ABA and the Floral Transition - Abscisic Acid: Metabolism, Transport and Signaling - page 357

ABA promotes flowering in Pharabitis , but only in coincidence with a compe-
tence window. Therefore, the location and timing of ABA application could play a
crucial role in determining how ABA affects flowering.
18.2.1 Clues from ABA Deficient Mutants in Arabidopsis
Extensive genetic screens coupled with physiological and biochemical analy-
ses have defined the biosynthetic steps required for ABA production (Nambara
and Marion-Poll 2005 ; Seo and Koshiba 2002 ). Unfortunately the phenotypes
of ABA biosynthetic mutants do not provide clues as to unambiguously assign a
role for endogenous ABA in flowering. Originally the ABA insensitive mutants
abi1 were shown to flower earlier under SDs, although this was not apparent
under LDs (Martínez-Zapater et al. 1994 ; Chandler et al. 2000 ). Similar con-
clusions were drawn for aba1 alleles impaired in ABA production grown under
LDs. However these experiments were carried out under in vitro conditions, not
in soil (Barrero et al. 2005 ). aba2 / glucose insensitive 1 mutants (Wassilewskija
background) exhibit a small early flowering phenotype under long and short
days conditions suggesting an inhibitory role for ABA during the floral transition
(Domagalska et al. 2010 ; Cheng et al. 2002 ). However, transgenic lines that over-
express 9 - CIS - EPOXYCAROTENOID DIOXYGENASE 3 (NCED3, a key enzyme
in the ABA biosynthesis) do not produce any consistent alterations in flowering
and, if anything, are slightly earlier flowering than wild type (Domagalska et al.
2010 ). Thus, increased levels of endogenous ABA do not recapitulate the effect of
exogenous ABA applications in producing the expected delay in flowering (Wang
et al. 2013b ). Further complicating the topic, flowering time alterations in ABA
deficient mutants appear to be ecotype-dependent in Arabidopsis. Under LDs low
expression of osmotic stress - responsive genes 6 ( los6 ) mutants (allelic to aba1 ,
C24 background) are early flowering and a similar observation was made for los5
(allelic to aba3 , C24 background) (Xiong et al. 2001 , 2002a ). In contrast, in the
Columbia background, aba1/npq1 , aba2 - 4 or aba1 T-DNA insertion mutants are
late flowering indicating that ABA is a positive regulator of flowering (Riboni
et al. 2013 ). In summary, the action of endogenous ABA appears to be highly
background specific although the cause for such specificity is currently unknown.
18.2.2 ABA Deficient Mutants in Other Plants
Studies from ABA-deficient mutants from tomato provide additional hints as to
the role of ABA in regulating the floral transition. The notabilis ( not ) mutant har-
bours a null allele of the tomato 9-CIS EPOXYCAROTENOID DIOXYGENASE1
( Le NCED1), the enzyme that catalyses the first dedicated step of ABA biosynthe-
sis (Burbidge et al. 1999 ). The flacca ( (flc) ) mutant contains a 6 base pairs deletion