Minisymposium 3: Global Change

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Abs # M0301: Molecular mechanisms of global isotope exchange: the role of carbonic anhydrase during C4 photosynthesis

Presenter:	Cousins, Asaph B.       Contact Presenter
Authors	Cousins, Asaph B. (A) (B)  Badger, Murray  (A) (B)  von Caemmerer, Susanne  (A)
Affiliations:	(A): The Australian National University (B): ARC Centre of Excellence in Plant Energy Biology

Isotope analysis of atmospheric CO₂, both ¹³CO₂ and the C¹⁸OO, is an important tool for monitoring changes in the global exchange of CO₂. However, to interpret the atmospheric CO₂ isotopic signature requires an understanding of the fractionation steps associated with specific processes that occur during leaf gas exchange. We use antisense technology coupled with online stable isotope analysis to study how changes in carbonic anhydrase (CA) influence the exchange of carbon and oxygen between C4-plants and their environment. The CO₂ concentrating mechanism in C4 plants is initiated in the mesophyll cells by CA, which catalyzes the reversible hydration reaction of CO₂ to bicarbonate (HCO₃^-). The rate of CA turnover is extremely rapid and it is generally assumed in C4 plants that the chemical and isotopic equilibrium of ¹⁸O and ¹³C between CO₂, H₂O and HCO₃^- is nearly complete. However, there is a large range of CA activity in C4 plants and the assumption of isotopic equilibrium is relatively untested. The isotope models predict that if the isotopic equilibrium is not complete then ¹³C discrimination will increase and discrimination against ¹⁸O will decrease. In Flaveria bidentis plants expressing various levels of CA, plants with 20% or greater CA activity as compared to wild-type, had normal photosynthetic rates and p_i/p_a but showed significant changes in C¹⁸OO and ¹³CO₂ isotope discrimination. CA activity plays an important role in determining ¹³CO₂ and the C¹⁸OO isotope discrimination in C4 plants and its influence on leaf level and global models of C4 photosynthesis need to be considered.