Characterization of two tea glutamate decarboxylase isoforms involved in GABA production

Genome-wide identification and expression analysis of the GAD family reveal their involved in embryogenesis and hormones responses in Dimocarpus longan Lour

Glutamate decarboxylase (GAD) is involved in GABA metabolism and plays an essential regulatory role in plant growth, abiotic stresses, and hormone response. This study investigated the expression mechanism of the GAD family during longan early somatic embryogenesis (SE) and identified 6 GAD genes based on the longan genome. Homology analysis indicated that DlGAD genes had a closer relationship with dicotyledonous plants. The analysis of cis-acting elements in the promoter region suggests that the GAD genes were associated with various stress responses and hormones. RNA sequencing (RNA-Seq) and the qRT-PCR data indicated that most DlGAD genes were highly expressed in the incomplete compact pro-embryogenic cultures (ICpEC) and upregulated in longan embryogenic callus (EC) after treatments with 2,4-D, high temperature (35 °C), IAA, and ABA. Moreover, the RNA-Seq analysis also revealed that DlGADs exhibit different expression patterns in various tissues and organs. The subcellular localization results showed that DlGAD5 was localized in the cytoplasm, suggesting that it played a role in the cytoplasm. Transient overexpression of DlGAD5 enhanced the expression levels of DlGADs and increased the activity of glutamate decarboxylase in longan embryogenic callus (EC), while the content of glutamic acid decreased. Thus, the DlGAD gene can play an important role in the early somatic embryogenesis of longan by responding to hormones such as IAA and ABA. DlGAD5 can affect the growth and development of longan by stimulating the expression of the DlGAD gene family, thereby increasing the GAD activity in the early SE of longan, participating in hormone synthesis and signaling pathways.

Unraveling the Potential of γ-Aminobutyric Acid: Insights into Its Biosynthesis and Biotechnological Applications

γ-Aminobutyric acid (GABA) is a widely distributed non-protein amino acid that serves as a crucial inhibitory neurotransmitter in the brain, regulating various physiological functions. As a result of its potential benefits, GABA has gained substantial interest in the functional food and pharmaceutical industries. The enzyme responsible for GABA production is glutamic acid decarboxylase (GAD), which catalyzes the irreversible decarboxylation of glutamate. Understanding the crystal structure and catalytic mechanism of GAD is pivotal in advancing our knowledge of GABA production. This article provides an overview of GAD's sources, structure, and catalytic mechanism, and explores strategies for enhancing GABA production through fermentation optimization, metabolic engineering, and genetic engineering. Furthermore, the effects of GABA on the physiological functions of animal organisms are also discussed. To meet the increasing demand for GABA, various strategies have been investigated to enhance its production, including optimizing fermentation conditions to facilitate GAD activity. Additionally, metabolic engineering techniques have been employed to increase the availability of glutamate as a precursor for GABA biosynthesis. By fine-tuning fermentation conditions and utilizing metabolic and genetic engineering techniques, it is possible to achieve higher yields of GABA, thus opening up new avenues for its application in functional foods and pharmaceuticals. Continuous research in this field holds immense promise for harnessing the potential of GABA in addressing various health-related challenges.

γ-Aminobutyric acid plays a key role in alleviating Glomerella leaf spot in apples

The fungal disease Glomerella leaf spot (GLS) seriously impacts apple production. As a nonprotein amino acid, γ-aminobutyric acid (GABA) is widely involved in biotic and abiotic stresses. However, it is not clear whether GABA is involved in a plant's response to GLS, nor is its molecular mechanism understood. Here, we found that exogenous GABA could significantly alleviate GLS, reduce lesion lengths, and increase antioxidant capacity. MdGAD1 was identified as a possible key gene for GABA synthesis in apple. Further analysis indicated that MdGAD1 promoted antioxidant capacity to improve apple GLS resistance in transgenic apple calli and leaves. Yeast one-hybrid analysis identified the transcription factor MdWRKY33 upstream of MdGAD1. Electrophoretic mobility shift assay, β-glucuronidase activity, and luciferase activity further supported that MdWRKY33 bound directly to the promoter of MdGAD1. The content of GABA and the transcription level of MdGAD1 in the MdWRKY33 transgenic calli were higher than that of the wild type. When MdWRKY33 transgenic calli and leaves were inoculated with GLS, MdWKRY33 positively regulated resistance to GLS. These results explained the positive regulatory effects of GABA on apple GLS and provided insight into the metabolic regulatory network of GABA.

Heat stress and excessive maturity of fruiting bodies suppress GABA accumulation by modulating GABA metabolism in Pleurotus ostreatus (Jacq. ex Fr.) P. Kumm

GABA is a health-promoting bioactive substance. Here, the GABA biosynthetic pathways were investigated, and then the dynamic quantitative changes in GABA and the expression levels of genes related to GABA metabolism under heat stress or at different developmental stages of fruiting bodies in Pleurotus ostreatus (Jacq. ex Fr.) P. Kumm were determined. We found that the polyamine degradation pathway was the main route of GABA production under growth normal condition. The accumulation of GABA and the expression of most genes related to GABA biosynthesis, including genes encoding glutamate decarboxylase (PoGAD-2), polyamine oxidase (PoPAO-1), diamine oxidase (PoDAO) and aminoaldehyde dehydrogenase (PoAMADH-1 and PoAMADH-2), were significantly suppressed by heat stress and the excessive maturity of fruiting bodies. Finally, the effects of GABA on the mycelial growth, heat tolerance and the morphogenesis and development of fruiting bodies were studied, the results showed that the deficiency of endogenous GABA inhibited the mycelial growth and primordial formation and aggravated heat damage, whereas exogenous application of GABA could improve thermotolerance and promote the development of fruiting bodies.

