Negev berry coaxes mice to ensure plant reproduction

While Israelis who eat sunflower seeds typically spit out the husks and often throw them near their feet, a spiny mouse living in the Negev is induced by a plant called sweet mignonette to eat the fruit and spit out the seeds. This mechanism, just discovered by researchers at the Technion- Israel Institute of Technology, helps the plant reproduce in an arid climate.

The discovery, carried out with assistance from other Israeli and American researchers and published on Thursday in the online edition of the journal Current Biology, shows that the plant uses a toxic “mustard oil bomb” to make the mouse serve its needs.

The plant, also called taily weed, is the first known case within a single species of what is known as the “directed deterrence” hypothesis that “the fruit is trying to have itself eaten by the right consumer that will spread its seeds.” The plant thus produces a fruit to deter a whole class of animals from destroying its seeds.

This mechanism makes the mice conduct “analytical chemistry experiments” to test the fruit for poisonous compounds and not bite into the seed. The plants are in effect handing out the orders. Until now, only primates have been shown to spit seeds.

The fleshy, fruited plant, known to botanists as Ochradenus Baccatus, grows a meter or two in height in wadis across an area ranging from Pakistan to Egypt and down to Ethiopia and Sudan.

The plant has radiating, narrow, spiny leaves and a central stalk with yellow-green flowers, immature white berries and mature black berries less than a centimeter in diameter. It produces thousands of berries throughout the year, and each fruit contains up to 20 seeds.

However, the plant’s powerful “bomb” – which tastes like mustard oil – is activated when an animal eating the berries chews the seed as well as the fruity pulp. Enzymes, which are called myrosinases, in the seed activate toxic substances called glucosinolates (GLSs) in the pulp, which otherwise would be harmless.

The reaction produces chemicals named thiocyanate, isothiocyanates and nitriles. Isothiocyanates are responsible for the characteristic hot flavor of mustard. This mustard bomb first was discovered in mustard plants and has been known for some time to deter insects from eating leaves of certain plants.

The mustard taste is powerful enough for the common spiny mouse, also known as the Cairo spiny mouse or the Acomys Cahirinus, to remember it and spit out most of the seeds while chewing the pulp.

The Technion study also found the mice, which are active at night, will revert to eating the seeds if the mustard bomb is deactivated.

The researchers, who videotaped the mice, found that the rodents intentionally pull the seed from the fruit pulp like a person eating seeded watermelon, thereby keeping the toxins from being activated.

The research was conducted by Technion doctoral student and ecologist Michal Samuni- Blank and physiologist Zeev Arad; colleagues at the University of Haifa, the University of Utah and the University of Wisconsin. It was funded by the US-Israel Bi-National Science Foundation, the Israel Science Foundation and US Agency for International Development’s Middle East Regional Cooperation program.