Protein Phosphorylation and Dephosphorylation in Plant Immune Signaling Systems - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 395

activated by ABA (Kobayashi et al. 2004 ). These SAPKs phosphorylated TRAB1,
which is a rice ortholog of the Arabidopsis ABFs (Kobayashi et al. 2005 ). ABA
induced the wheat SnRK, PKABA1, at the transcript level. The ABA-induced
PKAB1 phosphorylated TaABF, the wheat bZIP transcription factor (Johnson
et al. 2002 ). Several other CDPKs have been shown to be involved in ABA signal-
ing (Sheen 1996 ; Romeis et al. 2001 ; Choi et al. 2005 ). Two Arabidopsis CDPKs,
AtCPK10 and AtCPK30 activate an ABA-inducible promoter in maize leaf proto-
plasts (Sheen 1996 ). Another CDPK in Arabidopsis , AtCPK32, interacts with
C2-C3 conserved region of ABF4 (Choi et al. 2005 ). AtCPK32 has autophos-
phorylation activity and phosphorylates ABF4 in vitro. The CDPK has been shown
to be an ABA signaling component that positively modulates ABF4 function
(Choi et al. 2005 ).
A calcineurin B-like (CBL) protein kinase CIPK15 interacts with the calcium-
modulated protein phosphatases ABI1 and ABI2 and a CBL Ca 2+ -binding protein
ScaBP5. CIPK15 and one of its homologs CIPK2 are involved in ABA signaling as
negative regulators (Guo et al. 2002 ; Kim et al. 2003b ). The kinase substrate of
CIPK15 has been identifi ed as an AP2 transcription factor AtERT7 that negatively
regulates ABA signaling (Song et al. 2005 ).
Another SNF1-related protein kinase, AAPK (for ABA-activated protein kinase)
was detected in broad bean ( Vicia faba ). AAPK is stimulated by ABA and it is a
positive regulator of ABA response (Li et al. 2000 , 2002 ; Johnson et al. 2002 ).
Takahashi et al. ( 2007 ) showed that ABA induced binding of a 14-3-3 protein to
proteins with molecular masses of 61, 43 and 39 kDa. Autophosphorylation of
AAPK, which mediates anion channel activation and ABA-induced phosphoryla-
tion of the 61 kDa protein showed similar time courses. AAPK elicits the binding of
the 14-3-3 protein to the 61-kDa protein in vitro when AAPK was activated by
ABA. It has also been suggested that the 61 kDa protein may be a substrate for
AAPK and the 61 kDa protein is located upstream of H 2 O 2 and Ca 2+ , or on Ca 2+ -
independent signaling pathway (Takahashi et al. 2007 ).
An ABA-insensitive Vicia faba mutant, fi a (faba bean impaired in ABA-induced
stomatal closure) had been isolated. Unlike ABA, H 2 O 2 , and nitric oxide (NO)
induced stomatal closure in the fi a mutant. ABA did not induce production of either
reactive oxygen species or NO in the mutant. ABA also did not suppress inward-
rectifying K + currents or activate AAPK in mutant guard cells. These results suggest
that FIA functions as an early signal component upstream of AAPK activation in
ABA signaling in guard cells of Vicia faba (Sugiyama et al. 2012 ).
Arabidopsis OST1/SRK2E, an Arabidopsis ortholog of Vicia faba AAPK, which
is an ABA-activated, Ca 2+ -independent protein kinase, is involved in ABA signaling
and it acts upstream of ROS production (Mustilli et al. 2002 ; Yoshida et al. 2002 ).
Phosphorylation has been shown to function in ABA-induced ROS production and
I ca channel activation (Murata et al. 2001 ; Köhler and Blatt 2002 ).
ABA activates mitogen activated protein kinase (MAPK)-mediated signaling
system (Gomi et al. 2005 ; Wang and Song 2008 ). An Arabidopsis MAPK, AtMAPK3
and a rice MAPK, OsMAPK5, have been identifi ed as ABA-activated MAPKs
(Lu et al. 2002 ; Xiong and Yang 2003 ). It has been found that ABA and H 2 O 2