References

96 T. Nürnberger, B. Kemmerling
While plants clearly lack adaptive immunity, their innate immune system
consists of two mechanistically and evolutionarily related branches (Nürnberger
et al. 2004). PAMP-based activation of innate defense responses is
considered to form the molecular basis of plant species-specific or basal
resistance, a type of resistance that is effective in all plant cultivars of
a given species against all races of a pathogen species (Espinosa and Alfano
2004; Thordal-Christensen 2003). A current model predicts that evolution
of pathogen race-specific virulence mechanisms that help pathogens to
overcome basal defense responses facilitated the evolution of plant cultivarspecific
disease resistance in plants (Espinosa and Alfano 2004). This type
of pathogen race/plant cultivar-specific resistance constitutes the second
branch of the plant immune system and is governed by the direct or indirect
interaction of pathogen race-specific avirulence (Avr) factors and
plantcultivar-specificresistancegeneproducts.Thus,PAMPsandAvrfactors
can clearly be distinguished by their natural distribution (microbial
species-specific vs. microbial race-specific) and their plant species or plant
cultivar-specific activity profile. In summary, plant species (or plant noncultivar-specific)
and plant cultivar-specific resistance represent a pathogen
non-race-specific as well as a pathogen race-specific way of coping with attempted
microbial invasion, and should be considered as two distinct, but
evolutionarily interrelated types of resistance that constitute plant innate
immunity.
7.2
PAMPs as Triggers of Nonplant Cultivar-Specific
Innate Immune Responses
PAMP-based activation of innate immunity is almost invariably found in
multicellular eukaryotes (Aderem and Ulevitch 2000; Cook et al. 2004;
Imler and Hoffmann 2001; McGuinness et al. 2003). Major examples of
PAMPs triggering innate immune responses in various vertebrate and nonvertebrate
organisms comprise the lipopolysaccharide (LPS) envelope of
Gram-negative bacteria, peptidoglycans from Gram-positive bacteria, eubacterial
flagellin, methylated bacterial DNA fragments and fungal cell wall
derived glucans, chitins, mannans and proteins (Cook et al. 2004; Girardin
et al. 2002). Remarkably, many of these molecules act as triggers of immune
responses in various plant species as well (Boller 1995; Montesano
et al. 2003; Nürnberger et al. 2004; Vorwerk et al. 2004) (Tab. 7.1). Structural
conservation of the PAMPs recognized by plants and animals may
suggest a common ancestral origin of perception systems in the individual
hosts. However, such similarities seem not to extend to the minimum
structural requirements for eliciting immunity in both plants and animals.