The ‘mustard oil bomb’: not so easy to assemble?! Localization, expression and distribution of the components of the myrosinase enzyme system

Glucosinolates are plant secondary metabolites that are hydrolysed by the action of myrosinases into various products (isothiocyanates, thiocyanates, epithionitriles, nitriles, oxazolidines). Massive hydrolysis of glucosinolates occurs only upon tissue damage but there is also evidence indicating metabolism of glucosinolates in intact plant tissues. It was originally believed that the glucosinolate–myrosinase system in intact plants was stable due to a spatial separation of the components. This has been coined as the ‘mustard oil bomb’ theory. Proteins that form complexes with myrosinases have been described: myrosinase-binding proteins (MBPs) and myrosinase-associated proteins (MyAPs/ESM). The roles of these proteins and their biological relevance are not yet completely known. Other proteins of the myrosinase enzyme system are the epithiospecifier protein (ESP) and the thiocyanate-forming protein (TFP) that divert the glucosinolate hydrolysis from isothiocyanate production to nitrile/epithionitrile or thiocyanate production. Some glucosinolate hydrolysis products act as plant defence compounds against insects and pathogens or have beneficial health effects on humans. In this review, we survey and critically assess the available information concerning the localization, both at the tissular/cellular and subcellular level, of the different components of the myrosinase enzyme system. Data from the model plant Arabidopsis thaliana is compared to that from other glucosinolate-producing Brassicaceae in order to show common as well as divergent features of the ‘mustard oil bomb’ among these species.     

The genetic basis of constitutive and herbivore-induced ESP-independent nitrile formation in Arabidopsis

Glucosinolates are a group of thioglucosides that are components of an activated chemical defense found in the Brassicales. Plant tissue damage results in hydrolysis of glucosinolates by endogenous thioglucosidases known as myrosinases. Spontaneous rearrangement of the aglucone yields reactive isothiocyanates that are toxic to many organisms. In the presence of specifier proteins, alternative products, namely epithionitriles, simple nitriles, and thiocyanates with different biological activities, are formed at the expense of isothiocyanates. Recently, simple nitriles were recognized to serve distinct functions in plant-insect interactions. Here, we show that simple nitrile formation in Arabidopsis (Arabidopsis thaliana) ecotype Columbia-0 rosette leaves increases in response to herbivory and that this increase is independent of the known epithiospecifier protein (ESP). We combined phylogenetic analysis, a screen of Arabidopsis mutants, recombinant protein characterization, and expression quantitative trait locus mapping to identify a gene encoding a nitrile-specifier protein (NSP) responsible for constitutive and herbivore-induced simple nitrile formation in Columbia-0 rosette leaves. AtNSP1 is one of five Arabidopsis ESP homologues that promote simple nitrile, but not epithionitrile or thiocyanate, formation. Four of these homologues possess one or two lectin-like jacalin domains, which share a common ancestry with the jacalin domains of the putative Arabidopsis myrosinase-binding proteins MBP1 and MBP2. A sixth ESP homologue lacked specifier activity and likely represents the ancestor of the gene family with a different biochemical function. By illuminating the genetic and biochemical bases of simple nitrile formation, our study provides new insights into the evolution of metabolic diversity in a complex plant defense system.     

A thiocyanate-forming protein generates multiple products upon allylglucosinolate breakdown in Thlaspi arvense

