Recording true oxygen reduction capacity during photosynthetic electron transfer in Arabidopsis thylakoids and intact leaves

Reactive oxygen species (ROS) are generated in electron transport processes of living organisms in oxygenic environments. Chloroplasts are plant bioenergetics hubs where imbalances between photosynthetic inputs and outputs drive ROS generation upon changing environmental conditions. Plants have harnessed various site-specific thylakoid membrane ROS products into environmental sensory signals. Our current understanding of ROS production in thylakoids suggests that oxygen (O2) reduction takes place at numerous components of the photosynthetic electron transfer chain (PETC). To refine models of site- specific O2 reduction capacity of various PETC components in isolated thylakoids, the stoichiometry of oxygen production and consumption reactions, associated with H2O2 accumulation, was quantified using membrane inlet mass spectrometry and specific inhibitors. Combined with P700 spectroscopy and electron paramagnetic resonance spin trapping, we demonstrate that electron flow to PSI is essential for H2O2 accumulation during light-induced photosynthetic electron transport process. Further leaf disc measurements provided clues that H2O2 from PETC has a potential of increasing mitochondrial respiration and CO2 release. One sentence summaryPhotosynthetically derived H2O2 only accumulates at Photosystem I and may trigger cooperation with mitochondria during stress