Figure 1.

Plastid electron (e⁻) flow and indirect export of reducing equivalents. (A) Cyclic (red) and linear (blue) e⁻ flow/ transport (CEF and LEF, respectively) across the thylakoid membrane. Recycling e⁻ from reduced FD back to PQ by CEF allows for ATP production without NADPH formation. The pmf generated by LEF and CEF drives ATP synthesis at the ATP synthase complex. An excess of reduced PQH₂ can be avoided via PTOX activity (modified from Yamamoto and Shikanai, 2020). (B) Indirect export of reducing equivalents from the stroma to prevent PSI acceptor side limitation. The terminal PSI e⁻ acceptor NADP is resupplied to FNR through NADPH oxidation in the CBBC and by export of surplus reducing equivalents out of the chloroplast. Fatty acid synthesis in the light may provide another e⁻ sink. Reducing power is transferred to the cytosol via TPT in form of TP or through OMT as malate. 3-PGA can be reduced to TP by GapAB in the plastid and be regenerated in the cytosol by GapC or GapN. Reduction of oxaloacetate to malate is driven by plNAD-MDH or plNADP-MDH fueled by the respective redox pools. The CBBC interconnects the NAD(H) and NADP(H) redox pools. This is mediated by PGDH3. Stromal NADP⁺ synthesis relies on NAD⁺ import and phosphorylation by NADK2. 3-PGA, 3-Phosphoglyceric acid; ADP, adenosine diphosphate; ATP, adenosine triphosphate; CEF, cyclic electron flow; cy, cytosolic; Cyt b₆f, cytochrome b₆f; FAS, fatty acid synthase; FD, ferredoxin; FNR, ferredoxin:NADP(H) oxidoreductase; GapA/B, bispecific (NAD⁺/NADP⁺-dependent) glyceraldehyde 3-phosphate dehydrogenase; GapC, cytosolic glyceraldehyde 3-phosphate dehydrogenase; GapN, non-phosphorylating irreversible glyceraldehyde 3-phosphate dehydrogenase; H, hydrogen; LEF, linear electron flow; MDH, malate dehydrogenase; mt, mitochondrial; NAD(H), nicotinamide adenine dinucleotide; NADK2, nicotinamide adenine dinucleotide kinase 2; NADP(H), nicotinamide adenine dinucleotide phosphate; NDH, NADH dehydrogenase; NPQ, non-photochemical quenching; O₂, molecular oxygen; OAA, oxaloacetate; OMT, oxaloacetate malate transporter; PC, plastocyanin; PDCp, pyruvate dehydrogenase complex; pe, peroxisomal; PEP, 2-phosphoenolpyruvate; PGR5, proton gradient regulation 5; PGRL1, PGR5-like photosynthetic phenotype 1; Pi, inorganic phosphate; pl, plastid; pmf, proton motive force; PQ, plastoquinone; PQH₂, plastoquinol; PSI, photosystem I; PSII, photosystem II; PTOX, plastid terminal oxidase; TP, triose-phosphate; TPT, triose-phosphate/phosphate translocator (Created with BioRender.com).