PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 31

2.6.5 elf18 PAMP Epitope in Elongation Factor Tu (EF-Tu)

Elongation factor Thermo unstable (EF-Tu) is the abundant bacterial protein and is
involved in translation of bacterial mRNAs (Zipfel 2008 ). EF-Tu is recognized as a
PAMP in Arabidopsis and other members of the family Brassicaceae (Kunze et al.
2004 ). EF-Tu possesses all the characteristics of a typical PAMP; highly abundant,
high sequence conservation over thousands of bacterial species and vital for microbial
survival (Zipfel 2008 ). The PAMP epitope has been detected in the N terminus of
bacterial EF-Tu (Kunze et al. 2004 ; Zipfel et al. 2006 ). A highly conserved
N -acetylated 18 amino acid peptide, elf18, is suffi cient to trigger those responses
induced by full-length EF-Tu. Peptides derived from mitochondrial or plastid EF-Tu
are inactive as PAMPs, revealing that this perception is specifi c to the infectious
non-self (Zipfel 2008 ).
EF-Tu is mostly intracellular and surface localized in bacterial cell. It lacks
classical signal and transport sequences for secretion (Zipfel 2008 ). It is still not
known how EF-Tu inside the bacterial cell is recognized by the plant. Lysis of dying
bacteria in the plant cell during plant colonization may release suffi cient EF-Tu to
stimulate the receptor (Zipfel 2008 ).

2.6.6 Cold-Shock Protein (CSP22) as PAMP

Elicitation activity in some bacterial species was attributed to a cold-shock protein
rather than fl agellin (Felix and Boller 2003 ). An elicitor of defense responses found
in extracts of Micrococcus lysodeikticus was a member of the cold-shock protein
family (Felix and Boller 2003 ). The highly conserved RNA- binding motif RNP-1
of bacterial cold shock proteins (CSPs) acts as a PAMP. It triggers defense responses
in Solanaceous plants. The 22-amino acid core of RNP-1 named CSP22 is recog-
nized as elicitor by Solanaceous plants (Felix and Boller 2003 ).

2.6.7 Harpins with PAMP and Protein Secretion Structure

2.6.7.1 Several Different Harpins Are Produced

by Various Phytopathogenic Bacteria

Harpins are acidic, glycine rich, protease sensitive, and heat stable proteins that
are encoded by hrp genes present in several phytopathogenic bacteria including
members of the genera Erwinia , Pantoea , Pseudomonas , Xanthomonas , and
Ralstonia (Kvitko et al. 2007 ; Chen et al. 2008 ; Engelhardt et al. 2009 ; Tampakaki
et al. 2010 ; Boureau et al. 2011 ). Harpins are structurally unrelated proteins that
are produced and secreted by many bacterial pathogens and that share a number