Introduction - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 4

have been recognized in plasma membrane of plant cells (Nicaise et al. 2009 ;
Petutschnig et al. 2010 ; Shinya et al. 2010 ; Schulze et al. 2010 ; Segonzac and Zipfel
2011 ). The PRRs identifi ed to date are modular proteins harbouring an extracellular
domain consisting of leucine-rich repeat (LRR) or lysine motifs (LysM) (Saijo
2010 ; Segonzac and Zipfel 2011 ). Most of the PRRs are receptor-like kinases
(RLKs) and the sensors for extracellular molecules consisting of an extracellular
ligand-binding domain, a single transmembrane domain, and a cytosolic protein
kinase domain are called RLKs (Seifert and Blaukopf 2010 ). RLKs are proteins
with a “receptor” and a “signaling domain” in one molecule. The extracellular domains
of RLKs bind directly to legands to perceive extracellular signals (PAMPs), whereas
the cytoplasmic kinase domains transduce these signals into the cell (Bi et al. 2010 ).
PRRs interact with additional transmembrane proteins which act as signaling
amplifi ers to achieve their functionality (Zipfel 2009 ). PAMPs bind with PRRs and
induce conformational alteration in PRRs leading to their activation (Ali et al. 2007 ).
PAMPs trigger increased transcription of PRR genes and accumulation of PRR
proteins (Qutob et al. 2006 ; Lohmann et al. 2010 ). Most of the PRRs are receptor
kinases and the PAMPs induce autophosphorylation of the kinase domain of PRRs
(Kanzaki et al. 2008 ; Xiang et al. 2008 ).
The plasma membrane resident autophosphorylated PRRs are translocated to
endosomes and it helps to extend the signaling surface ensuring a robust and effi cient
cellular signaling system (Geldner and Robatzek 2008 ). PAMPs induce ubiquitin-
proteasome- or clathrin-mediated endocytosis of PRR proteins (Robatzek et al.
2006 ; Aker and de Vries 2008 ). PAMP-induced PRR-induced endocytosis has been
shown to be dependent on phosphorylation of the PRR (Robatzek et al. 2006 ).
PAMP-induced internalization of PRRs from the plasma membrane is closely
correlated with their immune function (Bar et al. 2009 ; Saijo 2010 ). The biogenesis
of trans-membrane PRRs may occur through the endoplasmic reticulum (ER) with
the aid of ER -resident chaperones (Dodds and Rathjen 2010 ; Popescu 2012 ). After
biosynthesis of PRR in ER, it is transported from the ER to the plasma membrane.
N -glycosylation of PRRs is required for transport of PRRs from ER to plasma
membrane (Häweker et al. 2010 ). Sustained activation of plasma membrane-
resident PRR signaling is important for mounting robust PAMP-triggered immunity
(Saijo 2010 ).

1.3 Second Messengers in PAMP Signal Transduction

The plant immune system uses several second messengers to encode information
generated by the PAMP/PRR signaling complex and deliver the information
downstream of PRRs to proteins which decode/interpret signals and initiate defense
gene expression (van Verk et al. 2008 ; Mersmann et al. 2010 ; Boudsocq et al. 2010 ;
Hwang and Hwang 2011 ). It is still not known how the PAMP signals are transmitted
downstream of PRR. In plant cells, the calcium ion is a ubiquitous intracellular
second messenger involved in numerous signaling pathways (Luan 2009 ; McAinsh