Unique Rubisco Subunit Enhances Plant Carbon Uptake

A unique Rubisco subunit from the hornwort plant carries a built-in extension that enables the enzyme to cluster into carbon-concentrating structures like algae, according to a new study – an innovation that successfully triggered similar condensates when introduced into Arabidopsis plants. The findings reveal a novel and evolutionarily independent solution for Rubisco condensation in land plants and open the door for engineering similar systems in agricultural crops to potentially boost nutrient efficiency and yield. Rubisco, or ribulose-1,5-biphosphate carboxylase/oxygenase, is the central enzyme responsible for carbon fixation in photosynthesis, but it also reacts with oxygen, leading to energy-wasting photorespiration. Many algae reduce this inefficiency by concentrating CO₂ around Rubisco inside specialized microcompartments called pyrenoids, which can dramatically boost local CO₂ levels and improve photosynthetic efficiency. This adaptation has received increasing attention, given its potential to increase yields in crop plants. Rubisco clustering is typically driven by specialized linker proteins that have evolved independently across lineages. However, these linkers often bind Rubisco in species-specific ways, and none of the known linker proteins are directly compatible with plant Rubisco, limiting their introduction to crop plants. Here, Tanner Robison and colleagues report the discovery of a distinctive variant of the Rubisco small subunit (RbcS) from the hornwort plant Anthoceros agrestis that provides a new solution to this challenge. According to Robison et al., this variant carries an ~100-amino acid extension at its C-terminal, termed the Sequestration Associated Region (STAR). Unlike previously known algal systems, which rely on separate linker proteins that bind Rubisco externally, RbcS-STAR appears to embed the condensation mechanism directly into the enzyme itself. In experiments, the authors show that attaching the STAR domain to native Arabidopsis RbcS was sufficient to trigger the formation of Rubisco condensates within chloroplasts. Further biochemical and structural analyses further revealed the molecular interactions that drive the assembly of these condensates. In a related Perspective, Moritz Meyer and Howard Griffiths discuss the study in greater detail.