Seed Dispersal Under the Threat of Mustard Bomb – Michal Samuni-Blank (2012)

Plants evolved diverse ways to disperse their seeds using animals. To give a few specific examples Long-wattled umbrella birds, Toucans and fruit eating bats are known for effective dispersal of palm tree seeds by eating the fruits. These dispersers regurgitate or defecate seeds after flying far away. On the other hand, most fruit eaters are also seed predators. Unlike birds and bats other animals such as agoutis, monkeys, peccaries and tapirs can chew and destroy the very same palm seeds. Not good for the plant.

Imagine a plant that detonates a mustard bomb in the mouths of seed predators who dare to chew and destroy its seeds. It must have been similar to throwing a chunk of wasabi in your mouth… Behavioural ecologist Michal Samuni-Blank and her colleagues describe a sub-Saharan desert plant taily weed (Ochradenus baccatus) which has evolved a solution to the seed predation problem.

The directed deterrence hypothesis states that secondary metabolites in fruit are to prevent fruit consumption by animals that do not disperse seeds. The sugary and water-rich berries of the desert-adapted taily weed fruit are available throughout the year. The 4-millimeter-wide berries of taily weed are laced with glucosinolates that are unreactive individually. However if chemicals come into contact with the myrosinase enzyme which is only found in the seeds an explosive reaction is catalyzed. Cracking open the berry’s seeds releases the enzyme, which mixes with glucosinolates in the flesh to produce the “mustard bomb” consisting of thiocyanates, isothiocyanates, and nitriles.

Animals normally would love to consume the seeds because seeds contain more protein. Indeed, when the rodents were given berries whose detonator enzyme myrosinase is inactivated they went ahead and ate the seeds. This behavior provides the evidence that this is a learned behavior. After a few unpleasant experiences the rodents learn to eat only the flesh of the fruit and spit out the seeds intact. The germination rates of the discarded seeds were more than twice of those left in the berries. The plant’s chemical defense has thus turned a seed consumer into a disperser.

If you want to learn more about the natural history of plants check out “Plants are Cool Too!” series.