Red Rubiscos and opportunities for engineering green plants.

Nature's vital, but notoriously inefficient, CO2-fixing enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) often limits the growth of photosynthetic organisms including crop species. Form I Rubiscos comprise eight catalytic large- and eight auxiliary small-subunits and can be classified into two distinct lineages - "red" and "green". While red-type Rubiscos (Form IC and ID) are found in Rhodophytes, their secondary symbionts and certain proteobacteria, green-type Rubiscos (Form IA and IB) exist in terrestrial plants, Chlorophytes, cyanobacteria and other proteobacteria. Eukaryotic red-type Rubiscos exhibit desirable kinetic properties, namely high specificity and high catalytic efficiency, with certain isoforms outperforming green-type Rubiscos. However, it is not yet possible to functionally express a high-performing red-type Rubisco in chloroplasts to boost photosynthetic carbon assimilation in green plants. Understanding the molecular and evolutionary basis for divergence between red- and green-type Rubiscos could help us to harness the superior CO2-fixing power of red-type Rubiscos. Here we review our current understanding about red-type Rubisco distribution, biogenesis and sequence-structure and present opportunities and challenges to utilising red-type Rubisco kinetics towards crop improvements.