Arabidopsis cDNA sequence encoding myrosinase

Comparative transcriptomic analyses of glucosinolate metabolic genes during the formation of Chinese kale seeds

BACKGROUND

To understand the mechanism of glucosinolates (GSs) accumulation in the specific organs, combined analysis of physiological change and transcriptome sequencing were applied in the current study. Taking Chinese kale as material, seeds and silique walls were divided into different stages based on the development of the embryo in seeds and then subjected to GS analysis and transcriptome sequencing.

RESULTS

The main GS in seeds of Chinese kale were glucoiberin and gluconapin and their content changed with the development of the seed. During the transition of the embryo from torpedo- to the early cotyledonary-embryo stage, the accumulation of GS in the seed was accompanied by the salient decline of GS in the corresponding silique wall. Thus, the seed and corresponding silique wall at these two stages were subjected to transcriptomic sequencing analysis. 135 genes related to GS metabolism were identified, of which 24 genes were transcription factors, 81 genes were related to biosynthetic pathway, 25 genes encoded catabolic enzymes, and 5 genes matched with transporters. The expression of GS biosynthetic genes was detected both in seeds and silique walls. The high expression of FMOGS-OX and AOP2, which is related to the production of gluconapin by side modification, was noted in seeds at both stages. Interestingly, the expression of GS biosynthetic genes was higher in the silique wall compared with that in the seed albeit lower content of GS existed in the silique wall than in the seed. Combined with the higher expression of transporter genes GTRs in silique walls than in seeds, it was proposed that the transportation of GS from the silique wall to the seed is an important source for seed GS accumulation. In addition, genes related to GS degradation expressed abundantly in the seed at the early cotyledonary-embryo stage indicating its potential role in balancing seed GS content.

CONCLUSIONS

Two stages including the torpedo-embryo and the early cotyledonary-embryo stage were identified as crucial in GS accumulation during seed development. Moreover, we confirmed the transportation of GS from the silique wall to the seed and proposed possible sidechain modification of GS biosynthesis may exist during seed formation.

The Imaging of Guard Cells of thioglucosidase (tgg) Mutants of Arabidopsis Further Links Plant Chemical Defence Systems with Physical Defence Barriers

The glucosinolate-myrosinase system is a well-known plant chemical defence system. Two functional myrosinase-encoding genes, THIOGLUCOSIDASE 1 (TGG1) and THIOGLUCOSIDASE 2 (TGG2), express in aerial tissues of Arabidopsis. TGG1 expresses in guard cells (GCs) and is also a highly abundant protein in GCs. Recently, by studying wild type (WT), tgg single, and double mutants, we showed a novel association between the glucosinolate-myrosinase system defence system, and a physical barrier, the cuticle. In the current study, using imaging techniques, we further analysed stomata and ultrastructure of GCs of WT, tgg1, tgg2 single, and tgg1 tgg2 double mutants. The tgg mutants showed distinctive features of GCs. The GCs of tgg1 and tgg1 tgg2 mutants showed vacuoles that had less electron-dense granular material. Both tgg single mutants had bigger stomata complexes. The WT and tgg mutants also showed variations for cell wall, chloroplasts, and starch grains of GCs. Abscisic acid (ABA)-treated stomata showed that the stomatal aperture was reduced in tgg1 single and tgg1 tgg2 double mutants. The data provides a basis to perform comprehensive further studies to find physiological and molecular mechanisms associated with ultrastructure differences in tgg mutants. We speculate that the absence of myrosinase alters the endogenous chemical composition, hence affecting the physical structure of plants and the plants' physical defence barriers.

Arabidopsis myrosinases link the glucosinolate-myrosinase system and the cuticle

Both physical barriers and reactive phytochemicals represent two important components of a plant's defence system against environmental stress. However, these two defence systems have generally been studied independently. Here, we have taken an exclusive opportunity to investigate the connection between a chemical-based plant defence system, represented by the glucosinolate-myrosinase system, and a physical barrier, represented by the cuticle, using Arabidopsis myrosinase (thioglucosidase; TGG) mutants. The tgg1, single and tgg1 tgg2 double mutants showed morphological changes compared to wild-type plants visible as changes in pavement cells, stomatal cells and the ultrastructure of the cuticle. Extensive metabolite analyses of leaves from tgg mutants and wild-type Arabidopsis plants showed altered levels of cuticular fatty acids, fatty acid phytyl esters, glucosinolates, and indole compounds in tgg single and double mutants as compared to wild-type plants. These results point to a close and novel association between chemical defence systems and physical defence barriers.

