It codes for the transcription factor TOC1, which affects the period of plants' circadian rhythms: built-in, malleable oscillations that repeat every 24 hours.
The gene codes for a transcriptional repressor, TOC1, one of five pseudo-response regulators (PRR) that mediate the period of the circadian clock in plants.
Millar developed an innovative forward genetic screen in which he linked a bioluminescent reporter, firefly (luciferase), to expression of CAB (chlorophyll-a,b binding protein—see Light-harvesting complexes of green plants) in Arabidopsis.
By measuring bioluminescence over the course of the day, Millar found CAB expression to display oscillatory patterns in constant light and to oscillate with a shorter period in toc1 mutant plants.
[7] Homologs of TOC1 have been found in lyrate rockcress, Brassica, papaya, cucumber, strawberry, soybean, lotus, apple, peach, western poplar (populus), castor bean, tomato, potato, grape vine, and chickpea.
Arabidopsis plants to which varying amounts of ABA were applied showed corresponding differences in TOC1 expression and in circadian period length.
Through computational modeling of this feedback loop, TOC1 was shown to be a clock-based influence on patterns of stoma opening and closure, which has traditionally been described as a mainly ABA-regulated process.
[1] To most efficiently use environmental resources such as light, plants generally synchronize their circadian rhythms to match the period of the environment.