The catalytic role of glutathione transferases in heterologous anthocyanin biosynthesis

Miniature dual-channel electrochemical 3D-printed sensing platform for enzyme-free screening of L-lactic acid in foodstuffs accompanying pH recognition

Accurate and rapid identification of expired and spoiled food is crucial for food conservation, reducing resource wastage, and preventing food poisoning. This paper presents a portable electrochemical sensing platform supported by a miniature electrochemical workstation and an intelligent pH recognition system, enabling detection of L-lactic acid (L-LA) in food products without enzyme involvement. The system was based on a Cu2O@CuO multifaceted extended spatial hexapod structure. The synthesized Cu2O@CuO was characterized by a well-defined multifaceted structure. Significant enzyme-free catalysis was exhibited, and pH responses dominated by anthocyanins were identified through an intelligent image acquisition system. Additionally, we developed an electrochemical detection device for pH assistance during target testing, addressing the limitations of current electrochemical sensors' complex signal acquisition components using 3D-printed fabrication techniques and smartphones. The proposed multifunctional electrochemical workbench based on Cu2O@CuO was found to offer a preferable linear detection range of 0.1-1000 μM for L-LA, with a low detection limit of 0.027 μM. The visualization of pH determination was introduced as a novel approach for developing advanced electrochemical workbenches. In conclusion, pH-assisted portable electrochemical detection systems hold great potential for immediate food safety identification, particularly in resource-limited areas, facilitating prompt diagnosis and ensuring food safety.

Making, Using, and Understanding Molecular Systems: The 57th Bürgenstock Conference

The 57th SCS Conference on Stereochemistry, better known as the Bürgenstock Conference, brought together a diverse range of chemistry expertise in Brunnen, Switzerland.

Overlooked and misunderstood: can glutathione conjugates be clues to understanding plant glutathione transferases?

Plant glutathione transferases (GSTs) constitute a large and diverse family of enzymes that are involved in plant stress response, metabolism and defence, yet their physiological functions remain largely elusive. Consistent with the traditional view on GSTs across organisms as detoxification enzymes, in vitro most plant GSTs catalyse glutathionylation, conjugation of the tripeptide glutathione (GSH; γ-Glu-Cys-Gly) onto reactive molecules. However, when it comes to elucidating GST functions, it remains a key challenge that the endogenous plant glutathione conjugates (GS-conjugates) that would result from such glutathionylation reactions are rarely reported. Furthermore, GSTs often display high substrate promiscuity, and their proposed substrates are prone to spontaneous chemical reactions with GSH; hence, single-gene knockouts rarely provide clear chemotypes or phenotypes. In a few cases, GS-conjugates are demonstrated to be biosynthetic intermediates that are rapidly further metabolized towards a pathway end product, explaining their low abundance and rare detection. In this review, we summarize the current knowledge of plant GST functions and how and possibly why evolution has resulted in a broad and extensive expansion of the plant GST family. Finally, we demonstrate that endogenous GS-conjugates are more prevalent in plants than assumed and suggest they are overlooked as clues towards the identification of plant GST functions. This article is part of the theme issue 'The evolution of plant metabolism'.

Comparative Characterization of Three Homologous Glutathione Transferases from the Weed Lolium perenne

The comparative analysis of homologous enzymes is a valuable approach for elucidating enzymes' structure-function relationships. Glutathione transferases (GSTs, EC. 2.5.1.18) are crucial enzymes in maintaining the homeostatic stability of plant cells by performing various metabolic, regulatory, and detoxifying functions. They are promiscuous enzymes that catalyze a broad range of reactions that involve the nucleophilic attack of the activated thiolate of glutathione (GSH) to electrophilic compounds. In the present work, three highly homologous (96-98%) GSTs from ryegrass Lolium perenne (LpGSTs) were identified by in silico homology searches and their full-length cDNAs were isolated, cloned, and expressed in E. coli cells. The recombinant enzymes were purified by affinity chromatography and their substrate specificity and kinetic parameters were determined. LpGSTs belong to the tau class of the GST superfamily, and despite their high sequence homology, their substrate specificity displays remarkable differences. High catalytic activity was determined towards hydroxyperoxides and alkenals, suggesting a detoxification role towards oxidative stress metabolites. The prediction of the structure of the most active LpGST by molecular modeling allowed the identification of a non-conserved residue (Phe215) with key structural and functional roles. Site-saturation mutagenesis at position 215 and the characterization of eight mutant enzymes revealed that this site plays pleiotropic roles, affecting the affinity of the enzyme for the substrates, catalytic constant, and structural stability. The results of the work have improved our understanding of the GST family in L. perenne, a significant threat to agriculture, sustainable food production, and safety worldwide.

