They may not have the muscles and reflexes of the animal kingdom, but that doesn’t mean plants just passively sit around when something starts chewing them up.
We know damage triggers a signal that spreads through the plant, mounting its defensive response. And we know part of that response is the release of smelly volatile organic compounds.
Now new research has shed new light on what those compounds do. They signal to other plants nearby that a threat is imminent, allowing them to go on the defensive too. In other words, they’re effectively a smelly warning cry.
In fact, this new study on Canada goldenrod (Solidago altissima) even found that the chemical compounds released are more similar in plants with a history of being attacked, whether or not they’re related.
In other words, it seems plants evolve a universal ‘language’ in areas where they’re under pressure of predation, to allow them to better warn others of damage.
“They kind of converge on the same language, or the same warning signs, to share the information freely,” said biologist André Kessler of Cornell University.
“The exchange of information becomes independent of how closely related the plant is to its neighbour.”
The researchers conducted their experiments in the plants’ natural environment, a field, using potted individual plants. In the centre of each group, a single plant was damaged by a herbivorous leaf beetle that eats goldenrods, Trirhabda virgata.
The damaged plant was covered with a fabric sleeve; the pots and the sleeve allowed the researchers to eliminate tactile and root-based communication.
As a control, the same experiment was set up with undamaged plants in the centre.
After several weeks of the plants getting chewed by herbivorous insects, the team collected the damage and control plants’ compound emissions by covering them in a polyethylene sleeve, pulling air over them for six hours, and filtering that air through charcoal traps.
The researchers also looked for compounds in the receiver plants around both the damage and control plants that could signify a defensive reaction.
They found that the plants in the damage group were more protected from herbivores than those in the control group – confirming the emitted compounds resulted in the receiver plants readying their defences.
We’re not yet sure how the receiver plants get the message, but the researchers believe the emitted chemical signals may interact with their cell membranes in some way.
The cool thing is, we already know the effects of some of these defences. For instance, the smell emitted by damaged grass can attract parasitic wasps. If that grass is being munched by insects, those parasitic wasps can help defend the grass by laying their eggs in the insects.
And some plants emit compounds that actively repel predators – like the tobacco plant, which repels female moths, preventing her from laying her eggs (which will then hatch into Very Hungry Caterpillars looking for leaves to munch).
“What we very often see when plants get attacked by pathogens or herbivores is, they change their metabolism,” Kessler said.
“But it’s not a random change – in fact, those chemical and metabolic changes are also helping them cope with those attackers. It’s very much like our immune system: though plants don’t have antibodies like we have, they can fight back with pretty nasty chemistry.”
But, interestingly, that move towards a common “immediate danger” language seems to indicate that plants are only willing to share information broadly when the threat can be mitigated by sharing the burden.
In areas where things are all fine and dandy, the chemical language they use to induce resistance to threats is only understood by their closest relatives.
“We code our language if we want to keep it private, and that’s exactly what happens there, but on a chemical level,” Kessler said. “That analogy is striking and not what we expected.”
But the findings could be useful. Figuring out a particular species’ danger chemicals and what they do could help develop compounds to keep insects at bay, which would be very useful for organic farming and reducing harmful pesticide use.
The research has been published in Current Biology.