PAMP Signaling in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 94

converted to ABA by the action of two enzymes, ABA3 and AAO3. ABA3 encodes
the molybdenum cofactor sulfurase that adds the sulfur atom to the molybdenum
center. The AAO3 encodes an aldehyde oxidase that requires molybdenum cofactor
for its activity. Abscisic aldehyde is converted to abscisic alcohol, which in turn is
converted to ABA through a shunt pathway (Wasilewska et al. 2008).
A regulator of G protein signaling (RGS) proteins, RGS1, has been identifi ed in
Arabidopsis (Chen et al. 2006 ). RGS1 signifi cantly stimulated the expression of
NCED encoding the 9- cis -epoxycarotenoid dioxygenase (NCED) enzyme, which
cleaves 9- cis xanthophylls to xanthoxin. The NCED is the fi rst committed step for
ABA synthesis (Chen et al. 2006 ; Wasilewska et al. 2008). RGS1 also triggered
increased expression of ABA2 gene (Chen et al. 2006 ), which encodes dehydroge-
nase/reductase involved in conversion of xanthoxin to abscisic acid aldehyde
(Wasilewska et al. 2008). These results suggest that RGS1 is involved in biosynthesis
of ABA (Chen et al. 2006 ).
ABA regulates several signaling systems, including Ca 2+ signaling complex,
ROS signaling pathway, and NO signaling system. ABA induces oscillations in
[Ca 2+ ] cyt by inducing both Ca 2+ release from intracellular stores and Ca 2+ infl ux from
the extracellular space (Hamilton et al. 2000; Pei et al. 2000; Klüsener et al. 2002).
ROS production precedes activation of Ca 2+ infl ux, and H 2 O 2 activates plasma mem-
brane Ca 2+ channels (Coelho et al. 2002). Cytosolic Ca 2+ elevation induced by ABA
activates slow (S-type) anion channels (Schroeder and Hagiwara 1990; Brault et al.
2004). ABA activates the plasma membrane anion channels and in several species,
this response is associated with changes in the cytoplasmic Ca 2+ concentration
(Marten et al. 2007). The expression of various calcium-dependent protein kinases
(CDPKs) of tobacco was also upregulated by ABA (Ludwig et al. 2004 ). ABA
response element binding factors (ABFs) have been shown to be activated by phos-
phorylation by protein kinases (Uno et al. 2000). ABA activates mitogen activated
protein kinase (MAPK)-mediated signaling system (Gomi et al. 2005; Wang and
Song 2008). ROS may act upstream of the MAPK cascade in the ABA signaling
system in maize leaves (Zhang et al. 2006 ).
The PAMP fl g22 triggers ABA synthesis in plants (Melotto et al. 2006 ). The
HAMP oligogalacturonates (OG) induced the enzyme molybdenum cofactor sulfu-
rase (ABA3), which is involved in the biosynthesis of ABA (Denoux et al. 2008 ).
The PAMP-induced ABA signaling system has been reported to be involved in sto-
matal closure (Hubbard et al. 2010 ). Stomata serve as passive ports of bacterial
entry during infection. They constitute one entry point for bacteria, which need to
reach apoplastic spaces to multiply and cause disease (Nicaise et al. 2009 ). The
stomata in the Arabidopsis leaf epidermis have been shown to act as innate immu-
nity gates to actively prevent bacteria from entering the plant leaf (Melotto et al.
2006 ). The PAMP fl g22 triggered closure of stomata which occurred within the fi rst
hour of contact with plant tissue (Melotto et al. 2006 ). Abscisic acid signaling sys-
tem has been reported to be involved in stomatal closure (Hubbard et al. 2010 ).
Flg22 triggered ABA synthesis, NO production, and OST1 (for OPEN STOMATA1)
kinase, which are required for stomatal closure. ABA increase was the critical early
event in stomatal closure induced by fl g22 (Melotto et al. 2006 ).