Component Name

Core Regulatory Networks of Flowering Time

A comprehensive overview of how plants integrate environmental and internal signals—such as nutrients, temperature, and day length—to control the critical transition to flowering.

Introduction: The Art of Timing

Flowering is a pivotal event in a plant's life, marking the switch from vegetative growth to reproduction. The timing of this transition is critical for ensuring reproductive success and maximizing crop yield. Plants have evolved sophisticated molecular networks to interpret a wide array of environmental cues. While photoperiod (day length) and vernalization (prolonged cold) are the classical master regulators, a growing body of research reveals the profound influence of other factors, including ambient temperature and nutrient availability.

This page explores the core molecular pathways that govern flowering time. We delve into how plants sense and process signals from their environment—be it the warmth of the air, the length of the day, or the richness of the soil—and how these signals are integrated at the molecular level through intricate genetic networks. Understanding this complex interplay is key to developing climate-resilient crops and managing plant lifecycles more effectively.

Nitrogen (N) Signal Pathway

In contrast to Phosphorus deficiency, moderate nitrogen stress often accelerates flowering. This is considered an "escape" strategy, allowing the plant to complete its life cycle and produce seeds before nitrogen resources are fully depleted. The molecular network controlling this process is equally complex, involving fine-tuning of a core energy sensor and the photoperiod pathway.

Core Mechanism: The SnRK1-FBH4 Collaboration

The interactive diagram below shows how, under different N conditions, the plant uses the energy-sensing kinase SnRK1 to regulate the transcription factor FBH4, ultimately affecting the expression of the florigen FT. Hover over the protein names to view their functions.

N-Sufficient (+N) Nucleus SnRK1 High Activity FBH4 (Phosphorylated/Inactive) CO FT ~ Standard Flowering N-Deficient (-N) Nucleus SnRK1 Low Activity FBH4 (Dephosphorylated/Active) CO FT ✓ Flowering Accelerated

Mechanism Summary:

  • N-Sufficient (+N): High energy levels keep the SnRK1 kinase active, which phosphorylates and inactivates the transcription factor FBH4, leading to standard flowering time.
  • N-Deficient (-N): Low energy deactivates SnRK1, allowing FBH4 to become active. Active FBH4 then teams up with CO to strongly induce FT expression, accelerating flowering.

Phosphorus (P) Signal Pathway

Phosphorus deficiency is a major limiting factor for global agricultural production. When plants are P-deficient, not only is their growth stunted, but their flowering time is also significantly delayed. Recent research has uncovered an elegant molecular mechanism that explains how plants sense P signals and translate them into flowering time regulation. The core of this mechanism lies in the "shuttling" and "retention" of key proteins in different cellular compartments.

Core Mechanism: GRP7's Nucleo-Cytoplasmic Shuttling

The interactive diagram below illustrates how, under different P conditions, the cell controls the expression of the flowering repressor FLC by regulating the location of the RNA-binding protein GRP7. Hover over the protein names in the diagram to view their functions.

P-Sufficient (+P) Nucleus Endoplasmic Reticulum (ER) Cytosol bGLU25 AtJAC1 GRP7 GRP7 FLC ✓ Flowering Promoted P-Deficient (-P) Nucleus Endoplasmic Reticulum (ER) Cytosol SCPL50 bGLU25 bGLU25 AtJAC1 GRP7 FLC ✗ Flowering Delayed

Mechanism Summary:

  • P-Sufficient (+P): bGLU25 is anchored in the ER. This allows GRP7 to freely enter the nucleus and repress FLC, thus promoting flowering.
  • P-Deficient (-P): The protease SCPL50 frees bGLU25, which then forms a complex with `AtJAC1` in the cytosol. This complex sequesters GRP7, preventing its nuclear import. Consequently, FLC remains active, severely delaying flowering.

Temperature Signal Pathway

Plants are highly sensitive to ambient temperature, which acts as a critical signal to align flowering with favorable seasons. Both high and low temperatures (excluding vernalizing cold) influence flowering time through distinct molecular pathways, primarily converging on the master florigen gene FT.

