Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 345

have shown that MAPK cascades are important components in the PAMP fl agellin
signaling system, there are also reports that fl g22 may trigger the plant immune
responses independent of MAPK cascades in Arabidopsis . The bik1 mutant is sig-
nifi cantly compromised in PAMP-induced resistance, but not the fl g22-induced
MAPK activation (Zhang and Zhou 2010 ).

7.9 Signals and Signaling Systems Activating MAPK

Cascades

Several MAP kinases have been shown to be involved in inducing resistance against
bacterial, oomycete, and fungal pathogens (Menke et al. 2004 ; Brader et al. 2007 ;
Zhang et al. 2007c ). These MAP kinase cascades may induce different signaling
systems and induce resistance or susceptibility to various types of pathogens (Xiong
and Yang 2003 ). Various signals activate MAP kinases. Fungal elicitor, H 2 O 2 , SA,
JA and ethylene activated BWMK1, the MAPK in rice cells (Cheong et al. 2003 ).
The kinase activity of BWMK1 was rapidly and transiently activated by all these
defense signals. The activity peaked 5-30 min after treatment. The transcript levels
of BWMK1 increased in a delayed manner after activation with the defense signals;
the increase was observed only 1 to 6 h after treatment (Cheong et al. 2003 ).
However, BWMK1 protein levels did not change. This indicates that the BWMK1
protein is maintained at steady-state levels in the cell, which would permit the plant
to respond rapidly to the external stimuli. The response consumes BWMK1 protein;
thus the plants produce new BWMK1 transcripts to maintain the baseline level of
protein. These observations suggest that BWMK1 activation is primarily achieved
by post-translational modifi cation (Cheong et al. 2003 ).
Several other MAPKs, including ERMK (for E licitor- r esponsive M AP K )
(Ligterink et al. 1997 ), SIPK (Zhang and Klessig 1997 ; Zhang et al. 1998 ), and
WIPK (Seo et al. 1995 ) have also shown to be activated by elicitors, SA, and JA.
The ERMK and WIPK are also activated by post-translational modifi cation (Seo
et al. 1995 ; Ligterink et al. 1997 ). LeMPK3 is a MAPK involved in defense response
in tomato and LeMPK3 was found to be transcriptionally up-regulated by both bac-
terial and fungal elicitors (Mayrose et al. 2004 ).
H 2 O 2 activates a MAPKK kinase from alfalfa, OMTK1 (oxidative stress-activated
MAP triple-kinase 1) (Nakagami et al. 2004 ). JA activates MKK3 - MPK6 cascade
in Arabidopsis (Takahashi et al. 2007a ) and OsBIMK1, a rice MAP kinase (Song
and Goodman 2002 ). Salicylic acid activates a 48-kD MAP kinase and SIPK in
tobacco (Zhang and Klessig 1997 ; Zhang and Liu 2001 ) and p48 and p44 MAPKs
in pea (Uppalapati et al. 2004 ). ABA activates p48 MAPK in pea (Uppalapati et al.
2004 ) and rice MAPK gene OsMAPK5 (Xiong and Yang 2003 ). Both nitric oxide
(NO) and SA activated SIPK in tobacco (Kumar and Klessig 2000 ). Studies with
transgenic NahG tobacco revealed that SA is required in the NO-mediated induction
of SIPK. These observations suggest that SIPK may function downstream of SA in
the NO signaling pathway (Kumar and Klessig 2000 ).