FAMA: A Molecular Link between Stomata and Myrosin Cells

Plants use sophisticated defense strategies against herbivores, including the myrosinase-glucosinolate system in Brassicales plants. This system sequesters myrosinase in myrosin cells, which are idioblasts in inner leaf tissues, and produces a toxic compound when cells are damaged by herbivores. Although the molecular mechanisms underlying myrosin cell development are largely unknown, recent studies have revealed that two key components, a basic helix-loop-helix (bHLH) transcription factor (FAMA) and vesicle trafficking factors (such as SYNTAXIN OF PLANTS 22), regulate the differentiation and fate determination of myrosin cells. FAMA also functions as a master regulator of guard cell (GC) differentiation. In this review, we discuss how FAMA operates two distinct genetic programs: the generation of myrosin cells in inner plant tissue and GCs in the epidermis.

Arabidopsis Myrosinase Genes AtTGG4 and AtTGG5 Are Root-Tip Specific and Contribute to Auxin Biosynthesis and Root-Growth Regulation

Plant myrosinases (β-thioglucoside glucohydrolases) are classified into two subclasses, Myr I and Myr II. The biological function of Myr I has been characterized as a major biochemical defense against insect pests and pathogens in cruciferous plants. However, the biological function of Myr II remains obscure. We studied the function of two Myr II member genes AtTGG4 and AtTGG5 in Arabidopsis. RT-PCR showed that both genes were specifically expressed in roots. GUS-assay revealed that both genes were expressed in the root-tip but with difference: AtTGG4 was expressed in the elongation zone of the root-tip, while AtTGG5 was expressed in the whole root-tip. Moreover, myrosin cells that produce and store the Myr I myrosinases in aboveground organs were not observed in roots, and AtTGG4 and AtTGG5 were expressed in all cells of the specific region. A homozygous double mutant line tgg4tgg5 was obtained through cross-pollination between two T-DNA insertion lines, tgg4E8 and tgg5E12, by PCR-screening in the F2 and F3 generations. Analysis of myrosinase activity in roots of mutants revealed that AtTGG4 and AtTGG5 had additive effects and contributed 35% and 65% myrosinase activity in roots of the wild type Col-0, respectively, and myrosinase activity in tgg4tgg5 was severely repressed. When grown in Murashiege & Skoog (MS) medium or in soil with sufficient water, Col-0 had the shortest roots, and tgg4tgg5 had the longest roots, while tgg4E8 and tgg5E12 had intermediate root lengths. In contrast, when grown in soil with excessive water, Col-0 had the longest roots, and tgg4tgg5 had the shortest roots. These results suggested that AtTGG4 and AtTGG5 regulated root growth and had a role in flood tolerance. The auxin-indicator gene DR5::GUS was then introduced into tgg4tgg5 by cross-pollination. DR5::GUS expression patterns in seedlings of F1, F2, and F3 generations indicated that AtTGG4 and AtTGG5 contributed to auxin biosynthesis in roots. The proposed mechanism is that indolic glucosinolate is transported to the root-tip and converted to indole-3-acetonitrile (IAN) in the tryptophan-dependent pathways by AtTGG4 and AtTGG5, and IAN is finally converted to indole-3-acetic acid (IAA) by nitrilases in the root-tip. This mechanism guarantees the biosynthesis of IAA in correct cells of the root-tip and, thus, a correct auxin gradient is formed for healthy development of roots.

