Effects of reduced carbonic anhydrase activity on CO2 assimilation rates in Setaria viridis: a transgenic analysis.

Hannah L Osborn,Hugo Alonso-Cantabrana,Robert E Sharwood,Robert T Furbank,Sarah Covshoff,John R Evans,Susanne Von Caemmerer

doi:10.1093/jxb/erw357

Abstract

In C4 species, the major β-carbonic anhydrase (β-CA) localized in the mesophyll cytosol catalyses the hydration of CO2 to HCO3-, which phosphoenolpyruvate carboxylase uses in the first step of C4 photosynthesis. To address the role of CA in C4 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 CO2 partial pressure (pCO2). At low pCO2, a strong correlation between CO2 assimilation rates and CA hydration rates was observed. C18O16O isotope discrimination was used to estimate the mesophyll conductance to CO2 diffusion from the intercellular air space to the mesophyll cytosol (gm) in control plants, which allowed us to calculate CA activities in the mesophyll cytosol (Cm). This revealed a strong relationship between the initial slope of the response of the CO2 assimilation rate to cytosolic pCO2 (ACm) and cytosolic CA activity. However, the relationship between the initial slope of the response of CO2 assimilation to intercellular pCO2 (ACi) and cytosolic CA activity was curvilinear. This indicated that in S. viridis, mesophyll conductance may be a contributing limiting factor alongside CA activity to CO2 assimilation rates at low pCO2.

Highlights

C4 plants have evolved a CO2-concentrating mechanism (CCM) that enables the elevation of CO2 around the active sites of Rubisco by a combination of anatomical and biochemical specialization (Hatch, 1987)
In S. viridis we identified four β-carbonic anhydrase (β-carbonic anhydrase (CA)) genes: Si002140m.g, Si002669m.g, Si030616m.g, and Si003882m.g
CA activity was not rate limiting for C4 photosynthesis in S. viridis

Summary

Introduction

C4 plants have evolved a CO2-concentrating mechanism (CCM) that enables the elevation of CO2 around the active sites of Rubisco by a combination of anatomical and biochemical specialization (Hatch, 1987). The key carboxylases in C4 plants are localized to different cellular compartments. Phosphoenolpyruvate carboxylase (PEPC) is localized to the cytosol of mesophyll cells and Rubisco to the chloroplasts of bundle sheath cells. For the CCM to operate effectively, PEPC activity must exceed Rubisco activity to balance leakage of CO2 out of the bundle sheath compartment. This maintains a high bundle sheath CO2 level but prevents wasteful overcycling of the mesophyll CO2 ‘pump’ (von Caemmerer and Furbank, 2003). As PEPC utilizes HCO3− and not CO2, the first committed enzyme of the C4 pathway is carbonic anhydrase (CA) which

Methods

Results

Discussion

Conclusion