Protein Phosphorylation and Dephosphorylation in Plant Immune Signaling Systems - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 393

MPK3, MPK4, and MPK6 are all activated by fungal and bacterial elicitors and
ROS (Kovtun et al. 2000 ; Nühse et al. 2000 ). To assess the potential involvement of
PDK1 in OXI1-MPK signaling, the expression of PDK1 in OXI1-MPK signaling
was ablated. The results showed that activation of MPK6 by the elicitor was PDK1-
dependent (Anthony et al. 2006 ). It suggests that MAPK signaling cascades func-
tion downstream of OXI1 and PTI1-2, resulting in the eventual activation of PR
genes. MAPK cascades may also play a role upstream and have been implicated in
the activation of the NADPH oxidase genes (Yoshioka et al. 2003 ). PA has the
potential to activate targets such as PDK1 and NADPH oxidase simultaneously; and
both of them trigger ROS by phosphorylation (Anthony et al. 2006 ).
The possible role of various phosphorylation events in ROS signaling system is
presented in Fig. 9.2.

9.12 Phosphorylation of Proteins Involved

in Ethylene-Signaling System

Protein phosphorylation and dephosphorylation have been shown to be involved in
the biosynthesis of ethylene. S-adenosylmethionine and 1-aminocyclopropane-1-
carboxylic acid (ACC) are the precursors of ethylene. S-adenosylmethionine is con-
verted to ACC by ACC synthase (ACS). ACC is oxidized by ACC oxidase (ACO)
to form ET (Vidhyasekaran 2007 ). Treatment with the protein kinase inhibitors
staurosporine or K-252 inactivated the ACS activity, whereas protein phosphatase
inhibitor calyculin A stimulated ACS activity (Spanu et al. 1991 ). These results sug-
gest that phosphorylation/dephosphorylation of ACS is involved in the increased
activity. Wang et al. ( 2002 ) suggested that ACS is unstable in vivo and present at low
abundance, and phosphorylation of ACS may increase its stability to sustain the
elevated activity.
An oomycete elicitor induced a dramatic increase in ACS activity in parsley cell
cultures (Chappell et al. 1984 ). Similar increase in ACS activity coincided with
activation of the MAPK SIPK followed by an increase in ET production in tobacco
(Kim et al. 2003a ). The protein kinase inhibitor H-7 (1-[5-isoquinolinylsulfonyl]-
2-methylpiperazine) and the protein phosphatase inhibitors vanadate and okadaic
acid inhibited the induction of ET in pea (Kwak and Lee 1997 ). These results
suggest that protein phosphorylation/dephosphorylation plays an important role in
ET signaling system.
MAPK cascade has been shown to negatively regulate ethylene signaling by
constitutive phosphorylation of downstream components that ultimately repress
the accumulation of EIN3 and its relatives (Ouaked et al. 2003 ). Binding of ethylene
to the receptors (ETR1, ETR2, EIN4, ERS1, and ERS2) blocks the MAPK cascade
by failing to activate the MAPKKK protein (CTR1). This in turn allows EIN3 and
its relatives to accumulate. The accumulated transcription factors induce the