ABA and the Floral Transition - Abscisic Acid: Metabolism, Transport and Signaling - page 358

in a molybdenum-cofactor sulfurase, which reduces the oxidizing capacity of alde-
hyde oxidases involved in the conversion of abscisic aldehyde to ABA, the final
step in ABA biosynthesis (Sagi et al. 1999 ). Despite their reduced ability to syn-
thesize ABA, not and lc mutants still contain considerable levels of ABA, amount-
ing to 47 and 21 % of wild type, respectively (Herde et al. 1999 ). Nevertheless,
these two mutants show an extended vegetative phase compared to the wild type,
indicating a positive role for ABA in the transition between vegetative and repro-
ductive growth in tomato (Carvalho et al. 2011 ).
More conclusive evidence in support of a promoting role for ABA in the transi-
tion to flowering comes from the analysis of SD plants (El-Antably and Waering
1966 ). In soybean, a preferential short day crop, ABA-related transcripts accu-
mulate in the SAM upon transferring to floral inductive conditions. Up-regulation
of biosynthetic genes, including NCED1 , translate into a significant increase in
ABA levels in SAMs from SD-treated plants, prior to the induction of floral genes
(Wong et al. 2009 ). This correlative evidence not only implies a floral-promoting
role for ABA in soybean, but also suggests the striking overlap of the floral signal-
ling pathway with that of abiotic stress responses.
18.3 ABA Interaction with the Photoperiod
Photoperiodic flowering is the result of complex interactions between the circa-
dian clock (an endogenous timekeeping mechanism) and external cues, which ulti-
mately results in the activation of a set of floral genes (Imaizumi 2010 ). Central
to photoperiod-dependent flowering is the pattern of accumulation of the flower-
ing protein CONSTANS (CO) (Putterill et al. 1995 ). CO expression is regulated
transcriptionally by the circadian clock through the GIGANTEA (GI)-FLAVIN-
BINDING, KELCH REPEAT, F-BOX (FKF1) complex (Imaizumi et al. 2005 ;
Sawa et al. 2007 ; Fornara et al. 2009 ; Song et al. 2012 ). However, CO protein
accumulates only under LDs when the CO mRNA coincides with the light phase
at the end of the day (Valverde et al. 2004 ; Suarez-Lopez et al. 2001 ). Stabilization
of CO depends on photoreceptors PHYTOCROME A, CRYPTOCHROME 1
and 2 (CRY1 and 2) which promote CO stability at the end of a long day, while
PHYTOCHROME B (PhyB) destabilizes CO protein in the morning (Valverde
et al. 2004 ). CO is the transcriptional regulator that promotes flowering by inducing
expression of the florigens genes FT and TWIN SISTER OF FT ( TSF ) in the phloem
companion cells of the leaves (An et al. 2004 ; Yamaguchi et al. 2005 ; Kardailsky
et al. 1999 ; Kobayashi et al. 1999 ). While favourable photoperiod triggers FT tran-
scription, it is the FT protein that moves through the phloem from the leaves to
the SAM to initiate the floral transition (Corbesier et al. 2007 ; Mathieu et al. 2007 ;
Jaeger and Wigge 2007 ; Notaguchi et al. 2008 ). In the shoot apex, FT interacts
with a set of b-ZIP transcription factors (FLOWERING D, FD and FD PARALOG,
FDP), whose expression is largely SAM-specific (Wigge et al. 2005 ; Abe et al.
2005 ; Jaeger et al. 2013 ). Here, the FT/FD heterodimer activates several MADS