Fig. 1: Benefits and examples of integrating natural and artificial Rubisco-containing CCM compartments into plant chloroplasts.

a The photosynthetic efficiency and productivity of C₃ crops (examples shown) is encumbered by Rubisco’s slow carboxylation rate, sub-saturating CO₂ affinity, and wasteful resource costs (ATP, NADPH consumption, CO₂ loss) from high photorespiratory flux. Asterisk (*) indicates crops amenable to plastome transformation. b Introducing a chloroplast CCM is envisaged to circumvent these deficiencies by introducing inorganic carbon (C_i) pumping systems to elevate stromal HCO₃⁻ levels, while housing Rubisco with carbonic anhydrase (CA) in a localising structure to elevate CO₂ supply. As seen in C₄-plants, a saturating CO₂ maximises Rubisco rate, limits oxygenic 2PG production and photorespiratory flux (indicated by thinner red dashed lines), all combining to improve the Calvin-Benson-Bassham (CBB) cycle flux and stimulate plant productivity (indicated by bolder black lines). Rubisco localising structures considered for integration into leaf chloroplasts include: (c) the multi-component pyrenoids from microalgae^12,13, and (d) carboxysomes from α- or β-cyanobacteria, which comprise a porous shell composed of several Cso and Ccm proteins, respectively¹¹. Rubisco in both systems is encased as phase-separated condensates by Rubisco-linking proteins. e This work examines the feasibility of developing Q. thermotolerans encapsulin as a genetically simple, Rubisco isoform-agnostic platform to develop CO₂-fixing compartments for downstream use in plants as a synthetic CCM.