Critical role for CCA1 and LHY in maintaining circadian rhythmicity in Arabidopsis

Circadian clocks are autoregulatory, endogenous mechanisms that allow organisms, from bacteria to humans, to advantageously time a wide range of activities within 24-hr environmental cycles [1]. CIRCADIAN CLOCK ASSOCIATED I (CCA1) and LATE ELONGATED HYPOCOTYL (LHY) are thought to be important components of the circadian clock in the model plant Arabidopsis [2-5]. The similar circadian phenotypes of lines overexpressing either CCA1 or LHY have suggested that the functions of these two transcription factors are largely overlapping. cca1-1 plants, which lack CCA1 protein, show a short-period phenotype for the expression of several genes when assayed under constant light conditions [5]. This suggests that LHY function is able to only partially compensate for the lack of CCA1 protein, resulting in a clock with a faster pace in cca1-1 plants. We have obtained plants lacking CCA1 and with LHY function strongly reduced, cca1-1 lhy-R, and show that these plants are unable to maintain sustained oscillations in both constant light and constant darkness. However, these plants exhibit some circadian function in light/dark cycles, showing that the Arabidopsis circadian clock is not entirely dependent on CCA1 and LHY activities.