| Summary: | <p>Improving photosynthesis is one of the most promising approaches to increase crop yield. To date, most efforts to enhance photosynthesis have focused on single-gene approaches. However, from a theoretical and experimental perspective, it is evident that multigene manipulations could lead to a greater impact. This DPhil project sets out to implement two innovative multigene metabolic engineering strategies for improved photosynthesis in Nicotiana tabacum. Firstly, combinatorial co-transformation was used with 12 transgenes, chosen to increase source
leaf photosynthesis and downstream carbon metabolism. A library of metabolic phenotypes was recovered, from which five lines were selected for showing increased stem growth under high light conditions. This phenotype was not reproducible in subsequent experiments, but further characterisation of the selected lines suggested that their specific transgene combinations may not have been beneficial for growth due to transgene interference effects. Secondly, a diel flux balance analysis model of primary metabolism in Arabidopsis leaves was used to design
a novel metabolic engineering strategy for improved leaf-energy efficiency and photosynthesis. A five-gene construct was generated that was predicted to result in increased mitochondrial ATP production, reduced chloroplast ATP export, and increased chloroplast NAD(P)H export. Contrary to the model predictions, transgene expression was found to correlate negatively with plant growth and photosynthesis. One possible explanation was a detrimental effect caused by the increased capacity of the chloroplast malate valve. This thesis constitutes one of the first investigations into the multigene engineering of photosynthesis and energy metabolism. Together, the results of this thesis highlight the complexity of manipulating plant central metabolism, particularly when targeting multiple transgenes, and suggests that further scrutiny of network behaviour and testing of transgene combinations will be needed to enhance photosynthesis and achieve the yield increases required to feed a growing population.
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