Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 339

7.4 MAPKK Kinase EDR1 Modulates SA-JA-ET Signaling

EDR1 (ENHANCED DISEASE RESISTANCE 1) is a MAPKK Kinase (MAPK-
KKK), which functions at the top of a MAP kinase cascade. The edr1 ( enhanced
disease resistance 1 ) gene encodes a putative MAPKKK, which negatively regu-
lates SA signaling system. All edr1 -associated phenotypes are suppressed by
mutations that block SA perception ( nim1 ) or reduce SA production ( pad4 and
eds1 ). The NahG transgene, which lowers endogenous SA levels, also suppressed
edr1 (Frye et al. 2001 ). These results suggest that EDR1 plays an important role in
SA-mediated defense responses. The ein2 mutation did not suppress edr1 -mediated
disease resistance, suggesting that ethylene and JA-induced responses are not
required for edr1 resistance (Frye et al. 2001 ). EDR1 gene has been isolated from
Arabidopsis and putative orthologs of EDR1 have been detected in rice and barley
(Frye et al. 2001 ).
The edr1 mutant exhibits enhanced resistance against the powdery mildew
pathogen Golovinomyces cichoracearum in Arabidopsis thaliana (Frye and
Innes 1998 ; Frye et al. 2001 ), suggesting that EDR1 acts as a negative regulator
of defense responses. Plant defensin PDF genes are downregulated in edr1
mutants. PDF1.2 (PR-12; defensin), is an important pathogenesis-related protein
involved in plant innate immune responses (Vidhyasekaran 2007 ) and its expres-
sion is triggered by the JA signaling system (Jung et al. 2007 ; Oñate-Sánchez
et al. 2007 ; Pré et al. 2008 ). MYC2 / JIN1 encodes a basic helix-loop-helix leucine
zipper transcription factor and differentially regulates JA-responsive defense
genes (Lorenzo et al. 2004 ). MYC2 is involved in repression of PDF1.2 expres-
sion and PDF1.2 was highly induced in edr1myc2 double mutant (Hiruma and
Takano 2011 ). It has been shown that EDR1 is critical for expression of plant
defensin genes and the MYC2 - encoded transcription factor represses defensin
expression. Inactivation of MYC2 fully restored defensin expression in edr1
mutants (Hiruma and Takano 2011 ). It suggests that EDR1 cancels MYC2 function
to regulate defensin expression.
The edr1 mutant of Arabidopsis confers resistance against bacterial and fun-
gal pathogens. When the edr1 plants were inoculated with the powdery mildew
pathogen Golovinomyces cichoracearum , the mutant plants showed increased
expression of several defense-related genes (Christiansen et al. 2011 ). Many of
the genes with elevated expression encoded WRKY transcription factors. EDR1
was found to be localized to the nucleus, suggesting that EDR1 could potentially
interact with transcription factors in the nucleus. Elevated expression of ROS-
related genes was also observed early during infection with the pathogen
(Christiansen et al. 2011 ).
EDR1 kinase domain displays autophosphorylation activity and phosphory-
lates the common MAP kinase substrate myelin basic protein. The EDR1 kinase
domain also phosphorylates a kinase-defi cient EDR1 protein, indicating that
EDR1 autophosphorylation can occur via an intermolecular mechanism (Tang
and Innes 2002 ).