Double-Edged Trees

Plants are often praised for their ability to clean the air and fight pollution. 

However, they may be polluting the environment while trying to protect themselves from pests. 

Scientists have long known that plants secrete isoprene, a natural chemical that they suspected was their way to resist pests, even as researchers weren’t sure how or why they produced this substance. 

Now, a new study, 40 years in the making, demonstrates how isoprene repels insects and how plants that don’t usually produce this chemical can secrete it in times of stress.  

Under extreme conditions, like very high temperatures, plants use up to 2 percent of their stored carbon to produce more isoprene. Spending so much valuable carbon to produce the chemical, diverting it away from growth and storage, indicates that isoprene must play a crucial role in their survival, according to New Atlas. 

To prove this, the team carried out lab experiments. 

They genetically engineered tobacco plants to emit isoprene and compared the outcomes with tobacco plants that didn’t produce the chemical. They found that tobacco hornworms (Manduca sexta) intensively fed on the non-emitting leaves, mostly avoiding the isoprene-emitting ones. 

Over a 10-day observation period, researchers monitored the plants’ reaction to the pests and found that they began responding to the pest infestation by producing isoprene within two hours of being attacked. 

When the chemical is activated, indigestible proteins kick in, giving insects that snack on the leaves a stomachache. Those proteins also prevent the worms from growing properly. 

However, the isoprene itself is not what is toxic to the insects. Instead, it causes a reaction within the plants that raises the levels of Jasmonic acid, making plants’ proteins tough for insects to digest. 

“The defense was not the isoprene itself, but the consequence of what isoprene did to the plant,” lead study author Tom Sharkey said in a statement. 

However, isoprene adds to air pollution and ranks among the most widely released hydrocarbons on the planet. When released into the atmosphere, this organic compound reacts with sunlight and nitrogen oxides – commonly emitted by vehicles and coal-burning power plants. These reactions form harmful substances like ground-level ozone, aerosols, and other harmful byproducts. 

Researchers now face a tough question: “Should we add isoprene to crop plants so that they’re protected against insects and put up with their effect on the ozone?” wondered Sharkey. “Or should we genetically engineer plants to turn off the isoprene synthase as much as we can to improve the atmosphere?”