Imbibition, but not release from stratification, sets the circadian clock in Arabidopsis seedlings

Circadian rhythms in the abundance of the CAT2 catalase mRNA were not seen in etiolated seedlings but developed upon illumination, These circadian osc illations were preceded by a rapid and transient induction of CAT2 mRNA abu ndance that varied strikingly according to the timing (circadian phase) of the onset of illumination. This variation oscillated with a circadian perio dicity of similar to 28 hr, indicating that the circadian oscillator is run ning in etiolated seedlings and regulates (gates) the induction of CAT2 by light. Moreover, because we assayed populations of seedlings, we infer that the individual clocks among populations of etiolated seedlings were synchr onized before the onset of illumination, What developmental or environmenta l signals synchronized the clocks among seedlings? Varying the phase of the onset of illumination relative to release from stratification failed to af fect the acute induction of CAT2, indicating that the temperature step from 4 to 22 degrees C associated with release from stratification did not rese t the circadian clock. However, the acute induction of CAT2 mRNA varied wit h time after imbibition, demonstrating that imbibition provides a signal ca pable of resetting the circadian clock and of synchronizing the clocks amon g populations of seedlings.