GABA Enhances Muskmelon Chloroplast Antioxidants to Defense Salinity-Alkalinity Stress

Salinity-alkalinity stress is a pivotal factor influencing plant growth, development, and yield. γ-Aminobutyric acid (GABA) protects plants against a variety of environmental stresses. However, it is remains largely unknown whether exogenous GABA increases the tolerance of Cucumis melon L. seedlings via effects on the chloroplast antioxidant system. In this study, the role of exogenous GABA application on the malondialdehyde content and antioxidant enzyme activities and the ascorbate-glutathione (AsA-GSH) cycle in seedlings of muskmelon was investigated. Plants were treated with foliar spraying of GABA (50 mM) under control or salinity-alkalinity stress conditions. Salinity-alkalinity stress induced cellular membrane damage. Treatment with GABA protected muskmelon seedlings from salinity-alkalinity stress by enhancing antioxidant enzyme activity and reducing malondialdehyde content. These effects of GABA resulted in maintenance of the membrane integrity of the muskmelon seedling. In addition, the status of both GSH and AsA redox played key roles in the regulation of the oxidative stress response in muskmelon seedlings under salinity-alkalinity stress.     

Gamma-amino butyric acid, glutamate dehydrogenase and glutamate decarboxylase levels in phylogenetically divergent plants

Gamma-amino butyric acid (GABA) is a nonprotein amino acid found in a wide range of organisms including plants. Several studies have shown that GABA plays different roles in plant metabolism including carbon–nitrogen metabolism, energy balance, signaling and development. It has been suggested that the occurrence of GABA and the enzymes related to GABA biosynthesis in prokaryotes and eukaryotes may be important in evolution and diversification. However, studies of GABA biosynthesis and GABA levels in an evolutionary context are restricted to sequenced plant genomes. In this study we aimed to compare the activities of GDH and GAD enzymes and total nitrogen, and the contents of total soluble protein, succinate, glutamate, proline and GABA in plants from different phylogenetic levels including Ulva lactuca, Pseudevernia furfuracea, Nephrolepsis exaltata, Ginkgo biloba, Pinus pinea, Magnolia grandiflora, Nymphaea alba, Urtica dioica, Portulaca oleraceae, Malva sylvestris, Rosa canina, Lavandula stoechas, Washingtonia filifera, Avena barbata and Iris kaempferi. The activities of GAD and GDH enzymes differed according to the species and were not always parallel to GABA levels. The discrepancy in the contents of succinate and GABA between higher and primitive plants was also prominent. Glutamate levels were high with a few exceptions and proline contents were at similar low values as compared to other amino acids. Our results support the hypothesis that the GABA shunt plays a key role in carbon and nitrogen partitioning via linking amino acid metabolism and the tricarboxylic acid cycle which is essential for higher plant species.     

Contribution of Gamma amino butyric acid (GABA) to salt stress responses of Nicotiana sylvestris CMSII mutant and wild type plants

Plants accumulate high levels of Gamma amino butyric acid (GABA) in response to different environmental stresses and GABA metabolism has different functions such as osmotic and pH regulation, bypass of tricarboxylic acid cycle, and C:N balance. The cytoplasmic male sterile (CMS) II mutant of Nicotiana sylvestris has a deletion in the mitochondrial gene nad7 which encodes the NAD7 subunit of complex I which causes increased leaf respiration, impaired photosynthesis, slower growth and increased amino acid levels. In this study we aimed to elucidate the role of GABA and GABA metabolism in different genotypes of the same plant system under salt stress (100mM NaCl) in short (24h) and long (7, 14 and 21 days) terms. We have investigated the differences in leaf fresh and dry weights, relative water content, photosynthetic efficiency (F(v)/F(m)), glutamate dehydrogenase (GDH, EC 1.4.1.4) and glutamate decarboxylase (GAD, EC 4.1.1.15) enzyme activities, GABA content and GAD gene expression profiles. GDH activity showed variations in CMSII and wild type (WT) plants in the first 24h. GAD gene expression profiles were in good agreement with the GAD enzyme activity levels in CMSII and WT plants after 24h. In long-term salinity, GAD activities increased in WT but, decreased in CMSII. GABA accumulation in WT and CMSII plants in short and long term was induced by salt stress. Variations in GDH and GAD activities in relation to GABA levels were discussed and GABA metabolism has been proposed to be involved in better performance of CMSII plants under long term salinity.     

