Introducing Transformative Plant Biotechnology: Boosting photosynthesis: engineering a pyrenoid-based CO2-concentrating mechanism into plants

The CO2-fixing enzyme Rubisco is considered a rate-limiting step in plant productivity due to its slow carboxylation rate and wasteful side reaction with O2. Improving the efficiency of Rubisco in crops would represent a transformative step-change in Plant Biotechnology. Green algae such as Chlamydomonas reinhardtii can concentrate CO2 around Rubisco in a structure called a pyrenoid, which significantly increases the efficiency of Rubisco carboxylation and reduces its oxygenase reaction. We aim to engineer a pyrenoid-based CO2 concentrating mechanism (pCCM) into plants to increase the performance of Rubisco and hence crop productivity and resilience. The pCCM in Chlamydomonas relies on several features to function, including the condensation of Rubisco into a phase-separated matrix, a network of thylakoid tubules that traverse the matrix, delivery of CO2 to Rubisco, and a diffusion barrier in the form of a starch sheath, which prevents escape of CO2. Previously we have engineered a pyrenoid-like Rubisco matrix in Arabidopsis. Here we show that introducing the Chlamydomonas proteins MITH1 and MITH2 allows thylakoid traversions to form across the matrix. In addition, the expression of the carbohydrate-binding proteins SAGA1 and SAGA2 leads to the recruitment of plate-like starch granules around the matrix. These findings take us a major step forward towards reconstituting a functional pCCM in plants, which when implemented in crops could help the challenge of increasing crop yields to ameliorate global food security concerns.