Abstract
Udotea flabellum is a marine, macroscopic green alga with C4-like photosynthetic characteristics, including little O2 inhibition of photosynthesis, a low CO2 compensation point, and minimal photorespiration; but it lacks anatomical features analogous to the Kranz compartmentation of C4 plants, and phosphoenolpyruvate carboxylase [PEPC; orthophosphate:oxaloacetate carboxy-lyase (phosphorylating), EC 4.1.1.31] activity is negligible. Phosphoenolpyruvate carboxykinase (PEPCK) activity (carboxylating) in Udotea extracts was equivalent to that of ribulose-bisphosphate carboxylase [Rubisco; 3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39]. When PEPCK activity was inhibited in vivo with 3-mercaptopicolinic acid (MPA), thallus photosynthesis decreased by 70% and became sensitive to O2. Codium decorticatum, a related species that lacks C4-like photosynthetic features and PEPCK activity, showed no increase in O2 inhibition upon exposure to MPA. Rubisco and PEPC activities in Udotea were not inhibited by MPA. Labeling of the early photosynthetic products malate and aspartate was reduced 66% by MPA, while intermediates of the photorespiratory carbon oxidation cycle showed a 3-fold increase. Udotea evolved O2 in the light in the absence of inorganic carbon, suggesting it had an endogenous carbon source for photosynthesis. Exogenous malate stimulated this process, while MPA inhibited it. PEPCK was not involved in Crassulacean acid metabolism or dark CO2 fixation. These MPA studies establish a direct link between PEPCK activity and the low O2 inhibition of photosynthesis and low photorespiration in Udotea. The data are consistent with carboxylation by a cytosolic PEPCK providing a C4 acid, such as malate, to the chloroplast for decarboxylation to elevate the CO2 concentration at the Rubisco fixation site. Udotea is to date the most primitive plant with a C4-like form of photosynthesis.
Full text
PDF
Images in this article
Selected References
These references are in PubMed. This may not be the complete list of references from this article.
- Holdsworth E. S., Bruck K. Enzymes concerned with beta-carboxylation in marine phytoplankter. Purification and properties of phosphoenolpyruvate carboxykinase. Arch Biochem Biophys. 1977 Jul;182(1):87–94. doi: 10.1016/0003-9861(77)90286-7. [DOI] [PubMed] [Google Scholar]
- Jomain-Baum M., Schramm V. L., Hanson R. W. Mechanism of 3-mercaptopicolinic acid inhibition of hepatic phosphoenolpyruvate carboxykinase (GTP). J Biol Chem. 1976 Jan 10;251(1):37–44. [PubMed] [Google Scholar]
- Lloyd G. M., Barrett J. Fasciola hepatica: inhibition of phosphoenolpyruvate carboxykinase, and end-product formation by quinolinic acid and 3-mercaptopicolinic acid. Exp Parasitol. 1983 Oct;56(2):259–265. doi: 10.1016/0014-4894(83)90071-1. [DOI] [PubMed] [Google Scholar]
- Moroney J. V., Kitayama M., Togasaki R. K., Tolbert N. E. Evidence for Inorganic Carbon Transport by Intact Chloroplasts of Chlamydomonas reinhardtii. Plant Physiol. 1987 Mar;83(3):460–463. doi: 10.1104/pp.83.3.460. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Pönsgen-Schmidt E., Schneider T., Hammer U., Betz A. Comparison of Phosphoenolpyruvate-Carboxykinase from Autotrophically and Heterotrophically Grown Euglena and Its Role during Dark Anaerobiosis. Plant Physiol. 1988 Feb;86(2):457–462. doi: 10.1104/pp.86.2.457. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Ray T. B., Black C. C. Inhibition of oxalacetate decarboxylation during C4 photosynthesis by 3-mercaptopicolin acid. J Biol Chem. 1976 Sep 25;251(18):5824–5826. [PubMed] [Google Scholar]
- Reiskind J. B., Seamon P. T., Bowes G. Alternative methods of photosynthetic carbon assimilation in marine macroalgae. Plant Physiol. 1988 Jul;87(3):686–692. doi: 10.1104/pp.87.3.686. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Salvucci M. E., Bowes G. Two photosynthetic mechanisms mediating the low photorespiratory state in submersed aquatic angiosperms. Plant Physiol. 1983 Oct;73(2):488–496. doi: 10.1104/pp.73.2.488. [DOI] [PMC free article] [PubMed] [Google Scholar]