JXB Advance Access originally published online on March 28, 2008
Journal of Experimental Botany 2008 59(7):1789-1798; doi:10.1093/jxb/erm373
RESEARCH PAPER |
The effects of Rubisco activase on C4 photosynthesis and metabolism at high temperature
1ARC Centre for Excellence in Plant Energy Biology, Research School of Biological Sciences, Australian National University, Canberra, ACT 2601, Australia
2Molecular Plant Physiology Group, Research School of Biological Sciences, Australian National University, PO Box 475, Canberra, ACT 2601, Australia
3School of Biomedical, Biomolecular and Chemical Sciences, University of Western Australia, Crawley, WA 6009, Australia
* To whom correspondence should be addressed. E-mail: Susanne.Caemmerer{at}anu.edu.au
The activation of Rubisco in vivo requires the presence of the regulatory protein Rubisco activase. This enzyme facilitates the release of sugar phosphate inhibitors from Rubisco catalytic sites thereby influencing carbamylation. T1 progeny of transgenic Flaveria bidentis (a C4 dicot) containing genetically reduced levels of Rubisco activase were used to explore the role of the enzyme in C4 photosynthesis at high temperature. A range of T1 progeny was screened at 25 °C and 40 °C for Rubisco activase content, photosynthetic rate, Rubisco carbamylation, and photosynthetic metabolite pools. The small isoform of F. bidentis activase was expressed and purified from E. coli and used to quantify leaf activase content. In wild-type F. bidentis, the activase monomer content was 10.6±0.8 µmol m–2 (447±36 mg m–2) compared to a Rubisco site content of 14.2±0.8 µmol m–2. CO2 assimilation rates and Rubisco carbamylation declined at both 25 °C and 40 °C when the Rubisco activase content dropped below 3 µmol m–2 (125 mg m–2), with the status of Rubisco carbamylation at an activase content greater than this threshold value being 44±5% at 40 °C compared to 81±2% at 25 °C. When the CO2 assimilation rate was reduced, ribulose-1,5-bisphosphate and aspartate pools increased whereas 3-phosphoglycerate and phosphoenol pyruvate levels decreased, demonstrating an interconnectivity of the C3 and C4 metabolites pools. It is concluded that during short-term treatment at 40 °C, Rubisco activase content is not the only factor modulating Rubisco carbamylation during C4 photosynthesis.
Key words: C4 photosynthesis, Flaveria bidentis, high temperature, Rubisco, Rubsico activase
Received 11 October 2007; Revised 25 November 2007 Accepted 18 December 2007
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