Functional expression and characterization of the myrosinase MYR1 from Brassica napus in Saccharomyces cerevisiae

Kinetic Study and Modeling of Wild-Type and Recombinant Broccoli Myrosinase Produced in E. coli and S. cerevisiae as a Function of Substrate Concentration, Temperature, and pH

The myrosinase enzyme hydrolyzes glucosinolates, among which is glucoraphanin, the precursor of the anticancer isothiocyanate sulforaphane (SFN). The main source of glucoraphanin is Brassicaceae; however, its natural concentration is relatively low, limiting the availability of SFN. An option to obtain SFN is its exogenous production, through enzymatic processes and under controlled conditions, allowing complete conversion of glucoraphanin to SFN. We characterized the kinetics of wild-type (BMYR) and recombinant broccoli myrosinases produced in E. coli (EMYR) and S. cerevisiae (SMYR) in terms of the reaction conditions. Kinetics was adjusted using empirical and mechanistic models that describe reaction rate as a function of substrate concentration, temperature, and pH, resulting in R2 values higher than 90%. EMYR kinetics differed significantly from those of BMYR and SMYR probably due to the absence of glycosylations in the enzyme produced in E. coli. BMYR and SMYR were subjected to substrate inhibition but followed different kinetic mechanisms attributed to different glycosylation patterns. EMYR (inactivation Ea = 76.1 kJ/mol) was more thermolabile than BMYR and SMYR. BMYR showed the highest thermostability (inactivation Ea = 52.8 kJ/mol). BMYR and EMYR showed similar behavior regarding pH, with similar pK1 (3.4 and 3.1, respectively) and pK2 (5.4 and 5.0, respectively), but differed considerably from SMYR.

Biochemical Characterization of a Novel Myrosinase Rmyr from Rahnella inusitata for High-Level Preparation of Sulforaphene and Sulforaphane

Myrosinase is a biotechnological tool for the preparation of sulforaphane and sulforaphene with a variety of excellent biological activities. In this study, a gene encoding the novel glycoside hydrolase family 3 (GH3) myrosinase Rmyr from Rahnella inusitata was heterologously expressed in Escherichia coli BL21 (DE3). The purified Rmyr shows the highest activity at 40 °C and pH 7.0; meanwhile, its half-life at 30 °C reaches 12 days, indicating its excellent stability. Its sinigrin-, glucoraphenin-, and glucoraphanin-hydrolyzing activities were 12.73, 4.81, and 6.99 U/mg, respectively. Rmyr could efficiently degrade the radish seed-derived glucoraphenin and the broccoli seed-derived glucoraphanin into sulforaphene and sulforaphane within 10 min with the highest yields of 5.07 mg/g radish seeds and 9.56 mg/g broccoli seeds, respectively. The highest conversion efficiencies of sulforaphane from glucoraphanin and sulforaphene from glucoraphenin reached up to 92.48 and 97.84%, respectively. Therefore, Rmyr is a promising and potent biocatalyst for efficient and large-scale preparation of sulforaphane and sulforaphene.

Broccoli Myrosinase cDNA Expression in Escherichia coli and Saccharomyces cerevisiae

Myrosinases (EC 3.2.1.147) are enzymes known for the generation of hydrolysis products that have a potential beneficial effect on human health. Their reaction mechanisms are widely studied, in order to improve and optimize secondary metabolite production processes. In this work, kinetic and biochemical properties of the broccoli myrosinase enzyme produced from its cDNA cloned in Escherichia coli and Saccharomyces cerevisiae were investigated. The results revealed that the thermal stability of the enzyme produced in S. cerevisiae was slightly higher (30 to 60 °C) than that of myrosinase produced in E. coli (20 to 50 °C). The effect of pH on the enzymatic activity was similar in both enzymes, with pH 3 being the optimum value under the reaction conditions used. The kinetic behavior of both enzymes was adjusted to the Michaelis–Menten model. The catalytic efficiency was up to 4 times higher in myrosinase produced in S. cerevisiae, compared to myrosinase produced in E. coli. The glycosylations present in the enzyme would be related to the formation of a dimeric quaternary structure and would not play an essential role in enzymatic activity, since both enzymes were biologically active. These results will probably allow the development of strategies for the production of bioactive metabolites of medical interest.

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.

