Optimization of synthetic photorespiratory bypass in Physcomitrium patens

The Calvin-Benson cycle (CBC) uses chemical energy to assimilate CO2 driving the biosynthesis of organic molecules. Energy-consuming reactions, although indispensable, may compete with CBC thereby reducing the potential for growth increase. This thesis explores the balance between essential energy-consuming pathways and their potential as targets for genetic improvement, focusing primarily on the impact of flavodiiron proteins (FLV) and photorespiration on photosynthetic efficiency. FLV acts as an alternative electron sink during the dark-to-light transition, before CBC enzymes are fully activated. Their loss in angiosperms suggests that their energy demands may have negatively impacted overall plant efficiency. We overexpress FLV without any significant advantage in photosynthetic efficiency and growth improvement under fluctuating light. Differently, protein overaccumulation appeared deleterious rather than beneficial for plants exposed to continuous medium or high light. Interestingly once we expressed heterologous FLV in WT Nicotiana tabacum, it increased electron transport capacity at the onset of light activation while competing with the native cyclic electron flow. RuBisCO catalyzes CBC first step by assimilating CO2. It can act also as oxygenase, producing a toxic molecule known as 2-phosphoglycolate. Photorespiration is a highly conserved pathway dedicated to the detoxification of this oxygenase byproduct and the recycling of carbon. However, it is a costly process that reduces the conversion efficiency of light into biomass by 20 to 50% with a significant impact on crops yield. We investigated on photorespiration importance in the moss Physcomitrium patens finding that its strong entanglement with CBC make it impossible the estimation of the solo photorespiration to the total photosynthetic efficiency. We also tested the impact of the introduction of a more efficient pathway able to detoxify 2-phosphoglycolate more efficiently. Resulting P. patens plants appeared larger and with an increased electron transport capacity.