Myrosin idioblast cell fate and development are regulated by the Arabidopsis transcription factor FAMA, the auxin pathway, and vesicular trafficking

Crucifer shoots harbor a glucosinolate-myrosinase system that defends against insect predation. Arabidopsis thaliana myrosinase (thioglucoside glucohydrolase [TGG]) accumulates in stomata and in myrosin idioblasts (MIs). This work reports that the basic helix-loop-helix transcription factor FAMA that is key to stomatal development is also expressed in MIs. The loss of FAMA function abolishes MI fate as well as the expression of the myrosinase genes TGG1 and TGG2. MI cells have previously been reported to be located in the phloem. Instead, we found that MIs arise from the ground meristem rather than provascular tissues and thus are not homologous with phloem. Moreover, MI patterning and morphogenesis are abnormal when the function of the ARF-GEF gene GNOM is lost as well as when auxin efflux and vesicular trafficking are chemically disrupted. Stomata and MI cells constitute part of a wider system that reduces plant predation, the so-called "mustard oil bomb," in which vacuole breakage in cells harboring myrosinase and glucosinolate yields a brew toxic to many animals, especially insects. This identification of the gene that confers the fate of MIs, as well as stomata, might facilitate the development of strategies for engineering crops to mitigate predation.

Characterization of a novel β-thioglucosidase CpTGG1 in Carica papaya and its substrate-dependent and ascorbic acid-independent O-β-glucosidase activity

Plant thioglucosidases are the only known S-glycosidases in the large superfamily of glycosidases. These enzymes evolved more recently and are distributed mainly in Brassicales. Thioglucosidase research has focused mainly on the cruciferous crops due to their economic importance and cancer preventive benefits. In this study, we cloned a novel myrosinase gene, CpTGG1, from Carica papaya Linnaeus. and showed that it was expressed in the aboveground tissues in planta. The recombinant CpTGG1 expressed in Pichia pastoris catalyzed the hydrolysis of both sinigrin and glucotropaeolin (the only thioglucoside present in papaya), showing that CpTGG1 was indeed a functional myrosinase gene. Sequence alignment analysis indicated that CpTGG1 contained all the motifs conserved in functional myrosinases from crucifers, except for two aglycon-binding motifs, suggesting substrate priority variation of the non-cruciferous myrosinases. Using sinigrin as substrate, the apparent K(m) and V(max) values of recombinant CpTGG1 were 2.82 mM and 59.9 μmol min⁻¹ mg protein⁻¹ , respectively. The K(cat) /K(m) value was 23 s⁻¹ mM⁻¹ . O-β-glucosidase activity towards a variety of substrates were tested, CpTGG1 displayed substrate-dependent and ascorbic acid-independent O-β-glucosidase activity towards 2-nitrophenyl-β-D-glucopyranoside and 4-nitrophenyl-β-D-glucopyranoside, but was inactive towards glucovanillin and n-octyl-β-D-glucopyranoside. Phylogenetic analysis indicated CpTGG1 belongs to the MYR II subfamily of myrosinases.

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.

Mapping of QTL for resistance against the crucifer specialist herbivore Pieris brassicae in a new Arabidopsis inbred line population, Da(1)-12 x Ei-2

Background

In Arabidopsis thaliana and other crucifers, the glucosinolate-myrosinase system contributes to resistance against herbivory by generalist insects. As yet, it is unclear how crucifers defend themselves against crucifer-specialist insect herbivores.

Methodology/Principal Findings

We analyzed natural variation for resistance against two crucifer specialist lepidopteran herbivores, Pieris brassicae and Plutella xylostella, among Arabidopsis thaliana accessions and in a new Arabidopsis recombinant inbred line (RIL) population generated from the parental accessions Da(1)-12 and Ei-2. This RIL population consists of 201 individual F₈ lines genotyped with 84 PCR-based markers. We identified six QTL for resistance against Pieris herbivory, but found only one weak QTL for Plutella resistance. To elucidate potential factors causing these resistance QTL, we investigated leaf hair (trichome) density, glucosinolates and myrosinase activity, traits known to influence herbivory by generalist insects. We identified several previously unknown QTL for these traits, some of which display a complex pattern of epistatic interactions.

Conclusions/Significance

Although some trichome, glucosinolate or myrosinase QTL co-localize with Pieris QTL, none of these traits explained the resistance QTL convincingly, indicating that resistance against specialist insect herbivores is influenced by other traits than resistance against generalists.

