Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 349

PAMP elicitor

Receptor

MAPKKK

ROS

MAPKK

MAPKs SIPK and
WIPK

Activation

Activation

Defense genes

Nbrboh B gene

NADPH oxidase

Fig. 7.5 MAP kinase cascade acting both upstream and downstream of ROS signaling system
(Yang et al. 2001 ; Yoshioka et al. 2003 )

(Yoshioka et al. 2003 ). It suggests that post-translational modifi cation of the
NADPH oxidase by phosphorylation may also be involved in ROS production. The
MAPK pathway may also act at downstream of H 2 O 2 , which activates MAPKs
(SIPK and WIPK in tobacco). The activated MAPKs trigger defense gene activation
(Fig. 7.5 ; Yang et al. 2001 ; Yoshioka et al. 2003 ).
The role of MAPK cascade in the generation of H 2 O 2 has been demonstrated in
Arabidopsis (Ren et al. 2002 ). Transgenic Arabidopsis plants expressing active
mutants of two MAPK kinases, AtMEK4 and AtMEK5, were developed (Ren et al.
2002 ). The external signal stimulated the activation of the endogenous MAPKs and
generation of H 2 O 2 (Ren et al. 2002 ).
Some MAP kinase may control H 2 O 2 accumulation by the action of catalase
(Xing et al. 2008 ). A catalase ( CAT1 ) transcript was induced in an abscisic acid
(ABA)-dependent way in Arabidopsis thaliana and the induction was abolished in
the T-DNA insertion mutant mkk1 , a gene encoding a MAPK kinase. Overexpression
of AtMKK1 signifi cantly enhanced ABA-dependent CAT1 expression and H 2 O 2
production (Xing et al. 2008 ). Another component in the MAPK cascade, MPK6
(a MAP kinase) was also involved in signal transduction. The mpk6 mutant blocked