Food Lipids: Chemistry, Nutrition, and Biotechnology

of the seed pod reversed senescnece (the cotyledons rejuvenate) with increased LOX
activity [153]. These phenomena evidently are not in agreement with a role for LOX
in senescence. It is worth mentioning that these contradictive results are all based
on in vitro measurements, and the role of LOXs in developmental processes cannot
readily be assessed until more information is available on in vivo LOX product
formation in plant tissues under different conditions.
Increased LOX activity in oilseeds during the early stages of germination has
recently been presumed to be involved in the initiation of the mobilization of storage
lipids. In this hypothesis, Feussner et al. [99] propose that during early stages of
cucumber seed germination, a LOX form is induced and is activated by binding to
the lipid body membrane. The active enzyme then oxygenates the esterified fatty
acids located in the lipid storage organelles to form hydroperoxides, both in the
storage triacylglycerols and in the phospholipids of the lipid body monolayer membrane.
The oxygenated fatty acids are preferentially cleaved and subsequently released
into the cytoplasm. In the cytosol, the hydroperoxide derivatives are utilized
via �-oxidation to serve as a major carbon source for the seedling before full photosynthesis
is developed. However studies of direct association of LOX with lipid
bodies in germinating soybean seeds in vivo have been negative [159]. LOXs have
also been proposed to be involved in nitrogen partitioning and storage in plants.
During soybean seed development, members of LOX multigene family, so-called
vegetative LOX, function in nitrogen and assimilate partitioning, mechanisms
evolved by plants to temporarily store and subsequently remobilize nutrients to meet
specific needs. The levels of gene transcript and protein accumulation of vegetable
LOXs in mature soybean generally increase in response to increasing levels of available
nitrogen within cells and tissues. LOX genes are regulated in response to plant
nitrogen status in both a developmental- and tissue-specific manner. Removal of
developing pods, a strong assimilate sink, causes a reallocation of nitrogen and other
assimilates to vegetative LOX [160–162]. Kolomiets et al. [163] recently reported
that the potato LOX1 class of genes is involved in potato tuber enlargement.
It has been suggested that LOX may also have been involved in the formation
of biological functional compounds such as phytoalexin, abscisic acid, and ethylene,
as well as in the regulation of the Calvin cycle, the response to wounding and
pathogen attack [81,94]. Antisense-mediated depletion of a potato LOX reduces
wound infection of proteinase inhibitors with increased weight gain of insect pests
and tuber yields. However, the regulatory role of LOX is not caused by its involvement
in the wound-induced increase of JA, as wild-type and LOX3-deficient plants
have similar jasmonate levels after wounding [164].
A more universal role for plant LOX, however, apparently is to provide fatty
acid hydroperoxide substrates for several enzyme systems designated as the HPLS
pathway, the HPDS (allene oxide synthetase) pathway, the HPIS pathway, and the
HPPR/HPEP pathway.
2. Hydroperoxide Metabolism
a. Hydroperoxide Dehydratase Pathway–Jasmonic Acid Biosynthesis. HPDS
(EC 4.2.1.92), also called allene oxide synthetase, is the first enzyme of this pathway
(Fig. 10) that leads to the biosynthesis of (�)7-isojasmonic acid, a plant growth
regulator. The enzyme catalyzes the dehydration of a fatty acid hydroperoxide to
form an allene oxide [165,166]. The allene oxide 12,13S-epoxy-9Z,11E,15Z-octa-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.