substrate for peroxidases. Fungal attack or elicitor application also induces SOD in several plant species
or cultivars that display an HR. Bowler et al. (Ref. 14 and references therein) hypothesize that SOD acti-
vation may aid in the strengthening of cell walls or possibly kill pathogens directly through H 2 O 2 , although
the H 2 O 2 involved in lignification may arise from other sources (reviewed in Ref. 15).
C. Phytoalexins
Phytoalexins have been isolated from a number of plant species and appear to be specific to a particular
plant family. For example, the Leguminosae produce (iso)flavonoids primarily, whereas the Solanaceae
produce sesquiterpenes and the Umbelliferae mainly manufacture coumarin derivates [6]. Phytoalexins
are toxic to both specific pathogens and the plants themselves and thus may contribute to the necrosis as-
sociated with the HR [16]. It should be noted, however, that questions remain as to whether phytoalexin
synthesis is merely a response to infection as very few studies have conclusively demonstrated a role for
phytoalexins in the defense response to pathogens [17]. In addition, several phytoalexins are also required
for normal growth and development [18]. Although it is almost certain that some phytoalexins play a role
in the defense response, they probably act in concert with other defense responses or proteins [17,18].
- (Iso)flavonoid-Derived Phytoalexins
Chalcone synthase (CHS) is the start of the (iso)flavonoid branch of the phenylpropanoid pathway. This
enzyme is active in normal growth, development, and metabolism of plants and is ultimately responsible
for many plant pigments (e.g., anthocyanins) [7]. In the Leguminosae, where it has been studied exten-
sively, CHS is highly inducible by pathogen attack [19]. This is accomplished by the differential regula-
tion of several isozymes, some of which are constitutive while others are specific to pathogenic attack
[20]. Numerous other enzymes associated with this class of phytoalexins are inducible at both the tran-
script and protein levels in response to biotic and abiotic stresses [18]. Although evidence for the specific
mechanisms by which isoflavonoid phytoalexins achieve their toxicity is generally lacking, accumulated
evidence suggests that they cause dysfunctions in the plasma membrane or tonoplast [21].
- Coumarin-Derived Phytoalexins
Coumarins are also derivatives of the phenylpropanoid pathway but the branch mechanisms remain un-
resolved [12]. Hahlbrock and Scheel [12] suggested that glucosides or glucose esters are key intermedi-
ates, providing a means of safely sequestering potentially self-toxic compounds until they are needed. As
with the (iso)flavonoid-synthesizing enzymes, coumarin pathway enzymes are stimulated by elicitor
treatment.
- Terpenoid-Derived Phytoalexins
The other major family of phytoalexins is derived from the terpenoid biosynthetic pathway. This pathway
also operates during normal development and metabolism, producing such compounds as abscisic acid
(ABA), giberellins, chlorophyll, carotenoids, and phytosterols [22]. Terpenoids arise via mevalonic acid,
with hydroxymethyl glutarate reductase (HMGR) as the putative key regulatory enzyme for the entire
pathway. The activity of several critical enzymes in the central pathway, including HMGR, increases as
a result of pathogen attack, as evidenced by increases in gene transcription and translation. Data also ex-
ist concerning the de novo synthesis of several enzymes specific to terpenoid phytoalexin biosynthesis.
For example, farnesyl pyrophosphate transferase and casbene synthase have been shown to be synthe-
sized de novo in response to elicitors, leading to the formation of diterpenoid phytoalexins [23–25]. Re-
search has uncovered evidence for an alternative terpenoid biosynthetic pathway in higher plants that does
not involve mevalonic acid (reviewed in Ref. 26), but it is not known whether this pathway is important
in biotic stress defense.
D. Pathogenesis-Related (PR) Proteins
Much of the data regarding PR proteins is correlative rather than causal; however, there are reports that
strongly suggest that PR proteins are an important component of the defense response in vivo. The PR
proteins are compositionally quite diverse, falling into 10 or 11 families as defined by Kombrink and
Somssich [27] or van Loon et al. [28], respectively. Although there is general agreement in their classifi-
cation schemes, Kombrink and Sommsich combine the PR-4 and PR-5 families of Van Loon et al., insert
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