GABA negatively regulates adventitious root development in poplar

Whole-Genome Sequence Analysis of Flammulina filiformis and Functional Validation of Gad, a Key Gene for γ-Aminobutyric Acid Synthesis

Flammulina filiformis is one of the widely produced edible fungi worldwide. It is rich in γ-aminobutyric acid (GABA), a non-protein amino acid with important physiological functions in humans. To investigate the functions of key genes in the GABA metabolic pathway of F. filiformis, we isolated the monokaryon Fv-HL23-1 from the factory-cultivated F. filiformis strain Fv-HL23 and then sequenced and assembled the genome using the PacBio Sequel and Illumina NovaSeq sequencing platforms. The results showed that the genome comprised 140 scaffolds with a total length of 40.96 Mb, a GC content of 49.62%, an N50 of 917,125 bp, and 14,256 protein-coding genes. Phylogenetic analysis based on the whole genome revealed a close evolutionary relationship of Fv-HL23-1 with Armillaria mellea, Lentinula edodes, and Schizophyllum commune. A total of 589 carbohydrate-active enzymes were identified in the genome of Fv-HL23-1, suggesting its strong lignocellulose degradation ability, and 108 CYP450 gene family members were identified, suggesting important functions such as resistance to stress, secondary metabolite synthesis, and growth and development. The F. filiformis proteins glutamate decarboxylase 1 (Ff-GAD1) and glutamate decarboxylase 2 (Ff-GAD2), which may be responsible for GABA synthesis, were identified by protein alignment. Molecular docking analysis showed that Ff-GAD2 may have better catalytic activity than Ff-GAD1. To verify the function of Ff-gad2, its heterologous expression in the mycelia of the mononuclear Hypsizigus marmoreus was analyzed. Compared with wild type, the GABA content of mycelia was increased by 85.40-283.90%, the growth rate was increased by 9.39 ± 2.35%, and the fresh weight was increased by 18.44 ± 7.57%. Ff-GAD2 may play a catalytic role in GABA synthesis. In addition, the expression of the full-length Ff-gad2 gene was increased by 7.96 ± 1.39 times compared with the exon expression level in H. marmoreus mycelia, suggesting that the intron may contribute to the heterologous expression of Ff-GAD2. Based on whole-genome sequencing, we analyzed the enzyme system related to the important life activities of F. filiformis, focusing on the function of Ff-GAD, a key enzyme in the GABA synthesis pathway. The results lay a foundation for elucidating the GABA metabolism pathway of edible fungi and developing targeted breeding strategies for GABA-producing edible fungi.

The Role of Glutamate Metabolism and the GABA Shunt in Bypassing the Tricarboxylic Acid Cycle in the Light

Glutamate is an essential amino acid in both the energy and biosynthetic processes in plant cells. The aim of this work was to study changes in glutamate metabolism upon irradiation of maize (Zea mays L.) leaves with light of different spectral compositions, as well as to identify mechanisms regulating the work of enzymes involved in the studied process. A study was conducted of light-induced changes in glutamate metabolism in maize leaves, mediated by redirecting the glutamate flow to the γ-aminobutyric acid (GABA) shunt. Glutamate dehydrogenase (GDH) was more active in darkness, and the irradiation by red light inhibited the expression of both the Gdh1 and Gdh2 genes. EGTA and ruthenium red abolished the effects of light, indicating the participation of Ca2+ ions in phytochrome signal transduction. Contrary to GDH, glutamate decarboxylase (GAD) activity was moderately higher in the light, stimulated by red light, while far-red light reversed the effect. The effect of light on Gad expression was more pronounced than on GAD activity. Irradiation by red light also resulted in the increase in activity of GABA transaminase (GTA), which was abolished by far-red light. The third enzyme of the GABA shunt, succinic semialdehyde dehydrogenase (SSADH), was also activated by light. The effect of light on the expression of Ssadh1, but not on Ssadh2, was phytochrome-dependent. It is concluded that irradiation by light shifts glutamate metabolism from GDH to GAD with the activation of GABA transaminase and SSADH. This suggests that the GABA pathway plays a role in the maintenance of the tricarboxylic acid cycle in the light via bypassing its reactions when the 2-oxoglutarate dehydrogenase complex is inhibited and the cycle switches to the open mode.

