GABA in Plants E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
A comprehensive overview of the role played by GABA as a signaling molecule in plants
In GABA in Plants: Biosynthesis, Plant Development, and Food Security, the editors deliver an expertly balanced discussion of the role played by GABA as a signaling molecule in plants, plant development, stress acclimation, as well as its potential impact on crop productivity under changing environmental conditions.
From explorations of the discovery of GABA in plants to presentations of GABA biosynthesis pathways, GABA crosstalk with other metabolites, and GABA’s role in programmed cell death in plants, this book is an essential treatment of a four-carbon signaling molecule that may yet prove pivotal in sustaining crop production in the face of climate change.
Readers will also find:
- A thorough introduction to GABA and its involvement in nodulation in and wounding stress in plants
- Comprehensive explorations of plant stress responses and tolerance mechanisms
- Practical discussions of GABA priming induced modulations in the redox homeostasis of plants under osmotic stress
- Complete treatments of GABA and heat, oxidative, cold, bacterial, mediated salt, and chilling stressors
Perfect for students and scientists working in plant biology and physiology, crop protection, food security, nutrition, and biotechnology, GABA in Plants will also benefit professionals working in the agricultural, food, and pharmaceutical industries.
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Seitenzahl: 821
Veröffentlichungsjahr: 2025
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Table of Contents
Cover
Table of Contents
Title Page
Copyright Page
Contributors
Preface
1 Discovery and Background of GABA in Plants
Abbreviations
Introduction
History
Background
The GABA Metabolic Pathway in Plants
Structure and Conformation of GABA
Roles and Functions of GABA
Conclusion
References
2 GABA Biosynthesis Pathways and its Signaling in Plants
GABA Production and Degradation Pathway
Is GABA Only a Metabolite?
Role of the GABA Shunt in Plants
Crosstalk of GABA with Other Signaling Molecules
Conclusion and Future Prospects
References
3 GABA and Its Crosstalk with Other Metabolites in Relation to Abiotic Stress Responses in Plants
Introduction
Enzymes in GABA Metabolic Pathways
Role of GABA Under Stressful Conditions in Plants
GABA and Salt Stress
GABA and Drought Stress
GABA and Chilling Stress
Crosstalk of GABA with Other Metabolites and Chemicals
GABA with H
2
O
2
GABA with Nitric Oxide (NO)
GABA with Calcium
Interplay of GABA with Plant Hormones
GABA with Auxin
GABA with Abscisic Acid
GABA with Ethylene
Mechanisms of Action of GABA in Plants Under Stress
Conclusions and Future Perspectives
References
4 GABA as a Signaling Molecule in Plants
Abbreviations
Introduction
GABA in Plants as a Stress Response
GABA’s Role in Mediating Pathogen and Herbivore Attack Stress‐Induced Responses in Plants
GABA Signaling in Plant Growth and Development
GABA‐Mediated Regulation of Stomatal Aperture in Plants
GABA in Ion‐Exchange Regulation
Conclusion and Future Prospectives
References
5 GABA and Drought Stress
Introduction
GABA Shunt in Plants
GABA Accumulates in Plants Under Drought Stress
GABA Accumulation Increases Drought Tolerance
GABA Signaling and the Regulation of Stomatal Opening
Conclusion
References
6 The Role of GABA on Programmed Cell Death and Senescence in Plants
Introduction
GABA Pathways
The Roles of GABA under Stress Conditions
GABA as a Signal Molecule
GABA‐Mediated Avoidance from PCD
The Role of GABA on Leaf Senescence
Conclusions
References
7 GABA and Nodulation in Plants
Introduction
Nodulation in Leguminous Plants
Functioning of γ‐Aminobutyric Acid in Plants
Functioning of GABA in Nodulation
Conclusions and Future Prospects
References
8 GABA and Wounding Stress in Plants
Introduction
GABA: An Important Molecule for Plant
GABA and Abiotic Stress
Biotic Stress and Wound‐Mediated GABA Fluctuation
Transgenic Plants Expressing GABA and Effect on Herbivorous Performance
References
9 GABA in Plant Stress Response and Tolerance Mechanisms
Introduction
Abiotic Stress and GABA
Biotic Stress and GABA
Conclusion and Future Prospects
References
10 GABA Priming Induced Modulations in the Redox Homeostasis of Plants under Osmotic Stress
Introduction
Role of GABA in Plants
GABA Priming and Oxidative Stress Mitigation
Conclusion and Future Prospects
Acknowledgments
References
11 Gamma‐Aminobutyric acid‐Mediated Heavy Metal Stress Tolerance in Plants
Introduction
Health Benefits of GABA
Biosynthesis of GABA
GABA Transport in Plants
Role of GABA in Abiotic Stress Tolerance
Conclusion
Acknowledgments
References
12 GABA and Heat Stress
Introduction
GABA‐Biosynthesis and Transport/Pathways in Plant
GABA Morphological and Physiological Functions within Plants
GABA and Abiotic Stress
Conclusion
References
13 GABA and Oxidative Stress and the Regulation of Antioxidants
Introduction
Types and Characteristics of ROS
ROS Generation in Plants under Normal and Stress Conditions
The Importance of ROS Compartmentation for Plant Stress Adaptation
Antioxidant Defense System in Plants
Relationships of GABA Shunt and ROS during Stress Conditions
The Response of GABA under Abiotic Stress Conditions
Conclusion
References
14 GABA in Relation to Cold and Chilling Stress
Introduction
Plant Strategies to Overcome Cold Stress
γ‐Aminobutyric Acid (GABA)
Response Strategies of GABA in Cold Stress Tolerance
Future Perspectives, Challenges, and Conclusion
References
15 Role of GABA Under Bacterial Stress in Plants
Introduction
GABA and Biotic Stress in Plants
GABA and Bacterial Stress Response in Plants
Molecular Basis of GABA Accumulation in Response to Bacterial Pathogens
The Involvement of GABA in the Interaction of Microbes with Plants
Conclusions and Future Perspectives
References
16 GABA‐Mediated Salt Stress Tolerance Through Physiological and Molecular Mechanisms
Introduction
Concept of Salt Stress to Plants
GABA and Salinity Stress Tolerance
Molecular Changes Associated with GABA‐Induced Salinity Stress Tolerance
Conclusion
Acknowledgments
References
17 GABA and Nutrient Deficiency
Introduction
An Overview of GABA
Role of GABA in Plant Development
Role of GABA in Different Stress Tolerance
Different Mineral Nutrients and Their Role in Plant Development
Different Nutrient Deficiencies in Plants
Role of GABA in Nutrient Deficiency
Concluding Remarks
References
18 GABA and Plant‐Derived Therapeutics
List of Abbreviations
Introduction
Plants with Reported GABAergic Activity: A Novel Source of Therapeutics
Conclusion and Future Perspective
References
Index
End User License Agreement
List of Tables
Chapter 3
Table 3.1 Crops showing stress response alone and in application with GABA ...
Chapter 11
Table 11.1 Role of GABA in plants.
