Agriculture Reference
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a negative effect on ABA sensitivity (Chen et al.
2010
). Previously, these factors
have been demonstrated to form hetero- and homocomplexes and to be involved
in defense response (Xu et al.
2006
). WRKY18 and WRKY40 expression is rap-
idly induced by ABA, and it has been proposed that the two WRKY proteins
regulate the expression of WRKY60, whose ABA induction is delayed compared
with WRKY18 and WRKY40. The molecular basis of WRKY40 function in ABA
response has been reported (Shang et al.
2010
). It binds the W-box in the promot-
ers of major regulatory genes of ABA and stress responses, such as
ABI4
,
ABF4
,
MYB2
,
DREB1A,
and
DREB2A
, and represses their expression. This repression is
relieved in the presence of ABA, which suppresses WRKY40 expression and, at
the same time, recruits WRKY40 from nucleus to cytosol to promote its binding
to the C-terminus of the chloroplast-localized ABA receptor, ABAR (Shen et al.
2006
). An elaborate model of cooperative and antagonistic interactions between
the three WRKY proteins has been proposed recently (Liu et al.
2012
).
11.9 NAC Proteins
NAC family TFs play a variety of roles in plant development and biotic/abi-
otic stress responses, including hormone signaling (Nakashima et al.
2012
;
Nuruzzaman et al.
2013
). The drought-inducible NAC-domain protein
RD26/ANAC72 is inducible by ABA, and its over- or underexpression enhances
or represses, respectively, ABA sensitivity, indicating its positive regulatory role in
ABA response (Fujita et al.
2004
). RD26/NAC72 has been isolated as one of the
ERD1
promoter-binding proteins together with two other NAC TFs, ANAC19 and
ANAC55 (Tran et al.
2004
). These two TFs interact with RHA2, a RING-type E3
ligase that positively regulates ABA response during germination and early seed-
ling growth (Jiang et al.
2009
). Another NAC TF ATAF1 regulates ABA biosyn-
thesis by binding to the promoter region of NCED3 to up-regulate its expression
(Jensen et al.
2013
). Expression of the NAC TF VNI2 is ABA-inducible, and it
regulates ABA- and/or stress-responsive genes. It has been suggested that VNI2
integrates ABA signal into leaf senescence (Yang et al.
2011
). Another NAC pro-
tein ANAC96 modulates ABA response by cooperatively interacting with ABFs.
Specifically, it interacts with ABF2 and ABF4 and synergistically activates ABA-
responsive genes (Xu et al.
2013
).
11.10 MYB/MYC Proteins
A number of MYB class TFs (Yanhui et al.
2006
) have been reported that are
involved in ABA response. AtMYB2, which binds the ABA/dehydratin-responsive
promoter of
RD22
, is inducible by ABA and various abiotic stresses (Abe et al.
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