Transcription Factors Involved in ABA Signaling - Abscisic Acid: Metabolism, Transport and Signaling - page 221

ABFs/AREBs were isolated as CE3-binding proteins in our one-hybrid screens
(unpublished observation).
The in vivo functions of ABF2/AREB1, ABF3, and ABF4/AREB2 in ABA-
dependent stress response have been demonstrated by transgenic approaches.
Constitutive overexpression of the three bZIP proteins resulted in enhanced ABA
sensitivity with concomitant increase in stress tolerance, whereas their knockout
mutations resulted in decrease in ABA sensitivity and stress tolerance (Finkelstein
et al. 2005 ; Kim 2006 ; Yoshida et al. 2010 ). These phenotypic changes were
accompanied by changes in the expression levels of a large number of ABA and/
or stress-responsive genes (Abdeen et al. 2010 ; Fujita et al. 2005a ; Yoshida et al.
2010 ).
ABI5, which is the same as AtDPBF1 (Kim et al. 2002 ), has been isolated based
on genetic screens to isolate ABA response mutants (Finkelstein 1994 ; Finkelstein
and Lynch 2000 ; Lopez-Molina and Chua 2000 ) and belongs to the ABF/AREB sub-
family. ABI5 plays an important role in ABA response in seeds and during early stage
of seedling establishment, and its mutations result in ABA-insensitive germination
and seedling growth (Finkelstein 1994 ; Lopez-Molina et al. 2002 ). Transcriptome
analysis of the transgenic plants overexpressing ABI5 demonstrated that 59 gene are
up-regulated by ABI5 in ABA-dependent manner (Reeves et al. 2011 ).
11.4 Modulation of ABFs/AREBs/ABI5 Functions
by Phosphorylation
Not only the expression of ABF/AREB/ABI5 family members is induced by ABA,
but their activities are also regulated by ABA posttranslationally (Uno et al. 2000 ;
Yoshida et al. 2010 ). The most important aspect of ABA-dependent posttransla-
tion modification of the bZIP factors is phosphorylation. Various studies indicate
that ABFs/AREBs are phosphorylated by SnRK2.2/SnrK2D, SnRK2.3/SnRK2I,
and SnRK2.6/SnRK2E/OST1 (Fujii et al. 2007 , 2009 ; Fujii and Zhu 2009 ; Fujita
et al. 2009 , 2013 ; Furihata et al. 2006 ; Sirichandra et al. 2010 ). The three SnRK2s,
which are key regulators of ABA response, are key players in the phosphorylation
of ABFs/AREBs/ABI5. For instance, ABF1, ABF2/AREB1, and ABI5 are phos-
phorylated by the three SnRKs and by SnRK2.7/SnRK2F and SnRK2.8/SnRK2C
as well (Furihata et al. 2006 ; Fujii et al. 2007 ). ABF3 is phosphorylated by
SnRK2.6/SnRK2E/OST1 in vivo (Sirichandra et al. 2010 ). ABI5 also is phospho-
rylated by the three SnRK2s (Fujii et al. 2007 ; Nakashima et al. 2009a ). Recently,
FyPP1 and FyPP3, which encode PP6 phosphatase catalytic subunits, have been
reported to negatively regulate ABA response (Dai et al. 2013 ). The phosphatases act
antagonistically with SnRKs by dephosphorylating ABI5, and thereby destabilizing it.
Several calcium-dependent protein kinases (CDPKs), which play positive
regulatory roles in ABA response, are known to phosphorylate ABFs/AREBs.
CPK32, for example, phosphorylates ABF4, and its overexpression enhances