First published online March 16, 2006; 10.1104/pp.106.077776
Plant Physiology 141:232-242 (2006)
© 2006 American Society of Plant Biologists
BIOENERGETICS AND PHOTOSYNTHESIS
Carbonic Anhydrase and Its Influence on Carbon Isotope Discrimination during C4 Photosynthesis. Insights from Antisense RNA in Flaveria bidentis1
Asaph B. Cousins*,
Murray R. Badger and
Susanne von Caemmerer
Molecular Plant Physiology Group (A.B.C., M.R.B., S.V.C.) and Australian Research Council Centre of Excellence in Plant Energy Biology (M.R.B.), Research School of Biological Sciences, Australian National University, Canberra, Australian Capital Territory 2601, Australia
In C4 plants, carbonic anhydrase (CA) facilitates both the chemical and isotopic equilibration of atmospheric CO2 and bicarbonate (HCO3–) in the mesophyll cytoplasm. The CA-catalyzed reaction is essential for C4 photosynthesis, and the model of carbon isotope discrimination ( 13C) in C4 plants predicts that changes in CA activity will influence 13C. However, experimentally, the influence of CA on 13C has not been demonstrated in C4 plants. Here, we compared measurements of 13C during C4 photosynthesis in Flaveria bidentis wild-type plants with F. bidentis plants with reduced levels of CA due to the expression of antisense constructs targeted to a putative mesophyll cytosolic CA. Plants with reduced CA activity had greater 13C, which was also evident in the leaf dry matter carbon isotope composition ( 13C). Contrary to the isotope measurements, photosynthetic rates were not affected until CA activity was less than 20% of wild type. Measurements of 13C, 13C of leaf dry matter, and rates of net CO2 assimilation were all dramatically altered when CA activity was less than 5% of wild type. CA activity in wild-type F. bidentis is sufficient to maintain net CO2 assimilation; however, reducing leaf CA activity has a relatively large influence on 13C, often without changes in net CO2 assimilation. Our data indicate that the extent of CA activity in C4 leaves needs to be taken into account when using 13C and/or 13C to model the response of C4 photosynthesis to changing environmental conditions.
1 This work was supported by a National Science Foundation international postdoctoral fellowship (to A.B.C.).
The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Susanne von Caemmerer (susanne.caemmerer{at}anu.edu.au).
Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.106.077776.
* Corresponding author; e-mail asaph.cousins{at}anu.edu.au; fax 61–2–61255075.
Received January 25, 2006;
returned for revision March 12, 2006;
accepted March 13, 2006.
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