ABA and the Floral Transition - Abscisic Acid: Metabolism, Transport and Signaling - page 361

with three proteins with RNA recognition motifs including FCA, FLK and FPA
(Lim et al. 2004 ; Macknight et al. 1997 ; Schomburg et al. 2001 ). Thus CBC may
modulate the sensitive step of interaction between FLC mRNA and the regulators
of FLC. Despite highlighting the importance of RNA processing in both ABA sig-
nalling and FLC regulation, ABH1 action does not provide clues as to whether and
how ABA might play a specific regulatory role in FLC transcript processing.
The above-mentioned mutants show clear defects related to ABA responses,
yet their pleiotropic nature does not provide clues as to the precise role of ABA
in flowering. The recent identification of PYRABACTIN RESISTANCE (PYR)/
REGULATORY COMPONENT OF ABA RECEPTOR (RCAR) proteins as ABA
receptors (Ma et al. 2009 ; Park et al. 2009 ) define the early events in ABA signal
transduction.
Three classes of protein represent key ABA signalling nodes: PYR/RCARs,
PROTEIN PHOSPHATASE 2Cs (PP2Cs) and SUCROSE NON-FERMENTING1-
SNF1-RELATED PROTEIN KINASE 2s (SnRK2s). In ABA signalling, PYR/RCARs
act as ABA receptors, PP2Cs act as negative regulators of the pathway and SnRK2s
act as positive regulators of downstream signalling (Cutler et al. 2010 ; Umezawa et al.
2010 ). ABA binds to the PYR/RCAR receptors and this complex inhibits the action
of group A PP2C proteins allowing SnRK2s activation and phosphorylation of their
targets. In the absence of ABA, PP2Cs dephosphorylate SnRK2s thus preventing their
activity (Umezawa et al. 2009 ; Vlad et al. 2009 ; Soon et al. 2012 ). Interestingly, bio-
chemical differences in the PYR/RCAR-PP2C interaction exist, depending on the
PYR/RCAR isoform and this suggests that the mechanism of ABA perception and sig-
nalling is more complex than anticipated (Hubbard et al. 2010 ).
In support for a positive role of ABA in flowering, triple pp2c mutants are sig-
nificantly earlier flowering compared to wild type under LDs (Riboni et al. 2013 ).
There are increased levels of FT transcripts accumulation in pp2c triple mutants,
suggesting that ABA acts upstream of FT transcription. SnRK2s activity is up-reg-
ulated following phosphatase down regulation, implying that SnRK2-type kinases
could mediate ABA signalling upstream of FT upregulation (Fujii et al. 2009 ;
Umezawa et al. 2009 ). In agreement with this model SnRK2.6/OST1 overexpress-
ing plants produce a small flowering acceleration under LDs (Zheng et al. 2010 ).
However, the triple snrk2 mutants are early flowering under LDs (Wang et al.
2013a , b ), arguing in favour of a negative role of ABA in flowering. The similar
early flowering phenotype of triple snrk2 and triple pp2c mutants argues against
a simple signalling cascade converging onto the activation of FT . Since SnRK2s
targets are diverse and involved in several independent processes one could specu-
late that their deregulation might produce pleiotropic effects (Wang et al. 2013a ;
Umezawa et al. 2013 ). A more detailed description of the SnRK2s and PP2Cs
regulative network would therefore help in discriminating ABA-direct from
ABA-indirect effects. Biochemical and genetic studies indicate that several kinds
of reversible protein phosphorylation events play an important role in the regu-
lation of ABA signalling. Phosphoproteomic screens indicate that several flower-
ing proteins are targeted by SnRK2s, although less is known about the regulatory
role of phosphorylation on the activity of these proteins (Wang et al. 2013a ). ABA