Ocimene, a Key Floral and Foliar Volatile Involved in Multiple Interactions Between Plants and Other Organisms

molecules Review β-Ocimene, a Key Floral and Foliar Volatile Involved in Multiple Interactions between Plants and Other Organisms Gerard Farré-Armengol 1,2,3,* , Iolanda Filella 1,2, Joan Llusià 1,2 and Josep Peñuelas 1,2 1 CSIC, Global Ecology Unit CREAF-CSIC-UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain; [email protected] (I.F.); [email protected] (J.L.); [email protected] (J.P.) 2 CREAF, Cerdanyola del Vallès, 08193 Barcelona, Catalonia, Spain 3 Department of Ecology and Evolution, University of Salzburg, Hellbrunnerstraße 34, 5020 Salzburg, Austria * Correspondence: [email protected] or [email protected]; Tel.: +43-662-8044-5517 Received: 18 April 2017; Accepted: 3 July 2017; Published: 13 July 2017 Abstract: β-Ocimene is a very common plant volatile released in important amounts from the leaves and flowers of many plant species. This acyclic monoterpene can play several biological functions in plants, by potentially affecting floral visitors and also by mediating defensive responses to herbivory. The ubiquity and high relative abundance of β-ocimene in the floral scents of species from most plant families and from different pollination syndromes (ranging from generalism to specialism) strongly suggest that this terpenoid may play an important role in the attraction of pollinators to flowers. We compiled abundant evidence from published studies that supports β-ocimene as a generalist attractant of a wide spectrum of pollinators. We found no studies testing behavioural responses of pollinators to β-ocimene, that could directly demonstrate or deny the function of β-ocimene in pollinator attraction; but several case studies support that the emissions of β-ocimene in flowers of different species follow marked temporal and spatial patterns of emission, which are typical from floral volatile organic compound (VOC) emissions that are involved in pollinator attraction. Furthermore, important β-ocimene emissions are induced from vegetative plant tissues after herbivory in many species, which have relevant functions in the establishment of tritrophic interactions. We thus conclude that β-ocimene is a key plant volatile with multiple relevant functions in plants, depending on the organ and the time of emission. Experimental behavioural studies on pure β-ocimene conducted with pollinating insects will be necessary to prove the assumptions made here. Keywords: trans-β-ocimene; (E)-β-ocimene; floral scent; dominant VOCs 1. Introduction More than 1700 volatile organic compounds (VOCs) have been identified in the floral scents of flowering plants [1]. The richness of VOCs emitted by flowers is certainly very high. Floral VOCs, however, are not equally distributed across the phylogeny of flowering plants, so that the commonness and predominance of these compounds in floral scents varies widely among species. Common floral VOCs have a widespread phylogenetic distribution, which means that they are present in the floral scents of many species from different plant families. Plants generally synthesize and emit species-specific floral VOC mixtures to attract pollinators by mixing several of these common VOCs [2]. These specific floral VOC mixtures attract specialist pollinators that have evolved an innate preference for them [3] but also provide generalist pollinators with a reliable signal to effectively forage on their floral resources [4]. Some less common floral VOCs are only present in plants that are pollinated by specific pollinator groups with specific innate preferences for those VOCs [5–7]. Some of the Molecules 2017, 22, 1148; doi:10.3390/molecules22071148 www.mdpi.com/journal/molecules Molecules 2017, 22, 1148 2 of 9 Molecules 2017, 22, 1148 2 of 9 rarestcommunication floral VOCs channels can constitute between very specialist specific andpollinat reliableors communicationand their host plants, channels even between when specialistthey are pollinatorsemitted within and a their mixture host that plants, includes even more when common they are VOCs emitted [3,8]. within a mixture that includes more commonMost VOCs floral [ 3VOCs,8]. range from being relatively common to rare, but some are very common in the floralMost scents floral VOCsof species range from from all being plant relatively families. common These volatiles, to rare, butwhich some are are essentially very common obligate in thecomponents floral scents of any of speciesfloral scent, from have all plantgreat families.potential for These attracting volatiles, pollinators. which are The essentially commonest obligate floral componentsVOCs, namely of anybenzaldehyde, floral scent, limonene, have great β-ocimene potential forand attracting linalool, are pollinators. not only Thevery commonest common but floral are VOCs,also