Spermine Pre-Treatment Improves Some Physiochemical Parameters and Sodium Transporter Gene Expression of Pumpkin Seedlings under Salt Stress

It is known that polyamines (PAs) including spermine (Spm) enhance the abiotic stress tolerance of crops. Here, the effects of hydroponic Spm pre-treatment on the amelioration of the adverse effects of salinity were investigated in pumpkin (Cucurbita pepo L.) that is an economically important horticultural crop and sensitive to salinity, especially at the establishment stage. For this purpose, 10-day-old, uniform-sized seedlings were transplanted to plastic containers containing Hoagland nutrient solution. Spm was added at 0.1 and 1 mM to the hydroponic medium for 5 days before stress. The plants were treated with 40 and 80 mM NaCl for inducing salinity stress. Salt stress reduced the plant growth and potassium content in roots, and these detrimental effects were alleviated when plants were pre-treated with Spm. Salinity stress caused a significant increase in sodium and γ-amino butyric acid (GABA) content when compared with controls. Spm pre-treatment ameliorated these salinity stress effects by increasing sodium content of root and leaves GABA content. Expression analysis of two sodium transporter genes, salt overly sensitive1 (SOS1) and Na⁺/H⁺ exchanger (NHX1) revealed that their expression was differentially induced in roots of plants treated either with salinity or Spm. These results suggest that Spm via the overexpression of the NHX1 gene substantially increased the tolerance to stress by sequestering excess Na+ into the vacuoles and sustaining a better cellular environment. Moreover, Spm has potential to scavenge directly free radical and to alleviate growth inhibition and promote the activity of antioxidant system enzymes in pumpkin seedlings under salt stress.     

Putative role of γ -aminobutyric acid (GABA) as a long-distance signal in up-regulation of nitrate uptake in Brassica napus L.

The relationship between nitrate influx, BnNrt2 nitrate transporter gene expression and amino acid composition of phloem exudate was investigated during N‐deprivation (short‐term experiment) and over a growth cycle (long‐term experiment) in Brassica napus L. The data showed a positive correlation between γ‐aminobutyric acid (GABA) in phloem exudate and nitrate uptake in the short‐ and the long‐term experiments. The hypothesis that this non‐protein amino acid could up‐regulate nitrate uptake via a long‐distance signalling pathway was tested by providing an exogenous GABA supply to the roots. The effect of GABA was compared with the effects of Gln, Glu and Asn, each known to be inhibitors of nitrate uptake. The results showed that GABA treatment induced a significant increase of BnNrt2 mRNA expression, but had less effect on nitrate influx. By contrast, Gln, Glu and Asn significantly reduced nitrate influx and BnNrt2 mRNA expression compared with the control plants. This study provides the first evidence that GABA may act as a putative long‐distance inter‐organ signal molecule in plants in conjunction with negative control exerted by Gln. The up‐regulation effect of GABA on nitrate uptake is discussed in the context of its role in N metabolism, nutritional stress and the recent discovery of a putative role of GABA as a signal molecule in plant development.     

An Appraisal of Ancient Molecule GABA in Abiotic Stress Tolerance in Plants,and Its Crosstalk with Other Signaling Molecules

Gamma-aminobutyric acid (GABA), a non-proteinaceous amino acid, is reported in prokaryotes and eukaryotes, since ancient times. However, it has gained attention in the present time because of its rapid accumulation during stressed conditions in plants as well as in the cyanobacteria. In plants, it regulates the number of physiological processes such as pollen tube growth, root growth, TCA cycle, N₂-metabolism, and osmoregulation. Several biotic and abiotic stresses prevail in the environment, which lead to enhanced accumulation of reactive oxygen species (ROS) thus causing oxidative damage. However, a rapid increase in the accumulation of GABA during stress in various plant forms like bacteria, cyanobacteria, fungi, and plants indicates its putative role in stress regulation and acclimation. This review summarizes the biosynthesis of GABA, its role in abiotic stress tolerance, and its crosstalk with ROS, nitric oxide, Ca⁺² ions, phytohormones, and polyamines in stress acclimation.

     

Higher accumulation of gamma-aminobutyric acid induced by salt stress through stimulating the activity of diamine oxidases in Glycine max (L.) Merr. roots.

