in membrane permeability [48]. It is also believed that self-toxicity to the plant cell is minimized by the
presence of a proteolytically cleaved precursor that may shield the cell’s membrane system from the ac-
tive domains of the thionins [49].
- Plant Defensins
Plant defensins are another widespread family of small, sulfur-rich proteins, structurally unrelated to the
thionins, although originally classified as -thionins [50]. They have antimicrobial activity in vitro (Ref.
50 and references therein). They are found primarily in seeds, although research indicates that they are also
expressed in vegetative and fruit tissues [51,52] (Wisniewski et al., unpublished). The family has homo-
logues found in mammals and insects, where they have also been shown to have antimicrobial activities.
Plant defensins are all similar in size and have several conserved residues, including eight Cys, two Gly,
an aromatic residue, and a Glu residue. Broekaert et al. [50] indicate that plant defensins can be broadly
classified into two groups based on the in vitro morphogenic effects on treated fungal hyphae. One group
causes reduced hyphal elongation with an increase in hyphal branching. The other group does not induce
marked morphological distortions. The different classifications are based on the activity against particular
fungi [50]. Plant defensins are generally not active against bacteria. Arabidopsis thalianahas been shown
to have five plant defensin genes, some of which are constitutively expressed, while others are differen-
tially regulated [52]. This is consistent with the observations that some plant defensins are constitutive
while others are stimulated by pathogen attack [50]. It has been shown that jasmonic acid (see later) can
induce the expression of plant defensins as well as thionins [53]. Terras et al. [54] indicate that the two
radish (Raphinus sativusL.) defensins are inducible by methyl jasmonate but not by SA. This report and
others suggest that plant defensins may form a portion of the biotic stress defense repertoire separate from
SAR. In turn, jasmonic acid functions in a signaling capacity both in normal development and in response
to wounding and pathogen attack independently of salicylic acid (see later).
- Lectins
Lectins constitute a large family of proteins occurring in numerous plant taxa. Lectins are primarily con-
fined to seeds, bark and vegetative organs and are typically viewed as nitrogen storage proteins. All
lectins share the property of binding particular carbohydrates, and many have been shown to have activ-
ities that can be considered defensive in nature. The evidence for active synthesis in response to pathogen
attack, however, is not strong. Van Damme et al. [55] summarized much of what is known about lectins
in a comprehensive review. Briefly, the following lectin families appear to have some form of putative
defensive capability. Legume lectins are suggested to bind to gut endothelial cells in animals, where they
elicit noxious effects. Chitin-binding lectins are a broad subfamily, some of whose members fall into the
class I chitinase family (see PR-3). Type 2 ribosome-inactivating proteins (RIPs) bind carbohydrates in
addition to inactivating ribosomes. It is felt that this class of lectins confers defense against mammalian
herbivores but members of this class also affect some insects. In addition, this class may act as suicide
proteins upon a breach of their vacuolar containment, disrupting protein synthesis and inducing cell death.
Monocot mannose-binding lectins are poorly understood but are presumed to have activity against in-
sects, possibly by binding gut proteins with mannose moieties. The jaculin subfamily of lectins is also
poorly understood, but feeding trials with insects have demonstrated an inhibitory effect. The Cucur-
bitaceae phloem lectins appear to interact with another phloem protein upon rupture of the phloem, form-
ing a gel, thus blocking the phloem. The presumed function of this protein would be to prevent the trans-
port of infectious or pathogenic agents via the phloem.
- Alternative Oxidase
The alternative oxidase (AOX) is the terminal oxidase of the cyanide-resistant alternative pathway found
in mitochondria. It has been shown that both gene transcript and protein levels increase upon infection in
tobacco resistant to tobacco mosaic virus. AOX is inducible by SA and sufficient correlative evidence ex-
ists to implicate AOX in interrupting viral movement and replication, either directly or indirectly [56–58].
More research is required to determine the mechanisms involved and how widespread this activity is
across different plant families and viral pathogens.
- Glutathione S-Transferases
GlutathioneS-transferases (GSTs) catalyze the conjugation of glutathione to a variety of substrates that
are typically hydrophobic, electrophilic, and cytotoxic in nature, thus detoxifying them [59]. Some class
662 ARTLIP AND WISNIEWSKI