Minisymposium 15: Photosynthesis

Abs # 28004: Carbon dioxide and the efficiency of oil synthesis during seed development

Presenter:	Goffman, Fernando D, fgoffman@lycos.com
Authors	Goffman, Fernando D (A)   Schwender, Joerg  (A)   Ruckle, Mike  (A)   Ohlrogge, John  (A)   Shachar-Hill, Yair  (A)
Affiliations:	(A): Michigan State University

The efficiency with which seeds can store carbon is crucial to plant fitness. Oil is the primary reserve material in most seeds but the conversion of photosynthate sugars via glycolysis to oil is inefficient with one third of carbon being dissipated as CO₂. Here we report our findings, made using a newly developed assay, on the buildup of CO₂ in the developing seeds of Brassica napus (oilseed rape) and Glycine max (soybean). Very high levels (10-35 mM) of CO₂ accumulate in the seed when oil synthesis is at its fastest. Using embryos cultured under conditions that mimic in planta growth, and with isotopic labeling it is possible to account accurately for the allocation of carbon taken up by the developing embryos as sugars and amino acids into oil, protein, CO₂ and other biomass. The results show that the efficiency of the conversion of imported carbon into storage products is dependent on ambient light levels. During two weeks of culture, the carbon storage efficiency (expressed as a percentage of total carbon uptake) of rapeseed embryos cultured at 0, 50 and 150 µmol photons m^-2 s^-1 was 60%, 86% and 94%, respectively. The ratio of oil produced to CO₂ released also differed among the light treatments, being 1.1, 4.2 and 12.5 respectively. In soybean embryos, light intensity also affected both the carbon storage efficiency (74% and 83%), and the oil to CO₂ ratio (0.29 and 0.72) when cultured at 0 and 50 µmol photons m^-2 s^-1, respectively. Additionally, we measured the CO₂ refixation capacity of rapeseed embryos under similar culture conditions. Embryo refixation capacity was ~11- and 3-fold higher at 150 and 50 µmol photons m^-2 s^-1, respectively, as compared to the dark treatment. Thus, the results indicate that the efficiencies of carbon storage and oil synthesis are strongly dependent on light intensity, suggesting that the ability of the embryos to reassimilate internally produced inorganic carbon plays a main role in increasing carbon use efficiency. These findings have implications for the role of light and RuBisCO in seeds of the many plant species whose embryos are green during development.