Floral Volatiles: from Biosynthesis to Function
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bs_bs_banner Plant, Cell and Environment (2014) 37, 1936–1949 doi: 10.1111/pce.12314 Original Article Floral volatiles: from biosynthesis to function Joëlle K. Muhlemann, Antje Klempien & Natalia Dudareva Department of Biochemistry, Purdue University, West Lafayette, IN 47907, USA ABSTRACT both attraction of pollinators and defence against florivores and pathogens. Based on their biosynthetic origin, floral Floral volatiles have attracted humans’ attention since antiq- VOCs can be divided into three major classes: terpenoids, uity and have since then permeated many aspects of our lives. phenylpropanoids/benzenoids, and fatty acid derivatives Indeed, they are heavily used in perfumes, cosmetics, flavour- (Fig. 1). In addition, sulphur- and nitrogen-containing com- ings and medicinal applications. However, their primary func- pounds contribute to the attraction of pollinators to flowers tion is to mediate ecological interactions between flowers and by mimicking food or brood sources such as carrion or dung a diverse array of visitors, including pollinators, florivores (Wiens 1978; Faegri & van der Pijl 1979; Jürgens et al. 2006). and pathogens. As such, they ultimately ensure the plants’ However, to date, little is known about the biosynthetic path- reproductive and evolutionary success. To date, over 1700 ways leading to the formation of these compounds. floral volatile organic compounds (VOCs) have been identi- fied. Interestingly, they are derived from only a few biochemi- BIOSYNTHETIC PATHWAYS AND GENES cal networks, which include the terpenoid, phenylpropanoid/ INVOLVED IN THE FORMATION OF benzenoid and fatty acid biosynthetic pathways. These FLORAL VOLATILES pathways are intricately regulated by endogenous and exter- nal factors to enable spatially and temporally controlled Biosynthesis of terpenoid compounds emission of floral volatiles, thereby fine-tuning the ecological Terpenoids are the largest class of floral volatiles and interactions facilitated by floral volatiles. In this review, we encompass 556 scent compounds, which are derived from will focus on describing the biosynthetic pathways leading to two common interconvertible five-carbon (C5) precursors: floral VOCs, the regulation of floral volatile emission, as well isopentenyl diphosphate (IPP) and its allylic isomer as biological functions of emitted volatiles. dimethylallyl diphosphate (DMAPP) (McGarvey & Croteau 1995). In plants, these C5 precursors are synthesized from two Key-words: benzenoids; floral scent; florivory; phenylpro- independent and compartmentally separated pathways, the panoids; pollination; regulation; terpenoids; volatile organic mevalonic acid (MVA) and the methylerythritol phosphate compounds. (MEP) pathways, which contribute to terpenoid biosynthesis in a species- and/or organ-specific manner (Vranova et al. INTRODUCTION 2013). The MEP pathway operates in plastids (Hsieh et al. Plants are sessile organisms that need to constantly adapt to 2008) and is mainly responsible for the formation of volatile ∼ ∼ changing environments for their survival and reproduction. mono- (C10) and diterpenes (C20)( 53 and 1% of total floral For this environmental adaptation, plants have evolved a terpenoids, respectively) (Knudsen & Gershenzon 2006), wide array of specialized metabolites, also called plant sec- whereas the MVA pathway is distributed among the cytosol, ondary metabolites or plant natural products. To date, over endoplasmic reticulum and peroxisomes (Simkin et al. 2011; 200 000 specialized metabolites have been described (Dixon Pulido et al. 2012), and gives rise to precursors for volatile ∼ & Strack 2003), out of which approximately 1% corresponds sesquiterpenes (C15)(28% of total floral terpenoids). to floral volatile organic compounds (VOCs) identified in 90 While being compartmentally separated, these isoprenoid different angio- and gymnosperm families (Knudsen et al. biosynthetic pathways are connected via a metabolic ‘cross- 2006). VOCs are lipophilic liquids with low molecular weight talk’ mediated by yet unidentified transporter(s) (Bick & and high vapour pressure at ambient temperatures. Physical Lange 2003; Flügge & Gao 2005). Such connectivity of the properties of these compounds allow them to freely cross pathways allows the MEP pathway, often with a higher cellular membranes and be released into the surrounding carbon flux than the MVA route, to support biosynthesis of environment (Pichersky et al. 2006). Biosynthesis of VOCs cytosolically formed terpenoids as was demonstrated in veg- occurs in all plant organs: roots, stems, leaves, fruits, seeds, as etative tissue (Laule et al. 2003; Ward et al. 2011), fruits well as flowers, which were found to release the highest (Gutensohn et