Characterisation of alternative electron transport and photosynthetic carbon assimilation during light–dark transitions in Physcomitrium patens and Nicotiana tabacum

Photosynthetic organisms regulate photosynthetic electron transport to balance biomass production with photoprotection under fluctuating environmental conditions. Alternative electron flow (AEF) around photosystem I (PSI) is important for PSI oxidation and photoprotection when downstream electron acceptors are limiting. AEF comprises cyclic electron flow (CEF) and pseudocyclic electron flow (PCEF), the latter mediated by flavodiiron proteins (FLVs) in non-angiosperms. However, the regulation of AEF during transitions into and out of darkness remain poorly understood. Here, activation kinetics of PCEF and CEF were investigated, as well as their relative contribution to photosynthetic electron flow. Physcomitrium patens is ideal for AEF research, possessing the complete set of proteins implicated in both CEF and PCEF pathways, including FLVs. Photosynthetic measurements of P. patens wild-type (WT) and mutants deficient in AEF components were compared. In AEF mutants, dark incubation disrupted electron flow which was comparatively stable during continuous illumination. Acceptor-side limitation of PSI, the fraction of PSI reaction centres limited by downstream electron acceptor availability, rapidly increased with dark incubation in AEF mutants whilst remaining low in WT. Similar responses to darkness were observed in transgenic lines of Nicotiana tabacum expressing moss FLVs, showing the relevance of a rapid modulation of AEF across different species.