Poster: Photosynthesis (carbon)

Abs # 230: Role of carbonic anhydrase (CA) isoforms in C₄ Hydrilla leaves

Presenter:	Rao, Vidya S., vidyarao@clas.ufl.edu
Authors	Rao, Vidya S. (A)   Rao, Srinath K. (A)   Reiskind, Julia B. (A)   Bowes, George  (A)
Affiliations:	(A): Department of Botany, University of Florida

Carbonic anhydrase (CA) plays a role in photosynthesis by catalyzing the equilibrium among the forms of dissolved inorganic carbon related to the extant pH. Its role in the Hydrilla system is unclear. Hydrilla, nominally a C₃ plant, has a facultative C₄ system with fixation events separated between the cytosol and chloroplast, a characteristic that makes it an ideal model for transforming C₃ crop species. In C₄ Hydrilla CA would be a key cytosolic element to supply HCO₃^- to PEPC, but its presence in the chloroplast stroma, where CO₂ is concentrated at pH 8.2, could be detrimental. This issue needs resolution to produce a transgenic C₄ rice plant, as C₃ rice chloroplasts presumably contain CA. In C₄ Hydrilla leaf extracts CA specific activity is higher than in C₃. The ratio between soluble and pellet-associated activities also differs, with 1:1 and 2:1 ratios in C₄ and C₃ leaves, respectively. Because of the facultative nature of Hydrilla’s photosynthesis, we postulate the presence of cytosolic and chloroplastic CA isoforms, the former with a role in the C₄ system. Employing RT-PCR and with the help of a degenerate primer pair designed from published plant CAs, a partial cDNA was isolated, cloned and sequenced. A database (Blastx) search with this sequence revealed close identities to the chloroplastic isoform in the C₃ plant, Nicotiana tabacum (80%) and the cytosolic isoform in the C₄ plant, Flaveria bidentis (74%) in addition to other plant CAs. Northern analysis indicated that this isoform was overexpressed in C₄ Hydrilla leaves. Isolation, cloning and characterization of the full-length of this and other isoforms are underway. Supported by USDA NRICGP 2002-35318-12540.