Editorial: Plant Glutathione Transferases: Diverse, Multi-Tasking Enzymes With Yet-to-Be Discovered Functions

Glutathione S-transferase: A versatile and dynamic enzyme

The dynamic and versatile group of enzymes referred to as glutathione S-transferases (GSTs) play diverse roles in cellular detoxification, safeguarding hosts from oxidative damage, and performing various other functions. This review explores different classes of GST, existence of polymorphisms in GST, functions of GST and utilizations of GST inhibitors in treatment of human diseases. The study indicates that the cytosolic GSTs, mitochondrial GSTs, microsomal GSTs, and bacterial proteins that provide resistance to Fosfomycin are the major classes. Given a GST, variation in its expression and function among individuals is due to the presence of polymorphic alleles that encode it. Genetic polymorphism might result in the modification of GST activity, thereby increasing individuals' vulnerability to harmful chemical compounds. GSTs have been demonstrated to play a regulatory function in cellular signalling pathways through kinases, S-Glutathionylation, and in detoxification processes. Various applications of bacterial GSTs and their potential roles in plants were examined. Targeting GSTs, especially GSTP1-1, is considered a potential therapeutic strategy for treating cancer and diseases linked to abnormal cell proliferation. Their role in cancer cell growth, differentiation, and resistance to anticancer agents makes them promising targets for drug development, offering prospects for the future.

Proteome- and metabolome-level changes during early stages of clubroot infection in Brassica napus canola

Clubroot is a destructive root disease of canola (Brassica napus L.) caused by Plasmodiophora brassicae Woronin. Despite extensive research into the molecular responses of B. napus to P. brassicae, there is limited information on proteome- and metabolome-level changes in response to the pathogen, especially during the initial stages of infection. In this study, we have investigated the proteome- and metabolome- level changes in the roots of clubroot-resistant (CR) and -susceptible (CS) doubled-haploid (DH) B. napus lines, in response to P. brassicae pathotype 3H at 1-, 4-, and 7-days post-inoculation (DPI). Root proteomes were analyzed using nanoflow liquid chromatography coupled with tandem mass spectrometry (nano LC-MS/MS). Comparisons of pathogen-inoculated and uninoculated root proteomes revealed 2515 and 1556 differentially abundant proteins at one or more time points (1-, 4-, and 7-DPI) in the CR and CS genotypes, respectively. Several proteins related to primary metabolites (e.g., amino acids, fatty acids, and lipids), secondary metabolites (e.g., glucosinolates), and cell wall reinforcement-related proteins [e.g., laccase, peroxidases, and plant invertase/pectin methylesterase inhibitors (PInv/PMEI)] were identified. Eleven nucleotides and nucleoside-related metabolites, and eight fatty acids and sphingolipid-related metabolites were identified in the metabolomics study. To our knowledge, this is the first report of root proteome-level changes and associated alterations in metabolites during the early stages of P. brassicae infection in B. napus.

OsGSTU34, a Bz2-like anthocyanin-related glutathione transferase transporter, is essential for rice (Oryza sativa L.) organs coloration

Anthocyanins are a flavonoid compound known as one of the most important chromogenic substances. They play several functions, including health promotion and sustaining plants during adverse conditions. They are synthesized at the endoplasmic reticulum and sequestered in the vacuole. In this work, we generated knock-out lines of OsGSTU34, a glutathione transporter's tau gene family, with no transgene line and off-target through CRISPR/Cas9 mutagenesis and highlighted the loss of pigmentation in rice flowers, leaves, stems, shoots, and caryopsis. The anthocyanin quantification in the wild-type BLWT and mutant line BLG34-8 caryopsis showed that cyanidin-3-O-glucoside (C3G) and peonidin-3-O-glucoside (P3G) were almost undetectable in the mutant line. A tandem mass tag (TMT) labeling proteomic analysis was conducted to elucidate the proteomic changes in the BLWT and BLG34-8. The result revealed that 1175 proteins were altered, including 408 that were down-regulated and 767 that were upregulated. The accumulation of the OsGSTU34-related protein (Q8L576), along with several anthocyanin-related proteins, was down-regulated. The enrichment analysis showed that the down-regulated proteins were enriched in different pathways, among which the phenylpropanoid biosynthesis pathway, flavonoid biosynthesis metabolites, and anthocyanin biosynthesis pathway. Protein interaction network prediction revealed that glutathione-S-transferase (Q8L576) was connected to the proteins involved in the flavonoid and anthocyanin biosynthesis pathways, such as flavanone 3-dioxygenase 1 (Q7XM21), leucoanthocyanidin dioxygenase 1 (Q93VC3), 4-coumarate-CoA ligase 2 (Q42982), phenylalanine ammonia-lyase (P14717), chalcone synthase 1 (Q2R3A1), and 4-coumarate-CoA ligase 5 (Q6ZAC1). However, the expression of the most important anthocyanin biosynthesis gene was not altered, suggesting that only the transport mechanism was affected. Our findings highlight new insight into the anthocyanin pigmentation in black rice and provide new perspectives for future research.

