Protein Phosphorylation and Dephosphorylation in Plant Immune Signaling Systems - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 384

PAMP chitin treatment in Arabidopsis (Peck et al. 2001 ). Perception of fl g22
activates MAP kinase activity within minutes and it peaks within 5-10 min and
diminishes after 60 min in Arabidopsis suspension-cultured cells (Nühse et al.
2000 ). Transient protein phosphorylation is involved in various defense signal
transduction systems triggered by PAMPs (Dietrich et al. 1990 ; Felix et al. 1991 ;
Lecourieux-Ouaked et al. 2000 ; Nühse et al. 2000 , 2003 ; Peck et al. 2001 ; Romeis
et al. 2001 ; Lecourieux et al. 2002 , 2006 ).

9.3 Protein Phosphorylation Is Carried Out by Different

Protein Kinases

Protein phosphorylation is carried out by different protein kinases. Protein kinase
superfamily has been classifi ed into fi ve main groups. These include “AGC” group
(protein kinase A, G, and C group), CaMK group (calcium and calmodulin-dependent
protein kinase group), CMGC group (cyclin-dependent kinase group), PTK group
(protein Tyr kinase group), and “other” group. The AGC group is represented by the
cyclic nucleotide-dependent kinases (protein kinase A [PKA] and protein kinase G
[PKG]) and the calcium-phospholipid-dependent kinases (protein kinase C [PKC]).
This group is regulated by cAMP (cyclic adenosine monophosphate), cGMP (cyclic
guanosine monophosphate), diacylglycerol, and Ca 2+ (Stone and Walker 1995 ).
Protein kinase C has been reported to induce phosphorylation in soybean (Dröge-
Laser et al. 1997 ) and potato (Després et al. 1995 ; Subramaniam et al. 1997 ).
The CaMK group of protein kinases includes the calcium-/calmodulin-dependent
and SNF1 (Suc nonfermenting1)/AMP-activated protein kinase families. Calcium-
dependent protein kinases (CDPKs) are the most common protein kinases in plants
(Ellard-Ivey et al. 1999 ; Harmon et al. 2000 ; Romeis et al. 2000 , 2001 ; Cheng et al.
2002 ; Zhang et al. 2002 ; Harmon 2003 ; Hrabak et al. 2003 ; Boudsocq et al. 2010 ).
The CDPKs constitute one of the largest families of protein kinases. Thirty-four
different genes encoding CDPKs have been detected in A. thaliana (Harmon et al.
2000 ; Cheng et al. 2002 ; Hrabak et al. 2003 ).
The CMGC group contains MAPK (mitogen-activated protein kinase), CDK
(cyclin-dependent kinase), GSK-3 (glycogen synthase kinase-3), and CKII (casein
kinase II) families. MAPKs are also known as ERKs (extracellular-regulated pro-
tein kinases) and these are Ser-Thr protein kinases activated by dual phosphoryla-
tion. The enzyme responsible for this dual phosphorylation, MAPKK (MAPK
kinase), represents an unusual class of protein kinases that will phosphorylate on
Ser, Thr, and Tyr residues (Stone and Walker 1995 ). A MAPK cascade generally
involves MAP kinase kinase kinase (MAPKK) - MAPKK - MAPK module that
transduces extracellular signals through the receptors into a wide range of intracellular
responses (He et al. 2007 ). In this module, a MAPKKK phosphorylates and activates
a MAPK. Activated MAPK is imported into the nucleus, where it phosphorylates
and activates specifi c downstream signaling components such as transcription
factors (Ligterink and Hirt 2000 ). In Arabidopsis, 68 MAPKKKs (MTKs), 10 MAPKKs
(MKKs), and 20 MAPKs (MPKs) have been identifi ed (He et al. 2007 ). Activation