Global organization of phenylpropanoid and anthocyanin pathways revealed by proximity labeling of trans-cinnamic acid 4-hydroxylase in Petunia inflata petal protoplasts

The phenylpropanoid pathway is one of the main carbon sinks in plants, channeling phenylalanine towards thousands of products including monolignols, stilbenes, flavonoids and volatile compounds. The enzymatic steps involved in many of these pathways are well characterized, however the physical organization of these enzymes within the plant cell remains poorly understood. Proximity-dependent labeling allows untargeted determination of both direct and indirect protein interactions in vivo, and therefore stands as an attractive alternative to targeted binary assays for determining global protein-protein interaction networks. We used TurboID-based proximity labeling to study protein interaction networks of the core phenylpropanoid and anthocyanin pathways in petunia. To do so, we coupled the endoplasmic reticulum (ER) membrane anchored cytochrome P450 cinnamic acid 4-hydroxylase (C4H, CYP73A412) from Petunia inflata to TurboID and expressed it in protoplasts derived from anthocyanin-rich petunia petals. We identified multiple soluble enzymes from the late anthocyanin pathway among enriched proteins, along with other C4H isoforms, and other ER membrane anchored CYPs. Several of these interactions were subsequently confirmed by bimolecular fluorescence complementation (BiFC). Our results suggest that C4H co-localizes with enzymes from the phenylpropanoid- and downstream anthocyanin pathways, supporting the idea that C4H may serve as ER anchoring points for downstream metabolic pathways. Moreover, this study demonstrates the feasibility of using protoplasts to perform global mapping of protein network for enzymes in their native cellular environment.

Conserved amino acid residues and gene expression patterns associated with the substrate preferences of the competing enzymes FLS and DFR

BACKGROUND

Flavonoids, an important class of specialized metabolites, are synthesized from phenylalanine and present in almost all plant species. Different branches of flavonoid biosynthesis lead to products like flavones, flavonols, anthocyanins, and proanthocyanidins. Dihydroflavonols form the branching point towards the production of non-colored flavonols via flavonol synthase (FLS) and colored anthocyanins via dihydroflavonol 4-reductase (DFR). Despite the wealth of publicly accessible data, there remains a gap in understanding the mechanisms that mitigate competition between FLS and DFR for the shared substrate, dihydroflavonols.

RESULTS

An angiosperm-wide comparison of FLS and DFR sequences revealed the amino acids at positions associated with the substrate specificity in both enzymes. A global analysis of the phylogenetic distribution of these amino acid residues revealed that monocots generally possess FLS with Y132 (FLSY) and DFR with N133 (DFRN). In contrast, dicots generally possess FLSH and DFRN, DFRD, and DFRA. DFRA, which restricts substrate preference to dihydrokaempferol, previously believed to be unique to strawberry species, is found to be more widespread in angiosperms and has evolved independently multiple times. Generally, angiosperm FLS appears to prefer dihydrokaempferol, whereas DFR appears to favor dihydroquercetin or dihydromyricetin. Moreover, in the FLS-DFR competition, the dominance of one over the other is observed, with typically only one gene being expressed at any given time.

CONCLUSION

This study illustrates how almost mutually exclusive gene expression and substrate-preference determining residues could mitigate competition between FLS and DFR, delineates the evolution of these enzymes, and provides insights into mechanisms directing the metabolic flux of the flavonoid biosynthesis, with potential implications for ornamental plants and molecular breeding strategies.

