Coordinate changes in carbon partitioning and plastidial metabolism duringthe development of oilseed rape embryo

Measurements of metabolic fluxes in whole embryos and isolated plastids hav e revealed major changes in the pathways of carbon utilization during cotyl edon filling by oilseed rape (Brassica napus L.) embryos. In the early coty ledon stage (stage A), embryos used sucrose (Suc) predominantly for starch synthesis. Plastids isolated from these embryos imported glucose-6-phosphat e (Glc-6-P) and partitioned it to starch and fatty acids synthesis and to t he oxidative pentose phosphate pathway in the ratio of 2:1:1 on a hexose ba sis. Of the substrates tested, Glc-6-P gave the highest rates of fatty acid synthesis by the plastids and pyruvate was used weakly. By the mid-to late -cotyledon stage (stage C), oil accumulation by the embryos was rapid, as w as their utilization of Suc for oil synthesis in vitro. Plastids from C-sta ge embryos differed markedly from those of stage-A embryos: (a) pyruvate up take and utilization for fatty acid synthesis increased by respectively 18- and 25-fold; (b) Glc-6-P partitioning was predominantly to the oxidative p entose phosphate pathway (respective ratios of 1:1:3); and (c) the rate of plastidial fatty acid synthesis more than doubled. This increased rate of f atty synthesis was dependent upon the increase in pyruvate uptake and was m ediated through the induction of a saturable transporter activity.