Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 330

MEKK1

MKK4 MKK4 MKK5 MKK5 MKK1 MKK1 MKK2 MKK2

MPK3 MPK6 MPK3 MPK6 MPK4 MPK6 MPK4 MPK6

Fig. 7.1 MAPK pathways with the common MAPKKK component MEKK1 in Arabidopsis
thaliana

(also called as MEKI) and MKK2 have been detected in Arabidopsis (Ichimura et al.
1998 ; Nakagami et al. 2004 ). A MAPKK, NbMKK1, has been detected in Nicotiana
benthamiana and the protein is localized to the nucleus (Takahashi et al. 2007b ). The
MAPKKKs MEKK1, ANP1 (Arabidopsis NPK1-related 1), and CTR1 (constitutive
triple-response 1) in Arabidopsis and NPK1 (a homolog of ANP1) in tobacco have
been reported to be the components of MAPK cascade (Kovtun et al. 2000 ; Nakagami
et al. 2004 ). OMTK1 ( o xidative stress-activated M AP t riple k inase 1 ) is a MAPKKK
from alfalfa (Nakagami et al. 2004 ).
MAPKKKs can feed into multiple MAPK pathways (Fig. 7.1 ). For example,
in Arabidopsis the MAPKKK MEKK1 acts in the MEKK1 - MKK4 - MPK3,
MEKK1 - MKK4 - MPK6, MEKK1 - MKK5 - MPK3, MEKK1 - MKK5 -
MPK6, MEKK1 - MKK1 - MPK4, MEKK1 - MKK1 - MPK6, MEKK1 -
MKK2 - MPK4, and MEKK1 - MKK2 - MPK6 pathways (Ichimura et al. 1998 ;
Mizoguchi et al. 1998 ; Asai et al. 2002 ; Teige et al. 2004 ; Mészáros et al. 2006 ;
Takahashi et al. 2007a ). By contrast, MAPKKs usually have restricted substrate
specifi city, functioning mainly in a single cascade. Four MAPKKs - MKK2,
MKK3, MKK4, and MKK5 - activate MPK6 in Arabidopsis (Takahashi et al.
2007a ). MKK2, MKK3, MKK4, and MKK5 interact with MPK6 to constitute
different MAPK complexes to transduce different signals and cross talk with
each other (Takahashi et al. 2007a ). The MKK3 activates four MAPKs, MPK1,
MPK2, MPK7, and MPK14 (D

ό

czi et al. 2007 ). The MKK2 activates two MAP
kinases, MPK4 and MPK6 (Brader et al. 2007 ). Another distinct MAP kinase
module in Arabidopsis consists of MEKK1-MKK1 and MKK2-MPK4 (Ichimura
et al. 1998 ; Mizoguchi et al. 1998 ; Zhang et al. 2012a ). Although MKK1 and
MKK2 form complexes with identical upstream and downstream kinases, they
appear to have distinctly different functions. While MKK1 is activated by biotic
elicitors, MKK2 is activated by abiotic stress signals (Teige et al. 2004 ). MPK4
is phosphorylated and activated by MKK1 (Huang et al. 2000 ; Matsuoka et al.