Food Lipids: Chemistry, Nutrition, and Biotechnology

Since 1971, when this physiologic role was first suggested by Anderson [183], further
evidence has strengthened the hypothesis. For example, water stress (desiccation)
induces the accumulation of some characteristic polypeptides in barley leaves. The
application of methyl jasmonate to healthy, unstressed leaves results in induction of
the same polypeptides [184]. When soybean plants were stressed by three treatments:
removal of seed pods, block of phloem export and water deficit, any of these three
treatments induces the synthesis of leaf vegetative storage proteins (VSPs), a group
of proteins that are thought to be repositories for nitrogen and can be mobilized
under conditions favorable for growth and transported to developing plant organs.
Application of methyl jasmonate to soybean plants mimics the effects of these treatments
by inducing VSP synthesis in healthy soybean plants [85,186]. In transgenic
Arabidopsis plants, where cosuppression reduces chloroplast LOX2 accumulation,
the wound-induced accumulation of jasmonate is absent, and there is less VSP
mRNA formation compared to the control in response to wounding [187]. Gaseous
methyl jasmonate promotes tendril coiling in Bryonia dioica Jacq., and the kinetics
of the coiling parallels that of coiling induced by mechanical stimulation [188].
Cultured cells of Rauwolfia canescens and Eschscholtzia californica (California
poppy) can be induced by a yeast elicitor to synthesize low molecular weight defensive
compounds derived from the phenylpropanoid pathway [189]. The elicitor
also enhances sharply the concentration of jasmonic acid within 30 minutes after
exposure. When jasmonins are applied to cell cultures in the absence of the yeast
elicitor, they independently induce the synthesis of defensive compounds. Proteinase
inhibitors are low molecular weight proteins and interfere with insect digestion,
prompting a decline in feeding by the insect. Synthesis of the inhibitors is activated
either by insects chewing on the leaf or by mechanical damage to the leaf. Accumulation
of the inhibitors is both local and systemic. Application to the leaf of
jasmonic acid, or its biosynthetic precursors, leads to the same specific induction of
both proteinase inhibitor mRNA and protein synthesis. In addition, exposure of tomato
plants to gaseous methyl jasmonate, or to plant species that synthesize methyl
jasmonate (e.g., sagebrush), also stimulates proteinase inhibitor synthesis [190,191].
Treatment of detached tomato leaves with either LOX inhibitors, ETYA and SHAM,
or HPDS inhibitor aspirin blocks the wound-induced accumulation of both protein
inhibitors and jasmonic acid. The inhibitory effect of aspirin on protein inhibitors
can be overcome by addition of exogenous 12-oxo PDA or jasmonic acid [192].
Based on the above observations, a jasmonate cascade was suggested in higher
plants [115]. However, understanding the physiological roles of jasmonic acid in
these many different processes is complicated by the observation that intermediates
in the synthesis of jasmonic acid are also active in at least some of these processes.
For example, 12-oxo-PDA has been shown to be a far more effective inducer of
tendril coiling in Bryonia dioica than jasmonic acid itself [193,194] and to act as an
elicitor of alkaloid biosynthesis in Eschscholtzia californica cell cultures [195]. The
levels of jasmonic acid, 12-oxo-PDA, and other intermediates of oxylipin synthesis
vary considerably among species [196], giving rise to the suggestion that the relative
and absolute concentrations of different compounds of the ‘‘oxylipin signature’’ may
provide flexibility to this multifunctional octodecanoid signaling system [197]. Koch
et al. [198] found that early, intermediate, and late compounds of the oxylipin cascade
leading to JA have differential effects on the induction of volatile biosynthesis.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.