Abscisic Acid Machinery Is under Circadian Clock Regulation at Multiple Levels

Abscisic acid (ABA) is recognized as the key hormonal regulator of plant stress physiology. This phytohormone is also involved in plant growth and development under normal conditions. Over the last 50 years the components of ABA machinery have been well characterized, from synthesis to molecular perception and signaling; knowledge about the fine regulation of these ABA machinery components is starting to increase. In this article, we review a particular regulation of the ABA machinery that comes from the plant circadian system and extends to multiple levels. The circadian clock is a self-sustained molecular oscillator that perceives external changes and prepares plants to respond to them in advance. The circadian system constitutes the most important predictive homeostasis mechanism in living beings. Moreover, the circadian clock has several output pathways that control molecular, cellular and physiological downstream processes, such as hormonal response and transcriptional activity. One of these outputs involves the ABA machinery. The circadian oscillator components regulate expression and post-translational modification of ABA machinery elements, from synthesis to perception and signaling response. The circadian clock establishes a gating in the ABA response during the day, which fine tunes stomatal closure and plant growth response.