Sexually deceptive orchids

Plants use a diverse range of visual and olfactory cues to attract animal pollinators, with floral volatiles often particularly critical for securing insect pollination. More than 1700 different compounds have been documented in plant floral odours, however relatively few studies have determined the precise role of these compounds in pollinator attraction. There are even fewer cases where we have a comprehensive understanding of the physiology, biochemistry and biosynthesis of the floral volatile compounds involved in pollination.

The sexually deceptive orchids are a group of plants for which specific semiochemicals play a key role in pollination. These orchids lure specific male insect pollinators to their flower by emitting volatile semiochemicals that mimic the female sex pheromone. Remarkably, this pollination strategy has evolved independently in orchids on at least four continents (Africa, Australia, Europe and South America), and new discoveries continue to be made within this family.

A recent paper in Annals of Botany studies the sexually deceptive Chiloglottis trapeziformis orchid, which attracts its specific male wasp pollinators by means of chiloglottones, a newly discovered class of volatiles with unique structures. The authors find that while initiation of chiloglottone biosynthesis requires only UV-B light, sustained production requires both UV-B and de novo protein synthesis. The amount of chiloglottone in flowers reflects the interplay between developmental stage, time and intensity of UV-B exposure, de novo protein synthesis, and feedback loops that cap the internal amount.

Amarasinghe, R., Poldy, J., Matsuba, Y., Barrow, R. A., Hemmi, J. M., Pichersky, E., & Peakall, R. (2015) UV-B light contributes directly to the synthesis of chiloglottone floral volatiles. Annals of Botany, 115(4): 693-703
Australian sexually deceptive Chiloglottis orchids attract their specific male wasp pollinators by means of 2,5-dialkylcyclohexane-1,3-diones or ‘chiloglottones’, representing a newly discovered class of volatiles with unique structures. This study investigated the hypothesis that UV-B light at low intensities is directly required for chiloglottone biosynthesis in Chiloglottis trapeziformis. Chiloglottone production occurs only in specific tissue (the callus) of the labellum. Cut buds and flowers, and whole plants with buds and flowers, sourced from the field, were kept in a growth chamber and interactions between growth stage of the flowers and duration and intensity of UV-B exposure on chiloglottone production were studied. The effects of the protein synthesis inhibitor cycloheximide were also examined. Chiloglottone was not present in buds, but was detected in buds that were manually opened and then exposed to sunlight, or artificial UV-B light for ≥5 min. Spectrophotometry revealed that the sepals and petals blocked UV-B light from reaching the labellum inside the bud. Rates of chiloglottone production increased with developmental stage, increasing exposure time and increasing UV-B irradiance intensity. Cycloheximide did not inhibit the initial production of chiloglottone within 5 min of UV-B exposure. However, inhibition of chiloglottone production by cycloheximide occurred over 2 h of UV-B exposure, indicating a requirement for de novo protein synthesis to sustain chiloglottone production under UV-B. The sepals and petals of Chiloglottis orchids strongly block UV-B wavelengths of light, preventing chiloglottone production inside the bud. While initiation of chiloglottone biosynthesis requires only UV-B light, sustained chiloglottone biosynthesis requires both UV-B and de novo protein biosynthesis. The internal amounts of chiloglottone in a flower reflect the interplay between developmental stage, duration and intensity of UV-B exposure, de novo protein synthesis, and feedback loops linked to the starting amount of chiloglottone. It is concluded that UV-B light contributes directly to chiloglottone biosynthesis. These findings suggest an entirely new and unexpected biochemical reaction that might also occur in taxa other than these orchids.