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.

Exogenous γ-aminobutyric acid improves the photosynthesis efficiency, soluble sugar contents, and mineral nutrients in pomegranate plants exposed to drought, salinity, and drought-salinity stresses

BACKGROUND

γ-aminobutyric acid (GABA), as a regulator of many aspects of plant growth, has a pivotal role in improving plant stress resistance. However, few studies have focused on the use of GABA in increasing plants' resistance to interactional stresses, such as drought-salinity. Therefore, the focus of this study was to examine the effect of foliar application of GABA (0, 10, 20, and 40 mM) on growth indices and physio-biochemical parameters in plants of two pomegranate cultivars, 'Rabab' and 'Atabaki' exposed to drought, salinity, and drought-salinity.

RESULTS

Under stress conditions, the photosynthetic capacity of two pomegranate cultivars, including transpiration rate, net photosynthetic rate, intercellular carbon dioxide concentration, stomatal conductance of water vapour, and mesophyll conductance, was significantly reduced. This resulted in a decrease in root morphological traits such as fresh and dry weight, diameter, and volume, as well as the fresh and dry weight of the aerial part of the plants. However, the application of GABA reversed the negative effects caused by stress treatments on growth parameters and maintained the photosynthetic capacity. GABA application has induced the accumulation of compatible osmolytes, including total soluble carbohydrate, starch, glucose, fructose, and sucrose, in charge of providing energy for cellular defense response against abiotic stresses. Analysis of mineral nutrients has shown that GABA application increases the absorption of potassium, potassium/sodium, magnesium, phosphorus, manganese, zinc, and iron. As concentration increased up to 40 mM, GABA prevented the uptake of toxic ions, sodium and chloride.

CONCLUSIONS

These findings highlight the potential of GABA as a biostimulant strategy to enhance plant stress tolerance.

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.

Resilient and sustainable production of peanut (Arachis hypogaea) in phosphorus-limited environment by using exogenous gamma-aminobutyric acid to sustain photosynthesis

Globally, many low to medium yielding peanut fields have the potential for further yield improvement. Low phosphorus (P) limitation is one of the significant factors curtailing Arachis hypogaea productivity in many regions. In order to demonstrate the effects of gamma-aminobutyric acid (GABA) on peanuts growing under P deficiency, we used a pot-based experiment to examine the effects of exogenous GABA on alleviating P deficiency-induced physiological changes and growth inhibition in peanuts. The key physiological parameters examined were foliar gas exchange, photochemical efficiency, proton motive force, reactive oxygen species (ROS), and adenosine triphosphate (ATP) synthase activity of peanuts under cultivation with low P (LP, 0.5 mM P) and control conditions. During low P, the cyclic electron flow (CEF) maintained the high proton gradient (∆pH) induced by low ATP synthetic activity. Applying GABA during low P conditions stimulated CEF and reduced the concomitant ROS generation and thereby protecting the foliar photosystem II (PSII) from photoinhibition. Specifically, GABA enhanced the rate of electronic transmission of PSII (ETRII) by pausing the photoprotection mechanisms including non-photochemical quenching (NPQ) and ∆pH regulation. Thus, GABA was shown to be effective in restoring peanut growth when encountering P deficiency. Exogenous GABA alleviated two symptoms (increased root-shoot ratio and photoinhibition) of P-deficient peanuts. This is possibly the first report of using exogenous GABA to restore photosynthesis and growth under low P availability. Therefore, foliar applications of GABA could be a simple, safe and effective approach to overcome low yield imposed by limited P resources (low P in soils or P-fertilizers are unavailable) for sustainable peanut cultivation and especially in low to medium yielding fields.

Primary Metabolite Screening Shows Significant Differences between Embryogenic and Non-Embryogenic Callus of Tamarillo (Solanum betaceum Cav.)