Heterologous Expression and Characterization of Tea (Camellia sinensis) Polyamine Oxidase Homologs and Their Involvement in Stresses

Polyamine oxidase (PAO) is a key enzyme maintaining polyamine homeostasis, which affects plant physiological activities. Until now, the gene members and function of PAOs in tea (Camellia sinenesis) have not been fully identified. Here, through the expression in Escherichia coli and Nicotiana benthamiana, we identified six genes annotated as CsPAO in tea genome and transcriptome and determined their enzyme reaction modes and gene expression profiles in tea cultivar 'Yinghong 9'. We found that CsPAO1,2,3 could catalyze spermine, thermospermine, and norspermidine, and CsPAO2,3 could catalyze spermidine in the back-conversion mode, which indicated that the precursor of γ-aminobutyric acid might originate from the oxidation of putrescin but not spermidine. We further investigated the changes of CsPAO activity with temperature and pH and their stability. Kinetic parameters suggested that CsPAO2 was the major PAO modifying polyamine composition in tea, and it could be inactivated by β-hydroxyethylhydrazine and aminoguanidine. Putrescine content and CsPAO2 expression were high in tea flowers. CsPAO2 responded to wound, drought, and salt stress; CsPAO1 might be the main member responding to cold stress; anoxia induced CsPAO3. We conclude that in terms of phylogenetic tree, enzyme characteristics, and expression profile, CsPAO2 might be the dominant CsPAO in the polyamine degradation pathway.

Genome-wide identification of GAD family genes suggests GhGAD6 functionally respond to Cd2+ stress in cotton

Glutamate decarboxylase (GAD) mainly regulated the biosynthesis of γ-aminobutyric acid (GABA) and played an important role in plant growth and stress resistance. To explore the potential function of GAD in cotton growth, the genome-wide identification, structure, and expression analysis of GAD genes were performed in this study. There were 10, 9, 5, and 5 GAD genes identified in G. hirsutum, G. barbadense, G. arboreum, and G. raimondii, respectively. GAD was divided into four clades according to the protein motif composition, gene structure, and phylogenetic relationship. The segmental duplication was the main way of the GAD gene family evolution. Most GhGADs respond to abiotic stress. Clade Ⅲ GAD was induced by Cd²⁺ stress, especially GhGAD6, and silencing GhGAD6 would lead to more serious Cd²⁺ poisoning in cotton. The oxidative damage caused by Cd²⁺ stress was relieved by increasing the GABA content. It was speculated that the decreased expression of GhGAD6 reduced the content of GABA in vivo and caused the accumulation of ROS. This study will further expand our understanding of the relationship between the evolution and function of the GhGAD gene family and provide new genetic resources for cotton breeding under environmental stress and phytoremediation.

CsCuAOs and CsAMADH1 Are Required for Putrescine-Derived γ-Aminobutyric Acid Accumulation in Tea

Polyamines are a potential source of γ-aminobutyric acid (GABA) in plants under abiotic stress. However, studies on GABA enrichment in tea mostly focus on the GABA shunt, while the correlation between polyamine degradation and GABA formation in tea is largely unknown. In this study, tea plants responded to exogenous putrescine, resulting in a significant increase in GABA content, while the glutamate level did not change. At the same time, five copper-containing amine oxidase (CuAO) and eight aminoaldehyde dehydrogenase (AMADH) genes involved in the putrescine-derived GABA pathway were identified from the Tea Plant Information Archive. Expression analysis indicated that CsCuAO1, CsCuAO3 as well as CsAMADH1 were induced to play an important function in response to exogenous putrescine. Thus, the three genes were cloned and the catalytic efficiency of soluble recombinant proteins was determined. CsCuAOs and CsAMADH1 exhibited indispensable functions in the GABA production from putrescine in vitro. Subcellular localization assays indicated that CsAMADH1 was localized in plastid, while both CsCuAO1 and CsCuAO3 were localized in peroxisome. In addition, the synergistic effects of CsCuAOs and CsAMADH1 were investigated by a transient co-expression system in Nicotiana benthamiana. Our data suggest that these three genes regulate the accumulation of GABA in tea by participating in the polyamine degradation pathway and improve the content of GABA in tea to a certain extent. The results will greatly contribute to the production of GABA tea.

Comprehensive analysis of putative dihydroflavonol 4-reductase gene family in tea plant

One identified dihydroflavonol 4-reductases (DFR) encoding gene (named as CsDFRa herein) and five putative DFRs (named as CsDFRb1, CsDFRb2, CsDFRb3, CsDFRc and CsDFRd) in tea (Camellia sinensis) have been widely discussed in recent papers concerning multi-omics data. However, except for CsDFRa, their function and biochemical characteristics are not clear. This study aims to compare all putative CsDFRs and preliminarily evaluate their function. We investigated the sequences of genes (coding and promoter regions) and predicted structures of proteins encoded, and determined the activities of heterologously expressed CsDFRs under various conditions. The results showed that the sequences of five putative CsDFRs were quite different from CsDFRa, and had lower expression levels as well. The five putative CsDFRs could not catalyze three dihydroflavonol substrates. The functional CsDFRa had the strongest affinity with dihydroquercetin, and performed best at pH around 7 and 35°C but was not stable at lower pHs or higher temperatures. Single amino acid mutation at position 141 modified the preference of CsDFRa for dihydroquercetin and dihydromyricetin, and also weakened its stability. These data suggest that only CsDFRa works in the pathway for generating anthocyanidins and catechins. This study provides new insights into the function of CsDFRs and may assist to develop new strategies to manipulate the composition of tea flavonoids in the future.