Molecular Cloning, Expression and Characterisation of a Bacterial Myrosinase from Citrobacter Wye1

Glucosinolates are plant natural products which on degradation by myrosinases give rise to the beneficial bioactive isothiocyanates. Recently, a myrosinase activity was detected in a Citrobacter strain isolated from soil. This enzyme was purified enabling its amino acid sequence and gene sequence (cmyr) to be determined. In order to study this myrosinase it was necessary to establish an expression system that would enable future work such as a structural determination of the protein to be carried out. The myrosinase gene was amplified, cloned and expressed in Escherichia coli with a 6XHis-tag. The heterologous expression of cmyr enabled relatively large amounts of myrosinase to be produced (3.4 mg cmyr/100 ml culture). Myrosinase activity was determined by mixing substrate and enzyme and determining glucose release. Optimum pH and temperature were determined to be pH 6.0 and 25 °C for the Ni-NTA purified protein. The kinetic parameters of the purified myrosinase were determined using sinigrin as a substrate. K_m and V_max were estimated as 0.18 mM and 0.033 mmol/min/mg respectively for sinigrin under optimum conditions and compared to other kinetic data for myrosinases. The substrate specificity of myrosinase was determined having the highest affinity for sinigrin followed by glucoiberin, progoitrin, glucoerucin, glucoraphanin and glucotropaeolin.

Optimisation of Recombinant Myrosinase Production in Pichia pastoris

Myrosinase is a plant defence enzyme catalysing the hydrolysis of glucosinolates, a group of plant secondary metabolites, to a range of volatile compounds. One of the products, isothiocyanates, proved to have neuroprotective and chemo-preventive properties, making myrosinase a pharmaceutically interesting enzyme. In this work, extracellular expression of TGG1 myrosinase from Arabidopsis thaliana in the Pichia pastoris KM71H (Mut^S) strain was upscaled to a 3 L laboratory fermenter for the first time. Fermentation conditions (temperature and pH) were optimised, which resulted in a threefold increase in myrosinase productivity compared to unoptimised fermentation conditions. Dry cell weight increased 1.5-fold, reaching 100.5 g/L without additional glycerol feeding. Overall, a specific productivity of 4.1 U/L_medium/h was achieved, which was 102.5-fold higher compared to flask cultivations.

Glucosinolate structural diversity, identification, chemical synthesis and metabolism in plants

The glucosinolates (GSLs) is a well-defined group of plant metabolites characterized by having an S-β-d-glucopyrano unit anomerically connected to an O-sulfated (Z)-thiohydroximate function. After enzymatic hydrolysis, the sulfated aglucone can undergo rearrangement to an isothiocyanate, or form a nitrile or other products. The number of GSLs known from plants, satisfactorily characterized by modern spectroscopic methods (NMR and MS) by mid-2018, is 88. In addition, a group of partially characterized structures with highly variable evidence counts for approximately a further 49. This means that the total number of characterized GSLs from plants is somewhere between 88 and 137. The diversity of GSLs in plants is critically reviewed here, resulting in significant discrepancies with previous reviews. In general, the well-characterized GSLs show resemblance to C-skeletons of the amino acids Ala, Val, Leu, Trp, Ile, Phe/Tyr and Met, or to homologs of Ile, Phe/Tyr or Met. Insufficiently characterized, still hypothetic GSLs include straight-chain alkyl GSLs and chain-elongated GSLs derived from Leu. Additional reports (since 2011) of insufficiently characterized GSLs are reviewed. Usually the crucial missing information is correctly interpreted NMR, which is the most effective tool for GSL identification. Hence, modern use of NMR for GSL identification is also reviewed and exemplified. Apart from isolation, GSLs may be obtained by organic synthesis, allowing isotopically labeled GSLs and any kind of side chain. Enzymatic turnover of GSLs in plants depends on a considerable number of enzymes and other protein factors and furthermore depends on GSL structure. Identification of GSLs must be presented transparently and live up to standard requirements in natural product chemistry. Unfortunately, many recent reports fail in these respects, including reports based on chromatography hyphenated to MS. In particular, the possibility of isomers and isobaric structures is frequently ignored. Recent reports are re-evaluated and interpreted as evidence of the existence of "isoGSLs", i.e. non-GSL isomers of GSLs in plants. For GSL analysis, also with MS-detection, we stress the importance of using authentic standards.

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.

