ABA and the Floral Transition - Abscisic Acid: Metabolism, Transport and Signaling - page 366

hormone Ethylene are increased in aba mutants plants, and may contribute to their
late flowering (LeNoble et al. 2004 ; Riboni et al. 2013 ). Ethylene in turn suppresses
flowering through the downregulation of GAs biosynthesis (Achard et al. 2007 ).
ABA-GAs cross-talk plays a fundamental role during the germination process where
ABA counteracts GA biosynthesis and signalling, thus triggering seed dormancy as
GAs beak dormancy to allow germination (Weiss and Ori 2007 ). Interestingly in the
context of the transition to flowering, ABA may act in parallel to GAs with respect
to FT upregulation (Riboni et al. 2013 ). GAs are absolutely required for flowering
under SDs, when the inductive photoperiodic pathway is not active (Wilson et al.
1992 ; Reeves and Coupland 2001 ). Under LDs the role of GAs is less pronounced
but still present. Different lines of evidence suggest that GAs promote the floral tran-
sition at different sites, by positively regulating FT and TSF transcription (in paral-
lel with CO) in leaves and by stimulating the SQUAMOSA PROMOTER BINDING
PROTEIN - like ( SPL ) genes, upstream of SOC1 in the shoot apical meristem (Porri
et al. 2012 ; Galv￣o et al. 2012 ; Hisamatsu and King 2008 ). Both ABA and GAs
appear to share a strong photoperiodic dependency in the way they can activate FT
expression. ABA does not activate FT or TSF under SDs. Similarly, the upregula-
tion of FT following applied GA is far more effective under LDs compared to SDs
(Hisamatsu and King 2008 ). Taken together these results suggest that ABA and GAs
require a photoperiodic component for their ability to upregulate FT, although ABA
appears more stringent than GAs in this requirement. Whilst ABA and GAs are pro-
posed to both converge to florigen transcription under LDs, the role of ABA sig-
nalling under SDs is opposite to that of GAs. ABA could play an important role in
the SAM by counteracting positive flowering signals from GAs. The molecular basis
for these mechanisms is unknown and further experiments are needed to reveal the
underlying photoperiodic component(s) and the spatial context involved.
18.8 Concluding Remarks
ABA is mainly known as a stress hormone and its role in flowering is only beginning
to emerge. Independent reports suggest ABA acting as a promoter of flowering in SD
plants. In contrast a consensus as to the precise mode of action of ABA in LD plants
is lacking. This begs to the question as to whether the ABA-mediated effects on flow-
ering are conserved in plants and if so, their relationship with photoperiod. Ecotype/
species specific effects, but also cultivation practices and conditions might affect the
observed variability in LD plants/backgrounds and need to be investigated further.
Although progresses have been made in linking ABA signalling to FT expres-
sion the molecular basis fora photoperiod-ABA cross talk is unknown. Similarly,
the mechanism and physiological significance of ABA in the transcriptional/post
transcriptional control of FLC remain poorly understood. Key questions remain
as to how early phosphorylation signals are integrated in the photoperiodic or
autonomous floral pathways, whether they involve other components alongside the
PYR/RCAR module and their spatial/temporal modes of regulation. Downstream