Abstract
In incompatible host-pathogen interactions, damage
caused by the pathogen remains restricted as a result of
the plant's defensive response. Most effective is the
hypersensitive reaction, in which the cells around the
infection site rapidly necrose. This response is associated
with a coordinated and integrated set of metabolic
alterations that are instrumental in impeding further
pathogen ingress, as
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well as in enhancing the capacity of
the host to limit subsequent infection by different types of
pathogens [27, 77]. Altered ion fluxes across the plant cell
membrane, generation of active oxygen species, changes
in the phosphorylation state of regulatory proteins and
transcriptional activation of plant defense systems culminate
in cell death at the site of infection, local
accumulation of phytoalexins and cell wall rigidification
as a result of callose, lignin and suberin deposition [31, 89].
In addition, various novel proteins are induced which are
collectively referred to as ``pathogenesis-related proteins ''
(PRs). These PRs, defined as proteins coded for by the
host plant but induced specifically in pathological or
related situations [4, 81], do not only accumulate locally
in the infected leaf, but are also induced systemically,
associated with the development of systemic acquired
resistance (SAR) against further infection by fungi,
bacteria and viruses. Induction of PRs has been found in
many plant species belonging to various families [78],
suggestive of a general role for these proteins in adaptation
to biotic stress conditions. SAR, likewise, is a generally
occurring phenomenon, that engenders an enhancement
of the defensive capacity of plants in response to
necrotizing infections [70]. Since some of the tobacco PRs
were identified as chitinases [45] and b-1,3-glucanases
[38] with potential antifungal activity, it has often been
suggested that the collective set of PRs may be effective in
inhibiting pathogen growth, multiplication and/or
spread, and be responsible for the state of SAR [42, 65].
Originally, five main classes of PRs (PR-1-5) were
characterized by both biochemical and molecular-biological
techniques in tobacco [9, 80]. Thereupon, in 1994
a unifying nomenclature for PRs was proposed based on
their grouping into families sharing amino acid sequences,
serological relationship, and/or enzymatic or biological
activity. By then 11 families (PR-1--11) were recognized
and classified for tobacco and tomato [81] (cf. Table 1).
Criteria used for the inclusion of new families of PRs were
that (i) protein(s) must be induced by a pathogen in
tissues that do not normally express the protein(s), and
(ii) induced expression must have been shown to occur in
at least two different plant-pathogen combinations, or
expression in a single plant-pathogen combination must
have been confirmed independently in diåerent laboratories.
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