Functional expression of Saccharomyces cerevisiae CYP51A1 encoding lanosterol-14-demethylase in tobacco results in bypass of endogenous sterol biosynthetic pathway and resistance to an obtusifoliol-14-demethylase herbicide inhibitor

Genetic variation in plant CYP51s confers resistance against voriconazole, a novel inhibitor of brassinosteroid-dependent sterol biosynthesis

Brassinosteroids (BRs) are plant steroid hormones with structural similarity to mammalian sex steroids and ecdysteroids from insects. The BRs are synthesized from sterols and are essential regulators of cell division, cell elongation and cell differentiation. In this work we show that voriconazole, an antifungal therapeutic drug used in human and veterinary medicine, severely impairs plant growth by inhibiting sterol-14α-demethylation and thereby interfering with BR production. The plant growth regulatory properties of voriconazole and related triazoles were identified in a screen for compounds with the ability to alter BR homeostasis. Voriconazole suppressed growth of the model plant Arabidopsis thaliana and of a wide range of both monocotyledonous and dicotyledonous plants. We uncover that voriconazole toxicity in plants is a result of a deficiency in BRs that stems from an inhibition of the cytochrome P450 CYP51, which catalyzes a step of BR-dependent sterol biosynthesis. Interestingly, we found that the woodland strawberry Fragaria vesca, a member of the Rosaceae, is naturally voriconazole resistant and that this resistance is conferred by the specific CYP51 variant of F. vesca. The potential of voriconazole as a novel tool for plant research is discussed.

Non-target-site herbicide resistance: a family business

We have witnessed a dramatic increase in the frequency and diversity of herbicide-resistant weed biotypes over the past two decades, which poses a threat to the sustainability of agriculture at both local and global levels. In addition, non-target-site mechanisms of herbicide resistance seem to be increasingly implicated. Non-target-site herbicide resistance normally involves the biochemical modification of the herbicide and/or the compartmentation of the herbicide (and its metabolites). In contrast to herbicide target site mutations, fewer non-target mechanisms have been elucidated at the molecular level because of the inherently complicated biochemical processes and the limited genomic information available for weedy species. To further understand the mechanisms of non-target-site resistance, we propose an integrated genomics approach to dissect systematically the functional genomics of four gene families in economically important weed species.

Optimized expression and catalytic properties of a wheat obtusifoliol 14alpha-demethylase (CYP51) expressed in yeast. Complementation of erg11Delta yeast mutants by plant CYP51

CYP51s form the only family of P450 proteins conserved in evolution from prokaryotes to fungi, plants and mammals. In all eukaryotes, CYP51s catalyse 14alpha-demethylation of sterols. We have recently isolated two CYP51 cDNAs from sorghum [Bak, S., Kahn, R.A., Olsen, C. E. & Halkier, B.A. (1997) Plant J. 11, 191-201] and wheat [Cabello-Hurtado, F., Zimmerlin, A., Rahier, A., Taton, M., DeRose, R., Nedelkina, S., Batard, Y., Durst, F., Pallett, K.E. & Werck-Reichhart, D. (1997) Biophys. Biochem. Res. Commun. 230, 381-385]. Wheat and sorghum CYP51 proteins show a high identity (92%) compared with their identity with their fungal and mammalian orthologues (32-39%). Data obtained with plant microsomes have previously suggested that differences in primary sequences reflect differences in sterol pathways and CYP51 substrate specificities between animals, fungi and plants. To investigate more thoroughly the properties of the plant CYP51, the wheat enzyme was expressed in yeast strains overexpressing different P450 reductases as a fusion with either yeast or plant (sorghum) membrane targeting sequences. The endogenous sterol demethylase gene (ERG11) was then disrupted. A sorghum-wheat fusion protein expressed with the Arabidopsis thaliana reductase ATR1 showed the highest level of expression and activity. The expression induced a marked proliferation of microsomal membranes so as to obtain 70 nmol P450.(L culture)-1, with CYP51 representing 1.5% of microsomal protein. Without disruption of the ERG11 gene, the expression level was fivefold reduced. CYP51 from wheat complemented the ERG11 disruption, as the modified yeasts did not need supplementation with exogenous ergosterol and grew normally under aerobic conditions. The fusion plant enzyme catalysed 14alpha-demethylation of obtusifoliol very actively (Km,app = 197 microm, kcat = 1.2 min-1) and with very strict substrate specificity. No metabolism of lanosterol and eburicol, the substrates of the fungal and mammalian CYP51s, nor metabolism of herbicides and fatty acids was detected in the recombinant yeast microsomes. Surprisingly lanosterol (Ks = 2.2 microM) and eburicol (Ks = 2.5 microm) were found to bind the active site of the plant enzyme with affinities higher than that for obtusifoliol (Ks = 289 microM), giving typical type-I spectra. The amplitudes of these spectra, however, suggested that lanosterol and eburicol were less favourably positioned to be metabolized than obtusifoliol. The recombinant enzyme was also used to test the relative binding constants of two azole compounds, LAB170250F and gamma-ketotriazole, which were previously reported to be potent inhibitors of the plant enzyme. The Ks of plant CYP51 for LAB170250F (0.29 microM) and gamma-ketotriazole (0.40 microM) calculated from the type-II sp2 nitrogen-binding spectra were in better agreement with their reported effects as plant CYP51 inhibitors than values previously determined with plant microsomes. This optimized expression system thus provides an excellent tool for detailed enzymological and mechanistic studies, and for improving the selectivity of inhibitory molecules.

