The way the dioecious plant Actinidia deliciosa attracts bees: critical role of volatile terpenes released from kiwifruit flowers of both genotypes

Abstract

Plants are sessile organisms which have evolved a wide range of strategies to adjust to their environment. As an example, many species of flowering plants need to be pollinated by an insect. To attract pollinators and seed dispersers and thus to ensure their reproductive and evolutionary success, they release diverse blends of volatile compounds from their flowers. The bewildering array of structures identified in floral scents, which may contain up to 100 different molecules, is dominated by terpenoids, with monoterpenes representing the most abundant components, followed by sesquiterpenes (Knudsen and Gershenzon, 2006). During the last decade, substantial progress has been made in plant volatile research as a result of the development of sensitive methods for dynamic headspace sampling and improvements in the analysis of volatiles by gas chromatography-mass spectrometry. In the context of current research dealing with the terpenoid composition of floral scents, including, for instance, Arabidopsis (Chen et al., 2003), Clarkia (Dudareva et al., 1996), Antirrhinum majus (Dudareva et al., 2003) or Vitis vinifera (Lucker et al., 2004), Nieuwenhuizen et al. (2009) report in this issue a comparative analysis of volatile terpenes released from male and female flowers of Actinidia deliciosa, a dioecious plant for which efficient insect pollination is particularly critical. The kiwifruit flowers of both male and female genotypes are shown to display a similar terpene profile in which two sesquiterpenes, (E,E)-a-farnesene and germacrene D, and one monoterpene, (E)-b-ocimene, are the main constituents. Interestingly, the monoterpene has been found only in petals, suggesting that it plays a specific role. The emission of volatile compounds was observed predominantly during the day when pollinators were at work, but the molecular mechanisms responsible for co-ordinated expression of the corresponding genes and the potential effect of light intensity remain to be investigated. In the next step, Nieuwenhuizen et al. (2009) used a functional genomics approach to identify the terpene synthases (TPS), which could account for the production of these volatile terpenes in both male and female flowers. To date, more than 100 TPS that synthesize mono-, sesqui-, and diterpenes have been isolated and characterized from different plant species. They belong to the terpene synthase gene family, which comprises seven subfamilies designated as Tps-a to Tps-g (Bohlmann et al., 1998; Trapp and Croteau, 2001). One of the most outstanding properties of these enzymes is their ability to make multiple products from a single prenyl diphosphate substrate. As an example, only two sesquiterpene synthases were found to be responsible for the synthesis of the 20 sesquiterpenes identified in the Arabidopsis floral scent (Tholl et al., 2005). However, some TPS also produce single products. Two specialized single-product monoterpene synthases are responsible for the biosynthesis of myrcene and (E)-bocimene, the two major terpenoids of snapdragon floral scent (Dudareva et al., 2003). In A. deliciosa, the authors identified two genes, referred as to AdGDS1 and AdAFS1, which encode two distinct sesquiterpene synthases belonging to subfamilies Tps-a and Tps-f, respectively. The substrates of these enzymes are prenyl diphosphates. Heterologous expression of these cloned Tps indicates that AdGDS1 uses only farnesyl diphosphate (FPP) as a substrate to form (+)-germacrene D as the sole compound while AdFS1 is able to accept both FPP and geranyl diphosphate (GPP) and catalyses the synthesis of afarnesene and (E)b-ocimene, respectively. Interestingly, AdAFS1 belongs to the Tps-f subfamiliy, which comprises only monoterpene synthases. When transiently expressed in Nicotiana benthamiana plants, AdGDS1 and AdAFS induced the formation of the same volatile terpenes as those found for the recombinant proteins in vitro. As the synthesis of terpene compounds is dependent on the availability of substrates for the TPS, the authors addressed the question of their cellular site of synthesis. In plants, all terpenoids arise from the five-carbon precursors isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP), which are derived from the two alternative pathways. Sesquiterpene synthesis is expected to occur in the cytosol from FPP synthesized by the mevalonate pathway whereas monoterpene synthesis should take place in the plastids, from GPP arising from the methyl