Plants use chemical bouquet to fight insects

Subtle levels of light influenced by shaded forest understory can prompt changes in the compounds that make up the defence bouquet

CHILLIWACK, B.C. — To survive, plants have learned to use a variety of chemical compounds to confuse insects and keep them at bay.

But researchers at Michigan State University have found that plants not only use a bouquet of compounds, but also different types of defence bouquets to reduce damage from insects.

Andrea Glassmire, a post-doctoral scholar in the Department of Entomology, said individual plants are made up of several chemicals, which she calls a defence bouquet.

“Many studies, however, examining the effects of plant chemical defence typically focus on a single compound rather than the bouquet. The idea that chemicals interact to produce a combined effect greater than the sum of their separate effects is well known, but applying this concept is extremely hard because there are several chemicals in plants and isolating each of these compounds to test this idea would require tons of plant material and time.”

She said that an alternative way to analyze the bouquet of chemical compounds in a plant is by calculating a diversity index, a measure incorporating the total number of different compounds and the proportion that they are present in the sample.

“Plants having more diverse bouquets have more kinds of chemicals, while plants having less diverse bouquets have one main kind of chemical.”

Glassmire and her colleagues focused the research on Piper kelleyi, a relative of black pepper that grows in Equador and Peru. Glassmire is trained in tropical ecology and Equador was the location of the research.

“The distribution range of Piper kelleyi is at the top of the Andes Mountains and I observed that Piper kelleyi was host to an unusually high diverse community of caterpillars.”

The study focused on answering why there were so many different kinds of caterpillars feeding on this one plant. Glassmire found strong evidence to support that it was due to the type of defence bouquet present in the leaves. One of the surprising results showed how subtle levels of light influenced by the shaded forest understory prompted changes in the defence bouquet. Lower levels of light actually increased the defensive effectiveness of the compounds.

“We found that high levels of light activated a particular chemical pathway that produced one kind of chemical,” she said. “Conversely, low levels of light not only activated the light chemical pathway, but it also allowed for other chemical pathways to produce different kinds of chemicals.”

As a result, insect damage was reduced by up to 37 percent when the plant produced the bouquet blend of compounds in low light. Insects had greater difficulty consuming plants with the bouquet blend than eating plants with a single compound.

“We were so excited about this result,” she said. “Insect damage was dependent on whether the bouquet (comprised) different chemicals or had mostly the same chemical (and) damage was significantly reduced if the chemicals were different. I think it makes it more difficult for insects to digest.”

Plants have chemicals not only in their leaves but ones they emit into the air as aromas. It was the chemicals in the leaves that she focused on given the leaves were what the insects ate.

“Insects have an extremely sensitive sense of smell and taste, allowing them to differentiate between different chemical blends. The bouquet definitely matters, and our results suggest that insects have a harder time when feeding on bouquet blends of different chemicals versus having all the same kind of chemicals.”

How this applies to agriculture may be based on a different approach to crop planting. She said that the study provides strong evidence that planting polycultures of crop varieties as opposed to monocultures will reduce insect damage. Insects have difficulty adjusting to neighbouring plants that are chemically different and a polyculture of crop varieties would provide that chemical barrier.

“One feasible method for increasing chemical differences on commercial and small-scale farms is for growers to use “in-the-bag” seed mixtures of varieties,” she said. “This approach would be economically and agriculturally feasible because varieties can be planted and harvested together. Plant breeders can engineer varieties that are agronomically identical except for having different chemical phenotypes. Subtle differences in chemical defence would reduce insect damage but would not affect the fruits or vegetables that humans consume. Seed mixtures would also be cost-effective because varieties could be marketed as assorted colours and receive higher prices for this variety.”

As an example, she said that a commercial tomato (similar to Roma tomatoes) has been backcrossed multiple times with the wild tomato species endemic to the Andean region. The resulting hybrid varieties are nearly identical to the commercial variety, except varieties have unique chemical traits that come from the wild tomato species.

“This is a powerful system because we know how these varieties differ from each other based on their genetics and chemistry and so we can plant varieties next to each other that have very different chemical profiles.”

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