Assessment of Methodological Pipelines for the Determination of Isothiocyanates Derived from Natural Sources

Isothiocyanates are biologically active secondary metabolites liberated via enzymatic hydrolysis of their sulfur enriched precursors, glucosinolates, upon tissue plant disruption. The importance of this class of compounds lies in their capacity to induce anti-cancer, anti-microbial, anti-inflammatory, neuroprotective, and other bioactive properties. As such, their isolation from natural sources is of utmost importance. In this review article, an extensive examination of the various parameters (hydrolysis, extraction, and quantification) affecting the isolation of isothiocyanates from naturally-derived sources is presented. Overall, the effective isolation/extraction and quantification of isothiocyanate is strongly associated with their chemical and physicochemical properties, such as polarity-solubility as well as thermal and acidic stability. Furthermore, the successful activation of myrosinase appears to be a major factor affecting the conversion of glucosinolates into active isothiocyanates.

The Cellular and Subcellular Organization of the Glucosinolate–Myrosinase System against Herbivores and Pathogens

Glucosinolates are an important class of secondary metabolites in Brassicales plants with a critical role in chemical defense. Glucosinolates are chemically inactive but can be hydrolyzed by myrosinases to produce a range of chemically active compounds toxic to herbivores and pathogens, thereby constituting the glucosinolate–myrosinase defense system or the mustard oil bomb. During the evolution, Brassicales plants have developed not only complex biosynthetic pathways for production of a large number of glucosinolate structures but also different classes of myrosinases that differ in catalytic mechanisms and substrate specificity. Studies over the past several decades have made important progress in the understanding of the cellular and subcellular organization of the glucosinolate–myrosinase system for rapid and timely detonation of the mustard oil bomb upon tissue damage after herbivore feeding and pathogen infection. Progress has also been made in understanding the mechanisms that herbivores and pathogens have evolved to counter the mustard oil bomb. In this review, we summarize our current understanding of the function and organization of the glucosinolate–myrosinase system in Brassicales plants and discuss both the progresses and future challenges in addressing this complex defense system as an excellent model for analyzing plant chemical defense.

Arabidopsis Glutathione-S-Transferases GSTF11 and GSTU20 Function in Aliphatic Glucosinolate Biosynthesis

Glutathione (GSH) conjugation with intermediates is required for the biosynthesis of glucosinolate (GSL) by serving as a sulfur supply. Glutathione-S-transferases (GSTs) primarily work on GSH conjugation, suggesting their involvement in GSL metabolism. Although several GSTs, including GSTF11 and GSTU20, have been recently postulated to act in GSL biosynthesis, molecular evidence is lacking. Here, we demonstrated that GSTF11 and GSTU20 play non-redundant, although partially overlapping, roles in aliphatic GSL biosynthesis. In addition, GSTU20 plays a more important role than GSTF11, which is manifested by the greater loss of aliphatic GSLs associated with GSTU20 mutant and a greater number of differentially expressed genes in GSTU20 mutant compared to GSTF11 mutant. Moreover, a double mutation leads to a greater aggregate loss of aliphatic GSLs, suggesting that GSTU20 and GSTF11 may function in GSL biosynthesis in a dosage-dependent manner. Together, our results provide direct evidence that GSTU20 and GSTF11 are critically involved in aliphatic GSL biosynthesis, filling the knowledge gap that has been speculated in recent decades.

Liquiritin elicitation can increase the content of medicinally important glucosinolates and phenolic compounds in Chinese kale plants

BACKGROUND

Brassica oleracea var. alboglabra (Chinese kale) is an important vegetable grown in southern China. This study was aimed at searching for environmentally friendly and affordable approaches to increase the production of medicinally relevant glucosinolates and phenolic compounds in Chinese kale plants. For this purpose, the foliar application of liquiritin at 0 (control), 250, 500 and 750 ppm was tested starting from the four-leaf stage and repeated every two weeks until plants were two months old.