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.

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.

Guard cell- and phloem idioblast-specific expression of thioglucoside glucohydrolase 1 (myrosinase) in Arabidopsis

Thioglucoside glucohydrolase 1 (TGG1) is one of two known functional myrosinase enzymes in Arabidopsis. The enzyme catalyzes the hydrolysis of glucosinolates into compounds that are toxic to various microbes and herbivores. Transgenic Arabidopsis plants carrying beta-glucuronidase and green fluorescent protein reporter genes fused to 0.5 or 2.5 kb of the TGG1 promoter region were used to study spatial promoter activity. Promoter activity was found to be highly specific and restricted to guard cells and distinct cells of the phloem. No promoter activity was detected in the root or seed. All guard cells show promoter activity. Positive phloem cells are distributed in a discontinuous pattern and occur more frequent in young tissues. Immunocytochemical localization of myrosinase in transverse and longitudinal sections of embedded material show that the TGG1 promoter activity reflects the position of the myrosinase enzyme. In the flower stalk, the myrosinase-containing phloem cells are located between phloem sieve elements and glucosinolate-rich S cells. Our results suggest a cellular separation of myrosinase enzyme and glucosinolate substrate, and that myrosinase is contained in distinct cells. We discuss the potential advantages of locating defense and communication systems to only a few specific cell types.

Myrosinase: gene family evolution and herbivore defense in Brassicaceae

Glucosinolates are a category of secondary products present primarily in species of the order Capparales. When tissue is damaged, for example by herbivory, glucosinolates are degraded in a reaction catalyzed by thioglucosidases, denoted myrosinases, also present in these species. Thereby, toxic compounds such as nitriles, isothiocyanates, epithionitriles and thiocyanates are released. The glucosinolate-myrosinase system is generally believed to be part of the plant's defense against insects, and possibly also against pathogens. In this review, the evolution of the system and its impact on the interaction between plants and insects are discussed. Further, data suggesting additional functions in the defense against pathogens and in sulfur metabolism are reviewed.

Myrosinase: gene family evolution and herbivore defense in Brassicaceae

Glucosinolates are a category of secondary products present primarily in species of the order Capparales. When tissue is damaged, for example by herbivory, glucosinolates are degraded in a reaction catalyzed by thioglucosidases, denoted myrosinases, also present in these species. Thereby, toxic compounds such as nitriles, isothiocyanates, epithionitriles and thiocyanates are released. The glucosinolate-myrosinase system is generally believed to be part of the plant's defense against insects, and possibly also against pathogens. In this review, the evolution of the system and its impact on the interaction between plants and insects are discussed. Further, data suggesting additional functions in the defense against pathogens and in sulfur metabolism are reviewed.

Cloning sulfur assimilation genes of Brassica juncea L.: cadmium differentially affects the expression of a putative low-affinity sulfate transporter and isoforms of ATP sulfurylase and APS reductase

The heavy-metal accumulator Brassica juncea L. is a high-biomass crop able to extract heavy-metal ions from the soil, a substantial part being translocated from root to shoot. Previous work has shown that Cd accumulation is accompanied by massive formation of phytochelatins (PCs). Rapid de novo synthesis of PCs in roots and leaves requires an increased synthesis of the tripeptide glutathione (GSH), which in turn depends on increased sulfur assimilation. Therefore. we have cloned cDNAs for three enzymes involved in sulfur assimilation, i.e. a putative low-affinity sulfate transporter (LAST) and two isoforms each for ATP sulfurylase (ATPS) and APS reductase (APSR). As degradation of glucosinolates might provide an additional sulfur source under stress, we also cloned a myrosinase (MYR). RNA blot analysis of transcript amounts indicated that upon Cd exposure (25 microM) the expression of ATPS and APSR in roots and leaves of 6-week-old Brassica juncea plants was strongly increased, whereas the expression of MYR was unaffected. LAST transcripts were significantly reduced in the root but remained unchanged in the leaves. Concomitant with Cd induction of ATPS and APSR mRNAs, cysteine concentrations in roots and leaves increased by 81% and 25%, respectively, whereas GSH concentrations decreased in roots and leaves by 39% and 48%, respectively. In agreement with our previous report on Cd induction of gamma-glutamylcysteine synthetase in B. juncea, the results indicate coordinate changes of expression for several sulfur assimilation enzymes in response to an increased demand for cysteine during PC synthesis.

