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Photoperiod Pathway in the Regulation of Flowering in Plants
Release time:
2025-02-25
Flowering is a crucial phase transition in the life cycle of higher plants, marking the shift from vegetative growth to reproductive growth. It plays a vital role in plant reproduction and environmental adaptation.
The timing of flowering in higher plants is regulated by both external environmental signals and internal cues. External signals mainly include temperature (high and low temperatures) and light (light intensity, light quality, and photoperiod), while internal signals involve hormones (primarily gibberellins), plant age, and carbohydrate accumulation.
Studies in the model plant Arabidopsis thaliana have identified six major regulatory pathways controlling flowering time: the photoperiod pathway, autonomous pathway, vernalization pathway, gibberellin (GA) pathway, temperature pathway, and age pathway (Fornara et al., 2010; Kinoshita & Richter, 2020).
In recent years, research on flowering time regulation has primarily focused on these six pathways. Today, We will introduce one of the key pathways influencing plant flowering—the photoperiod pathway.
Figure 1. Genetic Pathways Regulating Flowering Behavior (Using Arabidopsis thaliana as an Example)
The diagram is divided into three sections from top to bottom: upstream regulatory pathways, downstream flowering integrator genes, and floral meristem identity genes. The dashed box represents the six pathways involved in flowering regulation in Arabidopsis thaliana and their associated genes.
Introduction to Photoperiod
Plants are highly sensitive to environmental changes. Light is a major environmental cue that profoundly influences plant growth, development, and stress responses. It not only provides energy for photosynthesis but also serves as an essential signaling molecule in plants.
A key aspect of how plants perceive light is its duration, known as the photoperiod. The photoperiod refers to the length of time a plant is exposed to light within a 24-hour cycle. It plays a crucial role in regulating various aspects of plant biology, including flowering time, vegetative growth, and adaptation to adverse conditions.
Based on their response to photoperiod, plants can be classified into short-day (SD) plants, long-day (LD) plants, or day-neutral plants. This classification depends on whether plants require short days, long days, or are unaffected by day length to induce or accelerate flowering.
Light Signaling Pathway
Light receptors are a group of proteins in plants that perceive light signals and trigger cellular responses. Based on their absorption spectra, plant light receptors can be categorized into the following types:
01. light receptor - "light signal receiver"
Photoreceptors are a series of proteins in plants that sense light signals and trigger cellular responses. According to the different absorption spectra, plant light receptors can be mainly divided into the following categories:
1.Phytochromes (Phys): These primarily detect red light (600-700 nm) and far-red light (700-750 nm). Phytochromes exist in two interconvertible forms: Pr (red-light-absorbing) and Pfr (far-red-light-absorbing). They play crucial roles in various developmental processes, including seed germination, seedling de-etiolation, shade avoidance responses, circadian rhythms, flowering, and senescence.
2.Cryptochromes (CRYs): These perceive UV-A and blue light and are also known as blue light receptors.
3.Phototropins (PHOTs): Another class of blue light receptors, primarily involved in phototropism (growth towards light).
4.UVR8 (UV RESISTANCE LOCUS 8): This receptor senses UV-B light and plays an important role in plant growth, development, and stress responses.
Light receptors perceive environmental light signals and transmit them through intricate signaling networks, leading to various physiological and biochemical responses in plants, such as photosynthesis, growth, development, and stress adaptation.
Upon selective absorption of specific wavelengths, light receptors undergo photochemical activation, converting light energy into biological signals. These signals are subsequently amplified through complex signaling cascades, ultimately regulating the expression of target genes and triggering physiological and biochemical changes within the plant.
Advances in Light Receptor Research
With advancements in molecular biology and genetics, research on light receptors has deepened significantly. Scientists have uncovered the structure, function, and molecular mechanisms of light receptors, highlighting their critical roles in plant growth, development, and stress responses.
In 2024, Professor Ji-Gang Li’s research group at China Agricultural University published a study in the Journal of Integrative Plant Biology, titled “Regulation of cryptochromes-mediated blue light signaling by the ABI4-PIF4 module”. This study revealed that ABI4 interacts directly with the blue light receptor CRYs and the growth-promoting factor PIF4, promoting PIF4 protein accumulation under blue light. This interaction regulates photomorphogenesis, proposing a molecular model in which the ABI4-PIF4 module modulates CRY-mediated light signaling.
Figure 3. The ABI4-PIF4 Molecular Module Regulating CRY-Mediated Photomorphogenesis
02. Circadian Clock-The "Timekeeper" of Organisms
The circadian clock system in plants is composed of a complex signaling network, with core components exhibiting transcriptional activity. Some genes regulate the expression of other genes at specific times, forming cyclic regulatory loops that generate rhythmic oscillations (oscillators) to control the expression of downstream output genes.
As an internal timekeeping mechanism, the circadian clock aligns intracellular molecular, biochemical, physiological, and behavioral rhythms with external environmental changes, ensuring that organisms adapt effectively to their surroundings. These rhythms typically follow a near-24-hour cycle.
The core component of a mechanical clock is an oscillator, such as a pendulum, mechanical resonator, or quartz circuit, which generates stable periodic oscillations to regulate the movement of the clock hands. Similarly, in biological systems, the oscillator consists of a feedback loop network composed of transcriptional repressors and activators, operating through both positive and negative feedback mechanisms.
The diagram below illustrates the core oscillator structure of the circadian clock in Arabidopsis thaliana.
Figure 4. Simplified Model of the Core Oscillator in the Arabidopsis Circadian Clock
Light signals, received through photoreceptors, influence both the period and phase of circadian oscillations, ensuring that circadian clock-regulated processes align precisely with the solar cycle.
