For years, we’ve admired plants for their ability to cleanse the air and help fight air pollution. However, our green ally might be contributing to air pollution in an unexpected way. While they may be just trying to fend off pests, this particular self-defense mechanism is also harming the environment.
Researchers have long known that plants secrete a natural chemical called isoprene, but exactly why and how plants did this was sort of an enigma. Though it was hypothesized to be a pest-resisting tactic, there was no empirical evidence proving this. Now, a study nearly four decades in the making has been published in Science Advances, showing how isoprene acts as a natural insecticide to protect plant leaves from insects.
Plants tend to produce more isoprene under extreme environmental conditions like heat stress. For this, plants release up to 2% of their stored carbon to synthesize isoprene. The allocation of substantial resources towards isoprene formation indicates its importance. To demonstrate the pivotal role of this compound, researchers at Michigan State University conducted lab experiments.
In the greenhouse trials, researchers bioengineered tobacco plants to emit isoprene and compared the observations with tobacco plants that didn’t produce isoprene. The team found that hungry tobacco hornworms (Manduca sexta) intensively attacked the non-emitting leaves, leaving the isoprene-emitting leaves alone.
The 10 days of observation showed that plants responded to the pest infestation within two hours. Results revealed that insects that fed on isoprene leaves showed stunted growth, likely stemming from digestive problems triggered by exposure to the chemical, unlike those that munched on the normal leaves.
But the isoprene doesn’t work the way you might think. The isoprene itself is not toxic to insects. Instead, it triggers a reaction inside plants that elevates the levels of Jasmonic acid. This makes the plant’s proteins tough for insects to digest.
“The defense was not the isoprene itself, but the consequence of what isoprene did to the plant,” says the lead author, Tom Sharkey.
However, this natural pest control mechanism comes with an environmental cost. Isoprene is a volatile organic compound (VOC) produced by plants like oak and poplar trees. It’s the second-highest emitted hydrocarbon on Earth, following methane emissions from human activities. Once in the environment, it interacts with sunlight and nitrogen oxides to form ground-level ozone, a harmful pollutant linked to respiratory problems and environmental concerns.
Now, the discovery of this isoprene mechanism has set researchers up with a tough choice: should we breed more isoprene-emitting plants to curb pest attacks and improve food security, or constrain commercial crops' ability to produce this chemical in the hopes of protecting the air we breathe?
“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?” Sharkey asked, summing up the dilemma future bioengineers will face.
The study has been published in Science Advances
Source: Michigan State University
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