The chloroplast NADH dehydrogenase-like complex influences the photosynthetic activity of the moss Physcomitrella patens

Alternative electron pathways contribute to regulation of photosynthetic light reactions to adjust to metabolic demands in dynamic environments. The chloroplast NADH dehydrogenase-like (NDH) complex mediates the cyclic electron transport pathway around photosystem I (PSI) in different cyanobacteria, algae and plant species, but it is not fully conserved in all photosynthetic organisms. In order to assess how this complex's physiological role changed during plant evolution, we isolated Physcomitrella patens lines knocked out of the NDHM gene that encodes for a subunit fundamental for the activity of the complex. ndhm knock-out (KO) mosses indicated high PSI acceptor side limitation upon abrupt changes in illumination. In P. patens, pseudo-cyclic electron transport mediated by Flavodiiron proteins (FLVs) was also shown to prevent PSI overreduction in plants exposed to light fluctuations. flva ndhm double KO mosses had altered photosynthetic performance and growth defects under fluctuating light compared to wild-type and single KO mutants. The results evidenced that while NDH contribution to electron transport is minor compared to FLV, NDH still participates in modulating photosynthetic activity, and it is critical to avoid PSI photoinhibition, especially when FLVs are inactive. The functional overlap between NDH- and FLV-dependent electron transport supports PSI activity and prevents its photoinhibition under light variations.