PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 69

2010e ). XB24 enhances XA21 autophosphorylation and its ATPase activity is
required for this function. In plant a silencing of Xb24 expression enhances XA21-
mediated disease resistance (Chen et al. 2010e ). Association between XB15 and
XA21 is compromised while the association between XB15 and XA21 is enhanced
upon Ax21 triggering (Park et al. 2008 ; Chen et al. 2010e ). It is possible that the
regulation by XB24 occurs before Ax21 recognition while regulation by XB15
occurs after Ax21 recognition (Park et al. 2010a , b ).

2.16 Translocation of PRRs from Plasma Membrane

to Endocytic Compartments

2.16.1 Endocytosis of PRRs

Receptor endocytosis appears to be a common phenomenon in plant defense
signaling system (Altenbach and Robatzek 2007 ; Chinchilla et al. 2007a ; Robatzek
2007 ; Geldner and Robatzek 2008 ; Groen et al. 2008 ; Chen et al. 2010a ). The
endocytic machinery regulates the space and the time of signal transduction and
processing in the cell (Irani and Russinova 2009 ). Plasma membrane resident
receptors may be translocated into endosomes and it helps to extend the signaling
surface ensuring a robust and effi cient cellular signaling system (Geldner and
Robatzek 2008 ). Translocation of PRRs from plasma membrane to endocytic com-
partments has been widely reported (Fliegmann et al. 2004 ; Gross et al. 2005 ;
Robatzek et al. 2006 ; Leborgne-Castel et al. 2008 ).
The PRR FLS2 is found localized at the plasma membrane. When activated by
the PAMP fl g22, FLS2 is translocated to endocytic compartments. The induced
FLS2 endocytosis is dependent on cytoskeleton and proteasome function. FLS2
lacks a Yxx

= hydrophobic residue), which is
known to play a role in clathrin-dependent endocytosis, but contains a PEST-like
motif, which is reported to mediate receptor endocytosis via mono-ubiquitination.
Single mutations in the PEST-like motif or at a conserved, potentially phosphory-
lated, residue in the JM region impaired FLS2 endocytosis (Robatzek et al. 2006 ).
Endocytosis of the PRR FLS2 has been shown to be important for the PAMP fl g22-
induced defense signaling system (Chinchilla et al. 2007b ).
The tomato PRR LeEIX2 belongs to a superclade of leucine-rich repeat receptor-
like proteins (RLP) with a signal for receptor-mediated endocytosis (Bar et al. 2010 ).
It carries the Yxx

Φ motif (Y = Tyr, x = any amino acid, Φ

motif, which is involved in endocytosis, in its C-terminal part.
Mutation in this endocytosis motif resulted in abolishment of HR induction in
response to the PAMP EIX, suggesting that endocytosis plays a key role in mediating
the signal generated by EIX that leads to HR induction (Ron and Avni 2004 ). EIX
triggers internalization of the LeEIX2 receptor on endosomes, which is dependent on
an intact cytoskeleton. LeEIX2 is internalized on highly motile endosome 15-20 min
after EIX application (Bar and Avni 2008 ). Similar swift endocytic process has been
reported in fl g22-activated FLS2 internalization (Robatzek et al. 2006 ).

Φ