Alain Hehn

Cytochromes P450 for engineering herbicide tolerance

In recent years, genome sequencing has revealed that cytochromes P450 (P450s) constitute the largest family of enzymatic proteins in higher plants. P450s are mono-oxygenases that insert one atom of oxygen into inert hydrophobic molecules to make them more reactive and hydrosoluble. Besides their physiological functions in the biosynthesis of hormones, lipids and secondary metabolites, P450s help plants to cope with harmful exogenous chemicals including pesticides and industrial pollutants, making them less phytotoxic. The recovery of an increasing number of plant P450 genes in recombinant form has enabled their use in experimentation, which has revealed their extraordinary potential for engineering herbicide tolerance, biosafening, bioremediation and green chemistry.

Biosynthesis of coumarins in plants: a major pathway still to be unravelled for cytochrome P450 enzymes

Coumarins (1,2-benzopyrones) are ubiquitously found in higher plants where they originate from the phenylpropanoid pathway. They contribute essentially to the persistence of plants being involved in processes such as defense against phytopathogens, response to abiotic stresses, regulation of oxidative stress, and probably hormonal regulation. Despite their importance, major details of their biosynthesis are still largely unknown and many P450-dependent enzymatic steps have remained unresolved. Ortho-hydroxylation of hydroxycinnamic acids is a pivotal step that has received insufficient attention in the literature. This hypothetical P450 reaction is critical for the course for the biosynthesis of simple coumarin, umbelliferone and other hydroxylated coumarins in plants. Multiple P450 enzymes are also involved in furanocoumarin synthesis, a major class of phytoalexins derived from umbelliferone. Several of them have been characterized at the biochemical level but no monooxygenase gene of the furanocoumarin pathway has been identified yet. This review highlights the major steps of the coumarin pathway with emphasis on the cytochrome P450 enzymes involved. Recent progress and the outcomes of novel strategies developed to uncover coumarin-committed CYPs are discussed.

Crosstalk and differential response to abiotic and biotic stressors reflected at the transcriptional level of effector genes from secondary metabolism

Plant secondary metabolism significantly contributes to defensive measures against adverse abiotic and biotic cues. To investigate stress-induced, transcriptional alterations of underlying effector gene families, which encode enzymes acting consecutively in secondary metabolism and defense reactions, a DNA array (MetArray) harboring gene-specific probes was established. It comprised complete sets of genes encoding 109 secondary product glycosyltransferases and 63 glutathione-utilizing enzymes along with 62 cytochrome P450 monooxygenases and 26 ABC transporters. Their transcriptome was monitored in different organs of unstressed plants and in shoots in response to herbicides, UV-B radiation, endogenous stress hormones, and pathogen infection. A principal component analysis based on the transcription of these effector gene families defined distinct responses and crosstalk. Methyl jasmonate and ethylene treatments were separated from a group combining reactions towards two sulfonylurea herbicides, salicylate and an avirulent strain of Pseudomonas syringae pv. tomato. The responses to the herbicide bromoxynil and UV-B radiation were distinct from both groups. In addition, these analyses pinpointed individual effector genes indicating their role in these stress responses. A small group of genes was diagnostic in differentiating the response to two herbicide classes used. Interestingly, a subset of genes induced by P. syringae was not responsive to the applied stress hormones. Small groups of comprehensively induced effector genes indicate common defense strategies. Furthermore, homologous members within branches of these effector gene families displayed differential expression patterns either in both organs or during stress responses arguing for their non-redundant functions.

