Cyanogenesis
Many economically important food plants are highly cyanogenic and have caused numerous cases of acute cyanide poisoning of animals including man. Additionally, in areas of the world where cyanogenic plants such as cassava and lima beans comprise the major item of the diet, chronic cyanide poisoning and associated pathological conditions still exist. It is highly desirable that the toxicity of cyanogenic plants to humans and livestock be reduced. This is achievable by: (a) selective breeding to produce low-cyanogen varieties, as was accomplished for almonds, (b) screening of natural populations for low-cyanogen varieties, (c) mutagenesis of protoplasts or cell cultures with subsequent regeneration of plants having desired mutant genotypes, or (d) genetic engineering.
In essence cyanogenic plants cannot continually release cyanide as this would harm the plants themselves; a harmless precursor is stored in tissues such as leaves, and this is then converted to cyanide when the leaves are damaged. The amount of cyanide is insufficient to deter or harm mammals (Montgomery 1969), but acts as a deterrent against invertebrates such as slugs and snails (Jones 1962, 1973; Angseesing 1974).
Cyanogenesis in Higher Plants and Animals (https://onlinelibrary.wiley.com/doi/abs/10.1002/9780470015902.a0001921.pub2)
Cyanogenesis describes the ability of living organisms to liberate hydrogen cyanide from stored cyanogenic glycosides, cyanogenic lipids or cyanohydrins on tissue damage by hydrolysis and/or decomposition. It has been described for over 2650 species of higher plants and some animals. Together with plant β-glucosidases and hydroxynitrile lyases cyanogenic glycosides are the constituent part of a preformed defence system. Today it's generally accepted that cyanogenic glycosides also may serve as storage form for reduced nitrogen and sugar. Chemically, cyanogenic glycosides are glycosides of α-hydroxynitriles (cyanohydrins). The structures are biogenetically related to only a few precursor amino acids. Biogenetically closely related are some noncyanogenic glucosides of β- and γ-hydroxynitriles (cyanoglucosides). Their biological function is not fully understood until today. Many food plants are cyanogenic and great efforts are made to optimise detoxification. Humans are able to detoxify considerable amounts of HCN. Although acute intoxication is rare, the daily consumption of subacute amounts of cyanogenic glycosides leads to chronic diseases, caused by increased plasma levels of the human detoxification products. The presence of cyanogenic glycosides in the animal kingdom appears to be restricted to Arthropoda. Some insects, often aposematically coloured, either de novo synthesise cyanogenic glycosides and/or sequester them from their host plants.
Aridity shapes cyanogenesis cline evolution in white clover (Trifolium repens L.)
The cyanogenic polymorphism in Trifolium repens L. (white clover)
White clover releases toxic cyanide when its leaf tissues are damaged. This chemical defense, a response called cyanogenesis, helps it to deter insect pests.
Screening Crop Plants for toxins (https://www.sciencedaily.com/releases/2010/05/100514075153.htm)
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.