Cassava is one of the most important staple food crops in the developing world but is also one of the most poisonous, its tissues releasing large quantities of cyanide when they are macerated during eating. Although the cyanide can be effectively removed by thorough cooking or by fermentation, chronic cynaide poisoning due to consumption of poorly processed cassava is common throughout tropical Africa and leads to serious neurological conditions. To solve this problem, a team of researchers led by Richard Sayre at the Danforth Plant Science Center, St Louis, Missouri have created genetically engineered cassava plants that contain an enzyme that improves the processing efficiency of the crop. Reporting their findings in Plos One, they show that not only do the transgenic cassava roots contain less cyanide after processing, but that the high amount of the added enzyme also significantly raised the total protein content of the root. This is a major added benefit because cassava contains the lowest protein to energy ratio of any crop leading to protein energy malnutrition if it is the primary food source.
Plants that produce cyanide in their tissues do so as a protective mechanism to discourage their consumption by herbivorous animals and insects. However, cyanide is as toxic to plants as it is to animals and so to prevent self-poisoning, cyanide is only release when then cells of the plants are disrupted, as would occur when an animal or insect chews the tissue. This relies on the accumulation of non-toxic cyanide-containing compounds called cyanogenic glucosides in one part of the cell (the vacuole) being kept separate from an enzyme that can act on these compounds. When the cell is disrupted by chewing, the two are mixed and the enzyme causes the release of the cyanide. It is this enzyme, hydroxynitrile lyase, that the Danforth team have increased. The processing of cassava is based on allowing cyanide, which is volatile, to evaporate. The increased amount of hydroxynitrile lyase ensures a rapid conversion of cyanogenic glucosides to cyanide and reduces the risk that cyanogenic glucosides could remain in the tissue after processing such that cyanide is still produced.
The beauty of this approach is that it does not interfere with the natural cyanogenic process, meaning that the pest-resistance of the plants is not affected. This is in contrast to previous attempts to solve the cyanide problem by preventing the synthesis of the cyanogenic glucosides in the first place.
Plants that produce cyanide in their tissues do so as a protective mechanism to discourage their consumption by herbivorous animals and insects. However, cyanide is as toxic to plants as it is to animals and so to prevent self-poisoning, cyanide is only release when then cells of the plants are disrupted, as would occur when an animal or insect chews the tissue. This relies on the accumulation of non-toxic cyanide-containing compounds called cyanogenic glucosides in one part of the cell (the vacuole) being kept separate from an enzyme that can act on these compounds. When the cell is disrupted by chewing, the two are mixed and the enzyme causes the release of the cyanide. It is this enzyme, hydroxynitrile lyase, that the Danforth team have increased. The processing of cassava is based on allowing cyanide, which is volatile, to evaporate. The increased amount of hydroxynitrile lyase ensures a rapid conversion of cyanogenic glucosides to cyanide and reduces the risk that cyanogenic glucosides could remain in the tissue after processing such that cyanide is still produced.
The beauty of this approach is that it does not interfere with the natural cyanogenic process, meaning that the pest-resistance of the plants is not affected. This is in contrast to previous attempts to solve the cyanide problem by preventing the synthesis of the cyanogenic glucosides in the first place.
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