Cytochromes P-450 from cassava (Manihot esculenta Crantz) catalyzing the first steps in the biosynthesis of the cyanogenic glucosides linamarin and lotaustralin. Cloning, functional expression in Pichia pastoris, and substrate specificity of the isolated recombinant enzymes

The first committed steps in the biosynthesis of the two cyanogenic glucosides linamarin and lotaustralin in cassava are the conversion of L-valine and L-isoleucine, respectively, to the corresponding oximes. Two full-length cDNA clones that encode cytochromes P-450 catalyzing these reactions have been isolated. The two cassava cytochromes P-450 are 85% identical, share 54% sequence identity to CYP79A1 from sorghum, and have been assigned CYP79D1 and CYP79D2. Functional expression has been achieved using the methylotrophic yeast, Pichia pastoris. The amount of CYP79D1 isolated from 1 liter of P. pastoris culture exceeds the amounts that putatively could be isolated from 22,000 grown-up cassava plants. Each cytochrome P-450 metabolizes L-valine as well as L-isoleucine consistent with the co-occurrence of linamarin and lotaustralin in cassava. CYP79D1 was isolated from P. pastoris. Reconstitution in lipid micelles showed that CYP79D1 has a higher k(c) value with L-valine as substrate than with L-isoleucine, which is consistent with linamarin being the major cyanogenic glucoside in cassava. Both CYP79D1 and CYP79D2 are present in the genome of cassava cultivar MCol22 in agreement with cassava being allotetraploid. CYP79D1 and CYP79D2 are actively transcribed, and production of acyanogenic cassava plants would therefore require down-regulation of both genes.     

The cyanogenic glucoside composition of Zygaena filipendulae (Lepidoptera: Zygaenidae) as effected by feeding on wild-type and transgenic lotus populations with variable cyanogenic glucoside profiles

Zygaena larvae sequester the cyanogenic glucosides linamarin and lotaustralin from their food plants (Fabaceae) as well as carry out de novo biosynthesis of these compounds. In this study, Zygaena filipendulae were reared on wild-type Lotus corniculatus and wild-type and transgenic L. japonicus plants with differing content and ratios of the cyanogenic glucosides linamarin and lotaustralin and of the cyanoalkenyl glucosides rhodiocyanoside A and D. LC-MS analyses, free choice feeding experiments and developmental studies were used to examine the effect of varying content and ratios of these secondary metabolites on the feeding preferences, growth and development of Z. filipendulae. Larvae reared on cyanogenic L. corniculatus developed faster compared to larvae reared on L. japonicus although free choice feeding trials demonstrated that the latter plant source was the preferred food plant. Larvae reared on acyanogenic L. corniculatus showed decelerated development. Analysis of different life stages and tissues demonstrate that Z. filipendulae strive to maintain certain threshold content and ratios of cyanogenic glucosides regardless of the composition of the food plants. Despite this, the ratios of cyanogenic glucosides in Z. filipendulae remain partly affected by the ratio of the food plant due to the high proportion of sequestering that takes place.     

The evolutionary appearance of non‐cyanogenic hydroxynitrile glucosides in the Lotus genus is accompanied by the substrate specialization of paralogous β–glucosidases resulting from a crucial amino acid substitution

