Minisymposium 12: Photosynthetic Carbon Metabolism

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Abs # M1203: RuBisCO without the Calvin Cycle improves the carbon efficiency of developing green seeds

Presenter:	Schwender, Joerg       Contact Presenter
Authors	Schwender, Joerg  (A)   Goffman, Fernando  (B)   Ohlrogge, John  (B)   Shachar-Hill, Yair  (B)
Affiliations:	(A): Brookhaven National Laboratory (B): Michigan State University
Web Site:	http://www.biology.bnl.gov/plantbio/schwender.html

Efficient storage of carbon in seeds is crucial to plant fitness and to agricultural productivity. Oil is a major reserve material in most seeds and these oils provide the largest source of renewable reduced carbon chains available from nature. However, the conversion of carbohydrate to oil via glycolysis results in the loss of one third of the carbon as CO₂ at the pyruvate dehydrogenase reaction. By using stable isotope labeling and mass balancing with cultured embryos we could show that in developing embryos of Brassica napus L. (oilseed rape) RuBisCO (Ribulose 1,5-bisphosphate carboxylase/oxygenase) acts without the Calvin cycle and in a novel metabolic context to increase the efficiency of carbon use during the formation of oil. As compared to glycolysis, the partial bypass of glycolysis via RubisCO provides 20% more acetyl-CoA for fatty acid synthesis and results in 40% less loss of carbon as CO2. Analysis of the metabolic network involved in catabolism of sugars to acetyl-CoA (glycolsysis / pentose phosphate pathway / RubisCO) by elementary flux modes analysis revealed four sets of flux modes, distinct in their carbon conversion efficiency and their dependence on NADPH and ATP from outside the network. This analysis predicts that the involvement of RubisCO provides more carbon efficient sugar breakdown but has to be balanced by specific amounts of ATP and NADPH to be contributed by the photosynthetic light reactions within the developing embryo. Beyond the analysis of metabolism in green oil seeds this study is relevant to non-foliar photosynthesis in general which is often associated to green tissues (e.g. stems, flowers), with net CO₂ output. Schwender et al.: Nature 432, 779-782 (2004).