Protein Phosphorylation and Dephosphorylation in Plant Immune Signaling Systems - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 385

of the MAPK cascades triggers massive transcript changes and confers resistance to
multiple pathogens (He et al. 2007 ).
The CDK (cyclin-dependent kinase) family protein kinases have a regulatory
subunit, cyclin, and a catalytic subunit, CDK. In addition to their interaction with
cyclins, CDKs are themselves regulated by protein phosphorylation. Phosphorylation
of a Thr residue is required for kinase activation, whereas phosphorylation of a Tyr
residue serves an inhibitory function (Stone and Walker 1995 ). The GSK-3 family
includes ASKs (Apoptosis signal regulating kinases). ASKs autophosphorylate on
Ser, Thr, and Tyr. The CKII family protein kinases phosphorylate and promote the
DNA-binding activity of G-box binding factor 1, a transcription factor that binds to
the plant G-box promoter element (Klimczak et al. 1995 ).
PTK group includes Tyr-specifi c protein kinases, which have not been detected
in plants (Stone and Walker 1995 ). Several other protein kinases, which do not fall
into any of the above four groups have been detected in plants. These include pro-
tein kinases belonging to His kinase family. His kinase family protein kinase takes
part in ethylene signaling system. In Arabidopsis , ethylene is perceived by a family
of fi ve membrane-bound receptors (ETR1, ERS1, ETR2, EIN4, and ERS2), which
transmit the signal to downstream effectors Among them the ETR1 receptor shows
His kinase activity, while others show Ser/Thr kinase activity (Wang et al. 2006 ).
The other group of protein kinases includes receptor-like kinases (RLKs). These
RLKs are serine/threonine kinases (Shiu and Bleecker 2001a , b ). Large numbers of
RLKs have been detected in plants (Wang et al. 2005a , b ; Benschop et al. 2007 ) and
at least 340 genes encoding putative RLKs have been detected in Arabidopsis
genome (Montesano et al. 2003 ). RLKs are transmembrane proteins that recognize
an extracellular signal, in the form of a polypeptide ligand. Ligand-binding leads to
autophosphorylation on the cytoplasmic kinase domain, a requirement for propaga-
tion (Stone and Walker 1995 ).

9.4 PAMPs/Elicitors Activate Receptor-Like Kinases

Plants rely on recognition of elicitors to activate defense signaling. Defense signal-
ing is initiated through interaction of the elicitor with a plasma membrane-localized
receptor (Zipfel et al. 2004 ; Benschop et al. 2007 ). Most of the receptors belong to
the huge family of receptor-like kinases/receptor protein kinases (RLKs/RPKs;
Tena et al. 2011 ). About 610 receptor-like kinases (RLKs) have been identifi ed in
the Arabidopsis thaliana genome (Shiu and Bleecker 2001b ). Several receptor-like
kinases (RLKs) have been found to be phosphorylated in response to elicitors in
A. thaliana (Benschop et al. 2007 ). For example, the general elicitor fl agellin is
recognized in Arabidopsis through a conserved 22-amino acid sequence (fl g22).
Recognition involves the receptor-like kinase FLS2, which activates a downstream
response that includes the production of ROS, ethylene biosynthesis, activation of a
MAPK cascade, and activation of defense gene expression (Navarro et al. 2004 ;
Zipfel et al. 2004 ). The PAMP has been shown to activate the receptor kinase by