The Nonmevalonate Pathway Supports Both Monoterpene and Sesquiterpene Formation in Snapdragon Flowers
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The nonmevalonate pathway supports both monoterpene and sesquiterpene formation in snapdragon flowers Natalia Dudareva*†, Susanna Andersson‡, Irina Orlova*, Nathalie Gatto‡, Michael Reichelt‡, David Rhodes*, Wilhelm Boland‡, and Jonathan Gershenzon‡ *Department of Horticulture and Landscape Architecture, Purdue University, West Lafayette, IN 47907; and ‡Max Planck Institute for Chemical Ecology, Beutenberg Campus, Hans-Knoell-Strasse 8, D-07745 Jena, Germany Edited by Rodney B. Croteau, Washington State University, Pullman, WA, and approved December 1, 2004 (received for review October 4, 2004) Terpenoids, the largest class of plant secondary metabolites, play derived from farnesyl diphosphate. Emission of these com- essential roles in both plant and human life. In higher plants, the pounds follows diurnal rhythms and is controlled by a circadian five-carbon building blocks of all terpenoids, isopentenyl diphos- clock (15). The presence of monoterpenes and a sesquiterpene phate (IPP) and dimethylallyl diphosphate, are derived from two in a single mixture allowed us to examine the possible interac- independent pathways localized in different cellular compart- tions between the two IPP biosynthetic pathways. By using stable ments. The methylerythritol phosphate (MEP or nonmevalonate) isotope-labeled, pathway-specific precursors supplied to cut pathway, localized in the plastids, is thought to provide IPP and snapdragon flowers, we show that only one of the two pathways, dimethylallyl diphosphate for hemiterpene, monoterpene, and the plastid-localized MEP pathway, provides IPP precursors for diterpene biosynthesis, whereas the cytosol-localized mevalonate both plastidial monoterpene and cytosolic sesquiterpene biosyn- pathway provides C5 units for sesquiterpene biosynthesis. Stable thesis. The MEP pathway operates in a rhythmic manner con- 2 isotope-labeled, pathway-specific precursors (1-deoxy-[5,5- H2]-D- trolled by the circadian clock and determines the rhythmicity of 2 xylulose and [2,2- H2]-mevalolactone) were supplied to cut snap- terpenoid emission. The trafficking of IPP occurs unidirection- dragon flowers, which emit both monoterpenes and the sesqui- ally from the plastids to cytosol. terpene, nerolidol. We show that only one of the two pathways, the plastid-localized MEP pathway, is active in the formation of Materials and Methods volatile terpenes. The MEP pathway provides IPP precursors for Plant Material, Chemicals, and Stable Isotope-Labeled Compounds. both plastidial monoterpene and cytosolic sesquiterpene biosyn- Snapdragon (A. majus) Maryland True Pink cv. (Ball Seed, West thesis in the epidermis of snapdragon petals. The trafficking of IPP Chicago, IL) was grown under standard greenhouse conditions occurs unidirectionally from the plastids to cytosol. The MEP as described in ref. 16. The pathway-specific inhibitors fosmido- pathway operates in a rhythmic manner controlled by the circadian mycin and mevinolin were purchased from Molecular Probes clock, which determines the rhythmicity of terpenoid emission. and Sigma, respectively. Before use, the lactone of mevinolin was converted to the open acid form as described in ref. 17. Pathway- 2 2 floral scent ͉ terpenoids ͉ volatiles ͉ cross talk ͉ mevalonic acid pathway specific precursor 1-deoxy-[5,5- H2]-D-xylulose ([ H2]-DOX) was synthesized from commercially available 2,3-O-isopropyli- dene-D-tartrate as described in refs. 18 and 19. Racemic [2,2- soprenoids, the largest family of natural products, play numer- 2 2 H2]-mevalolactone ([ H2]-MVL) was prepared from commer- Ious vital roles in basic plant processes, including respiration, cially available unlabeled rac-mevaloactone by proton exchange. photosynthesis, growth, development, reproduction, defense, Commercial mevalolactone (0.13 g, 1 mM) was dissolved in 2 and adaptation to environmental conditions (1, 2). Plant- deuterated methanol (CH3O H, 5 ml) and treated with 1,5- produced terpenoids also are essential nutrients in human diets diazabicyclo[3.4.0]non-5-ene (10 l, 0.08 mM) to catalyze the and are used as chemotherapeutic agents with antitumor activ- isotope exchange. After stirring for 24 h at room temperature, ities (3). Although isoprenoids are extraordinarily diverse, all the solvent was removed in vacuum and the product was purified originate through the condensation of the universal five-carbon by chromatography on silica gel by using ether:methanol (20:1, precursors, isopentenyl diphosphate (IPP) and dimethylallyl vol͞vol) for elution. According to 1H NMR and MS, the product 2 2 diphosphate (DMAPP). In higher plants, two independent consisted of [2,2- H2]-MVL (Ͼ98% H) and Ϸ30% of the acyclic 