Repeated loss of plastid NDH during evolution of land plants

Advances in DNA sequencing technology have led to a rapid increase in the number of species with organelle genomes and even complete nuclear genomes being sequenced. Thousands of plastid genomes from across all major clades of land plants are now available, and one of the surprising findings is the recurring event of complete or functional loss of genes involved in cyclic electron transport during photosynthesis - the ndh genes that encode subunits of the chloroplast NADH dehydrogenase-like (NDH) complex. Gene loss in non-photosynthetic, heterotrophic plants may be expected, but the increasing number of losses being discovered in autotrophic plants questions the role and potential dispensability of the ndh genes and the entire NDH complex. With a focus on autotrophic plants, the present review compiles published evidence about the loss of both plastid and nuclear encoded NDH genes, providing an overview spanning all major clades of land plants. Current knowledge about the function of NDH and the possible reasons behind repeated loss is discussed. More than 100 independent events of plastid ndh gene loss have been reported from autotrophic land plants, and strong evidence exists that these losses go hand in hand with the loss of nuclear encoded NDH genes. While loss is almost inevitable in heterotrophic plants and common among carnivorous plants, it occurs in what appears to be random among normal, autotrophic plants. No single underlying reason for the events of loss can be discerned although a link to nutrient acquisition prevails. Even in autotrophic plants, the NDH complex may simply be dispensable due to the existence of an alternative and major pathway of cyclic electron transport around photosystem I.