The alternative mechanisms of chloroplast and mitochondria electron transport chains in the moss Physcomitrella patens.

In photosynthetic cells, chloroplast and mitochondria are key metabolism and energy supply players. The electron transport chains on these organelles are flexibly modulated in a dynamic environment owing to the existence of alternative mechanisms. Here, we get deeper insight into the alternative mechanisms in photosynthetic and respiratory electron transport chains and the interaction and function coordination between chloroplast and mitochondria. In this thesis, we aimed to: (1) Compare the ability of cyclic electron transport facilitated by PGRL1/PGR5 and NDH complex and pseudo-cyclic electron transport (PCET) mediated by the flavodiiron proteins to regulate the photosynthetic electron transport and carbon fixation in response to multiple light fluctuation regimes. (2) Explore the role of alternative oxidase located in the respiratory chain and the interaction between mitochondria and chloroplast bioenergetic metabolism. In Chapter I, we outline the several mechanisms of the thylakoid membrane and CO2 assimilation and how these processes regulate light harvesting efficiency and electron transport in dynamic light for photoprotection. We summarize recent advances in the respiratory alternative pathway, especially focusing on its role in stress responses and reductants and energy exchanges between plastids and mitochondria. We also briefly review the advantages of moss Physcomitrella patens as a model plant for research. In Chapter II, the wild type, flva KO mutant (depleted in pseudo-cyclic electron transport) or pgrl1/ndhm KO mutant (depleted in cyclic electron transport) in the moss Physcomitrium patens were exposed to light fluctuations of different intensities. Here, concluded that two mechanisms exhibit distinctive time kinetics and electron transport capacity. FLV modulated a fast increase in electron transport capacity, while CET activation was slower and showed a smaller impact on electron transport capacity after an increase in illumination. CET impact was stable, and FLV impact was transient and undetectable after 3 minutes from the light change. Furthermore, CET inactivation also impacted the CO2 assimilation rate after a light transition. Chapter III presents the comparison of physiology analysis of wild type, aox knockout and AOX overexpressing lines in multiple growth conditions. Here, we demonstrated AOX in Physcomitrium patens had a high electron transport capacity and high complementarity with the cytochrome pathway. The higher AOX capacity in overexpression line resulted in a detrimental effect on growth and CO2 assimilation rate. We studied photosynthesis and growth performances of these lines at respiratory chain inhibitors treatment and proposed that the operational respiratory chain is necessary for maintaining photosynthetic electron transport chain, especially for the ATPase activity regulation in the chloroplast. In conclusion, this thesis delves into the flexibility of stress response strategies in photosynthetic and respiratory electron transport chains and provides new insights and evidences into metabolic interactions between chloroplasts and mitochondria.