RESULTS

Foliar application of liquiritin in Chinese kale plants significantly increased glucosinolates and total phenolic content, in a dose-dependent manner. Compared with control plants, 2.3- and 1.9-fold increases in yields of glucosinolates and total phenolic content, respectively, were corroborated in Chinese kale plants treated with 750 ppm of liquiritin. Along with rises in the content of eight different glucosinolates, liquiritin elicitation effectively increased the concentration of glycosilated and acylated flavonoids and hydroxycinnamic acids. The expression of genes involved in glucosinolate and phenolic biosynthesis was significantly higher in liquiritin-treated plants as compared to controls.

CONCLUSIONS

Liquiritin elicitation is a feasible and environmentally friendly practice for increasing the production of medicinally important glucosinolates and phenolic compounds in Chinese kale, which may improve this plant's value as a nutraceutical food. This study also contributes to understanding the molecular mechanisms underlying liquiritin elicitation. This is the first report documenting the use of liquiritin for an elicitation purpose in plants. © 2019 Society of Chemical Industry.

Increased Glucosinolate Production in Brassica oleracea var. italica Cell Cultures Due to Coronatine Activated Genes Involved in Glucosinolate Biosynthesis

In this work, the effect of different elicitors and culture conditions on the production of glucosinolates in broccoli cell cultures was studied. The results showed that 0.5 μM coronatine was the best elicitor for increasing glucosinolate production (205-fold increase over untreated cells after 72 h of treatment). Furthermore, the expression levels of some genes related to the biosynthetic pathway of glucosinolates as well as three Myb transcription factors also have been studied. The highest glucosinolate levels found in coronatine-treated cells were closely correlated with the highest gene expression levels of Cyp79b2, Cyp83b1, St5a, Myb51, and Myb122 after 6 h of treatment. The data shown in this study provide new insight into the key metabolic steps involved in the biosynthesis of glucosinolates, which will be of use for future applications of metabolic engineering techniques in broccoli.

Chemodiversity of the Glucosinolate-Myrosinase System at the Single Cell Type Resolution

Glucosinolates (GLSs) are a well-defined group of specialized metabolites, and like any other plant specialized metabolites, their presence does not directly affect the plant survival in terms of growth and development. However, specialized metabolites are essential to combat environmental stresses, such as pathogens and herbivores. GLSs naturally occur in many pungent plants in the order of Brassicales. To date, more than 200 different GLS structures have been characterized and their distribution differs from species to species. GLSs co-exist with classical and atypical myrosinases, which can hydrolyze GLS into an unstable aglycone thiohydroximate-O-sulfonate, which rearranges to produce different degradation products. GLSs, myrosinases, myrosinase interacting proteins, and GLS degradation products constitute the GLS-myrosinase (GM) system ("mustard oil bomb"). This review discusses the cellular and subcellular organization of the GM system, its chemodiversity, and functions in different cell types. Although there are many studies on the functions of GLSs and/or myrosinases at the tissue and whole plant levels, very few studies have focused on different single cell types. Single cell type studies will help to reveal specific functions that are missed at the tissue and organismal level. This review aims to highlight (1) recent progress in cellular and subcellular compartmentation of GLSs, myrosinases, and myrosinase interacting proteins; (2) molecular and biochemical diversity of GLSs and myrosinases; and (3) myrosinase interaction with its interacting proteins, and how it regulates the degradation of GLSs and thus the biological functions (e.g., plant defense against pathogens). Future prospects may include targeted approaches for engineering/breeding of plants and crops in the cell type-specific manner toward enhanced plant defense and nutrition.