Microbiota: a mediator to transform glucosinolate precursors in cruciferous vegetables to the active isothiocyanates

Isothiocyanates (ITCs), such as sulforaphane (SFN), exhibit powerful biological functions in fighting cancers, and cardiovascular and neurodegenerative diseases. They normally exist as glucosinolates (GLSs) in cruciferous vegetables, which are not themselves bioactive until they are degraded by myrosinase to form ITCs. Myrosinase coexists in the same plants but is normally kept apart from GLSs in different apparatus. A key point is that myrosinase is temperature sensitive and can be inactivated upon exposure to temperatures over 60 °, as typically occurs during cooking. However, studies using animal models and population trials have suggested that human gut bacteria might act like an 'organ' in that they can secrete their own myrosinase. In this review, the hydrolysis of GLS by myrosinase is discussed, with an important focus on the gut microflora and their myrosinase-producing roles. © 2017 Society of Chemical Industry.

Purification and characterization of β-glucosidase from greater wax moth Galleria mellonella L. (Lepidoptera: Pyralidae)

The greater wax moth, Galleria mellonella, is one of the most ruinous pests of honeycomb in the world. Beta-glucosidases are a type of digestive enzymes that hydrolytically catalyzes the beta-glycosidic linkage of glycosides. Characterization of the beta-glucosidase in G. mellonella could be a significant stage for a better comprehending of its role and establishing a safe and effective control procedure primarily against G. mellonella and also some other insect pests. Laboratory reared final instar stage larvae were randomly selected and homogenized for beta-glucosidase activity assay and subsequent analysis. The enzyme was purified to apparent homogeneity by salting out with ammonium sulfate and using sepharose-4B-l-tyrosine-1-naphthylamine hydrophobic interaction chromatography. The purification was 58-fold with an overall enzyme yield of 29%. The molecular mass of the protein was estimated as ca. 42 kDa. The purified beta-glucosidase was effectively active on para/ortho-nitrophenyl-beta-d-glucopyranosides (p-/o-NPG) with Km values of 0.37 and 1.9 mM and Vmax values of 625 and 189 U/mg, respectively. It also exhibits different levels of activity against para-nitrophenyl-β-d-fucopyranoside (p-NPF), para/ortho-nitrophenyl β-d-galactopyranosides (p-/o-NPGal) and p-nitrophenyl 1-thio-β-d-glucopyranoside. The enzyme was competitively inhibited by beta-gluconolactone and also was very tolerant to glucose against p-NPG as substrate. The Ki and IC50 values of δ-gluconolactone were determined as 0.021 and 0.08 mM while the enzyme was more tolerant to glucose inhibition with IC50 value of 213.13 mM for p-NPG.

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.

Campesin, a thermostable antifungal peptide with highly potent antipathogenic activities

An 9.4-kDa antifungal peptide designated as campesin was isolated from seeds of the cabbage Brassica campestris. The isolation procedure involved affinity chromatography on Affi-gel blue gel, ion exchange chromatography on Q-Sepharose and Mono S, and gel filtration on Superdex 75 and Superdex Peptide. The peptide was adsorbed on the first three chromatographic media. It exerted an inhibitory action on mycelial growth including Fusarium oxysporum and Mycosphaerella arachidicola, with an IC(50) of 5.1 microM and 4.4 microM, respectively. The peptide was characterized by remarkable thermostability and pH stability. It inhibited proliferation of HepG2 and MCF cancer cells with an IC(50) of 6.4 microM and 1.8 microM, and the activity of HIV-1 reverse transcriptase with an IC(50) of 3.2 microM. It demonstrated lysolecithin binding activity.

Myrosinases from root and leaves of Arabidopsis thaliana have different catalytic properties

Myrosinases (EC 3.2.1.147) are beta-thioglucoside glucosidases present in Brassicaceae plants. These enzymes serve to protect plants against pathogens and insect pests by initiating breakdown of the secondary metabolites glucosinolates into toxic products. Several forms of myrosinases are present in plants but the properties and role of different isoenzymes are not well understood. The dicot plant model organism Arabidopsis thaliana seems to contain six myrosinase genes (TGG1-TGG6). In order to compare the different myrosinases, cDNAs corresponding to TGG1 from leaves and TGG4 and TGG5 from roots were cloned and overexpressed in Pichia pastoris. The His-tagged recombinant proteins were purified using affinity chromatography and the preparations were homogenous according to SDS-PAGE analysis. Myrosinase activity was confirmed for all forms and compared with respect to catalytic activity towards the allyl-glucosinolate sinigrin. There was a 22-fold difference in basal activity among the myrosinases. The enzymes were active in a broad pH range, are rather thermostable and active in a wide range of salt concentrations but sensitive to high salt concentrations. The myrosinases showed different activation-inhibition responses towards ascorbic acid with maximal activity around 0.7-1 mM. No activity was registered towards desulphosinigrin and this compound did not inhibit myrosinase activity towards sinigrin. All myrosinases also displayed O-beta-glucosidase activity, although with lower efficiency compared to the myrosinase activity. The differences in catalytic properties among myrosinase isozymes for function in planta are discussed.

