Abstract
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.
Highlights
A long-term strategy to enhance global crop photosynthesis and yield involves the introduction of cyanobacterial CO2-concentrating mechanisms (CCMs) into plant chloroplasts
The structures reported here mimic the gross structure of carboxysomes from Cyanobium but lack specific components expected to be required for full functionality in an operating CCM
12.3 nm Simplified carboxysomes in CyLS-S1S2 plants resembled those from Cyanobium but with predictable differences
Summary
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, concentrating it as CO2 around the enzyme Rubisco in specialized protein micro-compartments called carboxysomes. The cyanobacterial CCM is a single-cell, bipartite system that first generates a high intracellular bicarbonate (HCO3−) pool through action of membrane-bound inorganic carbon (Ci) transporters and CO2-converting complexes[7,8,9] (Fig. 1a). This HCO3− pool is utilized by subcellular micro-compartments called carboxysomes, which encapsulate the cell’s complement of.
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