An unconventional Rubisco small subunit underpins the CO ₂ -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 <i>Arabidopsis</i>, 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.