2 2 pathways located in separate intracellular compartments are methyl ester of [2,2- H2]-mevalonic acid (Ͼ98% H). Because involved in the biosynthesis of IPP and DMAPP. In the cytosol, mevalolactone easily passes membranes and is readily opened in 2 IPP is derived from the classic mevalonic acid (MVA) pathway the plant cell to give mevalonic acid, the lactone [ H2]-MVL was that starts from the condensation of acetyl-CoA (4, 5), whereas used for feeding experiments. in plastids, IPP is formed from pyruvate and glyceraldehyde 3-phosphate via the methylerythritol phosphate (MEP or non- Labeling Experiments and Volatile Analysis. All labeling experi- mevalonate) pathway (6–8). Initial research indicated that the ments were performed in a growth chamber under conditions of cytosolic pool of IPP serves as a precursor of farnesyl diphos- phate (FPP, C15) and, ultimately, sesquiterpenes and triterpenes, whereas the plastidial pool of IPP provides the precursors of This paper was submitted directly (Track II) to the PNAS office. geranyl diphosphate (GPP, C10) and geranylgeranyl diphosphate Abbreviations: IPP, isopentenyl diphosphate; DMAPP, dimethylallyl diphosphate; MVA, 2 mevalonic acid; MVL, racemic [2,2- H2]-mevalolactone; MEP, methylerythritol phosphate; (GGPP, C20) and, ultimately, monoterpenes, diterpenes, and GPP, geranyl diphosphate; DOX, 1-deoxy-D-xylulose; DXP, 1-deoxy-D-xylulose-5-phosphate; tetraterpenes. However, cross talk between these two different DXPS, DXP synthase; DXR, 1-deoxy-D-xylulose-5-phosphate reductoisomerase; HMGR, 3-hy- IPP biosynthetic pathways has been documented (9–14), and the droxy-3-methylglutaryl-CoA reductase; amu, atomic mass units. relative contributions of each pathway to the biosynthesis of the Data deposition: The sequences reported in this paper have been deposited in the GenBank various classes of terpenes remain uncertain. database [accession nos. AY770406 (for snapdragon DXR gene) and AY770407 (for snap- Terpenoids emitted from snapdragon flowers (Antirrhinum dragon DXPS gene)]. † majus) include three monoterpenes, myrcene, (E)--ocimene, To whom correspondence should be addressed. E-mail: [email protected]. PLANT BIOLOGY and linalool, derived from GPP, and a sesquiterpene, nerolidol, © 2005 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0407360102 PNAS ͉ January 18, 2005 ͉ vol. 102 ͉ no. 3 ͉ 933–938 Downloaded by guest on September 23, 2021 21°C, 50% relative humidity, 150 mol⅐mϪ2⅐sϪ1 light intensity, and a 12-h photoperiod. Although emission of floral volatiles from snapdragon flowers is developmentally regulated, 5- to 8-day-old flowers emit very similar amounts of volatiles (15, 16). Thus, to minimize the effect of developmental regulation, 5-day-old flowers were cut from the plants and transferred to small glass beakers (three flowers per beaker) filled with 10 ml of 5% (wt͞vol) sucrose in tap water (control). For feeding 2 2 experiments, [ H2]-DOX and [ H2]-MVL were supplied to 5% (wt͞vol) sucrose at concentrations of 2 mg͞ml and 3 mg͞ml, respectively. For inhibition experiments, either fosmidomycin or mevinolin was added to 5% (wt͞vol) sucrose to a final concen- tration of 100 M. The chosen concentrations of inhibitors had no effect on flower appearance for the duration of the experi- ments. Emitted volatiles were collected by a closed-loop strip- ping method (20) every 3 h for 60 h under normal light͞dark conditions (12-h photoperiod) and for 81 h under constant dark conditions. Volatiles were eluted from Porapak Q traps (80͞100 mesh size; Alltech Associates) with 200 lofCH2Cl2, and 2 g of nonyl acetate was added as an internal standard. The amount and isotope abundance of emitted terpenoids were analyzed by GC-MS on a Hewlett–Packard 6890 gas chromatograph (injector temperature 220°C and splitless injection with volume of 1 l) coupled to a Hewlett–Packard 5973 quadrupole mass selective detector. Separation was performed on a DB-5MS column (30 m ϫ 0.25 mm ϫ 0.25 m film; Agilent Technologies, Bo¨blingen, Germany) with helium as the carrier gas. The temperature program was as follows: initial oven temperature of 40°C for 3 min, increased at 5°C͞min to 120°C, followed by an increase of 10°C͞min to 180°C and final heating of the column at 300°C for 3 min. GC-MS was performed with a transfer-line temperature of 230°C, a source temperature of 230°C, a quadrupole temper- ature of 150°C, an ionization potential of 70 electron volts, and a scan range of m͞z from 50 to 300. For quantification, repre- sentative selected ion peaks of each compound were integrated and the amounts were calculated in relation to the response of the internal standard at a m͞z of 69. Response curves for the quantified compounds relative to the internal standard were generated by injecting a mixture of equal amounts of commercial standards (all from