Flavodiiron Protein Activity Outcompetes Cyclic Electron Transport When Expressed in Angiosperm Nicotiana tabacum.

In conditions of excess illumination, alternative electron transport pathways in the thylakoid membranes protect the photosynthetic apparatus against damage from eventual over-reduction. Two main pathways downstream of photosystem I (PSI) enable alternative electron flow, mitigating PSI acceptor-side limitation, while contributing to ATP biosynthesis without reducing NADP to NADPH: cyclic electron transport (CET) and pseudo-cyclic electron transport (PCET). Flavodiiron proteins (FLV) are crucial enzymes in PCET, found in all photosynthetic organisms but lost during the evolution of angiosperms. The absence of FLV coding sequences in angiosperm genomes raises intriguing questions about their role and function in photosynthetic organisms. Previous studies utilizing heterologous expression have already demonstrated that FLV can indeed function in angiosperms. In this study, Physcomitrium patens FLVA and FLVB coding sequences were stably expressed in wild-type Nicotiana tabacum, a model crop species. Transgenic lines exhibited significantly increased PCET rates, with FLV-dependent electron transport competing for electrons with CET, particularly under sudden increases in light intensity that limited acceptor-side limitation. These findings indicate that heterologously expressed FLVs are not only active but can also play a critical role in protecting from over-reduction the photosynthetic apparatus of Nicotiana tabacum under fluctuating light conditions.