ABA and the Floral Transition - Abscisic Acid: Metabolism, Transport and Signaling - page 356

18.2 Is ABA a Flowering Hormone?
Traditionally ABA has been mainly associated with the control of germination
and different stress responses, principally those connected to drought stress (Fujita
et al. 2011 ; Finkelstein et al. 2002 ). Experiments have shown that exogenous
applications of ABA cause flowering time alterations, suggesting that ABA might
be an endogenous component affecting the floral transition (Domagalska et al.
2010 ; Blazquez et al. 1998 ; Wang et al. 2013b ). However, from a genetic stand-
point, flowering alterations caused by ABA signalling or biosynthetic mutants are
not dramatic, although this does not rule out an important role for ABA in flower-
ing. Thus, how ABA participates in the floral transition has been elusive thus far.
Hormone signalling ensures developmental flexibility to plants (Santner et al.
2009 ). It is therefore not surprising that several hormonal pathways have been
shown to modulate the floral transition. In the context of the floral transitions ABA
is generally considered as an inhibitor of flowering, although in some experiments
it appeared to be a promoter. For example, endogenous ABA is a positive regula-
tor of the DE response via the upregulation of the key floral gene FLOWERING
LOCUS T ( FT ) (Riboni et al. 2013 ). However, exogenous ABA applications inhibit
flowering in Arabidopsis and other plants including darnel ( Lolium temulentum ),
(Blazquez et al. 1998 ; Domagalska et al. 2010 ; King and Evans 1977 ). A genome-
wide transcript analysis showed that FT is downregulated following ABA appli-
cations. However a similar downregulation occur also in ABA insensitive 1 - 1
( abi1 - 1 ) mutants (Hoth et al. 2002 ). Whole plant ABA applications thus negatively
regulate FT expression, independent of the well-established endogenous ABA sig-
nalling. Alternatively, the negative effect of ABA on FT is non specific and/or indi-
rect. One hypothesis is that ABA applications in Arabidopsis might be hampered
by the intrinsic spatial and temporal-dependent regulation of ABA signalling, thus
making it difficult to distinguish which experimental design fully mimics endog-
enous ABA action. In this respect, root applications of ABA promote flowering,
which is in contrast with other experiments using exogenous ABA (Koops et al.
2011 ).
Circadian fluctuation in ABA levels, gating of ABA signalling as well as tis-
sue specific ABA signalling and transport are well documented in plants (Lee et al.
2006 ; Legnaioli et al. 2009 ; Covington et al. 2008 ; Endo et al. 2008 ). Perhaps
whole plant applications of ABA subvert such fine tuned mechanism of endoge-
nous ABA metabolism. This could be indeed be the case as in the short day plant
Pharabitis nil exogenous ABA applications during the first part of the inductive
short day are inhibitory whilst ABA promotes flowering when applications occurs
at the end of the night (Wilmowicz et al. 2011 ). Quantifications of endogenous
ABA levels indicate that the hormone reach a maximum at the end of the inductive
night, coincidently with the temporal window when ABA applications are most
effective. The pharmacological manipulation of endogenous ABA by treatments
with ABA biosynthesis inhibitors at the end of the night results in a delay in flow-
ering (Wilmowicz et al. 2008 ). These data suggest a gating mechanism whereby