ABA Signaling and Circadian Clock - Abscisic Acid: Metabolism, Transport and Signaling - page 380

peak at the end of the day (Fukushima et al. 2009 ). Interestingly, NCED, one of
the limiting steps in ABA synthesis de novo in response to drought and salt stress,
showed diurnal oscillation in its abundance similar to ABA (Thompson et al.
2000 ). Although the circadian regulation of ABA metabolism is clear, little is
known about the molecular mechanisms implicated.
The first evidences of a molecular link between ABA metabolism and the
circadian clock were provided by an extensive metabolomic analysis of the
prr9 / prr7 / prr5 triple mutant (Fukushima et al. 2009 ). This mutant displays
defects in biosynthetic pathways involved in chlorophyll, carotenoid, tocoph-
erol, and ABA. Particularly, it shows a significant induction of the carotenoid
and ABA biosynthetic pathways, resulting in constitutive high ABA content,
thus suppressing diurnal oscillation (Fukushima et al. 2009 ). Not surprisingly,
prr9 / prr7 / prr5 mutant displayed drought resistance, freezing tolerance, and upreg-
ulation of cold-responsive genes (Nakamichi et al. 2009 ).
19.5 Circadian Modulation of the ABA Core
Signaling Network
Although the transcriptomical analysis indicated an important role of the clock
modulating ABA-mediated responses, the molecular mechanisms involved in that
regulation are largely unknown. Some key elements in ABA signaling exert robust
diurnal oscillation, most likely due to clock regulation. Among them should be
highlighted elements implicated in the ABA perception such as PYL3 , RCAR1 , and
ABAR / CHLH , and essential ABA signal transduction elements such as SnRK2.6 ,
ABI1 , ABF3, and HAB1 (Seung et al. 2012 ).
Molecularly, TOC1 has emerged as a central component acting as a molecular
switch between clock and ABA pathways, especially in response to drought stress.
TOC1 represses the candidate ABA receptor ABAR/CHLH (Legnaioli et al. 2009 ;
Shen et al. 2006 ; Wu et al. 2009 ) by direct binding to its promoter region, thus reg-
ulating its diurnal expression. Furthermore, TOC1 directly interacts and modulates
the activity of key proteins downstream ABA perception such as the phytochrome-
interacting factor (PIF) family proteins and the CBF/DREB1 transcription factors,
both implicated in drought and cold responses (Kidokoro et al. 2009 ), as well as ABA
INSENSITIVE3 (ABI3), an essential factor in seed germination and drought tolerance
(Kurup et al. 2000 ).
Although TOC1 has been proven to have a pervasive role connecting circadian
clock with ABA signaling, the presence of CCA1 binding elements in the promot-
ers of genes encoding several ABA signaling components, including ABAR / CHLH ,
suggests that other circadian clock components might be implicated in this regula-
tion (Pokhilko et al. 2013 ). Indeed, mutations in CCA1 and LHY genes result in
plant hypersensitivity to salt, osmotic, and heat stress (Kant et al. 2008 ). Hence,
more exhaustive studies are still needed to undercover the intricate mechanisms
that allow the circadian clock to precisely modulate the ABA responses.