A role for differential Rubisco activase isoform expression in C4 bioenergy grasses at high temperature

Rubisco activase (Rca) facilitates the release of sugar‐phosphate inhibitors at Rubisco catalytic sites during CO2 fixation. Most plant species express two Rca isoforms, the larger Rca‐α and the shorter Rca‐β, either by alternative splicing from a single gene or expression from separate genes. The mechanism of Rubisco activation by Rca isoforms has been intensively studied in C3 plants. However, the functional role of Rca in C4 plants where Rubisco and Rca are located in a much higher [CO2] compartment is less clear. In this study, we selected four C4 bioenergy grasses and the model C4 grass setaria (Setaria viridis) to investigate the role of Rca in C4 photosynthesis. All five C4 grass species contained two Rca genes, one encoding Rca‐α and the other Rca‐β, which were positioned closely together in the genomes. A variety of abiotic stress‐related motifs were identified in the Rca‐α promoter of each grass, and while the Rca‐β gene was constantly highly expressed at ambient temperature, Rca‐α isoforms were expressed only at high temperature but never surpassed 30% of Rca‐β content. The pattern of Rca‐α induction on transition to high temperature and reduction on return to ambient temperature was the same in all five C4 grasses. In sorghum (Sorghum bicolor), sugarcane (Saccharum officinarum), and setaria, the induction rate of Rca‐α was similar to the recovery rate of photosynthesis and Rubisco activation at high temperature. This association between Rca‐α isoform expression and maintenance of Rubisco activation at high temperature suggests that Rca‐α has a functional thermo‐protective role in carbon fixation in C4 grasses by sustaining Rubisco activation at high temperature. Activation of Rubisco, the enzyme responsible for carboxylation in photosynthesis, by Rubisco activase (Rca) has been well studied in C3 but not in C4 plants. We examined the role of Rca in four C4 bioenergy grasses and a model C4 grass under various conditions. Each species contained genes for both the large (Rca‐α) and short (Rca‐β) Rca forms, but while Rca‐β was highly expressed in all conditions tested, Rca‐α was only expressed at high temperature. Rca‐α induction upon transition to high temperature matched recovery of photosynthesis and Rubisco activation, suggesting Rca‐α provides a thermoprotective role in C4 grass photosynthesis.