Food Lipids: Chemistry, Nutrition, and Biotechnology

erides, although PUFAs in both forms appear to be oxygenated by soy L-2 [107]. In
vivo, alteration of C 16-PUFA composition does not impact C 6-aldehyde production
by Arabidopsis leaves [209]. Although various HPLS enzymes have not been investigated
completely in regard to their stereoselectivity for hydroperoxide substrates, a
few have been identified as specific for hydroperoxides with the S configuration.
Thus far, none has been identified as R-specific.
HPLS-catalyzed reactions in plant tissues are commonly triggered by wounding,
resulting in characteristic plant odors. The C 6-aldehydes formed by HPLS from
C 18-PUFAs have been demonstrated in vitro to be effective biocides, with �,�-unsaturated
hexenal showing considerably more inhibitory action [86]. The other cleavage
product, 12-oxo-10E-dodecenoic acid, is the so-called wound hormone or traumatin
[224]. It stimulates cell division near the wound site, resulting in the formation
of a protective callus around the wound. Thus, HPLS may provide two active defense
agents in the wounding reponse of plant tissues: (1) a short chain aldehyde to destroy
or inhibit the attacking pests and (2) a wound-healing agent to protect and mend the
damaged tissue. The 9-carbon aldehydes resulting from cleavage of 9-hydroperoxy
fatty acids probably have similar functions; however, no role has yet been proposed
for 9-oxo-nonanoic acid.
The first in vivo evidence of C 6-aldehyde-involved plant defense was supplied
by Croft et al. [87]. These investigators noted that the inoculation of Phaseolus
vulgaris L. leaves with an avirulent strain of Pseudomonas syringae pv. phaseolicola
Burkholdera resulted in a burst of hexenals and hexenols. However, a virulent, compatible
strain of P. syringae pv. phaseolicola did not elicit the burst. The burst occurred
after 15 to 24 hours, which corresponds to the time of the hypersensitive
response of plant tissues to the pest invasion. An in vitro experiment showed that
one of the hexenals, E2-hexenal, was particularly inhibitory to the growth of P.
syringae pv. phaseolicola. Because the hexenal/hexenol burst occurred before phytoalexin
accumulation, these compounds were hypothesized to be early volatile phytoalexins
important to early plant responses.
Aerial treatment of Arabidopsis seedlings with 10 �M concentrations of trans-
2-hexenal for 4 or 24 hours induced several genes known to be involved in plants
defense response, including phenylpropanoid-related genes as well as genes of the
LOX pathway. trans-2-Hexenal induction thus closely mimicked the group of genes
induced by methyl jasmonate. However, trans-2-hexenal exerted a moderate inhibitory
effect on root length relative to similar concentrations of methyl jasmonate and
was approximately 10-fold less effective than methyl jasmonate at inducing anthocyanin
accumulation in Arabidopsis seedlings. It is suggested that C 6 volatiles of the
LOX pathway may act as a wound signal in plants but result in a moderate plant
response relative to MJ at both the physiological and molecular level [225].
In addition, Vaughn and Gardner [226] found that 4-hydroxy-E2-nonenal has
high fungitoxicity. In animal studies, the 4-hydroxyalkenals had other physiological
effects [227] including modulation of gene expression and activation of phospholipase
and glucose-6-phosphate dehydrogenase.
HPLS has also been proposed to participate in the oxidative catabolism of
triacylglycerols during the carbon mobilization in germinating seedlings [228] and
regulation of cellular levels of fatty acid hydroperoxide in plant tissues. Griffiths et
al. [229] found that fatty acid hydroperoxide levels were within the range of 0.6–
1.7% relative to the total fatty acid content of plant tissues for both photosynthetic
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