Researchers tested tobacco plants engineered to emit isoprene; after 10 days, hornworms feeding on those leaves were smaller than those on ordinary plants

In a Michigan State University greenhouse, whiteflies infesting a batch of tobacco plants swarmed every plant except one: a genetically modified variety engineered to emit a chemical called isoprene. That accidental observation became the starting point for a research program spanning roughly 40 years, and the results, published in 2025, finally explain a long-standing puzzle in plant science. Isoprene is a substance that appeared to offer no benefit to the plants that make it, yet researchers at Michigan State University have now identified several. Those benefits, however, come with a trade-off that could matter for anyone breathing city air.

A plant’s chemical defense

Isoprene is the most abundant non-methane hydrocarbon emitted into Earth’s atmosphere. Methane is the only hydrocarbon emitted in greater volume, and unlike isoprene, a large share of methane emissions also come from natural sources such as wetlands, not solely human activity. Oak and poplar trees are among the main sources of isoprene emissions. Isoprene itself has no detectable scent, so its presence in a forest goes unnoticed in a way that aromatic compounds, like the pinenes that give pine forests their characteristic smell, do not.

For years, nobody knew why plants produced this substance. It takes away carbon that could otherwise be used for growth. Researchers led by Tom Sharkey’s lab at Michigan State found the answer: isoprene release triggers a spike in the plant hormone jasmonic acid, which makes proteins in the plant’s leaves harder for insects to digest. The findings were published in Science Advances under the title “Isoprene deters insect herbivory by priming plant hormone responses.”

Insects feeding on isoprene-emitting leaves struggle to digest their food and grow more slowly as a result. In greenhouse trials, hornworms that fed only on isoprene-emitting leaves were visibly smaller after just 10 days than those that fed on ordinary leaves. As Sharkey put it: “The defense was not the isoprene itself, but the consequence of what isoprene did to the plant.”

The soybean plot twist

Scientists assumed that major crops such as soybeans had lost the ability to produce isoprene throughout evolution. That assumption was overturned in 2022, when researchers found two intact isoprene-producing genes in soybean DNA.

According to a separate 2025 follow-up study titled “Cryptic isoprene emission of soybeans,” published in the journal Proceedings of the National Academy of Sciences, damaged soybean leaves release a small amount of isoprene, meaning the plants can apparently switch this defense trait on when needed. That raises the possibility of engineering soybean crops to activate the trait more broadly, making them more resilient to pests.

The atmospheric cost

However, isoprene isn’t just a defence mechanism for plants. It is a key component in the creation of ground-level ozone and smog. When reacting with sunlight and nitrogen oxide emissions from vehicles and power plants, this creates the smoggy air that plagues cities such as Los Angeles and Houston during hot summer afternoons. Having more isoprene in the atmosphere, particularly from crops grown across millions of acres of American farmland, could cause problems.

This is why the researchers are cautious rather than celebratory. Sharkey summarized the dilemma this way: whether to activate isoprene production in crops to help them resist pests, at the cost of more ozone, or to suppress the trait to protect air quality.

Why this matters

This finding is a useful reminder of the trade-offs involved in growing food sustainably while keeping air breathable. There’s no clean answer here: turning on a plant’s natural pest defense could also mean dirtier air in nearby cities. The discovery doesn’t suggest soybeans pose any hidden danger, nor does it mean pesticide-free farming is imminent, but it does show that one of the atmosphere’s most common molecules plays a far larger ecological role than previously understood.