Lotus japonicus, like several other legumes, biosynthesizes the cyanogenic α–hydroxynitrile glucosides lotaustralin and linamarin. Upon tissue disruption these compounds are hydrolysed by a specific β–glucosidase, resulting in the release of hydrogen cyanide. Lotus japonicus also produces the non‐cyanogenic γ‐ and β–hydroxynitrile glucosides rhodiocyanoside A and D using a biosynthetic pathway that branches off from lotaustralin biosynthesis. We previously established that BGD2 is the only β–glucosidase responsible for cyanogenesis in leaves. Here we show that the paralogous BGD4 has the dominant physiological role in rhodiocyanoside degradation. Structural modelling, site‐directed mutagenesis and activity assays establish that a glycine residue (G211) in the aglycone binding site of BGD2 is essential for its ability to hydrolyse the endogenous cyanogenic glucosides. The corresponding valine (V211) in BGD4 narrows the active site pocket, resulting in the exclusion of non‐flat substrates such as lotaustralin and linamarin, but not of the more planar rhodiocyanosides. Rhodiocyanosides and the BGD4 gene only occur in L. japonicus and a few closely related species associated with the Lotus corniculatus clade within the Lotus genus. This suggests the evolutionary scenario that substrate specialization for rhodiocyanosides evolved from a promiscuous activity of a progenitor cyanogenic β–glucosidase, resembling BGD2, and required no more than a single amino acid substitution.     

Cassava plants with a depleted cyanogenic glucoside content in leaves and tubers. Distribution of cyanogenic glucosides, their site of synthesis and transport, and blockage of the biosynthesis by RNA interference technology

Transgenic cassava (Manihot esculenta Crantz, cv MCol22) plants with a 92% reduction in cyanogenic glucoside content in tubers and acyanogenic (<1% of wild type) leaves were obtained by RNA interference to block expression of CYP79D1 and CYP79D2, the two paralogous genes encoding the first committed enzymes in linamarin and lotaustralin synthesis. About 180 independent lines with acyanogenic (<1% of wild type) leaves were obtained. Only a few of these were depleted with respect to cyanogenic glucoside content in tubers. In agreement with this observation, girdling experiments demonstrated that cyanogenic glucosides are synthesized in the shoot apex and transported to the root, resulting in a negative concentration gradient basipetal in the plant with the concentration of cyanogenic glucosides being highest in the shoot apex and the petiole of the first unfolded leaf. Supply of nitrogen increased the cyanogenic glucoside concentration in the shoot apex. In situ polymerase chain reaction studies demonstrated that CYP79D1 and CYP79D2 were preferentially expressed in leaf mesophyll cells positioned adjacent to the epidermis. In young petioles, preferential expression was observed in the epidermis, in the two first cortex cell layers, and in the endodermis together with pericycle cells and specific parenchymatic cells around the laticifers. These data demonstrate that it is possible to drastically reduce the linamarin and lotaustralin content in cassava tubers by blockage of cyanogenic glucoside synthesis in leaves and petioles. The reduced flux to the roots of reduced nitrogen in the form of cyanogenic glucosides did not prevent tuber formation.     

Diversification of an ancient theme: hydroxynitrile glucosides

Many plants produce cyanogenic glucosides as part of their chemical defense. They are alpha-hydroxynitrile glucosides, which release toxic hydrogen cyanide (HCN) upon cleavage by endogenous plant beta-glucosidases. In addition to cyanogenic glucosides, several plant species produce beta- and gamma-hydroxynitrile glucosides. These do not release HCN upon hydrolysis by beta-glucosidases and little is known about their biosynthesis and biological significance. We have isolated three beta-hydroxynitrile glucosides, namely (2Z)-2-(beta-D-glucopyranosyloxy)but-2-enenitrile and (2R,3R)- and (2R,3S)-2-methyl-3-(beta-D-glucopyranosyloxy)butanenitrile, from leaves of Ribesuva-crispa. These compounds have not been identified previously. We show that in several species of the genera Ribes, Rhodiola and Lotus, these beta-hydroxynitrile glucosides co-occur with the L-isoleucine-derived hydroxynitrile glucosides, lotaustralin (alpha-hydroxynitrile glucoside), rhodiocyanosides A (gamma-hydroxynitrile glucoside) and D (beta-hydroxynitrile glucoside) and in some cases with sarmentosin (a hydroxylated rhodiocyanoside A). Radiolabelling experiments demonstrated that the hydroxynitrile glucosides in R. uva-crispa and Hordeum vulgare are derived from L-isoleucine and L-leucine, respectively. Metabolite profiling of the natural variation in the content of cyanogenic glucosides and beta- and gamma-hydroxynitrile glucosides in wild accessions of Lotus japonicus in combination with genetic crosses and analyses of the metabolite profile of the F2 population provided evidence that a single recessive genetic trait is most likely responsible for the presence or absence of beta- and gamma-hydroxynitrile glucosides in L. japonicus. Our findings strongly support the notion that the beta- and gamma-hydroxynitrile glucosides are produced by diversification of the cyanogenic glucoside biosynthetic pathway at the level of the nitrile intermediate.     

