Die Bedeutung von Fermentation, Photosynthese und Pyrophosphat fu?r das U?berleben von Pflanzen unter Sauerstoffmangel

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Oxygen deficiency stress leads to inhibition of mitochondrial respiration in plant cells, resulting in accumulation of NADH and a decrease in the ATP content. Tolerant rice plants can survive oxygen deficiency better than wheat or potato plants. The aim of this PhD-work was to examine, which biochemical processes are responsible for plant tolerance against low oxygen stress. The studies were focused on the analysis of ethanolic fermentation, photosynthesis and the function of pyrophosphate (PPi) as an alternative and compensatory energy source during ATP deficiency. The rate of ethanolic fermentation in rice and wheat plants during anoxia was measured by using a laser-based photoacoustic system during darkness and illumination. These data were replenished by the measurements of carbohydrate contents and enzyme activities as well as photosynthetic parameters. Additionally, experiments with transgenic potato plants with altered PPi metabolism were performed under hypoxia. By using ethanolic fermentation, plants can regenerate NAD and maintain ATP formation during glycolysis. It could be shown that tolerant rice plants produced much higher amounts of ethanol during anoxia in darkness compared to sensitive wheat plants. The high fermentation rate mainly occurred in the shoots as a result of high activities of fermentative enzymes as well as high availability of carbohydrates. Nevertheless, rice plants could not survive more than 24 h of anoxia in the dark because the carbohydrates became depleted. Illumination during anoxic stress extended survival of plants. Ethanolic fermentation rate was reduced during light exposure of plants, indicating that other energy-producing processes can compensate. However, it could be shown that the complete photosynthesis was slowed down during oxygen deficiency due to CO2 deficiency. It is likely that cyclic electron transport could at least partially contribute to ATP production during these conditions. In the past, it was speculated that PPi could replace ATP for phosphorylating processes. With transgenic potato plants expressing E. coli pyrophosphatase and therefore containing less PPi it was demonstrated that PPi is a significant alternative energy donor for sucrose cleavage during hypoxia. However, in transgenic potato plants with a reduction of synthesis and activity of PPi-dependent phosphofructokinase it could not be demonstrated that these plants suffer more from oxygen- and ATP-deficiency than the control plants, although the transformants were almost completely dependent on the activity of the ATP-dependent isoforms. 9 Additionally, during this PhD-work seven sequences, coding for the ATP-dependent phosphofructokinase, were found in the Arabidopsis genome. These sequences are essential for future experiments to explore the significant functions of ATP- and PPi-dependent forms of phosphofructokinases.