At the same time, the circadian clock modulates certain photoreceptor components (such as GIGANTEA (GI) and EARLY FLOWERING 3 (ELF3)), thereby influencing how light affects the circadian rhythm. This interaction ultimately establishes a specific day-night rhythm, which directly or indirectly regulates the expression of downstream genes, allowing plants to adapt to cyclic environmental changes (Yanovsky & Kay, 2001).
A 2020 study published in The Plant Journal, titled "Photoperiod sensing of the circadian clock is controlled by EARLY FLOWERING 3 and GIGANTEA", highlights that ELF3 and GI are key components of the circadian clock in the photoperiodic pathway and play a crucial role in regulating flowering time. The genetic relationship between ELF3 and GI, as well as their impact on growth and phase transition phenotypes under circadian clock control, has been a major focus of research.
The study reveals that ELF3 and GI exert growth inhibition at different times of the day:
ELF3 primarily suppresses growth at night,GI exerts its inhibitory effects during the daytime.
Together, these components enable the oscillator to synchronize endogenous cellular mechanisms with external environmental signals. When both genes are absent, the circadian oscillator fails to synchronize with the light-dark cycle, disrupting normal plant responses to photoperiodic changes.
Figure 5: Photoperiod-Responsive Flowering and Hypocotyl Elongation Require Functional ELF3 and GI
03. Flowering-Related Genes
The CO (CONSTANS)/ CO-like (CONSTANS-like, COL) gene family plays a crucial role in the photoperiod pathway as key downstream output genes. They regulate flowering time by transmitting light signals to FLOWERING LOCUS T (FT). COL and FT serve as major integrators in the Arabidopsis photoperiod-induced flowering pathway, playing an essential role in coordinating light signals with developmental timing.
The COL genes belong to the zinc finger transcription factor family, which is vital for plant growth and development. They were initially identified in Arabidopsis mutants and contain a C-terminal CCT domain and an N-terminal B-box zinc finger domain. As a phloem-specific transcriptional activator, CO induces flowering by upregulating the expression of FT and TSF genes. Research has demonstrated that COL genes play a pivotal role in controlling flowering through the photoperiod pathway by integrating signals from the circadian clock, light perception, and meristem-related genes, all of which are recognized as key regulators of flowering time.
Upstream Regulation of the Photoperiod Pathway and Its Effect on Flowering Genes
A key question in photoperiodic flowering regulation is how the upstream regulatory pathway influences downstream flowering-related genes. One example illustrating this regulation involves FKF1, an F-box protein that is regulated by the photoperiod and primarily functions in the afternoon.
FKF1: A Photoperiod-Regulated F-Box Protein
FKF1 acts as a blue light receptor and participates in light signal transduction in plants. It has a unique LOV (Light, Oxygen, or Voltage) domain, allowing it to absorb blue light and undergo conformational changes that enable protein-protein interactions.
A 2013 study published in Science, titled "FKF1 conveys crucial timing information for CONSTANS stabilization in the photoperiodic flowering pathway," revealed a new model in which FKF1 plays a crucial role in stabilizing CO and promoting flowering.
Previous studies established that CO is a zinc finger transcription factor that promotes flowering by activating FT (FLOWERING LOCUS T) expression. FT acts as a long-distance signaling molecule, migrating from the leaves to the shoot apical meristem through the vascular system, thereby inducing flowering. As a result, the stability of CO is critical for FT expression and flowering regulation. The transcription of CO is negatively regulated by CDF1 (CYCLING DOF FACTOR 1), a protein controlled by the circadian clock.
How FKF1 Regulates CO Stability
Through experimental evidence, the study demonstrated that FKF1 interacts with the plant-specific protein GI (GIGANTEA) to form a protein complex in the afternoon, leading to the degradation of CDF1. Since CDF1 acts as a transcriptional repressor of CO, its degradation removes the repression on CO transcription, thereby inducing FT expression and promoting flowering.
Previous research also indicated that FKF1 interacts with GI in a blue light-dependent manner. The 2013 study further confirmed this by using immunoprecipitation assays, demonstrating that FKF1-CO interactions are also dependent on blue light. Under blue light conditions, the immunoprecipitation of HA-FKF1 with CO-3F6H was significantly higher than under red light conditions.
Thus, FKF1 not only promotes CO transcription by degrading CDF1, but it also directly binds to CO, stabilizing the CO protein under blue light conditions, ensuring efficient FT activation and flowering induction.
Figure 6: CDF1 Protein Inhibits FT Transcription Independently of CO Transcription
The study proposes a new model for the roles of CDF1 and FKF1 in FT regulation. Under long-day conditions in the afternoon, FKF1 stabilizes the CO protein. FKF1 interacts with CO through its LOV domain, and this interaction is enhanced by blue light. Additionally, FKF1 degrades CDF1, which acts as a repressor of both CO and FT transcription.
Figure 7: Partial Photoperiod Pathway Model in Arabidopsis
This article provides a brief introduction to the photoperiod pathway in plant flowering. The photoperiod pathway in plant flowering is an important mechanism that tightly links the flowering signal with the timing of flowering. This pathway mainly relies on light receptors within the plant, which are capable of sensing light signals of different wavelengths, including red light, far-red light, blue light, and ultraviolet light. Upon sensing the light signals, these receptors initiate a series of signaling processes that affect the expression of genes related to flowering. The changes in the expression of these genes further regulate the plant's growth and development processes, ultimately influencing flowering.
Research has shown that the composition of genes involved in light-dependent flowering pathways is relatively conserved across different species. However, the regulatory roles of each gene may vary between species to adapt to different environmental conditions. Of course, the photoperiod regulation pathway is not isolated; various flowering-related pathways and signaling pathways in plants are intertwined. Ultimately, through the integration of genes, these pathways regulate the transition to the flowering stage, achieving precise control over flowering.
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