Polyamines (PAs) are assumed to perform their functions through their oxidative product such as gamma-aminobutyric acid (GABA) formation. However, there is only limited information on the interrelation between PA degradation and GABA accumulation under salt stress. In order to reveal a quantitative correlation between PA oxidation and GABA accumulation, the effects of treatments with different NaCl concentrations, along with aminoguanidine (AG, a specific inhibitor of diamine oxidases (DAO; EC: 1.4.3.6)) and a recovery test from salt stress on endogenous free PAs, gamma-aminobutyric acid (GABA) accumulation and DAO activity were determined in roots of soybean [Glycine max (L.) Merr.] cultivar Suxie-1. The results showed that the levels of putrescine (Put), cadaverine (Cad), and spermidine (Spd) decreased significantly with increasing salt concentrations. This occurred because salt stress strongly promoted DAO activity to stimulate PA degradation. GABA accumulation increased with growing NaCl concentrations, about an 11- to 17-fold increase as compared with the control plants. AG treatment increased the accumulation of endogenous free PAs as a result of a strong retardation of DAO activity, but decreased GABA accumulation. The recovery for 6 days in 1/2 Hoagland solution from 100mM NaCl stress resulted in a decrease in DAO activity, a rebound of PA levels and a simultaneous reduction of GABA content. A close correlation was observed between the changes in DAO activity and GABA accumulation. The results indicated that higher GABA accumulation (about 39%) induced by salt stress could come from PA degradation, suggesting that PAs might perform their functions through GABA formation under salt stress.     

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.

γ-Aminobutyric acid (GABA) encodes a plant signal that links primary metabolism to physiological status to fine tune plant responses to the environment.     

Changes in γ-aminobutyric acid concentration,gas exchange,and leaf anatomy in <Emphasis Type="Italic">Eucalyptus</Emphasis> clones under drought stress and rewatering

Drought stress promotes biochemical and physiological alterations in plant metabolism that limit growth and yield. This study investigated the accumulation of γ-aminobutyric acid (GABA) in plant tissue, the stomatal conductance (gs) and changes in leaf anatomy in Eucalyptus following drought stress situation. In this study, eight Eucalyptus clones were evaluated under normal water supply (control) and drought stress conditions (stress). For the control treatment, plants were irrigated every day with an automated system until the soil was saturated, and for the stress treatment, drought stress was imposed by non-irrigation of plants, and pots were covered using plastic sheeting to avoid rainfall and humidity. This study has shown that: (1) all clones decreased gs with increasing vapor pressure deficit (D) in both treatments. All plastics and drought-tolerant clones (except GG) presented lower stomatal sensitivity to D under stress conditions than drought-sensitive clones; (2) GABA concentrations increased fast after drought stress, but we could not find correlation with these changes and resistance to water stress; and (3) all clones increased the number of stomata and reduced leaf thickness after water stress. The finding is that GABA is a fast stress-signaling molecule in Eucalyptus, but the response of gs to D is a best physiological variable to differentiate drought-tolerant and drought-sensitive Eucalyptus clones.     

Nitric oxide, γ‐aminobutyric acid,and mannose pretreatment influence metabolic profiles in white clover under water stress

Nitric oxide (NO), γ‐aminobutyric acid (GABA), and mannose (MAS) could be important regulators of plant growth and adaptation to water stress. The application of sodium nitroprusside (SNP, a NO donor), GABA, and MAS improved plant growth under water‐sufficient conditions and effectively mitigated water stress damage to white clover. The metabonomic analysis showed that both SNP and GABA application resulted in a significant increase in myo‐inositol content; the accumulation of mannose was commonly regulated by SNP and MAS; GABA and MAS induced the accumulation of aspartic acid, quinic acid, trehalose, and glycerol under water deficit. In addition, citric acid was uniquely up‐regulated by SNP associated with tricarboxylic acid (TCA) cycle under water stress. GABA specially induced the accumulation of GABA, glycine, methionine, and aconitic acid related to GABA shunt, amino acids metabolism, and TCA cycle in response to water stress. MAS uniquely enhanced the accumulation of asparagine, galactose, and D‐pinitol in association with amino acids and sugars metabolism under water stress. SNP‐, GABA‐, and MAS‐induced changes of metabolic profiles and associated metabolic pathways could contribute to enhanced stress tolerance via involvement in the TCA cycle for energy supply, osmotic adjustment, antioxidant defense, and signal transduction for stress defense in white clover.     

