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Plant and Cell Physiology Advance Access published online on August 27, 2006
Plant and Cell Physiology, doi:10.1093/pcp/pcj091
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© The Author 2006. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received May 19, 2006
Accepted July 15, 2006

Regular Paper

"Light regulation of the photosynthetic phosphoenolpyruvate carboxylase (PEPC) in Hydrilla verticillata"

Srinath Rao 1, Julia Reiskind 1 *, and George Bowes 1

1 University of Florida - Botany 220 Bartram Hall, PO Box 118526, Gainesville, Florida 32611-8526 USA

* To whom correspondence should be addressed.
Julia Reiskind, E-mail: reiskind{at}botany.ufl.edu


  Abstract

The submersed monocot, Hydrilla verticillata (L.f.) Royle, is a facultative C4 NADP-ME plant in which the C4 and Calvin cycles co-exist in the same cell. Futile cycling is avoided by an intracellular separation of carboxylases between the cytosol and chloroplasts. Of the two sequenced H. verticillata phosphoenolpyruvate carboxylase (PEPC) isoforms, hvpepc3 and hvpepc4, transcript expression of the latter was substantially upregulated during C4 induction, especially in the light. Western blots revealed two PEPC specific bands in C3 and C4 leaf extracts; the lower band dominated in the C4 and underwent posttranslational phosphorylation in the light as determined by immunological studies. This band likely represents the photosynthetic isoform, HVPEPC4, despite the lack of the C4 signature serine (Flaveria residue 774; Hydrilla 779). In C4 leaves PEPC activity increased 14-fold, was enhanced by leaf exposure to light, and showed allosteric regulation. Glucose-6-phosphate acted as a positive effector, but malate was inhibitory with I50 values of 0.4 and 0.2 mM in the light and dark, respectively, similar to those of other C4 PEPC isoforms. By contrast, in C3 leaves transcript expression of both isoforms was weak with little evidence of diel regulation, and the PEPC proteins showed essentially no indication of phosphorylation. PEPC activity in C3 leaves was low, light independent, and followed Michaelis-Menten kinetics. It was tolerant to malate, with ten-fold higher I50 values than the PEPC from C4 leaves. These data suggest that hvpepc4 encodes the C4 photosynthetic PEPC, and hvpepc3 an anaplerotic form.

Keywords: C4 photosynthesis; Hydrilla verticillata; N-terminal phosphorylation; PEPC expression; PEPC kinetics; Phosphoenolpyruvate carboxylase (PEPC).
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