GABA operates upstream of H+-ATPase and improves salinity tolerance in Arabidopsis by enabling cytosolic K+ retention and Na+ exclusion

Heterologous expression and purification of glutamate decarboxylase-1 from the model plant Arabidopsis thaliana: Characterization of the enzyme's in vitro truncation by thiol endopeptidase activity

Plant glutamate decarboxylase (GAD) is a Ca2+-calmodulin (CaM) activated enzyme that produces γ-aminobutyrate (GABA) as the first committed step of the GABA shunt. Our prior research established that in vivo phosphorylation of AtGAD1 (AT5G17330) occurs at multiple N-terminal serine residues following Pi resupply to Pi-starved cell cultures of the model plant Arabidopsis thaliana. The aim of the current investigation was to purify recombinant AtGAD1 (rAtGAD1) following its expression in Escherichia coli to facilitate studies of the impact of phosphorylation on its kinetic properties. However, in vitro proteolytic truncation of an approximate 5 kDa polypeptide from the C-terminus of 59 kDa rAtGAD1 subunits occurred during purification. Immunoblotting demonstrated that most protease inhibitors or cocktails that we tested were ineffective in suppressing this partial rAtGAD1 proteolysis. Although the thiol modifiers N-ethylmaleimide or 2,2-dipyridyl disulfide negated rAtGAD1 proteolysis, they also abolished its GAD activity. This indicates that an essential -SH group is needed for catalysis, and that rAtGAD1 is susceptible to partial degradation either by an E. coli cysteine endopeptidase, or possibly via autoproteolytic activity. The inclusion of exogenous Ca2+/CaM facilitated the purification of non-proteolyzed rAtGAD1 to a specific activity of 27 (μmol GABA produced/mg) at optimal pH 5.8, while exhibiting an approximate 3-fold activation by Ca2+/CaM at pH 7.3. By contrast, the purified partially proteolyzed rAtGAD1 was >40 % less active at both pH values, and only activated 2-fold by Ca2+/CaM at pH 7.3. These results emphasize the need to diagnose and prevent partial proteolysis before conducting kinetic studies of purified regulatory enzymes.

Relationship between the GABA Pathway and Signaling of Other Regulatory Molecules

GABA (gamma-aminobutyric acid) is an amino acid whose numerous regulatory functions have been identified in animal organisms. More and more research indicate that in plants, this molecule is also involved in controlling basic growth and development processes. As recent studies have shown, GABA plays an essential role in triggering plant resistance to unfavorable environmental factors, which is particularly important in the era of changing climate. The main sources of GABA in plant cells are glutamic acid, converted in the GABA shunt pathway, and polyamines subjected to oxidative degradation. The action of GABA is often related to the activity of other messengers, including phytohormones, polyamines, NO, H2O2, or melatonin. GABA can function as an upstream or downstream element in the signaling pathways of other regulators, acting synergistically or antagonistically with them to control cellular processes. Understanding the role of GABA and its interactions with other signaling molecules may be important for developing crop varieties with characteristics that enable adaptation to a changing environment.

GABA promotes peroxisome proliferation in Triticum monococcum leaves

Although peroxisomes are integral for both primary and secondary metabolism, how developmental changes affect activity of peroxisomes remains poorly understood. Here, we used published RNA-seq data to analyze the expression patterns of genes encoding 21 peroxisome metabolic pathways at successive developmental stages of Zea mays and Oryza sativa. Photorespiration was the most represented pathway in adult leaf relative to the juvenile stages. Components of reactive oxygen species (ROS)/reactive nitrogen species (RNS) metabolism, NADPH regeneration, and catabolism of polyamines were also enriched at later stages of leaf differentiation. The most commonly upregulated gene in differentiated leaves across all datasets of both species was BETAINE ALANINE DEHYDROGENASE (BADH). BADH functions in catabolism of polyamines where it converts 4-aminobutyraldehyde (ABAL) to 4-aminobutyrate (GABA). We tested the outcome of RNA-seq analysis by qRT-PCR in developing Triticum monococcum ssp. monococcum (Einkorn) seedlings. Consistent with the outcomes of RNA-seq analysis, transcription of BADH and CATALASE3 (CAT3) were upregulated in older seedlings. CAT3 is an essential peroxisome biogenesis factor and a key enzyme of ROS homeostasis. Furthermore, exogenous application of GABA resulted in higher peroxisome abundance and transcriptional upregulation of BADH and a gene encoding another peroxisome biogenesis factor responsible for peroxisome fission, PEROXIN11C (PEX11C), in leaves. We propose that GABA contributes to regulation of peroxisome fission machinery during leaf differentiation.

Endogenous γ-Aminobutyric Acid Accumulation Enhances Salinity Tolerance in Rice

Rice is an important food crop worldwide but is usually susceptible to saline stress. When grown on soil with excessive salt, rice plants experience osmotic, ionic, and oxidative stresses that adversely affect growth performance. γ-Aminobutyric acid (GABA) is a nonproteinogenic amino acid that plays an important role in the metabolic activities of organisms. Glutamate decarboxylase (GAD) is the rate-limiting enzyme in GABA metabolism. Here, we genetically modified rice GAD by overexpression or CRISPR-mediated genome editing. These lines, named gad3-ox1 and gad3-ox2 or gad1/3-ko, were used to explore the effects of endogenous GABA accumulation on salt tolerance in rice. Both the gad3-ox1 and gad3-ox2 lines exhibited significant accumulation of the GABA content, whereas the gad1/3-ko line presented a reduced GABA content in vivo. Notably, the two overexpression lines were markedly resistant to salt stress compared with the wild-type and knockout lines. Furthermore, our results demonstrated that endogenous GABA accumulation in the gad3-ox1 and gad3-ox2 lines increased the contents of antioxidant substances and osmotic regulators, decreased the content of membrane lipid peroxidation products and the Na+ content, and resulted in strong tolerance to salt stress. Together, these data provide a theoretical basis for cultivating rice varieties with strong salt tolerance.

Effects of superoxide radical on photosynthesis and K+ and redox homeostasis in quinoa and spinach

Methyl viologen (MV), also known as paraquat, is a widely used herbicide but has also been reported as highly toxic to different life forms. The mode of its operation is related to superoxide radical (O2.-) production and consequent oxidative damage. However, besides the damage to key macromolecules, reactive oxygen species (ROS; to which O2.- belongs) are also known as regulators of numerous ion transport systems located at cellular membranes. In this study, we used MV as a tool to probe the role of O2.- in regulating membrane-transport activity and systemic acquired tolerance in halophytic Chenopodium quinoa and glycophytic spinach plants. Both plant species showed growth reduction in terms of reduced shoot length, lower shoot fresh and dry weight, photosynthesis rate, and chlorophyll contents; however, quinoa showed less reduction in growth compared with spinach. This whole plant response was further examined by measuring the ion concentration, gene expression of ion transporters, activation of antioxidants, and osmolyte accumulation. We observed that at the mechanistic level, the differences in growth in response to MV were conferred by at least four complementary physiological mechanisms: (1) higher K+ loss from spinach leaves resulted from higher expression of MV-induced plasma membrane-based depolarization-activated K+ efflux GORK channel, (2) higher activation of high-affinity K+ uptake transporter HAK5 in quinoa, (3) higher antioxidant production and osmolyte accumulation in quinoa as compared with spinach, and (4) maintaining a higher rate of photosynthesis due to higher chlorophyll contents, and efficiency of photosystem II and reduced ROS and MDA contents. Obtained results also showed that MV induced O2.- significantly reduced N contents in both species but with more pronounced effects in glycophytic spinach. Taken together this study has shown the role of O2.- in regulating membrane ion transport and N metabolism in the leaves of halophyte vs. glycophyte in the context of oxidative stress tolerance.

MdSINA2-MdNAC104 Module Regulates Apple Alkaline Resistance by Affecting γ-Aminobutyric Acid Synthesis and Transport

Soil alkalization is an adverse factor limiting plant growth and yield. As a signaling molecule and secondary metabolite, γ-aminobutyric acid (GABA) responds rapidly to alkaline stress and enhances the alkaline resistance of plants. However, the molecular mechanisms by which the GABA pathway adapts to alkaline stress remain unclear. In this study, a transcription factor, MdNAC104 is identified, from the transcriptome of the alkaline-stressed roots of apple, which effectively reduces GABA levels and negatively regulates alkaline resistance. Nevertheless, applying exogenous GABA compensates the negative regulatory mechanism of overexpressed MdNAC104 on alkaline resistance. Further research confirms that MdNAC104 repressed the GABA biosynthetic gene MdGAD1/3 and the GABA transporter gene MdALMT13 by binding to their promoters. Here, MdGAD1/3 actively regulates alkaline resistance by increasing GABA synthesis, while MdALMT13 promotes GABA accumulation and efflux in roots, resulting in an improved resistance to alkaline stress. This subsequent assays reveal that MdSINA2 interacts with MdNAC104 and positively regulates root GABA content and alkaline resistance by ubiquitinating and degrading MdNAC104 via the 26S proteasome pathway. Thus, the study reveals the regulation of alkaline resistance and GABA homeostasis via the MdSINA2-MdNAC104-MdGAD1/3/MdALMT13 module in apple. These findings provide novel insight into the molecular mechanisms of alkaline resistance in plants.