Evolution of a Novel Phenolic Pathway for Pollen Development

From Retrogene to Phenolic Metabolism Metabolic plasticity, which involves the creation of new genes, is an essential feature of plant adaptation and speciation. Studying plants from the mustard family, Matsuno et al. (p. 1688) show that variants of the cytochrome P450 enzyme family were derived through retroposition, duplication, and subsequent mutaton. Evolutionary changes increased the volume of the substrate pocket altering with what sorts of substrates the enzymes could interact. The enzymes formed the basis for a new metabolic pathway, the products of which include constituents of pollen and of phenylpropanoid metabolism. Gene copying and positive Darwinian selection promoted the emergence of a phenolic pathway in Brassicaceae. Metabolic plasticity, which largely relies on the creation of new genes, is an essential feature of plant adaptation and speciation and has led to the evolution of large gene families. A typical example is provided by the diversification of the cytochrome P450 enzymes in plants. We describe here a retroposition, neofunctionalization, and duplication sequence that, via selective and local amino acid replacement, led to the evolution of a novel phenolic pathway in Brassicaceae. This pathway involves a cascade of six successive hydroxylations by two partially redundant cytochromes P450, leading to the formation of N1,N5-di(hydroxyferuloyl)-N10-sinapoylspermidine, a major pollen constituent and so-far-overlooked player in phenylpropanoid metabolism. This example shows how positive Darwinian selection can favor structured clusters of nonsynonymous substitutions that are needed for the transition of enzymes to new functions.

The isolation and mapping of a novel hydroxycinnamoyltransferase in the globe artichoke chlorogenic acid pathway

BackgroundThe leaves of globe artichoke and cultivated cardoon (Cynara cardunculus L.) have significant pharmaceutical properties, which mainly result from their high content of polyphenolic compounds such as monocaffeoylquinic and dicaffeoylquinic acid (DCQ), and a range of flavonoid compounds.ResultsHydroxycinnamoyl-CoA:quinate hydroxycinnamoyltransferase (HQT) encoding genes have been isolated from both globe artichoke and cultivated cardoon (GenBank accessions DQ915589 and DQ915590, respectively) using CODEHOP and PCR-RACE. A phylogenetic analysis revealed that their sequences belong to one of the major acyltransferase groups (anthranilate N-hydroxycinnamoyl/benzoyltransferase). The heterologous expression of globe artichoke HQT in E. coli showed that this enzyme can catalyze the esterification of quinic acid with caffeoyl-CoA or p-coumaroyl-CoA to generate, respectively, chlorogenic acid (CGA) and p-coumaroyl quinate. Real time PCR experiments demonstrated an increase in the expression level of HQT in UV-C treated leaves, and established a correlation between the synthesis of phenolic acids and protection against damage due to abiotic stress. The HQT gene, together with a gene encoding hydroxycinnamoyl-CoA:shikimate/quinate hydroxycinnamoyltransferase (HCT) previously isolated from globe artichoke, have been incorporated within the developing globe artichoke linkage maps.ConclusionA novel acyltransferase involved in the biosynthesis of CGA in globe artichoke has been isolated, characterized and mapped. This is a good basis for our effort to understand the genetic basis of phenylpropanoid (PP) biosynthesis in C. cardunculus.

Molecular Cloning and Functional Characterization of Psoralen Synthase, the First Committed Monooxygenase of Furanocoumarin Biosynthesis

Ammi majus L. accumulates linear furanocoumarins by cytochrome P450 (CYP)-dependent conversion of 6-prenylumbelliferone via (+)-marmesin to psoralen. Relevant activities, i.e. psoralen synthase, are induced rapidly from negligible background levels upon elicitation of A. majus cultures with transient maxima at 9-10 h and were recovered in labile microsomes. Expressed sequence tags were cloned from elicited Ammi cells by a nested DD-RT-PCR strategy with CYP-specific primers, and full-size cDNAs were generated from those fragments correlated in abundance with the induction profile of furanocoumarin-specific activities. One of these cDNAs representing a transcript of maximal abundance at 4 h of elicitation was assigned CYP71AJ1. Functional expression in Escherichia coli or yeast cells initially failed but was accomplished eventually in yeast cells after swapping the N-terminal membrane anchor domain with that of CYP73A1. The recombinant enzyme was identified as psoralen synthase with narrow substrate specificity for (+)-marmesin. Psoralen synthase catalyzes a unique carbon-chain cleavage reaction concomitantly releasing acetone by syn-elimination. Related plants, i.e. Heracleum mantegazzianum, are known to produce both linear and angular furanocoumarins by analogous conversion of 8-prenylumbelliferone via (+)-columbianetin to angelicin, and it was suggested that angelicin synthase has evolved from psoralen synthase. However, (+)-columbianetin failed as substrate but competitively inhibited psoralen synthase activity. Analogy modeling and docked solutions defined the conditions for high affinity substrate binding and predicted the minimal requirements to accommodate (+)-columbianetin in the active site cavity. The studies suggested that several point mutations are necessary to pave the road toward angelicin synthase evolution.

