PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 108

encodes an ubiquitin E3-ligase domain and it ubiquitinates the PRR FLS2 to
promote its degradation (Gimenez-Ibanez et al. 2009b ). E3 ligases play a key role
in the ubiquitin-proteasome-mediated degradation of protein. The effector catalyzes
polyubiquitination of the kinase domain of FLS2, resulting in degradation of the
PRR. The E3 ubiquitin ligase activity of AvrPtoB seems to be required for full
enhancement of virulence of the bacterial pathogen and degradation of the receptor
occurs during the bacterial infection (Göhre et al. 2008 ).
The effector AvrPtoB also inhibits PTI by targeting another PRR CERK1
(CHITIN ELICITOR RECEPTOR KINASE 1) for degradation (Gimenez-Ibanez
et al. 2009a , b ). To fi nd out the role of the effector AvrPtoB in inhibiting the PAMP-
triggered immunity by degrading the PRR CERK1, transgenic Arabidopsis plants
expressing avrPtoB from an inducible promoter were developed. Treatment of these
plants with chitin elicited generation of ROS, induction of defense gene expression
and deposition of callose into cell walls. All of these responses were suppressed
effi ciently by prior induction of avrPtoB transgene with dexamethasone. The PAMP
chitin treatment-induced activation of MAPKs was also abolished by AvrPtoB
(Gimenez-Ibanez et al. 2009b ). These results suggest that FLS2 and CERK1 are the
targets of AvrPtoB, leading to their degradation (Göhre et al. 2008 ; Shan et al. 2008 ;
Gimenez-Ibanez et al. 2009b ; Hann et al. 2010 ).

2.31.3 Effectors May Bind the Receptor Kinase PRRs

to Block PAMP-Triggered Immunity

Virulent pathogens may suppress the defense signal transduction-mediated by plant
transmembrane receptor kinases, the PRRs (Xiang et al. 2008 ). Pseudomonas syrin-
gae effector protein AvrPto suppresses host defenses by directly targeting the trans-
membrane receptor kinases involved in bacterial perception (Zipfel and Rathjen
2008 ). P. syringae injects two sequence-distinct effectors, AvrPto and AvrPtoB, to
intercept convergent defense responses stimulated by PAMPs. The AvrPto binds
receptor kinases, including Arabidopsis FLS2 and EFR and tomato LeFLS2, to
block plant defense responses in the plant cell (Xiang et al. 2008 ). Xiang et al.
( 2011 ) provided evidences to show that FLS2 is targeted by the P . syringae effector
AvrPto in plants. AvrPto is a kinase inhibitor that inhibits the PRR receptor kinase
activity (Xiang et al. 2008 ). The bacterial effector AvrPtoB targets the LysM recep-
tor kinase CERK1 to promote bacterial virulence (Gimenez-Ibanez et al. 2009a ). It
directly targets the kinase domains of FLS2, CERK1, and Pto (Shan et al. 2008 ).

2.31.4 Effectors May Prevent Interaction of Co-receptor

BAK1 with PAMPs

PRRs appear to interact with some transmembrane proteins that act as signaling
adapters or amplifi ers to achieve their full functionality (Zipfel 2009 ). BAK1 is an