Abstract
Zinc deficiency in humans is a serious problem worldwide with an estimated one third of populations at risk for insufficient zinc in diet, which leads to impairment of cognitive abilities and immune system function. The goal of this research was to increase the bioavailable zinc in the edible portion of cassava roots to improve the overall zinc nutrition of populations that rely on cassava as a dietary staple. To increase zinc concentrations, two Arabidopsis thaliana genes coding for ZIP1 and MTP1 were overexpressed with a tuber-specific or constitutive promoter. Eighteen transgenic events from four constructs, out of a total of 73 events generated, showed significantly higher zinc concentrations in the edible portion of the storage root compared to the non-transgenic controls. The zinc content in the transgenic lines ranged from 4 to 73 mg/kg dry weight (DW) as compared to the non-transgenic control which contained 8 mg/kg. Striking changes in whole plant phenotype such as smaller plant size and chlorotic leaves were observed in transgenic lines that over accumulated zinc. In a confined field trial five transgenic events grown for 12 months showed a range of zinc concentrations from 18 to 217 mg/kg DW. Although the overexpression of zinc transporters was successful in increasing the zinc concentrations in 25% of the transgenic lines generated, it also resulted in a decrease in plant and tuber size and overall yield due to what appears to be zinc deficiency in the aerial parts of the plant.
Highlights
Zinc is an essential mineral for both plants and animals
Production of Transgenic High-Zinc Cassava Plants Agrobacterium-mediated genetic transformation (Taylor et al, 2012) with five gene constructs containing AtZIP1 and AtMTP1 genes resulted in a total of 217 putative independent transgenic plant lines recovered from tissues selected on antibiotic-containing medium
Of the 217 plants regenerated, 47% of PAT:AtMTP1, 44% of PAT:AtZIP1, 68% of figwort mosaic virus (FMV):AtZIP1, 22% of FMV:AtMTP1, and 10% of FMV:AtZIP1-PAT:AtMTP1 plant lines survived and formed starchy roots whereas the remainder died in the greenhouse or did not form tubers within 4 months after transfer to soil
Summary
Zinc deficiency in humans is widespread, ranking fifth among the most important health risk factors in developing countries after obesity, iodine deficiency, iron deficiency, and vitamin A deficiency (WHO, 2002). Cassava (Manihot esculenta Crantz) is cultivated mainly for its edible starchy root and is an important calorie source for low-income populations in Sub-Saharan Africa and other tropical regions of the world. Overexpression of zinc transporters in cassava under optimal growing conditions (El-Sharkawy et al, 1990), making it an attractive source of food. An excellent source of carbohydrates, cassava storage roots are low in protein, vitamins, and micronutrients such as zinc and iron. A survey of 600 cassava clones showed that the range of zinc concentration found in edible portions of the storage roots was between 2.6 and 37 mg/kg, with an average of 7.5 mg/kg (Chávez et al, 2005). To provide the minimum daily required amount of zinc for individuals eating between 500 and 1000 g of fresh cassava each day, it would be necessary to create a bio-fortified cassava product with at least six times higher zinc levels in the edible portion of the root (Sayre et al, 2011)
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