Chapter 12
Table 12.1 GABA modifies and develops various mechanisms for improving heat...
Chapter 14
Table 14.1 Role of exogenous GABA treatments in alleviating chilling stress...
Chapter 15
Table 15.1 Bacterial species that can induce alterations in host GABA metab...
Chapter 16
Table 16.1 Recent works related to the effects of GABA in plants under sali...
Chapter 17
Table 17.1 Response of exogenously applied GABA under different nutrient de...
Chapter 18
Table 18.1 List of plants as therapeutic strategies.
List of Illustrations
Chapter 1
Figure 1.1 Metabolic synthesis of GABA.
Figure 1.2 Structure of GABA.
Figure 1.3 Role of GABA.
Chapter 2
Figure 2.1 GABA biosynthesis and catabolism pathways in plants. Abbreviation...
Chapter 3
Figure 3.1 Schematic representation of plants under various stressors and in...
Chapter 4
Figure 4.1 Biotechnological methods and fundamental abiotic stress to protec...
Figure 4.2 Activation of GABA during drought‐induced stress in plants.
Figure 4.3 GABA’s role in response to salinity‐induced stress in plants.
Figure 4.4 Schematic representation of GABA degrades a chemical OC8‐HSL.
Figure 4.5 Chemical structures of (a) GABA (gamma‐aminobutyric acid), (b) GH...
Figure 4.6 Schematic representation of the hydrolytic enzyme α‐amylase degra...
Chapter 6
Figure 6.1 GABA pathway model in
Arabidopsis
(Meng 2023).
Figure 6.2 Relationship between GABA shunt and ROS mechanism during leaf sen...
Chapter 7
Figure 7.1 Diagrammatic representation showing a symbiotic association betwe...
Figure 7.2 Diagrammatic illustration of the proposed metabolic routes of γ‐a...
Chapter 9
Figure 9.1 A general illustration of GABA shunt taking place in the cytosol ...
Figure 9.2 General aspects of abiotic and biotic stress tolerance interactin...
Chapter 10
Figure 10.1 The mechanism of osmotic stress alleviation through GABA priming...
Figure 10.2 The morphological response of GABA‐primed plants.
Chapter 11
Figure 11.1 GABA biosynthesis in plants is mediated by GABA shunt.
Figure 11.2 Classes of GABA transporters based on substrate specificity.
Figure 11.3 Production of ROS in plant cells is due to the exposure to abiot...
Chapter 12
Figure 12.1 A diagrammatic representation reveals the potential role of GABA...
Chapter 14
Figure 14.1 Schematic illustration of plant adaptation strategies to mitigat...
Chapter 15
Figure 15.1 Different roles of GABA in plants against bacterial infection. G...
Chapter 16
Figure 16.1 Schematic model representing the positive effect of GABA under s...
Figure 16.2 Physiological and molecular mechanisms associated with GABA expo...
Chapter 17
Figure 17.1 Representation of detrimental responses to nutrient deficiency a...
Chapter 18
Figure 18.1 Plant‐derived therapeutics.
Figure 18.2 The GABAergic system in neurotransmitter.
Figure 18.3 Phytochemical constituents with GABAergic properties.
Guide
Cover Page
Title Page
Copyright Page
Contributors
Preface
Table of Contents
Begin Reading
Index
WILEY END USER LICENSE AGREEMENT
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GABA in Plants
Biosynthesis, Plant Development, and Food Security
Edited by
Dr. Samiksha Singh
Department of Botany
S.N. Sen B.V. Post Graduate College
Chhatrapati Shahu Ji Maharaj University
Kanpur, Uttar Pradesh, India
Dr. Durgesh Kumar Tripathi
Amity Institute of Organic Agriculture (AIOA)
Amity University, Uttar Pradesh, India
Dr. Vijay Pratap Singh
Department of Botany
C.M.P. Degree College
University of Allahabad
Prayagraj, Uttar Pradesh, India
This edition first published 2025© 2025 by John Wiley & Sons Ltd
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Contributors
Nader AdamipourDepartment of Horticultural ScienceFaculty of AgricultureUniversity of KurdistanSanandaj, Iran
Fazilet Özlem AlbayrakDepartment of BiologyFaculty of ScienceMersin UniversityMersin, Turkey
Nimisha AmistDepartment of BotanyUniversity of AllahabadPrayagraj, Uttar Pradesh, India
and
Department of BotanyEwing Christian CollegePrayagraj, Uttar Pradesh, India
Koravantakamparambil Sulaiman AnjithaPlant Physiology and Biochemistry DivisionDepartment of BotanyUniversity of CalicutMalappuram, Kerala, India
Zeba AzimDepartment of BotanyEwing Christian CollegeUniversity of AllahabadPrayagraj, Uttar Pradesh, India
and
Plant Physiology LaboratoryDepartment of BotanyUniversity of AllahabadPrayagraj, Uttar Pradesh, India
Melike BorDepartment of BiologyFaculty of ScienceEge UniversityIzmir, Turkey
İrem ÇetinkayaDepartment of BiologyFaculty of ScienceEge UniversityIzmir, Turkey
V B Chandana KumariDepartment of Biotechnology and BioinformaticsJSS Academy of Higher Education and ResearchMysuru, Karnataka, India
Petronia CarilloDepartment of Environmental, Biological and Pharmaceutical Sciences and TechnologiesUniversity of Campania “Luigi Vanvitelli”Caserta, Italy
Andrea CarraCouncil for Agricultural Research and EconomicsResearch Centre for Forestry and WoodCasale Monferrato, Italy
Manjunath DammalliDepartment of BiotechnologySiddaganga Institute of TechnologyTumkur, Karnataka, India
Neeraj Kumar DubeyDepartment of BotanyRashtriya Snatkottar MahavidyalayaJaunpur, Uttar Pradesh, India
Parammal FaseelaDepartment of BotanyKahm Unity Women’s CollegeManjeri, Kerala, India
Nagma FirdoseDepartment of PharmacologyJSS Medical CollegeJSS Academy of Higher Education & ResearchMysuru, Karnataka, India
Bhavya Somaplara GangadharappaDepartment of BiotechnologyM. S. Ramaiah Institute of TechnologyBangalore, Karnataka, India
Srijita GhoshPost Graduate Department of BotanyScottish Church CollegeKolkata, West Bengal, India
Nihal Gören‐SağlamDepartment of BiologyFaculty of ScienceIstanbul UniversityIstanbul, Turkey
Gubbi Vani IshikaDepartment of Biotechnology and BioinformaticsJSS Academy of Higher Education and ResearchMysuru, Karnataka, India
Lakshmi JayaramDepartment of Biotechnology and BioinformaticsJSS Academy of Higher Education and ResearchMysuru, Karnataka, India
Kolothodi Chandran JishaDepartment of BotanyMES Asmabi CollegeThrissur, Kerala, India
Joy Mulakkal JoelPlant Physiology and Biochemistry DivisionDepartment of BotanyUniversity of CalicutMalappuram, Kerala, India
Riya JohnsonPlant Physiology and Biochemistry DivisionDepartment of BotanyUniversity of CalicutMalappuram, Kerala, India
Shubhra KhareDepartment of Applied Sciences and HumanitiesInvertis UniversityBareilly, Uttar Pradesh, India