The molecular basis of flavonoid biosynthesis response to water, light, and temperature in grape berries

Flavonoids, including proanthocyanidins (PAs), anthocyanins and flavonols are essential secondary metabolites that contribute to the nutritional value and sensory quality of grape berry and red wine. Advances in molecular biology technology have led to substantial progress in understanding the regulation of flavonoid biosynthesis. The influence of terroir on grape berries and wine has garnered increasing attention, yet its comprehensive regulatory network remains underexplored. In terms of application, environmental factors such as water, light, and temperature are more easily regulated in grapevines compared to soil conditions. Therefore, we summarize their effects on flavonoid content and composition, constructing a network that links environmental factors, hormones, and metabolites to provide a deeper understanding of the underlying mechanisms. This review enriches the knowledge of the regulatory network mechanisms governing flavonoid responses to environmental factors in grapes.

Recent advances in engineering microorganisms for the production of natural food colorants

Food colorants are frequently added to processed foods since color is an important tool in the marketing of food products, influencing consumer perceptions, preferences, and purchasing behavior. While synthetic dyes currently dominate the food colorant market, consumer concern regarding their safety and sustainability is driving a demand for their replacement with naturally derived alternatives. However, natural colorants are costly compared to their synthetic counterparts as the pigment content in the native sources is usually very low and extraction can be challenging. Recent advances in the engineering of microbial metabolism have sparked interest in the development of cell factories capable of producing natural colorants from renewable resources. This review summarizes major developments within metabolic engineering for the production of nature-identical food colorants by fermentation. Additionally, it highlights common applications, formulations, and physicochemical characteristics of prevalent pigment classes. Lastly, it outlines a workflow for accelerating the optimization of cell factories for the production or derivatization of nature-identical food colorants.

Genetic factors explaining anthocyanin pigmentation differences

BACKGROUND

Anthocyanins are important contributors to coloration across a wide phylogenetic range of plants. Biological functions of anthocyanins span from reproduction to protection against biotic and abiotic stressors. Owing to a clearly visible phenotype of mutants, the anthocyanin biosynthesis and its sophisticated regulation have been studied in numerous plant species. Genes encoding the anthocyanin biosynthesis enzymes are regulated by a transcription factor complex comprising MYB, bHLH and WD40 proteins.

RESULTS

A systematic comparison of anthocyanin-pigmented vs. non-pigmented varieties was performed within numerous plant species covering the taxonomic diversity of flowering plants. The literature was screened for cases in which genetic factors causing anthocyanin loss were reported. Additionally, transcriptomic data sets from four previous studies were reanalyzed to determine the genes possibly responsible for color variation based on their expression pattern. The contribution of different structural and regulatory genes to the intraspecific pigmentation differences was quantified. Differences concerning transcription factors are by far the most frequent explanation for pigmentation differences observed between two varieties of the same species. Among the transcription factors in the analyzed cases, MYB genes are significantly more prone to account for pigmentation differences compared to bHLH or WD40 genes. Among the structural genes, DFR genes are most often associated with anthocyanin loss.

CONCLUSIONS

These findings support previous assumptions about the susceptibility of transcriptional regulation to evolutionary changes and its importance for the evolution of novel coloration phenotypes. Our findings underline the particular significance of MYBs and their apparent prevalent role in the specificity of the MBW complex.

Poplar glutathione S-transferase PtrGSTF8 contributes to reactive oxygen species scavenging and salt tolerance

Glutathione S-transferases (GSTs) constitute a protein superfamily encoded by a large gene family and play a crucial role in plant growth and development. However, their precise functions in wood plant responses to abiotic stress are not fully understood. In this study, we isolated a Phi class glutathione S-transferase-encoding gene, PtrGSTF8, from poplar (Populus alba × P. glandulosa), which is significantly up-regulated under salt stress. Moreover, compared with wild-type (WT) plants, transgenic tobacco plants exhibited significant salt stress tolerance. Under salt stress, PtrGSTF8-overexpressing tobacco plants showed a significant increase in plant height and root length, and less accumulation of reactive oxygen species. In addition, these transgenic tobacco plants exhibited higher superoxide dismutase, peroxidase, and catalase activities and reduced malondialdehyde content compared with WT plants. Quantitative real-time PCR experiments showed that the overexpression of PtrGSTF8 increased the expression of numerous genes related to salt stress. Furthermore, PtrMYB108, a MYB transcription factor involved in salt resistance in poplar, was found to directly activate the promoter of PtrGSTF8, as demonstrated by yeast one-hybrid assays and luciferase complementation assays. Taken together, these findings suggest that poplar PtrGSTF8 contributes to enhanced salt tolerance and confers multiple growth advantages when overexpressed in tobacco.

