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(Huang et al.
2004
; Zhang et al.
2005
). In rice, ABA suppresses the expression of
SUB1A
(also encoding an ERF protein), and over-expression of
SUB1A
resulted in
an ABA-hypersensitive phenotype, thereby activating stress-inducible gene expres-
sion and improving drought and submergence resistance (Fukao et al.
2011
).
Some members of the MYB transcription factor family also regulate ABA-
related stress responses in plants. Transgenic tobacco constitutively express-
ing
AmMYB1
, a single-repeat MYB isolated from the salt-tolerant mangrove tree
Avicennia marina
, showed reduced sensitivity to ABA but enhanced tolerance
to salinity stress, suggesting that AmMYB1 may function as a regulator of ABA
and stress signaling (Ganesan et al.
2012
). In rice, the expression of
OsMYB3R
-
2
was induced by cold, drought, and salt stress. Transgenic Arabidopsis plants
over-expressing
OsMYB3R
-
2
showed increased tolerance to cold, drought, salt,
and ABA treatments (Dai et al.
2007
). A subsequent study demonstrated that
OsMYB3R
-
2
may play an important role in the cold stress signaling pathway
modulated by the cell cycle and a putative DREB/CBF pathway in rice (Ma et al.
2009a
). In addition, over-expression of the R2R3-type MYB gene
OsMYB2
in rice
enhanced salt and dehydration tolerance, and the transgenic plants were more sen-
sitive to ABA (Yang et al.
2012
).
A few zinc finger transcription factors are also involved in the regulation of
ABA and stress responses. Transgenic rice over-expressing a zinc finger gene
ZFP245
showed increased sensitivity to exogenous ABA and tolerance to cold
and drought stresses by regulating proline levels and reactive oxygen species-scav-
enging activities (Huang et al.
2009
). Over-expression of another zinc finger gene
ZFP179
in rice also resulted in hypersensitivity to exogenous ABA and increased
salt tolerance at the seedling stage, and it was suggested that
ZFP179
was involved
in both the ABA-dependent and ABA-independent pathways (Sun et al.
2010
).
Nuclear factor Y (NF-Y) is a heterotrimeric transcription factor complex com-
posed of the NF-YA, NF-YB, and NF-YC proteins. Over-expression of
GmNFYA3
,
encoding the NF-YA subunit of the NF-Y complex in soybeans, resulted in
reduced leaf water loss and enhanced drought tolerance in Arabidopsis (Ni et al.
2013
). In addition, the transcript levels of ABA biosynthesis (
ABA1
,
ABA2
), ABA
signaling (
ABI1
,
ABI2
), and stress-responsive genes (
RD29A
and
CBF3
) were gen-
erally higher in
GmNFYA3
-over-expression plants than in the controls, suggesting
that
GmNFYA3
functions in the positive modulation of drought stress tolerance,
partially through the regulation of ABA-related pathways (Ni et al.
2013
).
22.5 Manipulating Posttranscriptional Regulators of ABA
Signaling for Improving Stress Tolerance
Increasing evidence suggests that ABA signaling may also be regulated at the
posttranscriptional or posttranslational levels. SDIR1 is a RING finger E3 ligase
which positively regulates stress-responsive ABA signaling in Arabidopsis, and
over-expression of
SDIR1
leads to enhanced ABA-induced stomatal closure
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