METABOLIC BYPASS OF THE TRICARBOXYLIC-ACID CYCLE DURING LIPID MOBILIZATION IN GERMINATING OILSEEDS - REGULATION OF NAD(-DEPENDENT ISOCITRATE DEHYDROGENASE VERSUS FUMARASE())

Biosynthesis of sucrose from triacylglycerol requires the bypass of th e CO2-evolving reactions of the tricarboxylic acid (TCA) cycle. The re gulation of the TCA cycle bypass during lipid mobilization was examine d. Lipid mobilization in Brassica napus was initiated shortly after im bibition of the seed and proceeded until 2 d postimbibition, as measur ed by in vivo [1-C-14]acetate feeding to whole seedlings. The activity of NAD(+)-isocitrate dehydrogenase (a decarboxylative enzyme) was not detected until 2 d postimbibition. RNA-blot analysis of B. napus seed lings demonstrated that the mRNA for NAD(+)-isocitrate dehydrogenase w as present in dry seeds and that its level increased through the 4 d o f the experiment. This suggested that NAD(+)-isocitrate dehydrogenase activity was regulated by posttranscriptional mechanisms during early seedling development but was controlled by mRNA level after the 2nd or 3rd d. The activity of fumarase (a component of the nonbypassed secti on of the TCA cycle) was low but detectable in B. napus seedlings at 1 2 h postimbibition, coincident with germination, and increased for the next 4 d. RNA-blot analysis suggested that fumarase activity was regu lated primarily by the level of its mRNA during germination and early seedling development. It is concluded that posttranscriptional regulat ion of NAD(+)-isocitrate dehydrogenase activity is one mechanism of re stricting carbon flux through the decarboxylative section of the TCA c ycle during lipid mobilization in germinating oilseeds.