Signal Perception and Transduction in Plant Innate Immunity - Communication in Plants: Neuronal Aspects of Plant Life - page 99

an extracellular leucine-rich repeat (LRR) domain and transduce the PAMP
signal through a cytoplasmic Drosophila Toll/Interleukin-1 receptor (TIR)
domain (Cook et al. 2004; Underhill and Ozinsky 2002). Plant binding pro-
teins for numerous microbe-derived compounds have been kinetically and
biochemically characterized, but in only a very few cases purification of
the receptor protein or isolation of the encoding gene has been achieved
(Montesano et al. 2003; Nürnberger et al. 2004). Fabaceae species recognize
1,6- β β -branched heptaglucosides (HGs) from the cell walls of

the phytopathogenic oomycete, Phytophthora sojae , for activation of plant
defense through binding to a 75-kDa HG-binding protein (HGP) (Mithöfer
et al. 1999). Intriguingly, soybean HGP harbors endoglucanase activity that
releases oligomeric 1,3-

-linked, 1,3-

β -D-oligoglucosides consisting of at least four moi-

eties from complex glucans (Fliegmann et al. 2004). Thus, upon contact
with Phytophthora , HGP may facilitate release of oligoglucoside fragments
from the oomycete cell wall that are appropriate PAMPs for binding to
the glucan-binding domain of the same protein. As HGP does not show
any transmembrane domain it is likely to interact in concert with other
proteins in PAMP perception and intracellular signal transduction. Re-
markably, PAMP perception systems in animals are often multicomponent
complexes as well (Underhill and Ozinsky 2002).
The flagellin receptor, FLS2, from Arabidopsis thaliana constitutes
a transmembrane receptor protein kinase with an extracellular LRR domain
(LRR-receptor-like kinase, LRR-RLK) (Gomez-Gomez and Boller 2000). Ex-
tracellular LRR domains are also found in the Drosophila To l l r e c e p t o r a n d
in the ten vertebrate TLRs (Cook et al. 2004; Underhill and Ozinsky 2002),
all of which are implicated in the activation of innate immune responses.
Little sequence similarity between the LRR domains of FLS2 and the animal
flagellin receptor, TLR5, and the aforementioned fact that both receptors
recognize different structures of eubacterial flagellin make it very likely that
both receptors evolved independently as a result of convergent evolution
(Gomez-Gomez and Boller 2000; Hayashi et al. 2001). This is even more
evident as FLS2 and TLR5 also differ in the structure of their cytoplas-
mic domains. While FLS2 harbors a cytoplasmic Ser/Thr protein kinase
domain (Gomez-Gomez and Boller 2000), TLR5 carries an intracellular
TIRdomainthatisindirectlyassociatedwiththeinterleukin-1-receptor
associated kinase (IRAK) via the adaptor protein MyD88 (Hayashi et al.
2001). It is remarkable, however, that LRR domains have evolved indepen-
dently for extracellular ligand recognition, which likely reflects the unique
features of this domain for mediating intermolecular interactions. In that
respect it is worth noting that the plant brassinosteroid receptor, BRI1,
directly binds the brassinolide ( a plant steroid hormone) through one of
the LRR stretches (Kinoshita et al. 2005). This suggests that LRR domains
may not only facilitate protein-protein interactions, but may even act as