Cyanogenesis in cassava and its molecular manipulation for crop improvement

While cassava is one of the most important staple crops worldwide, it has received the least investment per capita consumption of any of the major global crops. This is in part due to cassava being a crop of subsistence farmers that is grown in countries with limited resources for crop improvement. While its starchy roots are rich in calories, they are poor in protein and other essential nutrients. In addition, they contain potentially toxic levels of cyanogenic glycosides which must be reduced to safe levels before consumption. Furthermore, cyanogens compromise the shelf life of harvested roots due to cyanide-induced inhibition of mitochondrial respiration, and associated production of reactive oxygen species that accelerate root deterioration. Over the past two decades, the genetic, biochemical, and developmental factors that control cyanogen synthesis, transport, storage, and turnover have largely been elucidated. It is now apparent that cyanogens contribute substantially to whole-plant nitrogen metabolism and protein synthesis in roots. The essential role of cyanogens in root nitrogen metabolism, however, has confounded efforts to create acyanogenic varieties. This review proposes alternative molecular approaches that integrate accelerated cyanogen turnover with nitrogen reassimilation into root protein that may offer a solution to creating a safer, more nutritious cassava crop. Cassava is the only staple crop whose nutritional quality is compromised by toxic levels of cyanogens. Recent progress has led to a greater understanding of their metabolism and potential amelioration.