A pseudo-beta-glucosidase in Arabidopsis thaliana: correction by site-directed mutagenesis, heterologous expression, purification, and characterization

Since At2g25630 is an intronless gene with a premature stop codon, its cDNA encoding the predicted mature beta-glucosidase isoenzyme was synthesized from the previously isolated Arabidopsis thaliana genomic DNA. The stop codon was converted to a sense codon by site-directed mutagenesis. The native and mutated cDNA sequences were separately cloned into the vector pPICZalphaB and expressed in Pichia pastoris. Only the cells transformed with mutated cDNA-vector construct produced the active protein. The mutated recombinant beta-glucosidase isoenzyme was chromatographically purified to apparent homogeneity. The molecular mass of the protein is estimated as ca. 60 kD by SDS-PAGE. The pH optimum of activity is 5.6, and it is fairly stable in the pH range of 5.0-8.5. The purified recombinant beta-glucosidase is effectively active on para-/ortho-nitrophenyl-beta-D-glucopyranosides (p-/o-NPG) and 4-methylumbelliferyl-beta-D-glucopyranoside (4-MUG) with K(m) values of 1.9, 2.1, 0.78 mM and k(cat) values of 114, 106, 327 nkat/mg, respectively. It also exhibits different levels of activity against para-/ortho-nitrophenyl-beta-D-fucopyranosides (p-/o-NPF), amygdalin, prunasin, cellobiose, gentiobiose, and salicin. The enzyme is competitively inhibited by gluconolactone and p-nitrophenyl-1-thio-beta-D-glucopyranoside with p-NPG, o-NPG, and 4-MUG as substrates. The enzyme is found to be very tolerant to glucose inhibition. The catalytic role of nucleophilic glutamic acid in the motif YITENG of beta-glucosidases and mutated recombinant enzyme is discussed.

The beta-glucosidases responsible for bioactivation of hydroxynitrile glucosides in Lotus japonicus

Lotus japonicus accumulates the hydroxynitrile glucosides lotaustralin, linamarin, and rhodiocyanosides A and D. Upon tissue disruption, the hydroxynitrile glucosides are bioactivated by hydrolysis by specific beta-glucosidases. A mixture of two hydroxynitrile glucoside-cleaving beta-glucosidases was isolated from L. japonicus leaves and identified by protein sequencing as LjBGD2 and LjBGD4. The isolated hydroxynitrile glucoside-cleaving beta-glucosidases preferentially hydrolyzed rhodiocyanoside A and lotaustralin, whereas linamarin was only slowly hydrolyzed, in agreement with measurements of their rate of degradation upon tissue disruption in L. japonicus leaves. Comparative homology modeling predicted that LjBGD2 and LjBGD4 had nearly identical overall topologies and substrate-binding pockets. Heterologous expression of LjBGD2 and LjBGD4 in Arabidopsis (Arabidopsis thaliana) enabled analysis of their individual substrate specificity profiles and confirmed that both LjBGD2 and LjBGD4 preferentially hydrolyze the hydroxynitrile glucosides present in L. japonicus. Phylogenetic analyses revealed a third L. japonicus putative hydroxynitrile glucoside-cleaving beta-glucosidase, LjBGD7. Reverse transcription-polymerase chain reaction analysis showed that LjBGD2 and LjBGD4 are expressed in aerial parts of young L. japonicus plants, while LjBGD7 is expressed exclusively in roots. The differential expression pattern of LjBGD2, LjBGD4, and LjBGD7 corresponds to the previously observed expression profile for CYP79D3 and CYP79D4, encoding the two cytochromes P450 that catalyze the first committed step in the biosyntheis of hydroxynitrile glucosides in L. japonicus, with CYP79D3 expression in aerial tissues and CYP79D4 expression in roots.

