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which function in the ABA-independent stress response pathway (Nakashima
et al.
2009b
), may cooperate to regulate ABA- and/or stress-responsive genes.
Overexpression of
RAP2.6
enhances ABA and stress responses, indicating
that it is a positive regulator of ABA response (Zhu et al.
2010
). ADAP (ARIA-
interacting AP2 domain protein) interacts with ARIA, which, in turn, interacts
with ABF2/AREB1 to positively regulate ABA response, and functions as a posi-
tive regulator in ABA-dependent growth regulation and stress response (Lee et al.
2009
). Another AP2 subfamily protein CHO1 mediates ABA response during ger-
mination by repressing GA biosynthesis (Yano et al.
2009
).
A group of AP2/ERF family proteins (AtERF1, 2, 5, 13, 15, ERF1, ORA59,
RAP2.4, etc.) that bind the coupling element CE1 has been isolated by yeast
one-hybrid screen (Lee et al.
2010c
). The proteins, named CE1-binding factors
(CEBFs), belong to the B-3 or A-6 subfamily of AP2/ERF superfamily. Analysis
of
in planta
functions of three CEBFs showed that AtERF13 is a positive regula-
tor of ABA response during seedling growth, whereas RAP2.4 and RAP2.4L are
involved in stress response.
AP2/ERF family TFs which negatively regulate ABA response have also been
reported. ABR1 represses ABA response during seed germination and down-
regulates ABA-responsive genes (Pandey et al.
2005
). AtERF7 acts as tran-
scriptional repressor, probably as part of a transcriptional repressor complex
containing Sin3-histone deacetylase, and reduces ABA response during seed ger-
mination and in guard cells (Song et al.
2005
). Similarly, AtERF4 negatively regu-
lates ABA response as well as ethylene response (Yang et al.
2005
). Most of the
AP2/DREB/ERF family TFs mentioned above bind the DRE, CE1 element, or eth-
ylene response element (i.e., GCC box) and are also involved in abiotic stress or
ethylene response.
11.8 WRKY Proteins
Several WRKY family TFs (Eulgem et al.
2000
) are known to be involved in ABA
response (Rushton et al.
2012
). Disruption of
WRKY2
confers ABA hypersensitiv-
ity during germination and early seedling growth, and its expression is induced by
ABA in an ABI3- and ABI5-dependent manner (Jiang and Yu
2009
). WRKY63/
ABO3 plays both positive and negative regulatory role (Ren et al.
2010
). Its loss of
function mutant is hypersensitive to ABA during germination and seedling growth.
However, the mutant is impaired in the stomatal regulation and susceptible to
water-deficit condition. It has been demonstrated that WRKY63 binds the W-box
(PyTGACPy) in the
ABF2
promoter and positively regulates ABF2 expression
(Ren et al.
2010
). WRKY8, on the other hand, mediates mainly defense response,
but it also positively regulates
ABI4
expression (Chen et al.
2013a
). Thus, WRKY8
may be involved in the cross talk between ABA and defense signaling.
Chen et al. (
2010
) showed that WRKY18 and WRKY60 play positive role in
ABA inhibition of seed germination and root elongation, whereas WRKY40 has
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