Mustard-bomb means of seed dispersal

A plant that detonates a mustard bomb in the mouth of any animal tempted to chew up rather than spit out its seeds has been described by scientists in Israel.

Publishing in Current Biology, Technion-Israel Institute of Technology, Haifa, behavioural ecologist Michal Samuni-Blank and her colleagues describe how the sub-Saharan desert plantOchradenus baccatushas evolved an ingenious strategy to encourage mice feasting on its fruits to disperse its seeds, rather than consume them.

The sugary and water-rich berries of these desert-adapted plants, which fruit continuously throughout the year, are packed with molecules called glucosinolates. These are normally innocuous. But locked away within the seeds inside the fruits is an enzyme called myrosinase.

If this comes into contact with the glucosinolates in the fruit flesh - for instance if an unwary mouse takes a bite out of both pips and fruit - it converts them into a toxic cocktail of thiocyanates, isothiocyanates, and nitriles known collectively as a "mustard bomb".

This, the team found, is deterrent enough to ensure that rodents that eat the fruits are careful to spit out the seeds first, helpfully dispersing them.

The evidence that this is a learned response, and that the animals would prefer to consume the protein-rich seeds, is that, offered samples of the berries containing a deactivated myrosinase and hence incapable of detonating a mustard bomb, the animals began consuming the seeds.

By resorting to this clever tactic, the plant, say the researchers, deters seed predators but has "no such effect on individuals of the same species acting as seed dispersers."