Tamarillo is a solanaceous tree that has been extensively studied in terms of in vitro clonal propagation, namely somatic embryogenesis. In this work, a protocol of indirect somatic embryogenesis was applied to obtain embryogenic and non-embryogenic callus from leaf segments. Nuclear magnetic resonance spectroscopy was used to analyze the primary metabolome of these distinct calli to elucidate possible differentiation mechanisms from the common genetic background callus. Standard multivariate analysis methods were then applied, and were complemented by univariate statistical methods to identify differentially expressed primary metabolites and related metabolic pathways. The results showed carbohydrate and lipid metabolism to be the most relevant in all the calli assayed, with most discriminant metabolites being fructose, glucose and to a lesser extent choline. The glycolytic rate was higher in embryogenic calli, which shows, overall, a higher rate of sugar catabolism and a different profile of phospholipids with a choline/ethanolamine analysis. In general, our results show that a distinct primary metabolome between embryogenic and non-embryogenic calli occurs and that intracellular levels of fructose and sucrose and the glucose to sucrose ratio seem to be good candidates as biochemical biomarkers of embryogenic competence.

Melatonin Treatment Enhances the Growth and Productivity of Useful Metabolites in the In Vitro Culture of Spirodela polyrhiza

Spirodela polyrhiza (Araceae family) is a duckweed species that serves as a potential resource for feed, food, bioremediation, and pharmaceutical applications. In this study, we assessed the effects of different concentrations of melatonin (0, 0.1, 1, and 10 μM) on the growth of S. polyrhiza during in vitro culture and the metabolic profiles and productivities of useful metabolites using gas chromatography-mass spectrometry coupled with multivariable statistical analysis. We found that exogenous melatonin significantly improved the total dry weight and altered the metabolic profiles of S. polyrhiza cultures. Melatonin significantly enhanced the cellular production of useful metabolites, such as γ-aminobutyric acid, dopamine, threonine, valine, and phytosterols. The volumetric productivities (mg/L) of γ-aminobutyric acid, dopamine, campesterol, β-sitosterol, and stigmasterol were the highest in the presence of 10 μM melatonin on day 12. Moreover, the productivities of ascorbic acid and serotonin were the highest in the presence of 1 μM melatonin on day 12. Therefore, melatonin could be used to enhance the production of biomass and useful metabolites during large-scale S. polyrhiza cultivation in cosmetic, food/feed, and pharmaceutical industries.

Application of γ-aminobutyric acid (GABA) improves fruit quality and rootstock drought tolerance in apple

GABA (γ-aminobutyric acid) plays a multifaceted role in plant growth, fruit quality, and tolerance to abiotic stresses. However, its physiological roles and mechanisms in the fruit quality and response to long-term drought stress in apple remain unelucidated. To investigate the effect of GABA on apple fruit quality and drought tolerance, we sprayed exogenous GABA on apple cultivar "Cripps Pink" and irrigated rootstock M.9-T337 with GABA, respectively. Results showed that exogenous GABA could effectively improve the fruit quality of "Cripps Pink", including increased sugar-to-acid ratio, flesh firmness, pericarp malleability, and GABA content, as well as reduced fruit acidity. In addition, pretreatment of M.9-T337 plants with GABA improved their tolerance to both long- and short-term drought stress. Specifically, 1 mM exogenous GABA increased the net photosynthetic rate, relative leaf water content, root-to-shoot ratio, and water use efficiency under long-term drought stress, and delayed the increased of the relative electrolyte leakage under short-term drought stress. RNA-seq analysis identified 1271 differentially expressed genes (DEGs) between nontreated and GABA-pretreated plants under short-term drought stress. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of these DEGs revealed that GABA may enhance plant drought resistance by upregulating the expression of genes related to "Biosynthesis of secondary metabolites", "MAPK signaling pathway", "Glutathione metabolism", and "Carbon fixation in photosynthetic organisms". In conclusion, these results revealed that exogenous GABA can improve fruit quality and enhance drought tolerance in apple.

