Several oilseed rape species are grown commercially.
Brassica napus is the most commonly grown oilseed rape with substantial acreage in Canada, Europe, and China, whereas B.
carinata is grown in parts of Africa. Breeding in the 1960s led to
“double-low” varieties—low in erucic acid (22:1 n-9) and glucosinolates—originally named canola in Canada. Such varieties—
termed low erucic acid rapeseed (LEAR) in Europe—are used
for edible purposes, whereas the original—or high erucic acid
rapeseed (HEAR)—variety cultivars are put to industrial uses.
Sunflower, the fourth most important plant oil crop, is
grown mainly in southern Europe, Russia, Ukraine, and Argentina. The original cultivars contained over 60% linoleic acid but
due to the reduction in demand for such seed oils resulting from
lower intake recommendations high oleic (18:1n-9) and high
oleic-palmitic acid (16:0) varieties have been bred. Recently,
efforts have been made to create high stearic acid (18:0) lines.
tHe BIocHeMIstry oF
Fundamental to an understanding of
crop oil production is knowledge about
the biosynthesis of triacylglycerols
(TAGs) and, more importantly, how
synthesis of this storage lipid is controlled. Over the last couple of decades,
information about plant oil biosynthesis has increased markedly even to the
point of new enzymes being discovered.
A detailed description of the biochemistry of oil accumulation is beyond the
scope of this article, but references for
further reading are provided (page 80).
A simplified picture of TAG accumulation is shown in Fig. 2 (page 80).
Acyl chains (fatty acids) are synthesized
de novo in the plastidial compartment of
plant cells after which they can be modified both within the plastid as well as on
the endoplasmic reticulum before being
assembled onto a glycerol backbone.
The main oil crops synthesize fatty
acids predominantly of 16 or 18 carbons
in length. Palmitic acid is the most abun-
dant saturated fatty acid, while its 18C
analogue, stearic acid, is rapidly desatu-
rated successively at the Δ9, Δ12, and
Δ15 positions to produce oleic, lin-
oleic, and α-linolenic acid (18:3n-3),
respectively. D9-desaturation occurs in the plastid. However, in
many oil crops, the majority of fatty acid modifications are per-
formed on the endoplasmic reticulum. Desaturation and chain
elongation are the most common modifications, but certain
important crops are capable of hydroxylations, production of ring
structures, and desaturations of unusual specificity. The PUFA
are produced by the action of fatty acid desaturases (FAD2 and
FAD3) which act on phosphatidylcholine (PtdCho) as the pre-
ferred substrate. Channelling of fatty acids on and off PtdCho is
an important point of regulation and has been referred to as “acyl-
editing” by Phil Bates and John Ohlrogge.
Once fatty acids have been synthesized, they are incorporated into TAG by a route known as the Kennedy pathway. This
pathway contains four steps (Fig. 3, page 81), three of which
are acylations which add fatty acids to the three carbons of the
continued on next page
FIg. 1. The World’s major oil crops.
Clockwise from top left these are
oilseed rape (canola), soybean, oil
palm, and sunflower which account
for about 12%, 24%, 9%, and 22% of
total oil production, respectively.