predominant namely benzaldehyde, components limonene, of the floralβ-ocimene scents of and many linalool, species are [9]. not They only belong very common to the two but most are alsoprominent predominant chemical components groups of plant of the VOCs, floral scentsthe terpenoids of many and species the benzenoids [9]. They belong [10,11]. to the two most prominentHere we chemical review groups the current of plant info VOCs,rmation the terpenoidson the importance and the benzenoidsof β-ocimene [10 ,in11 ].plants, focusing especiallyHere weon reviewits biological the current functions information and its onubiquitous the importance presence of βin-ocimene floral scents. in plants, We compiled focusing especiallyabundant onevidence its biological from functionspublished and studies its ubiquitous supporting presence that inβ-ocimene floral scents. may We play compiled a very abundant relevant evidencefunction as from a general published pollinator studies supportingattractant. We that furtherβ-ocimene emphasize may play that a veryβ-ocimene relevant may function not only as amediate general pollinator pollinator attraction attractant. to We flowers further but emphasize plays several that β biological-ocimene mayfunctions not only in plants, mediate which pollinator vary attractiondepending to on flowers the organ but and plays the several time of biological emission. functions in plants, which vary depending on the organ and the time of emission. 2. Biosynthesis of β-Ocimene 2. Biosynthesis of β-Ocimene β-Ocimene (3,7-dimethyl-1,3,6-octatriene) is a monoterpenoid with the chemical formula β C10H16-Ocimene. It has two (3,7-dimethyl-1,3,6-octatriene) stereoisomers, cis- and trans is a- monoterpenoidβ-ocimene (or with(Z)- and the chemical (E)-β-ocimene, formula respectively), C10H16. It has twowhich stereoisomers, are the cis cisand- and transtrans forms-β-ocimene of the (orcentral (Z)-and double (E)-β bond-ocimene, (Figure respectively), 1). The trans which isomer are the iscis moreand transcommonforms and of the more central abundantly double bond emitted (Figure in1 floral). The scentstrans isomer than the is more cis isomer common [1,9]. and It more is synthesized abundantly emittedfrom the in precursors floral scents isopentenyl than the cis pyrophosphateisomer [1,9]. It (IPP) is synthesized and dimethylallyl from the pyrophosphate precursors isopentenyl (DMAPP) pyrophosphatevia the methyl-erythritol-phosphate (IPP) and dimethylallyl pyrophosphate(MEP) pathway. (DMAPP) Monoterpenes via the methyl-erythritol-phosphate are obtained by the (MEP)transformation pathway. of Monoterpenes DMAPP and areIPP obtainedinto geranyl by the diphosphate transformation (GPP) of inside DMAPP the andchloroplasts, IPP into geranyland the diphosphatelater synthesis (GPP) of terpene inside compounds the chloroplasts, from andGPP the by later enzymes synthesis called of terpene terpene synthases compounds (TPS), from which GPP byare enzymes very diversified called terpene across the synthases phylogeny (TPS), of the which plant are kingdom very diversified [10,12–15]. across β-Ocimene the phylogeny is synthesized of the plantby the kingdom enzyme [10 ,12(E–)-15β].-ocimeneβ-Ocimene synthase is synthesized (with bythe the enzymesystematic (E)- βname-ocimene geranyl-diphosphate synthase (with the systematicdiphosphate-lyase name geranyl-diphosphate ((E)-β-ocimene-forming)), diphosphate-lyase which transforms ((E)-β-ocimene-forming)), GPP into trans which-β-ocimene transforms and GPPdiphosphate. into trans -β(E-ocimene)-β-Ocimene and diphosphate.synthase produces (E)-β-Ocimene mostly synthase trans-β produces-ocimene mostly(94–97%),trans -βbut-ocimene other (94–97%),monoterpenes but other such monoterpenes as cis-β-ocimene such or as myrcenecis-β-ocimene are produced or myrcene in low are proportions produced in (2–4% low proportions and 1–2%, (2–4%respectively) and 1–2%, [16–19]. respectively) Cis-β-ocimene [16–19 is]. thusCis -lessβ-ocimene common is than thus trans less- commonβ-ocimene than in floraltrans -scents;β-ocimene it is inproduced floral scents; and emitted it is produced in small and but emitted detectable in small amounts but detectableonly in species amounts where only trans in species-β-ocimene where is transproduced-β-ocimene in moderately is produced high in amounts. moderately high amounts. FigureFigure 1. Chemical structurestructure ofofthe the two twoβ β-ocimene-ocimene stereoisomers, stereoisomers,cis cis- and- andtrans trans-β-ocimene-β-ocimene (also (also referred referred to asto (asZ)- (Z and)- and (E)- β(E-ocimene,)-β-ocimene, respectively). respectively). 3.3. Functions of β-Ocimene in Flowers OcimeneOcimene isis likely likely to playto play major major ecological ecological roles inroles flowers in flowers due to its due commonness to its commonness

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