beta-Glucosidases as detonators of plant chemical defense

Some plant secondary metabolites are classified as phytoanticipins. When plant tissue in which they are present is disrupted, the phytoanticipins are bio-activated by the action of beta-glucosidases. These binary systems--two sets of components that when separated are relatively inert--provide plants with an immediate chemical defense against protruding herbivores and pathogens. This review provides an update on our knowledge of the beta-glucosidases involved in activation of the four major classes of phytoanticipins: cyanogenic glucosides, benzoxazinoid glucosides, avenacosides and glucosinolates. New aspects of the role of specific proteins that either control oligomerization of the beta-glucosidases or modulate their product specificity are discussed in an evolutionary perspective.

Biology and biochemistry of glucosinolates

Glucosinolates are sulfur-rich, anionic natural products that upon hydrolysis by endogenous thioglucosidases called myrosinases produce several different products (e.g., isothiocyanates, thiocyanates, and nitriles). The hydrolysis products have many different biological activities, e.g., as defense compounds and attractants. For humans these compounds function as cancer-preventing agents, biopesticides, and flavor compounds. Since the completion of the Arabidopsis genome, glucosinolate research has made significant progress, resulting in near-complete elucidation of the core biosynthetic pathway, identification of the first regulators of the pathway, metabolic engineering of specific glucosinolate profiles to study function, as well as identification of evolutionary links to related pathways. Although much has been learned in recent years, much more awaits discovery before we fully understand how and why plants synthesize glucosinolates. This may enable us to more fully exploit the potential of these compounds in agriculture and medicine.

A novel approach for reliable activity determination of ascorbic acid depending myrosinases

Up to now, a wide array of methods for the determination of myrosinase activity has been described. These vary from the simple photometric estimation to highly sophisticated assays using radioactively labelled substrates. However, ascorbic acid--an effective activator of myrosinases--interferes with most of these enzyme tests. Unfortunately, in the past, such interferences were disregarded in many scientific examinations of myrosinases. Whereas such failings have less effects when the activation of myrosinases is not very distinctive, they are quite relevant in all cases where myrosinases are completely inactive in the absence of ascorbic acid. In this paper, the current methods for myrosinase determination are reviewed critically with special emphasis on putative interferences with ascorbic acid. Thereafter, an alternative and interference-free HPLC-based quantification method of the enzymatically produced glucose is presented. Due to the benzoylation of glucose, it becomes possible to quantify even those exiguous glucose concentrations, which are indispensable for correct determination of kinetic enzyme data in the presence of ascorbic acid. Using this new method, the activity of Tropaeolum majus myrosinase towards glucotropaeolin was analyzed. This enzyme shows a distinctive activation by ascorbic acid with maximal activation at a concentration of about 2 mM.

Nucleotide variation at the myrosinase-encoding locus, TGG1, and quantitative myrosinase enzyme activity variation in Arabidopsis thaliana

The Arabidopsis thaliana TGG1 gene encodes thioglucoside glucohydrolase (myrosinase), an enzyme catalysing the hydrolysis of glucosinolate compounds. The enzyme is involved in plant defence against some insect herbivores, and is present in species of the order Capparales (Brassicales). Nucleotide variation was surveyed by sequencing c. 2.4 kb of the TGG1 locus in a sample of 28 worldwide A. thaliana accessions, and one Arabidopsis lyrata ssp. lyrata individual. Myrosinase activity was quantified for 27 of these same A. thaliana accessions, plus five additional others. Overall, estimated nucleotide diversity in A. thaliana was low compared to other published A. thaliana surveys, and the frequency distribution was skewed toward an excess of low-frequency variants. Furthermore, comparison to the outgroup species A. lyrata demonstrated that A. thaliana exhibited an excess of high-frequency derived variants relative to a neutral equilibrium model, suggesting a selective sweep. A. thaliana accessions differed significantly in total myrosinase activity, but analysis of variance detected no statistical evidence for an association between quantitative enzyme activity and alleles at the TGG1 myrosinase-encoding locus. We thus conclude that other, unsurveyed factors primarily affect the observed myrosinase activity levels in this species. The pattern of nucleotide variation was consistent with a model of positive selection but might also be compatible with a completely neutral model that takes into account the metapopulation behaviour of this highly inbreeding species which experienced a relatively recent worldwide expansion.

