A plant’s ultraviolet secret is hidden in broad daylight

You might use number 5 or number 19 to attract a male, but the for the Australian Chiloglottis orchids, the scent of seduction starts with 2,5-dialkylcyclohexane-1,3-diones, helpfully also called chiloglottones. The orchid isn’t trying to attract another orchid, it’s trying to entice the male of the Neozeleboria wasp genus. The aim isn’t to attract the wasp to nectar and pollen. Instead Chiloglottis orchids are sexually deceptive. They act rather like the European orchids in the video below, attempting to pull in a wasp so that can attach pollen that way.

The lures used by the orchids are the chiloglottones, which mimic the scent of a female wasp. It’s a complex organic compound and chiloglottones are remarkably specific in attracting pollinators. A team of scientists based in Canberra and Michigan set out to study them. The scent has to work with the floral display, so they examined what tissues made the chiloglottones, and how they did it. What they also considered is whether or not sunlight was needed to synthesise the chemicals and, if so, what wavelengths the flower was using.

Falara et al. examined two orchids. Chiloglottis trapeziformis attracts the wasp Neozeleboria cryptoides when it flowers in the spring. C. seminuda is pollinated by another wasp of the same genus that hasn’t been fully described yet, [N. sp. (proxima2)]. They both use the chemical chiloglottone 1 to attract their pollinators. Taking the flowers back to the lab, they were able to subject the flowers to a number of light treatments to see how they reacted.

Their first result was that C. trapeziformis produced chiloglottones from both fresh and mature flowers, but not from the whole flower. The scent was specifically isolated in the callus that the orchid used to entice wasps. C. seminuda had a slightly wider distribution of chiloglottones, but the plant’s architecture was different in how the lure was set up in the flower, which may explain that.

As far as the need for light, it looks like it is necessary to produce the scent. Initially this doesn’t look to be true. They found the flowers produced a scent night and day – which seems to suggest light isn’t an issue. However, what they did was deprive them of light. The flowers would stop producing chiloglottone. Then they introduced bursts of light. Chiloglottone began again but, the the light burst was brief, the scent emission would drop off. it was only with longer illumination that the plants continued to emit chiloglottones, but not all light is the same.

What really produced chiloglottones was not any light in the visible spectrum ~390-700 nm, but ultraviolet light, in particular UV-B ~300nm. UV-B is the band of ultraviolet that can give you painful sunburn. A lot of it is blocked by the Earth’s atmosphere, but it can still be present on sunny days. The advantage for flowers is that UV-B photons are high energy. C. seminuda has such an appetite for UV-B that just a couple of hours of UV-B was enough to more than double the amount of chiloglottones that it produces in the wild.

Falara et al. also tested the plants with UV-C light ~254nm. This is not a label you’ll find on many suncreams, because UV-C is usually blocked by the atmosphere at sea-level. It was well worth testing though as the shorter wavelength gives it a higher energy punch. Once again the flowers were happy, producing normal amounts of chiloglottones with a couple of hours exposure.

On the other hand there wasn’t such a great response at UV-A levels ~368nm or under visible violet ~420nm. It was these results that helped show that the plants are definitely using ultraviolet in natural sunlight.

Quite why the plants want ultraviolet is a mystery. The simplest answer would be that there’s a biochemical reaction that directly uses the light, but Falara et al. say that they’re not aware of an enzymatic reaction that uses ultraviolet. This would explain how the plants respond rapidly to ultraviolet. Another possibility they raise is that the ultraviolet light is not directly involved in the reaction, but instead UV receptors signal the production of chemicals. They point to the identification of a UV-B photoreceptor, UVR8. It could be this is used to stimulate the production of the perfume indirectly.

My mentioning of sunburn above highlights that we think of UV-B as bad, but Falara et al. point to new research that shows UV-B light could also be connected with a plant’s secondary metabolism. So far this tends to be seen as a response to produce protection from UV radiation. The reliance of Chiloglottis orchids on UV light for scent leads Falara et al. to prediction that research is going to uncover plenty of other metabolic effects for ultraviolet light, beyond plant stress.

It raises the possibility that a whole series of unknown interactions could have literally been hiding in broad daylight.

Image

Chiloglottis trapeziformis by photobitz/Flickr. [cc]BY-NC-ND[/cc]