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

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

The role of reactive oxygen species in regulation of the plasma membrane H+-ATPase activity in Masson pine (Pinus massoniana Lamb.) roots responding to acid stress

To understand the role of reactive oxygen species (ROS) in regulation of the plasma membrane (PM) H+-ATPase in acid-stressed Masson pine roots, different acidity (pH 6.6 as the control, pH 5.6 and pH 4.6) of simulated acid rain (SAR) added with and without external chemicals (H2O2, enzyme inhibitors and ROS scavenger) was prepared. After 30 days of SAR exposure, the plant morphological phenotype attributes, levels of cellular ROS and lipid peroxidation, enzymatic activities of antioxidants, PM nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and PM H+-ATPase activity in pine seedlings were measured. Compared with the control, the growth of pine seedlings exposed to SAR in the presence or absence of H2O2 was well-maintained, but the application of Na3VO4, 1,3-dimethyl-2-thiourea, N, N-dimethylthiourea (DMTU) and diphenyleneiodonium chloride (DPI) caused a substantial growth inhibition. In addition, SAR exposure, SAR with H2O2 treatment, and SAR with Na3VO4 treatment increased the cellular H2O2 content, O2- content and malondialdehyde (MDA) content, while the use of DMTU and DPI lead to relatively low levels. Similarly, the enzymatic activities of antioxidants, PM NADPH oxidase and PM H+-ATPase in acid stressed pine seedlings elevated with the increasing acidity. A significant stimulation of these enzymatic activities obtained from SAR with H2O2 treatment was observed, whereas which decreased obviously with the addition of Na3VO4, DMTU and DPI (P < 0.05). Moreover, a positive correlation was found between plant morphological attributes and the PM H+-ATPase activity (P < 0.05). Besides, the PM H+-ATPase activity positively correlated with the cellular ROS contents and the enzymatic activities of antioxidants and PM NADPH oxidase (P < 0.05). Therefore, the PM H+-ATPase is instrumental in the growth of pine seedlings resisting to acid stress by enhancing its activity. The process involves the signaling transduction of cellular ROS and coordination with PM NADPH oxidase.

Small but powerful: RALF peptides in plant adaptive and developmental responses

Plants live in a highly dynamic environment and require to rapidly respond to a plethora of environmental stimuli, so that to maintain their optimal growth and development. A small plant peptide, rapid alkalization factor (RALF), can rapidly increase the pH value of the extracellular matrix in plant cells. RALFs always function with its corresponding receptors. Mechanistically, effective amount of RALF is induced and released at the critical period of plant growth and development or under different external environmental factors. Recent studies also highlighted the role of RALF peptides as important regulators in plant intercellular communications, as well as their operation in signal perception and as ligands for different receptor kinases on the surface of the plasma membrane, to integrate various environmental cues. In this context, understanding the fine-print of above processes may be essential to solve the problems of crop adaptation to various harsh environments under current climate trends scenarios, by genetic means. This paper summarizes the current knowledge about the structure and diversity of RALF peptides and their roles in plant development and response to stresses, highlighting unanswered questions and problems to be solved.

Gamma-aminobutyric acid (GABA) improves salinity stress tolerance in soybean seedlings by modulating their mineral nutrition, osmolyte contents, and ascorbate-glutathione cycle

BACKGROUND

In plants, GABA plays a critical role in regulating salinity stress tolerance. However, the response of soybean seedlings (Glycine max L.) to exogenous gamma-aminobutyric acid (GABA) under saline stress conditions has not been fully elucidated.

RESULTS

This study investigated the effects of exogenous GABA (2 mM) on plant biomass and the physiological mechanism through which soybean plants are affected by saline stress conditions (0, 40, and 80 mM of NaCl and Na2SO4 at a 1:1 molar ratio). We noticed that increased salinity stress negatively impacted the growth and metabolism of soybean seedlings, compared to control. The root-stem-leaf biomass (27- and 33%, 20- and 58%, and 25- and 59% under 40- and 80 mM stress, respectively]) and the concentration of chlorophyll a and chlorophyll b significantly decreased. Moreover, the carotenoid content increased significantly (by 35%) following treatment with 40 mM stress. The results exhibited significant increase in the concentration of hydrogen peroxide (H2O2), malondialdehyde (MDA), dehydroascorbic acid (DHA) oxidized glutathione (GSSG), Na+, and Cl- under 40- and 80 mM stress levels, respectively. However, the concentration of mineral nutrients, soluble proteins, and soluble sugars reduced significantly under both salinity stress levels. In contrast, the proline and glycine betaine concentrations increased compared with those in the control group. Moreover, the enzymatic activities of ascorbate peroxidase, monodehydroascorbate reductase, glutathione reductase, and glutathione peroxidase decreased significantly, while those of superoxide dismutase, catalase, peroxidase, and dehydroascorbate reductase increased following saline stress, indicating the overall sensitivity of the ascorbate-glutathione cycle (AsA-GSH). However, exogenous GABA decreased Na+, Cl-, H2O2, and MDA concentration but enhanced photosynthetic pigments, mineral nutrients (K+, K+/Na+ ratio, Zn2+, Fe2+, Mg2+, and Ca2+); osmolytes (proline, glycine betaine, soluble sugar, and soluble protein); enzymatic antioxidant activities; and AsA-GSH pools, thus reducing salinity-associated stress damage and resulting in improved growth and biomass. The positive impact of exogenously applied GABA on soybean plants could be attributed to its ability to improve their physiological stress response mechanisms and reduce harmful substances.

CONCLUSION

Applying GABA to soybean plants could be an effective strategy for mitigating salinity stress. In the future, molecular studies may contribute to a better understanding of the mechanisms by which GABA regulates salt tolerance in soybeans.

Exogenous application of salicylic acid ameliorates salinity stress in barley (Hordeum vulgare L.)

Barley (Hordeum vulgare L.) is a significant cereal crop belonging to Poaceae that is essential for human food and animal feeding. The production of barley grains was around 142.37 million tons in 2017/2018. However, the growth of barley was influenced by salinity which was enhanced by applying a foliar spray of salicylic acid. The current study investigated to evaluated the potential effect of SA on the barley (Hordeum vulgare L.) plants under salinity stress and its possible effects on physiological, biochemical, and growth responses. The experiment was conducted at Postgraduate Research Station (PARS), University of Agriculture; Faisalabad to assess the influence of salicylic acid on barley (Hordeum vulgare L.) under highly saline conditions. The experiment was conducted in a Completely Randomized Design (CRD) with 3 replicates. In plastic pots containing 8 kg of properly cleaned sand, two different types of barley (Sultan and Jau-17) were planted. The plants were then watered with a half-strength solution of Hoagland's nutritional solution. After the establishment of seedlings, two salt treatments (0 mM and 120 mM NaCl) were applied in combining three levels of exogenously applied salicylic acid (SA) (0, 0.5, and 1 mg L-1). Data about morphological, physiological, and biochemical attributes was recorded using standard procedure after three weeks of treatment. The morpho-physiological fresh weight of the shoot and root (48%), the dry mass of the shoot and root (66%), the plant height (18%), the chlorophyll a (30%), the chlorophyll b (22%), and the carotenoids (22%), all showed significant decreases. Salinity also decreased yield parameters and the chl. ratio (both at 29% and 26% of the total chl. leaf area index). Compared to the control parameters, the following data was recorded under salt stress: spike length, number of spikes, number of spikelets, number of tillers, biological yield, and harvest index. Salicylic acid was used as a foliar spray to lessen the effects of salinity stress, and 1 mg L-1 of salicylic acid proved more effective than 0.5 mg L-1. Both varieties show better growth by applying salicylic acid (0 mg L-1) as a control, showing normal growth. By increasing its level to (0.5 mg L-1), it shows better growth but maximized growth occurred at a higher level (1 mg L-1). Barley sultan (Hordeum vulgare L.) is the best variety as compared to Jau-17 performs more growth to mitigate salt stress (0mM and 120mM NaCl) by improving morpho-physiological parameters by enhancing plan height, Root and shoot fresh and dry weights, as well as root and shoot lengths, photosynthetic pigments, area of the leaves and their index, and yield attributes and reduce sodium ions.