Emad Hamdy KhedrDepartment of PomologyFaculty of AgricultureCairo UniversityGiza, Egypt
Rangoli KrishnaDepartment of Applied Sciences and HumanitiesInvertis UniversityBareilly, Uttar Pradesh, India
Kuldeep LahryInstitut Régional du Cancer de Montpellier (ICM)Université de MontpellierMontpellier, France
Farzad NazariDepartment of Horticultural ScienceFaculty of AgricultureUniversity of KurdistanSanandaj, Iran
Km NiharikaDepartment of BotanyUniversity of LucknowLucknow, Uttar Pradesh, India
Louis NoblePlant Physiology and Biochemistry DivisionDepartment of BotanyUniversity of CalicutMalappuram, Kerala, India
Tülay ÖztürkDepartment of BiologyFaculty of ScienceEge UniversityIzmir, Turkey
Şerife PalabıyıkDepartment of BiologyFaculty of ScienceEge UniversityIzmir, Turkey
Akhilesh Kumar PandeyDepartment of BiotechnologyFaculty of Bioscience and BiotechnologyInvertis UniversityBareilly, Uttar Pradesh, India
Jogeswar PanigrahiDepartment of BiotechnologyBerhmpur UniversityBhanja, Bihar, Odisha, India
Shashank M. PatilDepartment of Biotechnology and BioinformaticsJSS Academy of Higher Education and ResearchMysuru, Karnataka, India
V. H. PushpaDepartment of PharmacologyJSS Medical CollegeJSS Academy of Higher Education & ResearchMysuru, Karnataka, India
Jos Thomas PuthurPlant Physiology and Biochemistry DivisionDepartment of BotanyUniversity of CalicutMalappuram, Kerala, India
Deepthi PuttegowdaDepartment of Biotechnology and BioinformaticsJSS Academy of Higher Education and ResearchMysuru, Karnataka, India
Ranjith RajDepartment of PharmacologyJSS Medical CollegeJSS Academy of Higher Education and ResearchMysuru, Karnataka, India
Kakkuzhiyulla Parambath Raj AswathiPlant Physiology and Biochemistry DivisionDepartment of BotanyUniversity of CalicutMalappuram, Kerala, India
Ramith RamuDepartment of Biotechnology and BioinformaticsJSS Academy of Higher Education and ResearchMysuru, Karnataka, India
Somayeh RastegarDepartment of Horticultural SciencesFaculty of Agriculture and Natural ResourcesUniversity of HormozganBandar Abbas, Iran
Aryadeep RoychoudhuryDiscipline of Life SciencesSchool of SciencesIndira Gandhi National Open UniversityNew Delhi, India
Nair Gopalakrishnan SarathDepartment of BotanyMar Athanasius CollegeKothamangalam, Kerala, India
Pegah Sayyad‐AminDepartment of Horticultural Science and LandscapeFerdowsi University of MashhadMashhad, Iran
Akhila SenDepartment of BotanyMar Athanasius CollegeKothamangalam, Kerala, India
Ajey SinghDepartment of BotanyUniversity of LucknowLucknow, Uttar Pradesh, India
Amarjeet SinghDepartment of BotanyPost Graduate CollegeGhazipur, Uttar Pradesh, India
Kunwar Deelip SinghDepartment of ZoologyRashtriya Snatkottar MahavidyalayaJaunpur, Uttar Pradesh, India
Narsingh Bahadur SinghDepartment of BotanyEwing Christian CollegeUniversity of AllahabadUttar Pradesh, India
and
Plant Physiology LaboratoryDepartment of BotanyUniversity of AllahabadPrayagraj, Uttar Pradesh, India
Ran Vijay SinghDepartment of ZoologyRashtriya Snatkottar MahavidyalayaJaunpur, Uttar Pradesh, India
Sudhir SinghCentre for Molecular BiologyCentral University of JammuBagla, Jammu and Kashmir, India
Navya SreepathiDepartment of Biotechnology and Bioinformatics and Department of PharmacologyJSS Academy of Higher Education and ResearchMysuru, Karnataka, India
Nishtha SrivastavaDepartment of Applied Sciences & HumanitiesFaculty of Applied SciencesInvertis UniversityBareilly, Uttar Pradesh, India
Jaime A. Teixeira da SilvaIndependent ResearcherIkenobe, Kagawa‐ken, Japan
Filiz VardarDepartment of BiologyFaculty of ScienceMarmara UniversityIstanbul, Turkey
Mathew VeenaPlant Physiology and Biochemistry DivisionDepartment of BotanyUniversity of CalicutMalappuram, Kerala, India
Ravi Kumar YadavDepartment of BotanyKashi Naresh Government P. G. CollegeBhadohi, Uttar Pradesh, India
Satyendra Kumar YadavDepartment of ZoologyRashtriya Snatkottar MahavidyalayaJaunpur, Uttar Pradesh, India
Vijay Bahadur YadavDepartment of BotanyRashtriya Snatkottar MahavidyalayaJaunpur, Uttar Pradesh, India
Preface
A four‐carbon non‐proteinogenic amino acid, gamma‐aminobutyric acid (GABA), initially discovered as a metabolite in plants during the mid‐20th century in potato tubers. But, the focus shifted to animals after the revelation of its function in neurotransmission especially due to its role in calming anxiety. Recently, this was accompanied by the release of CRISPR‐edited and GABA enriched tomatoes in the market. Eventually, the research has also been performed for elucidating function of GABA in plants after the finding of its active role in combating various biotic and abiotic stresses. Because of its unique cellular working, recently, GABA has also been identified as a signalling molecule regulating plant development and stress acclimation. Therefore, it would’nt be unexpected that this four carbon signalling molecule may play a very pivotal role in sustaining crop production in the present scenario of climate change.
The current global population is near 8 billion which is expected to increase to 10 billion by 2050. So, we will be in need of 30% extra crop yields. But current global crop production is severely hampered by the unexpected episode of climate change. Therefore, researchers should seek some strategies for increasing crop production for feeding the global population. Since GABA plays very promising roles in plant growth and development, and stress acclimation, this four carbon metabolite/signaling molecule could be a very promising agent in boosting crop productivity under changing environmental conditions.
In the present book we have tried to compile potential roles of GABA in plant biology from developmental and stress acclimation points of view. In this edited book, attempts has been made to put together plant GABA related research from leading laboratories around the globe in one place, in order to make it easily accessible to researchers, students, academicians, etc. In this book, 18 book chapters have been compiled. These book chapters encompass knowledge about discovery and biosynthetic mechanisms of GABA, GABA crosstalk with other signaling molecules during plant development, and biotic and abiotic stress tolerance, and GABA and plant derived therapeutics. We believe that this book will be useful for the readers from academic and non‐academic backgrounds.
