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theses

Photosynthesis, the physico-chemical process converting sunlight into chemical energy, is the basis to feed the world and fuel the planet. To satisfy the growing demand for food and fuel, the efficiency of the natural photosynthesis needs to be optimized for maximum crop yield, while the photosynthetically assimilated carbon needs to be more sophisticatedly recruited for generating energy-dense renewable products. There are two objectives of this dissertation, the first is to explore the feasibility to boost biomass yield of crop plants by genetically engineering their photosystem II (PSII), and the second is to create robust microalgal transgenic strains with enhanced lipid content and CO2 utilization efficiency, which will contribute to microalgal biofuel production as well as CO2 mitigation.
In Chapter 2, we explore whether the prokaryotic design principal of PSII D1 subunit is applicable in a higher plant model Nicotiana tabacum. By introducing single point mutations into tobacco psbA gene (coding for the reaction center D1 subunit of Photosystem II) to mimic the cyanobacterial high-light and low-light D1 isoforms, the tobacco mutants exhibit the biophysical traits of the prokaryotic PSII. The tobacco mutant expressing the engineered high light isoform exhibits higher photosynthetic efficiency, higher tolerance to photoinhibition and increased biomass production under the tested light conditions. The only benefit of incorporating the cyanobacterial low light mutation into tobacco D1 protein is restricted to improving the Water Oxidizing Complex catalytic efficiency at low light intensity, while the biomass yield was impaired at all the tested light conditions.
In Chapter 3, Nannochloropsis oceanica CCMP1779 (N.o1779), the emerging oleaginous model alga, is chosen for application of the “push and pull” strategy to enhance its lipid productivity by metabolic engineering. The regulatory importance of citrate synthase (CIS) in directing carbon flux towards protein synthesis pathway, and the functional role of glycerol 3-phosphate dehydrogenase (G3PDH) in diverting carbon precursors from glycolysis to TAG assembly are fully examined in the transgenic strains of N.o1779. Downregulation of a putative endogenous gene encoding CIS via RNA interference technology and expression of a yeast gene encoding the cytosolic G3PDH lead to higher accumulation of the storage lipid triacylglycerols (TAGs) and increased abundance of the lipid building block free fatty acids, advancing our understanding of the genetic and molecular basis of algal TAG metabolism.
In Chapter 4, the goal was to create a robust industrial strain that can be cultivated in the open culture using flue gas as carbon source. By applying insertional mutagenesis combined with high-throughput screening strategy to the oleaginous microalga N.o1779, a winning mutant was successfully identified for its advantages in photoautotrophic growth and intrinsic photosynthetic efficiency under both normal growth condition and acidic environment. The genome sequencing project of this mutant currently in progress will potentially unlock the regulatory mechanism responsible for its beneficial phenotypes.
In summary, my dissertation advances the understanding of the PSII design principal and the central carbon metabolism in the oxygenic photosynthetic organisms. Novel genetic engineering strategies have also been developed throughout this dissertation to improve biomass productivity in a higher plant and enhance lipid productivity and carbon utilization in a eukaryotic microalga.

ETD_9312

Ph.D.

Includes bibliographical references

by Yuan Zhang

doi:10.7282/t3-bkq3-ty29

ETD doctoral

The author owns the copyright to this work.