Truncation of the calmodulin binding domain in rice glutamate decarboxylase 4 (OsGAD4) leads to accumulation of γ-aminobutyric acid and confers abiotic stress tolerance in rice seedlings

GABA (Gamma-aminobutyric acid) is a non-protein amino acid widely known as major inhibitory neurotransmitter. It is synthesized from glutamate via the enzyme glutamate decarboxylase (GAD). GAD is ubiquitous in all organisms, but only plant GAD has ability to bind Ca2+/calmodulin (CaM). This kind of binding suppresses the auto-inhibition of Ca2+/calmodulin binding domain (CaMBD) when the active site of GAD is unfolded resulting in stimulated GAD activity. OsGAD4 is one of the five GAD genes in rice genome. It was confirmed that OsGAD4 has ability to bind to Ca2+/CaM. Moreover, it exhibits strongest expression against several stress conditions among the five OsGAD genes. In this study, CRISPR/Cas9-mediated genome editing was performed to trim the coding region of CaMBD from the OsGAD4 gene, to remove its autoinhibitory function. DNA sequence analysis of the genome edited rice plants revealed the truncation of CaMBD (216 bp). Genome edited line (#14-1) produced 11.26 mg GABA/100 g grain, which is almost nine-fold in comparison to wild type. Short deletion in the coding region for CaMBD yielded in mutant (#14-6) with lower GABA content than wild type counterpart. Abiotic stresses like salinity, flooding and drought significantly enhanced GABA accumulation in #14-1 at various time points compared to wild-type and #14-6 under the same stress conditions. Moreover, upregulated mRNA expression in vegetative tissues seems correlated with the stress-responsiveness of OsGAD4 when exposed to the above-mentioned stresses. Stress tolerance of OsGAD4 genome edited lines was evidenced by the higher survival rate indicating the gene may induce tolerance against abiotic stresses in rice. This is the first report on abiotic stress tolerance in rice modulated by endogenous GABA.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01460-1.

The GABA shunt contributes to ROS homeostasis in guard cells of Arabidopsis

γ-Aminobutyric acid (GABA) accumulates rapidly under stress via the GABA shunt pathway, which has been implicated in reducing the accumulation of stress-induced reactive oxygen species (ROS) in plants. γ-Aminobutyric acid has been demonstrated to act as a guard-cell signal in Arabidopsis thaliana, modulating stomatal opening. Knockout of the major GABA synthesis enzyme Glutamate Decarboxylase 2 (GAD2) increases the aperture of gad2 mutants, which results in greater stomatal conductance and reduces water-use efficiency compared with wild-type plants. Here, we found that the additional loss of GAD1, GAD4, and GAD5 in gad2 leaves increased GABA deficiency but abolished the more open stomatal pore phenotype of gad2, which we link to increased cytosolic calcium (Ca2+ ) and ROS accumulation in gad1/2/4/5 guard cells. Compared with wild-type and gad2 plants, glutamate was ineffective in closing gad1/2/4/5 stomatal pores, whereas lowering apoplastic calcium, applying ROS inhibitors or complementation with GAD2 reduced gad1/2/4/5 guard-cell ROS, restored the gad2-like greater stomatal apertures of gad1/2/4/5 beyond that of wild-type. We conclude that GADs are important contributors to ROS homeostasis in guard cells likely via a Ca2+ -mediated pathway. As such, this study reveals greater complexity in GABA's role as a guard-cell signal and the interactions it has with other established signals.

GABA: a key player of abiotic stress regulation

Abiotic stress is considered as the main culprit for reduction of global food production. Recent studies have reported GABA as a major regulator of abiotic stress and thus opening new avenues in research on emerging roles of GABA in abiotic stress acclimation in plants.

Exogenous γ-Aminobutyric Acid Can Improve Seed Germination and Seedling Growth of Two Cotton Cultivars under Salt Stress

Excessive salt content in soil has adverse effects on cotton production, especially during the germination and seedling stages. γ-aminobutyric acid (GABA) is an important active substance that is expected to improve the resistance of plants to abiotic stresses. This study focused on two cotton cultivars (Gossypium hirsutum L.: Tahe 2 and Xinluzhong 62) and investigated the impact of exogenous GABA (0, 1, 2, 3, and 4 mM) on seed germination, seedling growth, and related morphological, physiological, and biochemical indicators under salt stress (150 mM NaCl). The results showed that salt stress significantly reduced the germination rate and germination index of cotton seeds (decreased by 20.34% and 32.14% for Tahe 2 and Xinluzhong 62, respectively), leading to decreased seedling height and biomass and causing leaf yellowing. Salt stress induced osmotic stress in seedlings, resulting in ion imbalance (marked reduction in K+/Na+ ratio) and oxidative damage. Under salt stress conditions, exogenous GABA increased the germination rate (increased by 10.64~23.40% and 2.63~31.58% for Tahe 2 and Xinluzhong 62, respectively) and germination index of cotton seeds, as well as plant height and biomass. GABA treatment improved leaf yellowing. Exogenous GABA treatment increased the content of proline and soluble sugars, with varying effects on betaine. Exogenous GABA treatment reduced the Na+ content in seedlings, increased the K+ content, and increased the K+/Na+ ratio (increased by 20.44~28.08% and 29.54~76.33% for Tahe 2 and Xinluzhong 62, respectively). Exogenous GABA treatment enhanced the activities of superoxide dismutase and peroxidase, and reduced the accumulation of hydrogen peroxide and malondialdehyde, but had a negative impact on catalase activity. In conclusion, exogenous GABA effectively improved cotton seed germination. By regulating osmoprotectant levels, maintaining ion homeostasis, and alleviating oxidative stress, GABA mitigated the adverse effects of salt stress on cotton seedling growth.

Role of Acetic Acid and Nitric Oxide against Salinity and Lithium Stress in Canola (Brassica napus L.)

In this study, canola (Brassica napus L.) seedlings were treated with individual and combined salinity and lithium (Li) stress, with and without acetic acid (AA) or nitric acid (NO), to investigate their possible roles against these stresses. Salinity intensified Li-induced damage, and the principal component analysis revealed that this was primarily driven by increased oxidative stress, deregulation of sodium and potassium accumulation, and an imbalance in tissue water content. However, pretreatment with AA and NO prompted growth, re-established sodium and potassium homeostasis, and enhanced the defense system against oxidative and nitrosative damage by triggering the antioxidant capacity. Combined stress negatively impacted phenylalanine ammonia lyase activity, affecting flavonoids, carotenoids, and anthocyanin levels, which were then restored in canola plants primed with AA and NO. Additionally, AA and NO helped to maintain osmotic balance by increasing trehalose and proline levels and upregulating signaling molecules such as hydrogen sulfide, γ-aminobutyric acid, and salicylic acid. Both AA and NO improved Li detoxification by increasing phytochelatins and metallothioneins, and reducing glutathione contents. Comparatively, AA exerted more effective protection against the detrimental effects of combined stress than NO. Our findings offer novel perspectives on the impacts of combining salt and Li stress.

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.

Glutamate decarboxylase-1 is essential for efficient acclimation of Arabidopsis thaliana to nutritional phosphorus deprivation

Glutamate decarboxylase (GAD) is a Ca2+ -calmodulin-activated, cytosolic enzyme that produces γ-aminobutyrate (GABA) as the committed step of the GABA shunt. This pathway bypasses the 2-oxoglutarate to succinate reactions of the tricarboxylic acid (TCA) cycle. GABA also accumulates during many plant stresses. We tested the hypothesis that AtGAD1 (At5G17330) facilitates Arabidopsis acclimation to Pi deprivation. Quantitative RT-PCR and immunoblotting revealed that AtGAD1 transcript and protein expression is primarily root-specific, but inducible at lower levels in shoots of Pi-deprived (-Pi) plants. Pi deprivation reduced levels of the 2-oxoglutarate dehydrogenase (2-OGDH) cofactor thiamine diphosphate (ThDP) in shoots and roots by > 50%. Growth of -Pi atgad1 T-DNA mutants was significantly attenuated relative to wild-type plants. This was accompanied by: (i) an > 60% increase in shoot and root GABA levels of -Pi wild-type, but not atgad1 plants, and (ii) markedly elevated anthocyanin and reduced free and total Pi levels in leaves of -Pi atgad1 plants. Treatment with 10 mM GABA reversed the deleterious development of -Pi atgad1 plants. Our results indicate that AtGAD1 mediates GABA shunt upregulation during Pi deprivation. This bypass is hypothesized to circumvent ThDP-limited 2-OGDH activity to facilitate TCA cycle flux and respiration by -Pi Arabidopsis.

