PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 65

presence of one type of PAMP seems to serve as an indicator of injury or danger in
general (Zipfel et al. 2006 ). Treatment with fl g22 upregulates the transcription of
genes encoding PROPEP family precursors and both PEPR receptors, and AtPep1
treatment induces the transcription of FLS2, the fl g22 receptor (Zipfel et al. 2004 ;
Ryan et al. 2007 ).

2.13.3 PRRs May Act Additively in Perception of PAMP

Arabidopsis efr1 , fl s2 and fl s2 efr1 mutants displayed more severe disease symptoms
than the wild-type plants and allowed more bacterial growth when spray-inoculated
with P . syringae pv. tomato compared with wild-type plants. It was also observed
that efr1 and fl s2 efr1 mutants were more susceptible to P . syringae pv. tomato com-
pared with wild type and fl s2 , respectively, suggesting that FLS2 and EFR can act
additively in perception of the PAMPs of this bacterium and triggering innate immu-
nity (Nekrasov et al. 2009 ).

2.13.4 PRRs Bind with PAMPs for Their Activation

The PRRs bind with PAMPs. CERK1 binds polymeric chitin oligomers (Petutschnig
et al. 2010 ). FLS2 binds with the PAMP fl g22 (Chinchilla et al. 2006 ; Boutrot et al.
2010 ). PRRs such as FLS2 and EFR contain conserved cysteine residues fl anking
the LRR domain, which could form intermolecular disulfi de bridges allowing stable
homo- and heterodimerization or coupling to signaling molecules (van der Hoorn
et al. 2005 ; Kolade et al. 2006 ). Chinchilla et al. ( 2006 ) showed that FLS2 itself is
suffi cient to mediate fl g22 binding and there is no evidence for the involvement of
additional proteins on fl g22 binding. However, a reduction in the fl uidity of FLS2
upon stimulation by fl g22 was observed, indicative of the formation of larger
complexes (Ali et al. 2007 ). The binding of an extracellular ligand (PAMP) induces
a conformational alteration in PRRs leading to their activation (Ali et al. 2007 ).

2.13.5 Ethylene Regulates Transcription of PRRs

on PAMP Perception

It is still not fully understood how the PAMP recognition by PRR leads to increased
transcription of PRR genes. Boutrot et al. ( 2010 ) showed that ethylene is an integral
part of PAMP-triggered immunity. Plants mutated in the key ethylene-signaling
protein EIN2 are impaired in all FLS2-mediated responses, correlated with reduced
FLS2 transcription and protein accumulation. The EIN3 and EIN3-like transcrip-
tion factors, which depend on EIN2 activity for their accumulation, directly control
FLS2 expression. The results suggest a direct role for ethylene in transcription of
the PRR FLS2 (Boutrot et al. 2010 ).