Genetic Screening Identifies Cyanogenesis-Deficient Mutants of Lotus japonicus and Reveals Enzymatic Specificity in Hydroxynitrile Glucoside Metabolism

Cyanogenesis, the release of hydrogen cyanide from damaged plant tissues, involves the enzymatic degradation of amino acid–derived cyanogenic glucosides (α-hydroxynitrile glucosides) by specific β-glucosidases. Release of cyanide functions as a defense mechanism against generalist herbivores. We developed a high-throughput screening method and used it to identify cyanogenesis deficient (cyd) mutants in the model legume Lotus japonicus. Mutants in both biosynthesis and catabolism of cyanogenic glucosides were isolated and classified following metabolic profiling of cyanogenic glucoside content. L. japonicus produces two cyanogenic glucosides: linamarin (derived from Val) and lotaustralin (derived from Ile). Their biosynthesis may involve the same set of enzymes for both amino acid precursors. However, in one class of mutants, accumulation of lotaustralin and linamarin was uncoupled. Catabolic mutants could be placed in two complementation groups, one of which, cyd2, encoded the β-glucosidase BGD2. Despite the identification of nine independent cyd2 alleles, no mutants involving the gene encoding a closely related β-glucosidase, BGD4, were identified. This indicated that BGD4 plays no role in cyanogenesis in L. japonicus in vivo. Biochemical analysis confirmed that BGD4 cannot hydrolyze linamarin or lotaustralin and in L. japonicus is specific for breakdown of related hydroxynitrile glucosides, such as rhodiocyanoside A. By contrast, BGD2 can hydrolyze both cyanogenic glucosides and rhodiocyanosides. Our genetic analysis demonstrated specificity in the catabolic pathways for hydroxynitrile glucosides and implied specificity in their biosynthetic pathways as well. In addition, it has provided important tools for elucidating and potentially modifying cyanogenesis pathways in plants.     

Characterization and expression profile of two UDP-glucosyltransferases, UGT85K4 and UGT85K5, catalyzing the last step in cyanogenic glucoside biosynthesis in cassava

Manihot esculenta (cassava) contains two cyanogenic glucosides, linamarin and lotaustralin, biosynthesized from l ‐valine and l ‐isoleucine, respectively. In this study, cDNAs encoding two uridine diphosphate glycosyltransferase (UGT) paralogs, assigned the names UGT85K4 and UGT85K5, have been isolated from cassava. The paralogs display 96% amino acid identity, and belong to a family containing cyanogenic glucoside‐specific UGTs from Sorghum bicolor and Prunus dulcis. Recombinant UGT85K4 and UGT85K5 produced in Escherichia coli were able to glucosylate acetone cyanohydrin and 2‐hydroxy‐2‐methylbutyronitrile, forming linamarin and lotaustralin. UGT85K4 and UGT85K5 show broad in vitro substrate specificity, as documented by their ability to glucosylate other hydroxynitriles, some flavonoids and simple alcohols. Immunolocalization studies indicated that UGT85K4 and UGT85K5 co‐occur with CYP79D1/D2 and CYP71E7 paralogs, which catalyze earlier steps in cyanogenic glucoside synthesis in cassava. These enzymes are all found in mesophyll and xylem parenchyma cells in the first unfolded cassava leaf. In situ PCR showed that UGT85K4 and UGT85K5 are co‐expressed with CYP79D1 and both CYP71E7 paralogs in the cortex, xylem and phloem parenchyma, and in specific cells in the endodermis of the petiole of the first unfolded leaf. Based on the data obtained, UGT85K4 and UGT85K5 are concluded to be the UGTs catalyzing in planta synthesis of cyanogenic glucosides. The localization of the biosynthetic enzymes suggests that cyanogenic glucosides may play a role in both defense reactions and in fine‐tuning nitrogen assimilation in cassava.     

