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XA21-mediated resistance. Cross-linking experiments suggested that Ax21
directly binds XA21. Ax21 is conserved in most species of Xanthomonas and the
tyrosine sulfation is required for its recognition by XA21 (Shen et al. 2002; Lee
et al. 2006b , 2009 ).
The PAMP Ax21 protein carries two predicted tyrosine sulfation sites. An
Ax21-derived synthetic peptide (17-amino acid) containing a sulfated tyro-
sine-22 (axY s 22) is suffi cient for Ax21 activity, whereas peptides lacking tyro-
sine sulfation and peptide variants carrying alanine in place of the tyrosine are
inactive (Lee et al. 2009 ). The peptide axY s 22 directly binds to XA21 (Lee et al.
2009 ). Although all Xanthomonas oryzae pv. oryzae ( Xoo ) strains tested carry
ax21 (Lee et al. 2009 ), Xoo strains lacking the sulfation and/or secretion systems
can no longer elicit the XA21-mediated defense response (de Silva et al. 2004 ).
These results suggest that sulfation on the axY s 22 peptide is critical for XA21/
Ax21 recognition in rice.
The sulfated protein Ax21 is secreted by Xoo through type I secretion system
(Zhang and Zhou 2010 ). The genes raxA , raxB , and raxC encoding components of
a bacterial type I secretion system have been detected in Xoo . Xoo mutants carry-
ing knockouts in any of these genes lose the ability to trigger XA21-mediated
immunity and are no longer able to secrete Ax21 (Lee et al. 2006b ). The genes
involved in sulfation, raxST , raxR , and raxP , were also detected in Xoo . Tha raxST
encodes tyrosine sulfotransferase and the raxR and raxP genes are involved in
synthesis of 3
-phosphosulfate (PAPS). RaxST may utilize
PAPS to transfer a sulfuryl group to Ax21 (Lee et al. 2006b ). These results suggest
Ax21 is sulfated and secreted through the bacterial type 1 secretion system (Park
et al. 2010b ).
Ax21 is present in all sequenced Xanthomonas species, in Xyllella fastidiosa ,
the causal agent of Pierce's disease on grapes, and in the human pathogen,
Stenotrophomonas maltophila (Lee et al. 2009 ). Thus, Ax21 satisfi es a key aspect
of the defi nition of PAMPs. A Xoo mutant strain lacking Ax21 was unable to trig-
ger XA21-mediated immunity (Park et al. 2010b ). It shows that Ax21 is a PAMP
triggering defense responses.
-phosphoadenosine 5
2.6.9
Rhamnolipids as PAMPs
Rhamnolipids derived from Pseudomonas aeruginosa , an opportunistic pathogen of
plants, were identifi ed as PAMPs recognized by grapevine (Varnier et al. 2009 ). They
trigger the early signaling events including Ca 2+ infl ux, mitogen-activated protein
kinase activation and reactive oxygen species production, which are the characteris-
tic components in PAMP-triggered immunity (Varnier et al. 2009 ). The rhamnolipids
effi ciently protected grapevine against Botrytis cinerea (Varnier et al. 2009 ).
Rhamnolipids potentiate defense responses induced by another PAMP chitosan in
grapevine (Vatsa et al. 2010 ). Rhamnolipids were able to stimulate defense genes in
tobacco, wheat, and Arabidopsis thaliana , besides grapevine (Vatsa et al. 2010 ).
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