Genome-wide identification and characterization of glutathione S-transferase gene family in quinoa (Chenopodium quinoa Willd.)

The present investigation was envisaged for large scale in-silico genome wide identification and characterization of glutathione S-transferases (GSTs) in Chenopodium quinoa. In this study, a total of 120 GST genes (CqGSTs) were identified and divided into 11 classes of which tau and phi were highest in numbers. The average protein length of protein was found to be 279.06 with their corresponding average molecular weight of 31,819.4 kDa. The subcellular localization analysis results showed that proteins were centrally localized in the cytoplasm followed by chloroplast, mitochondria and plastids. Structural analysis revealed the presence of 2 -14 exons in CqGST genes. Most of the proteins possessed two exon one intron organization. MEME analysis identified 15 significantly conserved motifs with a width of 6-50 amino acids. Motifs 1, 3, 2, 5, 6, 8, 9 and 13 were found specifically in tau class family; motifs 3, 4, 5, 6, 7 and 9 were found in phi class gene family, while motifs 3, 4, 13 and 14 were found in metaxin class. Multiple sequence alignment revealed highly conserved N-terminus with active site serine (Ser; S) or cysteine (Cys; C) residue for the activation of GSH binding and GST catalytic activity. The gene loci were found to be unevenly distributed across 18 different chromosomes with a maximum of 17 genes located on chromosome number 7. Dominance of alpha helix was followed by coil, extended strand and beta turns. Gene duplication analysis revealed that segmental duplication and purifying type selection were highest in number and found to be main source of expansion of GST gene family. Cis acting regulatory elements analysis showed the presence of 21 different elements involved in stress, hormone and light response and cellular development. The evolutionary relationship of CqGST proteins carried out using maximum likelihood method revealed that all the tau and phi class GSTs were closely associated with those of G. max, O. sativa and A. thaliana. Molecular docking of GST molecules with the fungicide metalaxyl showed that the CqGSTF1 had the lowest binding energy. The comprehensive study of CqGST gene family in quinoa provides groundwork for further functional analysis of CqGST genes in the species at molecular level and has potential applications in plant breeding.

The Glutathione S-Transferase PtGSTF1 Improves Biomass Production and Salt Tolerance through Regulating Xylem Cell Proliferation, Ion Homeostasis and Reactive Oxygen Species Scavenging in Poplar

Glutathione S-transferases (GSTs) play an essential role in plant cell detoxification and secondary metabolism. However, their accurate functions in the growth and response to abiotic stress in woody plants are still largely unknown. In this work, a Phi class Glutathione S-transferase encoding gene PtGSTF1 was isolated from poplar (P. trichocarpa), and its biological functions in the regulation of biomass production and salt tolerance were investigated in transgenic poplar. PtGSTF1 was ubiquitously expressed in various tissues and organs, with a predominant expression in leaves and inducible expression by salt stress. Transgenic poplar overexpressing PtGSTF1 showed improved shoot growth, wood formation and improved salt tolerance, consistent with the increased xylem cell number and size under normal condition, and the optimized Na⁺ and K⁺ homeostasis and strengthened reactive oxygen species scavenging during salt stress. Further transcriptome analyses demonstrated that the expressions of genes related to hydrolase, cell wall modification, ion homeostasis and ROS scavenging were up- or down-regulated in transgenic plants. Our findings imply that PtGSTF1 improves both biomass production and salt tolerance through regulating hydrolase activity, cell wall modification, ion homeostasis and ROS scavenging in transgenic poplar, and that it can be considered as a useful gene candidate for the genetic breeding of new tree varieties with improved growth under salt stress conditions.

Cold stratification-induced dormancy removal in apple (Malus domestica Borkh.) seeds is accompanied by an increased glutathione pool in embryonic axes

A reduced form of glutathione (GSH) is an essential metabolite that participates in the control of reactive oxygen species (ROS) levels in cells. GSH plays a pivotal role in seed biology as a modulator of seed viability and germination. The GSH:GSSG ratio and half-cell reduction potential (E_GSSG/2GSH) serve as indicators of the oxidative status in seeds. Apple (Malus domestica Borkh.) seeds are deeply dormant, and this state is removed by long-term cold stratification. The aim of our work was to examine the modification of GSH and GSSG content, GSH:GSSG ratio and E_GSSG/2GSH in the embryonic axes isolated from apple seeds subjected to cold stratification for 7, 14, 21 and 40 d. Our data indicated that cold stratification increased the generation of free radicals in the embryonic axes, which correlated with an alteration in the expression of genes encoding Rboh, particularly RbohC. GSH and GSSG levels increased during prolonged cold stratification of apple seeds. This was accompanied by the modification of glutathione reductase and glutathione peroxidase-like activities, which did not match their transcript levels. The steady-state GSH:GSSG ratio and E_GSSG/2GSH in the axes of embryos subjected to cold stratification indicated no impact of the dormancy removal treatment on apple seed viability. We suggest that the glutathione system is an important component of the redox network and is involved in the management of the seed transition from dormant to nondormant states.

