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BIOENERGETICS AND PHOTOSYNTHESIS

Carbonic Anhydrase and Its Influence on Carbon Isotope Discrimination during C₄ Photosynthesis. Insights from Antisense RNA in Flaveria bidentis¹

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.

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.