Cytochrome P450 CYP79A2 from Arabidopsis thaliana L. Catalyzes the conversion of L-phenylalanine to phenylacetaldoxime in the biosynthesis of benzylglucosinolate

Glucosinolates are natural plant products gaining increasing interest as cancer-preventing agents and crop protectants. Similar to cyanogenic glucosides, glucosinolates are derived from amino acids and have aldoximes as intermediates. We report cloning and characterization of cytochrome P450 CYP79A2 involved in aldoxime formation in the glucosinolate-producing Arabidopsis thaliana L. The CYP79A2 cDNA was cloned by polymerase chain reaction, and CYP79A2 was functionally expressed in Escherichia coli. Characterization of the recombinant protein shows that CYP79A2 is an N-hydroxylase converting L-phenylalanine into phenylacetaldoxime, the precursor of benzylglucosinolate. Transgenic A. thaliana constitutively expressing CYP79A2 accumulate high levels of benzylglucosinolate. CYP79A2 expressed in E. coli has a K(m) of 6.7 micromol liter(-1) for L-phenylalanine. Neither L-tyrosine, L-tryptophan, L-methionine, nor DL-homophenylalanine are metabolized by CYP79A2, indicating that the enzyme has a narrow substrate specificity. CYP79A2 is the first enzyme shown to catalyze the conversion of an amino acid to the aldoxime in the biosynthesis of glucosinolates. Our data provide the first conclusive evidence that evolutionarily conserved cytochromes P450 catalyze this step common for the biosynthetic pathways of glucosinolates and cyanogenic glucosides. This strongly indicates that the biosynthesis of glucosinolates has evolved based on a cyanogenic predisposition.     

Genomic clustering of cyanogenic glucoside biosynthetic genes aids their identification in Lotus japonicus and suggests the repeated evolution of this chemical defence pathway

Cyanogenic glucosides are amino acid-derived defence compounds found in a large number of vascular plants. Their hydrolysis by specific β-glucosidases following tissue damage results in the release of hydrogen cyanide. The cyanogenesis deficient1 (cyd1) mutant of Lotus japonicus carries a partial deletion of the CYP79D3 gene, which encodes a cytochrome P450 enzyme that is responsible for the first step in cyanogenic glucoside biosynthesis. The genomic region surrounding CYP79D3 contains genes encoding the CYP736A2 protein and the UDP-glycosyltransferase UGT85K3. In combination with CYP79D3, these genes encode the enzymes that constitute the entire pathway for cyanogenic glucoside biosynthesis. The biosynthetic genes for cyanogenic glucoside biosynthesis are also co-localized in cassava (Manihot esculenta) and sorghum (Sorghum bicolor), but the three gene clusters show no other similarities. Although the individual enzymes encoded by the biosynthetic genes in these three plant species are related, they are not necessarily orthologous. The independent evolution of cyanogenic glucoside biosynthesis in several higher plant lineages by the repeated recruitment of members from similar gene families, such as the CYP79s, is a likely scenario.     

