Plants can alert neighbours to looming threats

Airborne chemicals known as volatile organic compounds can be used to warn other plants of incoming insect pests

In a new study from Cornell University in New York, researchers have discovered that plants can alert their neighbours when under attack by pests through the release of recognizable smells.

Smells, or airborne chemicals known as volatile organic compounds, are described as a form of language for plants. An incoming insect pest can prompt plants to send out warning signs.

In response to a plant’s chemical alert, neighbouring plants will prime or directly induce their chemical defences.

“It had been known for a while that plants produce volatile organic compounds (VOCs) and that the emissions of those compounds are induced by environmental stresses, specifically herbivory,” said Andre Kessler, professor of ecology and evolutionary biology.

“Initial research on poplar, sagebrush and wild tobacco had also established that these herbivory-induced VOCs can be perceived by a number of other organisms such as herbivores or predators seeking their prey.”

With the alert, surrounding plants could use the VOC-encoded information that a herbivore was approaching and prepare their own defences before it arrived. Kessler said that this was widely viewed by scientists as “eavesdropping” on a neighbour’s signal. The receiving plant, using this chemical information, could gain a competitive advantage over the emitter of the information.

The research team studied a species of goldenrod, Solidago altissima, native to the northeastern United States, and monitored the impact of a particular herbivore, the goldenrod leaf beetle.

“When we started working with goldenrod, we found that these plants can gain a fitness benefit from being able to spread the risk of damage to their neighbours,” said Kessler. “VOC-mediated plant-to-plant communication was one hypothesized mechanism to accelerate a more even distribution of herbivory across a plant population.”

He said that plants are immobile organisms yet, in order to survive and reproduce, they have to be able to respond to constantly changing environmental circumstances. They need to receive and process information. The question arose as to whether the sharing of information is conceptually similar to the sharing that goes on among animals and humans. Is there something like plant behaviour, or an adaptation of their normal characteristics, in response to environmental cues?

“The clear answer is yes, but we need to accept that how information is transmitted between plants may be different than in animals, for example, more chemical, less visual.”

The concept of eavesdropping was hypothesized as a major ecological function. The perception of a neighbour’s wound signal would lead to the evolution of a private channel communication. If a neighbouring plant gains a competitive benefit by using the information another plant has provided, it was assumed that this information would somehow be encoded and private so that a plant could use that information and not share it with others. But what was a surprising result of the research was that goldenrod plants had evolved to spread the risk.

“In goldenrod, plants gain benefit from spreading the risk of damage evenly among members of the population,” said Kessler. “It means that even the emitter of the information gains a benefit of sharing the information about the presence of herbivores as this information sharing is speeding up the spreading of damage.”

It appears the plants use the same warning signs to share information regardless of how closely related the plant is to its neighbour.

Kessler said when plants receive a warning signal, they change their metabolism and emit a set of defensive compounds that can attract predatory insects, or parasitoids, that will kill the herbivore and save the plant.

Kessler said the defence system activation is similar to immune systems in animals except plants rely more on actual defence compounds that slow or kill the attacker rather than the use of immune system cells.

He said they know from their research on goldenrod that, based on the VOC signal coming from a plant, the neighbouring plant could identify the pest, whether beetle or moth caterpillar, and increase its own defences by activating toxic metabolites so that, by the time the pest arrives, the neighbouring plant is as resistant as the plant actually being attacked.

An indirect defence is when a plant’s chemical compounds attract a predatory insect to feed on the plant’s pest.

“In the end, they use a plant wound signal that simply signals the presence of a herbivore,” said Kessler. “What is done with this information by other members of the plant’s community and (by how much) each receiver of that information affects the emitter plant’s fitness in return is largely unknown and likely different in every system. However, knowing about the information flow and the organisms affected by it allows us to manipulate the information flow to the advantage of, let’s say, a crop plant in agriculture.”

He said farmers have considered using plant-to-plant interactions in organic agriculture to protect crops, especially in intercropping systems.

His team is involved in work on a system in Kenya called “push-pull” developed by the International Center for Insect Physiology and Ecology. It is based on manipulating the flow of information to control a pest in corn fields.

“However, as the systems are different from that in Kenya, we probably need to use different species combinations, which requires some investment into this type of research. We have already started some pilot projects in the U.S. and Colombia to adapt local systems to a push-pull pest control technology.”

The study was published in Current Biology.

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