The chloroplast NADH dehydrogenase-like complex: evolutionary considerations

The chloroplast NADH dehydrogenase (NDH)-like complex facilitates the ferredoxin-dependent reduction of plastoquinone, coupled with proton translocation across the thylakoid membrane, thereby mediating cyclic electron transport around photosystem I (PSI). The NDH complex evolved from the cyanobacterial counterpart and it forms a large supercomplex with two copies of the PSI complex in angiosperms. In Arabidopsis, NDH-deficient mutants exhibit impaired oxidation of PSI during low-light phases under fluctuating light conditions. Despite its important physiological function clarified in angiosperms, the NDH complex has been lost in certain lineages of eukaryotic phototrophs, including some green and red algae, as well as specific gymnosperms. This loss is likely to be compensated by alternative regulatory mechanisms involving the PROTON GRADIENT REGULATION 5 protein or flavodiiron proteins (Flv). This review article provides an overview of the current knowledge on the evolution of the NDH-PSI supercomplex through the acquisition of new subunits. We also summarize the evolutional loss of the NDH complex, primarily focusing on monocotyledonous plants to extensively investigate the loss of the NDH complex in angiosperms, which had lost Flv genes early in their evolution. In monocots, loss of the NDH complex is relatively rare and occurred mostly in Orchidaceae (Asparagales) and among submerged aquatic plants in Alismatales. These findings support the idea that the NDH complex is crucial for maintaining optimal photosynthetic activity in terrestrial angiosperms exposed to harsh light environments.