Signal Perception and Transduction in Plant Innate Immunity - Communication in Plants: Neuronal Aspects of Plant Life - page 95

7 Signal Perception and Transduction

in Plant Innate Immunity

Thorsten Nürnberger, Birgit Kemmerling

Abstract Innate immunity is an ancient form of defense against microbial infection that
is shared by plants, insects and vertebrates. Like animals, plants have evolved perception
systems for multiple, highly invariant pathogen-associated molecular patterns (PAMP) that
trigger basal or non-cultivar-specific defense responses in plants. Non-self-recognition is
brought about by specific plant receptors that are structurally similar to Toll-like receptors
mediating PAMP perception and activation of innate immune responses in animals. In
addition to PAMP-mediated plant innate immunity, disease-resistance programs are often
also initiated through plant cultivar-specific recognition of microbial race-specific viru-
lence factors, a recognition specificity that is not known from animals. Plant species and
plant cultivar-specific resistance represent evolutionarily linked types of immunity that are
collectively referred to as the plant innate immune system. Signal transduction cascades
that mediate activation of innate immune responses comprise elements that are common to
both forms of plant immunity, such as alterations in cytoplasmic calcium levels, mitogen-
activated protein kinase activities or reactive oxygen species production. Forward genetic
approaches have, however, identified loci that contribute specifically to either plant species
or plant cultivar-specific immunity.

7.1
Introduction
Immunity of multicellular organisms to microbial invasion is based upon
the host's ability to discriminate between “self” and potentially dangerous
“nonself ” structures (Hoebe et al. 2004). Phylogenetically ancient defense
mechanisms, termed the innate immune system, make use of nonclonal,
germline-encoded pattern recognition receptors that recognize pathogen-
associated molecular patterns (PAMPs) (Medzhitov and Janeway 1997).
These structures are unique to microbes (not found in potential hosts), are
invariant among microorganisms of a given class, and are indispensable
for microbial physiology (Medzhitov and Janeway 2002). Innate immunity
appears to operate in virtually all higher organisms, including vertebrates,
insects and plants (Medzhitov and Janeway 2002; Nürnberger and Brun-
ner 2002). In contrast to this broad phylogenetic distribution of innate
immune systems, adaptive immunity arose only recently in evolution and
is restricted to jawed vertebrates (Hoebe et al. 2004). This type of immu-
nity is based on somatic recombination mechanisms that generate a large
repertoire of antigen receptors which are clonally distributed on T- and
B-lymphocytes (Kasahara et al. 2004).