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.