An unconventional Rubisco small subunit underpins the CO2-concentrating organelle in land plants.

In many algae, photosynthesis is boosted by biophysical CO2-concentrating mechanisms, which pack the CO2-fixing enzyme Rubisco into liquid-like organelles called pyrenoids. Engineering C3 crops with algal pyrenoids could increase yields, but progress is hampered by the deep evolutionary divide between crops and algae. Here, we show that hornworts, the only land plants with pyrenoids, have an unusual Rubisco small subunit, RbcS-STAR, the C-terminal coiled-coil domain of which oligomerizes to mediate Rubisco condensation. RbcS-STAR is incorporated into the Rubisco holoenzyme and thus acts as an "innate" Rubisco linker. Expression of RbcS-STAR in pyrenoid-lacking species, including Arabidopsis, is sufficient to induce pyrenoid-like condensates. Our findings reveal a mechanism for Rubisco condensation and a potential route to introduce pyrenoids into crop plants.