Exploring Enzyme-Mimicking Metal-Organic Frameworks for CO2 Conversion through Vibrational Spectra-Based Machine Learning

In pursuing the benefits of natural enzyme catalysts while overcoming their limitations, we find metal-organic frameworks (MOFs), renowned for their highly tunable functionalities, stand out in biomimetic applications. We used unsupervised machine learning on density functional theory-computed vibrational infrared and Raman spectral features to screen 300 Zn-MOFs for CO2 conversion, similar to carbonic anhydrase (CA). Our findings confirmed that MOFs with spectroscopic attributes closely resembling those of CA hold the potential for replicating CA's electronic and catalytic properties. Unlike previous studies that relied on heuristic or trial-and-error methods and focused on geometric configurations, our research uses vibrational spectral features to explore structure-property relationships, making them more accessible through spectroscopy. Moreover, we highlight vibrational spectral features as efficient carriers for highly dimensional chemical information, enabling the simultaneous optimization of multiple performance parameters. These findings pave the way for pioneering designs of enzyme-mimetic MOFs and concurrently expand the application scope of spectroscopic tools in biomimetic catalysis.

Highlights from the 57th Bürgenstock Conference on Stereochemistry 2024

Herein, we share an overview of the scientific highlights from speakers at the latest edition of the longstanding Bürgenstock Conference.

Downregulation of a Phi class glutathione S-transferase gene in transgenic torenia yielded pale flower color

The members of glutathione S-transferase (GST) belonging to the Phi class of the GST family are known to play a role in anthocyanin transport to the vacuole. We isolated a GST orthologue from the torenia petal cDNA library. Transgenic plants transcribing GST double stranded RNA were generated from a torenia cultivar having blue flowers. These plants exhibited a range of flower colors, from blue to almost white. Quantitative RT-PCR confirmed the downregulation of the GST transcript, accompanied by a decrease in anthocyanin levels in the petals of the transgenic plants, whereas flavone levels remained unchanged. These results suggest that GST is involved in anthocyanin transport in torenia petals, and that anthocyanins and flavones are likely transported to the vacuole through different mechanisms.

The road less taken: Dihydroflavonol 4-reductase inactivation and delphinidin anthocyanin loss underpins a natural intraspecific flower colour variation

Visual cues are of critical importance for the attraction of animal pollinators, however, little is known about the molecular mechanisms underpinning intraspecific floral colour variation. Here, we combined comparative spectral analysis, targeted metabolite profiling, multi-tissue transcriptomics, differential gene expression, sequence analysis and functional analysis to investigate a bee-pollinated orchid species, Glossodia major with common purple- and infrequent white-flowered morphs. We found uncommon and previously unreported delphinidin-based anthocyanins responsible for the conspicuous and pollinator-perceivable colour of the purple morph and three genetic changes underpinning the loss of colour in the white morph - (1) a loss-of-function (LOF; frameshift) mutation affecting dihydroflavonol 4-reductase (DFR1) coding sequence due to a unique 4-bp insertion, (2) specific downregulation of functional DFR1 expression and (3) the unexpected discovery of chimeric Gypsy transposable element (TE)-gene (DFR) transcripts with potential consequences to the genomic stability and post-transcriptional or epigenetic regulation of DFR. This is one of few known cases where regulatory changes and LOF mutation in an anthocyanin structural gene, rather than transcription factors, are important. Furthermore, if TEs prove to be a frequent source of mutation, the interplay between environmental stress-induced TE evolution and pollinator-mediated selection for adaptive colour variation may be an overlooked mechanism maintaining floral colour polymorphism in nature.