MOLECULAR-GENETIC ANALYSIS OF PLANT CYTOCHROME P450-DEPENDENT MONOOXYGENASES

Cytochrome P450-dependent monooxygenases are a large group of heme-containing enzymes, most of which catalyze NADPH- and O2-dependent hydroxylation reactions. The cloning of plant P450s has been hampered because these membrane-localized proteins are typically present in low abundance and are often unstable to purification. Since the cloning of the first plant P450 gene in 1990, there has been an explosion in the rate at which genes encoding plant P450s have been identified. These successes have largely been the result of advances in purification techniques, as well as the application of alternative methods such as mutant- and PCR-based cloning strategies. The availability of these cloned genes has made possible the analysis of P450 gene regulation and may soon reveal aspects of the evolution of P450s in plants. This new knowledge will significantly improve our understanding of many metabolic pathways and may permit their manipulation in the near future.

Cloning and functional expression in yeast of a cDNA coding for an obtusifoliol 14alpha-demethylase (CYP51) in wheat

Screening of a wheat cDNA library with an heterologous CYP81B1 probe from Helianthus tuberosus led to the isolation of a partial cDNA coding a protein with all the characteristics of a typical P450 with high homology (32-39% identity) to the fungal and mammalian CYP51s. Extensive screening of several wheat cDNA libraries isolated a longer cDNA (W516) coding a peptide of 453 amino acids. Alignment of W516 with other P450 sequences revealed that it was missing a segment corresponding to the N-terminal membrane anchor of the protein. The corresponding segment from the yeast lanosterol 14alpha-demethylase was linked to the partial wheat cDNA and the chimera expressed in Saccharomyces cerevisiae. Compared to microsomes from control yeasts, membranes of yeast expressing the chimera catalysed 14alpha-demethylation of obtusifoliol with an increased efficiency relative to lanosterol demethylase activity. W516 is thus a plant member of the most ancient and conserved P450 family, CYP51.

Isolation and reconstitution of the heme-thiolate protein obtusifoliol 14alpha-demethylase from Sorghum bicolor (L.) Moench

The heme-thiolate (cytochrome P450) enzyme which catalyzes the 14alpha-demethylation of obtusifoliol has been isolated from microsomes prepared from etiolated seedlings of Sorghum bicolor (L.) Moench. The obtusifoliol 14alpha-demethylase is a key enzyme in plant sterol biosynthesis and a target for the design of phyla-specific sterol 14alpha-demethylase inhibitors. Microsomal cytochrome P450s were solubilized by using the detergents Renex 690 and reduced Triton X-100, and the obtusifoliol 14alpha-demethylase was isolated by DEAE ion exchange and dye affinity column chromatography. The isolated enzyme has an absorption spectrum characteristic for low spin cytochrome P450s and produces a Type I binding spectrum with obtusifoliol as substrate. Binding spectra were not obtained with lanosterol, campesterol, sitosterol, or stigmasterol. Obtusifoliol 14alpha-demethylase has an apparent molecular mass of 53 kDa and is estimated to constitute approximately 20% of the total cytochrome P450 content of the microsomal membranes and about 0.2% of the total microsomal protein. Gas chromatography-mass spectrometry analysis of reconstitution experiments with dilauroylphosphatidylcholine micelles containing isolated obtusifoliol 14alpha-demethylase and sorghum NADPHcytochrome P450 oxidoreductase demonstrated the conversion of obtusifoliol (4alpha,14alpha-dimethyl-5alpha-ergosta-8, 24(28)-dien-3beta-ol) to 4alpha-methyl-5alpha-ergosta-8,14, 24(28)-trien3beta-ol, the 14alpha-demethylated product of obtusifoliol with a double bond introduced at the Delta14 position. The N-terminal amino acid sequence of the protein is MDLADIPQ/KQQRLMAGXALVV. Five internal sequences were obtained after endoproteinase Lys-C and Glu-C digestion. The fragment AAGAFSYISFGGGRH aligns with the unique heme binding domain of mammalian and yeast sterol 14alpha-demethylases which belong to the CYP51 family. Therefore it is conceivable that the obtusifoliol 14alpha-demethylase from plants also belongs to the CYP51 family, the only P450 family so far known to be conserved across the phyla.