Hornwort pyrenoids: a terrestrial exception with engineering lessons

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) underpins nearly all primary production yet remains a slow, error-prone enzyme because it catalyzes both carboxylation and oxygenation, the latter initiating photorespiration and reducing net carbon gain. Many organisms mitigate these limitations not by improving Rubisco selectivity for CO2 directly, but by modifying its local environment using CO₂-concentrating mechanisms (CCMs). In algae, a prominent biophysical CCM strategy is the pyrenoid: a phase-separated, Rubisco-rich condensate coupled to bicarbonate transport, local carbonic anhydrase activity, and diffusion barriers that elevate CO₂ at Rubisco active sites. Although pyrenoids have been most intensively studied in algal models, a pyrenoid-based CCM has evolved independently in a single land-plant lineage-the hornworts-providing a powerful comparative system for understanding how chloroplast organization can be tuned to terrestrial CO₂-delivery constraints. Here we synthesize a century of hornwort pyrenoid research in ecological, phylogenetic, and mechanistic context. We summarize bryophyte anatomical and microhabitat features that impose strong CO₂ diffusion limitation, and compare hornwort and algal pyrenoids in ultrastructure, molecular parts lists, and regulation. We highlight emerging models for hornwort pyrenoid formation, inorganic-carbon delivery, CO₂ generation and recapture, and recent biochemical/structural work revealing distinctive hornwort Rubisco properties and biogenesis. Finally, we discuss how hornwort pyrenoids complement efforts to engineer algal pyrenoid components into C₃ crops, and propose modular, hybrid engineering strategies that leverage hornwort compatibility with the embryophyte chloroplast, while selectively importing algal modules. Together, hornwort pyrenoids illustrate both the convergent logic and the lineage-specific solutions of biophysical CO₂ concentration, and they open avenues for mechanistic discovery and photosynthesis engineering.