Complex Sexual Deception in an Orchid Is Achieved by Co-opting Two Independent Biosynthetic Pathways for Pollinator Attraction

Abstract

Sexually deceptive orchids lure their specific male pollinators using volatile semiochemicals that mimic female sex pheromones. To date, the semiochemicals known to be involved consist of blends of chemically and biosynthetically related compounds. In contrast, we report that (S)-β-citronellol and 2-hydroxy-6-methylacetophenone, two biosynthetically distinct compounds, are the active semiochemicals in Caladenia plicata, which is pollinated by male Zeleboria sp. thynnine wasps. They are also sex pheromone components of the female Zeleboria. A 1:4 blend elicits a high rate of attempted copulation (∼70%) in bioassays, equivalent to rates observed at orchid flowers. Whereas β-citronellol is well known, 2-hydroxy-6-methylacetophenone appears to be previously unknown as a floral volatile. Production of the two compounds is restricted to glandular sepal tips; thus, differential expression analysis of contrasting floral tissue transcriptomes was employed to illuminate the biosynthesis. As expected, production of (S)-β-citronellol commences with the terpene synthase GES1 catalyzing the irreversible conversion of geranyl diphosphate (GPP) to geraniol. Contrary to prediction, biosynthesis subsequently proceeds in three steps, commencing with the oxidation of geraniol to geranial by alcohol dehydrogenase ADH3, followed by the enantioselective reduction of a double bond in geranial by geranial reductase GER1 to give (S)-β-citronellal. Finally, ADH3-catalyzed reduction of (S)-β-citronellal results in (S)-β-citronellol. In line with previous work on insects showing that 2-hydroxy-6-methylacetophenone is derived from a polyketide pathway, we report a differentially expressed polyketide synthase (PKS) gene candidate. Thus, in this unique example of sexual deception, pollination is achieved by co-opting and regulating two independent biosynthetic pathways of floral volatile compounds. VIDEO ABSTRACT.

Tissue-Specific Floral Transcriptome Analysis of the Sexually Deceptive Orchid Chiloglottis trapeziformis Provides Insights into the Biosynthesis and Regulation of Its Unique UV-B Dependent Floral Volatile, Chiloglottone 1

“…We have uncovered a similar situation in the biosynthesis of floral volatiles in the case of Caladenia plicata . Here, the biosynthesis of one key active semiochemical, the monterpene ( S )-β-citronellol, proceeds via a novel multistep route in which only two of the three genes involved its biosynthesis are differentially expressed between the sepal tips (active) and column (non-active) tissues (Xu et al, 2017). Our findings in this present study, suggest that not all steps involved in chiloglottone 1 biosynthesis are UV-B responsive and that precursor supply (i.e., activated 3-oxohexanoic acid and 2-hexenoic acid) is not limiting in the callus.…”

“…(2) A specific blend of alkyl- and hydroxymethyl-pyrazines in Drakaea glyptodon (Bohman et al, 2014). (3) Unique (methylthio)phenols in Caladenia crebra (Bohman et al, 2017), and (4) A combination of two biosynthetically unrelated compounds (2-hydroxy-6-methylacetophenone and ( S )-β-citronellol) in Caladenia plicata (Xu et al, 2017). …”

Identification of (Z)-8-Heptadecene and n-Pentadecane as Electrophysiologically Active Compounds in Ophrys insectifera and Its Argogorytes Pollinator

“…In addition to comparing observations between various Ophrys systems, it is of further interest to extend this comparison to other sexually deceptive orchids with known semiochemistry. Such cases are predominantly Australian, where the pollinator attractants in hammer orchids and spider orchids, unlike in Ophrys, have been found to be stored in relatively large amounts within the floral tissue [21,23,25].…”

“…In Australian sexually deceptive orchids, 1,3-cyclohexanediones (chiloglottones) have been identified as pollinator attractants in Chiloglottis [18], as have hydroxymethylpyrazines and a β-hydroxylactone (drakolide) in Drakaea [19][20][21][22], (methylthio)phenols, acetophenones and monoterpenes in Caladenia [23][24][25], and tetrahydrofuran acid derivatives in Cryptostylis [26].…”

Volatiles from Merino fleece evoke antennal and behavioural responses in the Australian sheep blow fly Lucilia cuprina

“…The GCMS-EAD set-up comprised a HP 5890 GC connected to a 5972 mass selective detector (MSD) equipped with a HP-5 ms column (30 m × 0.25 mm × 0.25 μm) (Hewlett-Packard, Palo Alto, CA, USA), using helium at 2 mL/min as the carrier. The gas flow was split with a Y-glass between the MSD and the EAD, as described previously (Bohman & Peakall, 2014;Xu et al, 2017). For EAD, the GC effluent was passed through a Syntech conditioner (Syntech, Kirchzarten, Germany) containing a heated (250 ∘ C) transfer line, with the outlet placed in a purified and humidified air stream, where the electrodes holding the antenna were positioned.…”