Glucosinolates, amino acid-derived thioglycosides found in plants of the Brassicales order, are one of the best studied classes of plant secondary metabolites. Together with myrosinases and supplementary proteins known as specifier proteins, they form the glucosinolate–myrosinase system that upon tissue damage gives rise to a number of biologically active glucosinolate breakdown products such as isothiocyanates, epithionitriles and organic thiocyanates involved in plant defense. While isothiocyanates are products of the spontaneous rearrangement of the glucosinolate aglycones released by myrosinase, the formation of epithionitriles and organic thiocyanates depends on both myrosinases and specifier proteins. Hydrolysis product profiles of many glucosinolate-containing plant species indicate the presence of specifier proteins, but only few have been identified and characterized biochemically. Here, we report on cDNA cloning, heterologous expression and characterization of TaTFP, a thiocyanate-forming protein (TFP) from Thlaspi arvense L. (Brassicaceae), that is expressed in all plant organs and can be purified in active form after heterologous expression in Escherichia coli. As a special feature, this protein promotes the formation of allylthiocyanate as well as the corresponding epithionitrile upon myrosinase-catalyzed hydrolysis of allylglucosinolate, the major glucosinolate of T. arvense. All other glucosinolates tested are converted to their simple nitriles when hydrolyzed in the presence of TaTFP. Despite its ability to promote allylthiocyanate formation, TaTFP has a higher amino acid sequence similarity to known epithiospecifier proteins (ESPs) than to Lepidium sativum TFP. However, unlike Arabidopsis thaliana ESP, its activity in vitro is not strictly dependent on Fe²⁺ addition to the assay mixtures. The availability of TaTFP in purified form enables future studies to be aimed at elucidating the structural bases of specifier protein specificities and mechanisms. Furthermore, identification of TaTFP shows that product specificities of specifier proteins can not be predicted based on amino acid sequence similarity and raises interesting questions about specifier protein evolution.     

Epithiospecifier protein activity in broccoli: the link between terminal alkenyl glucosinolates and sulphoraphane nitrile

The chemical nature of the hydrolysis products from the glucosinolate-myrosinase system depends on the presence or absence of supplementary proteins, such as epithiospecifier proteins (ESPs). ESPs (non-catalytic cofactors of myrosinase) promote the formation of epithionitriles from terminal alkenyl glucosinolates and as recent evidence suggests, simple nitriles at the expense of isothiocyanates. The ratio of ESP activity to myrosinase activity is crucial in determining the proportion of these nitriles produced on hydrolysis. Sulphoraphane, a major isothiocyanate produced in broccoli seedlings, has been found to be a potent inducer of phase 2 detoxification enzymes. However, ESP may also support the formation of the non-inductive sulphoraphane nitrile. Our objective was to monitor changes in ESP activity during the development of broccoli seedlings and link these activity changes with myrosinase activity, the level of terminal alkenyl glucosinolates and sulphoraphane nitrile formed. Here, for the first time, we show ESP activity increases up to day 2 after germination before decreasing again to seed activity levels at day 5. These activity changes paralleled changes in myrosinase activity and terminal alkenyl glucosinolate content. There is a significant relationship between ESP activity and the formation of sulforaphane nitrile in broccoli seedlings. The significance of these findings for the health benefits conferred by eating broccoli seedlings is briefly discussed.     

Isolation of the epithiospecifier protein from oil-rape (Brassica napus ssp. oleifera) seed and its characterization

The epithiospecifier protein (ESP) is a myrosinase (MYR) cofactor, which is necessary to drive the MYR-catalyzed hydrolysis of some specific glucosinolates towards the production of cyanoepithioalkanes instead of isothiocyanates and nitriles. ESP was isolated from Brassica napus seeds by anionic exchange and gel filtration chromatography. ESP showed a molecular weight of about 39 kDa and pI 5.3. The amino acid sequence of several tryptic peptides of ESP (accounting for about 50% of the total sequence) made it possible to establish the high similarity (81% identity) with a hypothetical 37 kDa protein (TrEMBL data base accession number Q39104) and several jasmonate-inducible proteins from Arabidopsis thaliana. This observation suggests that ESP is likely to be involved in jasmonate-mediated defence and disease resistance mechanisms.     

The Arabidopsis Epithiospecifier Protein Promotes the Hydrolysis of Glucosinolates to Nitriles and Influences Trichoplusia ni Herbivory