Recent advances of γ-aminobutyric acid: Physiological and immunity function, enrichment, and metabolic pathway

γ-aminobutyric acid (GABA) is a non-protein amino acid which naturally and widely occurs in animals, plants, and microorganisms. As the chief inhibitory neurotransmitter in the central nervous system of mammals, it has become a popular dietary supplement and has promising application in food industry. The current article reviews the most recent literature regarding the physiological functions, preparation methods, enrichment methods, metabolic pathways, and applications of GABA. This review sheds light on developing GABA-enriched plant varieties and food products, and provides insights for efficient production of GABA through synthetic biology approaches.

The Role of γ-Aminobutyric Acid (GABA) in the Occurrence of Adventitious Roots and Somatic Embryos in Woody Plants

The occurrence of adventitious roots and somatic embryos is a crucial step in micropropagation that frequently limits the application of this technique in woody plants. Recent studies demonstrated that they can be negatively or positively regulated with γ-aminobutyric acid (GABA), which is a four-carbon non-proteinous amino acid that not only acts as a main inhibitory neurotransmitter in mammals. It has been reported that GABA affects plant growth and their response to stress although its mode of action is still unclear. This review dealt with the effects of GABA on adventitious root formation and growth as well as on somatic embryogenesis. Furthermore, we focused on discussing the interaction of GABA with phytohormones, such as auxin, ethylene, abscisic acid, and gibberellin, as well as with the carbon and nitrogen metabolism during adventitious root development. We suggested that research on GABA will contribute to the application of micropropagation in the recalcitrant fruit and forest species.

Exogenous GABA improves the resistance of apple seedlings to long-term drought stress by enhancing GABA shunt and secondary cell wall biosynthesis

Drought stress is an important factor limiting apple production. γ-Aminobutyric acid (GABA) exists widely in plants and participates in the response to abiotic stress as a metabolite or signaling molecule. The role of exogenous GABA in apple plants, response to long-term drought stress remains unclear. Our study confirmed that exogenous GABA affects the drought resistance of apple plants under long-term drought stress. We found that 1 mM exogenous GABA improved the resistance of apple seedlings to long-term drought stress. The plants showed better growth, less reactive oxygen radical accumulation, less damage to cell membranes and greater active photosynthetic capacity. Under long-term drought stress, exogenous GABA facilitated GABA shunt, resulting in more accumulation of organic acids, namely citric acid, succinic acid and malic acid, in roots and stems of apple seedlings. In addition, exogenous GABA upregulated the expression of cellulose-related genes and lignin-related genes, and activated secondary cell wall-related transcription factors to synthesize more cellulose and lignin. A multiple factorial analysis confirmed that the GABA shunt and the biosynthesis of cellulose and lignin substantially contributed to the growth of apple seedlings with the application of exogenous GABA under long-term drought stress. Our results suggested that exogenous GABA improved the resistance of apple seedlings to long-term drought stress by enhancing GABA shunt and secondary cell wall biosynthesis.

Review of the Mechanisms by Which Transcription Factors and Exogenous Substances Regulate ROS Metabolism under Abiotic Stress

Reactive oxygen species (ROS) are signaling molecules that regulate many biological processes in plants. However, excess ROS induced by biotic and abiotic stresses can destroy biological macromolecules and cause oxidative damage to plants. As the global environment continues to deteriorate, plants inevitably experience abiotic stress. Therefore, in-depth exploration of ROS metabolism and an improved understanding of its regulatory mechanisms are of great importance for regulating cultivated plant growth and developing cultivars that are resilient to abiotic stresses. This review presents current research on the generation and scavenging of ROS in plants and summarizes recent progress in elucidating transcription factor-mediated regulation of ROS metabolism. Most importantly, the effects of applying exogenous substances on ROS metabolism and the potential regulatory mechanisms at play under abiotic stress are summarized. Given the important role of ROS in plants and other organisms, our findings provide insights for optimizing cultivation patterns and for improving plant stress tolerance and growth regulation.