A phosphate-starvation inducible beta-glucosidase gene (psr3.2) isolated from Arabidopsis thaliana is a member of a distinct subfamily of the BGA family

We have previously isolated a phosphate starvation-response (psr) cDNA clone, psr3.1, from Brassica nigra which encodes a beta-glucosidase. Southern blots of Arabidopsis thaliana genomic DNA probed with the psr3.1 cDNA indicated that this gene exists as a single locus. A genomic library of A. thaliana was screened at high stringency to isolate the corresponding genomic clone. The resultant clone was coined psr3.2 because of its sequence divergence from isolated psr3.1 cDNA clones. Northern blotting with probes derived from the coding region of the genomic clone showed that this gene is expressed at high levels in P(i)-starved roots and the enhancement occurred within two days of growth in medium lacking P(i). The expression of this gene is repressed by heat shock and anaerobic conditions, and it is not significantly induced by high salinity, or by nitrogen or sulfur deprivation. Sequence analysis of the genomic clone revealed the existence of 13 exons interrupted by 12 AT-rich introns and it possessed a high homology with the B. nigra psr3.1 as well as various other beta-glucosidase genes from other species. Sequence similarity and divergence percentages between the deduced amino acid sequences of the psr3 clones and other beta-glycosidases suggests that they should be included along with two other Brassicaceae genes in a distinct subfamily of the BGA glycosidase gene family. The presence of an endoplasmic reticulum retention signal at the carboxy terminus indicates the likely cellular location of PSR3.2. The possible metabolic and regulatory roles of this enzyme during the P(i)-starvation response are discussed.

The unusual 5' splicing border GC is used in myrosinase genes of the Brassicaceae

Myrosinase (thioglucosidase glucohydrolase; EC 3.2.3.1) is a group of isoenzymes in the Brassicaceae, which hydrolyze glucosinolates. Genes encoding myrosinase contain 12 exons and 11 introns. Sequence comparison of two myrosinase genes from Arabidopsis thaliana, TGG1 and TGG2, with the corresponding cDNA from leaves, showed preferential use of a GC dinucleotide as the 5' splicing border in intron 1 instead of an adjacent GT dinucleotide four bp further 3'. This 5' GC splice site is conserved in all seven myrosinase genes characterized from three different species. Likewise, in the 3' region of intron 1 two AG dinucleotides are located seven bp apart. Only the most 5' of these dinucleotides was found to be used in splicing. Sequence analyses of TGG1 cDNA isolated from seeds, siliques and vegetative tissue using reverse transcription PCR showed that the splicing pattern of this intron is identical in these tissues for TGG1. The GT and the most 3' AG dinucleotides mentioned above have been assumed to be the intron borders of intron 1 in several myrosinase genes. The present investigation shows that this assumption is not correct.

Temporal and spatial expression of amygdalin hydrolase and (R)-(+)-mandelonitrile lyase in black cherry seeds

In black cherry (Prunus serotina Ehrh.) macerates, the cyanogenic diglucoside (R)-amygdalin undergoes stepwise degradation to HCN catalyzed by amygdalin hydrolase (AH), prunasin hydrolase, and (R)-(+)-mandelonitrile lyase (MDL). A near full-length AH cDNA clone (pAH1), whose insert encodes the isozyme AH I, has been isolated and sequenced. AH I exhibits several features characteristic of beta-glucosidases of the BGA family, including their likely nucleophile center (isoleucine-threonine-glutamic acid-asparagine-glycine) and acid catalyst (asparagine-glutamic acid-proline/isoleucine) motifs. The temporal expression of AH and MDL in ripening fruit was analyzed by northern blotting. Neither mRNA was detectable until approximately 40 days after flowering (DAF), when embryos first became visible to the naked eye. Both mRNAs peaked at approximately 49 DAF before declining to negligible levels when the fruit matured (82 DAF). Taken together with enzyme activity data, these time courses suggest that AH and MDL expression may be under transcriptional control during fruit maturation. In situ hybridization analysis indicated that AH transcripts are restricted to the procambium, whereas MDL transcripts are localized within cotyledonary parenchyma cells. These tissue-specific distributions are consistent with the major locations of AH and MDL protein in mature seeds previously determined by immunocytochemistry (E. Swain, C.P. Li, and J.E. Poulton [1992] Plant Physiol 100:291-300).

