Synchrony of plant cellular circadian clocks with heterogeneous properties under light/dark cycles

Masaaki Okada,Tomoaki Muranaka,Shogo Ito,Tokitaka Oyama

doi:10.1038/s41598-017-00454-8

Abstract

Individual cells in a plant can work independently as circadian clocks, and their properties are the basis of various circadian phenomena. The behaviour of individual cellular clocks in Lemna gibba was orderly under 24-h light/dark cycles despite their heterogeneous free-running periods (FRPs). Here, we reveal the entrainment habits of heterogeneous cellular clocks using non-24-h light/dark cycles (T-cycles). The cellular rhythms of AtCCA1::LUC under T = 16 h cycles showed heterogeneous entrainment that was associated with their heterogeneous FRPs. Under T = 12 h cycles, most cells showed rhythms having ~24-h periods. This suggested that the lower limit of entrainment to the light/dark cycles of heterogeneous cellular circadian clocks is set to a period longer than 12 h, which enables them to be synchronous under ~24-h daily cycles without being perturbed by short light/dark cycles. The entrainment habits of individual cellular clocks are likely to be the basis of the circadian behaviour of plant under the natural day–night cycle with noisy environmental fluctuations. We further suggest that modifications of EARLY FLOWERING3 (ELF3) in individual cells deviate the entrainability to shorter T-cycles possibly by altering both the FRPs and light responsiveness.

Highlights

Background fluctuationsDaily cycles Ratio of cellsPeriod of T-cycles (h)In general, circadian clocks can skip a light/dark or temperature cycle(s) if the environmental cycle period is short, for example, about half the length of the free-running periods (FRPs); this phenomenon has been described as a hallmark of clock entrainment in many organisms[9, 17, 19,20,21]
Individual cellular clocks were entrained in an FRP-dependent manner to light/dark cycles with a range of period lengths, and LgELF3 was involved in the entrainment process by maintaining the proper FRPs of cellular circadian rhythms and by gating the light signal
Individual cells in the same L. gibba tissue showed the circadian rhythms of AtCCA1::LUC + (AtCCA1)::LUC with a relatively wide range of FRPs (Fig. 1), the heterogeneity of FRPs was almost eliminated under light/dark cycles with periods of 24 h or 20 h2

Summary

Introduction

Background fluctuationsDaily cycles Ratio of cellsPeriod of T-cycles (h)In general, circadian clocks can skip a light/dark or temperature cycle(s) if the environmental cycle period is short, for example, about half the length of the FRP; this phenomenon has been described as a hallmark of clock entrainment in many organisms[9, 17, 19,20,21]. Many cells in the control samples having T = 12 h cycles showed robust rhythms with periods of 24 h or longer, suggesting that the L. gibba was unable to recognize this T-cycle as a day. Those cellular rhythms with a period of 24 h showed 1:2 synchronization (frequency demultiplication). This suggests that their phases responded to light/dark transitions to be entrained to the T-cycle, thought the phase transition is latent. Since daily environmental cycles include unpredictable background fluctuations, such as transient gloom from clouds passing the sun, it is important for the circadian clock to be entrained properly to the daily cycles without being perturbed by such short period fluctuations

Methods

Results

Conclusion