PLANT PHYSIOLOGY , Vol 104, Issue 3 827-837, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Metabolic Evidence for the Involvement of a [delta]4-Palmitoyl-Acyl Carrier Protein Desaturase in Petroselinic Acid Synthesis in Coriander Endosperm and Transgenic Tobacco Cells
E. B. Cahoon and J. B. Ohlrogge
Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 48824
We have previously demonstrated that the double bond of petroselinic acid
(18:1[delta]6cis) in coriander (Coriandrum sativum L.) seed results from
the activity of a 36-kD desaturase that is structurally related to the
[delta]9-stearoyl-acyl carrier protein (ACP) desaturase (E.B. Cahoon, J.
Shanklin, J.B. Ohlrogge [1992] Proc Natl Acad Sci USA 89: 11184-11188). To
further characterize the biosynthetic pathway of this unusual fatty acid,
14C-labeling experiments were conducted using developing endosperm of
coriander. Studies were also performed using suspension cultures of
transgenic tobacco (Nicotiana tabacum L.) that express the coriander 36-kD
desaturase, and as a result produce petroselinic acid and
[delta]4-hexadecenoic acid. When supplied exogenously to coriander
endosperm slices, [1-14C]palmitic acid and stearic acid were incorporated
into glycerolipids but were not converted to petroselinic acid. This
suggested that petroselinic acid is not formed by the desaturation of a
fatty acid bound to a glycerolipid or by reactions involving acyl-coenzyme
As (CoA). Instead, evidence was most consistent with an acyl-ACP route of
petroselinic acid synthesis. For example, the exogenous feeding of
[1-14C]lauric acid and myristic acid to coriander endosperm slices resulted
in the incorporation of the radiolabels into long-chain fatty acids,
including primarily petroselinic acid, presumably through
acyl-ACP-associated reactions. In addition, using an in vitro fatty acid
biosynthetic system, homogenates of coriander endosperm incorporated
[2-14C]malonyl-CoA into petroselinic acid, of which a portion was detected
in a putative acyl-ACP fraction. Furthermore, analysis of transgenic
tobacco suspension cultures expressing the coriander 36-kD desaturase
revealed significant amounts of petroselinic acid and [delta]4-hexadecenoic
acid in the acyl-ACP pool of these cells. Also presented is evidence
derived from [U-14C]nonanoic acid labeling of coriander endosperm, which
demonstrates that the coriander 36-kD desaturase positions double bonds
relative to the carboxyl end of acyl-ACP substrates. The data obtained in
these studies are rationalized in terms of a biosynthetic pathway of
petroselinic acid involving the [delta]4 desaturation of palmitoyl-ACP by
the 36-kD desaturase followed by two-carbon elongation of the resulting
[delta]4-hexadecenoyl-ACP.
