The aroma of freshly cut grass is one of nature’s most recognizable and enjoyable scents, so you might be surprised to learn that it's actually the smell of chemical warfare that's been taking place right under our noses for longer than humans have walked the Earth.
In a fascinating study led by researchers at National Taiwan University (NTU), scientists have uncovered intricate details about the sophisticated biochemical alarm systems that plants set off when wounded – and how insects found ways to manipulate those signals in an evolutionary arms race that dates back an estimated 100 million years.
The smell of freshly cut grass is a nostalgic scent for me – it reminds me of summers spent with my family in rural New Zealand – but our enjoyment of it is merely a byproduct of one of nature's fights for survival.
When plants are damaged – by weather, lawn mowers or hungry herbivores – they release compounds called green leaf volatiles (GLVs) into the air as a distress signal to alert nearby growth. And it's these GLVs, which are six-carbon molecules, that combine to produce that cut-grass scent. One chemical, Z-3-hexenal, gives off a fresh leafy aroma, while E-2-hexenal adds a sharper-smelling scent.
These also help repel herbivores, suppress pathogens, and even attract predators that attack the insects feeding on the plant. This sort of chemical power is critical for a sessile organism that can't just pull up its roots and relocate away from a threat.
In this new study, scientists homed in on the enzyme hexenal isomerase (Hi), which controls the balance of the GLVs released into the atmosphere. It can even manipulate these compounds, converting Z-3-hexenal into E-2-hexenal. Essentially, the enzyme can make small but powerful tweaks to the chemical recipe with the "ingredients" the plant produces, boosting the effectiveness of the distress signal.
Why should we care, besides now knowing that the smell of cut grass is our lawn in a state of distress? Well, what we enjoy as the smell of nature is part of an arms race that's older than our species.
During their research, the scientists were surprised to find that plants aren't the only organisms capable of manipulating these chemical signals. The team discovered that moth and butterfly larvae have evolved their own versions of the Hi enzyme in saliva, which means they can modify the distress signals plants release.
One species – the tobacco hornworm moth (Manduca sexta) – puzzled the scientists, though. Its saliva contains highly active Hi that boosts the production of the sharper-smelling compounds in damaged tomato leaves.
Why? We don't know exactly, as it sounds both counterproductive and a waste of energy for the caterpillar. However, a 2010 study hypothesized that higher levels of specific compounds stimulated by the caterpillar's saliva could have some sort of microbial benefit. But more than 15 years on, scientists still don't really know.
That oddity aside, the study highlights how plants and insects independently evolved remarkably similar biochemical weapons. While both can manipulate the airborne alarm chemicals released during damage, they do so using entirely different protein families. It's a classic example of convergent evolution, where unrelated organisms arrive at similar solutions through separate evolutionary paths.
The researchers also traced the emergence of these compound-changing mechanisms back to the rise of flowering plants during the Cretaceous period, suggesting the smell of freshly cut grass, while a simple joy for us humans, is also the scent of an ancient chemical war between plants and insect predators that continues to this day.
While there's still so much we don't know about this small-scale biological arms race – unraveling the science of chemosignaling is one of the most challenging areas of plant and animal science – at least we can be thankful that nature gave us a pleasantly scented battlefield.
"The study connects plant and insect physiology, chemical ecology, molecular function, and evolutionary analysis to offer a new perspective on plant–insect coevolution," says first and co-corresponding author Yu-Hsien Lin from NTU.
"The next time we notice the familiar scent of freshly cut grass, it may be worth remembering that this is not just the smell of plants – it may also be a chemical distress signal, and a trace of a plant–insect arms race that has unfolded over millions of years."
The research was published Nature Ecology & Evolution.
Source: National Taiwan University
Fact-checked by Mike McRae
