ABA Signaling and Circadian Clock - Abscisic Acid: Metabolism, Transport and Signaling - page 383

19.7.1 Stomatal Movements
Circadian regulation in stomatal aperture had been largely reported in well-watered
plants. The circadian clock allows to anticipate dawn and dusk transitions, promoting
stomata opening before dawn, to maximize the CO 2 uptake necessary for photosynthe-
sis, and stomata closure at dusk, avoiding unnecessary water loss without any increase
in carbon fixation. For instance, disruption of the circadian function by CCA1 overex-
pression results in impaired stomatal movements failing to anticipate dawn and dusk.
In these plants, stomata do not start to open before dawn and remain open at dusk,
until the dark induces closure (Dodd et al. 2005 ). Additionally, CCA1-overexpressing
plants showed increased water consumption and reduced carbon fixation levels, indi-
cating the importance of this regulation for plant fitness (Dodd et al. 2005 ).
During water-deficit stress, ABA reduces the size of the stomatal pore.
However, ABA is less effective at closing stomata in the morning than in the after-
noon (Correia et al. 1995 ). Thus, the ABA-mediated stomata closure might be
gated by the circadian clock. This may ensure that stomata are open to facilitate
CO 2 uptake in the cool of the morning, when transpiration is lower, but are closed
in the heat of the afternoon if water supply is limiting.
Although the circadian gating of the ABA-mediated stomatal closure is
essential to enhance fitness in plants, the molecular mechanisms implicated are
still largely unknown. The first molecular evidences were provided by studies
using plants miss-expressing TOC1 . These plants show impaired ABA-mediated
stomata closure and altered drought tolerance. While TOC1 overexpression
leads to drought hypersensitivity and reduced sensitivity to ABA, plants with
reduced TOC1 expression show increased drought tolerance and ABA hyper-
sensitivity. TOC1 effect on stomata aperture requires a functional ABAR/CHLH
as TOC1 directly binds to its promoter (Legnaioli et al. 2009 ). ABAR/CHLH is
also necessary for ABA-mediated induction of TOC1 expression (see above).
Interestingly, the TOC1 promoter region contains two ABA-responsive elements
(ABRE), commonly associated to ABA-induced genes through PP2C/SnRK2
pathway (Yoshida et al. 2010 ; Nakashima and Yamaguchi-Shinozaki 2013 ).
Together with current circadian clock models, these findings allowed to build
a simple mathematical model able to explain the interaction between the cir-
cadian clock and ABA to control stomata aperture (Fig. 19.4 ) (Pokhilko et al.
2013 ). This model, although is still simple and might suffer important revisions
in the future, challenges the current model of ABA perception (see Chap. 7 ) and
represents an important step toward the understanding of this crucial process for
plant adaptation to the changing environmental conditions in water supply.
19.7.2 Cold Response
The exposure to non-damaging low temperature triggers the cold response
and the induction of freezing tolerance. Clock gating of the cold responses was