Under illumination, the photosynthetic light reactions are the main source of NADPH in the chloroplasts. During this process, electrons are transferred from water molecules to reduce NADP+to NADPH via FNR, and NADPH is mainly consumed by the Calvin cycle. Malate accumulating in chloroplasts can be converted to pyruvate and provides NADPH via NADP-ME4. Pyruvate is then oxidized to acetyl-CoA and produces NADH in the light. NADH produced during this reaction can be consumed by clNAD-MDH during the conversion of OAA to malate. In the light, this step can also be catalyzed by clNADP-MDH. Excess reducing equivalents are exported from chloroplasts to the cytosol in the form of malate and triose-P; the latter is mainly used for sucrose biosynthesis. Malate can also be a source of cytosolic NADPH and NADH via the actions of NADP-ME13 and cyNAD-MDH, respectively. np-GAPDH and cytosolic OPPP can also supply cytosolic NADPH. The conversion of PEP into OAA in the cytosol is light dependent, offering an important source of cytosolic OAA during the day. Cytosolic OAA enters the mitochondria and is converted into malate by mtNAD-MDH. This reaction and the mETC consume large amounts of mitochondrial NADH provided by the glycine decarboxylation step of photorespiration via GDC. The excess NADH can also be consumed by NDA1-2 (A1-2), with NDA1 (A1) being significantly induced in the light. During the daytime, the tricarboxylic acid (TCA) cycle is not actually a cycle, as the activities of mtPDC, citrate synthase, NAD-ICDH, and OGDH are suppressed. Surplus mitochondrial NADH can be exported to the cytosol in the form of malate. Some cytosolic malate is imported into peroxisomes to generate NADH via pNAD-MDH and is then consumed in the hydroxypyruvate reduction step of photorespiration via HPR1. The remaining surplus cytosolic malate and synthesized sucrose are stored in the vacuole. Citrate accumulating in the vacuole at night can be released during the day via conversion to isocitrate in the cytosol and mitochondria by cyNADP-ICDH and cmNADP-ICDH, respectively, offering a supply of NADPH. Cytosolic isocitrate is transported to peroxisomes and can supply NADPH via pNADP-ICDH. In the light, cytosolic nitrogen assimilation is the major cytosolic NADH sink. Under stress conditions, the NADPH oxidase RboH can consume cytosolic NADPH. TrxRs also consume NADPH in the cytosol (NTRA), chloroplasts (NTRC), and mitochondria (NTRB). The AsAGSH cycle can consume both NADH and NADPH in the cytosol, chloroplasts, peroxisomes, and mitochondria regardless of light conditions. In addition, NADPH in the matrix can be consumed by NDC1 (C1); cytosolic NADPH and NADH can be consumed by NDB1 (B1) and NDB2-4 (B2-4), respectively. DHAP, dihydroxyacetone phosphate; GDC, glycine decarboxylase; HPR1, hydroxypyruvate reductase 1; PEP, phosphoenolpyruvate; plPDC, plastidic pyruvate dehydrogenase complex; PSI, photosystem I; PSII, photosystem II.