Cloning of three A-type cytochromes P450, CYP71E1, CYP98, and CYP99 from Sorghum bicolor (L.) Moench by a PCR approach and identification by expression in Escherichia coli of CYP71E1 as a multifunctional cytochrome P450 in the biosynthesis of the cyanogenic glucoside dhurrin

A cDNA encoding the multifunctional cytochrome P450, CYP71E1, involved in the biosynthesis of the cyanogenic glucoside dhurrin from Sorghum bicolor (L.) Moench was isolated. A PCR approach based on three consensus sequences of A-type cytochromes P450 – (V/I)KEX(L/F)R, FXPERF, and PFGXGRRXCXG – was applied. Three novel cytochromes P450 (CYP71E1, CYP98, and CYP99) in addition to a PCR fragment encoding sorghum cinnamic acid 4-hydroxylase were obtained.Reconstitution experiments with recombinant CYP71E1 heterologously expressed in Escherichia coli and sorghum NADPH–cytochrome P450–reductase in L--dilaurylphosphatidyl choline micelles identified CYP71E1 as the cytochrome P450 that catalyses the conversion of p-hydroxyphenylacetaldoxime to p-hydroxymandelonitrile in dhurrin biosynthesis. In accordance to the proposed pathway for dhurrin biosynthesis CYP71E1 catalyses the dehydration of the oxime to the corresponding nitrile, followed by a C-hydroxylation of the nitrile to produce p-hydroxymandelonitrile. In vivo administration of oxime to E. coli cells results in the accumulation of the nitrile, which indicates that the flavodoxin/flavodoxin reductase system in E. coli is only able to support CYP71E1 in the dehydration reaction, and not in the subsequent C-hydroxylation reaction.CYP79 catalyses the conversion of tyrosine to p-hydroxyphenylacetaldoxime, the first committed step in the biosynthesis of the cyanogenic glucoside dhurrin. Reconstitution of both CYP79 and CYP71E1 in combination with sorghum NADPH-cytochrome P450–reductase resulted in the conversion of tyrosine to p-hydroxymandelonitrile, i.e. the membranous part of the biosynthetic pathway of the cyanogenic glucoside dhurrin. Isolation of the cDNA for CYP71E1 together with the previously isolated cDNA for CYP79 provide important tools necessary for tissue-specific regulation of cyanogenic glucoside levels in plants to optimize food safety and pest resistance.     

The biosynthesis of cyanogenic glucosides in seedlings of cassava (Manihot esculenta Crantz).

A microsomal system catalyzing the in vitro synthesis of the aglycones of the two cyanogenic glucosides linamarin and lotaustralin has been isolated from young etiolated seedlings of cassava (Manihot esculenta Crantz). A prerequisite to obtain active preparations is the complete removal of the endosperm pellicle covering the cotyledons before seedling homogenization. The rates of conversion of the parent amino acids valine and isoleucine to their cyanohydrins are 19 and 6 nmol/h/mg protein, respectively. The conversion rates for the corresponding oximes (2-methylpropanal oxime and 2-methylbutanal oxime) are 475 and 440 nmol/h/mg protein and for the nitriles (2-methylpropionitrile and 2-methylbutyronitrile) 45 and 75 nmol/h/mg protein. With the exception of 2-cyclopentenylglycine, none of the additionally tested amino acids are metabolized, whereas a broad substrate specificity is observed using oximes and nitriles as substrates. The in vitro biosynthesis is photoreversibly inhibited by carbon monoxide, demonstrating the involvement of cytochrome P450 in the hydroxylation processes. All tissues of the cassava seedling contain cyanogenic glucosides. The microsomal enzyme system responsible for their synthesis is restricted to the cotyledons and their petioles. This demonstrates that the cyanogenic glucosides are actively transported to other parts of the seedling. The enzyme activity decreases with the height of the etiolated seedling and is barely detectable in seedlings above 75 mm.     

Biosynthesis of cyanogenic glucosides in butterflies and moths: Effective incorporation of 2-methylpropanenitrile and 2-methylbutanenitrile into linamarin and lotaustralin by Zygaena and Heliconius species (Lepidoptera)

¹⁴C-labelled 2-methylpropanenitrile and 2-methylbutanenitrile were administered to larvae and imagines of Heliconius melpomone and to larvae of Zygaena trifolii and the incorporation into the cyanogenic glucosides, linamarin and lotaustralin, was measured. Both species incorporated the precursors at all stages tested, at a high level of 15–72%, thereby indicating that the nitriles are probale intermediates in the lepidopteran biosynthesis of linamarin and lotaustralin from valine and isoleucine respectively.     

