Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 332

Asai et al. 2002 ; Menke et al. 2005 ; Takahashi et al. 2007a ; Gao et al. 2008 ;
Ren et al. 2008 ; Pitzschke et al. 2009b ; Liu et al. 2011 ). MPK3, MPK6, MKK4, and
MKK5 form a cascade that positively regulates plant defenses (Pitzschke et al.
2009b ). MPK3 has been shown to be required for camalexin accumulation upon
Botrytis cinerea infection (Ren et al. 2008 ). Inactivation of MPK3 and MPK6 by the
P . syringae effector HopA/1 and inactivation of MKKs by the P . syringae effector
HopF2 severely impair PAMP-induced defenses and render plants highly suscepti-
ble to nonpathogenic P . syringae bacteria (Zhang et al. 2007b ; Wang et al. 2010 ).
MPK3 and MPK6 have been shown to be required for priming of defense responses
during induced resistance (Beckers et al. 2009 ).
Arabidopsis MPK3 and MPK6 have been shown to be positive regulators of
plant immune responses. These MAPKs are activated by pathogens, PAMP elici-
tors and DAMPs (endogenous elicitors) (Asai et al. 2002 ; Bethke et al. 2012 ). The
bacterial PAMP fl g22 and the oligogalacturonides elicitor of host plant origin
activated MPK3 and MPK6 in Arabidopsis (Galletti et al. 2011 ). Analysis of single
mapk mutants revealed that lack of MPK3 increased basal susceptibility to the
fungal pathogen Botrytis cinerea , but did not signifi cantly affect elicitors-induced
disease resistance. By contrast, lack of MPK6 had no effect on basal resistance
but suppresses fl g22- and OGs- induced resistance to B. cinerea . Overexpression
of the AP2C1 phosphatase led to impaired fl g22- and OGS-induced phosphoryla-
tion of both MPK3 and MPK6 (Galletti et al. 2011 ). These results suggest that
both MPK3 and MPK6 are involved in plant innate immunity, but their mode of
action may vary.
Root treatment with N -3-oxo-tetradecanoyl- L -homoserine lactone (HSL) induced
resistance against Pseudomonas syringae pv. tomato DC3000 in Arabidopsis . HSL
treatment promoted a stronger activation of MPK3 and MPK6 when challenged with
fl g22, followed by a higher expression of the defense-related transcription factors
WRKY22 and WRKY29 , and the PR-1 gene (Schikora et al. 2011 ). These studies
show that MPK3 and MPK6 are involved in triggering induced resistance.

7.2.2 Arabidopsis thaliana MPK4 Negatively Regulates

Plant Immune Responses

MPK4, its upstream MAP kinase kinases MKK1 and MKK2, and the MAP kinase
kinase kinase MEKK1 form a cascade that negatively regulates defenses in
Arabidopsis because loss-of-function mutations in this cascade result in constitutive
activation of defenses (Petersen et al. 2000 ; Mészáros et al. 2006 ; Suarez-Rodriguez
et al. 2007 ; Gao et al. 2008 ; Qiu et al. 2008a ; Pitzschke et al. 2009b ). The mpk4
plants exhibit constitutive systemic acquired resistance, including elevated salicylic
acid levels and increased resistance to virulent pathogens (Petersen et al. 2000 ). The
results suggest that MPK4 may negatively regulate SA signaling system. The induc-
tion of JA-responsive genes was blocked in mpk4 mutant plants, suggesting that
MPK4 positively regulates the JA pathway (Petersen et al. 2000 ; Gao et al. 2008 ;