PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 68

the rice bacterial blight pathogen Xanthomonas oryzae pv. oryzae , which secretes the
PAMP Ax21 (Chen et al. 2010e ). The PRR XA21 protein is present on the plasma
membrane after transit from the endoplasmic reticulum (ER) (Park et al. 2010a , b ),
where it recognizes the PAMP Ax21. XB24 physically associates with XA21 and
uses ATP to promote phosphorylation of certain Ser/Thr sites on XA21, keeping the
XA21 protein in an inactive state (Chen et al. 2010e ). Upon recognition of the PAMP
Ax21, XA21 may be dissociated from the negative regulator XB24 and/or XB24-
promoted autophosphorylation may be removed (Chen et al. 2010e ). When the function
of XB24 is nullifi ed, the XA21 kinase becomes activated, triggering downstream of
defense responses (Chen et al. 2010e ). These results suggest that PAMP may activate
PRR by removing the action of negative regulators of PRR.

2.15 Negative Regulation of PRR Signaling

Improperly regulated plant immune responses can lead to the overexpression of
defense-related genes and cell death (Park et al. 2008 ; Schweswinger and Zipfel
2008 ). It is therefore necessary that the PRR signaling components, as well as the
PRRs themselves, are tightly regulated. Although PRR-mediated immune responses
are clearly essential for innate immunity in plants, sustained or highly induced
immune response can be harmful. It is therefore necessary that PRR signaling
through non-RD kinases be under tight negative regulation.
One important class of negative regulators is protein phosphatase 2Cs
(PP2Cs). Arabidopsis PP2C, kinase-associated protein phosphatase (KAPP)
interacts with BAK1 and the PRR FLS2 (Gómez-Gómez et al. 2001 ). In
Arabidopsis , FLS2 is negatively regulated by KAPP, which blocks activated
FLS2 signaling and attenuates the downstream innate immune response (Gómez-
Gómez et al. 2001 ). Overexpression of KAPP in Arabidopsis results in loss of
sensitivity to fl agellin treatment, suggesting that KAPP negatively regulates the
FLS2-mediated immune response (Gómez-Gómez et al. 2001 ).
XB15, a PP2C phosphatase, is a PRR binding protein in rice. It dephosphorylates
autophosphorylated XA21 and negatively regulates the PRR XA21-mediated innate
immune responses (Wang et al. 2006 ; Park et al. 2008 ). Phosphorylation of certain
residues is required for activation of XA21 function. These residues may be dephos-
phorylated by XB15 to down-regulate XA21 activity (Park et al. 2008 ). XB15,
another rice protein, negatively regulates the XA21-mediated innate immune
response (Park et al. 2008 ). Overexpression of the Xb15 in an XA21 rice line com-
promised resistance to X . oryzae pv. oryzae (Park et al. 2008 ).
Another rice PRR binding protein, XB24, is an ATPase and it regulates XA21-
mediated immunity (Chen et al. 2010e ). XB24 displays signifi cant ATP hydrolysis
activity, while XB24 mutant containing a single aminoacid change Ser154 with Ala
had only negligible ATPase activity, indicating that amino acid Ser154 is essential
for its ATPase activity (Chen et al. 2010e ). XB24 promotes autophosphorylation of
the XA21 protein in vitro . XB24 is not transphosphorylated by the XA21 protein in
the absence or presence of X. oryzae pv. oryzae expressing AX21 (Chen et al.