Cyclic Electron Flow Mechanisms in Photosynthetic Systems

Cyclic electron flow (CEF) around photosystem I shuttles electrons from ferredoxin back to the plastoquinone pool, driving proton translocation across the thylakoid membrane without concomitant NADPH production. Two principal routes have been characterised: one dependent on the PROTON GRADIENT REGULATION 5 protein and one mediated by the chloroplast NADH dehydrogenase-like (NDH) complex. By adjusting the ATP/NADPH output ratio, CEF fulfils key roles in balancing energy supply with metabolic demand, sustaining CO₂ assimilation under low-light or stress conditions, and protecting photosystems from photoinhibition via enhanced non-photochemical quenching. Structural breakthroughs have uncovered supercomplex assemblies between NDH and photosystem I that stabilise electron transfer under high irradiance, while studies in cyanobacteria and green algae reveal complementary electron sinks such as flavodiiron-protein-driven Mehler-like reactions. Together, these pathways form a dynamic network that underpins photosynthetic acclimation, informs crop engineering for improved yield under fluctuating environments, and inspires bioengineering approaches for solar energy conversion.

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Cyclic Electron Flow Mechanisms in Photosynthetic Systems publication trend

The graph below shows the total number of articles in cyclic electron flow mechanisms in photosynthetic systems across all publications each year (not limited to Nature Index journals).

Technical terms

Cyclic electron flow (CEF): Pathway routing electrons from photosystem I back to the plastoquinone pool to generate a proton gradient without NADPH synthesis.

NADH dehydrogenase-like (NDH) complex: Multi-subunit chloroplast enzyme mediating one CEF route by transferring electrons from ferredoxin to plastoquinone.

PROTON GRADIENT REGULATION 5 (PGR5): Thylakoid membrane protein essential for one CEF pathway and for establishing the ΔpH that drives photoprotection.

Cytochrome b₆f complex: Thylakoid membrane protein assembly linking photosystem II and photosystem I, facilitating electron transfer and proton pumping via the Q-cycle.

Mehler reaction: Photoreduction of oxygen at photosystem I, producing reactive oxygen species that act as an alternative electron sink and contribute to ΔpH formation.

References

1. Cryo-EM structure of the NDH–PSI–LHCI supercomplex from Spinacia oleracea. Nature Structural & Molecular Biology (2025).
2. Photosystem I cyclic electron flow via chloroplast NADH dehydrogenase-like complex performs a physiological role for photosynthesis at low light. Scientific Reports (2015).
3. Cyanobacterial Oxygenic Photosynthesis is Protected by Flavodiiron Proteins. Life (2015).
4. Characterization of PetM cytochrome b6f subunit 7 domain-containing protein in tomato. Horticulture Research (2023).
5. How Does Cyclic Electron Flow Alleviate Photoinhibition in Arabidopsis?. Plant Physiology (2008).
6. Combined Increases in Mitochondrial Cooperation and Oxygen Photoreduction Compensate for Deficiency in Cyclic Electron Flow in Chlamydomonas reinhardtii. The Plant Cell (2014).