School of BioSciences - Theses
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ItemEffects of elevated atmospheric carbon dioxide on iron metabolism in bread wheat (Tritium aestivum L.)( 2019)Atmospheric carbon dioxide concentrations have increased from pre-industrial levels of 280 ppm to a current level of 406 ppm and are predicted to reach 550 ppm by the year 2050. The rise in atmospheric carbon dioxide is known to directly alter the photosynthetic activity of C3 crops resulting in enhanced photosynthetic carbon fixation and, as a consequence, an increase in water use efficiency, biomass and grain yield. Yet, there is considerable evidence indicating a concomitant reduction in the content of essential mineral micronutrients such as iron in cereal grain. Wheat is the second most consumed staple crop in the world and it is an important source of calories for the human population, however it contains low grain iron concentration. Under rising levels of carbon dioxide wheat grain will likely contain even lower iron concentration and thus, intensify the already existing acute problem of human iron malnutrition, which currently affects over two billion people worldwide. This project aims to study the reasons underlying decreased iron concentration in wheat grain under elevated carbon dioxide concentrations by investigating wheat iron metabolism in relation to its uptake and remobilisation in field settings and to assess the feasibility of lessening the decreased iron concentration in wheat grain using a transgenic biofortification approach. Field trials were conducted over two growing seasons at the Australian Grains Free Air Carbon Dioxide Enrichment facility to investigate changes in iron distribution of bread wheat grown under ambient and elevated carbon dioxide concentration. At maturity, grain iron concentration decreased under elevated carbon dioxide concentration by 25% in the first season and by 26% in the second season. Iron distribution analysis revealed that an increased proportion of iron remained in the lower leaf, flag leaf and bracts during grain filling under elevated carbon dioxide concentration, resulting in a decrease in the iron remobilisation from those organs to the grain. Iron is an essential micronutrient, not only for humans, but for all plants and is involved in several important biological processes, including photosynthesis, respiration and chlorophyll biosynthesis. Iron possesses chemical properties that make it suitable to associate with proteins as a cofactor in the form of heme and iron-sulphur cluster. In order to decipher whether altered iron distribution between organs under elevated carbon dioxide concentration was related to changes in metabolic processes, in which iron is involved, an untargeted and targeted metabolite analysis was performed in flag leaf, bracts and grain at grain filling under ambient and elevated carbon dioxide concentration. In addition, the expression of genes involved in iron long-distance transport, iron influx and efflux transport, iron chelation biosynthesis and iron storage were investigated. The results showed decreases in the levels of compounds involved in most metabolic processes related to iron, including those involved in photosynthesis, nitrogen assimilation and oxidative stress in all three organs under elevated carbon dioxide concentration, with increases in the compartmentalisation of iron in chloroplasts and vacuoles. The iron chelators nicotianamine and deoxymugineic acid; showed decreased concentration in the grain under elevated carbon dioxide concentration, with a decreased expression of the genes involved in their biosynthesis. Furthermore, a decreased expression of genes involved in iron long-distance transport from flag leaf and bracts into the grain was shown. Biofortification is the enrichment of staple crops with essential micronutrients through agricultural practices, conventional breeding and/or genetic engineering. Constitutive expression of nicotianamine synthase genes has been an effective genetic engineering strategy to increase iron concentration in cereals such as wheat. In order to investigate the effects of elevated carbon dioxide concentration on a transgenic iron biofortification trait in wheat, transgenic wheat constitutively overexpressing the nicotianamine synthase gene and corresponding null segregants were grown under ambient and elevated carbon dioxide concentration in a glasshouse setting. The analysis revealed that the transgenic iron biofortified wheat plants grown under elevated carbon dioxide concentration increased grain iron concentration as well as nicotianamine and deoxymugineic acid chelators compared to the null segregants. Overall the results of this PhD project indicate that the decreased iron concentration in wheat grain under elevated carbon dioxide concentration is associated with altered disruption of iron within the plant during grain filling, with a greater proportion of iron remaining in flag leaf and bracts at the expense of the grain. This study suggests that the iron surplus in flag leaf and bracts under elevated carbon dioxide concentration is related to a decreased expression of the long-distance iron transporters and chelator genes responsible for the loading of iron from flag leaf and bracts into the grain, where the iron is compartmentalised in chloroplasts and vacuoles to avoid toxicity. Iron biofortified wheat plants constitutively expressing the OsNAS gene show potential to counteract low grain iron concentration under elevated carbon dioxide concentration.