act against the proteolytic activity of microbe-secreted proteases as well as the proteinases found in the
digestive tract of animals, particularly insects. Consequently, animals are not deterred in the short term
from consuming plant tissue. Prolonged exposure to the inhibitors, however, will contribute to starvation
of the animal [39]. There are several classes of proteinases based on mechanism of action [45]. They are
named after the active residue or cofactor responsible for the proteolytic cleavage (i.e., serine-,
cysteine-, aspartic- and metalloproteinases). Not surprisingly, there are inhibitors for each of these pro-
teinase classes and all apparently work by competitive inhibition. Pathogen attack, fungal elicitors, and
wounding are all capable of inducing all of the types (classes) of proteinase inhibitors [39].
- PR-7 (Proteinases)
Despite the logical need for proteinases either as a component of the HR or to counter pathogens, little in-
formation exists about this family [27]. An alkaline endoproteinase has been shown to be inducible in re-
sponse to viroid infection and Phytophthorainfection in tomato leaves (summarized in Ref. 27).
- PR-8 (Lysozymes/Class III Chitinases)
This class consists of class III chitinases bearing no structural similarity to the chitinases from the PR-3
family. The PR-8 chitinases do bear sequence similarity to a bifunctional lysozyme/chitinase (Ref. 27 and
references therein). Basic and acidic isoforms exist, with the acidic isoforms devoid of lysozyme activ-
ity. Curiously, the class III chitinases demonstrate little or no antifungal properties, leaving their contri-
butions to pathogen defense debatable except for the lysozyme activity of the basic isoforms [27].
- PR-9 (Peroxidases)
This family of enzymes varies widely in function and structure but all catalyze the oxidation of a substrate
in the presence of H 2 O 2. As such, the activity of this family is diverse, ranging from lignification and
suberization of cell walls (see earlier) to potentially participating in signal transduction (see later). Some
of these processes occur in the absence of pathogen or related stimuli and are part of normal growth and
development [27].
- PR-10 (Intracellular PR proteins)
Little is known of this class aside from sequence similarity and size range (16 to 18 kDa [27]). It has been
speculated that they are intracellular in nature, which may be consistent with observed ribonuclease se-
quence similarities and activities (Ref. 46 and references therein). Lo et al. [46] presented data indicating
a role for these proteins in degrading stress- or pathogen-specific RNAs as well as in developmental reg-
ulation of normal plant metabolism.
E. Non-PR Proteins
Numerous other peptides and proteins have been identified across a variety of plant species that accumu-
late in response to pathogens. For example, Segura et al. [47] reported that a peptide from potato, snakin-
1, is active against both a bacterium and a fungus and has homology to hemotoxic snake venoms. These
other peptides and proteins have not been yet been included in a formal classification scheme. This is due
to very limited knowledge of the extent of their expression or the fact that most reports concern changes
in a particular gene transcript with no attendant protein data. In addition, SA appears to be crucial for the
expression of most, if not all, of the defense molecules already described, while the groups that follow do
not necessarily require SA for induction.
- Thionins
Thionins are a family of small (approximately 5 kDa), sulfur-rich polypeptides shown to exist in several
families of plants and have a putative role in plant defense [48]. Thionins were originally described as
toxic factors in many cereal seeds [49] and classified as - or -. More recently, they have been shown
to be synthesized in leaves as well, with the highest abundance occurring in the epidermal cell walls [49].
Because the cell wall of the epidermis is often the primary site of pathogenic attack, thionins may act as
a first line of defense. Thionin expression is regulated both developmentally and by pathogen attack, with
pathogenic attack causing enhanced transcription and translation of thionin mRNA [49]. Non–pathogen-
induced expression of thionins is limited to the period of growth preceding emergence from the soil.
Thionin toxicity is hypothesized to arise from an amphipathic tertiary structure that may cause an increase
INDUCTION OF PROTEINS IN RESPONSE TO STRESSES 661