Biotechnological approaches in glucosinolate production

Glucosinolates (GLSs) are sulfur-rich, amino acid-derived defense compounds characteristic of the Brassicales order. In the past, GLSs were mostly known as anti-nutritional factors in fodder, biopesticides in agriculture, and flavors in condiments such as mustard. However, in recent times, GLSs have received increased attention as promoters of human health. This has spurred intensive research towards generating rich sources of health-promoting GLSs. We provide a comprehensive overview of the biotechnological approaches applied to reach this goal. This includes optimization of GLS production and composition in native, GLS-producing plants, including hairy root and cell cultures thereof, as well as synthetic biology approaches in heterologous hosts, such as tobacco and the microbial organisms Escherichia coli and Saccharomyces cerevisiae. The progress using these different approaches is discussed.

Biosynthesis and bioactivity of glucosinolates and their production in plant in vitro cultures

Glucosinolates are biologically active compounds which are involved in plant defense reaction. The use of plant in vitro cultures and genetic engineering is a promising strategy for their sustainable production. Glucosinolates are a class of secondary metabolites found mainly in Brassicaceae, which contain nitrogen and sulfur in their structures. Glucosinolates are divided into three groups depending on the amino acid from which they are biosynthesized. Aliphatic glucosinolates are generally derived from leucine, valine, methionine, isoleucine and alanine while indole and aromatic glucosinolates are derived from tryptophan and phenylalanine or tyrosine, respectively. These compounds are hydrolyzed by the enzyme myrosinase when plants are stressed by biotic and abiotic factors, obtaining different degradation products. Glucosinolates and their hydrolysis products play an important role in plant defense responses against different types of stresses. In addition, these compounds have beneficial effect on human health because they are strong antioxidants and they have potent cardiovascular, antidiabetic, antimicrobial and antitumoral activities. Due to all the properties described above, the demand for glucosinolates and their hydrolysis products has enormously increased, and therefore, new strategies that allow the production of these compounds to be improved are needed. The use of plant in vitro cultures is emerging as a biotechnological strategy to obtain glucosinolates and their derivatives. This work is focused on the biosynthesis of glucosinolates and the bioactivity of these compounds in plants. In addition, a detailed study on the strategies used to increase the production of several glucosinolates, in particular those synthesized in Brassicaceae, using in vitro plant cultures has been made. Special attention has been paid for increasing the production of glucosinolates and their derivatives using metabolic engineering.

CB5C affects the glucosinolate profile in Arabidopsis thaliana

Cytochrome b5 (CB5) proteins are small heme-binding proteins, that influence cytochrome P450 activity. While only one CB5 isoform is found in mammals, higher plants have several isoforms of these proteins. The roles of the many CB5 isoforms in plants remain unknown. We hypothesized that CB5 proteins support the cytochrome P450 enzymes of plant specialized metabolism and found CB5C from Arabidopsis thaliana to co-express with glucosinolate biosynthetic genes. We characterized the glucosinolate profiles of 2 T-DNA insertion mutants of CB5C, and found that long-chained aliphatic glucosinolates were reduced in one of the mutant lines - a phenotype that was exaggerated upon methyl-jasmonate treatment. These results support the hypothesis, that CB5C influences glucosinolate biosynthesis, however, the mode of action remains unknown. Furthermore, the mutants differed in their biomass response to methyl jasmonate treatment. Thereby, our results highlight the varying effects of T-DNA insertion sites, as the 2 analyzed alleles show different phenotypes.

Genomic origin, expression differentiation and regulation of multiple genes encoding CYP83A1, a key enzyme for core glucosinolate biosynthesis, from the allotetraploid Brassica juncea

