PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 70

EFR is a glycosylated transmembrane protein and therefore needs to enter the
secretory pathway to mature and to reach its fi nal plasma membrane destination
(Li et al. 2009b ). It is localized at the PM, albeit with signifi cant amounts of the
receptor in the endo-membrane compartments (Nekrasov et al. 2009 ; Saijo et al.
2009 ; Häweker et al. 2010 ). The bacterial PAMP LPS was found to bind to the
plasma membrane and to become internalized into vesicles in tobacco suspension-
cultured cells (Gross et al. 2005 ). LPS uptake was abolished by amantadine, an inhibitor
of receptor-mediated endocytosis (Gross et al. 2005 ). It suggests that the receptor of
LPS may be translocated to the vesicle.
The

-glucan binding protein (GBP) in soybean is devoid of any transmembrane
domain. Electron microscopy unraveled localization of GBP at the cytoplasmic face of
the cell wall and to vesicles at the plasma membrane. It is suggested that GBP interacts
with a receptor-like protein or RLK that is targeted for receptor-mediated endocytosis
(Fliegmann et al. 2004 ). The PRR XA21 is localized in plasma membrane and may be
endocytosed to initiate resistance responses during pathogen infection in rice (Chen
et al. 2010a ). XA21 is primarily localized to the ER but also to the PM (Park et al.
2010a , b ). The chaperone complex interacts with rice CERK1 in the endoplasmic retic-
ulum (ER) and mediates its maturation and transport to the PM (Chen et al. 2010c ).

β

2.16.2 Clathrin-Mediated Endocytosis

The PRRs are translocated to endosomes through two different pathways. The fi rst
one is clathrin-mediated while the other one is ubiquitination-mediated (Robatzek
et al. 2006 ). PRRs containing a Yxx motif show clathrin-dependent endocytosis,

whereas the PRRs containing PEST-like motif undergo endocytosis via mono-
ubiquitination (Gómez-Gómez and Boller 2000 ; Ron and Avni 2004 ; Robatzek
et al. 2006 ; Bar and Avni 2008 ).
Clathrin is a basketlike network of protein molecules that forms on the cell
membrane in response to the attachment of ligands (PAMPs) to receptors (PRRs).
It coats the endocytotic vesicles which bud off from the membrane and it becomes
inside surface of the coated vesicle during endocytosis. Clathrin-mediated endo-
cytosis of PRR has been shown to be dependent on reactive oxygen species
(ROS) production.
The oomycete PAMP cryptogein has been shown to stimulate endocytosis in
tobacco cells. Internalization of the lipophilic dye FM4-64, which is a marker of
endocytosis, was stimulated a few minutes after addition of cryptogein to tobacco
cells. The cryptogein increased clathrin-mediated endocytosis. In the presence of
cryptogein, increases in both FM4-64 internalization and clathrin-mediated endocy-
tosis are specifi cally blocked upon treatment with tyrphostin, a receptor-mediated
endocytosis inhibitor (Leborgne-Castel et al. 2008 ). The kinetics of the transient
increase in clathrin-coated pits at the plasma membrane coincided with that of transi-
tory reactive oxygen species (ROS) production occurring within the fi rst 15 min after
elicitation. In tobacco cells expressing the NADPH oxidase gene NtrbohD antisense

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