Transcription Factors Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 219

et al. ( 2008 ) demonstrated, by whole-genome expression profiling using a til-
ing array, that more than 6,300 genes are up- or down-regulated by ABA (Matsui
et al. 2008 ). These and other studies (Hoth et al. 2002 ; Seki et al. 2002 ; Takahashi
et al. 2004 ; Zeller et al. 2009 ) show that up to 25 % of the Arabidopsis genes are
affected by ABA. ABA plays a key role in seed development and stress response,
and the studies indicate that large fraction of the ABA-regulated genes are seed-
specific genes, such as storage protein and LEA genes, and that over 60 % of the
ABA-regulated genes are also responsive to various abiotic stresses.
Approximately 10 % of the ABA-responsive genes are associated with tran-
scription (Nemhauser et al. 2006 ; Seki et al. 2002 ), which include most of the
major classes of transcription factors (TFs), such as bZIP, AP2/ERF, MYB, HB,
zinc finger, WRKY, etc. Not all ABA-responsive TFs may be involved in ABA
regulation of gene expression, and, conversely, it is certain that TFs whose expres-
sion is not ABA-dependent are involved in ABA-regulated gene expression.
Nonetheless, the genome-wide transcriptome studies suggest that several hundreds
of TFs may be responsive to ABA. Furthermore, the list of TFs involved in ABA
response and/or stress responses in other plant species is ever-increasing.
Several reviews on ABA-dependent transcription in general and in association
with stress response are available (Busk and Pages 1998 ; Cutler et al. 2010 ; Fujita
et al. 2011 ; Nakashima et al. 2009b ). Also, a large number of general reviews on
various classes of TFs are available (see below). In this review, I will focus on
the Arabidopsis TFs whose functions in ABA signaling have been experimentally
determined and which can be related to the core ABA signaling pathway men-
tioned in previous chapters.
11.2 Cis -Elements
A number of ABA response elements have been determined by promoter analy-
ses of typical ABA-responsive genes, such as Em , RAB , and RD29A (Busk and
Pages 1998 ). Most ubiquitous elements are those possessing the PyACGTGGC
core sequence. The core element is similar to the G-box, CACGTG, which is pre-
sent in many light-regulated genes (Menkens et al. 1995 ). The conserved core
sequence is generally known as ABA response element (ABRE). Originally found
in the monocot LEA genes (Guiltinan et al. 1990 ; Marcotte et al. 1989 ; Mundy
et al. 1990 ), the element was found to be present in numerous ABA-responsive
genes (Gomez-Porras et al. 2007 ; Yazaki and Kikuchi 2005 ; Zhang et al. 2005a ).
Later experiments showed that a single ABRE is not sufficient to induce ABA-
dependent gene expression: It usually functions in combination with other cis -
elements for full ABA induction. The additional elements, named “coupling
elements,” may be the ABRE itself or one of other cis -elements, such as cou-
pling element 1 (CE1) (Shen and Ho 1995 ), coupling element 3 (CE3) (Shen
et al. 1996 ), Motif III (Ono et al. 1996 ), and Hex III (Lam and Chua 1991 ).
Except CE1, these coupling elements share the CGCGTG consensus. CE3 has