Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 333

Qiu et al. 2008a ; Pitzschke et al. 2009a ). Overexpression of an inactive form of
MPK4 failed to complement mpk4 phenotypes, indicating that kinase activity is
required for MPK4 function (Petersen et al. 2000 ).
The 'Three Kinase Module' including MPK4 shows that MEKK1 and MKK1/2
act upstream of MPK4. The mekk1/mkk1/mkk2 double mutants display similar con-
stitutive defense responses, including elevated levels of SA and ROS, constitutive
PR gene expression, and pathogen resistance (Petersen et al. 2000 ; Gao et al. 2008 ;
Qiu et al. 2008b ). It has also been shown that both MPK4 and MEKK1 interact with
MKK1 and MKK2 in vivo (Gao et al. 2008 ). MEKK1 and MKK1/2 have also been
shown to be essential for activation of MPK4 (Ichimura et al. 2006 ; Suarez-
Rodriguez et al. 2007 ; Gao et al. 2008 ). Many defense-related genes were similarly
deregulated in mekk1 , mkk1/2 , and mpk4 mutants (Qiu et al. 2008b ; Pitzschke et al.
2009a ). Collectively these studies suggest the existence of three kinase module
involving MEKK1-MKK1/2-MPK4 in Arabidopsis thaliana .

7.2.3 Arabidopsis thaliana MPK11 in Plant Immune

Responses

Flg22 treatment is known to increase MPK11 expression in Arabidopsis thaliana
(Bethke et al. 2012 ). MPK11 is an MPK4 paralog and has overlapping function with
MPK4 in regulating cell division (Bethke et al. 2012 ). MPK11 transcripts accumu-
late following pathogen infection or treatments with PAMPs, pathogen-derived
molecules, or resistance-inducing chemicals such as benzothiadiazole. MPK11
expression is rapidly induced within 10-15 min after fl g22 treatment. MPK11 inter-
acted with an ethylene response transcription factor (ERF104). MPK11 is activated
during fl g22 treatment (Bethke et al. 2012 ). MPK11 constitutes a fourth MAPK
activated by fl g22, in addition to MPK3, MPK4, and MPK6. MPK4 and MPK11 are
nearly identical at the amino acid sequence level and similar in molecular weight
(Bethke et al. 2012 ). MPK11 differed in its function from MPK4, although it was
structurally similar to MPK4. In contrast to reports for mpk4 (Petersen et al. 2000 ),
no enhanced expression of PR genes such as PR1 , PR2 , or PR5 was observed in the
mpk11 mutant. The levels of SA and its conjugates were not elevated in the mpk11
mutant relative to wild-type plants, whereas levels in mpk4 mutants were at least
20-fold higher (Bethke et al. 2012 ).

7.2.4 Arabidopsis MPK9 and MPK12 Positively Regulate

ROS-Mediated ABA Signaling

The MAP kinases MPK9 and MPK12 have been shown to be preferentially
expressed in stomata guard cells and positively regulate ROS-mediated ABA signal-
ing (Jammes et al. 2009 ). The MAPKs act upstream of anion channels in guard cell