The multiple BjuCYP83A1 genes formed as a result of polyploidy have retained cell-, tissue-, and condition-specific transcriptional sub-functionalization to control the complex aliphatic glucosinolates biosynthesis in the allotetraploid Brassica juncea. Glucosinolates along with their breakdown products are associated with diverse roles in plant metabolism, plant defense and animal nutrition. CYP83A1 is a key enzyme that oxidizes aliphatic aldoximes to aci-nitro compounds in the complex aliphatic glucosinolate biosynthetic pathway. In this study, we reported the isolation of four CYP83A1 genes named BjuCYP83A1-1, -2, -3, and -4 from allotetraploid Brassica juncea (AABB genome), an economically important oilseed crop of Brassica genus. The deduced BjuCYP83A1 proteins shared 85.7-88.4 % of sequence identity with A. thaliana AtCYP83A1 and 84.2-95.8 % among themselves. Phylogenetic and divergence analysis revealed that the four BjuCYP83A1 proteins are evolutionary conserved and have evolved via duplication and hybridization of two relatively simpler diploid Brassica genomes namely B. rapa (AA genome) and B. nigra (BB genome), and have retained high level of sequence conservation following allopolyploidization. Ectopic over-expression of BjuCYP83A1-1 in A. thaliana showed that it is involved mainly in the synthesis of C4 aliphatic glucosinolates. Detailed expression analysis using real-time qRT-PCR in B. juncea and PromoterBjuCYP83A1-GUS lines in A. thaliana confirmed that the four BjuCYP83A1 genes have retained ubiquitous, overlapping but distinct expression profiles in different tissue and cell types of B. juncea, and in response to various elicitor treatments and environmental conditions. Taken together, this study demonstrated that transcriptional sub-functionalization and coordinated roles of multiple BjuCYP83A1 genes control the biosynthesis of aliphatic glucosinolates in the allotetraploid B. juncea, and provide a framework for metabolic engineering of aliphatic glucosinolates in economically important Brassica species.

Crambe tataria: The Glucosinolate/Myrosinase System in vivo and in vitro

The Crambe tataria glucosinolate/myrosinase system in seeds and leaves of in vivo and in vitro regenerated plantlets, and two callus cell lines was investigated. It was demonstrated that in all the extracts glucosinolates were present and the myrosinase system was operative. There appears to be no discrimination between the glucosinolates used as substrates, but the hydrolysis rates were different regardless of the nature of the side chain. This is one of the first studies demonstrating that undifferentiated cells are able to synthetize glucosinolates and have an operating myrosinase system.

Functional characterization of Arabidopsis thaliana isopropylmalate dehydrogenases reveals their important roles in gametophyte development

• Isopropylmalate dehydrogenases (IPMDHs) catalyze the oxidative decarboxylation of 3-isopropylmalate (3-IPM) in leucine biosynthesis in microorganisms. The Arabidopsis thaliana genome contains three putative IPMDH genes. • IPMDH2 and IPMDH3 proteins exhibited significantly higher activity toward 3-IPM than IPMDH1, which is indicative of a pivotal role in leucine biosynthesis. Single mutants of IPMDH2 or IPMDH3 lacked a discernible phenotype. Genetic analysis showed that ipmdh2 ipmdh3 was lethal in male gametophytes and had reduced transmission through female gametophytes. The aborted pollen grains were small, abnormal in cellular structure, and arrested in germination. In addition, half of the double mutant embryo sacs exhibited slowed development. • The IPMDH2/ipmdh2 ipmdh3/ipmdh3 genotype exhibited abnormal vegetative phenotypes, suggesting haplo-insufficiency of IPMDH2 in the ipmdh3 background. This mutant and a triple mutant containing one allele of IPMDH2 or IPMDH3 had decreased leucine biosynthetic enzyme activities and lower free leucine concentrations. The latter mutant showed changes in glucosinolate profiles different from those in the ipmdh1 mutant. • The results demonstrate that IPMDH2 and IPMDH3 primarily function in leucine biosynthesis, are essential for pollen development and are needed for proper embryo sac development.