Isolation and functional characterization of a cDNA coding a hydroxycinnamoyltransferase involved in phenylpropanoid biosynthesis in Cynara cardunculus L

BackgroundCynara cardunculus L. is an edible plant of pharmaceutical interest, in particular with respect to the polyphenolic content of its leaves. It includes three taxa: globe artichoke, cultivated cardoon, and wild cardoon. The dominating phenolics are the di-caffeoylquinic acids (such as cynarin), which are largely restricted to Cynara species, along with their precursor, chlorogenic acid (CGA). The scope of this study is to better understand CGA synthesis in this plant.ResultsA gene sequence encoding a hydroxycinnamoyltransferase (HCT) involved in the synthesis of CGA, was identified. Isolation of the gene sequence was achieved by using a PCR strategy with degenerated primers targeted to conserved regions of orthologous HCT sequences available. We have isolated a 717 bp cDNA which shares 84% aminoacid identity and 92% similarity with a tobacco gene responsible for the biosynthesis of CGA from p-coumaroyl-CoA and quinic acid. In silico studies revealed the globe artichoke HCT sequence clustering with one of the main acyltransferase groups (i.e. anthranilate N-hydroxycinnamoyl/benzoyltransferase). Heterologous expression of the full length HCT (GenBank accession DQ104740) cDNA in E. coli demonstrated that the recombinant enzyme efficiently synthesizes both chlorogenic acid and p-coumaroyl quinate from quinic acid and caffeoyl-CoA or p-coumaroyl-CoA, respectively, confirming its identity as a hydroxycinnamoyl-CoA: quinate HCT. Variable levels of HCT expression were shown among wild and cultivated forms of C. cardunculus subspecies. The level of expression was correlated with CGA content.ConclusionThe data support the predicted involvement of the Cynara cardunculus HCT in the biosynthesis of CGA before and/or after the hydroxylation step of hydroxycinnamoyl esters.

Cloning, characterization and regulation of a family of phi class glutathione transferases from wheat

Six phi (F) class glutathione transferases (GSTs) were cloned from bread wheat (Triticum aestivum L.) treated with the herbicide safener fenchlorazole ethyl and named TaGSTF1–6. Recombinant TaGSTFs were assayed for glutathione conjugating activity towards xenobiotics including herbicides and for glutathione peroxidase (GPOX) activity. TaGSTF1, which resembled ZmGSTF1, the dominant GST in maize (Zea mays), was highly active in conjugating 1-chloro-2,4-dinitrobenezene (CDNB) but had low activities towards chloroacetanilide, diphenyl ether and aryloxphenoxypropionate herbicides. TaGSTF2, TaGSTF3 and TaGSTF4 all resembled the safener-inducible ZmGSTF2, with TaGSTF2 and TaGSTF3 being highly active GPOXs and rapidly detoxifying chloroacetanilides. TaGSTF5 resembled ZmGSTF3, having limited conjugating and GPOX activity. TaGSTF6 contained both ZmGSTF1- and ZmGSTF2-like sequences but was most similar to ZmGSTF1 in detoxifying activity. The expression of TaGSTFs in wheat seedlings was enhanced upon exposure to fenchlorazole ethyl, herbicides or other chemical inducing treatments. TaGSTFs were also enhanced by treatment with the natural products caffeic acid, 7,4-dihydroxyflavone and naringenin. The CDNB-conjugating activity of TaGSTF1, and to a lesser extent TaGSTF6, was highly sensitive to inhibition by flavonoids, particularly the chalcone isoliquiritigenin. The other TaGSTFs were much less sensitive to such inhibition. It was subsequently determined that isoliquiritigenin underwent glutathione conjugation, though this reversible reaction did not require the intervention of any TaGSTF. The potential importance of GSTFs and glutathione conjugation in flavonoid metabolism is discussed.