Comprehensive Genome-Wide Identification and Transcript Profiling of GABA Pathway Gene Family in Apple (Malus domestica)

γ-Aminobutyric Acid (GABA), a four-carbon non-protein amino acid, is a significant component of the free amino acid pool in most prokaryotic and eukaryotic organisms. GABA is involved in pH regulation, maintaining C/N balance, plant development and defence, as well as a compatible osmolyte and an alternative pathway for glutamate utilization via anion flux. Glutamate decarboxylase (GAD, EC 4.1.1.15) and GABA transaminase (GABA-T, EC 2.6.1.19) are two key enzymes involved in the synthesis and metabolism of GABA. Recently, GABA transporters (GATs), protein and aluminium-activated malate transporter (ALMT) proteins which function as GABA receptors, have been shown to be involved in GABA regulation. However, there is no report on the characterization of apple GABA pathway genes. In this study, we performed a genome-wide analysis and expression profiling of the GABA pathway gene family in the apple genome. A total of 24 genes were identified including five GAD genes (namely MdGAD 1–5), two GABA-T genes (namely MdGABA-T 1,2), 10 GAT genes (namely GAT 1–10) and seven ALMT genes (namely MdALMT1–7). These genes were randomly distributed on 12 chromosomes. Phylogenetic analyses grouped GABA shunt genes into three clusters—cluster I, cluster II, and cluster III—which had three, four, and five genes, respectively. The expression profile analysis revealed significant MdGAD4 expression levels in both fruit and flower organs, except pollen. However, there were no significant differences in the expression of other GABA shunt genes in different tissues. This work provides the first characterization of the GABA shunt gene family in apple and suggests their importance in apple response to abiotic stress. These results can serve as a guide for future studies on the understanding and functional characterization of these gene families.

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.

Adventitious Rooting in Populus Species: Update and Perspectives

Populus spp. are among the most economically important species worldwide. These trees are used not only for wood and fiber production, but also in the rehabilitation of degraded lands. Since they are clonally propagated, the ability of stem cuttings to form adventitious roots is a critical point for plant establishment and survival in the field, and consequently for the forest industry. Adventitious rooting in different Populus clones has been an agronomic trait targeted in breeding programs for many years, and many factors have been identified that affect this quantitative trait. A huge variation in the rooting capacity has been observed among the species in the Populus genus, and the responses to some of the factors affecting this trait have been shown to be genotype-dependent. This review analyses similarities and differences between results obtained from studies examining the role of internal and external factors affecting rooting of Populus species cuttings. Since rooting is the most important requirement for stand establishment in clonally propagated species, understanding the physiological and genetic mechanisms that promote this trait is essential for successful commercial deployment.

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 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.

Effects of GABA and Vigabatrin on the Germination of Chinese Chestnut Recalcitrant Seeds and Its Implications for Seed Dormancy and Storage

Recalcitrant chestnut seeds are rich in γ-aminobutyric acid (GABA), which negatively regulates adventitious root development by altering carbon/nitrogen metabolism. However, little is known regarding the role of this metabolite in chestnut seeds. In this study, we investigated the effects of GABA changes on the germination of chestnut seeds treated with exogenous GABA and vigabatrin (VGB, which inhibits GABA degradation). Both treatments significantly inhibited seed germination and primary root growth and resulted in the considerable accumulation of H₂O₂, but the endogenous GABA content decreased before germination at 48 h. Soluble sugar levels increased before germination, but subsequently decreased, whereas starch contents were relatively unchanged. Changes to organic acids were observed at 120 h after sowing, including a decrease and increase in citrate and malate levels, respectively. Similarly, soluble protein contents increased at 120 h, but the abundance of most free amino acids decreased at 48 h. Moreover, the total amino acid levels increased only in response to VGB at 0 h. Accordingly, GABA and VGB altered the balance of carbon and nitrogen metabolism, thereby inhibiting chestnut seed germination. These results suggested that changes to GABA levels in chestnut seeds might prevent seed germination. The study data may also help clarify the dormancy and storage of chestnut seeds, as well as other recalcitrant seeds.