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

Sedoheptulose-1,7-bisphosphatase (SBPase) is one of the rate-limiting enzymes of the Calvin cycle, and increasing the abundance of SBPase in C₃ plants provides higher photosynthetic rates and stimulates 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 the Calvin cycle are localized to the bundle sheath cells. Here we tested what effect increasing abundance of SBPase would have on C₄ photosynthesis. Using green foxtail millet (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 localization of the protein. The abundance of ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit was not affected in transgenic plants overexpressing SBPase, and neither was leaf chlorophyll content or photosynthetic electron transport parameters. We found no association between SBPase content in S. viridis and saturating rates of CO₂ assimilation. Moreover, a detailed analysis of CO₂ assimilation rates at different CO₂ partial pressures, irradiances, and leaf temperatures showed no improvement of photosynthesis in plants overexpressing SBPase. We discuss the potential implications of these results for understanding the role of SBPase in regulation of C₄ photosynthesis.