another PR-4 family, and omit the PR-11 family. The classification scheme of Kombrink and Somssich
will be followed, as their rationale seems better justified when considering the apparent functions and re-
latedness of the family members.
- PR-1
This family currently has no function assigned to its members despite considerable effort and characteri-
zation at the gene, transcript, and protein levels [27]. The members are grouped together on the basis of
sequence data.
- PR-2 (-1,3-glucanses)
This family is well characterized. Glucanases act by hydrolyzing -1,3 glucan residues. This type of car-
bohydrate dominates fungal cell walls along with chitin [29]. Glucanase expression, as determined by
transcript and protein analyses, increases greatly upon exposure of plant tissue to pathogens or fungal elic-
itors [5]. The family is subdivided based on pI values and cellular and extracellular location (i.e., iso-
forms) that reflect differences in enzymatic and antifungal properties [27].
It has been shown that the isoforms can be under differential regulation [30–32]. Non–pathogen-in-
duced expression of glucanses has been reported in roots, stems, and flowers [5], with certain isoforms
specific to the normal development of the plant [32]. Glucanases are apparently induced in concert with
chitinases (PR-3 family) in a number of species [5]. The combination of the two enzymes has been shown
to have direct antifungal properties in vitro [33].
- PR-3 (Chitinases)
This family is also well characterized. Chitinases hydrolyze -1,4 acetylglucosamine linkages of chitin
polymers, which are a primary constituent of fungal walls [34]. Although plant secondary walls report-
edly contain chitin, those linkages resist chitinase, possibly due to glycolipid modification of the linkages
[35]. Chitinases are also induced by pathogens and microbial elicitors as well as by abiotic stresses such
as heavy metals and salt [36]. Kombrink and Somssich (Ref. 27 and references therein) indicate that four
classes of this family exist, based on primary structure, pI, enzymatic activity and antifungal properties.
- PR-4 (Chitin-Binding; Win-like Proteins)
This family is poorly understood and the grouping is based on sequence homology and a chitin-binding
motif. Although proteins have been isolated and their genes analyzed, no obvious function can be ascribed
to them; however, antimicrobial activity has been demonstrated in vitro. The chitin-binding abilities of
these proteins appear to be minimal [27].
- PR-5 (Thaumatin-like Proteins; Osmotins)
These proteins are structurally related to the sweet-tasting protein thaumatin, originally isolated from
Thaumatococcus danielliiBenth. [37]. Thaumatin and other members of this family have sequence ho-
mology with the maize trypsin/-amylase inhibitor family of proteins [38]. These inhibitors have binding
sites with varying specificity to both serine proteinases and -amylases in insects, animals, and pathogens
[39], and this family of proteins is also considered to be a part of the defense response of plants.
The thaumatin-like family has also been shown to have another defensive capability. A well-charac-
terized member of this family of proteins is osmotin, a 26-kDa salinity-inducible protein initially isolated
from cultured tobacco cells [40]. In whole tobacco plants, the expression of this protein is complex. Fun-
gal infection and salt induce the accumulation of osmotin in a tissue-specific manner with some accumu-
lation also induced by ethylene and tobacco mosaic virus [41]. In contrast, osmotin messenger RNA
(mRNA), but not protein, is inducible by ABA, wounding, and several abiotic stresses [41,42]. Vigers et
al. [43] showed that osmotin and the serologically related proteins zeamatin (from maize) and PR-5 (from
tobacco) had antifungal activity, causing the rapid bursting of hyphal tips. This is probably due to a mem-
brane-permeating ability that osmotin, along with a related class of proteins termed permatins, has been
shown to have [44]
- PR-6 (Proteinase Inhibitors)
Because of their small molecular mass, proteinase inhibitors are generally referred to as polypeptides
rather than structurally complex proteins. They are also classified as inducible defense molecules. They
660 ARTLIP AND WISNIEWSKI