Transcription Factors Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 223

98 genes are the targets of ABI3 (Monke et al. 2012 ). Most of the ABI3 target
genes are ABA-regulated and seed-specific genes, such as storage proteins, oleosin,
and LEA protein genes. One of the ABI3 target genes is ABI5, which acts down-
stream of ABI3 to arrest seedling establishment (Lopez-Molina et al. 2002 ). Several
studies indicate that ABI3 functions synergistically with ABI5 to regulate ABA-
responsive genes (Casaretto and Ho 2003 ; Gampala et al. 2002 ; Hobo et al. 1999 ;
Lim et al. 2013 ; Nakashima et al. 2006 ). ABI3 protein is unstable in the absence of
ABA, and it has been shown that AIP2, a RING-type E3 ligase, ubiquitinates ABI3
to promote its proteasomal degradation (Lopez-Molina et al. 2002 ; Zhang et al.
2005b ).
Initially, ABI4 has been isolated as ABA-insensitive mutant with seed-specific
defects (Finkelstein 1994 ). However, later studies showed that it plays diverse
roles in sugar signaling, stress response, lipid mobilization, chloroplast retrograde
signaling, root development, and biosynthesis of ABA and GA (Leon et al. 2012 ;
Shu et al. 2013 ; Wind et al. 2013 ). ABI4 belongs to the DREB/CBF subfamily of
the AP2/ERF superfamily (Sakuma et al. 2002 ), and its binding sites have been
determined in vitro employing maize ABI4 (Niu et al. 2002 ). The study shows that
its preferred binding sites contain the CACCG core sequence, which is similar to
the coupling element CE1. Reeves et al. ( 2011 ) showed that approximately 100
genes are induced by ectopically expressed ABI4. However, CE1 element is not
enriched in the promoters of the ABI4 target genes, although ABI4 binds them.
The result suggests that ABI4 may bind sequences other than CE1 in vivo, and it
is consistent with the observation that other AP2-domain proteins than ABI4 are
likely to bind the CE1 element (Shen et al. 2004 ). Also, strong synergy between
ABI4 and ABI5, ABF1, or ABF3 was observed in the activation of the promoters
lacking CE1 in yeast.
The functions of ABI3 and ABI4 in ABA signaling have been firmly estab-
lished. However, their position in the core ABA signaling pathway is not clearly
understood. ABI3 is epistatic to ABI5; that is, ABI3 acts upstream of ABI5 (Lopez-
Molina et al. 2002 ), and ABI5 expression is regulated by ABI3. How ABI3 is related
to upstream components of the core ABA signaling pathway and, if it functions in a
separate pathway which converges on ABI5 with the core pathway, the identity of
the upstream regulatory components remains to be determined. Similarly, ABA sign-
aling components functioning upstream of ABI4 remain to be determined.
11.7 AP2/ERF Domain Proteins
In addition to ABI4, several other AP2/ERF domain proteins have been reported
to function in ABA response. Maize DRE-binding proteins, DBF1 and DBF2,
are involved in the ABA regulation of RAB17 (Kizis and Pages 2002 ). DREB2C
interacts with ABFs and positively regulates ABA response (Lee et al. 2010b ).
Other DREBs, i.e., DREB1A and DREB2A, also interact with ABF2/AREB1
and ABF4/AREB2, suggesting that ABFs/AREBs and DREB family TFs,