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• W889952307 abstract "Green plants fix inorganic carbon dioxide (CO2) by Ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) in a process called photosynthesis. This carboxylation reaction of Rubisco generates two molecules of 3-phosphoglycerate (3-PGA) and finally leads to biomass production. Hence plants account for a significant share of the global organic carbon pool. While most plants use the original C3 type of photosynthesis for CO2 assimilation some of the most important crop plants on earth like maize (Zea mays) use a highly efficient type of photosynthesis, namely C4 photosynthesis. In C4 photosynthesis CO2 is prefixed by an oxygen (O2) insensitive enzyme into a C4 acid and concentrated in close vicinity of Rubisco. This leads to a high efficiency of photosynthesis through a reduced rate of photorespiration that is caused by the oxygenation reaction of Rubisco. Although Rubisco strongly favors CO2, the enzyme can also accept O2 as a substrate. The product of the oxygenation reaction is one molecule of 3-PGA and one molecule of 2-phosphoglycolate (2-PG). The latter can neither enter the Calvin cycle nor be converted to carbohydrates. This can lead to a dramatic decrease in biomass production since 2-PG has to be recycled at the cost of energy and reduction equivalents in the photorespiratory cycle. The photorespiratory cycle is a highly compartmentalized pathway involving metabolic reactions in the chloroplasts, peroxisomes, mitochondria, and the cytosol. The mass flow through this pathway is only excelled by photosynthesis and therefore requires a tight and fast connection between the different compartments. This is obtained through highly specific metabolite transporters that account for the shuttle of precursors, intermediates and products across the membranes. It has previously been shown that short-circuiting of the photorespiratory cycle led to enhanced biomass production in C3 plants. This important finding verifies the significance of research for molecular plant engineering as a significant share of the components of the pathway, such as metabolite transporters and regulatory proteins are still unknown. Therefore one aim of this thesis was the identification of photorespiratory metabolite transporters since all genes encoding enzymes required for appropriate function of the core cycle are known to date while no transporter has been characterized on the molecular level. In a reverse genetics screen Plastidic glycolate glycerate transporter 1 (PLGG1) was designated as a promising candidate. Molecular characterization of the candidate was used to identify expression pattern and subcellular localization. An A. thaliana (Arabidopsis thaliana) T-DNA insertional knockout mutant (plgg1-1) was isolated and characterized concerning its phenotype and steady state and dynamic metabolite accumulation. Biochemical characterization of the candidate was used to identify transport capacity and to define the role of PLGG1 in plant metabolism. The molecular characterization revealed that PLGG1 is expressed in all green tissues and that it is located at the chloroplast envelope. Analysis of the knockout mutant revealed that plgg1-1 plants show a photorespiratory phenotype and the photorespiratory metabolites glycolate and glycerate accumulate in the mutant. Finally the biochemical characterization indicated that glycolate and glycerate transport are impaired in the mutant. These analyses lead to the discovery of PLGG1 as the chloroplastidic glycolate glycerate transporter and thereby to the identification of the first transporter of the core photorespiratory pathway. Surprisingly PLGG1 was found to be high abundant in maize chloroplast what is somehow counter intuitive as PLGG1 is a photorespiratory transporter and the rate of photorespiration is reduced in the C4 plant maize. Until today the role of PLGG1 in C4 photosynthesis is still unresolved and emphasizes the question how the C4 cycle is organized and how the establishment of C4 photosynthesis is orchestrated since the C4 cycle is obviously not as linear and simple at is has been assumed. Therefore the second aim of this thesis was to set up a systems biology approach to find candidates for unidentified components of the C4 pathway, such as metabolite transporters and regulatory proteins. The third leaf of the C4 model plant maize was analyzed along a base-to-tip developmental gradient. Ten continuous leaf slices were analyzed individually and transcriptome analysis, oxygen sensitivity of photosynthesis, photosynthetic rate measurements, and chlorophyll and protein measurements revealed a gradual sink-to-source transition for the leaf without a binary on-off switch. Analysis of transcription factors exhibiting a similar expression pattern to key C4 enzymes along the leaf gradient designated in a list of putative regulatory proteins orchestrating the establishment of C4 photosynthesis. Finally, transcriptome and metabolome analysis, enzyme activity measurements, and quantification of selected metabolites showed that the C4 cycle is not linear but rather a branched cycle. These results led to a revised model of maize C4 photosynthesis." @default.
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• W889952307 date "2013-01-01" @default.
• W889952307 modified "2023-09-27" @default.
• W889952307 title "A systems biology approach and single gene analysis to identify transporters and regulatory proteins in the plant carbon cycle" @default.
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