The proteome landscape of the root cap reveals a role for the jacalin-associated lectin JAL10 in the salt-induced endoplasmic reticulum stress pathway

Rapid climate change has led to enhanced soil salinity, one of the major determinants of land degradation, resulting in low agricultural productivity. This has a strong negative impact on food security and environmental sustainability. Plants display various physiological, developmental, and cellular responses to deal with salt stress. Recent studies have highlighted the root cap as the primary stress sensor and revealed its crucial role in halotropism. The root cap covers the primary root meristem and is the first cell type to sense and respond to soil salinity, relaying the signal to neighboring cell types. However, it remains unclear how root-cap cells perceive salt stress and contribute to the salt-stress response. Here, we performed a root-cap cell-specific proteomics study to identify changes in the proteome caused by salt stress. The study revealed a very specific salt-stress response pattern in root-cap cells compared with non-root-cap cells and identified several novel proteins unique to the root cap. Root-cap-specific protein-protein interaction (PPI) networks derived by superimposing proteomics data onto known global PPI networks revealed that the endoplasmic reticulum (ER) stress pathway is specifically activated in root-cap cells upon salt stress. Importantly, we identified root-cap-specific jacalin-associated lectins (JALs) expressed in response to salt stress. A JAL10-GFP fusion protein was shown to be localized to the ER. Analysis of jal10 mutants indicated a role for JAL10 in regulating the ER stress pathway in response to salt. Taken together, our findings highlight the participation of specific root-cap proteins in salt-stress response pathways. Furthermore, root-cap-specific JAL proteins and their role in the salt-mediated ER stress pathway open a new avenue for exploring tolerance mechanisms and devising better strategies to increase plant salinity tolerance and enhance agricultural productivity.

Gamma-Aminobutyric Acid Accumulation Contributes to Citrus sinensis Response against 'Candidatus Liberibacter Asiaticus' via Modulation of Multiple Metabolic Pathways and Redox Status

Huanglongbing (HLB; also known as citrus greening) is the most destructive bacterial disease of citrus worldwide with no known sustainable cure yet. Herein, we used non-targeted metabolomics and transcriptomics to prove that γ-aminobutyric acid (GABA) accumulation might influence the homeostasis of several metabolic pathways, as well as antioxidant defense machinery, and their metabolism-related genes. Overall, 41 metabolites were detected in 'Valencia' sweet orange (Citrus sinensis) leaf extract including 19 proteinogenic amino acids (PAA), 10 organic acids, 5 fatty acids, and 9 other amines (four phenolic amines and three non-PAA). Exogenous GABA application increased most PAA in healthy (except L-threonine, L-glutamine, L-glutamic acid, and L-methionine) and 'Candidatus L. asiaticus'-infected citrus plants (with no exception). Moreover, GABA accumulation significantly induced L-tryptophan, L-phenylalanine, and α-linolenic acid, the main precursors of auxins, salicylic acid (SA), and jasmonic acid (JA), respectively. Furthermore, GABA supplementation upregulated most, if not all, of amino acids, phenolic amines, phytohormone metabolism-related, and GABA shunt-associated genes in both healthy and 'Ca. L. asiaticus'-infected leaves. Moreover, although 'Ca. L. asiaticus' induced the accumulation of H2O2 and O2•- and generated strong oxidative stress in infected leaves, GABA possibly stimulates the activation of a multilayered antioxidative system to neutralize the deleterious effect of reactive oxygen species (ROS) and maintain redox status within infected leaves. This complex system comprises two major components: (i) the enzymatic antioxidant defense machinery (six POXs, four SODs, and CAT) that serves as the front line in antioxidant defenses, and (ii) the non-enzymatic antioxidant defense machinery (phenolic acids and phenolic amines) that works as a second defense line against 'Ca. L. asiaticus'-induced ROS in citrus infected leaves. Collectively, our findings suggest that GABA might be a promising alternative eco-friendly strategy that helps citrus trees battle HLB particularly, and other diseases in general.

Role of Gamma-Aminobutyric Acid in Plant Defense Response

Gamma-aminobutyric acid (GABA) is a four-carbon non-protein amino acid that acts as a defense substance and a signaling molecule in various physiological processes, and which helps plants respond to biotic and abiotic stresses. This review focuses on the role of GABA's synthetic and metabolic pathways in regulating primary plant metabolism, redistributing carbon and nitrogen resources, reducing the accumulation of reactive oxygen species, and improving plants' tolerance of oxidative stress. This review also highlights the way in which GABA maintains intracellular pH homeostasis by acting as a buffer and activating H⁺-ATPase. In addition, calcium signals participate in the accumulation process of GABA under stress. Moreover, GABA also transmits calcium signals through receptors to trigger downstream signaling cascades. In conclusion, understanding the role of GABA in this defense response provides a theoretical basis for applying GABA in agriculture and forestry and feasible coping strategies for plants in complex and changeable environments.

Leaf sodium homeostasis controlled by salt gland is associated with salt tolerance in mangrove plant Avicennia marina

Avicennia marina, a mangrove plant growing in coastal wetland habitats, is frequently affected by tidal salinity. To understand its salinity tolerance, the seedlings of A. marina were treated with 0, 200, 400 and 600 mM NaCl. We found the whole-plant dry weight and photosynthetic parameters increased at 200 mM NaCl but decreased over 400 mM NaCl. The maximum quantum yield of primary photochemistry (Fv/Fm) significantly decreased at 600 mM NaCl. Transmission electron microscopy observations showed high salinity caused the reduction in starch grain size, swelling of the thylakoids and separation of the granal stacks, and even destruction of the envelope. In addition, the dense protoplasm and abundant mitochondria in the secretory and stalk cells, and abundant plasmodesmata between salt gland cells were observed in the salt glands of the adaxial epidermis. At all salinities, Na+ content was higher in leaves than in stems and roots; however, Na+ content increased in the roots while it remained at a constant level in the leaves over 400 mM NaCl treatment, due to salt secretion from the salt glands. As a result, salt crystals on the leaf adaxial surface increased with salinity. On the other hand, salt treatment increased Na+ and K+ efflux and decreased H+ efflux from the salt glands by the non-invasive micro-test technology, although Na+ efflux reached the maximum at 400 mM NaCl. Further real-time quantitative PCR analysis indicated that the expression of Na+/H+ antiporter (SOS1 and NHX1), H+-ATPase (AHA1 and VHA-c1) and K+ channel (AKT1, HAK5 and GORK) were up-regulated, and only the only Na+ inward transporter (HKT1) was down-regulated in the salt glands enriched adaxial epidermis of the leaves under 400 mM NaCl treatment. In conclusion, salinity below 200 mM NaCl was beneficial to the growth of A. marina, and below 400 mM, the salt glands could excrete Na+ effectively, thus improving its salt tolerance.

Comprehensive analysis of physiological and metabolomic responses to drought reveals specific modulation of acquired tolerance mechanisms in rice

Mild stresses induce "acquired tolerance traits" (ATTs) that provide tolerance when stress becomes severe. Here, we identified the genetic variability in ATTs among a panel of rice germplasm accessions and demonstrated their relevance in protecting growth and productivity under water-limited conditions. Diverse approaches, including physiological screens, association mapping and metabolomics, were adopted and revealed 43 significant marker-trait associations. Nontargeted metabolomic profiling of contrasting genotypes revealed 26 "tolerance-related-induced" primary and secondary metabolites in the tolerant genotypes (AC-39000 and AC-39020) compared to the susceptible one (BPT-5204) under water-limited condition. Metabolites that help maintain cellular functions, especially Calvin cycle processes, significantly accumulated more in tolerant genotypes, which resulted in superior photosynthetic capacity and hence water use efficiency. Upregulation of the glutathione cycle intermediates explains the ROS homeostasis among the tolerant genotypes, maintaining spikelet fertility, and grain yield under stress. Bioinformatic dissection of a major effect quantitative trait locus on chromosome 8 revealed genes controlling metabolic pathways leading to the production of osmolites and antioxidants, such as GABA and raffinose. The study also led to the identification of specific trait donor genotypes that can be effectively used in translational crop improvement activities.

Exogenous γ-aminobutyric acid (GABA) mitigated salinity-induced impairments in mungbean plants by regulating their nitrogen metabolism and antioxidant potential

BACKGROUND

Increasing soil salinization has a detrimental effect on agricultural productivity.Therefore, strategies are needed to induce salinity-tolerance in crop species for sustainable foodproduction. γ-aminobutyric acid (GABA) plays a key role in regulating plant salinity stresstolerance. However, it remains largely unknown how mungbean plants (Vigna radiata L.) respondto exogenous GABA under salinity stress.

