Protein Phosphorylation and Dephosphorylation in Plant Immune Signaling Systems - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 383

9.1 Protein Phosphorylation Plays Key Roles in Plant

Immune Signal Transduction

Phosphoproteomic studies have established that the information signal is transferred
to a protein by phosphorylation and this simple modifi cation of the protein causes a
change in its activity and localization. The incoming signal is transduced to the target
protein by a protein kinase and the signal alone is responsible for the change in activ-
ity of the target protein (Ferl 2004 ). Phosphatase activity may remove the signal and
return the protein to its original state, providing only transitional activity of the target
protein/enzyme. The activated kinases in turn may activate other kinases such that
cascades of phosphorylation events propagate to enzymatic or structural proteins,
where their phosphorylation may result in several changes in the activities of these
proteins (Ferl 2004 ; Vidhyasekaran 2007 ). It has been demonstrated that signals initi-
ate cascades while the protein kinases propagate the signaling processes (Ferl 2004 ).
Rapid and transient phosphorylation of several proteins involved in defense
signaling system has been reported. Phosphorylation of various protein kinases,
either by autophosphorylation or by other related kinases appears to be the crucial
factor in triggering phosphorylation of various defense signaling-related proteins.
Elicitor signals transiently activate various protein kinases within a few minutes
after application (Romeis et al. 2000 ; Vitart et al. 2000 ). This type of transition from
nonelicited to elicited form of protein kinase is caused by a phosphorylation event.
Protein phosphorylation has been shown to play a central role in the plant
immune response signaling (Peck et al. 2001 ; Zipfel et al. 2004 ; Benschop et al. 2007 ;
Tena et al. 2011 ). Protein kinases and protein phosphatases and their corresponding
protein substrates play key roles in the pathogen-associated molecular pattern
(PAMP)-plant pattern recognition receptor (PRR) mediated defense signal transduction
(Benschop et al. 2007 ; Tischner et al. 2010 ).

9.2 Protein Phosphorylation Is an Early

PAMP/Elicitor- Triggered Event

The proteins that make up the signal transduction pathway are present in the cell
prior to the perception of elicitor (Benschop et al. 2007 ). These proteins are acti-
vated by post-translational modifi cations and conformational changes. The most
widely recognized post-translational modifi cation involved in signal transduction is
protein phosphorylation (Benschop et al. 2007 ; Tischner et al. 2010 ). Early signal-
ing and induction of defense responses are likely mediated through plasma
membrane-associated proteins. Many membrane-associated proteins have one or
more phosphorylation sites (Nühse et al. 2004 ). Very early signaling appears to be
transient protein phosphorylation. The phosphorylation pattern of proteins was tran-
siently changed within 10 min in Arabidopsis thaliana after contact with Verticillium
longisporum conidia (Tischner et al. 2010 ). At least 30 proteins were differentially
phosphorylated within fi rst 4 min after the bacterial PAMP fl g22 or the fungal