Agriculture Reference
In-Depth Information
1997
). Overexpression of
AtMYB2
up-regulates ABA-responsive genes and confers
ABA hypersensitivity during germination and enhanced drought tolerance, indi-
cating that it is a positive regulator of ABA response (Abe et al.
2003
). Likewise,
AtMYB15
is ABA/stress-inducible, and its overexpression results in ABA hyper-
sensitivity and enhanced stress tolerance (Ding et al.
2009
). Transgenic plants
overexpressing another ABA-inducible MYB TF gene
AtMYB44
are also ABA
hypersensitive and drought tolerant, whereas its knockout mutants display opposite
phenotypes (Jung et al.
2008
). Interestingly, salt-induced expression of PP2C genes
(i.e.,
ABI1
,
ABI2
,
HAB1
, and
HAB2
), which are negative regulators of ABA signal-
ing, is repressed in the
AtMYB44
overexpression lines. Similar observations were
also made with
AtMYB20
: Its overexpression enhances salt tolerance and represses
the expression of
ABI1
and
ABI2
(Cui et al.
2013
). AtMYB20 directly binds the
MYB recognition sites in the
ABI1
promoter. Seo et al. demonstrated that
AtMYB96
,
which is ABA- and drought-inducible, is involved not only in ABA/drought
response but also in the regulation of lateral root development under drought condi-
tion (Seo et al.
2009
).
AtMYB52
is known to regulate secondary cell wall synthesis,
but a recent study showed that it also plays a positive regulatory role in ABA and
stress responses (Park et al.
2011
). On the other hand, MYC2 binds to the MYC rec-
ognition site in the
RD22
promoter mentioned above and positively regulates ABA
response and drought tolerance together with AtMYC2 (Abe et al.
2003
).
11.11 Homeodomain Proteins
Several homeodomain proteins, especially those belonging to the HD-Zip I sub-
family (Mukherjee et al.
2009
), are known to mediate ABA response. ATHB6 has
been isolated as an ABI1-interacting protein and shown to negatively regulate ABA
response in germination and stomatal closure (Himmelbach et al.
2002
). It has been
further demonstrated to be a target of CUL3-based ubiquitin E3 ligase (Lechner
et al.
2011
). On the other hand, two highly homologous and ABA/drought-induc-
ible homeodomain proteins, ATHB7 and ATHB12, are positive regulators of ABA
response during postgermination growth (Olsson et al.
2004
). Recently, ATHB12
has been demonstrated to regulate plant growth by suppressing the
GA20
oxidase
gene expression (Son et al.
2010
).
ATHB5
is also known as a positive regulator of
ABA response (Johannesson et al.
2003
). Its expression is ABA-inducible, and the
ABA inducibility is compromised in
abi3
and
abi5
mutants. Although its knock-
out mutant does not exhibit noticeable phenotypes, its overexpression lines are ABA
hypersensitive.
ATHB20
has been isolated based on gene expression profiling to
identify genes involved in seed dormancy (Barrero et al.
2010
). A T-DNA insertion
mutant of ATHB2 is more dormant than wild-type plants, whereas the germination
of its overexpression lines is partially insensitive to ABA.
ATHB17
, another HD-
Zip I subfamily homebox gene, positively regulates ABA response during seedling
establishment stage and under water-deficit condition (Park et al.
2013
).
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