Self-defending seeds

A two-layered protective coating that releases cyanide when bitten into could protect seeds from pests, say scientists in Switzerland. The system only becomes toxic when the layers are mixed, eliminating many of the problems of current pesticides.

Protecting agricultural crops against pests is a worldwide concern, particularly in developing countries, but pesticides can contaminate the surrounding environment, and can also hurt harmless insects like honey bees.

Wendelin Stark, of the Institute for Chemical and Bioengineering in Zurich, says his concerns over the effect of pesticides on the environment inspired him to look for alternative solutions. Stark and his team of researchers noticed that many types of plants already produce a form of natural protection against herbivores. ‘If you have ever opened an apple and found an insect inside, you may notice that the seeds are still fine,’ says Stark. This is because the seeds of apples contain the chemical amygdalin or other cyano-glycosides, which reacts with other enzymes to release the toxic chemical hydrogen cyanide, perhaps best known as the poison of choice in murder mysteries. In fact over 3000 species of plant can do this. Stark used this idea to develop an artificial coating that mimics the effect already seen in nature that can be applied to as-yet unprotected seeds such as wheat.

The coating consists of a layer of the enzyme hydroxynitrile lyase (HNL) and a layer of polylactic acid (PLA) containing the chemical malonitrile (MN), and the two layers are separated by a layer of pure PLA. These layers can be applied to seeds on a large scale by dipping the seeds into the different chemicals and letting them dry between layers. When a herbivore bites into the seed, the layers are merged, causing HNL and MN to react and produce hydrogen cyanide, a process known as cyanogenesis.

The advantage of this kind of protection is that it triggered by the herbivore itself, says Christian Krupke, an expert in pest management at Purdue University, Indiana, in the US. ‘Because the protectant is only activated by insect feeding, it remains durable and the threat of resistance development over time is lessened.’

The coating is only applied to the seeds so it is safe for humans to eat the fully-grown crops.

A disadvantage of the coating is that it can reduce the ability of the seed to germinate, probably due to the toxic effect of the coating on the seed itself. In future this may be solved by applying an initial layer of high density PLA, to reduce the diffusion of hydrogen cyanide towards the centre of the seed.