Samiksha Singh
Durgesh Kumar Tripathi
Vijay Pratap Singh
1Discovery and Background of GABA in Plants
Gubbi Vani Ishika1*, Deepthi Puttegowda1*, Ranjith Raj2, Manjunath Dammalli3, and Ramith Ramu1
1 Department of Biotechnology and Bioinformatics, JSS Academy of Higher Education and Research, Mysuru, Karnataka, 570015, India
2 Department of Pharmacology, JSS Medical College, JSS Academy of Higher Education and Research, Mysuru, Karnataka, 570015, India
3 Department of Biotechnology, Siddaganga Institute of Technology, Tumkur, Karnataka, 572103, India
* Equal contribution
Abbreviations
ALMT
Aluminum‐activated malate transporters
GABA
Gamma‐aminobutyric acid
GAD
Glutamic acid decarboxylase
GHB
Gamma hydroxybutyrate
MDA
Malondialdehyde
ROS
Reactive oxygen species
SlGAD
Solanum lycopersicum
glutamic acid decarboxylase
SSA
Succinic semialdehyde
Ssdh
Succinic semialdehyde dehydrogenase
TCAC
Tricarboxylic acid cycle
Introduction
Gamma‐aminobutyric acid (GABA) is a chemical that helps plants adapt to different growing conditions. GABA is a four‐carbon nonproteinogenic amino acid that is present in all plants and plant parts. It has been researched in both eukaryotes and prokaryotes. It was first identified in plants over seven decades ago in potato (Solanum tuberosum) tubers. Since then, extensive research has been conducted on its physiological significance, and it has been established that it functions as a signal molecule in plants in addition to being a metabolite. Food security and crop productivity are seriously at risk from the possible effects of climate change on plant development (Li et al. 2021). GABA is a four‐C, nonprotein amino acid that makes up 75% of the pool of free amino acids. Prokaryotic and eukaryotic species include GABA. Glutamic acid decarboxylase (GAD), an enzyme found in the cytosol, is the primary source of endogenous GABA. The so‐called GABA shunt, which is involved in a variety of physiological processes, including carbon flow into the tricarboxylic acid cycle (TCAC), cytosolic pH regulation, osmoregulation, signaling, and energy production, is how it is metabolized in the mitochondrial matrix (Shelp et al. 2017, 2021).
Its physiological function has been the subject of several research. Furthermore, it has been demonstrated that in plants, it serves as a signaling molecule as well as a metabolite. It has the ability to control plant growth and respond to both biotic and abiotic stresses, among other things (Seifikalhor et al. 2019). GABA functions as a signal for root growth, fruit ripening, pollen tube elongation (to enter the ovule), and seed germination in the context of Agrobacterium tumefaciens‐mediated plant gene transformation.
In response to drought stress, it emerges as a result of plant reactions that activate antioxidant enzymes and regulate stomatal opening; a high‐GABA concentration makes a plant more resilient to stress (Li et al. 2021). GABA's function in both stress‐adaptation (salinity, hypoxia/anoxia, drought, temperature, heavy metals, plant–insect interaction, and ROS‐related responses) and nonstress‐related biological pathways (e.g., plant–microbe interaction, contribution to the carbon and nitrogen metabolism, and regulation of signal transduction pathways) has been specifically highlighted (Seifikalhor et al. 2019 and Kaspal et al. 2021).
History
It was initially found that GABA was a naturally occurring compound in plants in the early to mid‐twentieth century, although the credit for its identification in plants goes to a number of researchers who studied different species of plants. The identification of GABA in plants is typically associated with the isolation and characterization of the compound in various plant tissues. These are some of the main figures who helped uncover GABA in plants. The year 1949 saw the discovery of it in plants (potato tuber) (Steward et al. 1949). However, later studies on animals showed that it was also significant in the brains of mammals as a neurotransmitter.
Research on plants gained additional speed when it was discovered that abiotic stress causes a rise in GABA concentration. The fact is that higher plants have been found to contain GABA, several animals, and bacteria as well as invertebrates. GABA was first identified in 1883 as a metabolic byproduct in microorganisms and plants. After the occurrence of GABA in potato tubers was confirmed in 1949, other reports of its presence in the brain emerged. Three distinct investigations on GABA in the mammalian central nervous system were given by Roberts and Frankel (1950) and Roberts (2007) in the same year in the same issue of Biological Chemistry.
Global experts are still working to establish GABA's status as an inhibitory neurotransmitter years later. Ernst Florey extracted a substance from horse brains in 1953 and administered it exogenously to cats and crayfish. They noticed inhibition in a few of their receptors in both studies and aversions. This substance was subsequently separated from the cow brain and recognized as GABA. It was also shown that artificially produced GABA inhibited the same crayfish receptors, confirming Florey's hypothesis that GABA functions in the brain as an inhibitory neurotransmitter. GABA was later proposed to serve as a metabolite and not be engaged in signaling because it is over 1000 times more abundant in the vertebrate brain than other neurotransmitters and participates in the Krebs cycle (GABA shunt). The lack of considerable GABA levels in invertebrates led to the conclusion that GABA is merely a metabolite and fails to meet the criteria for being a neurotransmitter (Florey and McLennan 1955).
Later studies on nematodes, however, demonstrated that GABA is, in fact, a potential neurotransmitter. The absence of it in invertebrates has been disproved by the discovery that it is also detected in crayfish and in larger quantities in Ascaris motor neurons. (Deo Rashmi et al. 2018). Ernest H. Wood and Albert F. Haight were among the first to identify and characterize GABA in plants in 1950. They described the presence of GABA in various plant tissues. Arthur M. Andrews and Ray B. Morrison in 1960 contributed to an understanding of GABA in plant tissues. The plasma membrane and organelle membranes are two examples of the membranes that GABA can pass through in a cell. Intercellular and intracellular GABA transport are also involved in this mechanism. It was not until 1999 that GABA transporters were found in plants, having first been identified in animals. According to Ramesh et al. (2018), Arabidopsis thaliana is one plant species that can effectively develop when given GABA as its main source of nitrogen (Ramesh et al. 2015). This suggests that plants do indeed have GABA transporters.
Background
The amino acid GABA, not part of the standard proteinogenic amino acids, was initially discovered in 1949 in potato tubers before its identification in animal brain extracts. Growing data in the 1950s and 1960s revealed that GABA may have an inhibitory neurotransmitter role in animals. It has been shown to inhibit crayfish stretch receptor neurons' impulses. However, GABA in mammalian nerve terminals was not identified until the study of Bloom and Iversen in 1971. About 10 years later, the activation of GABAA (ionotropic) and GABAB (metabotropic) receptors was found to be the primary mechanism responsible for GABA's inhibitory neurotransmitter effect. Mammals respond to excitatory neurotransmitters by activating GABAA receptors, which causes membrane hyperpolarization and a modest impact (Bloom and Iversen 1969; Watanabe and Fukuda 2015).
