Protein Phosphorylation and Dephosphorylation in Plant Immune Signaling Systems - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 394

transcription of a variety of ethylene-response genes such as the ERFs (Ouaked
et al. 2003 ).
ERFs (ethylene-responsive transcription factors) have been shown to bind
specifically to the GCC box of the defense genes (Yamamoto et al. 1999 ).
Trichoderma viride xylanase elicitor induced accumulation of mRNA for ERF2 in
tobacco cells. The elicitor-inducible accumulation of the transcription factor was
inhibited by staurosporine (an inhibitor of serine/threonine protein kinase) and
calcyculin A (an inhibitor of protein phosphatase). These results suggest that ERF2
might play a major role in the elicitor-induced GCC box-mediated transcription of
defense genes and that both protein kinase and protein phosphatase might be
involved, as positive regulators in the signal transduction pathway that leads to
expression of ERF2 and subsequent GCC box-mediated transcription of defense
genes (Yamamoto et al. 1999 ).

9.13 Phosphorylation of Proteins Involved

in Salicylic Acid Signaling System

Protein phosphorylation has been shown to be involved in SA-dependent signaling
system. Okadaic acid blocked SA-induced PR-1 gene expression (Conrath et al.
1987 ). Okadaic acid is an inhibitor of phosphorylation of protein kinase and
hence it suggests that phosphorylation is a component in SA-dependent PR gene
expression.

9.14 Protein Phosphorylation in ABA Signaling System

Protein phosphorylation is the important mechanism for ABA signaling (He and
Li 2008 ). Specifi c protein kinases are activated in response to ABA and they play
a positive role in ABA signaling (Li et al. 2000 ; Lu et al. 2002 ; Fujii et al. 2009 ).
ABA response element binding factors (ABFs) have been shown to be activated
by phosphorylation by protein kinases (Uno et al. 2000 ). SnRK (for SNF1-related
protein kinase) family of protein kinases, belonging to calcium-dependent protein
kinases (CDPKs) are involved in phosphorylation of ABF proteins (Fujii et al.
2009 ). The Arabidopsis genome contains 38 SnRKs, of which 10 are SnRK2s
(Hrabak et al. 2003 ). Five of these SnRK2s were shown to be activated by ABA
and they phosphorylated ABI1, ABF2 and ABF4 (Furihata et al. 2006 ; Yoshida
et al. 2006 ). SnRK2.2 and SnRK2.3 are the two protein kinases and the effect of
these protein kinases is mediated by phosphorylating ABFs and regulating ABA-
responsive genes (Fujii et al. 2009 ). In rice, SAPK8, SAPK9, and SAPK10, which
are homologous with SnRK2.2, SnRK2.3, and SnRK2.6 of Arabidopsis , were