Entrainment of the Arabidopsis circadian clock to skeleton photoperiods

Abstract

Entrainment aligns the phase of the circadian clock and its outputs with 24 h fluctuations in the environmental conditions. One approach for investigating entrainment involves the application of skeleton photoperiods. In these protocols, light pulses are used to demarcate dawn and dusk, with the organism otherwise remaining under constant darkness. Historically, the consequences of skeleton photoperiod entrainment have been investigated by tracking behavioural responses in mammalian and insect models. In comparison, effects of skeleton photoperiod entrainment upon the plant circadian oscillator have received less attention. Using promoter-luciferase reporters for the Arabidopsis thaliana circadian clock, we monitored clock function during both entrainment to skeleton photoperiods, and subsequent free-running conditions. This enabled us to access molecular responses of the plant circadian oscillator to skeleton light conditions. We identified that individual circadian oscillator components in Arabidopsis can entrain to skeleton photoperiods. When these skeleton photoperiods were extended to represent long-day conditions, the oscillator underwent a phase jump relative to intact photoperiod entrainment such that it interpreted the dusk light pulse as dawn, and vice versa. This mirrors phase jumps that have been reported for rodent and insect behaviour following entrainment to long skeleton photoperiods, supporting the idea that phase jumps under skeleton photoperiod entrainment are a universal feature of circadian entrainment. Our results provide important context for the interpretation of physiological and developmental phenotypes of plants under modified light conditions.