Changing planetary rotation rescues the biological clock mutant lhy cca1 of Arabidopsis thaliana

Background: Pervasive, 24-hour rhythms from the biological clock affect diverse biological processes in metabolism and behaviour, including the human cell division cycle and sleep-wake cycle, nightly transpiration and energy balance in plants, and seasonal breeding in both plants and animals. The clock mechanism in the laboratory model plant species Arabidopsis thaliana is complex, in part due to the multiple interlocking, negative feedback loops that link the clock genes. Clock gene mutants are powerful tools to manipulate and understand the clock mechanism and its effects on physiology. The LATE ELONGATED HYPOCOTYL and CIRCADIAN CLOCK ASSOCIATED 1 genes encode dawn-expressed, Myb-related repressor proteins that delay the expression of other clock genes until late in the day. Double mutant plants (lhy cca1) have low-amplitude, short-period rhythms that have been used in multiple studies of the plant circadian clock. Results: We used in vivo imaging of several luciferase (LUC) reporter genes to test how the rhythmic gene expression of wild-type and lhy cca1 mutant plants responded to light:dark cycles. Red, blue and red+blue light were similarly able to entrain these gene expression rhythms. The timing of expression rhythms in double mutant plants showed little or no response to the duration of light under 24h light:dark cycles (dusk sensitivity), in contrast to the wild type. As the period of the mutant clock is about 18h, we tested light:dark cycles of different duration (T cycles), simulating altered rotation of planet Earth. lhy cca1 double mutants regained as much dusk sensitivity in 20h T cycles as the wild type in 24h cycles, though the phase of the rhythm in the mutants was much earlier than wild type. The severe, triple lhy cca1 gi mutants also regained dusk sensitivity in 20h cycles. The double mutant showed some dusk sensitivity under 28h cycles. lhy cca1 double mutants under 28h cycles with short photoperiods, however, had the same apparent phase as wild-type plants. Conclusion: Simulating altered planetary rotation with light:dark cycles can reveal normal circadian performance in clock mutants that have been described as arrhythmic under standard conditions. The features rescued here comprise a dynamic behaviour (apparent phase under 28h cycles) and a dynamic property (dusk sensitivity under 20h cycles). These conditional clock phenotypes indicate that parts of the clock mechanism continue to function independently of LHY and CCA1, despite the major role of these genes in wild-type plants under standard conditions. Accessibility: Most results here will be published only in this format, citable by the DOI. Data and analysis are publicly accessible on the BioDare resource (www.biodare.ed.ac.uk), as detailed in the links below. Transgenic lines are linked to Stock Centre IDs below (Table 7).