The ELF3 zeitnehmer regulates light signalling to the circadian clock

Abstract

The circadian system regulates 24-hour biological rhythms and seasonal rhythms, such as flowering. Long-day flowering plants like Arabidopsis thaliana, measure day length with a rhythm that is not reset at lights-off, whereas short-day plants measure night length on the basis of circadian rhythm of light sensitivity that is set from dusk, early flowering 3 (elf3) mutants of Arabidopsis are aphotoperiodic and exhibit light-conditional arrhythmias. Here we show that the elf3-7 mutant retains oscillator function in the light but blunts circadian gating of CAB gene activation, indicating that deregulated phototransduction may mask rhythmicity. Furthermore, elf3 mutations confer the resetting pattern of short-day photoperiodism, indicating that gating of phototransduction may control resetting. Temperature entrainment can bypass the requirement for normal ELF3 function for the oscillator and partially restore rhythmic CAB expression. Therefore, ELF3 specifically affects light input to the oscillator, similar to its function in gating CAB activation, allowing oscillator progression past a light-sensitive phase in the subjective evening. ELF3 provides experimental demonstration of the zeitnehmer ('time-taker') concept.

EARLY FLOWERING 4Functions in Phytochrome B-Regulated Seedling De-Etiolation

“…EARLY FLOWERING 3 (ELF3) function is required for the light sensitivity of the central oscillator in the circadian cycle (McWatters et al, 2000). This gene encodes a nuclear-localized putative transcriptional regulator that interacts with phyB (Hicks et al, 2001;Liu et al, 2001), and loss-of-function elf3 mutant plants have a long hypocotyl phenotype (Zagotta et al, 1996).…”

Differential effects of light-to-dark transitions on phase setting in circadian expression among clock-controlled genes in Pharbitis nil

“…The core clock that mediates photoperiodic flowering in Pharbitis probably shares most of the Arabidopsis clock genes because, as was observed in several Arabidopsis mutants such as early flowering 3 ( elf3 ), 19 the lack of only a single gene function can cause deficiency in the mechanism that temporarily restricts the activity of light input to the clock. Therefore, in Pharbitis an orthologous clock system could remain linked to the same photoperiodic-flowering genes as in Arabidopsis but lack the restriction mechanism against light input, and thereby provide dusk-set rhythms.…”

“…Circadian phases in expression rhythms of the photosynthetic-gene CAB is strongly set by light exposure at dawn without being strongly affected by the timing of lights off. 18,19 Therefore, Pharbitis is likely to contain a clock system that controls photoperiodic flowering whose manner of light resetting is inconsistent with that of Arabidopsis. 17 …”

“…30 However, the Arabidopsis clock is mainly set by the dark-to-light transition at dawn, 18,19 so light qualities that mediate light-off resetting are still unknown in plants. To understand this, we checked the effects of transitions from each of these wavelengths of light to darkness on phase setting in circadian expression of Pn FTs .…”

“…17 Based on genetic and molecular studies on light resetting of the circadian clock in Arabidopsis, dusk resetting of the clock in Pharbitis may involve loss of a mechanism that limits the activity of light input to the clock component to dawn, causing its activity to be always on and suspending the activity of the clock at the particular phase during the light period till the dark period begins. 18,19 Red, far-red and blue light are likely to be responsible for dusk resetting of the Pharbitis clock, since transitions from any of these wavelengths of light to darkness could set circadian expression of Pn FT s (Figure 2A-L). These light qualities are responsible for entrainment of the Arabidopsis circadian clock as well, but in this plant activities of light input pathways that mediate between these wavelengths of light and the clock components are limited to dawn allowing it to be specifically reset at this time.…”