Functional specification of Arabidopsis isopropylmalate isomerases in glucosinolate and leucine biosynthesis

The Arabidopsis genome encodes one potential isopropylmalate isomerase (IPMI) large subunit and three potential IPMI small subunits, which in bacteria and archaea form heterodimers to catalyze the isomerization of 2-isopropylmalate to 3-isopropylmalate in leucine biosynthesis. We demonstrate here that AtLeuC physically interacts with AtLeuD proteins to form functional IPMIs. The IPMIs are localized to chloroplast stroma. Tissue-specific expression analysis revealed that the patterns of AtLeuD1 and AtLeuD2 expression are similar, but distinct from that of AtLeuD3. This result indicates functional redundancy of AtLeuD1 and AtLeuD2, and functional specification of AtLeuD3. Reverse genetics and metabolite profiling showed that AtLeuD1 and AtLeuD2 function redundantly in aliphatic glucosinolate biosynthesis, but AtLeuD3 is not likely to be involved in this pathway. The lethal phenotype of the atleud3 mutant suggests functional specification of AtLeuD3 in leucine biosynthesis. A defect in female gametophyte development was found to contribute to the mutant lethality, suggesting the important role of AtLeuD3 in female gametophyte development.

A redox-active isopropylmalate dehydrogenase functions in the biosynthesis of glucosinolates and leucine in Arabidopsis

We report a detailed functional characterization of an Arabidopsis isopropylmalate dehydrogenase (AtIPMDH1) that is involved in both glucosinolate biosynthesis and leucine biosynthesis. AtIPMDH1 shares high homology with enzymes from bacteria and yeast that are known to function in leucine biosynthesis. In plants, AtIPMDH1 is co-expressed with nearly all the genes known to be involved in aliphatic glucosinolate biosynthesis. Mutation of AtIPMDH1 leads to a significant reduction in the levels of free leucine and of glucosinolates with side chains of four or more carbons. Complementation of the mutant phenotype by ectopic expression of AtIPMDH1, together with the enzyme's substrate specificity, implicates AtIPMDH1 in both glucosinolate and leucine biosynthesis. This functional assignment is substantiated by subcellular localization of the protein in the chloroplast stroma, and the gene expression patterns in various tissues. Interestingly, AtIPMDH1 activity is regulated by a thiol-based redox modification. This work characterized an enzyme in plants that catalyzes the oxidative decarboxylation step in both leucine biosynthesis (primary metabolism) and methionine chain elongation of glucosinolates (specialized metabolism). It provides evidence for the hypothesis that the two pathways share a common origin, and suggests a role for redox regulation of glucosinolate and leucine synthesis in plants.

Benzylglucosinolate, benzylisothiocyanate, and myrosinase activity in papaya fruit during development and ripening

Papaya is a climacteric fruit that has high amounts of benzylglucosinolates (BG) and benzylisothiocyanates (BITC), but information regarding levels of BG or BITC during fruit development and ripening is limited. Because BG and BITC are compounds of importance from both a nutritional and a crop yield standpoint, the aim of this work was to access data on the distribution and changes of BG and BITC levels during fruit development and ripening. BG and BITC levels were quantified in peel, pulp, and seeds of papaya fruit. Volatile BITC was also verified in the internal cavity of the fruit during ripening. The influence of the ethylene in BG and BITC levels and mirosinase activity was tested by exposing mature green fruits to ethylene and 1-methylcyclopropene (1-MCP). The highest BG levels were detected in seeds, followed by the peel and pulp being decreased in all tissues during fruit development. Similarly, the levels of BITC were much higher in the seeds than the peel and pulp. The levels of BG for control and ethylene-treated fruit were very similar, increasing in the pulp and peel during late ripening but not changing significantly in seeds. On the other hand, fruit exposed to 1-MCP showed a decrease in BG amount in the pulp and accumulation in seed. The treatments did not result in clear differences regarding the amount of BITC in the pulp and peel of the fruit. According to the results, ethylene does not have a clear effect on BITC accumulation in ripening papaya fruit. The fact that BG levels in the pulp did not decrease during ripening, regardless of the treatment employed, and that papaya is consumed mainly as fresh fruit, speaks in favor of this fruit as a good dietary source for glucosinolate and isothiocyanates.