Signal Perception and Transduction in Plant Innate Immunity - Communication in Plants: Neuronal Aspects of Plant Life - page 98

For example, an N-terminal 22 amino acid fragment of bacterial flagellin is
sufficient to activate immune responses in various plants (Felix et al. 1999),
but the same domain is dispensable for flagellin perception by animal cells
(Donnelly and Steiner 2002). Thus, flagellin recognition may have arisen
independently in the two phylae, and likely results from convergent evolu-
tion. The evolutionary reason, however, why flagellin perception has arisen
twice in plants and animals to activate innate defense systems is likely
because flagellin represents a predominant and microbe-specific surface
signature that facilitated non-self-recognition.
PAMPs constitute highly conserved determinants typical of whole classes
of pathogens that are not found in potential host organisms and that are in-
dispensable for the microbial lifestyle. PAMP-like elicitors of plant immune
responses match these characteristics. For example, Pep-13 (Nürnberger
et al. 1994), a surface-exposed peptide motif of a Phytophthora cell wall
transglutaminase (TGase) serves as a recognition determinant for the acti-
vation of defense in various plants, such as parsley and potato, in response
to infection by various species of the genus Phytophthora (Brunner et al.
2002; Halim et al. 2005). Pep-13 sequences are highly conserved among
Phytophthora TGases, but are not found in plant proteins and the Pep-13
motif is essential not only for elicitor activity but also for TGase activity of
the protein. Furthermore, TGase isoforms containing the Pep-13 motif are
expressed at all stages of the life cycle of Phytophthora infestans including
plant infection, suggesting a crucial role of these enzymes in Phytophthora
biology (Fabritius and Judelson 2003). In a similar study, Felix and Boller
(2003) described a cold-shock-inducible RNA-binding protein from vari-
ous Gram-positive bacteria (RNP-1) that induced innate immune responses
in tobacco. Within RNP-1 a central region was defined that was conserved
among all RNP-1 orthologs tested from various bacteria. Remarkably, this
region was found to be indispensable not only for the RNA-binding activity
of the protein but also for its immune-stimulating capacity.
7.3
Plant Pattern Recognition Receptors Mediate
PAMP Perception and Activation
of Non-Cultivar-Specific Plant Defense
In animals (ranging from crustaceans to insects to vertebrates), PAMP per-
ception is brought about by pattern recognition receptors that distinguish
self from conserved microbial structures (Aderem and Ulevitch 2000; Cook
et al. 2004; Girardin et al. 2002; Medzhitov and Janeway 2002). Drosophila
Toll and mammalian Toll-like receptors (TLRs) recognize PAMPs through