Transcription Factors Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 220

been demonstrated to be functionally equivalent to the ABRE (Hobo et al. 1999 ),
suggesting that the ABRE and coupling elements are likely to be the same class
of ABA response element. Interestingly, the drought response element (DRE)
and the C-repeat (CRT) sequence (Baker et al. 1994 ; Yamaguchi-Shinozaki and
Shinozaki 1994 ), which share the CCGAC consensus, function as a coupling ele-
ment in the ABA-induced expression of RD20A (Narusaka et al. 2003 ). In the
case of maize RAB17 , DRE itself functions as an ABRE (Kizis and Pages 2002 ).
Other experimentally determined ABREs are the MYB and the MYC recogni-
tion sequences in RD22 (Abe et al. 2003 ). Mutations of these elements result in
reduced ABA induction of reporter expression in tobacco.
11.3 ABFs/AREBs/ABI5
A small subfamily of bZIP class TFs that bind to the G-box-type ABRE have
been isolated by yeast one-hybrid screens (Choi et al. 2000 ; Uno et al. 2000 ).
Subsequently, the TFs, which are named ABRE-binding factors (ABFs: ABF1,
ABF2, ABF3, and ABF4) or ABRE-binding proteins (AREBs: AREB1, AREB2,
and AREB3), have been demonstrated to mediate ABA and stress responses in
vivo (Fujita et al. 2005b ; Kang et al. 2002 ; Kim et al. 2004 ; Yoshida et al. 2010 ).
As mentioned in previous chapters, ABFs/AREBs constitute the core ABA signal-
ing pathway (i.e., PYR/PYL/RCAR-ABI1/2-SnRK2s-ABFs/AREBs) (Cutler et al.
2010 ; Fujii et al. 2009 ). Numerous other bZIP class TFs had been isolated based
on their in vitro interactions with the ABRE prior to the isolation of ABFs/AREBs
(Menkens et al. 1995 ), but their in planta functions in ABA response have not
been reported.
ABF/AREB family TFs are highly homologous to sunflower Dc3 promoter-
binding factors (DPBFs) (Kim et al. 1997 ; Kim and Thomas 1998 ) and their
Arabidopsis homologs AtDPBFs (AtDPBF1/ABI5, AtDPBF2, AtDPBF3, and
AtDPBF4) (Kim et al. 2002 ). Homologous bZIP factors have been reported in
monocot plants, such as rice, barley, and wheat (Casaretto and Ho 2003 ; Hobo
et al. 1999 ; Johnson et al. 2002 ). The Arabidopsis bZIP proteins constitute a
clade consisted of 9 members (Bensmihen et al. 2002 ; Fujita et al. 2005a ; Kim
et al. 2002 ; Kim 2006 ), including ABI5 (see below). The family members share
a highly conserved bZIP domain and additional short conserved regions, C1-C4.
Among the nine members, ABF1 - ABF4 are expressed in vegetative tissues, and
their expression is ABA-inducible. Additionally, their expression is stress induc-
ible; that is, ABF2/AREB1 , ABF3, and ABF4/AREB2 are highly inducible by high
salt and high osmolarity, whereas ABF1 is cold inducible (Choi et al. 2000 ; Fujita
et al. 2005a ). Other five members of the family (i.e., AtDPBF1/ABI5 , AtDPBF2 ,
AtDPBF3/AREB3 , AtDPBF4/EEL , and an unknown gene) are expressed mainly
in embryos. A binding site selection assay showed that the preferred binding site
of ABF1, one of the ABFs, is the CACGTGGC, the typical ABRE mentioned
above (Choi et al. 2000 ). It also could bind the coupling element CE3. In fact, only