The regulation of brain function and development in mammals is greatly influenced by GABA receptors. Still, in addition to neural cells, these receptors have also been identified in human organs and other tissues. The role of GABA as an animal signaling molecule has been extensively studied after 50 years. In contrast, its comprehension in plants has primarily revolved (Bouch and Fromm 2004). Since the 1990s, there has been growing data that supports GABA's classification as a carbon–nitrogen metabolite and suggests that it may really serve as a signaling drug in plants. The evidence for this statement encompasses several factors, such as the presence of GABA concentration gradients, compartmentalized GABA metabolism, the fluctuating concentration of GABA in plant tissue, and its quick increase in response to different stimuli (Yue et al. 2014).
The control of the plant anion channel family ALMTs by GABA, for example, has led to new insights into the functions of GABA in plants. These insights suggest that changing anion flux across cell membranes is a means of translation of GABA metabolism into membrane signaling. The sequence similarity between animal GABAA receptors and ALMTs is minimal, yet GABA increases GABAA channel activity in mammals while inhibiting ALMT activity in plants. Nonetheless, this regulation leads to a relatively hyperpolarized state in both plant and animal cells Li et al. 2016; Nonaka et al. 2017). The alteration in membrane potential due to GABA in plants, influencing tissue growth, strongly indicates its signaling role in plants. GABA's role in plants was initially confined to its definition as a carbon–nitrogen metabolite (Palacios et al. 1981). Nevertheless, since the 1990s, a growing body of evidence has pointed toward GABA's potential function as a signaling molecule in plants. Notable observations include the variability in GABA concentration in plant tissues, its rapid escalation in response to diverse stresses, the existence of concentration gradients, and the impact of GABA on plant growth and development Stress. Moreover, recent discoveries, such as the regulation of plant anion channels (ALMTs) by GABA, have challenged previous notions. While these ALMTs bear little resemblance to animal GABAA receptors, they exhibit sensitivity to GABA in a manner opposite to that observed in animals. Even though adult animal neurons and plants have different equilibrium potentials for chloride, this differential response causes cells to become hyperpolarized (Palacios et al. 1981; Ramesh et al. 2017).
Previously, arguments against GABA acting as a signal in both animals and plants were rooted in its ubiquitous presence and lack of localized gradients. However, analogs to the discoveries in animal biology, localized gradients, and the identification of specific receptors in plants challenge these assumptions, underscoring the potential signaling role of GABA in plant biology. In the area of plant research, there's research to describe the intricacies of GABA‐regulated ion channels and their connectivity to physiological processes. This entails exploring the dissimilarities and commonalities between ALMTs and animal GABAA receptors while illuminating GABA's unique impacts on plants and its potential intersections with other signaling molecules. The continual examination and exploration of GABA's multifaceted role in both animal and plant systems offer an intriguing avenue for further research, promising novel insights into the realm of biological signaling and cellular regulation ( Ramesh et al. 2017).
The GABA Metabolic Pathway in Plants
In plants, cytosolic GAD converts glutamate into GABA irreversibly. GABA may be produced via oxidative stress‐induced breakdown of polyamines or a nonenzymatic mechanism starting from proline. As shown in Figure 1.1, the expression of five GAD‐encoding genes in Arabidopsis fluctuates according to the activities and growth conditions of the plant in different plant organs (Bouche and Fromm 2004; Shelp et al. 2012c): GAD is a Ca2+‐dependent calmodulin‐binding protein that is induced by changes in the neutral pH and cytosolic Ca2+ levels in intact plants.
Research on the development of pollen tubes revealed that GAD is positively regulated on Ca2+ channels that depend on extracellular GABA signals, i.e., by administering GABA. As a result, variations in intracellular GABA levels stimulated the production of genes linked to metabolism and signaling. Through cytosolic acidification following plant cell wounding and vacuolar content release, GAD activation was demonstrated to be independent of Ca2+ (Carroll et al. 1994).
Figure 1.1 Metabolic synthesis of GABA.
In the mitochondrial membrane, GABA permease (GABA‐P) establishes a connection between the GABA shunt and the TCA cycle. This allows cytosolic GABA to be taken up into the mitochondria, where the TCA cycle ultimately provides energy and carbon skeletons. The GABA shunt, which skips two TCA cycle steps from α‐ketoglutarate to succinate to succinyl‐CoA, is used to break down GABA (Long et al. 2020). The GABA transaminase in the mitotic matrix changes GABA into succinic semialdehyde (SSA) (GABA‐T). According to Ling et al. (2013) and Ramos‐Ruiz et al. (2019), there are two types of GABA‐T that use either pyruvate or α‐ketoglutarate (GABA‐TK). As amino acceptors, GABA‐TP produces either glutamate or alanine as the final product. The TCA cycle components succinate and NADH is produced when SSA is broken down by SSA dehydrogenase (SSADH). The ultimate result of both chemicals is ATP, which is supplied by electrons to the electron transport chain in the mitochondria. The modulation of SSADH's negative feedback by ATP and NADH. A possible reason for the connection between GABA and pathways other than the TCAC, such as signaling routes. SSA reductase, on the other hand, can convert SSA to γ‐hydroxybutyrate (GHB). This enzyme is most likely present in the cytosol or in chloroplasts. Stress from the environment causes GHB to accumulate, and it has been shown that ssadh mutants of Arabidopsis that accumulate GHB are less active in lower chlorophyll concentrations. Fewer blooms and less substance than the wild variety. It's unclear how GABA and GHB are related at this time. By inhibiting the correct TCA cycle enzymes and then compensating with an enhanced flow through the GABA shunt, the GABA shunt's circumvention of the TCA cycle was shown (Ramos‐Ruiz et al. 2019). On the other hand, Arabidopsis mutants deficient in the mitochondrial GABA transporter displayed elevated TCA cycle activity and decreased absorption of GABA into the mitochondria. Plants include iron‐activated malate transporter proteins or ALMT (Meyer et al. 2006). According to theory, these represent GABA receptors that are often triggered by anions and inhibited by GABA, which is obtained from plant tissue. It has been suggested that the control of ALMT by GABA represents a signaling mechanism, perhaps by altering membrane potentials, which can then initiate physiological alterations throughout the whole plant. It is still unclear how ALMT is distributed and characterized in plant cells and across plant tissue (Ramos‐Ruiz et al. 2019; Bouché et al. 2003).
Structure and Conformation of GABA
GABA is primarily found as a zwitterion when the amino group is protonated, and the carboxyl group is deprotonated (Figure 1.2). The environment has an impact on its conformance. According to (Thacker and Popelier 2018), the electrostatic attraction between the two functional groups in the gas phase greatly favors a highly folded conformation. Quantum chemistry simulations indicate that the stabilization is around 50 kcal/mol. An extended shape is seen in the solid state, with a trans conformation at the amino end and a gauche conformation at the carboxyl end. This is caused by the packing interactions with the molecules in the vicinity. Solvation effects result in the identification of five distinct conformations in solution, some of which are folded and some of which are stretched. Since GABA has been shown to bind to many receptors with various conformations, its conformational flexibility is crucial to its biological function. To improve binding control, several GABA analogs used in pharmaceutical applications have more rigid structures (Majumdar and Guha 1988).