Glucosinolates are anionic thioglucosides that have become one of the most frequently studied groups of defensive metabolites in plants. When tissue damage occurs, the thioglucoside linkage is hydrolyzed by enzymes known as myrosinases, resulting in the formation of a variety of products that are active against herbivores and pathogens. In an effort to learn more about the molecular genetic and biochemical regulation of glucosinolate hydrolysis product formation, we analyzed leaf samples of 122 Arabidopsis ecotypes. A distinct polymorphism was observed with all ecotypes producing primarily isothiocyanates or primarily nitriles. The ecotypes Columbia (Col) and Landsberg erecta (Ler) differed in their hydrolysis products; therefore, the Col x Ler recombinant inbred lines were used for mapping the genes controlling this polymorphism. The major quantitative trait locus (QTL) affecting nitrile versus isothiocyanate formation was found very close to a gene encoding a homolog of a Brassica napus epithiospecifier protein (ESP), which causes the formation of epithionitriles instead of isothiocyanates during glucosinolate hydrolysis in the seeds of certain Brassicaceae. The heterologously expressed Arabidopsis ESP was able to convert glucosinolates both to epithionitriles and to simple nitriles in the presence of myrosinase, and thus it was more versatile than previously described ESPs. The role of ESP in plant defense is uncertain, because the generalist herbivore Trichoplusia ni (the cabbage looper) was found to feed more readily on nitrile-producing than on isothiocyanate-producing Arabidopsis. However, isothiocyanates are frequently used as recognition cues by specialist herbivores, and so the formation of nitriles instead of isothiocyanates may allow Arabidopsis to be less apparent to specialists.     

Characterisation of recombinant epithiospecifier protein and its over-expression in Arabidopsis thaliana

Epithiospecifier protein (ESP) is a protein that catalyses formation of epithionitriles during glucosinolate hydrolysis. In vitro assays with a recombinant ESP showed that the formation of epithionitriles from alkenylglucosinolates is ESP and ferrous ion dependent. Nitrile formation in vitro however does not require ESP but only the presence of Fe(II) and myrosinase. Ectopic expression of ESP in Arabidopsis thaliana Col-5 under control of the strong viral CaMV 35S promoter altered the glucosinolate product profile from isothiocyanates towards the corresponding nitriles.     

Evolution of specifier proteins in glucosinolate-containing plants

ABSTRACT: BACKGROUND: The glucosinolate-myrosinase system is an activated chemical defense system found in plants of the Brassicales order. Glucosinolates are stored separately from their hydrolytic enzymes, the myrosinases, in plant tissues. Upon tissue damage, e.g. by herbivory, glucosinolates and myrosinases get mixed and glucosinolates are broken down to an array of biologically active compounds of which isothiocyanates are toxic to a wide range of organisms. Specifier proteins occur in some, but not all glucosinolate-containing plants and promote the formation of biologically active non-isothiocyanate products upon myrosinase-catalyzed glucosinolate breakdown. RESULTS: Based on a phytochemical screening among representatives of the Brassicales order, we selected candidate species for identification of specifier protein cDNAs. We identified ten specifier proteins from a range of species of the Brassicaceae and assigned each of them to one of the three specifier protein types (NSP, nitrile-specifier protein, ESP, epithiospecifier protein, TFP, thiocyanate-forming protein) after heterologous expression in Escherichia coli. Together with nine known specifier proteins and three putative specifier proteins found in databases, we subjected the newly identified specifier proteins to phylogenetic analyses. Specifier proteins formed three major clusters, named AtNSP5-cluster, AtNSP1-cluster, and ESP/TFP cluster. Within the ESP/TFP cluster, but not within the AtNSP1 cluster, specifier proteins grouped according to the Brassicaceae lineage they were identified from. Non-synonymous vs. synonymous substitution rate ratios suggested purifying selection to act on specifier protein genes. CONCLUSIONS: Among specifier proteins, NSPs represent the ancestral activity. The data support a monophyletic origin of ESPs from NSPs. The split between NSPs and ESPs/TFPs happened before the appearance of lineage I and expanded lineage II of the Brassicaceae. TFP activity evolved from ESPs at least twice independently in different Brassicaceae lineages. The ability to form non-isothiocyanate products by specifier protein activity may provide plants with a selective advantage. The evolution of specifier proteins in the Brassicaceae demonstrates the plasticity of secondary metabolism within an activated plant defense system.     

