Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 329

7.1 MAPK Signaling Three-Kinase Modules

Mitogen-activated protein kinase (MAPK) cascades are major pathways downstream
of sensors/receptors that transduce extracellular stimuli into intracellular responses in
plants (Liu et al. 2003 ; Pedley and Martin 2005 ; Tena et al. 2011 ; Hettenhausen et al.
2012 ; Zhang et al. 2012a ). A typical MAPK signaling module consists of three inter-
connected protein kinases: a MAP kinase kinase kinase (MAPKKK or MEKK [for
M APK/ E xtracellular signal-regulated kinase K inase K inase]), a MAP kinase kinase
(MAPKK or MKK), and a MAP kinase (MAPK or MPK) (Mészáros et al. 2006 ).
MAP kinase cascade involves sequence of phosphorylation events (Hirt 2000 ).
MAPKs function at the bottom of the three-kinase cascade and are activated by
MAPKKs through phosphorylation on the Thr and Tyr residues in their activation
motif between the kinase subdomain VII and VIII. The activity of MAPKKs is, in
turn, regulated by MAPKKKs via phosphorylation of two Ser/Thr residues in the
activation loop of MAPKKs. MAPKKKs receive signals from upstream receptors/
sensors (Ichimura et al. 2002 ; Li et al. 2012 ).
MAPKs (MPKs) are serine/threonine protein kinases with two-lobed structure
(Hirt 2000 ). The active site contains the MAPK-specifi c T-X--Y (threonine-X-
tyrosine, where X denotes any amino acid) motif that is targeted by MAPKKS
(MKKs). The MAPKKs are dual-specifi city protein kinases that activate MAPKs
by phosphorylation of both the threonine and tyrosine residue of the T-X-Y motif
located between kinase subdomains VII and VIII (Ligterink and Hirt 2000 ; Liu
et al. 2000 ). MAPKKs are activated themselves by phosphorylation of two con-
served serine or threonine residues (S/TXXXS/T) by MAPKKKs (MEKKs) (Hirt
2000 ). MAPKKKs contain different regulatory motifs, proline-rich sequences
involved in Src homology3 binding, zinc fi nger motifs, leucine zippers, and binding
sites for G proteins (Hirt 2000 ). MAPKKKs can be activated by various elicitors,
pathogens and a wide range of stress stimuli (Teige et al. 2004 ).
MAP kinase cascade components are abundant in plants. There are more than 80
putative MEKKS, 10 MKKs, and at least 20 MPKs in Arabidopsis (Ichimura et al.
2002 ; Jonak et al. 2002 ; Nakagami et al. 2005 ). MAPKs constitute a large gene
family with 20 family members in Arabidopsis , 15 in rice and 21 in Populus spp.
(MAPK Group 2002 ; Hamel et al. 2006 ). Several MAPKKKs, MAPKKs, and
MAPKs have been identifi ed in tobacco (Zhang and Klessig 1998a , b ; Romeis et al.
1999 ; Yang et al. 2001 ), tomato (Mayrose et al. 2004 ; Kandoth et al. 2007 ; Stulemeijer
et al. 2007 ), rice (He et al. 1999 ; Agrawal et al. 2003 ; Cheong et al. 2003 ; Xiong and
Yang 2003 ), alfalfa (Cardinale et al. 2002 ), pea (Uppalapati et al. 2004 ), and pars-
ley (Ligterink et al. 1997 ). These also include salicylic acid-induced protein kinase
(SIPK) (Zhang and Klessig 1998b ; Yang et al. 2001 ), wound-induced protein
kinase (WIPK) (Seo et al. 1995 ; Zhang et al. 2000 ; Jin et al. 2003 ; Waller et al.
2006 ), elicitor-responsive MAPK (ERMK) (Ligterink et al. 1997 ), stress- activated
MAPK (SAMK) (Nakagami et al. 2004 ), and salt-induced MAPK (SIMK) (Nakagami
et al. 2004 ). Three MAPKKs, MKK1, PRKK, and SIMKK have been isolated from
alfalfa (Cardinale et al. 2002 ; Nakagami et al. 2004 ), and two MAPKKs, MKK1