PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 33

2.6.7.4 A Specifi c Region in Harpin with Pore Formation Function

May Be Involved in Delivery of the PAMP Residing Within
Harpin into Plant Cells

Many bacterial pathogens use type-III protein secretion systems (TTSS) to infect
plants. TTSS are molecular conduits that facilitate the injection of bacterial effec-
tors into plant cells to manipulate host physiology. Harpins from various P . syringae
pathovars form ion-conducting pores, suggesting a role of the harpin proteins in
effector delivery during infection (Lee et al. 2001b ; Fu et al. 2006 ). The harpin
HrpZ of P . syringae showed membrane-binding and pore-forming activities in
vitro , suggesting that it could be targeted to the host cell plasma membrane
(Haapalainen et al. 2011 ). HrpZ was found to interact with the lipid phosphatidic
acid and pore-formation by HrpZ in artifi cial lipid vesicles was found to be depen-
dent on the presence of phosphatidic acid. In addition, HrpZ was able to form pores
in vesicles prepared from Arabidopsis thaliana plasma membrane. These studies
suggest that the harpin HrpZ is targeted to the host cell plasma membrane and it
binds with the lipid layer.
HrpG forms dimers and higher order oligomers. The oligomerization was mainly
mediated by a region near the C-terminus of the protein and the same region was
also found to be essential for membrane pore formation. Phosphatidic acid binding
appears to be mediated by two regions separate in the primary structure. A 24-amino-
acid HrpG fragment found in the region was shown to be indispensable for the
oligomerization and pore formation functions (Haapalainen et al. 2011 ). The pore
formation activity of harpins may facilitate translocation of the PAMP found within
the harpin structure into plant cells and trigger the expression of genes involved in
defense signaling systems.

2.6.8 Ax21 Sulfated Protein as a PAMP

Ax21 (activator of XA21-mediated immunity) isolated from the rice bacterial
blight pathogen Xanthomonas oryzae pv. oryzae ( Xoo ) has been identifi ed as an
elicitor and it triggers hypersensitive reaction (HR) in rice cultivars expressing the
R protein XA21 (Lee et al. 2006b ). Xa21 was cloned in 1995 as a disease resistance
gene expressing resistance against wide range of Xoo strains and it was the fi rst
disease resistance gene cloned from rice (Song et al. 1995 ). The sequence of the
predicted protein encoded by the Xa21 gene carried both a leucine-rich repeat
motif and a serine-threonine kinase-like domain, suggesting a role in cell surface
recognition of a pathogen ligand and subsequent activation of an intracellular
defense response (Song et al. 1995 ). After 14 years, Lee et al. ( 2009 ) have shown
that XA21 is a pattern recognition receptor (PRR) and it recognizes a 194-amino
acid protein designated Ax21 as the pathogen ligand and as a pathogen-associated
molecular pattern (PAMP). A tyrosine-sulfated 17-amino acid synthetic peptide
corresponding to the N-terminus of Ax21 was fully active in eliciting