For millennia, plants and their predators have been locked in a bitter battle. Literally.
About 90 million years ago, ancestors of today’s Brassicale order — which includes wasabi, horseradish and mustard as well as cabbages and kale — figured out a way to fend off hungry caterpillars: sharp-tasting chemicals called glucosinolates that were poisonous to the snacking critters. But over the course of generations, the bugs developed defenses against the glucosinolates, a new study shows, and the plants had to churn out ever-stronger versions of the toxin “bomb.”
Eventually the fight escalated into an “evolutionary arms race,” researchers say, with defensive adaptations from caterpillars prompting sharper and more complex glucosinolates from the plants. And in this particular case of garden-variety chemical warfare, the winner wasn’t either of the combatants — it was humans.
Though toxic to tiny caterpillars, the cocktail of glucosinolates turned out to be pretty tasty to humans. We have them to thank for half our refrigerator condiments shelf — spicy wasabi, bitter horseradish, pungent mustard. They also add to the complex and highly particular tastes of cauliflower, capers, radishes and a variety of other vegetables.
“Why do you think plants have spices or any flavor at all? It’s not for us,” Chris Pires, a biologist at the University of Missouri and one of the lead authors of the study, told NPR. “They have a function. All these flavors are evolution.”
Though scientists have long known that sharp flavored plants co-evolved with the bugs that eat them, Pires’s study, published in the Proceedings of the National Academy of Sciences last week, was the first to map out exactly how the escalation occurred.
Pires and his colleagues started by lining up the evolutionary family trees of Brassicales and ancestors of the cabbage butterfly. They found that every time the plants evolved a new type of glucosinolate “bomb,” their family tree would branch into several new and more complexly flavored species.
“Most bugs don’t like it. It’s toxic,” Pires said of the chemical. “It turns their guts inside out.”
Eventually, the bugs would respond with new protein defenses and diversify themselves.
This escalation occurred three times over the course of the past 90 million years, according to Smithsonian; each time the combatants would raise the stakes not just by tweaking their genes to protect themselves but by creating entirely new copies — a rare occurrence in the natural world.
Peter Raven, professor emeritus of the Missouri Botanical Garden and a former University of Missouri curator who was not involved in the study, told the University of Missouri science blog that the finding confirms patterns of co-evolution that had long been suspected but never fully understood.
“The wonderful array of molecular and other analytical tools applied now under leadership of people like Chris Pires, provide verification and new insights that couldn’t even have been imagined then,” he said.
For his part, Pires sees a potential to harness the ability of the Brassicales to escalate their defenses by altering their genes to develop pest-resistant crops.
“It could open different avenues for creating plants and food that are more efficiently grown,” he said.