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transcriptional regulatory pathways of gene expression show up. One group is TFs called
dehydration response element binding protein (DREB) coming from AP2/ERF family and
another is NAC, AREB/ABF, WRKY and MYB group. The former work in the ABA-
independent pathway, latter known to be as the most responsive to ABA signaling under
drought.
The DREB proteins contain an ERF/AP2 DNA-binding domain that is quite conserved. The
TFs containing it are widely found in many plants, including Arabidopsis (Okamuro et al.
1997 ), tomato (Zhou et al. 1997 ), tobacco (Ohme-Takagi and Shinshi 1995 ), rice (Weigel
1995 ) and maize (Moose and Sisco 1996 ). And a conserved Ser/Thr-rich region next to
ERF/AP2 domain is considered to be responsible for DREB proteins phosphorylation (Liu et
al., 1998). Latter, Agarwal et al. found high sequence similarity exists in different DREB
proteins by amino acid alignment analysis (Agarwal et al. , 2006).
Generally, The DREB TFs could be divided into DREB1 and DREB2, and they participate in
signal transduction pathways under low temperature and dehydration respectively. They
belong to plants-distinctive ERF family. ERF proteins share a conserved domain binding to
the C-repeat CRT/dehydration responsive element (DRE) motif engaged in the expression of
cold and dehydration responsive genes (Agarwal et al., 2006). The expression of DREB2A
and its homolog DREB2B are induced by dehydration and high salt stress (Liu et al., 1998;
Nakashima et al., 2000 ). Furthermore, the expression of DREB genes is induced by abiotic
stresses at different time periods (Liu et al. 1998; Dubouzet et al. 2003). But for the character
of tissue-specific expression, the data is still in short.
For the ABA-dependent gene induction under water-deficit condition, we want to stress on
another group of TFs. Firsly, MYB, MYC, homeodomain TFs and a family of transcriptional
repressors (Cys2/His2-type zinc-finger proteins) are involved in the ABA response to water
deficit. The promoter region of Responsive to Dehydration 22 (RD22), who is induced by
ABA, contains MYC and MYB
cis
-element recognition sites. And MYC and MYB TFs only
accumulate after an increase of ABA concentration, and Over-expressing these TFs lead to
promoted ABA sensitivity and drought tolerance (Abe et al. 2003). Also other data indicate
that two R2R3-MYB TFs (AtMYB60 and AtMYB61) are directly involved in stomatal
dynamics in Arabidopsis regulated by light conditions, ABA and water stress (Liang et al.
2005). Very recently, in Arabidopsis, it has been strongly suggested that WRKY TFs possibly
act downstream of at least two ABA receptors, the cytoplasmic PYR⁄PYL⁄RCAR protein
phosphatase 2C-ABA complex and the chloroplast envelope-located ABAR-ABA complex.
And the promoter-binding experiments show that the target genes for WRKY TFs involved
in ABA signalling include
ABF2/4
,
ABI4/5
,
MYB2
,
DREB1a/2a
,
RAB18
,
RD29A
and
COR47
.
Other findings in a large sense prove us ome WRKY TFs are positive regulators of ABA-
mediated stomatal closure being relevant with drought responses (reviewed by Deena L.
Rushton et al., 2011). On the other hand, it has been realized earlier that the ABA response
element (ABRE) is bound by basic Leucine Zipper Domain (bZIP-type) TFs, and three
Arabidopsis bZIP TFs (AREB1/ABF2, AREB2/ABF4, and ABF3) are activated through
phosphorylation reacting to water deficit and ABA treatment. Other NAC domain proteins
ANAC019, ANAC055, and ANAC072 are also induced by the same treatment. And in guard
