Alternative electron transport pathways in photosynthesis: a confluence of regulation

- Several photosynthetic alternative electron transport pathways were recently identified.
- Some occur at the level of the thylakoid membrane (NDH1, proton gradient regulation 5/like 1 (PGR5/PGRL1), plastid terminal oxidase (PTOX), KEA3).
- Some others take place in the chloroplast stroma (FLVs, Mehler, MDH).
- Some transport metabolites across organelles (oxoglutarate/malate transporter (OMT), triose-P translocator (TPT), maltose exporter (MEX), hexose-P translocator (GPT), plastidal glycolate glycerate translocator (PLGG)).
- These pathways are interconnected and regulated by environmental changes.

Photosynthetic reactions proceed along a linear electron transfer chain linking water oxidation at photosystem II (PSII) to CO2 reduction in the Calvin-Benson-Bassham cycle. Alternative pathways poise the electron carriers along the chain in response to changing light, temperature and CO2 inputs, under prolonged hydration stress and during development. We describe recent literature that reports the physiological functions of new molecular players. Such highlights include the flavodiiron proteins and their important role in the green lineage. The parsing of the proton-motive force between ΔpH and Δψ, regulated in many different ways (cyclic electron flow, ATPsynthase conductivity, ion/H+ transporters), is comprehensively reported. This review focuses on an integrated description of alternative electron transfer pathways and how they contribute to photosynthetic productivity in the context of plant fitness to the environment.