Genotype, age, tissue, and environment regulate the structural outcome of glucosinolate activation

Glucosinolates are the inert storage form of a two-part phytochemical defense system in which the enzyme myrosinase generates an unstable intermediate that rapidly rearranges into the biologically active product. This rearrangement step generates simple nitriles, epithionitriles, or isothiocyanates, depending on the structure of the parent glucosinolate and the presence of proteins that promote specific structural outcomes. Glucosinolate accumulation and myrosinase activity differ by plant age and tissue type and respond to environmental stimuli such as planting density and herbivory; however, the influence of these factors on the structural outcome of the rearrangement step remains unknown. We show that the structural outcome of glucosinolate activation is controlled by interactions among plant age, planting density, and natural genetic variation in Arabidopsis (Arabidopsis thaliana) rosette leaves using six well-studied accessions. We identified a similarly complex interaction between tissue type and the natural genetic variation present within these accessions. This raises questions about the relative importance of these novel levels of regulation in the evolution of plant defense. Using mutants in the structural specifier and glucosinolate activation genes identified previously in Arabidopsis rosette leaves, we demonstrate the requirement for additional myrosinases and structural specifiers controlling these processes in the roots and seedlings. Finally, we present evidence for a novel EPITHIOSPECIFIER PROTEIN-independent, simple nitrile-specifying activity that promotes the formation of simple nitriles but not epithionitriles from all glucosinolates tested.     

Molecular models and mutational analyses of plant specifier proteins suggest active site residues and reaction mechanism

As components of the glucosinolate-myrosinase system, specifier proteins contribute to the diversity of chemical defenses that have evolved in plants of the Brassicales order as a protection against herbivores and pathogens. Glucosinolates are thioglucosides that are stored separately from their hydrolytic enzymes, myrosinases, in plant tissue. Upon tissue disruption, glucosinolates are hydrolyzed by myrosinases yielding instable aglucones that rearrange to form defensive isothiocyanates. In the presence of specifier proteins, other products, namely simple nitriles, epithionitriles and organic thiocyanates, can be formed instead of isothiocyanates depending on the glucosinolate side chain structure and the type of specifier protein. The biochemical role of specifier proteins is largely unresolved. We have used two thiocyanate-forming proteins and one epithiospecifier protein with different substrate/product specificities to develop molecular models that, in conjunction with mutational analyses, allow us to propose an active site and docking arrangements with glucosinolate aglucones that may explain some of the differences in specifier protein specificities. Furthermore, quantum-mechanical calculations support a reaction mechanism for benzylthiocyanate formation including a catalytic role of the TFP involved. These results may serve as a basis for further theoretical and experimental investigations of the mechanisms of glucosinolate breakdown that will also help to better understand the evolution of specifier proteins from ancestral proteins with functions outside glucosinolate metabolism.     

Crystal structure at 1.1 Angstroms resolution of an insect myrosinase from Brevicoryne brassicae shows its close relationship to beta-glucosidases

The aphid Brevicoryne brassicae is a specialist feeding on Brassicaceae plants. The insect has an intricate defence system involving a beta-D-thioglucosidase (myrosinase) that hydrolyses glucosinolates sequestered from the host plant into volatile isothiocyanates. These isothiocyanates act synergistically with the pheromone E-beta-farnesene to form an alarm system when the aphid is predated. In order to investigate the enzymatic characteristics of the aphid myrosinase and its three-dimensional structure, milligram amounts of pure recombinant aphid myrosinase were obtained from Echerichia coli. The recombinant enzyme had similar physiochemical properties to the native enzyme. The global structure is very similar to Sinapis alba myrosinase and plant beta-O-glucosidases. Aphid myrosinase has two catalytic glutamic acid residues positioned as in plant beta-O-glucosidases, and it is not obvious why this unusual enzyme hydrolyses glucosinolates, the common substrates of plant myrosinases which are normally not hydrolyzed by plant beta-O-glucosidases. The only residue specific for aphid myrosinase in proximity of the glycosidic linkage is Tyr180 which may have a catalytic role. The aglycon binding site differs strongly from plant myrosinase, whereas due to the presence of Trp424 in the glucose binding site, this part of the active site is more similar to plant beta-O-glucosidases, as plant myrosinases carry a phenylalanine residue at this position.     