Revisiting decade-old questions in proanthocyanidin biosynthesis: current understanding and new challenges

Proanthocyanidins (PAs), one of the most abundant natural polymers found in plants, are gaining increasing attention because of their beneficial effects for agriculture and human health. The study of PA biosynthesis has been active for decades, and progress has been drastically accelerated since the discovery of key enzymes such as Anthocyanidin Reductase (ANR), Leucoanthocyanidin Reductase (LAR), and key transcription factors such as Transparent Testa 2 (TT2) and Transparent Testa 8 (TT8) in the early 2000s. Scientists raised some compelling questions regarding PA biosynthesis about two decades ago in the hope that addressing these questions would lead to an enhanced understanding of PA biosynthesis in plants. These questions focus on the nature of starter and extension units for PA biosynthesis, the stereochemistry of PA monomers and intermediates, and how and where the polymerization or condensation steps work subcellularly. Here, I revisit these long-standing questions and provide an update on progress made toward answering them. Because of advanced technologies in genomics, bioinformatics and metabolomics, we now have a much-improved understanding of functionalities of key enzymes and identities of key intermediates in the PA biosynthesis and polymerization pathway. Still, several questions, particularly the ones related to intracellular PA transportation and deposition, as well as enzyme subcellular localization, largely remain to be explored. Our increasing understanding of PA biosynthesis in various plant species has led to a new set of compelling open questions, suggesting future research directions to gain a more comprehensive understanding of PA biosynthesis.

Metabolite and transcriptome analyses reveal the effects of salinity stress on the biosynthesis of proanthocyanidins and anthocyanins in grape suspension cells

Proanthocyanidins (PAs) and anthocyanins are flavonoids that contribute to the quality and health benefits of grapes and wine. Salinity affects their biosynthesis, but the underlying mechanism is still unclear. We studied the effects of NaCl stress on PA and anthocyanin biosynthesis in grape suspension cells derived from berry skins of Vitis vinifera L. Cabernet Sauvignon using metabolite profiling and transcriptome analysis. We treated the cells with low (75 mM NaCl) and high (150 mM NaCl) salinity for 4 and 7 days. High salinity inhibited cell growth and enhanced PA and anthocyanin accumulation more than low salinity. The salinity-induced PAs and anthocyanins lacked C5'-hydroxylation modification, suggesting the biological significance of delphinidin- and epigallocatechin-derivatives in coping with stress. The genes up-regulated by salinity stress indicated that the anthocyanin pathway was more sensitive to salt concentration than the PA pathway, and WGCNA analysis revealed the coordination between flavonoid biosynthesis and cell wall metabolism under salinity stress. We identified transcription factors potentially involved in regulating NaCl dose- and time-dependent PA and anthocyanin accumulation, showing the dynamic remodeling of flavonoid regulation network under different salinity levels and durations. Our study provides new insights into regulator candidates for tailoring flavonoid composition and molecular indicators of salt stress in grape cells.

Unveiling the evolutionary history of lingonberry (Vaccinium vitis-idaea L.) through genome sequencing and assembly of European and North American subspecies

Lingonberry (Vaccinium vitis-idaea L.) produces tiny red berries that are tart and nutty in flavor. It grows widely in the circumpolar region, including Scandinavia, northern parts of Eurasia, Alaska, and Canada. Although cultivation is currently limited, the plant has a long history of cultural use among indigenous communities. Given its potential as a food source, genomic resources for lingonberry are significantly lacking. To advance genomic knowledge, the genomes for 2 subspecies of lingonberry (V. vitis-idaea ssp. minus and ssp. vitis-idaea var. 'Red Candy') were sequenced and de novo assembled into contig-level assemblies. The assemblies were scaffolded using the bilberry genome (Vaccinium myrtillus) to generate a chromosome-anchored reference genome consisting of 12 chromosomes each with a total length of 548.07 Mb [contig N50 = 1.17 Mb, BUSCO (C%) = 96.5%] for ssp. vitis-idaea and 518.70 Mb [contig N50 = 1.40 Mb, BUSCO (C%) = 96.9%] for ssp. minus. RNA-seq-based gene annotation identified 27,243 and 25,718 genes on the respective assembly, and transposable element detection methods found that 45.82 and 44.58% of the genome were repeats. Phylogenetic analysis confirmed that lingonberry was most closely related to bilberry and was more closely related to blueberries than cranberries. Estimates of past effective population size suggested a continuous decline over the past 1-3 MYA, possibly due to the impacts of repeated glacial cycles during the Pleistocene leading to frequent population fragmentation. The genomic resource created in this study can be used to identify industry-relevant genes (e.g. anthocyanin production), infer phylogeny, and call sequence-level variants (e.g. SNPs) in future research.