The myrosinase gene family in Arabidopsis thaliana: gene organization, expression and evolution

Myrosinase (thioglucoside glucohydrolase, EC 3.2.3.1.) is in Brassicaceae species such as Brassica napus and Sinapis alba encoded by two differentially expressed gene families, MA and MB, consisting of about 4 and 10 genes, respectively. Southern blot analysis showed that Arabidopsis thaliana contains three myrosinase genes. These genes were isolated from a genomic library and two of them, TGG1 and TGG2, were sequenced. They were found to be located in an inverted mode with their 3' ends 4.4 kb apart. Their organization was highly conserved with 12 exons and 11 short introns. Comparison of nucleotide sequences of TGG1 and TGG2 exons revealed an overall 75% similarity. In contrast, the overall nucleotide sequence similarity in introns was only 42%. In intron 1 the unusual 5' splice border GC was used. Phylogenetic analyses using both distance matrix and parsimony programs suggested that the Arabidopsis genes could not be grouped with either MA or MB genes. Consequently, these two gene families arose only after Arabidopsis had diverged from the other Brassicaceae species. In situ hybridization experiments showed that TGG1 and TGG2 expressing cells are present in leaf, sepal, petal, and gynoecium. In developing seeds, a few cells reacting with the TGG1 probe, but not with the TGG2 probe, were found indicating a partly different expression of these genes.

Characterization of a new myrosinase in Brassica napus

A full-length cDNA clone defining the new myrosinase gene family MC in Brassica napus was isolated and sequenced. Southern hybridization showed that the MC family probably consists of 3 or 4 genes in B. napus. MC genes are expressed in the developing seed, but not in the vegetative tissues investigated. In situ hybridizations to developing seeds showed that the MC genes are expressed in the myrosin cells of the embryo axis and the cotyledons. Complexes with myrosinase and myrosinase-binding protein (MBP) were purified and characterized. Sequencing of peptides from myrosinases occurring in the complexes showed that the 70 kDa myrosinase is encoded by the MC genes, whereas the 65 kDa myrosinase is encoded by the MB genes. This is in contrast to the 75 kDa myrosinase which occurs in free form and is encoded by the MA genes. Deglycosylations of the myrosinase complexes and the free myrosinase showed that the molecular sizes of the myrosinases could be reduced significantly by this treatment, and that the size differences between the different myrosinases are mainly due to differences in glycosylation.

Characterization of rapeseed myrosinase-binding protein

Myrosinase-binding proteins (MBPs) were purified from seeds of Brassica napus L. (oilseed rape). The proteins were characterized with respect to amino-acid composition, peptide sequence and isoelectric points. Gel electrophoresis and Western blotting of protein extracts from mature seeds showed the existence of at least ten proteins reacting with a monoclonal anti-MBP antibody and ranging in molecular size from 110 to 30 kDa. Proteins other than MBP reacting with the anti-MBP antibody were assigned as myrosinase-binding protein-related proteins (MBPRPs). Two MBPRPs were purified by immunoaffinity chromatography and characterized with respect to partial amino-acid sequence. Sequence identities were found between MBP and MBPRP. Western blot analysis of protein extracts from different tissues of B. napus showed that MBPRP is present in the whole plant, whereas MBP mostly occurs in the mature seed. A double-antibody sandwich enzyme-linked immunosorbent assay (ELISA) was used to investigate the occurrence of MBP and MBPRP in developing seeds of some species in the Brassicaceae family.