Core Mechanism: PIF4 and CBFs as Key Temperature Responders

The interactive diagram below illustrates a detailed view of how plants respond to temperature. Multiple upstream signals regulate the central activator PIF4 in warm conditions, while the ICE1-CBF module governs the response to cool temperatures. Hover over the protein names in the diagram to view their functions.

Warm Temperature (~27°C) Nucleus phyB (Inactive) ELF3 (Phase Sep.) TCPs BZR1 PIF4 (Active) FT (High) ✓ Flowering Accelerated Low Temperature (~16°C) Nucleus ICE1 CBFs FLC (High) FT (Low) SOC1 (Low) ✗ Flowering Delayed

Mechanism Summary:

  • Warm Temperature: Light receptor phyB becomes inactive, and the Evening Complex component ELF3 undergoes phase separation. These events, along with activation by other factors like TCP and BZR1, lead to the accumulation and activation of the transcription factor PIF4. PIF4 then directly induces FT expression and accelerates flowering.
  • Low (Cool) Temperature: Low temperatures induce ICE1, which in turn activates the CBF transcription factors. CBFs then activate the major floral repressor FLC. High levels of FLC protein suppress the expression of floral integrators like FT and SOC1, thereby delaying flowering.

Photoperiod & Circadian Clock Pathway

Plants measure day length (photoperiod) to flower at the right time of year. This ability is governed by the interplay between an internal biological clock (the circadian clock) and external light signals. The circadian clock is an endogenous 24-hour oscillator that regulates thousands of genes, ensuring biological processes occur at the optimal time of day.

Core Mechanism: The Gated GI-CO-FT Module

The central mechanism involves the circadian clock "gating" the expression of a key activator, CONSTANS (CO). The interactive diagram below illustrates how the clock, composed of core genes, orchestrates various biological processes throughout the day, ensuring CO only accumulates under the right light conditions to activate the florigen FT. Hover over the gene names and process labels for more information.

Light Dark Chloroplast development Growth and development Photosynthesis Oxidation-reduction Biotic stress response Phosphorylation MsCCA1 MsLHY MsPRR9 MsPRR5 MsPRR3/7 MsTOC1 MsELF3 MsELF4 MsLUX MsGI ZT0 ZT6 ZT12 ZT18

Mechanism Summary:

  • The Clock: A series of interconnected transcriptional feedback loops (e.g., morning-expressed CCA1/LHY repressing evening-expressed genes) creates a robust daily rhythm.
  • The Gate: The clock restricts the expression of GI and CO to the afternoon.
  • The Coincidence: In long-day plants like Arabidopsis, if light is present when CO is expressed (i.e., on a long day), the CO protein is stabilized. In short-day plants, the logic is often reversed. Stabilized CO protein activates FT transcription, initiating the flowering process. This "external coincidence model" ensures plants only flower when days reach a critical length.
  • Seasonal Adaptation: In perennial plants like Alfalfa, the components of this pathway show seasonal expression dynamics, with the photoperiod pathway being most prominent in summer and autumn, while vernalization pathways dominate in spring.

Key References

Cho, H., Choi, I., Shahzad, Z., Brandizzi, F., & Rouached, H. (2025). Nutrient cues control flowering time in plants. Trends in Plant Science. (Manuscript provided as the basis for the N/P section).
Luo, X., Liu, X., Zheng, N., Song, C., & He, Y. (2025). Molecular Mechanisms of Temperature-Mediated Flowering Regulation: From Arabidopsis to Short-Day Crops. Plant, Cell & Environment, 1-18.
Zhang, Y., Zhao, H., Xiang, H., Zhang, J., & Wang, L. (2025). Seasonal and Diurnal Transcriptome Atlas in Natural Environment Reveals Flowering Time Regulatory Network in Alfalfa. Plant, Cell & Environment, 48, 4723-4739.