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The over expression of ABI5 results in a late flowering phenotype, while abi5
loss-of-function mutants are early flowering under LDs (Wang et al. 2013b ).
ABI5 acts as a transcriptional activator and binds to an ABA-responsive element
(ABRE), which are common elements in the promoters found in many ABA-
induced genes (Busk and Pag│s 1998 ; Zhang et al. 2005 ). A number of ABRE
binding factors (ABFs; also referred to as AREBs) were identified, including
ABF1, ABF2/AREB1, ABF3, ABF4/AREB2 (Uno et al. 2000 ; Choi et al. 2000 ).
The ABFs genes are upregulated following drought stress and their overexpression
results in plants with enhanced drought tolerance and altered expression of ABA/
stress-regulated genes (Fujita et al. 2005 ; Kang et al. 2002 ). Similarly to ABI5, the
overexpression of ABFs also results in late flowering, pointing to a negative role
for this clade of bZIPs in flowering (Fujita et al. 2005 ). However multiple ABFs
knock-out mutants do not display evident flowering alterations (Yoshida et al.
2010 ). Posttranslational modifications play a key role in ABI5 and AREB/ABF
activity since SnRK2s phosphorylate ABI5 and ABI5-like bZIP transcription fac-
tors in an ABA-dependent manner to promote gene expression (Furihata et al.
2006 ; Fujii et al. 2007 , 2009 ; Lopez-Molina et al. 2001 ; Wang et al. 2013b ).
ABI5 acts as a positive regulator of FLC , a MADS-box protein that suppresses
flowering (Michaels 1999 ). FLC levels are negatively regulated by the autono-
mous and vernalization pathways that promote flowering indirectly by repressing
FLC levels (Simpson 2004 ). ABA-dependent stimulation of SnRK2s promotes
ABI5 activity and FLC upregulation. Deletions of different phosphorylation sites
through targeted mutagenesis results in lack of ABI5 activity in vivo and no FLC
activation even in the presence of exogenous ABA applications. Transcript analy-
sis indicates that two important floral integrators ( FT and SOC1 ) are upregulated
in abi5 mutants, which is in agreement with a reduced FLC activity. Likewise, FT
and SOC1 are decreased in 35S :: ABI5 plants. However, neither 35S :: ABI5 nor abi5
mutant plants display changes in flowering under SDs.
Other important layers of regulation are likely to play a role in control-
ling bZIPs activity and, as a result, regulation of flowering by ABA. NUCLEAR
FACTOR - Y -type transcription factors interact with ABI5 and ABFs and participate
in FT expression by interacting with CO (Kumimoto et al. 2010 , 2013 ; Wenkel
et al. 2006 ). Furthermore, mutations in the FyPP1 and FyPP3 ( PHYTOCHROME -
ASSOCIATED SERINE/THREONINE PROTEIN PHOSPHATASE 1 and 3 ) cause
an ABA hypersensitive phenotype and an accumulation of hyper phosphorylated
ABI5 forms (Dai et al. 2013 ). Despite over accumulating ABI5 protein, fypp1
mutants are early flowering although it is not clear whether this can be directly
related to ABI5 or defective signalling in other pathways (Kim et al. 2002 ).
Other transcription factors mediate ABA response, but independent of SnRK2-
mediated phosphorylation. One such transcription factor is ABSCISIC ACID
INSENSITIVE 3 (ABI3), a B1/B3 transcription factor responsible for the posi-
tive and negative regulation of a plethora of ABA responses (Parcy et al. 1994 ;
Giraudat et al. 1992 ; Rohde et al. 2000a , b , 2002 ; M￶nke et al. 2012 ; Suzuki et al.
2003 ). Besides the well established role in seed germination and early seedling
development, ABI3 has broader function in vegetative development, including
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