ABA Signaling and Circadian Clock - Abscisic Acid: Metabolism, Transport and Signaling - page 382

19.7 Circadian Gating on ABA Responses

An essential aspect in circadian clock function lies in its ability to respond dif-
ferentially to external stimuli depending on the circadian time through a process
called gating (Millar and Kay 1996 ). A clear example of this capability is the light
input. While during the dusk and dawn, the circadian clock is extremely sensible to
light changes, allowing a fine-tuning of the period, in the middle of the night, light
signals (such as a thunder) are gated, thus not significantly affecting the circadian
clock pace. This phenomenon can be explained by the diurnal oscillation of pho-
toreceptors along with other factors. However, the gating phenomenon is not only
important in clock synchronization, but also is essential mediating the response to
stimuli in some downstream processes such as ABA signaling.
Although ABA has been typically associated with responses to rather ran-
dom environmental changes, most of these stressful conditions are more likely
to occur in a certain moment during the diurnal cycle (Fig. 19.3 ). Multiple lines
of emerging evidence indicate that the capacity to respond to those environ-
mental challenges is gated by the clock. This idea is in agreement with a more
efficient use of the resources and consequently and increased plant fitness. Two
of the most well-studied physiological processes where the ABA signaling is
clearly gated by the clock are the stomatal movements and the cold response.

Starvation

Cold stress

ZT0

ZT23

ZT22

Light harvesting
genes expression

ZT4

ROS scavenging

ZT19

solar radiation
stomata aperture

ZT6

ZT7

H 2 0 2 levels

ZT8

Susceptibility to
bacterial infection

ZT16

Heat stress

ZT11

ZT12

Drought stress

Herbivore feeding
Susceptibility to oomycetes

Fig. 19.3 Circadian oscillation of biotic and abiotic stress (adapted from Spoel and Van Ooijen
2014 ). Many biotic and abiotic stresses fluctuate over the day cycle showing a circadian oscilla-
tion. This figure represents the approximate Zeitgeber time ( ZT ) where those stresses reach their
maximum. ZT0 and ZT12 represent dawn and dusk, respectively