Regulation and function of specifier proteins in plants

Specifier proteins are responsible for the diversification of biologically active products formed upon myrosinase-catalyzed glucosinolate hydrolysis and are therefore assumed to have an impact on the defensive function of the glucosinolate–myrosinase system. Among glucosinolate hydrolysis products, the generation of epithionitriles and organic thiocyanates requires the presence of epithiospecifier protein (ESP) and thiocyanate-forming protein (TFP), respectively, while myrosinase alone is sufficient for the production of isothiocyanates. Both ESP and TFP also promote the formation of simple nitriles upon myrosinase-catalyzed glucosinolate hydrolysis. Only little is known about the biological effects of epithionitriles and thiocyanates. Moreover, simple nitriles have repeatedly been reported to be less toxic to plant pathogens and herbivorous insects than the correponding isothiocyanates. Thus, it has remained an open question how plants benefit from the presence of specifier proteins. In this review, we survey the biological effects of different types of glucosinolate hydrolysis products on insects and pathogens as well as the current knowlegde on the developmental, organ specific and stimuli-mediated regulation of specifier proteins. Integrating these findings can help us to better understand the ecological functions of plant specifier proteins as well as the co-evolution of glucosinolate-containing plants and their insect herbivores.     

ESP and ESM1 mediate indol-3-acetonitrile production from indol-3-ylmethyl glucosinolate in Arabidopsis

Glucosinolates are plant secondary metabolites that act as direct defenses against insect herbivores and various pathogens. Recent analysis has shown that methionine-derived glucosinolates are hydrolyzed/activated into either nitriles or isothiocyanates depending upon the plants genotype at multiple loci. While it has been hypothesized that tryptophan-derived glucosinolates can be a source of indole-acetonitriles, it has not been explicitly shown if the same proteins control nitrile production from tryptophan-derived glucosinolates as from methionine-derived glucosinolates. In this report, we formally test if the proteins involved in controlling aliphatic glucosinolate hydrolysis during tissue disruption can control production of nitriles during indolic glucosinolate hydrolysis. We show that myrosinase is not sufficient for indol-3-acetonitrile production from indol-3-ylmethyl glucosinolate and requires the presence of functional epithospecifier protein in planta and in vitro to produce significant levels of indol-3-acetonitrile. This reaction is also controlled by the Epithiospecifier modifier 1 gene. Thus, like formation of nitriles from aliphatic glucosinolates, indol-3-acetonitrile production following tissue disruption is controlled by multiple loci raising the potential for complex regulation and fine tuning of indol-3-acetonitrile production from indol-3-ylmethyl glucosinolate.     

Characterization of recombinant nitrile-specifier proteins (NSPs) of Arabidopsis thaliana: Dependency on Fe(II) ions and the effect of glucosinolate substrate and reaction conditions

Glucosinolates are plant secondary metabolites that are part of a plant defence system against pathogens and pests, the myrosinase-glucosinolate system, in which glucosinolates get activated by enzymic degradation through thioglucoside glucohydrolases called myrosinases. Epithiospecifier protein (ESP) and nitrile-specifier proteins (NSPs) divert myrosinase-catalyzed hydrolysis of a given glucosinolate from the formation of isothiocyanate to that of epithionitrile and/or nitrile. As the biological activity of glucosinolate hydrolysis products varies considerably, a detailed characterization of these specifier proteins is of utmost importance to understand their biological role. Therefore, the Arabidopsis thaliana AtNSP1, AtNSP2 and AtNSP5 and a supposed ancestor protein AtNSP-like1 were expressed in Escherichia coli and the activity of the purified recombinant proteins was tested in vitro on three highly different glucosinolates and compared to that of purified AtESP. As previously reported, only AtESP showed epithiospecifier activity on 2-propenylglucosinolate. We further confirmed that purified AtNSP1, AtNSP2 and AtNSP5, but not the ancestor AtNSP-like1 protein, show nitrile-specifier activity on 2-propenylglucosinolate and benzylglucosinolate. We now show for the first time that in vitro AtNSP1, AtNSP2 and AtNSP5 are able to generate nitrile from indol-3-ylmethylglucosinolate. We also tested the effect of different Fe(II) ion concentrations on the nitrile-specifier activity of purified AtNSP1, AtNSP2 and AtNSP5 on 2-propenylglucosinolate and benzylglucosinolate. AtNSP-related nitrile production was highly dependent on the presence of Fe(II) ions in the reaction assay. In the absence of added Fe(II) ions nitriles were only detected when benzylglucosinolate was incubated with AtNSP1. While AtNSP1 also exhibited overall higher nitrile-specifier activity than AtNSP2 and AtNSP5 at a given Fe(II) ion concentration, the pattern of nitrile formation in relation to Fe(II) ion concentrations depended on the AtNSP and the glucosinolate substrate. The pH of the solution also affected the reaction outcome, with a higher proportion of nitrile being produced at the higher pH for AtNSP2 and AtNSP5.     