METHODS

Thus, we evaluated the effect of exogenous GABA (1.5 mM) on the growth and physiobiochemicalresponse mechanism of mungbean plants to saline stress (0-, 50-, and 100 mM [NaCland Na2SO4, at a 1:1 molar ratio]).

RESULTS

Increased saline stress adversely affected mungbean plants' growth and metabolism. Forinstance, leaf-stem-root biomass (34- and 56%, 31- and 53%, and 27- and 56% under 50- and 100mM, respectively]) and chlorophyll concentrations declined. The carotenoid level increased (10%)at 50 mM and remained unaffected at 100 mM. Hydrogen peroxide (H2O2), malondialdehyde(MDA), osmolytes (soluble sugars, soluble proteins, proline), total phenolic content, andenzymatic activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), peroxidase(POD), glutathione reductase (GTR), and polyphenol oxidation (PPO) were significantlyincreased. In leaves, salinity caused a significant increase in Na+ concentration but a decrease inK+ concentration, resulting in a low K+/Na+ concentration (51- and 71% under 50- and 100- mMstress). Additionally, nitrogen concentration and the activities of nitrate reductase (NR) andglutamine synthetase (GS) decreased significantly. The reduction in glutamate synthase (GOGAT)activity was only significant (65%) at 100 mM stress. Exogenous GABA decreased Na+, H2O2,and MDA concentrations but enhanced photosynthetic pigments, K+ and K+/Na+ ratio, Nmetabolism, osmolytes, and enzymatic antioxidant activities, thus reducing salinity-associatedstress damages, resulting in improved growth and biomass.

CONCLUSION

Exogenous GABA may have improved the salinity tolerance of mungbean plants by maintaining their morpho-physiological responses and reducing the accumulation of harmfulsubstances under salinity. Future molecular studies can contribute to a better understanding of themolecular mechanisms by which GABA regulates mungbean salinity tolerance.

Phytohormones regulate the abiotic stress: An overview of physiological, biochemical, and molecular responses in horticultural crops

Recent changing patterns of global climate have turned out to be a severe hazard to the horticulture crops production. A wide range of biotic and abiotic stresses often affect plants due to their sessile nature. Horticultural crop losses are mainly caused by abiotic factors such as drought, salt, heat, cold, floods, and ultraviolet radiation. For coping up with these adversities, well-developed mechanisms have been evolved in plants, which play a role in perceiving stress signals and enabling optimal growth responses. Interestingly, the use of phytohormones for suppressing the impact of abiotic stress has gained much attention in recent decades. For circumvention of stress at various levels, including physiological, molecular, as well as biochemical, a sophisticated mechanism is reported to be provided by the phytohormones, thus labeling these phytohormones a significant role in plant growth and development. Phytohormones can improves tolerance against abiotic stresses by increasing seed germination, seedling growth, leaf photosynthesis, root growth, and antioxidant enzymes and reducing the accumulation of reactive oxygen species, malonaldehyde, and electrolyte leakage. Recent discoveries highlight the significant role of a variety of phytohormones including melatonin (MEL), Gamma-aminobutyric acid (GABA), jasmonic acid (JA), salicylic acid (SA), brassinosteroids (BRs), and strigolactones (SLs) in abiotic stress tolerance enhancement of horticultural plants. Thus, current review is aimed to summarize the developmental concepts regarding role of phytohormones in abiotic-stress mitigation, mainly in horticultural crops, along with the description of recent studies which identified the role of different phytohormones in stressed environments. Hence, such a review will help in paving the path for sustainable agriculture growth via involvement of phytohormones in enhancement of abiotic stress tolerance of horticultural crops.

Transcriptomic and functional characterization reveals CsHAK5;3 as a key player in K+ homeostasis in grafted cucumbers under saline conditions

Grafting can improve the salt tolerance of many crops. However, critical genes in scions responsive to rootstock under salt stress remain a mystery. We found that pumpkin rootstock decreased the content of Na⁺ by 70.24 %, increased the content of K⁺ by 25.9 %, and increased the K⁺/Na⁺ ratio by 366.0 % in cucumber scion leaves. RNA-seq analysis showed that ion transport-related genes were the key genes involved in salt stress tolerance in grafted cucumber. The identification and analysis of the expression of K⁺ transporter proteins in cucumber and pumpkin revealed six and five HAK5 members, respectively. The expression of CsHAK5;3 in cucumber was elevated in different graft combinations under salt stress and most notably in cucumber scion/pumpkin rootstock. CsHAK5;3 was localized to the plasma membrane, and a yeast complementation assay revealed that it can transport K⁺. CsHAK5;3 knockout in hairy root mutants decreased the K⁺ content of leaves (45.6 %) and roots (50.3 %), increased the Na⁺ content of leaves (29.3 %) and roots (34.8 %), and decreased the K⁺/Na⁺ ratio of the leaves (57.9 %) and roots (62.9 %) in cucumber. However, CsHAK5;3 overexpression in hairy roots increased the K⁺ content of the leaves (31.2 %) and roots (38.3 %), decreased the Na⁺ content of leaves (17.2 %) and roots (14.3 %), and increased the K⁺/Na⁺ ratio of leaves (58.9 %) and roots (61.6 %) in cucumber. In conclusion, CsHAK5;3 in cucumber can mediate K⁺ transport and is one of the key target pumpkin genes that enhance salt tolerance of cucumber grafted.

Interaction between selenium and essential micronutrient elements in plants: A systematic review

Nutrient imbalance (i.e., deficiency and toxicity) of microelements is an outstanding environmental issue that influences each aspect of ecosystems. Although the crucial roles of microelements in entire lifecycle of plants have been widely acknowledged, the effective control of microelements is still neglected due to the narrow safe margins. Selenium (Se) is an essential element for humans and animals. Although it is not believed to be indispensable for plants, many literatures have reported the significance of Se in terms of the uptake, accumulation, and detoxification of essential microelements in plants. However, most papers only concerned on the antagonistic effect of Se on metal elements in plants and ignored the underlying mechanisms. There is still a lack of systematic review articles to summarize the comprehensive knowledge on the connections between Se and microelements in plants. In this review, we conclude the bidirectional effects of Se on micronutrients in plants, including iron, zinc, copper, manganese, nickel, molybdenum, sodium, chlorine, and boron. The regulatory mechanisms of Se on these micronutrients are also analyzed. Moreover, we further emphasize the role of Se in alleviating element toxicity and adjusting the concentration of micronutrients in plants by altering the soil conditions (e.g., adsorption, pH, and organic matter), promoting microbial activity, participating in vital physiological and metabolic processes, generating element competition, stimulating metal chelation, organelle compartmentalization, and sequestration, improving the antioxidant defense system, and controlling related genes involved in transportation and tolerance. Based on the current understanding of the interaction between Se and these essential elements, future directions for research are suggested.

The mechanistic basis of sodium exclusion in Puccinellia tenuiflora under conditions of salinity and potassium deprivation

Soil salinity is a significant threat to global agriculture. Understanding salt exclusion mechanisms in halophyte species may be instrumental in improving salt tolerance in crops. Puccinellia tenuiflora is a typical salt-excluding halophytic grass often found in potassium-deprived saline soils. Our previous work showed that P. tenuiflora possesses stronger selectivity for K⁺ than for Na⁺ ; however, the mechanistic basis of this phenomenon remained elusive. Here, P. tenuiflora PutHKT1;5 was cloned and the functions of PutHKT1;5 and PutSOS1 were characterized using heterologous expression systems. Yeast assays showed that PutHKT1;5 possessed Na⁺ transporting capacity and was highly selective for Na⁺ over K⁺ . PutSOS1 was located at the plasma membrane and operated as a Na⁺ /K⁺ exchanger, with much stronger Na⁺ extrusion capacity than its homolog from Arabidopsis. PutHKT2;1 mediated high-affinity K⁺ and Na⁺ uptake and its expression levels were upregulated by mild salinity and K⁺ deprivation. Salinity-induced changes of root PutHKT1;5 and PutHKT1;4 transcript levels matched the expression pattern of root PutSOS1, which was consistent with root Na⁺ efflux. The transcript levels of root PutHKT2;1 and PutAKT1 were downregulated by salinity. Taken together, these findings demonstrate that the functional activity of PutHKT1;5 and PutSOS1 in P. tenuiflora roots is fine-tuned under saline conditions as well as by operation of other ion transporters/channel (PutHKT1;4, PutHKT2;1, and PutAKT1). This leads to the coordination of radial Na⁺ and K⁺ transport processes, their loading to the xylem, or Na⁺ retrieval and extrusion under conditions of mild salinity and/or K⁺ deprivation.

