Plants have it rough. In order to spread their seeds, they have to cater to the animals that eat them, and that means watching their offspring get eaten in the hopes that they'll be pooped out somewhere better. But when mice start thwarting the best-laid plans of vegetation, it's up to the plants to strike back — with chemical weapons.
A plant known as taily weed grows through much of Israel, Saudi Arabia, Egypt, and Sudan. It grows three to six feet tall, is grazed on by birds, lizards, camels, ibex, and mice. It also provides shelter, shade, and cover to animals, and it's feeling a little bit passive aggressive. The grass produces black berries year round. The berries hold its seeds, tiny black specks that mean the survival of the plant as a species. Some animals, like birds, tend to gulp the berries down without chewing, leaving the seeds intact to be spread throughout the area. Some animals prefer the grasses themselves to berries — which isn't ideal, but doesn't directly destroy the next generation. And then there are those damn rodents.
Specifically, there's the desert spiny mouse, which can eat and digest seeds, as well as berries. These mice enjoy a plant's largesse, without any intention of spreading seeds. In fact, the seeds just are bonus nutrition. Which is why the taily weed turned its fruit into booby trapped chemical bombs.
Inside the fruit are chemicals called glucosinolates. You probably haven't heard about them, and you don't need to. They're harmless, even in the fruit of the taily weed. Animals eat them without a problem. Unless, like the mice, they bite into the seeds. Inside each seed are enzymes called myrosinases. When the seed is bitten into, it releases the enzymes into the fruit. When the two chemicals combine, they turn the fruit from a sweet, enticing meal, to a world of pain. They combine to form a strong mustard flavor, pungent and spicy. It's not a problem for a human eating the berry, but for a mouse, it's like taking an entire mouthful of mustard in every bite. The whole berry becomes inedible.
If, that is, they eat the seeds. The plant has, over time and with the weapons at its disposal, gotten the mice to play nice. They now carefully wriggle out the seeds and spit them on the ground as they eat, obediently spreading the seeds without damaging them. What a difference surprise chemical warfare can make.
Study to be published in Current Biology.
Both Images: Michal Samuni-Blank, Technion-Israel Institute of Technology