Signal Perception and Transduction in Plant Innate Immunity - Communication in Plants: Neuronal Aspects of Plant Life - page 96

While plants clearly lack adaptive immunity, their innate immune system
consists of two mechanistically and evolutionarily related branches (Nürn-
berger 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 cultivar-
specific 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 in-
direct interaction of pathogen race-specific avirulence (Avr) factors and
plantcultivar-specificresistancegeneproducts.Thus,PAMPsandAvrfac-
tors 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 non-
cultivar-specific) and plant cultivar-specific resistance represent a pathogen
non-race-specific as well as a pathogen race-specific way of coping with at-
tempted 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 non-
vertebrate organisms comprise the lipopolysaccharide (LPS) envelope of
Gram-negative bacteria, peptidoglycans from Gram-positive bacteria, eu-
bacterial 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 im-
mune responses in various plant species as well (Boller 1995; Montesano
et al. 2003; Nürnberger et al. 2004; Vorwerk et al. 2004) (Tab. 7.1). Struc-
tural conservation of the PAMPs recognized by plants and animals may
suggest a common ancestral origin of perception systems in the individ-
ual hosts. However, such similarities seem not to extend to the minimum
structural requirements for eliciting immunity in both plants and animals.