PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 67

XA21 autophosphorylation occurs on multiple residues, some of which stimu-
late XA21 function and others of which inhibit XA21 function. Phosphorylation of
certain residues on XA21 negatively regulates XA21 function, whereas phosphory-
lation on other residues may be required for activation of XA21 function.
Autophosphorylation of the XA21 JM residues Ser686, Thr688 and Ser689 is
important for stabilization of the XA21 protein (Xu et al. 2006a ). The Thr705 resi-
due in the XA21 JM region is required for binding to XA21 binding proteins (XBs)
including XB3, XB10, XB15 and XB24 (Park et al. 2008 ; Chen et al. 2010d ). The
replacement of Thr705 residue by an alanine or a glutamic acid abolishes XA21
autophosphorylation and eliminates the interactions between XA21 and XB3, XB10,
XB15 and XB24 in rice. These results suggest that after being autophosphorylated,
Thr705 may transfer its phosphoryl group to another XA21 residue, which would
activate XA21 (Chen et al. 2010a ; Park et al. 2010b ).
The Phytophthora infestans PAMP INF1 treatment of Nicotiana benthamiana
results in autophosphorylation of the PRR NbLRK1. The autophosphorylation
signal was stronger at 10 min after INF1 treatment (Kanzaki et al. 2008 ). Ser/Thr
kinase domain of the tobacco PRR NgRLK1 shows autophosphorylation activity
(Kim et al. 2010 ). NgRLK1 undergoes a conformational change upon enzymatic
activation (Kim et al. 2010 ). The PAMP chitin oligomers and chitosan rapidly
induce autophosphorylation of the PRR CERK1 at multiple residues in the juxta-
membrane and kinase domain (Petutschnig et al. 2010 ). Kinase activity of CERK1
has been shown to be required for its chitin-dependent in vivo phosphorylation.
The PRR CERK1 binds polymeric chitin oligomers. Subsequently, ligand binding
leads to phosphorylation of CERK1 in the juxtamembrane and kinase domain
(Petutschnig et al. 2010 ).

2.14.2 PRR Autophosphorylation Is Essential for PRR

to Bind to Its Negative Regulators

The activity of PRRs may be negatively regulated by some PRR binding proteins.
Autophosphorylation of the rice PRR XA21 has been shown to be essential for XA21
to bind to its negative regulators including XB10 (OsWRKY62) (Peng et al. 2008 ),
XB15 (a PP2C phosphatase) (Park et al. 2008 ) and XB24 (an ATPase) (Chen et al.
2010e ). The replacement of Thr 705 residue by an alanine or glutamic acid abolishes
XA21 autophosphorylation and eliminates interactions between XA21 and the three
XA21-binding proteins (XB10, XB15, and XB24) in rice (Chen et al. 2010d ). It sug-
gests that PRR phosphorylation is important in binding these negative regulators.
Upon perception of PAMP, the PRR may be dissociated from the negative regula-
tors. This may result in nullifying the function of the negative regulators and activat-
ing the function of PRR. One of the negative regulators in rice, XB24, is an ATPase
(Chen et al. 2010e ). The activity of the PRR XA21 is negatively regulated by XB24.
Rice lines silenced for xb24 display enhanced XA21-mediated immunity (Chen et al.
2010e ). Association between XB24 and XA21 is compromised upon inoculation of