Nitric oxide,g-aminobutyric acid,and mannose pretreatment influence metabolic profiles in white clover under water stress

Nitric oxide(NO), g-aminobutyric acid(GABA),and mannose(MAS) could be important regulators of plant growth and adaptation to water stress. The application of sodium nitroprusside(SNP, a NO donor),GABA, and MAS improved plant growth under watersufficient conditions and effectively mitigated water stress damage to white clover. The metabonomic analysis showed that both SNP and GABA application resulted in a significant increase in myo-inositol content;the accumulation of mannose was commonly regulated by SNP and MAS; GABA and MAS induced the accumulation of aspartic acid, quinic acid, trehalose,and glycerol under water deficit. In addition, citric acid was uniquely up-regulated by SNP associated with tricarboxylic acid(TCA) cycle under water stress. GABAspecially induced the accumulation of GABA, glycine,methionine, and aconitic acid related to GABA shunt,amino acids metabolism, and TCA cycle in response to water stress. MAS uniquely enhanced the accumulation of asparagine, galactose, and D-pinitol in association with amino acids and sugars metabolism under water stress. SNP-, GABA-, and MAS-induced changes of metabolic profiles and associated metabolic pathways could contribute to enhanced stress tolerance via involvement in the TCA cycle for energy supply, osmotic adjustment, antioxidant defense, and signal transduction for stress defense in white clover.     

Metabolic pathways regulated by abscisic acid,salicylic acid and γ‐aminobutyric acid in association with improved drought tolerance in creeping bentgrass (Agrostis stolonifera)

Abscisic acid (ABA), salicylic acid (SA) and γ‐aminobutyric acid (GABA) are known to play roles in regulating plant stress responses. This study was conducted to determine metabolites and associated pathways regulated by ABA, SA and GABA that could contribute to drought tolerance in creeping bentgrass (Agrostis stolonifera). Plants were foliar sprayed with ABA (5 μM), GABA (0.5 mM) and SA (10 μM) or water (untreated control) prior to 25 days drought stress in controlled growth chambers. Application of ABA, GABA or SA had similar positive effects on alleviating drought damages, as manifested by the maintenance of lower electrolyte leakage and greater relative water content in leaves of treated plants relative to the untreated control. Metabolic profiling showed that ABA, GABA and SA induced differential metabolic changes under drought stress. ABA mainly promoted the accumulation of organic acids associated with tricarboxylic acid cycle (aconitic acid, succinic acid, lactic acid and malic acid). SA strongly stimulated the accumulation of amino acids (proline, serine, threonine and alanine) and carbohydrates (glucose, mannose, fructose and cellobiose). GABA enhanced the accumulation of amino acids (GABA, glycine, valine, proline, 5‐oxoproline, serine, threonine, aspartic acid and glutamic acid) and organic acids (malic acid, lactic acid, gluconic acid, malonic acid and ribonic acid). The enhanced drought tolerance could be mainly due to the enhanced respiration metabolism by ABA, amino acids and carbohydrates involved in osmotic adjustment (OA) and energy metabolism by SA, and amino acid metabolism related to OA and stress‐defense secondary metabolism by GABA.     

Aquaporin genes GintAQPF1 and GintAQPF2 from Glomus intraradices contribute to plant drought tolerance

Arbuscular mycorrhizal (AM) symbiosis, established between AM fungi (AMF) and roots of higher plants, occurs in most terrestrial ecosystems. It has been well demonstrated that AM symbiosis can improve plant performance under various environmental stresses, including drought stress. However, the molecular basis for the direct involvement of AMF in plant drought tolerance has not yet been established. Most recently, we cloned two functional aquaporin genes, GintAQPF1 and GintAQPF2, from AM fungus Glomus intraradices. By heterologous gene expression in yeast, aquaporin localization, activities and water permeability were examined. Gene expressions during symbiosis in expose to drought stress were also analyzed. Our data strongly supported potential water transport via AMF to host plants. As a complement, here we adopted the monoxenic culture system for AMF, in which carrot roots transformed by Ri-T DNA were cultured with Glomus intraradices in two-compartment Petri dishes, to verify the aquaporin gene functions in assisting AMF survival under polyethylene glycol (PEG) treatment. Our results showed that 25% PEG significantly upregulated the expression of two aquaporin genes, which was in line with the gene functions examined in yeast. We therefore concluded that the aquaporins function similarly in AMF as in yeast subjected to osmotic stress. The study provided further evidence to the direct involvement of AMF in improving plant water relations under drought stresses.     

