Inter-Organelle NAD Metabolism Underpinning Light Responsive NADP Dynamics in Plants

Upon illumination, photosystem I in chloroplasts catalyzes light-driven electron transport from plastocyanin to ferredoxin, followed by the reduction of NADP(+) to NADPH by ferredoxin: NADP+ reductase for CO2 fixation. At the beginning of photosynthesis, NADP(+) supply control is dominated by de novo NADP(+) synthesis rather than being recycled from the Calvin cycle. Importantly, ferredoxin distributes electrons to NADP(+) as well as to thioredoxins for light-dependent regulatory mechanisms, to cyclic electron flow for more adenosine triphosphate (ATP) production, and to several metabolites for reductive reactions. We previously demonstrated that the NADP(+) synthesis activity and the amount of the NADP pool size, namely the sum of NADP(+) and NADPH, varies depending on the light conditions and the ferredoxin-thioredoxin system. In addition, the regulatory mechanism of cytoplasmic NAD(+) supply is also involved in the chloroplastic NADP+ supply control because NAD(+) is an essential precursor for NADP(+) synthesis. In this mini-review, we summarize the most recent advances on our understanding of the regulatory mechanisms of NADP(+) production, focusing on the interactions, crosstalk, and co-regulation between chloroplasts and the cytoplasm at the level of NAD(+) metabolism and molecular transport.