Everything in Its Time
Plants anticipates dawn. Before the sun rises, they have already upregulated photosynthetic genes, opened stomata, and positioned leaves for maximum light capture. This is not a simple light response — it is a circadian rhythm, an endogenous oscillation with a period of approximately 24 hours that persists in constant conditions.
The adaptive significance of circadian regulation is profound: plants with a correctly entrained clock grow larger, photosynthesise more efficiently, and resist disease better than arrhythmic mutants. A plant that "knows" when dawn will arrive can pre-activate its photosynthetic machinery, gaining a 30–40% carbon assimilation advantage over a plant that responds only after light hits its leaves.
The Core Oscillator: A Three-Loop Model
The plant circadian oscillator is a transcription-translation feedback loop (TTFL) consisting of three interlocking loops:
Morning Loop
- CCA1 (CIRCADIAN CLOCK-ASSOCIATED 1) and LHY (LATE ELONGATED HYPOCOTYL) are MYB transcription factors that peak at dawn. They bind to the EE (evening element, AAAATATC) and CBS (CCA1-binding site, AAMAAGCT) motifs in target gene promoters.
- CCA1/LHY repress evening-expressed genes (particularly TOC1 and the evening complex) and activate day-expressed genes (photosynthesis, stomatal opening, cold response).
- As the day progresses, CCA1/LHY protein levels decline (phosphorylated by CK2 and degraded via the proteasome).
Evening Loop
- TOC1 (TIMING OF CAB EXPRESSION 1) is an evening-phased B-box transcription factor that peaks at dusk (CT 12–15, where CT = circadian time). TOC1 was originally identified by the toc1-1 mutant, which has a short period of 21 hours.
- TOC1 represses CCA1/LHY expression, closing the feedback loop: CCA1/LHY repress TOC1 at dawn → TOC1 protein accumulates through the day → TOC1 represses CCA1/LHY at night → CCA1/LHY rise at dawn.
- The Evening Complex (EC): LUX (LUX ARRHYTHMO), ELF3 (EARLY FLOWERING 3), and ELF4 form a protein complex that peaks in the early night. The EC represses TOC1 and other evening genes, adding an additional layer of negative regulation.
Additional Oscillators
- PRR (PSEUDO-RESPONSE REGULATOR) proteins: PRR9 (morning), PRR7 (midday), PRR5 (afternoon), PRR3 (evening) form a sequential cascade of transcriptional repressors that provide "gears" within the clock, ensuring robust oscillations across a range of temperatures.
- GI (GIGANTEA): A large nuclear protein with multiple roles — stabilises ZTL (an E3 ubiquitin ligase that degrades TOC1), regulates CO (CONSTANS) for flowering, and interacts with CCA1/LHY.
The combined system produces oscillations with a free-running period of ~24 hours at 22°C, with a Q₁₀ (temperature coefficient) close to 1.0 — meaning the period is temperature-compensated across a 10°C range, a defining feature of circadian clocks.
Entrainment: Setting the Clock
The circadian oscillator is entrained (synchronised) to the external day-night cycle by environmental signals called zeitgebers (German for "time-givers"):
- Light. The primary zeitgeber. Phytochrome B (red/far-red) entrains the clock via effects on CCA1/LHY and PRR9/PRR7. Cryptochromes (blue/UV-A) entrain via degradation of TOC1 through the ZTL photoreceptor. Light pulses at dawn advance the clock (phase advance); pulses at dusk delay it (phase delay).
- Temperature. A warm pulse at dawn or a cool pulse at dusk can entrain the clock independently of light. The PRR7 promoter is highly thermoresponsive — a 4°C warm pulse at dawn advances the clock by ~2 hours.
- Metabolic signals. Sugars from photosynthesis feed back to the clock. Trehalose-6-phosphate (a sugar signal) regulates CCA1 and PRR7 expression, linking carbon status to circadian timing.
Outputs: What the Clock Controls
The circadian clock gates (permits or prevents) a remarkable range of physiological processes:
Photosynthesis
- CAB (chlorophyll a/b binding) gene expression peaks at dawn, anticipating light. In arrhythmic mutants (cca1 lhy double), CAB expression is arrhythmic and photosynthetic efficiency drops by 30–40%.
- RuBisCO activase is clock-regulated, peaking in the morning when CO₂ assimilation is highest.
Stomatal Conductance
- Stomata open in anticipation of dawn, driven by a circadian rhythm in guard cell H⁺-ATPase activity that is independent of light. Under constant light, stomatal conductance continues to oscillate with a ~24-hour period for 3–5 cycles before damping.
Floral Induction
- CO (CONSTANS) is clock-regulated at both transcriptional and post-translational levels. CO protein is stabilised by light (via cryptochromes and phytochromes) and degraded in darkness (via COP1 ubiquitin ligase). In long-day plants, CO accumulates to high levels only on long days when the evening peak of CO mRNA coincides with daylight — a precise clock-photoreceptor coincidence.
- FT (FLOWERING LOCUS T) is the florigen signal, transcribed in leaf phloem in response to CO and transported to the shoot apex to initiate flowering.
Nyctinasty: Sleep Movements
- Many legumes (Mimosa, Samanea) and some houseplants (Marantaceae — Calathea, Maranta, Stromanthe) exhibit nyctinastic leaf movements: leaves fold upward at dusk and unfold at dawn. This rhythm is driven by pulvinar motor cells that actively transport K⁺ and Cl⁻ between flexor and extensor cells, changing their turgor.
- The rhythm persists in constant darkness for 3–5 days, confirming circadian control. In Samanea, the motor cell ion channels (KAT1-type K⁺ channels, SLAC1-type anion channels) are clock-regulated at the transcriptional level.
Immune Defence
- Pathogen-associated molecular pattern (PAMP) responses are clock-gated. Morning inoculations with Pseudomonas syringae produce smaller lesions than evening inoculations — the clock primes defence responses at dawn, when pathogens are most likely to invade through open stomata.
- Salicylic acid levels oscillate, peaking at dawn, and PR (pathogenesis-related) gene expression is clock-controlled.
Volatile Organic Compound (VOC) Emission
- Floral scent compounds (monoterpenes, benzenoids, fatty acid derivatives) are emitted on a circadian schedule, timed to coincide with pollinator activity. Night-blooming flowers (e.g., some orchids, jasmine) peak in VOC emission at dusk; day-blooming flowers peak at dawn.
Temperature Compensation: Clock Stability Across Seasons
The circadian period must remain ~24 hours across a range of growth temperatures (typically 12–30°C). Temperature compensation is achieved through several mechanisms:
- PRR7/PRR9 thermoresponsiveness: A 10°C temperature increase advances PRR7/PRR9 expression, which in turn advances CCA1. This feedforward loop counteracts the temperature-driven speeding up of all biochemical reactions.
- GI thermostability: GI protein is more stable at higher temperatures, providing a damping effect that prevents the clock from running too fast in warm conditions.
- CK2 phosphorylation: Casein kinase 2 phosphorylates CCA1, targeting it for degradation. CK2 activity increases with temperature, reducing CCA1 half-life and preventing period shortening.
The Clock and Houseplant Care
- Consistent light sched
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