CML8 and GAD4 function in (Z)-3-hexenol-mediated defense by regulating γ-aminobutyric acid accumulation in Arabidopsis

(Z)-3-hexenol, a small gaseous molecule, is produced in plants under biotic stress and induces defense responses in neighboring plants. However, little is known about how (Z)-3-hexenol induces plant defense-related signaling. In this study, we uncovered how (Z)-3-hexenol treatment enhances plant resistance to insect attacks by increasing γ-aminobutyric acid (GABA) contents in Arabidopsis leaves. First, (Z)-3-hexenol increases the intracellular content of calcium as secondary messenger in Arabidopsis leaf mesophyll cells. Both intracellular and extracellular calcium stores regulate changes in calcium content. Then, CML8 and GAD4 transmit calcium signaling to affect (Z)-3-hexenol induced GABA content and plant resistance. Herein, CML8 interaction with GAD4 was examined via yeast two-hybrid assays, firefly luciferase complementation imaging, and GST pull-down assays. These results indicate that (Z)-3-hexenol treatment increased the GABA contents in Arabidopsis leaves based on CML8 and GAD4, thus increasing plant resistance to the insect Plutella xylostella. This study revealed the mechanism of activating plant insect defense induced by (Z)-3-hexenol, which guides the study of volatiles as biological pest control.

Improved osmotic stress tolerance in brewer's yeast induced by wheat gluten peptides

The influences of three wheat gluten peptides (WGP-LL, WGP-LML, and WGP-LLL) on the osmotic stress tolerance and membrane lipid component in brewer's yeast were investigated. The results demonstrated that the growth and survival of yeast under osmotic stress were enhanced by WGP supplementation. The addition of WGP upregulated the expressions of OLE1 (encoded the delta-9 fatty acid desaturase) and ERG1 (encoded squalene epoxidase) genes under osmotic stress. At the same time, WGP addition enhanced palmitoleic acid (C16:1) content, unsaturated fatty acids/saturated fatty acids ratio, and the amount of ergosterol in yeast cells under osmotic stress. Furthermore, yeast cells in WGP-LL and WGP-LLL groups were more resistant to osmotic stress. WGP-LL and WGP-LLL addition caused 25.08% and 27.02% increase in membrane fluidity, 22.36% and 29.54% reduction in membrane permeability, 18.38% and 14.26% rise in membrane integrity in yeast cells, respectively. In addition, scanning electron microscopy analysis revealed that the addition of WGP was capable of maintaining yeast cell morphology and reducing cell membrane damage under osmotic stress. Thus, alteration of membrane lipid component by WGP was an effective approach for increasing the growth and survival of yeast cells under osmotic stress. KEY POINTS: •WGP addition enhanced cell growth and survival of yeast under osmotic stress. •WGP addition increased unsaturated fatty acids and ergosterol contents in yeast. •WGP supplementation improved membrane homeostasis in yeast at osmotic stress.

Evaluation of salt tolerance of oat cultivars and the mechanism of adaptation to salinity

Soil salinity is a threat to agricultural production worldwide. Oat (Avena sativa L.) is an irreplaceable crop in areas with fragile ecological conditions. However, there is a lack of research on salt tolerance evaluation of oat germplasm resources. Therefore, the purpose of this work was to evaluate the salt tolerance of oat cultivars and investigate the mechanism of salt-tolerant oat cultivars' adaptation to salinity. Salt tolerance of 100 oat cultivars was evaluated, and then two salt-tolerant cultivars and two salt-sensitive cultivars were used to compare their physiological responses and expression patterns of Na⁺- and K⁺-transport-related genes under salinity. Principal component analysis and membership function analysis had good predictability for salt tolerance evaluation of oat and other crops. The 100 oat cultivars were clustered into three categories, with three salt tolerance levels. Under saline condition, salt-tolerant cultivars maintained higher growth rate, leaf cell membrane integrity, and osmotic adjustment capability via enhancing the activities of antioxidant enzymes and accumulating more osmotic regulators. Furthermore, salt-tolerant cultivars had stronger capability to restrict root Na ⁺ uptake through reducing AsAKT1 and AsHKT2;1 expression, exclude more Na⁺ from root through increasing AsSOS1 expression, compartmentalize more Na ⁺ into root vacuoles through increasing AsNHX1 and AsVATP-P1 expression, and absorb more K⁺ through increasing AsKUP1 expression, compared with salt-sensitive cultivars. The evaluation procedure developed in this work can be applied for screening cereal crop cultivars with higher salt tolerance, and the elucidated mechanism of oat adaptation to salinity lays a foundation for identifying more functional genes related to salt tolerance.

Characterization of GABA-Transaminase Gene from Mulberry (Morus multicaulis) and Its Role in Salt Stress Tolerance

Gamma-aminobutyric acid (GABA) has been reported to accumulate in plants when subjected to salt stress, and GABA-transaminase (GABA-T) is the main GABA-degrading enzyme in the GABA shunt pathway. So far, the salt tolerance mechanism of the GABA-T gene behind the GABA metabolism remains unclear. In this study, the cDNA (designated MuGABA-T) of GABA-T gene was cloned from mulberry, and our data showed that MuGABA-T protein shares some conserved characteristics with its homologs from several plant species. MuGABA-T gene was constitutively expressed at different levels in mulberry tissues, and was induced substantially by NaCl, ABA and SA. In addition, our results demonstrated that exogenous application of GABA significantly reduced the salt damage index and increased plant resistance to NaCl stress. We further performed a functional analysis of MuGABA-T gene and demonstrated that the content of GABA was reduced in the transgenic MuGABA-T Arabidopsis plants, which accumulated more ROS and exhibited more sensitivity to salt stress than wild-type plants. However, exogenous application of GABA significantly increased the activities of antioxidant enzymes and alleviated the active oxygen-related injury of the transgenic plants under NaCl stress. Moreover, the MuGABA-T gene was overexpressed in the mulberry hairy roots, and similar results were obtained for sensitivity to salt stress in the transgenic mulberry plants. Our results suggest that the MuGABA-T gene plays a pivotal role in GABA catabolism and is responsible for a decrease in salt tolerance, and it may be involved in the ROS pathway in the response to salt stress. Taken together, the information provided here is helpful for further analysis of the function of GABA-T genes, and may promote mulberry resistance breeding in the future.

Potassium in plants: Growth regulation, signaling, and environmental stress tolerance

Potassium (K) is an essential element for the growth and development of plants; however, its scarcity or excessive level leads to distortion of numerous functions in plants. It takes part in the control of various significant functions in plant advancement. Because of the importance index, K is regarded second after nitrogen for whole plant growth. Approximately, higher than 60 enzymes are reliant on K for activation within the plant system, in which K plays a vital function as a regulator. Potassium provides assistance in plants against abiotic stress conditions in the environment. With this background, the present paper reviews the physiological functions of K in plants like stomatal regulation, photosynthesis and water uptake. The article also focuses upon the uptake and transport mechanisms of K along with its role in detoxification of reactive oxygen species and in conferring tolerance to plants against abiotic stresses. It also highlights the research progress made in the direction of K mediated signaling cascades.

Rewilding staple crops for the lost halophytism: Toward sustainability and profitability of agricultural production systems

Abiotic stress tolerance has been weakened during the domestication of all major staple crops. Soil salinity is a major environmental constraint that impacts over half of the world population; however, given the increasing reliance on irrigation and the lack of available freshwater, agriculture in the 21st century will increasingly become saline. Therefore, global food security is critically dependent on the ability of plant breeders to create high-yielding staple crop varieties that will incorporate salinity tolerance traits and account for future climate scenarios. Previously, we have argued that the current agricultural practices and reliance on crops that exclude salt from uptake is counterproductive and environmentally unsustainable, and thus called for a need for a major shift in a breeding paradigm to incorporate some halophytic traits that were present in wild relatives but were lost in modern crops during domestication. In this review, we provide a comprehensive physiological and molecular analysis of the key traits conferring crop halophytism, such as vacuolar Na⁺ sequestration, ROS desensitization, succulence, metabolic photosynthetic switch, and salt deposition in trichomes, and discuss the strategies for incorporating them into elite germplasm, to address a pressing issue of boosting plant salinity tolerance.

