Signal Perception and Transduction in Plant Innate Immunity - Communication in Plants: Neuronal Aspects of Plant Life - page 102

intracellular NBS-LRR proteins carrying an additional CARD domain
(NOD1, NOD2) are implicated in intracellular PAMP sensing in animals
(Girardin et al. 2002), while TIR-LRR proteins are implicated in PAMP
sensing at the animal cell plasma membrane (Underhill and Ozinsky 2002).
Thus, structural modules implicated in PAMP perception in animals do not
only resemble plant PAMP perception systems (FLS2 vs. TLR5), but also
perception systems for pathogen race-specific Avr proteins.
7.5
Intracellular Signal Transduction in Plant Innate Immunity
Signaling pathways mediating PAMP-induced defense responses employ
altered cytoplasmic Ca 2+ levels, reactive oxygen species (ROS), nitric ox-
ide (NO) and posttranslationally regulated mitogen-activated protein ki-
nase (MAPK) activity (Jonak et al. 2002; Nürnberger et al. 2004; Zhang
and Klessig 2001). Intriguingly, most of these components are important
for PAMP-induced activation of innate immune responses in animal cells
(Barton and Medzhitov 2003). NO production appears to be a common
characteristic of plants that are under pathogen attack (Clarke et al. 2000;
Delledonne et al. 1998; Durner et al. 1998). Although plant enzymes that are
homologous to human NO synthase (hNOS) have not been reported, phar-
macological evidence has been presented for a plant enzyme that mecha-
nistically resembles the human ortholog (Delledonne et al. 1998; Durner
et al. 1998). Recently, Zeidler et al. (2004) showed that AtNOS1, a plant-
specific NOS previously associated with hormone signaling in plants (Guo
et al. 2003), mediated LPS-induced NO production and PR gene expression
in A. thaliana (Zeidler et al. 2004). Importantly, inactivation of the AtNOS1
gene did not only abrogate LPS-induced NO production in these plants, but
also dramatically enhanced susceptibility of the mutant to P. s y r i n g a e pv.
tomato DC3000 infection.
MAPKs constitute central points of crosstalk in stress signaling, in-
cluding those that result in protection against microbial invasion (Gomez-
Gomez and Boller 2002; Jonak et al. 2002; Nürnberger and Scheel 2001;
Zhang and Klessig 2001). Transient posttranslational activation of MAPK
activityhasbeenreportedfromvariouspathogen-infectedplantsorfrom
plants that were infiltrated with different PAMPs. In particular, two isoforms
of a total of 20 Arabidopsis MAPKs (AtMPK3 and AtMPK6) and their ho-
mologous proteins in other plant species are responsive to PAMP treatment
or pathogen infection (Jonak et al. 2002; Zhang and Klessig 2001). In PAMP-
treated parsley cells, PcMPK3 and PcMPK6 translocate to the nucleus (Lee
et al. 2004; Ligterink et al. 1997), where they likely contribute to ROS-
independent, WRKY transcription factor-dependent PR gene expression