Thiamine in plants: Aspects of its metabolism and functions

Thiamine diphosphate (vitamin B₁) plays a fundamental role as an enzymatic cofactor in universal metabolic pathways including glycolysis, the pentose phosphate pathway, and the tricarboxylic acid cycle. In addition, thiamine diphosphate has recently been shown to have functions other than as a cofactor in response to abiotic and biotic stress in plants. Recently, several steps of the plant thiamine biosynthetic pathway have been characterized, and a mechanism of feedback regulation of thiamine biosynthesis via riboswitch has been unraveled. This review focuses on these most recent advances made in our understanding of thiamine metabolism and functions in plants. Phenotypes of plant mutants affected in thiamine biosynthesis are described, and genomics, proteomics, and metabolomics data that have increased further our knowledge of plant thiamine metabolic pathways and functions are summarized. Aspects of thiamine metabolism such as catabolism, salvage, and transport in plants are discussed.     

Gamma Aminobutyric Acid (GABA) and Plant Responses to Stress

4-aminobutyrate (GABA) is a non-protein amino acid that is widely distributed throughout the biological world. In animals, GABA functions as the predominant inhibitory neurotransmitter in the central nervous system by acting through the GABA receptors. The neuromuscular system enables animals to escape from environmental stresses. Being nonmotile, plants have evolved chemical responses to mitigate stress. Mechanisms by which GABA may facilitate these responses are discussed in this review. Environmental stresses increase GABA accumulation through two different mechanisms. Stresses causing metabolic and/or mechanical disruptions, resulting in cytosolic acidification, induce an acidic pH-dependent activation of glutamate decarboxylase and GABA synthesis. Extremely marked declines in cytosolic pH occur under oxygen deprivation, which is the primary stress factor in flooded soils, and this stress induces the greatest accumulation of GABA. Other stresses, including cold, heat, salt, and mild or transient environmental factors, such as touch, wind, rain, etc. rapidly increase cellular levels of Ca²⁺. Increased cytosolic Ca²⁺ stimulates calmodulin-dependent glutamate decarboxylase activity and GABA synthesis. A review of the kinetics of GABA accumulation in plants reveals a stress-specific pattern of accumulation that is consistent with a physiological role for GABA in stress mitigation. Recent physiological and genetic evidence indicates that plants may possess GAB A-like receptors that have features in common with the animal receptors. The mechanism of action of animal GABA receptors suggests a model for rapid amplification of ion-mediated signals and GABA accumulation in response to stress. Metabolic pathways that link GABA to stress-related metabolism and plant hormones are identified. The survival value of stress-related metabolism is dependent on metabolic changes occurring before stress causes irreversible damage to plant tissue. Rapid accumulation of GABA in stressed tissue may provide a critical link in the chain of events leading from perception of environmental stresses to timely physiological responses.     

Is GABA-shunt functional in endodormant grapevine buds under respiratory stress?

It has been suggested that a respiratory stress is part of the mechanism through which the dormancy-breaking compounds, hydrogen cyanamide (HC) and sodium azide, induce the release of buds from the endodormancy (ED) in grapevines. The accumulation of metabolites like succinate, alanine (Ala) and γ-amino butyric acid (GABA), together with the activation of the GABA-shunt pathway, is a general feature of plants in response to oxygen deprivation and to respiratory stress. Unexpectedly, in a previous study, we found that GABA applied exogenously to grapevine buds, down-regulated the expression of most genes encoding for antioxidant enzymes, suggesting that its accumulation under respiratory stress conditions could be deleterious for the bud. In order to analyze whether GABA accumulates under respiratory stress conditions in grapevine buds, we analysed in this study, the effect of hypoxia, the respiration inhibitor KCN and the dormancy breaker compound HC, on the level of GABA, and on the expression levels of the GABA-shunt genes (VvGAD, VvGABA-T, VvSSADH). Additionally, genes from the Ala fermentative pathway (VvAlaAT, VvAspAT) were also analysed. The results revealed that although the three treatments mentioned above, up-regulated the expression of VvGAD1, the content of GABA remained constant, while Ala content increased. The lack of GABA accumulation under respiratory stress is an important physiological fact in grapevine buds, since it avoids the down-regulation of antioxidant genes, and promotes the incorporation of succinate into the TCA cycle, a fact that would be important in the release of buds from the ED.     

