Regulatory Role of Circadian Clocks on ABA Production and Signaling, Stomatal Responses, and Water-Use Efficiency under Water-Deficit Conditions.

Plants deploy molecular, physiological, and anatomical adaptations to cope with long-term water-deficit exposure, and some of these processes are controlled by circadian clocks. Circadian clocks are endogenous timekeepers that autonomously modulate biological systems over the course of the day-night cycle. Plants' responses to water deficiency vary with the time of the day. Opening and closing of stomata, which control water loss from plants, have diurnal responses based on the humidity level in the rhizosphere and the air surrounding the leaves. Abscisic acid (ABA), the main phytohormone modulating the stomatal response to water availability, is regulated by circadian clocks. The molecular mechanism of the plant's circadian clock for regulating stress responses is composed not only of transcriptional but also posttranscriptional regulatory networks. Despite the importance of regulatory impact of circadian clock systems on ABA production and signaling, which is reflected in stomatal responses and as a consequence influences the drought tolerance response of the plants, the interrelationship between circadian clock, ABA homeostasis, and signaling and water-deficit responses has to date not been clearly described. In this review, we hypothesized that the circadian clock through ABA directs plants to modulate their responses and feedback mechanisms to ensure survival and to enhance their fitness under drought conditions. Different regulatory pathways and challenges in circadian-based rhythms and the possible adaptive advantage through them are also discussed.