Protein Phosphorylation and Dephosphorylation in Plant Immune Signaling Systems - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 389

14-3-3 proteins exist as dimers and hence they can bring together two different
proteins, or two different domains within one protein, showing a direct interaction
between the clients (Yaffe et al. 1997 ).

9.8 PAMP/Elicitor Triggers Phosphorylation of PEN

Proteins

Three PEN (for PENETRATION) proteins, PEN1, PEN2, and PEN3 have been
reported to be involved in penetration resistance by limiting pathogen entry into
host cells (Collins et al. 2003 ; Assad et al. 2004 ; Stein et al. 2006 ). PEN1, also
called SYP121, is a syntaxin, while PEN2 is a glycosyl hydrolase and PEN3 is an
ABC (ATP binding cassette) transporter (Collins et al. 2003 ; Lipka et al. 2005 ).
Expression of the genes encoding these proteins, PEN1 , PEN2 , and PEN3 , were
found to be induced in response to elicitor perception in A. thaliana . Both PEN1 and
PEN3 were found to be phosphorylated upon elicitor treatment (Benschop et al.
2007 ). PEN3 was phosphorylated on two residues in response to the elicitors and
PEN1 was phosphorylated on the N terminus (Ser-7). These phosphorylation sites
may be involved in the activation of the PEN proteins (Benschop et al. 2007 ). The
importance of PEN proteins in defense response was demonstrated by developing
mutants. Arabidopsis mutants, pen1 , pen2 , and pen3 , supported higher frequency
of penetration of Blumeria graminis f. sp. hordei into leaves (Stein et al. 2006 ).
Elicitor-induced phosphorylation of PEN proteins is involved in early defense
responses (Collins et al. 2003 ; Stein et al. 2006 ).

9.9 Protein Phosphorylation Involved in Early Defense

Signaling Events Triggered by PAMPs/Elicitors

Protein phosphorylation/dephosphorylation plays an important role in early signal-
ing events (Fig. 9.1 ). An oomycete elicitor induces calcium ion infl ux, anion effl ux,
and activation of a plasma membrane NADPH oxidase responsible for a transient
production of ROS in tobacco. All these effects were inhibited by staurosporine, a
protein kinase inhibitor, indicating that phosphorylation reactions occurred upstream
from these effects (Viard et al. 1994 ; Tavernier et al. 1995 ). Phosphorylation of
proteins involved in G-protein coupled signaling, Ca 2+ /calmodulin-dependent sig-
naling pathways, redox signaling system, and H + -ATPase regulation of intracellular
pH has been reported in tobacco cells treated with a bacterial elicitor (Fig. 9.1 ;
Gerber et al. 2006 ).
PAMP-triggered early events include G proteins modulated protein phosphoryla-
tion/dephosphorylation systems which trigger Ca 2+ infl ux. Phosphorylation of proteins
involved in G-protein coupled signaling has been reported in tobacco cells treated