In photosynthesis, final electron transfer from ferredoxin to NADP+ is accomplished by the flavo enzyme ferredoxin:NADP+ oxidoreductase (FNR). FNR is recruited to thylakoid membranes via integral membrane thylakoid rodanese-like protein TROL and soluble Tic62. TROL protein has recently been characterized by our group. FNR also represents a link between light-driven reactions and general metabolism (e.g. carbon fixation, nitrogen metabolism and fatty acid and chlorophyll biosynthesis). In this project, we address the fate of electrons downstream of photosystem I when TROL is absent. We will employ gene knock-out and knock-in approaches in the model plant Arabidopsis thaliana to study in vivo functions of TROL and its domains. We hypothesize that the interaction of FNR with TROL is dynamic, similar to FNR-Tic62 association, and in synchrony with changing environmental light conditions. Further, we propose that thylakoid lumen-exposed rhodanese domain of TROL is involved in perception of redox signals, while proline-rich swivel preceding FNR membrane attachment motif is responsible for the dynamic FNR biding and/or recruitment of FNR into different membrane domains. We will use elaborate biochemical and biophysical approaches to quantify free radical formation and electron flow and partitioning in various TROL mutants. We will aim to demonstrate that TROL-FNR branch point is integrated into plant stress responses. Finally, we want to demonstrate that the dynamic binding and release of FNR from TROL can control the flow of electrons prior to activation of the pseudo-cyclic electron transfer pathway. This mechanism may have been overlooked in present models of electron flow in oxygenic photosynthesis.