The most agronomically important of the cyanogenic crops is the tropical root crop cassava (Manihot esculenta, Crantz) with over 153 million tons of cassava roots being harvested annually in the tropics, primarily for direct human consumption. Cassava foods must be processed to remove cyanogens prior to consumption as exposure to lower levels of cyanide causes vomiting, nausea, palpitations, headaches and impaired vision. The long-term ingestion of large quantities of poorly processed cassava has also been shown to be associated with chronic cyanide-associated health disorders in several areas of Africa. Recent findings have also demonstrated an involvement of cyanogens in both amino acid content and post-harvest deterioration complementing its herbivore deterrent capabilities. The gene for beta-cyanoalanine synthase (beta-CAS), a mitochondrial enzyme that detoxifies cyanide to asparagine has been identified. The research proposed here plans to further characterize the cassava beta-CAS DNA sequence as well as clarify the function of this enzyme by introducing the gene into model plant Arabidopsis. Furthermore transgenic cassava over-expressing beta-CAS gene in the cytoplasm or the mitochondria will be produced. The expression of beta-CAS in the cytoplasm would divert cyanogens from root storage to the synthesis of amino acids, producing a cassava variety with enhanced amino acid pool and thus higher protein nutrition. In addition, roots over-expressing beta-CAS will be analyzed for post-harvest deterioration since the affinity of beta-CAS for cyanide should lead to a reduction in the available reactive-oxygen species, and thus to a longer shelf-life. The broader impact of this project is to develop a more nutritious food source for 600 million people world-wide while educating undergraduate and graduate students attending the University of Puerto Rico Mayaguez, all of whom belong to an underrepresented group (Hispanic) in biology.
