School of BioSciences - Theses
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ItemA metabolic engineering approach for iron and zinc biofortification of bread wheat (Triticum aestivum L.): impacts on plant growth, grain nutrition and food processing( 2019)Human iron (Fe) and zinc (Zn) deficiencies are among the most prevalent nutritional disorders in the world and manifest as a range of health issues including fatigue, impaired cognitive development and increased mortality. Micronutrient supplements and fortificants are frequently used to increase human Fe and Zn intakes yet these strategies require continuous investment and frequently miss rural populations; improving the density and/or bioavailability of Fe and Zn in staple crops (a process known as biofortification) represents a powerful alternative. Bread wheat (Triticum aestivum L.) is cultivated on more land than any other crop and is processed into a range of food products to supply ~20% of the daily calories consumed by humans. The wheat grain is mostly comprised of starch and protein, with micronutrients such as Fe and Zn concentrated in the outer aleurone layer of the grain. In the aleurone layer, Fe and Zn are complexed to compounds that inhibit their absorption (bioavailability) in the human gut and the aleurone layer is removed during grain milling to produce white flour. White flour (representing the inner wheat endosperm) contains low concentrations of dietary Fe and Zn, and high consumption of either wholemeal flour or white wheat flour coincides with high prevalences of human Fe and Zn deficiencies. Generating Fe and Zn biofortified wheat through conventional breeding in modern wheat cultivars is hindered by inherently low grain Fe and Zn concentrations and a lack of genetic variation for these traits. Nicotianamine (NA) is a low-molecular weight metal chelator present in all higher plants with high affinity for Fe2+, Zn2+, and other divalent metal cations. In graminaceous plant species such as wheat, NA serves as the biosynthetic precursor to 2’-deoxymugineic acid (DMA), a root-secreted mugineic acid family phytosiderophore that chelates ferric iron (Fe3+) in the rhizosphere for subsequent uptake by the plant. Both NA and/or DMA are the major chelators of Fe within white wheat flour, and NA is known to enhance Fe bioavailability in cereal grain. For these reasons, increasing the biosynthesis of NA/DMA through upregulation of nicotianamine synthase (NAS) genes has emerged as a popular strategy for Fe and Zn biofortification of cereal crops. In this study we employed constitutive expression (CE) of the rice (Oryza sativa L.) nicotianamine synthase 2 (OsNAS2) gene in bread wheat to upregulate the biosynthesis of NA and DMA, and evaluate plant growth, grain nutrition and food processing properties of CE-OsNAS2 wheat. Our lead CE-OsNAS2 wheat transgenic event (CE-1) demonstrated higher concentrations of Fe, Zn, NA and DMA in wholemeal flour, white flour and white bread, altered distribution of Fe in the grain, and higher white flour Fe bioavailability relative to null segregant (NS) control. Protein composition, dough rheology and breadmaking properties were similar between CE-1 and NS white flour, and a chicken (Gallus gallus) feeding study over a period of six weeks demonstrated that chickens consuming CE-1 white flour had improved Fe status, intestinal morphology and microbial populations relative to chickens that consumed NS white flour. Multi-location confined field trial (CFT) evaluation over three field seasons demonstrated no differences between CE-1 and NS agronomic performance apart from plant height. Throughout all CFTs, grain yield was negatively correlated with grain Fe, Zn, and protein concentrations yet not correlated with grain NA and DMA concentrations. White flour Fe bioavailability was positively correlated with white flour NA concentrations, and we determined NA to be the strongest enhancer of in vitro Fe bioavailability identified to date. Together these results suggest the proportion of Fe that is chelated to enhancers of bioavailability (such as NA and DMA) should be prioritized in future crop biofortification efforts and highlight new strategies for developing Fe and Zn biofortified wheat as a more nutritious staple food.