Plant Tricks Mouse Into Spitting Out Its Seeds

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Brett Smith for
A plant in the Israeli desert has developed a ℠toxic mustard bomb´ which causes the spiny mouse that eats its berries to spit out the seeds like a child munching on watermelon.
Researchers found that enzymes within the seeds of the sweet mignonette berries activate toxic substances in its pulp, which would otherwise go unnoticed, according to a new study published online in the journal Current Biology.
When an unknowing mouse chomps down on the seed, it releases the enzymes, resulting in the creation of isothiocyanates, which are responsible for the hot flavor of mustard. This so-called ℠mustard bomb´ first was discovered in mustard plants, and is known to deter and select for certain animals or insects.
Through this mechanism, animals learn not to eat the seeds with the fruit, spitting them out and dispersing the seeds so that they might later germinate. Interestingly, rodents observed in the experiment would often return to eat the seeds hours later, presumably after they had digested the pulp.
The study illustrates a phenomenon known as “directed deterrence.” According to the study´s co-author Denise Dearing and professor of biology at the University of Utah, “the fruit is trying have itself eaten by the right consumer — one that will spread its seeds.”
“The plant produces a fruit to deter a class of consumers that would destroy its seeds,” she added.
The best known example before the new study, Dearing noted, involved chili peppers, which deter mammals from eating their seeds because mammals feel pain induced by their ingredient, capsaicin. Because birds “don´t feel the heat at all, they tend not to crush the seeds while they are feeding, so they are good dispersers of chili pepper seeds,” she said.
The joint U.S.-Israel study also found that if the captive mice were presented with fruits that had the mustard enzyme deactivated, the mice left less than 20 percent of the seeds intact, compared with 74 percent of the seeds that still contained the active mustard bomb ingredient.
“Thus, when faced with a ℠disarmed´ mustard oil bomb, Acomys behaved as a seed predator,” the researchers wrote about their test subjects.
In another part of the study, the researchers left the berries in petri dishes located in both rocky crevices and under the sweet mignonette plants. They then videotaped the mice eating them, and counted more seeds that were left intact in the crevices than under the parent plants.
“The mice are actually dispersing the seeds to a suitable habitat for germination,” Dearing said. “Under the parent plant is a bad spot. A rocky crevice would be a cool, suitable location because it´s not in direct sunlight.”
The researchers also decided to see if the mice´s interactions with the seed had any effect on the potential for germination. They collected seeds spit out by mice in the field and attempted to germinate them in the lab. All the seeds from mature berries successfully germinated, in fact the seeds spit out by the mice germinated at twice the rate of seeds left inside intact fruit.
Image 2 (below): Immature berries of the Middle East desert shrub Ochradenus baccatus, also known as sweet mignonette or taily weed. If the seeds are chewed at the same time as the fruit pulp, the berries release a toxic “mustard oil bomb.” A new study by Israeli and University of Utah scientists shows that toxic reaction prompts spiny mice to spit out the seeds when they eat the berries, effectively helping the plant reproduce. Photo Credit: Michal Samuni-Blank, Technion-Israel Institute of Technology