Final report : generation of transgenic cassava having reduced cyanide toxicity -- August 29, 1990-October 30, 1993

In fact, a correlation has now been demonstrated between the cyanide content of poorly processed cassava and the occurrence of the neurological disorder called Konzo. This paper describes genetic research designed to develop cassava plants with reduced cyanide toxicity. The strategy was to isolate and overexpress the gene encoding linamarase, the enzyme that converts linamarin into acetone cyanohydrin. Most research objectives were achieved, and some were even expanded upon. The most significant accomplishments have been to: (1) isolate, characterize, and localize cassava hydroxynitrile lyase (HNL), which converts acetone cyanohydrin into cyanide; (2) isolate cassava cDNA clones for linamarase; (3) characterize the stability of acetone cyanohydrin in low pH media and its relevance to cassava processing procedures; (4) demonstrate that roots can synthesize linamarin; and (5) develop cassava regeneration systems suitable for transformation, and identify suitable Ti plasmid transformation vectors. Project researchers are now close to developing recombinant plants that overexpress the cyanogenic enzymes, linamarase and HNL. Moreover, due in part to project efforts, the international Working Group on Cassava Safety has recommended that most effective way to detoxify cassava using a recombinant DNA approach is a strategy based on (1) tissue-specific inhibition of linamarin synthesis, combined with (2) tissue-specific overexpression of the cyanogenic enzymes.