Clock genes and the solar rhythm

Circadian rhythms in plants and animals are self-sustaining endogenous daily cycles – a sort of internal clock. The accuracy of these clocks is maintained by environmental references known as zeitgebers – “time givers”. Plants are equipped with sensitive light sensors, and light is commonly the signal that plants use to synchronize their internal clocks to the environment.

A recent short review in Annals of Botany examines how the accuracy of these cycles is maintained across varying day length and light intensity at different seasons. Solar rhythm is controlled by a clock synchronized to noon and midnight, much the same way we set our clocks and watches to chronometer time. The solar rhythm allows the plant clock to be set and also allows the timing of genes cycling on solar time to be predicted in plants subjected to different photoperiods, cycle periods or dark/light ratios.

Yeang, H.Y. (2015) Cycling of clock genes entrained to the solar rhythm enables plants to tell time: data from arabidopsis. Annals of botany, 116(1), 15-22
An endogenous rhythm synchronized to dawn cannot time photosynthesis-linked genes to peak consistently at noon since the interval between sunrise and noon changes seasonally. In this study, a solar clock model that circumvents this limitation is proposed using two daily timing references synchronized to noon and midnight. Other rhythmic genes that are not directly linked to photosynthesis, and which peak at other times, also find an adaptive advantage in entrainment to the solar rhythm.
Fourteen datasets extracted from three published papers were used in a meta-analysis to examine the cyclic behaviour of the Arabidopsis thaliana photosynthesis-related gene CAB2 and the clock oscillator genes TOC1 and LHY in T cycles and N–H cycles. Changes in the rhythms of CAB2, TOC1 and LHY in plants subjected to non-24-h light:dark cycles matched the hypothesized changes in their behaviour as predicted by the solar clock model, thus validating it. The analysis further showed that TOC1 expression peaked ∼5·5 h after mid-day, CAB2 peaked close to noon, while LHY peaked ∼7·5 h after midnight, regardless of the cycle period, the photoperiod or the light:dark period ratio. The solar clock model correctly predicted the zeitgeber timing of these genes under 11 different lighting regimes comprising combinations of seven light periods, nine dark periods, four cycle periods and four light:dark period ratios. In short cycles that terminated before LHY could be expressed, the solar clock correctly predicted zeitgeber timing of its expression in the following cycle.
Regulation of gene phases by the solar clock enables the plant to tell the time, by which means a large number of genes are regulated. This facilitates the initiation of gene expression even before the arrival of sunrise, sunset or noon, thus allowing the plant to ‘anticipate’ dawn, dusk or mid-day respectively, independently of the photoperiod.