The emerging role of GABA as a transport regulator and physiological signal

While the proposal that γ-aminobutyric acid (GABA) acts a signal in plants is decades old, a signaling mode of action for plant GABA has been unveiled only relatively recently. Here, we review the recent research that demonstrates how GABA regulates anion transport through aluminum-activated malate transporters (ALMTs) and speculation that GABA also targets other proteins. The ALMT family of anion channels modulates multiple physiological processes in plants, with many members still to be characterized, opening up the possibility that GABA has broad regulatory roles in plants. We focus on the role of GABA in regulating pollen tube growth and stomatal pore aperture, and we speculate on its role in long-distance signaling and how it might be involved in cross talk with hormonal signals. We show that in barley (Hordeum vulgare), guard cell opening is regulated by GABA, as it is in Arabidopsis (Arabidopsis thaliana), to regulate water use efficiency, which impacts drought tolerance. We also discuss the links between glutamate and GABA in generating signals in plants, particularly related to pollen tube growth, wounding, and long-distance electrical signaling, and explore potential interactions of GABA signals with hormones, such as abscisic acid, jasmonic acid, and ethylene. We conclude by postulating that GABA encodes a signal that links plant primary metabolism to physiological status to fine tune plant responses to the environment.

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

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

Electrical Signaling of Plants under Abiotic Stressors: Transmission of Stimulus-Specific Information

Plants have developed complex systems of perception and signaling to adapt to changing environmental conditions. Electrical signaling is one of the most promising candidates for the regulatory mechanisms of the systemic functional response under the local action of various stimuli. Long-distance electrical signals of plants, such as action potential (AP), variation potential (VP), and systemic potential (SP), show specificities to types of inducing stimuli. The systemic response induced by a long-distance electrical signal, representing a change in the activity of a complex of molecular-physiological processes, includes a nonspecific component and a stimulus-specific component. This review discusses possible mechanisms for transmitting information about the nature of the stimulus and the formation of a specific systemic response with the participation of electrical signals induced by various abiotic factors.

γ-Aminobutyrate (GABA) Regulated Plant Defense: Mechanisms and Opportunities

Global climate change and associated adverse abiotic and biotic stress conditions affect plant growth and development, and agricultural sustainability in general. Abiotic and biotic stresses reduce respiration and associated energy generation in mitochondria, resulting in the elevated production of reactive oxygen species (ROS), which are employed to transmit cellular signaling information in response to the changing conditions. Excessive ROS accumulation can contribute to cell damage and death. Production of the non-protein amino acid γ-aminobutyrate (GABA) is also stimulated, resulting in partial restoration of respiratory processes and energy production. Accumulated GABA can bind directly to the aluminum-activated malate transporter and the guard cell outward rectifying K+ channel, thereby improving drought and hypoxia tolerance, respectively. Genetic manipulation of GABA metabolism and receptors, respectively, reveal positive relationships between GABA levels and abiotic/biotic stress tolerance, and between malate efflux from the root and heavy metal tolerance. The application of exogenous GABA is associated with lower ROS levels, enhanced membrane stability, changes in the levels of non-enzymatic and enzymatic antioxidants, and crosstalk among phytohormones. Exogenous GABA may be an effective and sustainable tolerance strategy against multiple stresses under field conditions.

Potassium physiology from Archean to Holocene: A higher-plant perspective

In this paper, we discuss biological potassium acquisition and utilization processes over an evolutionary timescale, with emphasis on modern vascular plants. The quintessential osmotic and electrical functions of the K⁺ ion are shown to be intimately tied to K⁺-transport systems and membrane energization. Several prominent themes in plant K⁺-transport physiology are explored in greater detail, including: (1) channel mediated K⁺ acquisition by roots at low external [K⁺]; (2) K⁺ loading of root xylem elements by active transport; (3) variations on the theme of K⁺ efflux from root cells to the extracellular environment; (4) the veracity and utility of the "affinity" concept in relation to transport systems. We close with a discussion of the importance of plant-potassium relations to our human world, and current trends in potassium nutrition from farm to table.

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

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

The versatile GABA in plants

Gamma-aminobutyric acid (GABA) is a ubiquitous four-carbon, non-protein amino acid. GABA has been widely studied in animal central nervous systems, where it acts as an inhibitory neurotransmitter. In plants, it is metabolized through the GABA shunt pathway, a bypass of the tricarboxylic acid (TCA) cycle. Additionally, it can be synthesized through the polyamine metabolic pathway. GABA acts as a signal in Agrobacterium tumefaciens-mediated plant gene transformation and in plant development, especially in pollen tube elongation (to enter the ovule), root growth, fruit ripening, and seed germination. It is accumulated during plant responses to environmental stresses and pathogen and insect attacks. A high concentration of GABA elevates plant stress tolerance by improving photosynthesis, inhibiting reactive oxygen species (ROS) generation, activating antioxidant enzymes, and regulating stomatal opening in drought stress. The transporters of GABA in plants are reviewed in this work. We summarize the recent research on GABA function and transporters with the goal of providing a review of GABA in plants.

Improving ATPase and PPase activities, nutrient uptake and growth of salt stressed ajowan plants by salicylic acid and iron-oxide nanoparticles

Salicylic acid and iron-oxide nanoparticles alleviated salt toxicity and improved plant growth by stimulating the activities of H⁺-ATPase and H⁺-PPase and preventing nutrient imbalance. Two factorial experiments were undertaken in a greenhouse during 2018 and 2019, to evaluate the impacts of SA (1 mM) and nano-Fe₂O₃ (3 mM) sprays at 7 leaves and flowering stages on vacuolar H⁺-pumps, growth and essential oil of salt-subjected (0, 4, 8 and 12 dS m^-1 NaCl) ajowan plants. Measurements of plant traits were started at about 12 days after the last foliar spray and continued up to maturity. The H⁺-ATPase and H⁺-PPase activities and root ATP content were enhanced under low salinity, but higher salinities reduced these parameters. Rising salinity enhanced Na uptake and translocation, endogenous SA and DPPH activity, while reduced K⁺/Na⁺ ratio and nutrients uptake, leading to a reduction in plant biomass. Treatment with SA, nano-Fe₂O₃ and their combination improved H⁺-pumps activities and ATP content in roots and leaves. The SA-related treatments caused the highest activities of H⁺-pumps in roots, but Fe-related treatments resulted in the highest activities of these pumps in leaves. Increasing H⁺-pumps activities reduced sodium uptake and translocation and enhanced nutrients uptake. Foliar treatments, especially SA + nano-Fe₂O₃ augmented endogenous SA, DPPH activity, and plant growth in salt-stressed plants. Essential oil contents of vegetative and inflorescence organs under severe salinity and seeds under moderate and severe salinities were enhanced. Maximum essential oil was obtained from seeds of SA + nano-Fe₂O₃-treated plants, which was strongly correlated with endogenous SA and DPPH. Nevertheless, the SA + nano-Fe₂O₃ was the best treatment for diminishing salt toxicity and improving ajowan plant growth and essential oil production.

Exogenous γ-aminobutyric acid (GABA)-induced signaling events and field performance associated with mitigation of drought stress in Phaseolus vulgaris L

Not much information is available to substantiate the possible role of γ -aminobutyric acid (GABA) signaling in mitigating water-deficit stress in snap bean (Phaseolus vulgaris L.) plants under semiarid conditions. Present work aims to investigate the role of exogenous GABA (foliar application; 0.5, 1 and 2 mM) in amelioration of drought stress and improvement of field performance on snap bean plants raised under two drip irrigation regimes (100% and 70% of water requirements). Water stress led to significant reduction in plant growth, leaf relative water content (RWC), cell membrane stability index (CMSI), nutrient uptake (N, P, K, Ca, Fe and Zn), pod yield and its content from protein and total soluble solids (TSS). Meanwhile, lipid peroxidation (malondialdehyde content- MDA), osmolyte content (free amino acids- FAA, proline, soluble sugars) antioxidative defense (activity of superoxide dismutase- SOD, catalase- CAT, peroxidase- POX and ascorbate peroxidase- APX) and the pod fiber content exhibited significantly increase due to water stress. Exogenous GABA application (especially at 2 mM) revealed partial normalization of the effects of drought stress in snap bean plants. GABA-induced mitigation of drought stress was manifested by improvement in growth, water status, membrane integrity, osmotic adjustment, antioxidant defense and nutrient acquisition. Furthermore, GABA application during water stress in snap bean plants resulted in improvement of field performance being manifested by increased pod yield and its quality attributes. To sum up, exogenous GABA appears to function as an effective priming molecule to alleviate drought stress in snap bean plants under semiarid conditions.