Figure 1.2 Structure of GABA.
Roles and Functions of GABA
GABA's several roles in plants have been thoroughly outlined in Figure 1.3. GABA is an intrinsic signaling molecule that regulates plant growth and development in addition to its role in metabolism (Renault et al. 2011; Carillo 2018). In addition, it is essential for controlling the conversion of carbon to nitrogen. There is strong evidence that GABA contributes to the development of tolerance to a range of environmental challenges, including salt, low light, drought, nitrogen deficiency, and temperature changes (Kinnersley and Turano 2000). It may also help to prolong and improve the quality of crops while they are in storage, in addition to promoting plant development and stress reduction through the strengthening of antioxidant defense systems.
Figure 1.3 Role of GABA.
It has been established that the recycling and redistribution of nitrogen during abiotic stress‐induced leaf aging is related to GABA metabolism (Jalil et al. 2017). In addition, studies have demonstrated that it plays a part in protecting plants from biotic stresses such as necrotrophic fungi and insects (Seifi et al. 2013; Bown and Shelp 2016). Michaeli and Fromm (2015) suggest that the various functions of GABA in plants are probably interrelated and difficult to distinguish apart.
Plant Development
Sexual reproduction in flowering plants is crucial for their life cycle. Fertilization cannot take place until the pollen grains rest on the stigma; instead, the pollen tube must grow through the pistil and toward the ovule. The amount of GABA present in the bloom influences the pollen tube's orientation and elongation. GABA creates a gradient along the pollen tube's path, reaching its highest levels near the micropyle, the entry point for the pollen tube into the ovule (Takayama et al. 2017). Studies on Arabidopsis mutants lacking POP2, a GABA‐T encoding gene, showed that excessive GABA hindered pollen tube growth and disrupted the gradient, leading to misdirected tubes that could not enter the ovule. Experiments on Picea wilsonii revealed that both low and high concentrations of GABA influenced pollen tube elongation, where low concentrations promoted growth while excessive amounts inhibited it. High levels of GABA can also impact pollen tube growth by interfering with calcium ion influx and GAD functions, which are crucial in calcium feedback control (Matsuyama et al. 2009).
GABA is involved in the ripening of fruit as well. Under normal circumstances, GABA levels in fruits peak during the fully grown green stage along with the immature seeds, then quickly decline as the fruits ripen and the tomato seeds become mature. Modifying Solanum lycopersicum GAD (SlGAD) genes in tomatoes influences GABA levels, affecting plant growth and development. Upregulation of SlGAD2 results in stunted growth and delayed flowering, while overexpressing SlGAD3 causes high GABA levels without causing morphological abnormalities. However, manipulating SlGAD3 leads to delayed fruit development due to altered ethylene production and sensitivity (Xu et al. 2010; Takayama et al. 2017).
In many different types of plants, GABA also influences seed germination and root growth. It influences root growth and facilitates the germination of many seeds. For instance, it activates gene expression related to seed starch degradation during germination. Excessive GABA inhibits primary root elongation and even prevents germination in certain seeds by altering the carbon–nitrogen balance. Furthermore, an abundance of GABA inhibits adventitious root growth in plants like poplar through various mechanisms affecting cell wall metabolism and phytohormone signaling (Zhao et al. 2017; Ramos‐Ruiz et al. 2019).
Carbon and Nitrogen Metabolic Balance
For plants to grow and produce, carbon and nitrogen are essential components. For the best possible plant development, these elements must be absorbed efficiently. GABA can serve as a nitrogen store in plants, and through the GABA shunt, carbon structures are integrated into the TCA cycle (Chen et al. 2020). For instance, A. thaliana exhibits robust growth when supplied with GABA as the primary nitrogen source. Medicago truncatula has a nitrogen deficit, which causes the GABA content in phloem exudates to nearly treble when 50% of the nodules are removed. Supplementing with GABA significantly boosts GABA concentration in nodules, decreases glutamate concentration in exudates, and restores N2 fixation postexcision after four to five days (Sulieman 2011).
GABA first increases during the “maturation‐drying” stage of Arabidopsis seeds, and it subsequently diminishes after germination. Changes in GAD affect the nitrogen‐to‐carbon ratio in Arabidopsis seeds, as shown by the accumulation of GABA in dry seeds in genetically modified plants, a decrease in the number of sugars and organic acids, and a noticeable increase in total protein and different amino acids (Scholz et al. 2015). Significant changes in the amounts of sucrose and starch result from interference with the GABA shunt, which impacts the carbon metabolism of the cell wall. Therefore, it is acknowledged that GABA is the essential component that connects the nitrogen and carbon metabolisms in plants (Ramos‐Ruiz et al. 2019; Li et al. 2021).
In poplar seedling stems cultivated in low‐nitrogen conditions, the application of exogenous GABA increases TCA intermediates and nonstructural carbs. In addition, in poplar, GABA significantly suppresses the low nitrogen‐induced rise in leaf antioxidant enzymes, suggesting that it regulates the C : N ratio for growth by conserving energy in nitrogen‐scarce environments (Sulieman and Schulze 2010; Žárský 2015).
Enhancement of Storage Quality and Shelf Life
The utilization of GABA was found to be an effective method in preserving postharvest quality and enhancing storage performance in various fruits like bananas, citrus, cucumber, peaches, and pears. Studies by different authors have shown that GABA treatment leads to increased antioxidant enzyme activity, reduction in peel browning, decreased levels of OSs and malondialdehyde, improved gene expression, heightened activities of antioxidant enzymes, sustained mitochondrial structure, and elevated endogenous GABA concentration in treated fruits (Malekzadeh et al. 2017). Notably, fruits treated with GABA during postharvest periods had superior quality in terms of reduced chilling injury index and weight loss. This was especially true for zucchini and cut flowers. The significant role that GABA components play in avoiding damage caused by cold stress at the cellular level was indicated by increased GABA content and the activity of associated enzymes (GABA‐T and GAD) during storage. Moreover, pre‐ and postharvest GABA treatments were observed to improve cut flower indices, with positive effects on chilling injury and browning when applied at different concentrations and stages, maintaining membrane integrity, enhancing antioxidant capacity, and increasing PAL enzyme activity. Therefore, it was found that using GABA could potentially lessen the effects of chilling harm in cut Anthurium flowers (Palma et al. 2019; Ramos‐Ruiz et al. 2019).
pH Regulation
Since GAD consumes H+ in an acidic environment, GAD activity increases, which raises the possibility that GABA, which is produced during stress, plays a role in regulating pH. Successful demonstrations of GABA build‐up in response to acidic conditions in the cytosol have also been made. Weak acid treatment of Asparagus sprengeri cells resulted in GABA accumulation and cytosolic pH alterations; GABA concentration increased to 300% in just a few seconds (Crawford et al. 1994). In addition, after ammonium absorption, GAD activity increased in cultivated carrot cells along with a pH decrease and an increase in GABA concentrations. A subsequent rise in pH resulted in a reduction in GAD activity, illustrating GABA's function in reducing cytosolic acidification (Snedden, 2018).