外源γ-氨基丁酸（GABA）对低氧胁迫下甜瓜幼苗根系GABA代谢及氨基酸含量的影响

采用水培法,通过准确控制营养液溶氧浓度,研究了外源γ-氨基丁酸（GABA）对低氧胁迫0~8 d ‘西域一号’甜瓜幼苗根系GABA代谢及氨基酸含量的影响.结果表明：与通气对照相比,低氧处理的甜瓜幼苗正常生长受到严重抑制,其根系谷氨酸脱羧酶（GAD）、谷氨酸脱氢酶（GDH）、谷氨酸合成酶（GOGAT）、谷氨酰胺合成酶（GS）、丙氨酸氨基转移酶（ALT）、天门冬氨酸氨基转移酶（AST）活性以及GABA、丙酮酸、丙氨酸、天冬氨酸含量均显著提高,而谷氨酸和α 酮戊二酸含量在处理4~8 d均显著降低.与低氧处理相比,外源GABA处理有效缓解了低氧胁迫对幼苗根系生长的抑制作用,同时甜瓜根系内源GABA、谷氨酸、α-酮戊二酸、天冬氨酸含量显著提高,但GAD、GDH、GOGAT、GS、ALT、AST活性在整个处理过程中均显著降低,丙酮酸和丙氨酸含量也显著降低.低氧同时添加GABA和γ-乙烯基 γ-氨基丁酸（VGB）处理显著降低了低氧胁迫下GABA的缓解效应.低氧胁迫下外源GABA被植物根系吸收后,通过反馈抑制GAD活性维持较高的Glu含量,保持植物体内碳、氮代谢平衡,维持正常生理代谢,从而缓解低氧胁迫对甜瓜幼苗的伤害.     

Does GABA Act as a Signal in Plants?: Hints from Molecular Studies

GABA is a non-protein amino acid that accumulates rapidly in plant tissues in response to biotic and abiotic stress. There have been a number of suggestions as to the role that GABA might play in plants, ranging from a straightforward involvement in N metabolism to a signal mediating plant-animal and plant-microbe interactions. It has also been several proposed that it might function as an intracellular signalling molecule in plants. Here, we discuss recent evidence that plant cells respond at the molecular level to the presence of applied GABA. We argue that these data might serve as the basis for investigating the possible signalling role for GABA in plant development and stress responses in more detail.Key Words: 14-3-3 proteins, GABA, signalling, gene expression, stress, senescenceGABA (γ-aminobutyric acid), which comprises a significant fraction of the free amino acid pool in plant cells, was first identified in potato tubers over half a century ago, but its functions remained obscure for many years. In animal systems, GABA is present at high levels in the brain where it acts as an important neurotransmitter. GABA is synthesised in a pathway known as the GABA shunt, which operates not only in the animals, but in bacteria, fungi and plants too.^1,2 The function of GABA in plants has attracted renewed attention in the last decade following the discovery that intracellular and/or extracellular GABA concentrations increase rapidly in response to a range of stresses. Subsequently, a number of possible roles for GABA and the GABA shunt in plants have been suggested.^1,2 These include acting as a buffering mechanism in C and N metabolism, cytosolic pH regulation, protection against oxidative stress and defence against herbivorous pests. It has been proposed recently that one common function of GABA might be to mediate interactions between plants and other organisms, including bacterial and fungal pathogens, nematodes and insect pests.³ On the other hand, because of the rapid increases in GABA concentration in response to stress, it has sometimes been inferred that GABA might act as an intracellular signalling molecule in plants. However, few studies have adopted molecular approaches to GABA function, and molecular responses are largely unknown.     

Gamma-aminobutyric acid mediates the nicotine biosynthesis in tobacco under flooding stress

Gamma-aminobutyric acid (GABA) is a four-carbon non-protein amino acid conserved from bacteria to plants and vertebrates. Increasing evidence supports a regulatory role for GABA in plant development and the plant′s response to environmental stress. The biosynthesis of nicotine, the main economically important metabolite in tobacco, is tightly regulated. GABA has not hitherto been reported to function in nicotine biosynthesis. Here we found that water flooding treatment (hypoxia) markedly induced the accumulation of GABA and stimulated nicotine biosynthesis. Suppressing GABA accumulation by treatment with glutamate decarboxylase inhibitor impaired flooding-induced nicotine biosynthesis, while exogenous GABA application directly induced nicotine biosynthesis. Based on these results, we propose that GABA triggers nicotine biosynthesis in tobacco seedlings subjected to flooding. Our results provide insight into the molecular mechanism of nicotine biosynthesis in tobacco plants exposed to environmental stress.