摘要： Photosynthetic organisms support cell metabolism by harvesting sunlight and driving the electron transport chain at the level of thylakoid membranes. Excitation energy and electron flow in the photosynthetic apparatus is continuously modulated in response to dynamic environmental conditions. Alternative electron flow around photosystem I plays a seminal role in this regulation contributing to photo-protection by mitigating over-reduction of the electron carriers. Different pathways of alternative electron flow coexist in the moss Physcomitrella patens, including cyclic electron flow mediated by the PGRL1/PGR5 complex and pseudo-cyclic electron flow mediated by the flavodiiron proteins FLV. In this work we generated P. patens plants carrying both pgrl1 and flva knock-out (KO) mutations. A comparative analysis of the WT, pgrl1, flva and pgrl1 flva lines suggests that cyclic and pseudo-cyclic processes have a synergic role in the regulation of photosynthetic electron transport. However, while both contribute to photosystem I protection from over-reduction by modulating electron flow following changes in environmental conditions, FLV activity is particularly relevant in the first seconds after a light change while PGRL1 has a major role upon sustained strong illumination.