How Does a Plant Internal Clock Regulate Daily Rhythms?

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How does a plant internal clock regulate daily rhythms? It uses a network of interlinked genes that produces cycles of roughly 24 hours. Those cycles set the timing of growth, metabolism, flowering and stress responses across the day and night. The network runs on feedback loops, in which clock proteins switch each other on and off over time. This article explains the main parts of that loop, the order in which the genes peak, and the signals that reset the timer. Most of the detailed research comes from the model plant Arabidopsis thaliana, so the examples focus on that species.

Quick answer

• The plant clock is a timing network built from transcription factors that produces rhythms close to 24 hours.
• The core loop centers on TOC1 and the dawn-expressed proteins CCA1 and LHY, which repress each other through negative feedback.
• Outside signals such as light and temperature reset the timer, and output pathways carry that timing into growth, flowering and stress responses.

Why plants keep an internal timer

The Earth’s rotation creates a repeating pattern of day and night. Many organisms have evolved an internal timer that matches this pattern, and it is called a circadian clock. One review describes the value of the clock as anticipation: it allows organisms to anticipate predictable environmental fluctuations and adjust biological processes accordingly. A plant with an internal schedule can prepare for a change before it arrives, instead of responding only after conditions have shifted.

The clock is not a single gene. It is a set of interacting components whose expression rises and falls in a repeating order. Plants show rhythmic biological activity with a period of about 24 hours, and the clock is the mechanism that generates this rhythm inside the plant.

The three-part structure of the clock

Plant biologists usually divide the clock into three parts: input pathways, core oscillators, and output pathways. Input pathways take in signals from outside the plant. The clock review calls these signals zeitgebers, and lists light and dark cycles, temperature, humidity, salinity, nutrients, pathogens, and pest attacks among them. The core oscillator is the timekeeping machinery inside the cell. Output pathways are the processes that the clock controls, including metabolism, hormone signaling, growth, flowering, and responses to stress.

This three-part description gives a practical way to read the research. Each part can be studied on its own, and the reviews discuss them in that order: the signals that reach the clock, the genes that form the oscillator, and the outputs that follow from those genes.

The core loop: TOC1, CCA1 and LHY

In Arabidopsis, the central loop involves TOC1, a member of the PSEUDO-RESPONSE REGULATOR (PRR) gene family that is expressed at dusk. It works with two dawn-expressed MYB-family transcription factors, CCA1 and LHY. The review on core oscillator feedback loops states that these proteins reciprocally repress each other’s expression through negative feedback. When one set of genes is high, it pushes the other set down, and that push-and-pull produces the daily rhythm.

The loop has more than one level of control. The same review notes that CCA1 and LHY repress TOC1 expression, and that CCA1 frequently uses H3 deacetylation to do so. This means the loop involves changes to how DNA is packaged around the genes, not only which genes are switched on. The second source, a review on chromatin modifications, describes the morning and evening loops as proposed to form a negative feedback loop that generates the circadian rhythm.

Because the loop depends on mutual repression, changing one of its parts affects the timing of the whole. Plants with altered CCA1 and LHY activity are reported to show shortened or lengthened rhythms, which fits the idea that these proteins sit at the center of the cycle.

How the genes take turns across the day

The clock components do not all peak at the same time. The chromatin review lays out the sequence. After dawn, PRR9 is the first of the PRR genes to be expressed, followed by PRR7 and PRR5 in the afternoon, and finally by TOC1 in the evening. The same review notes that the expression of CCA1 and LHY is also repressed by TOC1, PRR7, and PRR9. Because of this ordering, CCA1 and LHY are expressed only during a narrow window of time near dawn.

This schedule gives the clock its daily shape. Morning genes are expressed first, evening genes come later, and the cycle resets for the next day. The feedback network is what keeps this order going from one cycle to the next.

What the clock controls

The core oscillator generates rhythms that regulate a wide set of outputs. The review on transcriptional activity describes clock-controlled processes that include metabolism, hormone signaling, growth, photoperiodic flowering, physiological processes, and abiotic and biotic stress responses. Some links are direct. CCA1 directly represses the expression of FLOWERING LOCUS T (FT), a flowering signal, while LHY does not directly regulate FT expression. Two morning proteins can therefore influence flowering through different routes.

