Surveying the C3-C4 landscape for positive selection in the Calvin Benson Cycle enzyme SBPase

The Calvin-Benson cycle is the primary biochemical pathway for fixation of atmospheric carbon dioxide (CO2) in over 85% of plants, including most major crop species. The cycle has three main phases: the initial carboxylation of RuBP by Rubisco, the reductive phase, and the regeneration of RuBP. As atmospheric concentrations of CO2 increase, it is expected that control over carbon flux in the Calvin-Benson cycle will shift towards the rate of RuBP regeneration, making this a key target for future-proofing improvements to photosynthetic efficiency. The enzyme SBPase plays a crucial role within the regeneration phase of the cycle, and previous studies have shown that overexpression and increased activity of SBPase translates to yield gains in many plants including major crop species. However, the natural variation of this enzyme has been relatively unexplored. If an increase in CO2 concentrations were to impose selection pressure to improve the rate of RuBP regeneration, then variations in SBPase activity may be observed between C4 species, with their carbon-concentrating mechanism, and their C3 ancestors. The Flaveria (Asteraceae) genus includes C3, C3-4 intermediates and C4 species which provides a convenient model to assess selection pressure along a C3-4 gradient. The purpose of this study is to determine whether there is positive selection pressure across a range of photosynthetic organisms but particularly the Flaveria genus to search for adaptive evolution in the transition from C3 to C4 photosynthesis. Using a bioinformatics approach, the ratio of synonymous/non-synonymous residues has been assessed in 116 SBPase coding sequences to search for instances of positive selection. These residues may confer a higher performance of SBPase, providing targets for precise transgenic manipulation of SBPase. The characteristics of SBPase enzymes in the Flaveria genus should also be assessed to determine if there is a link between variants under selection pressure and enzyme activity.