Effects of reduced carbonic anhydrase activity on CO(2) assimilation rates in Setaria viridis: a transgenic analysis

In C(4) species, the major β-carbonic anhydrase (β-CA) localized in the mesophyll cytosol catalyses the hydration of CO(2) to HCO(3)(−), which phosphoenolpyruvate carboxylase uses in the first step of C(4) photosynthesis. To address the role of CA in C(4) photosynthesis, we generated transgenic Setaria viridis depleted in β-CA. Independent lines were identified with as little as 13% of wild-type CA. No photosynthetic defect was observed in the transformed lines at ambient CO(2) partial pressure (pCO(2)). At low pCO(2), a strong correlation between CO(2) assimilation rates and CA hydration rates was observed. C(18)O(16)O isotope discrimination was used to estimate the mesophyll conductance to CO(2) diffusion from the intercellular air space to the mesophyll cytosol (g(m)) in control plants, which allowed us to calculate CA activities in the mesophyll cytosol (C(m)). This revealed a strong relationship between the initial slope of the response of the CO(2) assimilation rate to cytosolic pCO(2) (AC(m)) and cytosolic CA activity. However, the relationship between the initial slope of the response of CO(2) assimilation to intercellular pCO(2) (AC(i)) and cytosolic CA activity was curvilinear. This indicated that in S. viridis, mesophyll conductance may be a contributing limiting factor alongside CA activity to CO(2) assimilation rates at low pCO(2).