CYP79F1 and CYP79F2 have distinct functions in the biosynthesis of aliphatic glucosinolates in Arabidopsis

Cytochromes P450 of the CYP79 family catalyze the conversion of amino acids to oximes in the biosynthesis of glucosinolates, a group of natural plant products known to be involved in plant defense and as a source of flavor compounds, cancer-preventing agents and bioherbicides. We report a detailed biochemical analysis of the substrate specificity and kinetics of CYP79F1 and CYP79F2, two cytochromes P450 involved in the biosynthesis of aliphatic glucosinolates in Arabidopsis thaliana. Using recombinant CYP79F1 and CYP79F2 expressed in Escherichia coli and Saccharomyces cerevisiae, respectively, we show that CYP79F1 metabolizes mono- to hexahomomethionine, resulting in both short- and long-chain aliphatic glucosinolates. In contrast, CYP79F2 exclusively metabolizes long-chain elongated penta- and hexahomomethionines. CYP79F1 and CYP79F2 are spatially and developmentally regulated, with different gene expression patterns. CYP79F2 is highly expressed in hypocotyl and roots, whereas CYP79F1 is strongly expressed in cotyledons, rosette leaves, stems, and siliques. A transposon-tagged CYP79F1 knockout mutant completely lacks short-chain aliphatic glucosinolates, but has an increased level of long-chain aliphatic glucosinolates, especially in leaves and seeds. The level of long-chain aliphatic glucosinolates in a transposon-tagged CYP79F2 knockout mutant is substantially reduced, whereas the level of short-chain aliphatic glucosinolates is not affected. Biochemical characterization of CYP79F1 and CYP79F2, and gene expression analysis, combined with glucosinolate profiling of knockout mutants demonstrate the functional role of these enzymes. This provides valuable insights into the metabolic network leading to the biosynthesis of aliphatic glucosinolates, and into metabolic engineering of altered aliphatic glucosinolate profiles to improve nutritional value and pest resistance.

Complex formation of myrosinase isoenzymes in oilseed rape seeds are dependent on the presence of myrosinase-binding proteins

The enzyme myrosinase (EC 3.2.3.1) degrades the secondary compounds glucosinolates upon wounding and serves as a defense to generalist pests in Capparales. Certain myrosinases are present in complexes together with other proteins such as myrosinase-binding proteins (MBP) in extracts of oilseed rape (Brassica napus) seeds. Immunhistochemical analysis of wild-type seeds showed that MBPs were present in most cells but not in the myrosin cells, indicating that the complex formation observed in extracts is initiated upon tissue disruption. To study the role of MBP in complex formation and defense, oilseed rape antisense plants lacking the seed MBPs were produced. Western blotting and immunohistochemical staining confirmed depletion of MBP in the transgenic seeds. The exclusive expression of myrosinase in idioblasts (myrosin cells) of the seed was not affected by the down-regulation of MBP. Using size-exclusion chromatography, we have shown that myrosinases with subunit molecular masses of 62 to 70 kD were present as free dimers from the antisense seed extract, whereas in the wild type, they formed complexes. In accordance with this, MBPs are necessary for myrosinase complex formation of the 62- to 70-kD myrosinases. The product formed from sinalbin hydrolysis by myrosinase was the same whether MBP was present or not. The performance of a common beetle generalist (Tenebrio molitor) fed with seeds, herbivory by flea beetles (Phyllotreta undulata) on cotyledons, or growth rate of the Brassica fungal pathogens Alternaria brassicae or Lepthosphaeria maculans in the presence of seed extracts were not affected by the down-regulation of MBP, leaving the physiological function of this protein family open.

Characterization of a Brassica napus myrosinase expressed and secreted by Pichia pastoris

In Brassica napus three different gene families with different temporal and tissue-specific expression and distribution patterns encode myrosinases (thioglucoside glucohydrolases, EC 3.2.3.1). Myrosinases encoded by the MA gene family are found as free and soluble dimers, while myrosinases encoded by the MB and MC gene families are mainly found in large insoluble complexes associated with myrosinase-binding proteins and myrosinase-associated proteins. These large complexes impede purification and characterization of MB and MC myrosinases from the plant. We used Pichia pastoris to express and secrete functional recombinant MYR1 myrosinase from B. napus to allow further characterization of myrosinase belonging to the MB gene family. The purified recombinant myrosinase hydrolyzes sinigrin with a K(m) of 1.0 mM; the specific activity and calculated k(cat)/K(m) were 175 U/mg and 1.9 x 10(5) s(-1) M(-1), respectively. A novel in-gel staining method for myrosinase activity is presented.

