Pyrenoids of hornworts: its mysterious evolutionary origin

Abstract

Cyanobacteria and many eukaryotic algae have a carbon-concentrating mechanism (CCM) within each cell or individual chloroplast to enable efficient photosynthesis even in CO₂-deficient environments. The ‘pyrenoids’, a chloroplast internal compartment responsible for CCM in eukaryotic algae, are absent in chloroplasts of land plants, and CCM at the cellular level appears to have been lost during the evolution of land plants. However, only hornworts have pyrenoids in their chloroplasts and are believed to possess CCM. This review summarizes the current status of research on the characteristics and functions of pyrenoids in hornworts and their evolutionary origin, and discusses why hornworts are the only land plants that possess pyrenoids.