ABA Regulation of Plant Responses to Drought and Salt Stresses - Abscisic Acid: Metabolism, Transport and Signaling - page 311

DREB2A-CA, which is more stable in the cell nucleus than the full DREB2A pro-
tein. Transgenic Arabidopsis overexpressing DREB2A-CA rather than full length
of DREB2A show strong drought tolerance as well as up-regulated expression of
stress-inducible genes. This indicates that the negative regulatory domain is responsi-
ble for regulating the stability of DREB2A, and it is important for DREB2A to func-
tion under drought stress conditions (Qin et al. 2008 ). The DREB2 gene homologs
have also been identified in crops such as rice, wheat, barley, maize, and pearl millet
(Agarwal et al. 2007 ; Dubouzet et al. 2003 ; Egawa et al. 2006 ; Qin et al. 2007 ; Shen
et al. 2003 ; Xue and Loveridge 2004 ), and most of these genes respond to drought
and high salinity. In contrast to the DREB2A in Arabidopsis , the functional forms of
the alternatively spliced DREB2A transcripts in wheat, barley, and maize are induci-
ble by stress conditions, although the non-functional transcripts are abundant (Egawa
et al. 2006 ; Qin et al. 2007 ; Xue and Loveridge 2004 ). Hence, different regulatory
mechanisms underlie DREB2A function in Arabidopsis and crops.
Although DREB1A/CBF3 and DREB1B/CBF1 are not rapidly induced by
drought and high salinity, overexpression of DREB1A or DREB1B in transgenic
Arabidopsis improves stress tolerance to drought and salt stresses (Jaglo-Ottosen
et al. 1998 ; Kasuga et al. 1999 ; Liu et al. 1998 ). This indicates that DREB1 targets
multiple genes. Most of the downstream target genes contain DRE/CRT elements
in their promoter regions and respond to stress tolerance, such as genes encod-
ing phospholipase C, sugar transport protein, LEA protein, and osmoprotectant
biosynthesis proteins (Maruyama et al. 2004 ). DREB1 homolog genes have been
isolated from rice, wheat, barley, maize, and sorghum (Brautigam et al. 2005 ;
Dubouzet et al. 2003 ; James et al. 2008 ; Qin et al. 2004 ; Skinner et al. 2005 ;
Vagujfalvi et al. 2005 ; Xiong and Fei 2006 ; Xue 2003 ; Zhao and Bughrara 2008 ).
Transgenic Arabidopsis or tobacco plants overexpressing DREB1 homolog genes
derived from other species exhibit enhanced drought stress tolerance as well as
a significant increase in the expression of stress-inducible genes under control
conditions (Dubouzet et al. 2003 ; Qin et al. 2004 ; Skinner et al. 2005 ; Xiong and
Fei 2006 ; Zhao and Bughrara 2008 ). Overexpressing transgenic rice DREB1A in
Arabidopsis accumulates osmoprotectants under non-stress conditions. The trans-
genic tall fescue, harboring DREB1A driven by the stress-inducible RD29A pro-
moter, showed a marked accumulation of proline under drought stress (Zhao et al.
2007 ). Transgenic rice overexpressing rice OsDREB1A or Arabidopsis DREB1B
caused expression of downstream stress-inducible genes (Ito et al. 2006 ; Lee et al.
2004 ). In summary, DREB1 proteins derived from crops function similar to those
in Arabidopsis .
In Arabidopsis , three NAC-type proteins, ANAC19 (At1g52890), ANAC055
(At3g15500), and ANAC072 (RD26, At4g27410), were isolated using yeast one-
hybrid screening. These three NAC proteins, together with the ZHFD1 protein, as
transcription activators, cooperatively regulate the expression of the ERD1 gene,
which causes acquired stress tolerance to drought and high salinity in transgenic
plants (Fujita et al. 2004 ; Tran et al. 2004 ). In rice, six NAC transcription factors
have been identified (Ooka et al. 2003 ). Among them, overexpression of SNAC1
and SNAC2 in transgenic rice resulted in enhanced drought and salt tolerances