Two Arabidopsis circadian oscillators can be distinguished by differential temperature sensitivity.

Circadian rhythms are widespread in nature and reflect the activity of an endogenous biological clock. In metazoans, the circadian system includes a central circadian clock in the brain as well as distinct clocks in peripheral tissues such as the retina or liver. Similarly, plants have distinct clocks in different cell layers and tissues. Here, we show that two different circadian clocks, distinguishable by their sensitivity to environmental temperature signals, regulate the transcription of genes that are expressed in the Arabidopsis thaliana cotyledon. One oscillator, which regulates CAB2 expression, responds preferentially to light-dark versus temperature cycles and fails to respond to the temperature step associated with release from stratification. The second oscillator, which regulates CAT3 expression, responds preferentially to temperature versus light-dark cycles and entrains to the release from stratification. Finally, the phase response curves of these two oscillators to cold pulses are distinct. The phase response curve of the oscillator component TOC1 to cold pulses is similar to that of CAB2, indicating that CAB2 is regulated by a TOC1-containing clock. The existence of two clocks, distinguishable on the basis of their sensitivity to temperature, provides an additional means by which plants may integrate both photoperiodic and temperature signals to respond to the changing seasons.