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Plants emit insect-repelling chemical that could secretly be poisoning our air
In the 2008 film 'The Happening', plants turned against humans in an eerie twist of nature, releasing invisible neurotoxins that drove people to their deaths. It was pure sci-fi horror, but the idea that plants can chemically shape the world around them isn't fiction.
In fact, the air we breathe carries traces of a quieter kind of warfare — not against people, but pests. Scientists at Michigan State University have cracked a 40-year mystery around isoprene, a natural chemical that some plants emit to repel hungry insects. But there's a twist: while it defends the plant, it could be polluting the air.
Isoprene is a colorless, volatile hydrocarbon, a simple organic compound made up of five carbon atoms and eight hydrogen atoms (C₅H₈).
It's naturally released by certain plants, especially in hot weather, and is one of the most abundant hydrocarbons emitted into the atmosphere, second only to methane.
Unlike the fragrant terpenes you'd smell in pine forests or poplar groves, isoprene is odorless yet highly reactive. Once released, it interacts with sunlight and nitrogen oxides from vehicle exhaust and industrial emissions, contributing to the formation of ozone, aerosols, and other pollutants that degrade air quality.
'Everyone understands what it smells like when you walk through a pine forest,' Sharkey said. 'In an oak grove, which has more hydrocarbons because it makes so much isoprene, you just don't notice it.'
Until now, scientists weren't entirely sure why some plants go to the trouble of producing isoprene at all, especially since it doesn't seem to help them grow. But the new research suggests the answer lies in defense, not just against heat stress, but against hungry insects.
In controlled greenhouse experiments, MSU researchers grew two types of tobacco plants — one genetically modified to emit isoprene, the other left unchanged. When whiteflies invaded, they swarmed the non-emitting plants while avoiding the isoprene producers altogether.
Further tests using hornworms confirmed the pattern. Worms that fed on isoprene-rich leaves grew smaller and weaker than those that didn't. But it wasn't the isoprene itself that harmed them. Instead, the chemical triggered a spike in jasmonic acid, a defense hormone that disrupts an insect's ability to digest protein.
'The defense was not the isoprene itself, but the consequence of what isoprene did to the plant,' Sharkey said.
Another surprise came from soybeans. Long believed to have lost the ability to make isoprene through evolution, soybeans were found to release it in small bursts when their leaves were damaged. The discovery suggests they still carry the gene to produce isoprene and switch it only under stress.
Researchers say this discovery could change the way we protect crops. But that upside comes with a downside. Isoprene is a hydrocarbon that can worsen air pollution, especially in areas where air quality is already poor. If more crops are genetically modified to emit isoprene, it could further harm the atmosphere. The findings also raise concerns about how soybeans might be contributing to air pollution.
'That's one of the questions that's most important to come out of this research,' Sharkey said. 'Should we add isoprene to crop plants so that they're protected against insects and put up with their effect on the ozone? Or should we genetically engineer plants to turn off the isoprene synthase as much as we can to improve the atmosphere?'
The findings come from two new studies published in Science Advances and the Proceedings of the National Academy of Science.