Different myrosinase and idioblast distribution in Arabidopsis and Brassica napus

Myrosinase (EC 3.2.3.1) is a glucosinolate-degrading enzyme mainly found in special idioblasts, myrosin cells, in Brassicaceae. This two-component system of secondary products and degradative enzymes is important in plant-insect interactions. Immunocytochemical analysis of Arabidopsis localized myrosinase exclusively to myrosin cells in the phloem parenchyma, whereas no myrosin cells were detected in the ground tissue. In Brassica napus, myrosinase could be detected in myrosin cells both in the phloem parenchyma and in the ground tissue. The myrosin cells were similar in Arabidopsis and B. napus and were found to be different from the companion cells and the glucosinolate-containing S-cells present in Arabidopsis. Confocal laser scanning immunomicroscopy analysis of myrosin cells in B. napus embryos showed that the myrosin grains constitute a continuous reticular system in the cell. These findings indicate that in the two species studied, initial cells creating the ground tissue have different potential for making idioblasts and suggest that the myrosinase-glucosinolate system has at least partly different functions. Several myrosinases in B. napus extracts are recovered in complex together with myrosinase-binding protein (MBP), and the localization of MBP was therefore studied in situ. The expression of MBP was highest in germinating seedlings of B. napus and was found in every cell except the myrosin cells of the ground tissue. Rapid disappearance of the MBP from the non-myrosin cells and emergence of MBP in the myrosin cells resulted in an apparent colocalization of MBP and myrosinase in 7-d-old seedlings.

The myrosinase-glucosinolate system in the interaction between Leptosphaeria maculans and Brassica napus

summary Leptosphaeria maculans causes blackleg disease, and resistance to this fungal pathogen is an important trait in the breeding of oilseed rape. A better comprehension of the role of the myrosinase-glucosinolate system in this context is of great value. The present study is the first to address effects on multiple components of this complex system, including concentrations of individual glucosinolates, product formation, myrosinase isoform distribution and activity, and levels of myrosinase binding proteins during the infection process. One resistant B. napus cultivar (Maluka) and one susceptible cultivar (Westar) were compared in the investigation. Our results show that the two cultivars had the same histological distribution, isoform expression, and activity of the myrosinase enzymes. The glucosinolate levels were also similar, with the exception of glucobrassicin and neoglucobrassicin, which were significantly lower in the resistant cultivar at 11 days post-infection. Growth of the fungus on the plant tissues did not alter glucosinolate levels, suggesting that L. maculans does not degrade these compounds. When the plants were starved of sulphur, and thereby depleted of glucosinolates, no increased susceptibility was observed. Hence, we suggest that the myrosinase-glucosinolate system does not determine the outcome of the interaction between B. napus and L. maculans.

Post-transcriptional maturation of the S receptor kinase of Brassica correlates with co-expression of the S-locus glycoprotein in the stigmas of two Brassica strains and in transgenic tobacco plants

The S-locus-encoded S receptor kinase (SRK) is an intrinsic plasma membrane protein that is viewed as the primary stigma determinant of specificity in the self-incompatibility response of Brassica spp. We analyzed two self-compatible mutant strains that express low levels of the S-locus glycoprotein (SLG), a cell wall-localized protein also encoded at the S locus that is coordinately expressed with SRK. We found that mutant stigmas synthesized wild-type levels of SRK transcripts but failed to produce SRK protein at any of the developmental stages analyzed. Furthermore, SRK was shown to form aberrant high-molecular mass aggregates when expressed alone in transgenic tobacco (Nicotiana tabacum) plants. This aggregation was prevented in tobacco plants that co-expressed SRK and SLG, but not in tobacco plants that co-expressed SRK and SLR1, an SLG-related secreted protein not encoded at the S locus. In analyses of protein extracts under reducing and non-reducing conditions, evidence of intermolecular association was obtained only for SLG, a fraction of which formed disulfide-linked oligomers and was membrane associated. The data indicate that, at least in plants carrying the S haplotypes we analyzed, SRK is an inherently unstable protein and that SLG facilitates its accumulation to physiologically relevant levels in Brassica stigmas.