METABOLIC BYPASS OF THE TRICARBOXYLIC-ACID CYCLE DURING LIPID MOBILIZATION IN GERMINATING OILSEEDS - REGULATION OF NAD(-DEPENDENT ISOCITRATE DEHYDROGENASE VERSUS FUMARASE())
Kl. Falk et al., METABOLIC BYPASS OF THE TRICARBOXYLIC-ACID CYCLE DURING LIPID MOBILIZATION IN GERMINATING OILSEEDS - REGULATION OF NAD(-DEPENDENT ISOCITRATE DEHYDROGENASE VERSUS FUMARASE()), Plant physiology, 117(2), 1998, pp. 473-481
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.