Compatible Osmolyte
Plants can be stressed by a variety of biotic and abiotic factors due to their immobility. One strategy to deal with stressful situations is to produce osmoprotectants or suitable chemicals like proline, GABA, glycine betaine, and carbs (Wang et al. 2020). GABA is widely distributed throughout all plants and plant sections in addition to its build‐up, especially under the role it plays as an efficient osmoprotectant molecule, which can be explained under stressful situations. Over time, a number of investigations have demonstrated the build‐up of GABA in response to diverse biotic and abiotic stressors. Numerous data indicate that GABA enhances a variety of metabolic processes, photosynthetic activities, relative water levels, osmolyte accumulation, and leaf turgor (Shelp et al. 2012a). Black pepper plants exposed to PEG stress responded well to a pretreatment with GABA, which produced a priming effect that increased the plants' ability to withstand osmotic stress. GABA treatment enhanced the ability of maize plants to photosynthesize, exchange gases, and maintain an antioxidant defense mechanism against oxidative stress. Research into GABA's function in plants is still under progress despite the fact that its role as a neurotransmitter in mammals is well documented. The role of GABA gradient in pollen tube guidance provides evidence in favor of the notion. Certain glutamate receptor‐like (GLRs) proteins have been found in plants, and it is anticipated that these proteins will be involved in the control of growth, wound healing, and plant defense signals. Certain GABA receptor agonists and antagonists, like picrotoxin and bicuculline, cause phenotypic changes in plants. Plant cell membranes have been found to include several GABA‐binding sites (Shelp et al. 2012b; Vijayakumari and Puthur 2016).
Biotic and Abiotic Stress
The plant's ability to withstand abiotic stress was enhanced by positive regulation of the GABA shunt and related pathways, while endogenous GABA levels were raised by exogenous GABA. It was discovered that applying exogenous GABA to a range of crops, such as lentils, melon, rice, and wheat, was beneficial in lowering germination or growth inhibition. When it was applied in response to unfavorable environmental conditions like extreme heat, drought, water, salt, light, or oxygen stress (Liu et al. 2011).
It has been demonstrated that elevated GAD expression and GABA accumulation are correlated with wheat cultivars' resistance to salt and osmotic stress, which showed a correlation between partially higher photosynthetic capability and antioxidant enzyme activity and improved wheat seedling development under salt stress. The restoration of growth in heat‐stressed rice seedlings was also followed by an increase in several antioxidant enzymes. Drought‐tolerant barley seedlings showed increased expression of GABA receptor genes. Under stress, the GABA shunt in plants that is controlled by Ca2+ through GAD was triggered. In plants under stress, GABA treatment resulted in increased GABA shunt activity, enhanced photosynthetic capacity, increased levels of endogenous GABA, and increased levels of antioxidant enzymes (Aghdam et al. 2015).
ROS increased in tandem with a reduction in malondialdehyde (MDA), a marker of oxidative stress. Aghdam et al. (2015) concluded that the treated plants' membrane integrity was preserved. Potentially, the enzymes that the GABA shunt avoids are impacted by oxidative stress. Thus, reduced efficiency of the TCAC could arise from stressed plants' ability to compensate for this sensitivity through the GABA shunt. To prevent ROS from rising, it appears that a functional GABA shunt is required.
Degradation of proteins and lipids is commonly reported in combination with abiotic stress‐induced leaf senescence. Products of degradation are sent to the metabolic process and then dispersed to budding plant sections or ripening fruits. GABA is produced when amino acids are partially converted to glutamate during the breakdown of proteins. Through increased activity of the GABA shunt, specifically GABA‐T, and additional downstream pathways, the plant's metabolic system is once again able to access the nitrogen and carbon skeletons. In these studies, Arabidopsis thaliana GABA‐T mutants were employed (Aghdam et al. 2015; Aghdam et al. 2016).
Temperature Stress
When plants are exposed to extreme temperatures, whether it's cold (low temperatures) or heat (high temperatures), GABA can accumulate in plant tissues. This accumulation helps plants cope with temperature‐induced stress by regulating cellular homeostasis and minimizing damage caused by temperature extremes. GABA stabilizes and preserves the integrity of proteins and cell membranes. When plants are stressed by high temperatures, they may receive some protection from the nonprotein amino acid GABA. This study proposes the hypothesis that a decrease in GABA concentrations within the cells of mung bean plants experiencing heat stress leads to increased susceptibility to heat‐related damage in their reproductive functions. There are two types of temperature such as low temperature and high temperature (Li et al. 2021).
Low Temperature
Cold temperatures are a major factor that can greatly limit plant productivity. When plants experience cold stress, they often show increased GABA levels. For example, when barley or wheat seedlings are exposed to freezing or low temperatures, GABA buildup increases noticeably, and genes involved in the GABA shunt pathway are prompted to express (Mazzucotelli et al. 2006). In the perennial grass Brachypodium sylvaticum, GABA levels significantly rise in response to freezing stress.
High Temperature
Elevated temperatures can be a critical factor limiting plant growth and development. Researchers have examined the relationship between heat stress and GABA in numerous plant species. The application of GABA appears to play a protective role for plants exposed to heat stress. When GABA was externally applied to four‐day‐old rice seedlings under heat stress conditions, it notably enhanced their growth and survival rates. This improvement was attributed to the enhancement of leaf turgor, the upregulation of osmoprotectants, and antioxidants (Li et al. 2016).
Similar to this, when GABA was administered exogenously, creeping bent grass showed enhanced heat tolerance. Numerous processes were regulated by GABA, including the TCAC, osmotic potential, photosynthesis, and metabolic balance. The application of GABA to heat‐stressed plants enhanced carbon fixation, assimilation, and leaf water status by promoting the synthesis of osmolytes and subsequently reducing oxidative damage (Priya et al. 2019).
Drought
Drought, much like the stresses mentioned earlier, acts as a significant impediment to crop development and production. Plants often store more GABA in response to drought stress. Many plants, including turnip, bean, soybean, and sesame, have detached leaves that exhibit this rise in GABA levels. There have also been reports of GABA accumulation brought on by drought in tomatoes, Phyllanthus species, and creeping bent grass. According to Ayenew et al. (2015), these results imply that the build‐up of GABA during drought is a particular reaction to this stress and helps control the stomata's opening to minimize water loss.
Also, white clover's ability to withstand drought is enhanced when the endogenous GABA content is raised externally. This improvement is attributable to upregulating the GABA shunt, proline metabolism, and polyamines. Improved nitrogen recycling, photosystem II protection, a reduction in drought‐induced cell elongation, fatty acid desaturation, wax biosynthesis, and a delay in leaf senescence in creeping bent grass are all linked to GABA's role in improving drought tolerance.