Modulation of Ion Transport Across Plant Membranes by Polyamines: Understanding Specific Modes of Action Under Stress

This work critically discusses the direct and indirect effects of natural polyamines and their catabolites such as reactive oxygen species and γ-aminobutyric acid on the activity of key plant ion-transporting proteins such as plasma membrane H+ and Ca2+ ATPases and K+-selective and cation channels in the plasma membrane and tonoplast, in the context of their involvement in stress responses. Docking analysis predicts a distinct binding for putrescine and longer polyamines within the pore of the vacuolar TPC1/SV channel, one of the key determinants of the cell ionic homeostasis and signaling under stress conditions, and an additional site for spermine, which overlaps with the cytosolic regulatory Ca2+-binding site. Several unresolved problems are summarized, including the correct estimates of the subcellular levels of polyamines and their catabolites, their unexplored effects on nucleotide-gated and glutamate receptor channels of cell membranes and Ca2+-permeable and K+-selective channels in the membranes of plant mitochondria and chloroplasts, and pleiotropic mechanisms of polyamines' action on H+ and Ca2+ pumps.

Response Mechanisms of Plants Under Saline-Alkali Stress

As two coexisting abiotic stresses, salt stress and alkali stress have severely restricted the development of global agriculture. Clarifying the plant resistance mechanism and determining how to improve plant tolerance to salt stress and alkali stress have been popular research topics. At present, most related studies have focused mainly on salt stress, and salt-alkali mixed stress studies are relatively scarce. However, in nature, high concentrations of salt and high pH often occur simultaneously, and their synergistic effects can be more harmful to plant growth and development than the effects of either stress alone. Therefore, it is of great practical importance for the sustainable development of agriculture to study plant resistance mechanisms under saline-alkali mixed stress, screen new saline-alkali stress tolerance genes, and explore new plant salt-alkali tolerance strategies. Herein, we summarized how plants actively respond to saline-alkali stress through morphological adaptation, physiological adaptation and molecular regulation.

Exogenous GABA promotes adaptation and growth by altering the carbon and nitrogen metabolic flux in poplar seedlings under low nitrogen conditions

Nitrogen (N) deficiency adversely affects tree growth. Additionally, γ-aminobutyric acid (GABA) is closely associated with growth and stress responses because of its effects on carbon (C) and N metabolism. However, little is known about its roles related to plant adaptations to N-deficient conditions. In this study, we analyzed the effects of GABA (0, 2 and 10 mM) applications on the growth traits and physiological responses of poplar (Populus alba × P. glandulosa '84K') seedlings under high N (HN) and low N (LN) conditions. We found that the added GABA interacted with N to affect more than half of the studied parameters, with greater effects in LN plants than in HN plants. Under LN conditions, the GABA application tended to increase poplar growth, accompanied by increased xylem fiber cell length and xylem width. In stems, exogenous GABA increased the abundance of non-structural carbohydrates (starch and sugars) and tricarboxylic acid cycle intermediates (succinate, malate and citrate), but had the opposite effect on the structural C contents (hemicellulose and lignin). Meanwhile, exogenous GABA increased the total soluble protein contents in leaves and stems, accompanied by significant increases in nitrate reductase, nitrite reductase and glutamine synthetase activities in leaves, but significant decreases in those (except for the increased glutamate synthetase activity) in stems. A multiple factorial analysis indicated that the nitrate assimilation pathway substantially influences poplar survival and growth in the presence of GABA under LN conditions. Interestingly, GABA applications also considerably attenuated the LN-induced increase in the activities of leaf antioxidant enzymes, including peroxidase and catalase, implying that GABA may regulate the relative allocation of C and N for growth activities by decreasing the energy cost associated with stress defense. Our results suggest that GABA enhances poplar growth and adaptation by regulating the C and N metabolic flux under N-deficient conditions.

GABA reverses salt-inhibited photosynthetic and growth responses through its influence on NO-mediated nitrogen-sulfur assimilation and antioxidant system in wheat

Gamma-aminobutyric acid (GABA) is a newly recognized signaling molecule participating in physiological processes, growth, and development of plants under optimal and stressful environments. In the present reported research, we investigated the role of GABA in imparting salt stress tolerance in wheat (Triticum aestivum L.). Exposure of wheat plants to 100 mM NaCl resulted in increased oxidative stress, glucose content, nitric oxide (NO) production together with reduced growth and photosynthetic traits of plants. Contrarily, GABA application improved nitrogen (N) metabolism, sulfur (S) assimilation, ion homeostasis, growth and photosynthesis under salt stress. Additionally, GABA mitigated oxidative stress induced by salt stress with the increased ascorbate-glutathione cycle and proline metabolism. The study with NO inhibitor, c-PTIO [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxy-3-oxide] in GABA experiment suggested that the impact of GABA on improvement of growth and photosynthesis under salt stress was mediated by NO and influenced N and S assimilation and antioxidant systems. The results suggested that the GABA has a significant potential in reversing the salt stress response in wheat plants, and GABA-mediated signals are manifested through NO.

GABA signaling in plants: targeting the missing pieces of the puzzle

The adaptation of plants to unstable environments relies on their ability to sense their surroundings and to generate and transmit corresponding signals to different parts of the plant to evoke changes necessary for optimizing growth and defense. Plants, like animals, contain a huge repertoire of intra- and intercellular signals, including organic and inorganic molecules. The occurrence of neurotransmitter-like signaling molecules in plants has been an intriguing field of research. Among these, γ-aminobutyric acid (GABA) was discovered in plants over half a century ago, and studies of its roles as a primary metabolite have been well documented, particularly in the context of stress responses. In contrast, evidence of the potential mechanism by which GABA acts as a signaling molecule in plants has only recently been reported. In spite of this breakthrough, the roles of GABA as a signaling molecule in plants have yet to be established and several aspects of the complexity of the GABA signaling system remain obscure. This review summarizes the uncertainties in GABA signaling in plants and suggests research directions and technologies that would help in answering unsolved questions.

Gamma-aminobutyric acid (GABA) alleviates salt damage in tomato by modulating Na+ uptake, the GAD gene, amino acid synthesis and reactive oxygen species metabolism

BACKGROUND

Salt stress is a serious abiotic stress that caused crop growth inhibition and yield decline. Previous studies have reported on the the synthesis of gamma-aminobutyric acid (GABA) and its relationship with plant resistance under various abiotic stress. However, the relationship between exogenous GABA alleviating plant salt stress damage and ion flux, amino acid synthesis, and key enzyme expression remains largely unclear. We investigated plant growth, Na⁺ transportation and accumulation, reactive oxygen species (ROS) metabolism and evaluated the effect of GABA on amino acids, especially SlGADs gene expression and the endogenous GABA content of tomato (Solanum lycopersicum L.) seedlings treated with or without 5 mmol·L^- 1 GABA under 175 mmol·L^- 1 NaCl stress.

RESULTS

Exogenous application of GABA significantly reduced the salt damage index and increased plant height, chlorophyll content and the dry and fresh weights of tomato plants exposed to NaCl stress. GABA significantly reduced Na⁺ accumulation in leaves and roots by preventing Na⁺ influx in roots and transportation to leaves. The transcriptional expression of SlGAD1-3 genes were induced by NaCl stress especially with GABA application. Among them, SlGAD1 expression was the most sensitive and contributed the most to the increase in glutamate decarboxylase (GAD) activity induced by NaCl and GABA application; Exogenous GABA increased GAD activity and amino acid contents in tomato leaves compared with the levels under NaCl stress alone, especially the levels of endogenous GABA, proline, glutamate and eight other amino acids. These results indicated that SlGADs transcriptional expression played an important role in tomato plant resistance to NaCl stress with GABA application by enhancing GAD activity and amino acid contents. GABA significantly alleviated the active oxygen-related injury of leaves under NaCl stress by increasing the activities of antioxidant enzymes and decreasing the contents of active oxygen species and malondialdehyde.

CONCLUSION

Exogenous GABA had a positive effect on the resistance of tomato seedlings to salt stress, which was closely associated with reducing Na⁺ flux from root to leaves, increasing amino acid content and strengthening antioxidant metabolism. Endogenous GABA content was induced by salt and exogenous GABA at both the transcriptional and metabolic levels.

Halotropism: Phytohormonal Aspects and Potential Applications

Halotropism is a sodium specific tropic movement of roots in order to obtain the optimal salt concentration for proper growth and development. Numerous results suggest that halotropic events are under the control and regulation of complex plant hormone pathway. This minireview collects some recent evidences about sodium sensing during halotropism and the hormonal regulation of halotropic responses in glycophytes. The precise hormonal mechanisms by which halophytes plant roots perceive salt stress and translate this perception into adaptive, directional growth forward increased salt concentrations are not well understood. This minireview aims to gather recently deciphered information about halotropism focusing potential hormonal aspects both in glycophytes and halophytes. Advances in our understanding of halotropic responses in different plant species could help these plants to be used for sustainable agriculture and other future applications.