Plant Circadian Rhythms

Abstract

Circadian clocks are found in most eukaryotic organisms. By allowing anticipation of daily and seasonal changes, they enable coordination of metabolism and lifecycle with the natural rhythms of the environment. Plant circadian rhythms are generated by a series of interlocking feedback loops of RNA (ribonucleic acid) and protein expression that respond to environmental cycles of light and temperature. They control essential processes in the plant's development, such as the transition to flowering or growth cessation, and thus influence yield, plant growth and biomass production. Many components of the clock are conserved across a wide variety of plant species and thus research in Arabidopsis translates into an understanding of the clock in agricultural crops or long‐living deciduous tree species such as hybrid aspen.

Key Concepts

  • Circadian clocks are found in both eukaryotes and bacteria.
  • Circadian clocks have a free‐running periodicity of about 24 h but are normally entrained to environmental cycles of light and temperature.
  • Temperature compensation is a key feature of the circadian clock and thus the free‐running period length varies relatively little across the range of ambient temperature.
  • The clock underlies many aspects of plant metabolism and physiology because it can detect and respond to both short‐term (the day:night cycle) and long‐term (the pattern of daylength variation across a year) changes in light and temperature.
  • The circadian clock of plants is made up of a series of interconnected transcription‐translation feedback loops (TTFLs) governing cycles of mRNA and protein expression.
  • Every plant cell contains its own clock. Clocks in different cells may be entrained independently of one another, although there appears to be a hierarchy of clocks within a plant dominated by the apex.
  • Plants with malfunctioning clocks suffer reductions in growth.
  • Many of the key components of the plant clock first described in the model species Arabidopsis thaliana are conserved across a wide range of species including trees such as hybrid aspen.

Keywords: Arabidopsis thaliana; bud set; circadian clock; dormancy; entrainment; photoperiodism; Populus; temperature compensation

Figure 1. Leaf movements of Arabidopsis thaliana seedlings over the course of a day and a night. Plants were entrained to LD 12 : 12 then placed in constant light. The molecular clock is superimposed, showing peak expression times under the same conditions of genes linked to the circadian clock. The roles of these genes are described in the text. White circle, subjective day; grey circle, subjective night.
Figure 2. Anatomy of a circadian rhythm. (a) Diagram of a rhythm entrained by LD cycles then released into constant light. Note that all the parameters of the rhythm are retained in free‐running conditions of constant light (LL). White bars, lights on; black bars, lights off; grey bars, subjective night. (b) Effect of exogenous sucrose on the amplitude of CAB2::LUC bioluminescence rhythmicity. Seedlings were grown on media containing increasing concentrations of sucrose in LD 12:12 and released into darkness at dusk (12 h after dawn).
Figure 3. Overview of the circadian network of Arabidopsis thaliana, together with its inputs and physiological outputs. Broad arrows indicate environmental and metabolic input pathways and physiological output pathways. ‘Metabolic sugar’ refers to the products of photosynthesis and starch degradation. The major elements of the transcription‐translation feedback loops (TTFLs) of the plant circadian system are shown in the five central blocks. Positive regulation is depicted in blue and negative regulation in red; the position of the arrowhead (positive effect) or bar (negative effect) indicates the direction of effect. Coupling between the rhythms produced by peroxiredoxin oxidation and the TTFL network is indicated by a dotted line as the link between these two circadian systems has not yet been revealed. ABA, abscisic acid; GAs, gibberellins; FLC, FLOWERING LOCUS C; all other gene and protein names abbreviated in this figure are given in full in the text. The intensity of shading in boxes containing TTFL components indicates time of expression, the lighter the earlier. A light grey background indicates expression around dawn and a black background means expression around dusk.
Figure 4. Phase response curves (PRCs) for Arabidopsis thaliana. This plots how much a light pulse (given at different times) causes the clock to speed up or slow down. Phase advances are plotted as positive changes, phase delays as negative changes. Red line, red light PRC; blue line, blue light PRC.
Figure 5. Photoperiod is a reliable cue for seasonal change but temperature and weather are unpredictable. These pictures were taken on 25th October (a) and 6th November (b) in northern Sweden (63 oN) in 2006. At high latitudes deciduous trees must stop growth early in order to be prepared to withstand harsh winter conditions. Winter may strike quickly and lack of preparation may lead to death.
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Further Reading

Causton HC, Feeney KA, Ziegler CA and O'Neill JS (2015) Metabolic cycles in yeast share features conserved among circadian rhythms. Current Biology 25 (8): 1056–1062.

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McWatters, Harriet G, and Eriksson, Maria E(May 2016) Plant Circadian Rhythms. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0020113.pub2]