Increased sedoheptulose-1,7-bisphosphatase content in the C4 species Setaria viridis does not affect photosynthesis

Sedoheptulose-1,7-bisphosphatase (SBPase) is one of the rate-limiting enzymes of the Calvin cycle, and, in C3 plants, increasing the abundance of SBPase is known to provide higher photosynthetic rates and stimulate biomass and yield. C4 plants usually have higher photosynthetic rates because they operate a biochemical CO2 concentrating mechanism between mesophyll and bundle sheath cells. In the C4 system, SBPase and other enzymes of Calvin cycle are localised to the bundle sheath cells. Here we tested what effect increasing abundance of SBPase would have on C4 photosynthesis. Using Setaria viridis, a model C4 plant of NADP-ME subtype, we created transgenic plants with 1.5 to 3.2-times higher SBPase content, compared to wild type plants. Transcripts of the transgene were found predominantly in the bundle sheaths suggesting the correct cellular localisation of the protein. Abundance of RBCL, the large subunit of Rubisco, was not affected in transgenic plants overexpressing SBPase, and neither was relative chlorophyll content or photosynthetic electron transport parameters. We found no correlation between SBPase content in S. viridis and saturating rates of CO2 assimilation. Moreover, detailed analysis of CO2 assimilation rates at different CO2 partial pressure, irradiance and leaf temperature, showed no improvement of photosynthesis in plants overexpressing SBPase. We discuss potential implications of these results for understanding the regulation of C4 photosynthesis.