Metabolic Pathways for S-Metolachlor Detoxification Differ Between Tolerant Corn and Multiple-Resistant Waterhemp

Herbicide resistance in weeds can be conferred by target-site and/or non-target-site mechanisms, such as rapid metabolic detoxification. Resistance to the very-long-chain fatty acid-inhibiting herbicide, S-metolachlor, in multiple herbicide-resistant populations (CHR and SIR) of waterhemp (Amaranthus tuberculatus) is conferred by rapid metabolism compared with sensitive populations. However, enzymatic pathways for S-metolachlor metabolism in waterhemp are unknown. Enzyme assays using S-metolachlor were developed to determine the specific activities of glutathione S-transferases (GSTs) and cytochrome P450 monooxygenases (P450s) from CHR and SIR seedlings to compare with tolerant corn and sensitive waterhemp (WUS). GST activities were greater (∼2-fold) in CHR and SIR compared to WUS but much less than corn. In contrast, P450s in microsomal extracts from CHR and SIR formed O-demethylated S-metolachlor, and their NADPH-dependent specific activities were greater (>20-fold) than corn or WUS. Metabolite profiles of S-metolachlor generated via untargeted and targeted liquid chromatography-mass spectrometry from CHR and SIR differed from WUS, with greater relative abundances of O-demethylated S-metolachlor and O-demethylated S-metolachlor-glutathione conjugates formed by CHR and SIR. In summary, our results demonstrate that S-metolachlor metabolism in resistant waterhemp involves Phase I and Phase II metabolic activities acting in concert, but the initial O-demethylation reaction confers resistance.

Crosstalk between the redox signalling and the detoxification: GSTs under redox control?

Reactive oxygen species (ROS), antioxidants and their reduction-oxidation (redox) states all contribute to the redox homeostasis, but glutathione is considered to be the master regulator of it. We aimed to understand the relationship between the redox potential and the diverse glutathione transferase (GST) enzyme family by comparing the stress responses of two tomato cultivars (Solanum lycopersicum 'Moneymaker' and 'Ailsa Craig'). Four-week-old plants were treated by two concentrations of mannitol, NaCl and salicylic acid. The lower H₂O₂ and malondialdehyde contents indicated higher stress tolerance of 'Moneymaker'. The redox status of roots was characterized by measuring the reduced and oxidized form of ascorbate and glutathione spectrophotometrically after 24 h. The redox potential of 'Ailsa Craig' was more oxidized compared to 'Moneymaker' even under control conditions and became more positive due to treatments. High-throughput quantitative real-time PCR revealed that besides overall higher expression levels, SlGSTs were activated more efficiently in 'Moneymaker' due to stresses, resulting in generally higher GST and glutathione peroxidase activities compared to 'Ailsa Craig'. The expression level of SlGSTs correlated differently, however Pearson's correlation analysis showed usually strong positive correlation between SlGST transcription and glutathione redox potential. The possible redox regulation of SlGST expressions was discussed.

Compensation of Mutation in Arabidopsis glutathione transferase (AtGSTU) Genes under Control or Salt Stress Conditions

Glutathione transferases (GSTs) play a crucial role in detoxification processes due to the fact of their glutathione (GSH) conjugating activity, and through glutathione peroxidase or dehydroascorbate reductase (DHAR) activities, they influence the redox state of GSH and ascorbate (AsA). The plant-specific tau (GSTU) group is the largest class of Arabidopsis GSTs, and their members are involved in responses to different abiotic stresses. We investigated the effect of salt stress on two-week-old Arabidopsis thaliana wild-type (Col-0), Atgstu19 and Atgstu24 mutant plants after applying 150 mM NaCl for two days. The Atgstu19 seedlings had lower GST activity and vitality both under control conditions and after salt stress than the wild-type, but the level of total ROS was similar to the Col-0 plants. The GST activity of the knockout Atgstu24 mutant was even higher under control conditions compared to the Col-0 plants, while the ROS level and its vitality did not differ significantly from the wild-type. Analysis of the AtGSTU expression pattern revealed that the mutation in a single AtGSTU gene was accompanied by the up- and downregulation of several other AtGSTUs. Moreover, elevated AsA and GSH levels, an altered GSH redox potential and increased DHAR and glutathione reductase activities could help to compensate for the mutation of AtGSTU genes. The observed changes in the mutants suggest that the investigated isoenzymes influence the redox homeostasis under control conditions and after NaCl treatment in Arabidopsis seedlings. These data indicate for the first time the more general role of a temporary shift of redox status as part of GST mechanisms and regulation.