ABA Regulation of Plant Responses to Drought and Salt Stresses - Abscisic Acid: Metabolism, Transport and Signaling - page 310

of CBF4 in transgenic Arabidopsis enhanced drought tolerance and up-regulated
DRE containing stress-responsive genes (Chinnusamy et al. 2004 ; Haake et al.
2002 ). The early response to dehydration1 gene ( ERD1 ), a NAC family member,
responded to drought and high salinity but not to ABA (Nakashima et al. 1997 ).
There are two types of cis-acting elements in the promoter region of ERD1 . One,
a MYC-like sequence (CATGTG) named NACRS, is recognized and bound by
NAC-type transcription factors (Tran et al. 2004 ). The other, a 14-bp rps1 site 1-like
sequence, binds to the zinc finger homeodomain (ZFHD) transcription factor. Both
cis-acting elements are necessary for the expression of ERD1 under dehydration
stress. The NAC protein functions as a transcription activator alone or in coopera-
tion with the ZFHD protein to regulate the expression of the ERD1 gene.
16.3 Comparison of Arabidopsis with Crops
In order to understand the regulatory mechanisms underlying ABA-inducible
gene expression under drought and salt stresses, transcription factors and interna-
tional cis-acting elements involved in ABA signaling were initially investigated
and reported in the dicotyledonous model plant Arabidopsis . Later, the survey
was extended to important agricultural crops such as rice, maize, wheat, and bar-
ley because of their significant commercial value. The functions and mechanisms
of transcription factors regulating gene expression in Arabidopsis and crops have
some similarities and some differences.
The most important transcription factors, AREB/ABFs, regulate ABA-dependent
gene expression by binding to the cis-acting elements like ABREs and CEs in the
promoter regions of target genes. In Arabidopsis , activation of AREB1 requires
multisite phosphorylation by ABA-responsive 42-KDa kinases. The overexpression
of the phosphorylated active form of AREB1 in transgenic plants up-regulated the
expression of ABA-inducible genes in the absence of ABA or under non-stressed
conditions (Furihata et al. 2006 ; Uno et al. 2000 ). In addition, AAPKs, such as
SnRK2.2, SnRK2.3, and SnRK2.6, that belong to the subclass ʲ SnRK2 family, a
central hub of ABA signaling, phosphorylate AREB1 to activate ABRE-regulated
gene expression (Fujii et al. 2007 ; Furihata et al. 2006 ; Kobayashi et al. 2004 ;
Umezawa et al. 2010 ). In rice, TRAB1, an ABF transcription factor, was rapidly
phosphorylated in response to ABA (Kagaya et al. 2002 ). Kobayashi et al. ( 2005 )
reported that the ABA-activated SnRK2 in rice phosphorylated TRAB1. OSRK1,
a protein kinase belonging to the SnRK2 rice family, phosphorylated OREB1, an
ABRE-binding factor (Chae et al. 2007 ). In wheat, PKAB1, the ortholog of wheat
SnRK2, phosphorylated TaABF, a wheat AREB protein (Johnson et al. 2002 ).
Hence, phosphorylation modifications by protein kinases to activate ABF-type tran-
scription factors play similar roles in the expression of AREB/ABF-regulated ABA-
responsive genes under drought and salt stresses in both Arabidopsis and crops.
In Arabidopsis , a negative regulatory domain occurs within the DREB2A pro-
tein. Deletion of this domain transforms DREB2A into the constitutively active