First responders to hungry insects
Wounded by insects, plants release two rounds of volatile molecules. The first one serves to seal the wounds and alarm the more distant part of the plant as well as any neighbouring plants who pick up the signal. The second round is to attract the natural enemies of the damage causing insect. Now a new study by Swiss scientists shows that surprisingly those bystander plants release those natural enemies attracting scents earlier than the wounded ones.
The first scents that are released, within seconds, are the so-called green leaf volatiles. These are pre-prepared molecules that escape the nibbled-on leaf when cells get damaged. Only hours till days later, the wounded plant releases a second folly of scents. This time containing indoles and terpenes. The job of those volatile molecules is to function as a call sign for predatory insects, especially those that are the natural enemies of the insects that are eating up the plant. The plant is only making and releasing this last group of volatile molecules on demand, as such it takes longer for them to be released.
Now, neighbouring plants are also responding by releasing volatile molecules that attract the natural enemies of the herbivory insects that caused the raising of the alarm. The important question is, however, who does what and in what order? Knowing this helps with understanding not only how plants protect themselves, but also how this plays out in a population level, like in a crop field.
Unexpected findings
Before investigating the timing of scent release, the researchers expected the neighbouring plants to respond slower. At best they expected the bystanders would release their scents about a minute after the wounded plant. That was however not what they found.
They started with measuring the volatiles wounded and bystander maize plants release after one was wounded. They noticed that only the wounded plant released green leaf volatiles. They did this within minutes. But the second round of volatile release, those containing indoles and terpenes, the bystander plants released about 15 to 80 minutes earlier than the wounded plants.
Subsequently the researchers wanted to know why those bystander plants where quicker to act. As the production of the volatiles is required for the release of that second round of volatiles, the researchers tested to see if this was the cause of the difference in release time. But to their surprise they noticed that the activation of the required genes happened just as fast in the wounded as in the bystander plants. So, no difference here.
Not getting out
Next the researchers suspected that a difference in the opening and closing of stomate, those pores in the leaf, might be responsible. The researchers noticed that wounded leaves closed their stomata surrounding the wound. The suggestion is that plants do this to prevent water loss. The opening of the stomata in bystander plants did not change after perceiving the green leaf volatiles of wounded plants.
This makes stomata a good candidate for being the culprit for the slower release of the second round of volatiles by wounded plants. The problem however, that both forcing the stomata open in the wounded plants as well as forcing the stomata close in the bystander plants come with loads of side effects that obscure their effects on volatile release. This prevented the researchers from confirming that this was indeed the case.
The last thing the researchers did was confirming that the green leaf volatiles are indeed responsible for inducing and releasing the volatiles of the bystander plants. The researchers created three types of plants that either did not produce any indoles, terpenes, or green leaf volatiles. Only when the wounded plant could not make any green leaf volatiles did the bystander plants not release their natural enemies attracting volatiles.
So, green leaf volatiles that are released by the wounded plant are picked up by neighbouring bystander plants, which used the signal to put out a call sign for predatory insects. This ability of neighbouring plants to pickup those green leaf volatiles is likely a side effect of the ability to pickup the green leaf volatiles released by their own more distant laying tissues. At the same time, on a population level this allows for both a quicker and stronger predatory insect attraction than if the wounded plant is the only one that put out a request for help. So those eavesdropping neighbours might be a benefit.
Literature
T.M. Cofer, L. Hu, G. Glauser, & M. Erb, (2026) Rapid green leaf volatile sensing allows intact plants to release induced volatiles earlier than wounded emitters, Proc. Natl. Acad. Sci. U.S.A. 123 (44) e2604047123, https://doi.org/10.1073/pnas.2604047123