Leucine-derived cyano glucosides in barley

Barley (Hordeum vulgare) seedlings contain five cyano glucosides derived from the amino acid L-leucine (Leu). The chemical structure and the relative abundance of the cyano glucosides were investigated by liquid chromatography-mass spectrometry and nuclear magnetic resonance analyses using spring barley cultivars with high, medium, and low cyanide potential. The barley cultivars showed a 10-fold difference in their cyano glucoside content, but the relative content of the individual cyano glucosides remained constant. Epiheterodendrin, the only cyanogenic glucoside present, comprised 12% to 18% of the total content of cyano glucosides. It is proposed that the aglycones of all five cyano glucosides are formed by the initial action of a cytochrome P450 enzyme of the CYP79 family converting L-Leu into Z-3-methylbutanal oxime and subsequent action of a less specific CYP71E enzyme converting the oxime into 3-methylbutyro nitrile and mediating subsequent hydroxylations at the alpha-, as well as beta- and gamma-, carbon atoms. Presence of cyano glucosides in the barley seedlings was restricted to leaf tissue, with 99% confined to the epidermis cell layers of the leaf blade. Microsomal preparations from epidermal cells were not able to convert L-[(14)C]Leu into the biosynthetic intermediate, Z-3-methylbutanal-oxime. This was only achieved using microsomal preparations from other cell types in the basal leaf segment, demonstrating translocation of the cyano glucosides to the epidermal cell layers after biosynthesis. A beta-glucosidase able to degrade epiheterodendrin was detected exclusively in yet a third compartment, the endosperm of the germinating seed. Therefore, in barley, a putative function of cyano glucosides in plant defense is not linked to cyanide release.     

Oximes, nitriles and 2-hydroxynitriles as precursors in the biosynthesis of cyanogenic glucosides

The biosynthesis of the cyanogenic glucosides, linamarin and prunasin, was investigated in linen-flax, peach and cherry-laurel shoots. It was shown that related 2-oximino acids, aldoximes, nitriles and 2-hydroxynitriles were generally good precursors of the aglycone moiety. Studies with double-labelled compounds confirmed the retention of the oximino nitrogen atom from 2-oximinoisovaleric acid and isobutyraldoxime in the biosynthesis of linamarin. A general pathway from amino acids to cyanogenic glucosides involving N-hydroxyamino acids, aldoximes, nitriles and 2-hydroxynitriles is proposed.     

The presence of CYP79 homologues in glucosinolate-producing plants shows evolutionary conservation of the enzymes in the conversion of amino acid to aldoxime in the biosynthesis of cyanogenic glucosides and glucosinolates

A cDNA encoding CYP79B1 has been isolated from Sinapis alba. CYP79B1 from S. alba shows 54% sequence identity and 73% similarity to sorghum CYP79A1 and 95% sequence identity to the Arabidopsis T42902, assigned CYP79B2. The high identity and similarity to sorghum CYP79A1, which catalyses the conversion of tyrosine to p-hydroxyphenylacetaldoxime in the biosynthesis of the cyanogenic glucoside dhurrin, suggests that CYP79B1 similarly catalyses the conversion of amino acid(s) to aldoxime(s) in the biosynthesis of glucosinolates. Within the highly conserved PERF and the heme-binding region of A-type cytochromes, the CYP79 family has unique substitutions that define the family-specific consensus sequences of FXP(E/D)RH and SFSTG(K/R)RGC(A/I)A, respectively. Sequence analysis of PCR products generated with CYP79B subfamily-specific primers identified CYP79B homologues in Tropaeolum majus, Carica papaya, Arabidopsis, Brassica napus and S. alba. The five glucosinolate-producing plants identified a CYP79B amino acid consensus sequence KPERHLNECSEVTLTENDLRFISFSTGKRGC. The unique substitutions in the PERF and the heme-binding domain and the high sequence identity and similarity of CYP79B1, CYP79B2 and CYP79A1, together with the isolation of CYP79B homologues in the distantly related Tropaeolaceae, Caricaceae and Brassicaceae within the Capparales order, show that the initial part of the biosynthetic pathway of glucosinolates and cyanogenic glucosides is catalysed by evolutionarily conserved cytochromes P450. This confirms that the appearance of glucosinolates in Capparales is based on a cyanogen predisposition. Identification of CYP79 homologues in glucosinolate-producing plants provides an important tool for tissue-specific regulation of the level of glucosinolates to improve nutritional value and pest resistance.     