Isolation and Functional Characterization of CYP71AJ4 Encoding for the First P450 Monooxygenase of Angular Furanocoumarin Biosynthesis

The biosynthesis of linear and angular furanocoumarins is still poorly understood at the molecular level, with only psoralen synthase (CYP71AJ1) identified from Ammi majus. Using cDNA probes inferred from CYP71AJ1, three orthologs were isolated from Apium graveolens (CYP71AJ2) and Pastinaca sativa (CYP71AJ3 and -4) and functionally expressed in yeast cells. CYP71AJ2 and CYP71AJ3 displayed psoralen synthase activity, whereas CYP71AJ4 only catalyzed the conversion of (+)-columbianetin to angelicin and negligible amounts of a hydroxylated columbianetin by-product. CYP71AJ4 thus constitutes the first fully characterized P450 monooxygenase specific for the angular furanocoumarin pathway. The angelicin synthase exhibited an apparent Km of 2.1 ± 0.4 μm for (+)-columbianetin and a kcat of 112 ± 14 min–1. Moreover, the use of 3′-deuterated (+)-columbianetin as substrate led to an almost complete “metabolic switch,” resulting in the synthesis of anti-3′-hydroxy-3′-deuterated(+)-columbianetin. This confirms that angelicin synthase attacks columbianetin by syn-elimination of hydrogen from C-3′. Sequence comparison between psoralen synthase (CYP71AJ3) and angelicin synthase (CYP71AJ4) showed 70% identity, whereas the identity dropped to 40% in those regions thought to provide the substrate recognition sites. Accordingly, CYP71AJ3 and CYP71AJ4 might be derived from a common ancestor of unknown functionality by gene duplication and subsequent molecular evolution.

Conservation and diversity of gene families explored using the CODEHOP strategy in higher plants

BackgroundAvailability of genomewide information on an increasing but still limited number of plants offers the possibility of identifying orthologues, or related genes, in species with major economical impact and complex genomes. In this paper we exploit the recently described CODEHOP primer design and PCR strategy for targeted isolation of homologues in large gene families.ResultsThe method was tested with two different objectives. The first was to analyze the evolution of the CYP98 family of cytochrome P450 genes involved in 3-hydroxylation of phenolic compounds and lignification in a broad range of plant species. The second was to isolate an orthologue of the sorghum glucosyl transferase UGT85B1 and to determine the complexity of the UGT85 family in wheat. P450s of the CYP98 family or closely related sequences were found in all vascular plants. No related sequence was found in moss. Neither extensive duplication of the CYP98 genes nor an orthologue of UGT85B1 were found in wheat. The UGT85A subfamily was however found to be highly variable in wheat.ConclusionsOur data are in agreement with the implication of CYP98s in lignification and the evolution of 3-hydroxylation of lignin precursors with vascular plants. High conservation of the CYP98 family strongly argues in favour of an essential function in plant development. Conversely, high duplication and diversification of the UGT85A gene family in wheat suggests its involvement in adaptative response and provides a valuable pool of genes for biotechnological applications. This work demonstrates the high potential of the CODEHOP strategy for the exploration of large gene families in plants.