Plant Physiol, October 2002, Vol. 130, pp. 876-886

Photosynthetic and Other Phosphoenolpyruvate Carboxylase Isoforms in the Single-Cell, Facultative C₄ System of Hydrilla verticillata¹

Department of Botany, 220 Bartram Hall, P.O. Box 118526, University of Florida, Gainesville, Florida 32611-8526

The submersed monocot Hydrilla verticillata (L.f.) Royle is a facultative C₄ plant. It typically exhibits C₃ photosynthetic characteristics, but exposure to low [CO₂] induces a C₄ system in which the C₄ and Calvin cycles co-exist in the same cell and the initial fixation in the light is catalyzed by phosphoenolpyruvate carboxylase (PEPC). Three full-length cDNAs encoding PEPC were isolated from H. verticillata, two from leaves and one from root. The sequences were 95% to 99% identical and shared a 75% to 85% similarity with other plant PEPCs. Transcript studies revealed that one isoform, Hvpepc4, was exclusively expressed in leaves during C₄ induction. This and enzyme kinetic data were consistent with it being the C₄ photosynthesis isoform. However, the C₄ signature serine of terrestrial plant C₄ isoforms was absent in this and the other H. verticillata sequences. Instead, alanine, typical of C₃ sequences, was present. Western analyses of C₃ and C₄ leaf extracts after anion-exchange chromatography showed similar dominant PEPC-specific bands at 110 kD. In phylogenetic analyses, the sequences grouped with C₃, non-graminaceous C₄, and Crassulacean acid metabolism PEPCs but not with the graminaceous C₄, and formed a clade with a gymnosperm, which is consistent with H. verticillata PEPC predating that of other C₄ angiosperms.