Cyanogenic glucosides in the biological warfare between plants and insects: the Burnet moth-Birdsfoot trefoil model system

Cyanogenic glucosides are important components of plant defense against generalist herbivores due to their bitter taste and the release of toxic hydrogen cyanide upon tissue disruption. Some specialized herbivores, especially insects, preferentially feed on cyanogenic plants. Such herbivores have acquired the ability to metabolize cyanogenic glucosides or to sequester them for use in their own predator defense. Burnet moths (Zygaena) sequester the cyanogenic glucosides linamarin and lotaustralin from their food plants (Fabaceae) and, in parallel, are able to carry out de novo synthesis of the very same compounds. The ratio and content of cyanogenic glucosides is tightly regulated in the different stages of the Zygaena filipendulae lifecycle and the compounds play several important roles in addition to defense. The transfer of a nuptial gift of cyanogenic glucosides during mating of Zygaena has been demonstrated as well as the possible involvement of hydrogen cyanide in male assessment and nitrogen metabolism. As the capacity to de novo synthesize cyanogenic glucosides was developed independently in plants and insects, the great similarities of the pathways between the two kingdoms indicate that cyanogenic glucosides are produced according to a universal route providing recruitment of the enzymes required. Pyrosequencing of Z. filipendulae larvae de novo synthesizing cyanogenic glucosides served to provide a set of good candidate genes, and demonstrated that the genes encoding the pathway in plants and Z. filipendulae are not closely related phylogenetically. Identification of insect genes involved in the biosynthesis and turn-over of cyanogenic glucosides will provide new insights into biological warfare as a determinant of co-evolution between plants and insects.     

Metabolon formation in dhurrin biosynthesis

Synthesis of the tyrosine derived cyanogenic glucoside dhurrin in Sorghum bicolor is catalyzed by two multifunctional, membrane bound cytochromes P450, CYP79A1 and CYP71E1, and a soluble UDPG-glucosyltransferase, UGT85B1 (Tattersall, D.B., Bak, S., Jones, P.R., Olsen, C.E., Nielsen, J.K., Hansen, M.L., H?j, P.B., M?ller, B.L., 2001. Resistance to an herbivore through engineered cyanogenic glucoside synthesis. Science 293, 1826-1828). All three enzymes retained enzymatic activity when expressed as fluorescent fusion proteins in planta. Transgenic Arabidopsis thaliana plants that produced dhurrin were obtained by co-expression of CYP79A1/CYP71E1-CFP/UGT85B1-YFP and of CYP79A1/CYP71E1/UGT85B1-YFP but not by co-expression of CYP79A1-YFP/CYP71E-CFP/UGT85B1. The lack of dhurrin formation upon co-expression of the two cytochromes P450 as fusion proteins indicated that tight interaction was necessary for efficient substrate channelling. Transient expression in S. bicolor epidermal cells as monitored by confocal laser scanning microscopy showed that UGT85B1-YFP accumulated in the cytoplasm in the absence of CYP79A1 or CYP71E1. In the presence of CYP79A1 and CYP71E1, the localization of UGT85B1 shifted towards the surface of the ER membrane in the periphery of biosynthetic active cells, demonstrating in planta dhurrin metabolon formation.