Agriculture Reference
In-Depth Information
The expression of ABA-responsive genes is important for plant adaptation
and survival under drought and salt stresses. Fine regulatory mechanisms are
necessary for a correct expression of these stress-inducible genes, and the tran-
scriptional regulation of gene expression responsive to drought and high salinity
depends on the interaction of transcription factors with regulatory elements, which
usually results in the expression of multiple stress-inducible genes, thus confer-
ring improved plant stress tolerance. The transcription regulatory circuits, which
comprise the main cis-acting elements, transcription factors, and other necessary
components, in response to drought and salt stresses in the dicotyledonous model
plant Arabidopsis, are shown in Fig. 16.1 . The primary regulatory elements and
transcription factors will be described next.
16.2.1 ABRE Element and bZIP-Type Transcription Factors
Many ABA-inducible genes contain a conserved ABA-responsive element, ABRE,
an 8-bp cis-acting sequence (PyACGTGGC), in the promoter regions. ABRE,
often together with a coupling element (CE), forms an ABA-responsive complex
(ABRC) (Hobo et al. 1999a ). This complex is the most important cis-acting DNA
element involved in ABA-responsive gene expression.
The ABRE element interacting with transcription factors, also known as
ABRE-binding factors (ABFs) or ABRE-binding proteins (AREBs), belong
to the basic leucine zipper (bZIP) family (Choi et al. 2000 ; Uno et al. 2000 ).
In Arabidopsis , approximately 75 distinct bZIP-type transcription fac-
tors have been identified (Jakoby et al. 2002 ). Among them, AREB1/ABF2,
AREB2/ABF4, and ABF3 were induced by drought, high salinity, and ABA
in vegetative tissues (Koornneef et al. 1982 ; Uno et al. 2000 ). Overexpression
of ABF3 and ABF4 in Arabidopsis led to ABA-hypersensitive and enhanced
drought-tolerant phenotypes, as well as an up-regulated expression of some
ABA-responsive genes (Kang et al. 2002 ). Overexpression of the active form
of ABF2/AREB1 also resulted in significantly improved drought tolerance in
transgenic Arabidopsis plants (Fujita et al. 2005 ; Kim et al. 2004 ). By contrast,
the areb1 areb2 abf3 triple mutant displays an ABA-insensitive phenotype and
reduced drought tolerance. Accordingly, the expression of stress-responsive
genes is largely impaired in the triple mutant (Yoshida et al. 2010 ). Another
bZIP-type transcription factor ABA-insensitive 5 (ABI5) induced by drought
and ABA in seeds and seedlings was cloned (Finkelstein and Lynch 2000 ;
Lopez-Molina and Chua 2000 ). The transcription factor responsible for ABA
regulation1 ( TRAB1 ), a homolog of ABI5 in rice, was induced by ABA, drought,
and salt stress (Hobo et al. 1999b ). In soybeans, “Group A”-type bZIP proteins
play important roles in ABA and stress signaling. For instance, the GmbZIP
gene endowed soybeans with salt tolerance (Liao et al. 2008 ). In short, the
bZIP-ABRE system is the key ABA-dependent signal transduction pathway
under drought and high salinity stresses.
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