Progress and challenges of engineering a biophysical CO ₂ -concentrating mechanism into higher plants

Growth and productivity in important crop plants is limited by the inefficiencies of the C ₃ photosynthetic pathway. Introducing CO ₂ -concentrating mechanisms (CCMs) into C ₃ plants could overcome these limitations and lead to increased yields. Many unicellular microautotrophs, such as cyanobacteria and green algae, possess highly efficient biophysical CCMs that increase CO ₂ concentrations around the primary carboxylase enzyme, Rubisco, to enhance CO ₂ assimilation rates. Algal and cyanobacterial CCMs utilize distinct molecular components, but share several functional commonalities. Here we outline the recent progress and current challenges of engineering biophysical CCMs into C ₃ plants. We review the predicted requirements for a functional biophysical CCM based on current knowledge of cyanobacterial and algal CCMs, the molecular engineering tools and research pipelines required to translate our theoretical knowledge into practice, and the current challenges to achieving these goals.