ABA and the Floral Transition - Abscisic Acid: Metabolism, Transport and Signaling - page 359

box-type transcription factors, namely SUPPRESSOR OF OVEREXPRESSION OF
CONSTANS 1 ( SOC1 ), APETALA1 , and FRUITFUL , responsible for triggering the
floral transition in the SAM (Abe et al. 2005 ; Wigge et al. 2005 ).
Genetic and expression data suggest a role for ABA in the activation of the
florigen genes (Riboni et al. 2013 ). First, drought conditions promote FT upreg-
ulation, resulting in a DE response (Riboni et al. 2013 ; Su et al. 2013 ). Second,
aba1 mutants display a reduced FT expression, especially under drought condi-
tions. Consistent with the idea of ABA stimulating flowering in a photoperiodic-
dependent manner, aba1 mutants do not show any obvious flowering phenotype
under SDs, when no photoperiodic-dependent activation of FT occurs. Also, no DE
response occurs under SDs, despite a substantial increase in endogenous ABA, nor
in gi mutants under LDs. Interestingly, ABA accumulates primarily in the vascular
tissue in Arabidopsis (Endo et al. 2008 ; Koiwai et al. 2004 ; Cheng et al. 2002 ),
overlapping with the site of FT expression. In addition to FT , FT -like genes are
present in Arabidopsis including TSF and MOTHER OF FT AND TFL1 ( MFT )
and they all appear to be positively regulated by ABA (Xi et al. 2010 ; Riboni et al.
2013 ). Taken together these data argue in favour for a positive role for endogenous
ABA in flowering via potentiation of florigen-like genes in a photoperiodic manner.
It is unclear how ABA might affect photoperiodic signalling. Drought stress
results in an increase in FT expression without affecting the physiological circa-
dian oscillation of FT (Riboni et al. 2013 ; Su et al. 2013 ). Because the pattern
of FT transcript accumulation is mainly dictated by variations in CO protein lev-
els, ABA might directly affect CO protein levels and/or activity. Supporting an
increase in CO transcription under drought conditions, FLOWERING BHLH 1
(FBH1) , a basic helix-loop-helix-type transcription factor and CO positive acti-
vator is phosphorylated in vivo following ABA signalling activation (Ito et al.
2012 ; Wang et al. 2013a ). However the role of phosphorylation on FBH1 activ-
ity is unclear. An alternative ABA target is EID1 - like protein 3 ( EDL3 ), a positive
regulator of ABA signalling and an activator of CO . EDL3 transcript is upregu-
lated following ABA applications. EDL3 encodes an F-box type protein that acts
upstream of CO and positively increase its mRNA levels, thus providing a link
between ABA and photoperiodic flowering (Koops et al. 2011 ). These observa-
tions suggest ABA acting upstream of CO. However, CO transcripts upon drought
conditions are slightly decreased despite GI and FKF1 being upregulated (Han
et al. 2013 ). A further possibility could be that ABA promotes GI activity by
facilitating its direct action on FT promoter, independent of CO. Other light sens-
ing proteins could be involved in this mechanism. Cryptochromes are emerging
as important general components of ABA-dependent signalling since the overex-
pression of CRY1 from wheat ( TaCRY1a ) in Arabidopsis results in an ABA hyper-
sensitive phenotype (Xu et al. 2009 ). Similarly to gi , cry2 mutants have defective
DE response, despite accumulating increased ABA levels (Riboni et al. 2013 ;
Boccalandro et al. 2012 ). Most importantly, CRY2 positively regulates FT expres-
sion through different mechanisms, including stabilization of GI (Saijo et al. 2008 ;
Zuo et al. 2011 ; Liu et al. 2008 ). Therefore, it is intriguing to speculate that CRY2
may participate in the GI- and ABA-dependent activation of FT .