Heterologous Expression of Carboxysomes in Plants

A long-term strategy to enhance global crop photosynthesis and yield involves the introduction of cyanobacterial CO2-concentrating-mechanisms (CCMs) into plant chloroplasts. Cyanobacterial CCMs enable relatively rapid CO2-fixation by elevating intracellular inorganic carbon as bicarbonate, then concentrating it as CO2 around the enzyme Rubisco in specialized protein micro-compartments called carboxysomes. To date, chloroplastic expression of carboxysomes has been elusive, requiring coordinated expression of almost a dozen proteins. Here we successfully produce simplified carboxysomes, isometric with those of the source organism Cyanobium, within tobacco chloroplasts. We replace the endogenous Rubisco large subunit gene with cyanobacterial Form-1A Rubisco large and small subunit genes, along with genes for two key α-carboxysome structural proteins. This minimal gene set produces carboxysomes which encapsulate the introduced Rubisco and enable autotrophic growth at elevated CO2. This result demonstrates the formation of α-carboxysomes from a reduced gene set, informing the step-wise construction of fully-functional α-carboxysomes in chloroplasts.