A role for differential Rubisco activase isoform expression in C<sub>4</sub> bioenergy grasses at high temperature
Sang Yeol Kim, Rebecca Slattery, Donald R. Ort
Abstract
Abstract Rubisco activase (Rca) facilitates the release of sugar‐phosphate inhibitors at Rubisco catalytic sites during CO 2 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 C 3 plants. However, the functional role of Rca in C 4 plants where Rubisco and Rca are located in a much higher [CO 2 ] 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‐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 C 4 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 C 4 grasses by sustaining Rubisco activation at high temperature.