Screening crop plants for toxins

Many plants, in response to predators or herbivores, release hydrogen cyanide to defend themselves. Cyanide precursors are kept in a compartment in the cell. Tissue damage allows them to break out of the compartment and mix with a degrading enzyme in the cell. This produces toxic, bitter hydrogen cyanide that repels the herbivore.

This mechanism, known as cyanogenesis, is found in two thirds of the main crop species eaten worldwide, including maize, sugar cane and some legumes. The major impacts on human health are seen when it is the edible part of the plant that produces cyanogenic compounds, such as in cassava roots. In fodder crops such as sorghum it can lead to livestock poisoning.

Without correct processing, high levels of hydrogen cyanide in the food can cause neural disease and permanent paralysis, a condition known as konzo. In drought conditions, the cyanide levels increase even higher.

Cassava is the third largest source of carbohydrates for human food in the world after wheat and rice. The bitter varieties, favoured by farmers because of their better resistance to pests, contain two cyanogenic compounds. Various processing methods are used to remove them, such as by soaking in water for several days.

Finding less toxic strains of these crops is a high priority, and a new genetic screen developed at the John Innes Centre will help in this search. Researchers, working on a collaborative program sponsored by the Danmarks Grundforskningsfonden (Danish National Research Foundation) with colleagues at the University of Copenhagen, developed a high-throughput way of detecting cyanogenesis-deficient mutant plants. Using the model legume Lotus japonicus, they screened more than 40,000 plants in just 10 days, identifying 44 cyanogenesis deficient mutants.

"We are keen to extend this work to crop plants and cassava is the big target. If we could set up a system we could get to a non-cyanogenic variety of cassava quite quickly," said Professor Cathie Martin of the John Innes Centre. "We're now looking to identify populations of cassava that we can screen so that we can get non-cyanogenic lines to trial for performance in the field."

The study also found that some mutants were deficient in cyanogenesis only in certain parts of the plant and not in others, suggesting, for example, it may be possible to find mutants that retain cyanogenesis in leaves but don't make the dangerous toxins in the edible roots of cassava. This would enable crops to keep their valuable defence mechanisms against pests, and yet reduce the considerable time required for preparation of food using cyanogenic crops and the risk to human health.

Dr Jonathan Clarke, Head of Business Development at the John Innes Centre, an institute of the Biotechnology and Biological Sciences Research Council, (BBSRC), is working with Prof Martin to apply this technology. "The effects of cyanogenic crops impact on the lives of some of the most vulnerable people in the world. The problem is increased during times of drought when the toxin levels increase and water for soaking is unavailable" said Dr Jonathan Clarke. "We have developed a simple, rapid, and low cost screen. We are now seeking funding to use this to produce non-cyanogenic cassava for Africa."