Hornworts reveal a spatial model for pyrenoid-based CO-concentrating mechanisms in land plants.

Pyrenoid-based CO-concentrating mechanisms (pCCMs) turbocharge photosynthesis by saturating CO around Rubisco. Hornworts are the only land plants with a pCCM. Owing to their closer relationship to crops, hornworts could offer greater translational potential than the green alga Chlamydomonas, the traditional model for studying pCCMs. Here we report a thorough investigation of a hornwort pCCM using the emerging model Anthoceros agrestis. The pyrenoids in A. agrestis exhibit liquid-like properties similar to those in Chlamydomonas, but they differ by lacking starch sheaths and being enclosed by multiple thylakoids. We found that the core pCCM components in Chlamydomonas, including BST, LCIB and CAH3, are conserved in A. agrestis and probably have similar functions on the basis of their subcellular localizations. The underlying chassis for concentrating CO might therefore be shared between hornworts and Chlamydomonas, and ancestral to land plants. Our study presents a spatial model for a pCCM in a land plant, paving the way for future biochemical and genetic investigations.