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-alpha and the shorter Rca-beta, 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 C-3 plants. However, the functional role of Rca in C-4 plants where Rubisco and Rca are located in a much higher [CO2] compartment is less clear. In this study, we selected four C-4 bioenergy grasses and the model C-4 grass setaria (Setaria viridis) to investigate the role of Rca in C-4 photosynthesis. All five C-4 grass species contained two Rca genes, one encoding Rca-alpha and the other Rca-beta, which were positioned closely together in the genomes. A variety of abiotic stress-related motifs were identified in the Rca-alpha promoter of each grass, and while the Rca-beta gene was constantly highly expressed at ambient temperature, Rca-alpha isoforms were expressed only at high temperature but never surpassed 30% of Rca-beta content. The pattern of Rca-alpha induction on transition to high temperature and reduction on return to ambient temperature was the same in all five C-4 grasses. In sorghum (Sorghum bicolor), sugarcane (Saccharum officinarum), and setaria, the induction rate of Rca-alpha was similar to the recovery rate of photosynthesis and Rubisco activation at high temperature. This association between Rca-alpha isoform expression and maintenance of Rubisco activation at high temperature suggests that Rca-alpha has a functional thermo-protective role in carbon fixation in C-4 grasses by sustaining Rubisco activation at high temperature.