A Single Nucleotide Polymorphism in a Phytoene Synthase Gene can Simplify Provitamin A Biofortification of Cassava
Ralf Welsch
ISB NEWS REPORT MARCH 2011
The need for biofortification of staple crops
Millions of poor people rely on staples to meet their daily calorie requirements. The existence of such energydense food is the result of genetic selection by humans since the beginning of agriculture, which was dominated by selection criteria related to yield increase. Biochemical diversity as an important food requirement, such as the need for micronutrients—i.e., vitamins and microelements—could not be considered. The resulting biochemical monotony in staples can be counterbalanced by food diversification, where possible. However, poverty reasons have led to predominant consumption of staples in large parts of the world—60% of calories consumed are from three crops: rice, corn and wheat. Consequently, a variety of micronutrient malnutrition diseases affecting large parts of the world’s population have emerged.
“Biofortification” of crop plants describes the idea of providing the micronutrients through the staple crop plant’s own biosynthetic (vitamins) or physiological (minerals) capacity. This can be accomplished by conventional classical breeding, when the desired micronutrient-rich germplasm is available, allowing the transfer of such nutritional traits into agriculturally relevant cultivars. In the absence of sufficient genetic variability or when plant breeding is ineffective, the introduction of the desired trait by genetic engineering is required.
An important scientific prerequisite for the application of genetic engineering is inter alia a toolbox of identified genes. The term “Nutritional Genomics” has been coined to describe a discipline of modern plant biology that focuses on the molecular elucidation of the biochemical pathways or physiological processes involved in micronutrient accumulation.
Biofortification as a necessary vitamin A deficiency intervention Vitamin A malnutrition is widespread in the tropics, leading to irreversible blindness and severely exacerbating infectious diseases due to its essential role in the immune response. According to the WHO, an estimated million preschool children are affected by vitamin A deficiency, with 250 000 – 500 000 children becoming blind every year, half of whom die within a year (WHO database of vitamin A deficiency;
http://www.who.int/vmnis/vitamina/data/en/index.html). Vitamin A denotes a group of compounds formed by the cleavage of provitamin A carotenoids, i.e., carotenoids containing at least one unsubstituted β-ionone ring, such as β- or α-carotene. Prevalence of Vitamin A deficiency among poor people with diets primarily based on rice prompted the development of Golden Rice, which is a rice cultivar that accumulates provitamin A carotenoids in the endosperm, the edible part of the rice grain. The absence of variability for the trait (no germplasm accession is known that develops yellow endosperm) required the use of genetic engineering for biofortification.
Cassava is another important staple crop, cited as the fifth most important crop worldwide. Its importance is even higher in arid areas, such as sub-Saharan Africa. Roots of commercial cassava cultivars are rich in starch, but low in proteins and micronutrients, including provitamin A carotenoids; thus biofortified cultivars with elevated levels of provitamin A are desirable. In contrast to rice, some yellow-rooted cassava varieties do exist, and thus, breeding efforts have yielded a three-fold increase in provitamin A content. However, cassava is vegetatively propagated and breeding is very tedious because of long breeding cycles. Moreover the very complex genetics of cassava renders varietal recovery extremely difficult. Thus, the identification of genes crucial for improvements of the provitamin A content in cassava storage roots can greatly assist in accelerating the development of provitamin A-rich cassava both by breeding and genetic engineering.
We therefore focused on the carotenoid biosynthesis pathway in cassava. We took advantage of existing cassava varieties differing in root carotenoid accumulation and found an allelic polymorphism in the gene coding for the rate-limiting enzyme of carotenoid biosynthesis. This polymorphism was found to determine root color differences caused by varying accumulations of provitamin A carotenoids..
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