ABA and the Floral Transition - Abscisic Acid: Metabolism, Transport and Signaling - page 365

LIGHT

GI

FBH1
EDL3

1

2

CO ABI3

ABA ABA

5

3

FLC

4

OXS2

FT

SOC1

Fig. 18.1 A schematic model summarizing different modes of action of ABA during the floral
transition. 1 ABA upregulates the transcription of EDL3 and affects FBH1 activity in an unknown
manner. Both these proteins are positive regulators of CONSTANS (CO). 2 ABA potentiates pho-
toperiod-stimulated GI activity, independent of CO. 3 ABA promotes degradation of ABI3, an
interactor of CO, which might lead to an increase in CO activity. 1 - 3 lead to activation of FT . 4
ABA promotes the activity of OXS2, a direct activator of SOC1 . 5 ABA induces FLC expression
via upregulation of ABI5 activity. FLC causes downregulation of FT and SOC1 expression

In conclusion, rather than operating in a hierarchal cascade of signalling events,
these data suggest ABA acting at multiple (transcriptional and post-transcriptional)
levels in the regulation of several transcription factors. Each class of transcription
factor positively or negatively regulates the floral transition by acting on a specific
floral integrator; OXS2-factors converge on SOC1, ABI3 on TSF / MFT and ABI5
on FLC . A future goal will be to understand how and where ABA co-ordinately
activates such a complex network of transcription factors, the underlying regula-
tory logic and ultimately its physiological significance (Fig. 18.1 ).

18.7 ABA Interaction with Other Hormonal Pathways
During the Floral Transition

Several phytohormones participate in the floral transition including Gibberellic
acid (GAs), Cytokinin, Salicylic Acid, Ethylene, Nitric Oxide and Brassinosterodis
(D'Aloia et al. 2011 ; Wilson et al. 1992 ; Achard et al. 2007 ; Martínez et al. 2004 ;
Domagalska et al. 2007 ; He et al. 2004 ). Due to the well-established hormone
cross-talk, ABA could affect flowering indirectly, e.g. by modifying other hormo-
nal pathways (Gazzarrini and McCourt 2003 ). The levels of the floral repressing