been found in several species of solanaceous plants. A survey of recorded systemin sequences in Gen-
Bank found no examples outside the Solanaceae, but it is possible that this polypeptide or its equivalent
exists in other plant families.
JASMONIC ACID. As noted before, jasmonic acid (JA) and its methyl ester (methyl jasmonate,
MeJA) are products of the lipid-derived octadecanoid pathway [53]. It is thought that the interaction of
elicitors, wounding, and/or cell wall fragments with a membrane receptor initiates the biosynthetic path-
way resulting in production of JA with or without the action of systemin [53,74]. JA and MeJA partici-
pate in several aspects of normal growth and development as well as in several biotic and abiotic stress
defense responses [53]. Numerous proteins appear to be regulated by JA and MeJA, including thionins,
defensins, some proteinase inhibitors, genes involved in phytoalexin biosynthesis, and other defense
genes, including osmotin. As noted, JA and MeJA may interact with SA and ethylene to induce some PR
proteins. Interestingly, SA has been noted to inhibit JA synthesis [53], suggesting some degree of signal
modulation. MeJa has also been proposed to function as a means of interplant communication, akin to
MeSA [75], but its significance under physiological conditions again remains uncertain.
GENTISIC ACID. Gentisic acid (GeA) is a derivative of salicylic acid. As noted by Belles et al. [76],
GeA has been reported to have antifungal properties in vitro. Belles et al. [76] reported that GeA synthe-
sis is induced in tomato in response to viral infection and can induce PR proteins that exogenously ap-
plied SA could not. The authors hypothesized that GeA may have a role complementary to that of SA
(Figure 1). Further research is necessary to determine what specific role(s) GeA has in biotic stress de-
fense responses and whether this compound is active in other species.
PROTEIN KINASES. Protein kinases act either directly in the activation of various defense proteins
or indirectly through transcription factor activation or synthesis. In some cases, MAP (mitogen-activated
protein) kinase cascades are involved. Indeed, Kumar and Klessig [77] have reported differential induc-
tion of MAP kinases in tobacco by NO, SA, ethylene, and JA. Most protein kinases are also counteracted
or modulated by specific phosphatases.
ION FLUXES. Ion fluxes, particularly Ca^2 , have been implicated in many aspects of signal trans-
duction. Ca^2 has been shown to activate individual proteins, and transcription factors, usually in concert
with protein kinases and MAP kinase cascades [78]. Changes in Ca^2 are also important in normal growth
and development as well as in signal transduction of several abiotic stress responses (Ref. 78 and refer-
ences therein). A full accounting of such fluxes and subsequent signaling events, however, is beyond the
scope of this chapter.
ETHYLENE. Ethylene is produced upon wounding or infection by pathogens. Exogenous application
of ethylene induces several PR proteins, indicating a role in biotic stress defense responses. Several lines
of evidence suggest that induced ethylene may be a symptom rather than a cause of defense responses. It
is likely that ethylene modulates such responses, acting together with JA or SA to induce several PR pro-
teins [71].
OTHER SIGNALS. It has been shown that action potentials akin to those observed in animal neural
systems exist in plants as a consequence of wounding. Induction of several defense protein transcripts and
proteins has been demonstrated to result from such action potentials. However, the mechanisms and the
place of action potentials in the signaling network are not understood [79].
III. ABIOTIC STRESSES
Abiotic stresses also induce a diverse array of proteins and, in some cases, similar proteins are induced
by different stresses. As noted before, biotic and abiotic stresses may induce similar or identical proteins
as well. Frequently considered abiotic stresses include light, temperature, nutrients, salinity, and air and
water pollutants. It is well known that an excess or deficit of any of these factors can greatly reduce growth
and reproduction [80].
A. Heat Stress
Perhaps the most studied response to environmental stress in plants is the response to elevated tempera-
ture. An increase of five degrees or more above the optimal growth temperatures of a plant defines heat
INDUCTION OF PROTEINS IN RESPONSE TO STRESSES 665