I and class III GSTs have been shown to be inducible at the transcriptional and translational levels by var-
ious environmental stresses that have oxidative stress in common, including pathogen attack [59]. The ox-
idative burst produces H 2 O 2 , which among other actions (see earlier) induces the synthesis of SA, several
GSTs, and glutathione peroxidases [59]. SA inhibits the catalase that normally degrades H 2 O 2 [57,59],
thus these enzymes may help detoxify lipid peroxides formed via H 2 O 2. The utilization of glutathione in
these processes causes an up-regulation of its synthesis, and the elevated glutathione levels have been
shown to induce a number of the biotic stress defense molecules already described [57].
F. Alkaloids
Alkaloids are a diverse group of nonproteinaceous compounds reported from many plant taxa. Whereas
much is known about their chemistry, biosynthesis, and medicinal and toxicological properties, it is cur-
rently unknown to what extent these compounds play a defensive role in biotic stress. As noted by
Kutchan [60], many alkaloids have cytotoxic effects, particularly toward insects. The biosynthesis of
these proteins is probably induced by various factors, but this area clearly requires further study.
G. Signal Transduction
An aspect of alterations in protein synthesis that has been alluded to but not described is that of the sig-
nal transduction pathways responsible for the expression of the various pathogen-induced proteins. In-
deed, a complex set of signal transduction pathways or networks exist which coordinate a plant’s de-
fensive response to biotic stress. It is often observed that several different types of proteins may be
synthesized in response to a particular pathogen, whereas another pathogen engenders a much different
protein response, and such responses are dependent on the tissue, tissue status, and numerous other con-
ditions. Thus, it is important to consider the signal transduction pathways or networks involved in the
induction of the proteins already discussed. Figure 1 has been compiled to help provide a context. Many
of the pathways illustrated have been shown to exist only in a small number of plant species (e.g., sys-
temin, see later), and some are still speculative (e.g., the roles of AOX and gentisic acid in viral resis-
tance). It should be noted that some abiotic stresses also induce the expression of some of the defense-
related genes. In particular, the enzymes responsible for phenylpropanoid-derived phytoalexins have
been shown to be induced by ultraviolet light and heavy metals. As mentioned before osmotin is also
induced by salt stress.
- Elicitors
Specific molecules (i.e., elicitors) are important in initiating the signal transduction pathways. Specific
proteins (avirulence factors), fragments of fungal cell walls, and fragments of plant cell walls hydrolyzed
by pathogens or produced by phytophagous insects serve as triggers of the defense system. Presumably,
this is accomplished by the elicitors binding to specific receptors that in turn initiate the various signal
transduction pathways (Figure 1). In some cases, the receptors have been identified and localized to the
plasma membrane.
- Signal Molecules
NITRIC OXIDE. Nitric oxide (NO) has been shown to be an integral part of the biotic stress response
network as shown in Figure 1. Its mode of action is manifested in at least two ways, through transcrip-
tional activation of various genes within the network and the hypersensitive response, with cyclic GMP
(cGMP) and cyclic ADP-ribose (cADPR) as possible intermediates [61–63]. The hypersensitive response
is not completely understood but may rely in part on direct synthesis of both NO and superoxide by NO
synthase. These compounds can synergistically destroy many cellular structures [64]. NO synthase has
also been shown to be induced in resistant plants [63]. Thus, NO synthase, although not specifically men-
tioned earlier, could also be considered a member of the cadre of defense proteins.
SALICYLIC ACID. Salicylic acid (SA) is a derivative of the phenylpropanoid pathway and is
widespread in plant species [65]. It is a key intermediary for numerous aspects of the plant defense re-
sponse, including LAR and SAR [57] (see Figure 1). Evidence for their role is quite strong in dicots and
less so in monocots. SA may also enable communication between infected and healthy plants via its
volatile methyl ester, methyl salicylate (MeSA) [66]. Although such a communication system between
sessile organisms has obvious benefits, its utility under physiological conditions has not yet been proved.
INDUCTION OF PROTEINS IN RESPONSE TO STRESSES 663