ABA Signaling and Circadian Clock - Abscisic Acid: Metabolism, Transport and Signaling - page 378

component called “X” modulating TOC1 activation of CCA1 and LHY , as TOC1
and CCA1 / LHY expressions are too far temporally (Fig. 19.2 ).
This model represented a breakthrough on the understanding of how the rhyth-
micity is generated and also inspired most of the works until now. However, the
model was incomplete as many other essential elements for a proper clock func-
tion had been described, and also did not include the important post-transcriptional
and post-translational regulation described by some of these components.
Among the several components of the clock that are not included in this model
should be highlighted CCA1 HIKING EXPEDITION CCA1 / LHY ( CHE ) (Pruneda-
Paz et al. 2009 ), a CCA1 repressor by direct binding to its promoter region. In
turn, CCA1 and probably LHY repress CHE expression, forming a new feedback
loop. Also, LUX ARRHYTHMO ( LUX ), a MYB factor that is expressed at dusk,
is essential for maintaining the rhythmicity (Hazen et al. 2005 ; Onai and Ishiura
2005 ). Mutations in this gene lead to the absence of rhythmicity in plants under
constant conditions. In terms of its function, LUX interacts with two essential
clock proteins, EARLY FLOWERING 3 (ELF3) and EARLY FLOWERING 4
(ELF4), forming a DNA-binding protein complex-denominated Evening Complex
(Nusinow et al. 2011 ).
The actual model of the circadian clock has undergone a significant revision
recently. TOC1 and the other PRRs proteins were shown to be transcription fac-
tors binding DNA through their conserved CCT domain (Gendron et al. 2012 ).
Unexpectedly, the PRRs proteins act as transcriptional repressors. Indeed, experi-
ments where TOC1 was acutely induced by either ABA or ethanol-responsive
transgenes led to immediate downregulation of CCA1 and LHY transcrip-
tion (Huang et al. 2012 ). These results challenged the well-establish model as
TOC1 does not act to promote LHY and CCA1 transcription but to repress it.
Furthermore, the identification of the Evening Complex allowed to postulate the
mechanism responsible for LHY and CCA1 activation. Mutations in either lux ,
elf3 or elf4 lead to higher TOC1 levels (Dixon et al. 2011 ; Helfer et al. 2011 ; Kikis
et al. 2005 ; Kolmos et al. 2009 ), suggesting that the Evening Complex might allow
the CCA1 and LHY expression at dawn by downregulating TOC1 and therefore
lift the repression (Pokhilko et al. 2012 ). Incorporating all these new evidence, a
new model arose where CCA1/LHY lead the sequential expression and repression
of the PRRs genes PRR9 , PRR7 , PRR5 , and TOC1 that keep CCA1 / LHY expres-
sion levels down and finally the Evening Complex release the repression before
dawn allowing the cycle to restart (Fig. 19.2 ).
19.3 Interaction Between the Circadian Clock
and ABA Signaling Networks
Many physiological processes show circadian rhythms, including essential pro-
cesses such as stomatal and leaf movements (Seung et al. 2012 ; Harmer et al.
2000 ; Dodd et al. 2005 ), osmotic and cold responses (Bieniawska et al. 2008 ;