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 through glycolysis results in the loss of one-third of the carbon as CO2. Here we 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 previously undescribed metabolic context to increase the efficiency of carbon use during the formation of oil. In comparison with glycolysis, the metabolic conversion we describe provides 20% more acetyl-CoA for fatty-acid synthesis and results in 40% less loss of carbon as CO2. Our conclusions are based on measurements of mass balance, enzyme activity and stable isotope labelling, as well as an analysis of elementary flux modes.

The conversion of photosynthate to seed storage reserves is crucial to plant fitness and agricultural production, yet quantitative information about the efficiency of this process is lacking. To measure metabolic efficiency in developing seeds, rapeseed (Brassica napus) embryos were cultured in media in which all carbon sources were [U-14 C]-labeled and their conversion into CO 2 , oil, protein, and other biomass was determined. The conversion efficiency of the supplied carbon into seed storage reserves was very high. When provided with 0, 50, or 150 mmol m 22 s 21 light, the proportion of carbon taken up by embryos that was recovered in biomass was 60% to 64%, 77% to 86%, and 85% to 95%, respectively. Light not only improved the efficiency of carbon storage, but also increased the growth rate, the proportion of 14 C recovered in oil relative to protein, and the fixation of external 14 CO 2 into biomass. Embryos grown at 50 mmol m 22 s 21 in the presence of 5 mM 1,1-dimethyl-3-(3,4-dichlorophenyl) urea (an inhibitor of photosystem II) were reduced in total biomass and oil synthesis by 3.2-fold and 2.8-fold, respectively, to the levels observed in the dark. To explore if the reduced growth and carbon conversion efficiency in dark were related to oxygen supplied by photosystem II, embryos and siliques were cultured with increased oxygen. The carbon conversion efficiency of embryos remained unchanged when oxygen levels were increased 3-fold. Increasing the O 2 levels surrounding siliques from 21% to 60% did not increase oil synthesis rates either at 1,000 mmol m 22 s 21 or in the dark. We conclude that light increases the growth, efficiency of carbon storage, and oil synthesis in developing rapeseed embryos primarily by providing reductant and/or ATP.In seed crops, yield is primarily a function of the production of assimilates by the leaves and other green parts of the plant and the utilization of these assimilates to synthesize reserve materials in the seeds. In addition to its importance to agricultural productivity, efficient storage of assimilates by seeds is essential to provide metabolic precursors and chemical energy to power the young seedling until it can capture its own energy from the sun. Within a species, seedling growth is positively correlated with seed size (Howe and Richter, 1982;Stanton, 1984;Vaughton and Ramsey, 1998;Sousa et al., 2003) and seedlings grown from large seeds have higher rates of establishment than those from small seeds (Black, 1958;Grime and Jeffery, 1965;Armstrong and Westoby, 1993;Burke and Grime, 1996). Thus, the amount of reserves stored in the seed will in large part determine the success of the young seedling.Seeds are not simply passive receptacles for the assimilates and minerals provided by the mother plant. They synthesize complex molecules from simple raw materials in relatively precise amounts and proportions (Egli, 1998). Green seeds are photosynthetically active and are able to fix carbon (Watson and Duffus, 1991;Eastmond et al., 1996). As a consequence, the light re...

SummaryThe efficiency with which developing sunflower embryos convert substrates into seed storage reserves was determined by labeling embryos with [U- 14 acids and amino acids was analyzed by NMR and GC-MS. The fluxes through intermediary metabolism were then quantified by computer-aided modeling. The resulting flux map accounted well for the labeling data, was in good agreement with the observed carbon efficiency, and was further validated by testing for agreement with gas exchange measurements. The map shows that the influx of malate into oil is low and that flux through futile cycles (wasting ATP) is low, which contrasts with the high rates previously determined for growing root tips and heterotrophic cell cultures.

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The balance between the vitamin E (tocochromanols) and polyunsaturated fatty acid (PUFA) contents mainly determines the susceptibility to lipid peroxidation and the storage stability of corn oil. In 1997, field experiments were conducted at two different locations to evaluate a collection of 30 corn hybrids for fatty acid profiles and tocochromanol contents. Hybrids differed significantly (p < 0.01) for major fatty acids, as well as for tocochromanol contents and composition. The major fatty acids were palmitic, oleic, and linoleic acids, whose contents were in the ranges 9.2-12.1%, 19.5-30.5%, and 53.0-65.3%, respectively. The tocopherol contents ranged as follows: alpha-tocopherol, 67-276 mg (kg of oil)(-1); beta-tocopherol, 0-20 mg (kg of oil)(-1); gamma-tocopherol, 583-1048 mg (kg of oil)(-1); delta-tocopherol, 12-71 mg (kg of oil)(-1); total tocopherol, 767-1344 mg (kg of oil)(-1). gamma-Tocopherol was the predominant derivative among all tocopherols. The tocotrienol contents were in the ranges 46-89, 53-164, and 99-230 mg (kg of oil)(-1) for alpha-, gamma-, and total tocotrienol contents, respectively. The tocotrienol profile was not characterized by the predominance of any tocotrienol homologue. alpha-Tocopherol was positively correlated with PUFA (r = 0.41) and with the vitamin E equivalent (vit E equiv) (r = 0.84), and it was not correlated with gamma-tocopherol. gamma-Tocopherol was highly correlated with total tocopherol and tocochromanol contents (r = 0.93 and r = 0.90, respectively), indicating that the contribution of this vitamer to the total tocochromanol content is the most important among all tocochromanols. The high positive correlation found between the vit E/PUFA ratio and the vit E equiv, as well as the absence of correlation between this ratio and PUFA indicates that a higher vit E/PUFA ratio can be easier achieved be increasing the vitamin E content than by modifying fatty acid profile in corn oil.