Cell Specific,Cross-Species Expression of Myrosinases in Brassica Napus,Arabidopsis Thaliana and Nicotiana Tabacum

A prototypical characteristic of the Brassicaceae is the presence of the myrosinase-glucosinolate system. Myrosinase, the only known S-glycosidase in plants, degrades glucosinolates, thereby initiating the formation of isothiocyanates, nitriles and other reactive products with biological activities. We have used myrosinase gene promoters from Brassica napus and Arabidopsis thaliana fused to the beta -glucuronidase (GUS) reporter gene and introduced into Arabidopsis thaliana, Brassica napus and/or Nicotiana tabacum plants to compare and determine the cell types expressing the myrosinase genes and the GUS expression regulated by these promoters. The A. thaliana TGG1 promoter directs expression to guard cells and phloem myrosin cell idioblasts of transgenic A. thaliana plants. Expression from the same promoter construct in transgenic tobacco plants lacking the myrosinase enzyme system also directs expression to guard cells. The B. napus Myr1.Bn1 promoter directs a cell specific expression to idioblast myrosin cells of immature and mature seeds and myrosin cells of phloem of B. napus. In A. thaliana the B. napus promoter directs expression to guard cells similar to the expression pattern of TGG1. The Myr1.Bn1 signal peptide targets the gene product to the reticular myrosin grains of myrosin cells. Our results indicate that myrosinase gene promoters from Brassicaceae direct cell, organ and developmental specific expression in B. napus, A. thaliana and N. tabacum.     

Tipping the scales--specifier proteins in glucosinolate hydrolysis

Glucosinolates are a group of secondary plant metabolites found in the Brassicales order that are beneficial components of our diet, determine the flavor of a number of vegetables and spices and have been implicated in pest management strategies. These properties, most of the biological activities and the pungent odor and taste associated with glucosinolate-containing plants are due to the products formed from glucosinolates by their hydrolytic enzymes, myrosinases, upon tissue disruption. Specifier proteins impact the outcome of glucosinolate hydrolysis without having hydrolytic activity on glucosinolates themselves. In the presence of specifier proteins, glucosinolate hydrolysis results in nitriles, epithionitriles and organic thiocyanates whose biological functions are currently unknown. In contrast, isothiocyanates formed in the absence of specifier proteins have been demonstrated to possess a variety of biological activities and are thought to protect plants from herbivore and pathogen attack. This review discusses the current knowledge on plant and insect specifier proteins with special emphasis on their biochemical properties and possible mechanisms of action.     

Shotgun proteomic profiling of five species of New Zealand Pachycladon

The genus Pachycladon consists of ten species of alpine plants, nine of which are endemic to New Zealand. The species are closely related to the model plant Arabidopsis thaliana with respect to their sequence divergence and chromosome synteny, occupy distinct geographical habitats in terms of both latitude and altitude, and display a range of morphologies. We have performed label‐free quantitative shotgun proteomic analysis of five different species of Pachycladon, namely P. cheesemanii (CH), P. exile (EX), P. fastigiatum (FA), P. enysii (EN) and P. novae‐zelandiae (NZ). The total non‐redundant data set for all five species contained 1489 proteins. The numbers of proteins identified reproducibly in each species ranged from 629 for CH to 987 for NZ, with 681 for EN, 741 for EX and 934 for FA. Previous metabolite‐based studies have shown that FA hydrolyzes glucosinolates completely to isothiocyanates while EN converts glucosinolates to nitriles. In this study, we observed high expression of ESP (At1g54040, epithiospecifying senescence regulator protein) and myrosinase 2 (At5g25980, glycosyl hydrolase family protein), which result in production of nitriles and epithionitriles, in EN and NZ, and we also observed higher expression of ESM1 (At3g14210, GDSL esterase/lipase), which mediates the formation of isothiocyanate, in FA.