Plants show a rapid systemic response to a wide range of environmental stresses, where the signals from the site of stimulus perception are transmitted to distal organs to elicit plant-wide responses. A wide range of signaling molecules are trafficked through the plant, but a trio of potentially interacting messengers, reactive oxygen species (ROS), Ca²⁺ and electrical signaling ('trio signaling') appear to form a network supporting rapid signal transmission. The molecular components underlying this rapid communication are beginning to be identified, such as the ROS producing NAPDH oxidase RBOHD, the ion channel two pore channel 1 (TPC1), and glutamate receptor-like channels GLR3.3 and GLR3.6. The plant cell wall presents a plant-specific route for possible propagation of signals from cell to cell. However, the degree to which the cell wall limits information exchange between cells via transfer of small molecules through an extracellular route, or whether it provides an environment to facilitate transmission of regulators such as ROS or H⁺ remains to be determined. Similarly, the role of plasmodesmata as both conduits and gatekeepers for the propagation of rapid cell-to-cell signaling remains a key open question. Regardless of how signals move from cell to cell, they help prepare distant parts of the plant for impending challenges from specific biotic or abiotic stresses.

Zooming In on Plant Hormone Analysis: Tissue- and Cell-Specific Approaches

Plant hormones are a group of naturally occurring, low-abundance organic compounds that influence physiological processes in plants. Our knowledge of the distribution profiles of phytohormones in plant organs, tissues, and cells is still incomplete, but advances in mass spectrometry have enabled significant progress in tissue- and cell-type-specific analyses of phytohormones over the last decade. Mass spectrometry is able to simultaneously identify and quantify hormones and their related substances. Biosensors, on the other hand, offer continuous monitoring; can visualize local distributions and real-time quantification; and, in the case of genetically encoded biosensors, are noninvasive. Thus, biosensors offer additional, complementary technologies for determining temporal and spatial changes in phytohormone concentrations. In this review, we focus on recent advances in mass spectrometry-based quantification, describe monitoring systems based on biosensors, and discuss validations of the various methods before looking ahead at future developments for both approaches.

Chloride on the Move

Chloride (Cl^-) is an essential plant nutrient but under saline conditions it can accumulate to toxic levels in leaves; limiting this accumulation improves the salt tolerance of some crops. The rate-limiting step for this process - the transfer of Cl^- from root symplast to xylem apoplast, which can antagonize delivery of the macronutrient nitrate (NO₃^-) to shoots - is regulated by abscisic acid (ABA) and is multigenic. Until recently the molecular mechanisms underpinning this salt-tolerance trait were poorly defined. We discuss here how recent advances highlight the role of newly identified transport proteins, some that directly transfer Cl^- into the xylem, and others that act on endomembranes in 'gatekeeper' cell types in the root stele to control root-to-shoot delivery of Cl^-.

Early changes of the pH of the apoplast are different in leaves, stem and roots of Vicia faba L. under declining water availability

Changes in pH of the apoplast have recently been discussed as an important factor in adjusting transpiration and water relations under conditions of drought via modulatory effect on abscisic acid (ABA) concentration. Using Vicia faba L., we investigated whether changes in the root, shoot and leaf apoplastic pH correlated with (1) a drought-induced reduction in transpiration and with (2) changes in ABA concentration. Transpiration, leaf water potential and ABA in leaves were measured and correlated with root and shoot xylem pH, determined by a pH microelectrode, and pH of leaf apoplast quantified by microscopy-based in vivo ratiometric analysis. Results revealed that a reduction in transpiration rate in the early phase of soil drying could not be linked with changes in the apoplastic pH via effects on the stomata-regulating hormone ABA. Moreover, drought-induced increase in pH of xylem or leaf apoplast was not the remote effect of an acropetal transport of alkaline sap from root, because root xylem acidified during progressive soil drying, whereas the shoot apoplast alkalized. We reason that other, yet unknown signalling mechanism was responsible for reduction of transpiration rate in the early phase of soil drying.

A Member of the 14-3-3 Gene Family in Brachypodium distachyon, BdGF14d, Confers Salt Tolerance in Transgenic Tobacco Plants

Plant 14-3-3 proteins are involved in diverse biological processes, but for the model monocotyledonous species, Brachypodium distachyon, their roles in abiotic stress tolerance are not well understood. In this study, a total of eight Bd14-3-3 genes were identified from B. distachyon and these were designated respectively as BdGF14a-BdGF14g. The qRT-PCR analyses of 3-month-old plants of B. distachyon showed that these genes were all expressed in the stems, leaves, and spikelets. By contrast, most of the plants had relatively lower transcriptional levels in their roots, except for the BdGF14g gene. The different expression profiles of the Bd14-3-3s under various stress treatments, and the diverse interaction patterns between Bd14-3-3s and BdAREB/ABFs, suggested that these gene products probably had a range of functions in the stress responses. The NaCl-induced Bd14-3-3 gene, BdGF14d, was selected for overexpression in tobacco. BdGF14d was found to be localized throughout the cell and it conferred enhanced tolerance to salt in the transgenic plants. Lowered contents of malondialdehyde, H₂O₂, and Na⁺, and lower relative electronic conductance (Rec%), yet greater activities of catalase and peroxidase, were observed in the overexpressing plants. Higher photosynthetic rate, transpiration rate, stomatal conductance, and water use efficiency were measured in the transgenic lines. Following abscisic acid (ABA) or NaCl treatment, stomatal aperture in leaves of the BdGF14d-overexpression plants was significantly lower than in leaves of the wild type (WT) controls. The stress-related marker genes involved in the ABA signaling pathway, the reactive oxygen species (ROS)-scavenging system, and the ion transporters were all up-regulated in the BdGF14d-overexpressing plants as compared with WT. Taken together, these results demonstrate that the Bd14-3-3 genes play important roles in abiotic stress tolerance. The ABA signaling pathway, the ROS-scavenging system, and ion transporters were all involved in enhancing the tolerance to salt stress in the BdGF14d-overexpression plants.

Improving stomatal functioning at elevated growth air humidity: A review

Plants grown at high relative air humidity (RH≥85%) are prone to lethal wilting upon transfer to conditions of high evaporative demand. The reduced survival of these plants is related to (i) increased cuticular permeability, (ii) changed anatomical features (i.e., longer pore length and higher stomatal density), (iii) reduced rehydration ability, (iv) impaired water potential sensitivity to leaf dehydration and, most importantly, (v) compromised stomatal closing ability. This review presents a critical analysis of the strategies which stimulate stomatal functioning during plant development at high RH. These include (a) breeding for tolerant cultivars, (b) interventions with respect to the belowground environment (i.e., water deficit, increased salinity, nutrient culture and grafting) as well as (c) manipulation of the aerial environment [i.e., increased proportion of blue light, increased air movement, temporal temperature rise, and spraying with abscisic acid (ABA)]. Root hypoxia, mechanical disturbance, as well as spraying with compounds mimicking ABA, lessening its inactivation or stimulating its within-leaf redistribution are also expected to improve stomatal functioning of leaves expanded in humid air. Available evidence leaves little doubt that genotypic and phenotypic differences in stomatal functioning following cultivation at high RH are realized through the intermediacy of ABA.