ABA Regulation of Plant Responses to Drought and Salt Stresses - Abscisic Acid: Metabolism, Transport and Signaling - page 309

16.2.2 Additional Cis-Acting Elements
and Transcription Factors
In addition to ABRE, MYC and MYB elements (Abe et al. 2003 ) and the NAC
recognition sites (NACRS) (Fujita et al. 2004 ; Tran et al. 2004 ) have also been
identified as functioning in ABA-inducible gene expression under drought and salt
stress conditions.
MYC and MYB recognition sites, present in the promoter region of the RD22
gene, play key roles in ABA-mediated dehydration-inducible expression of RD22 .
Transgenic Arabidopsis overexpressing AtMYC2, AtMYB2, or the two genes
simultaneously were hypersensitive to ABA and showed enhanced tolerance to
osmotic stress (Abe et al. 2003 ). Overexpressing OsMYB3R - 2, an R1R2R3-type
MYB transcription factor from rice, in transgenic Arabidopsis enhanced resistance
to drought and salt stresses as well as decreased sensitivity to ABA (Dai et al. 2007 ).
The cis-acting elements, which usually contain CATGT and harbor CACG
core sequences, that are bound by NAC transcription factors are called NACRS.
The Arabidopsis RD26/ANAC072 gene encodes a NAC protein, which can recog-
nize NACRSs in the promoters of some stress-inducible genes and activate their
expression. Moreover, the promoter itself also contains a NACRS that is induced
by drought, high salinity, and ABA. Transgenic plants overexpressing RD26 were
hypersensitive to ABA and showed up-regulated expression of ABA and stress-
inducible genes, whereas plants with the RD26 gene repressed exhibited the con-
verse phenotype (Fujita et al. 2004 ).
Additionally, an ABA-independent pathway exists in plants, which is mediated
by the interaction of other transcription factors and cis-acting elements in response
to drought and salt stresses. It has been widely reported that the dehydration-respon-
sive element/C-repeat (DRE/CRT), with a 9-bp core sequence (TACCGACAT),
acts in response to drought and high salinity through an ABA-independent signal
transduction pathway (Yamaguchi-Shinozaki and Shinozaki 1994 ). DREB2 , an
APETALA2 (AP2)-type transcription factor induced by drought and high salinity,
transactivates the promoter containing the DRE element of stress-responsive genes.
Transgenic Arabidopsis plants overexpressing the constitutively active DREB2A
(DREB2A-CA) showed enhanced drought tolerance (Sakuma et al. 2006 ). Maize
ZmDREB2A overexpression in Arabidopsis conferred drought stress tolerance as
did DREB2A-CA overexpression (Qin et al. 2007 ). Another AP2-type transcription
factor, DREB1/CBF , has downstream genes that always contain DRE/CRT elements
(Maruyama et al. 2004 ). Overexpressing DREB1A in transgenic rice increased
stress tolerance to drought and high salinity (Oh et al. 2005 ). The homolog of
DREB1 in rice, OsDREB1A, was induced by high salinity but not by ABA, and
its overexpression in transgenic Arabidopsis improved salt tolerance (Dubouzet
et al. 2003 ). The homolog of DREB1 in maize, ZmDREB1A, was also identified
(Qin et al. 2004 ), and ZmDREB1A overexpression in Arabidopsis led to enhanced
drought tolerance. CBF4 is the one DREB1 homolog in Arabidopsis to function in
an ABA-dependent manner; it is increased by drought and ABA. Overexpression