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The Biosynthesis of Erucic Acid in Developing Embryos of
Brassica rapa1
Xiaoming Bao,
Mike Pollard, and
John Ohlrogge*
Department of Botany and Plant Pathology, Michigan State
University, East Lansing, Michigan 48824
The prevailing hypothesis on the
biosynthesis of erucic acid in developing seeds is that oleic acid,
produced in the plastid, is activated to oleoyl-coenzyme A (CoA) for
malonyl-CoA-dependent elongation to erucic acid in the cytosol. Several
in vivo-labeling experiments designed to probe and extend this
hypothesis are reported here. To examine whether newly synthesized
oleic acid is directly elongated to erucic acid in developing seeds of
Brassica rapa L., embryos were labeled with
[14C]acetate, and the ratio of radioactivity of carbon
atoms C-5 to C-22 (de novo fatty acid synthesis portion) to carbon
atoms C-1 to C-4 (elongated portion) of erucic acid was monitored with time. If newly synthesized 18:1 (oleate) immediately becomes a substrate for elongation to erucic acid, this ratio would be expected to remain constant with incubation time. However, if erucic acid is
produced from a pool of preexisting oleic acid, the ratio of C in the 4 elongation carbons to 14C in the
methyl-terminal 18 carbons would be expected to decrease with time.
This labeling ratio decreased with time and, therefore, suggests the
existence of an intermediate pool of 18:1, which contributes at least
part of the oleoyl precursor for the production of erucic acid. The
addition of
2-[{3-chloro-5-(trifluromethyl)-2-pyridinyl}oxyphenoxy] propanoic
acid, which inhibits the homodimeric acetyl-CoA carboxylase, severely
inhibited the synthesis of [14C]erucic acid, indicating
that essentially all malonyl-CoA for elongation of 18:1 to erucate was
produced by homodimeric acetyl-CoA carboxylase. Both light and
2-[{3-chloro-5-(trifluromethyl)-2-pyridinyl}oxyphenoxy]-propanoic acid increased the accumulation of [14C]18:1 and the
parallel accumulation of [14C]phosphatidylcholine. Taken
together, these results show an additional level of complexity in the
biosynthesis of erucic acid.
1
This work was supported by a grant from the
Department of Energy (no. DE-FG02-87ER12729).
*
Corresponding author; e-mail ohlrogge{at}pilot.msu.edu; fax
1-517-353-1926.
Plant Physiol. (1998) 118: 183-190
Copyright Clearance Center: 0032-0889/98/118//08
© 1998 American Society of Plant Physiologists
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