al. 2013) and flowers (Laule et al. 2003; amounts and diversity of VOCs. In contrast to VOCs Dudareva et al. 2005; Ward et al. 2011). Indeed, the MEP released from other plant organs, which are exclusively pathway alone supports sesquiterpene biosynthesis in snap- involved in plant defense, floral VOCs assume functions in dragon flowers (Dudareva et al. 2005). Terpenoid research in flowers has predominantly focused Correspondence: N. Dudareva. E-mail: [email protected] on the isolation and characterization of terpene synthase 1936 © 2014 John Wiley & Sons Ltd Floral volatiles 1937 2000; Rohdich et al. 2003; Guirimand et al. 2012) with several excellent reviews devoted to this subject (McGarvey & Croteau 1995; Chappell 2002; Vranova et al. 2013). In brief, the MVA pathway starts from a stepwise condensation of three molecules of acetyl-CoA and consists of six enzymatic reactions while the MEP pathway begins with the condensa- tion of D-glyceraldehyde 3-phosphate and pyruvate and involves seven enzymatic reactions. Volatile terpenoids are synthesized from prenyl diphosphate precursors, which are produced from condensa- tion of IPP and DMAPP by prenyltransferases. Sequential head-to-tail condensation of two IPP and one DMAPP mol- ecules by farnesyl diphosphate (FPP) synthase in the cytosol leads to the formation of FPP,the precursor for sesquiterpenes (Fig. 2). Head-to-tail condensation of one DMAPP with one IPP molecule in plastids results in geranyl pyrophosphate (GPP) formation, the precursor of monoterpenes, and is cata- Figure 1. Major volatile classes emitted by flowers. Based on their biosynthetic origin, volatiles emitted by flowers can be lysed by the GPP synthase (GPPS) (Fig. 2). This enzyme was grouped into one of the three major volatile classes: terpenoids, found to be heterodimeric in Antirrhinum majus (snap- phenylpropanoids/benzenoid, and fatty acid derivatives. Each dragon) and Clarkia breweri, both of which have a floral scent volatile class is represented by a few typical floral scent compounds. bouquet rich in monoterpene compounds (Tholl et al. 2004). Analyses of tissue-specific, developmental and rhythmic (TPS) genes responsible for the final steps in terpenoid expression of the GPPS small subunit showed positive corre- biosynthesis, while genes and cognate enzymes of the MVA lation between expression and monoterpene emission in and MEP pathways were mainly characterized from vegeta- snapdragon flowers (Tholl et al. 2004), whereas no such corre- tive tissues (Cane 1999; Wise & Croteau 1999; Lange et al. lation was found for the large subunit, suggesting that the Figure 2. Schematic representation of terpenoid VOC biosynthesis. Synthesis of terpenoid VOCs occurs via the cytosolic mevalonic acid (MVA) and the plastidial methylerythritol phosphate (MEP) pathways, the former giving rise to sesquiterpenes and the latter to monoterpenes, diterpenes and volatile carotenoid derivatives. Crosstalk between both pathways is facilitated by the export of IPP from the plastid to the cytosol. Stacked arrows represent multiple biosynthetic steps. Volatile compounds are highlighted with a yellow background. DMAPP, dimethylallyl pyrophosphate; FPP, farnesyl pyrophosphate; FPPS, FPP synthase; G3P, glyceraldehyde-3-phosphate; GGPP, geranylgeranyl pyrophosphate; GGPPS, GGPP synthase; GPP, geranyl pyrophosphate; GPPS, GPP synthase; IPP, isopentenyl pyrophosphate; TPS, terpene synthase. © 2014 John Wiley & Sons Ltd, Plant, Cell and Environment, 37, 1936–1949 1938 J. K. Muhlemann et al. Table 1. List of biosynthetic genes involved in final steps of floral volatile formation Volatile Gene Species Reference Monoterpenoids 1,8-Cineole CitMTSL1 Citrus unshiu Shimada et al. 2005 NsCIN Nicotiana suaveolens Roeder et al. 2007 Linalool CbLIS Clarkia breweri Dudareva et al. 1996a AmNES/LIS-1 Antirrhinum majus Nagegowda et al. 2008 TPS10 Arabidopsis thaliana Ginglinger et al. 2013 TPS14 A. thaliana Ginglinger et al. 2013 Myrcene Am1e20 A. majus Dudareva et al. 2003 AmOc15 A. majus Dudareva et al. 2003 AlstroTPS Alstroemeria peruviana Aros et al. 2012 E-(β)-Ocimene Am0e23 A. majus Dudareva et al. 2003 CitMTSL4 C. unshiu Shimada et al. 2005 Sesquiterpenoids α-Farnesene AdAFS1 Actinidia deliciosa Nieuwenhuizen et al. 2009 Germacrene D AdGDS1 A. deliciosa Nieuwenhuizen et al. 2009 VvGerD Vitis vinifera Lucker et al. 2004 FC0592 Rosa hybrida Guterman et al. 2002 Nerolidol AmNES/LIS-2 A. majus Nagegowda et al. 2008 AcNES1 Actinidia chinensis Green et al. 2012 Valencene VvVal V. vinifera Lucker et al. 2004 Benzenoids/phenylpropanoids Benzaldehyde AmBALDH A. majus Long et al. 2009 Benzylacetate CbBEAT C. breweri Dudareva et al. 1998 Benzylbenzoate PhBPBT P. hybrida Boatright et al. 2004 Eugenol PhEGS P. hybrida Koeduka et al. 2006 Isoeugenol PhIGS P. hybrida Koeduka et al. 2006 Isomethyleugenol CbIEMT