Heavy Metals
Heavy metals are significant soil pollutants that can lead to food contamination by accumulating in the edible parts of crop plants. Heavy metal stress frequently leads to the build‐up of ROS in addition to ion toxicity. The amount of GABA in rice roots increases when heavy metals such as chromium (Cr) are present, according to metabolome analysis. Similarly, higher levels of GABA have been seen in soybean plants under stress from zinc (Zn) and copper (Cu) (Kang et al. 2015).
GABA concentrations were lower in plants treated with higher (100 μM) Zn concentrations than in Nicotiana tabacum plants treated with moderate (10 μM) Zn values. GABA treatment boosted the resistance of rice seedlings to arsenic (As [III]) stress by encouraging the expression of genes associated with the GABA shunt, triggering the antioxidant enzyme system, and successfully lowering accumulation (Li et al. 2021).
It's interesting to note that long‐term GABA accumulation – as opposed to short‐term, potentially hazardous GABA elevation – is more effective in promoting As (III) tolerance (Kumar et al. 2017). Conversely, in the presence of cadmium (Cd) stress, endogenous GABA content appeared to decrease in duckweed. Under Cd stress conditions, the addition of exogenous GABA promoted the shedding of root‐like structures (rhizoid abscission), while the addition of glutamate (Glu) facilitated rhizoid abscission. In addition, it's worth noting that having a high GABA content does not always guarantee enhanced plant tolerance to metal stress (Ludewig et al. 2008).
ROS
Abiotic stress conditions can lead to the accumulation of ROS in plants. In response to abiotic stress, the GABA shunt is initiated, which essentially limits the generation of ROS in plant tissues (Bouché et al. 2003). The final stage of the GABA shunt is catalyzed by an enzyme known as succinic‐semialdehyde dehydrogenase (SSADH). Studies have shown that mutants with T‐DNA insertions in the SSADH gene, also known as ssadh SSADH mutants, display specific traits such as reduced leaf area, necrotic lesions, bleached leaves, shorter hypocotyls, and fewer flowers. These mutants, according to Ludwig et al. (2008), show increased susceptibility to heat and UV stress, which leads to the build‐up of significant amounts of ROS, which eventually induces cell death in stressed tissues.
Interestingly, it was found that GHB, a consequence of SSA, was five times more numerous in ssadh mutants than in wild‐type Arabidopsis. In ssadh mutants, treatment with γ‐vinyl‐γ‐aminobutyrate, a specific GABA transaminase inhibitor (GABA‐T/POP2), or a mutation in the POP2 gene reduced cell death, increased proliferation, and inhibited ROS production. Boa et al. (2015) reported that the symptoms of tomato mutants with silenced SlSSADH, which were obtained by the virus‐induced gene silencing (VIGS) system, were comparable to those of Arabidopsis ssadh mutants. Among these were twisted leaves, slowed growth, and an overabundance of ROS. GHB dehydrogenase can convert GHB into SSA, while SSA reductase can convert SSA into GHB in both plants and animals. In ssadh mutants, SSA and/or GHB build up because SSADH is unable to convert SSA to succinate. Furthermore, compared to wild‐type plants, pop2, and pop2ssadh mutant plants have much higher GABA levels. However, the phenotypes of the double pop2ssadh mutants revert to those of the wild type, suggesting that the high‐GABA level is not responsible for the phenotype of the ssadh mutants. Instead, increased SSA and/or GHB levels are the cause of the characteristics linked to ssadh mutants (Ludewig et al. 2008).
Salt
Salinity in the soil is a major environmental stressor that affects agricultural productivity worldwide (Snedden, 2018). There have been three main hypothesized cellular responses for salt tolerance in plants: tissue tolerance to the build‐up of Na+ ions, the capacity to reject Na+ ions, and tolerance to osmotic stress. Numerous researches have shown the build‐up of defense compounds in plants and have illuminated the processes behind plants' reactions to salinity (Wang et al. 2018). As an illustration, it has been discovered that the pop2‐1 mutant, which is GABA‐T defective, is susceptible to salt stress but not osmotic stress. However, salt tolerance rather than sensitivity was shown by the pop2‐5 mutant, which accumulates high quantities of GABA in its roots (Liu et al. 2011).
The variations in the reactions of these distinct pop2 mutants could be related to the GABA levels in each of them, as excessive accumulation of GABA above a particular threshold can be harmful to plants. Research employing mutants with decreased capacity to produce GABA, such as pop2‐5 and gad1,2, has demonstrated that GABA activates H+‐ATPase, diminishes Na+ absorption, restricts H2O2‐induced K+ efflux, and decreases ROS levels (Su et al. 2019). In addition, GAD activity was not consistently linked with GABA content, despite the fact that GABA accumulation was observed in Nicotiana sylvestris and cytoplasmic male sterile (CMS) II plants during both short‐ and long‐term salt stress. Conversely, on the seventh day of the salt treatment, the GABA content of N. tabacum plants treated with 500 mM NaCl increased, possibly due to the high‐saline level and differences in plant developmental phases (Ludewig et al. 2008). The GABA content of the plants originally dropped on the first and third day of salt treatment.
Additional studies have shown that in saline conditions, the levels of GABA and GAD mRNA in poplar and other wheat cultivars increase significantly. Important metabolic enzymes required for the tricarboxylic acid (TCA) cycle were physicochemically hindered by salt in wheat leaves; however, the elevated activity of the GABA shunt offered an alternative carbon source that allowed the TCA cycle to continue operating within mitochondria. By avoiding salt‐sensitive enzymes, this enabled wheat plants to have increased leaf respiration (Che‐Othman et al. 2020). When applied to plants that are stressed by salt, exogenous GABA has also been shown to increase the content of endogenous GABA, activate enzymatic antioxidant activity, lessen plant damage from salt, and improve salt tolerance in maize, white clover, muskmelon, germinated hull‐less barley, and tomato. Hormones and other stimuli are linked to the build‐up of GABA in response to salt. Research has shown that durum wheat exposed to high nitrogen or high light treatments in combination with salt accumulates GABA. GABA serves as a temporary nitrogen storage area as well. It has been shown that giving plants exposed to NaCl GABA has an impact on how much ethylene, H2O2, and ABA (abscisic acid) are produced. Exogenous GABA has been demonstrated in studies to modulate several genes related to ABA and ethylene (Vijayakumari and Puthur 2016; Li et al. 2021).
Conclusion
GABA has a different purpose in plants than it does as an animal neurotransmitter. GABA is largely used by plants as a nonprotein amino acid and a signaling molecule in a variety of physiological processes. When it comes to reacting to environmental stresses like salt, drought, and disease attacks, GABA is a key player. These stressors cause plants to store GABA, which aids in ion balance regulation, oxidative stress management, and cellular homeostasis maintenance. The importance of GABA in plant biology can be attributed to its capacity to improve stress tolerance, provide damage protection, and facilitate general environmental adaptation. Understanding GABA's function can help create plant varieties that are more resilient and tolerant of stress, which makes it an important target for research in crop improvement and agriculture.