CCA1 and LHY also influence starch accumulation and stress responses, in addition to their role in maintaining the rhythm. The PRR genes contribute to flowering too. They do this by enhancing the accumulation of the CO protein, which increases CO binding to the FT promoter and promotes flowering in response to day length.

TOC1 has outputs of its own. Seedlings that overexpress TOC1 are more sensitive to light and have extremely short hypocotyls, the stems that emerge from seeds. TOC1 also interacts with PIF proteins and represses their activity on shared target genes, which connects the clock to light-driven growth.

For a related look at the day-length side of this system, see how plants sense day length. The seasonal timing of growth is covered in how a plant bud bursts after winter chilling.

How light and temperature reset the timer

A clock that ran unchanged would drift away from the real day. Plants avoid this by resetting the timer with input signals. The clock review states that external signals communicate with the core oscillator through relevant receptors, which links the internal rhythm to the environment. Light is the most familiar of these signals, but the list of zeitgebers is longer than light alone.

Temperature is one of the clearly described resetting inputs. The chromatin review notes that heat shock factor B2b mediates the repression of PRR7 expression, and that this affects temperature compensation and clock resetting. After heat exposure, PRR7 and HsfB2b regulate the expression of output genes, which links a temperature change directly to growth and development.

How we chose these sources

The two sources are open-access review articles hosted on PubMed Central. The first covers core oscillator feedback loops and output pathways in the plant clock. The second covers how chromatin modifications and transcriptional regulation fit into the clock. Both were fetched directly from their PMC addresses, and both returned HTTP 200 on the check dated 2026-10-28. Each claim in the article is tied to a passage in one of these two reviews, and claims that could not be tied to a passage were removed.

What we could not verify

Most of the detailed gene work in these reviews comes from Arabidopsis. Several output examples are reported in rice, maize, Medicago, sugar beet, and other crops, and the reviews describe those species separately. The article therefore does not present the Arabidopsis gene map as a complete guide to every garden plant. The sources also focus on molecular networks rather than on the timing of any particular plant in an ordinary yard.

What the sources do not say

The sources do not give one schedule that applies to every garden plant. They do not provide field measurements of how far the clock shifts under local weather. They also do not describe steps a gardener can take to adjust a plant’s clock, and this article does not offer such steps.

Frequently asked questions

Is the plant clock the same as the biological clock in animals?

The sources say that plants, like animals, show rhythmic biological activity with a period of about 24 hours. The components named in the plant reviews, including PRR, TOC1, CCA1 and LHY, belong to the plant clock network, so the gene-level details described here apply to plants.

Does the plant clock keep running at night?

According to the reviews, yes. The core oscillator produces approximately 24-hour rhythms, and gene expression changes through the night. TOC1 is expressed in the evening, and CCA1 and LHY are expressed in a narrow window near dawn, so the clock keeps shifting its gene activity through the dark hours.

What is a zeitgeber in plants?

A zeitgeber is an external signal that helps set the internal clock. The review lists light and dark cycles, temperature, humidity, salinity, nutrients, pathogens, and pest attacks as zeitgebers. These signals reach the core oscillator through receptors and help align the clock with the surrounding environment.

Why does the plant clock affect flowering?

The clock controls flowering through several routes. CCA1 directly represses FLOWERING LOCUS T, while the PRR genes help increase accumulation of the CO protein, which promotes FT expression. Because FT is a flowering signal, the timing of these clock proteins can shape when a plant flowers in response to day length.

Can light alone explain how the plant clock works?

No. Light is one of several zeitgebers that the review lists, and temperature is another important input. The chromatin review shows that heat effects reach the clock through HsfB2b and PRR7, so a clock driven only by light would miss part of the picture.

Sources

• Transcriptional activation and repression in the plant circadian clock: Revisiting core oscillator feedback loops and output pathways. https://pmc.ncbi.nlm.nih.gov/articles/PMC12365851/ (HTTP 200, opened 2026-10-28)
• The Plant Circadian Clock and Chromatin Modifications. https://pmc.ncbi.nlm.nih.gov/articles/PMC6266252/ (HTTP 200, opened 2026-10-28)