Molecular Control of Flowering

Floral Induction

  • Floral Induction: The process where the shoot apical meristem (SAM) transitions from vegetative growth to reproductive (flowering) growth. Most calorie intake is derived from fruits/seeds produced during the reproductive phase.

Photoperiodic Control of Flowering

  • Photoperiodism: The regulation of flowering by the length of day and night.
  • Short-day plants (long-night plants): Flowering is induced by long periods of darkness.
  • Long-day plants (short-night plants): Flowering is induced by short periods of darkness.
  • A brief flash of light during the dark period can prevent flowering in short-day plants, effectively mimicking a long-day condition.
  • Example: Chrysanthemums are short-day plants native to China. Covering them with a black cloth can artificially induce flowering.

Discovery of Florigen

  • Chailakhyan (1937): Demonstrated that a flowering stimulus is produced in the leaves of Chrysanthemum. Even if the apex is in long days, flowering occurs if the leaves are in short days, and vice versa.
  • The flowering stimulus must be mobile, suggesting a hormonal signal.
  • Chailakhyan termed this unknown flowering hormone 'florigen' (blossom-former).
  • Identification of florigen took approximately 70 years.

Hamner's Experiments (1942)

  • Cocklebur (Xanthium strumarium): A short-day plant used to study the flowering stimulus.
  • Experiments involving plants with two shoots showed that if one shoot is exposed to short days, both shoots flower.
  • Even with only 1/8 of a leaf exposed to short days, both shoots can flower.
  • If the shoot in short days has no leaves, neither shoot flowers, indicating the importance of leaves in perceiving the photoperiodic signal.

Zeevaart's Experiments (1965)

  • Perilla frutescens: A short-day plant used to study the flowering stimulus.
  • A single leaf from a plant in short days, when repeatedly grafted onto plants in long days, can induce flowering.
  • A single leaf from a plant in long days does not induce flowering.

Movement of Florigen

  • Florigen moves from the leaves to the apex.
  • Photoassimilates (organic products of CO2CO_2 fixation) move from leaf to apex via the phloem transport system.
  • Phloem transport rate: 50-100 cm/hour.

Takeba & Takimoto (1966)

  • Studied the rate of 'florigen' movement in 'Japanese morning glory' (Ipomoea nil).
  • Ipomoea nil has been a floricultural plant in Japan since the 8th century, with the Japanese publishing its genome in 2016.
  • Floral stimulus from 'donor leaves' is produced in the last 2 hours of a 16-hour dark period.
  • Florigen moved 102 cm in 2 hours, which approximates to ~50 cm/hour, similar to phloem transport rates.
  • Cutting off donor leaves at the end of the 16-hour dark period still allowed the 'receptor bud' to flower.

Nature of Florigen

  • Is 'florigen' the same substance in short-day and long-day plants?
  • Zeevaart (1982) conducted grafting experiments of related species to investigate this.

Grafting Experiments

  • Crassulaceae:
    • Kalanchoë blossfeldiana: Short-day plant (SDP)
    • Sedum spectabile: Long-day plant (LDP)
    • Echeveria harmsii: SLDP (requires short then long days to flower)
    • Bryophyllum crenatum & Kalanchoë daigremontianum: LSDP (requires long then short days to flower)
  • 'Florigen' from the stock can induce flowering in the scion.
  • The same signal may be used in both long-day and short-day plants.
  • Florigen transmission occurs between flowering types.
    • LDP stock induces flowering in SDP scion.
    • SLDP stock induces flowering in LDP scion.
    • LSDP stock induces flowering in LDP scion.
    • SLDP stock induces flowering in LSDP scion.

Flowering Mutants of Arabidopsis thaliana

  • Arabidopsis thaliana is a 'facultative' long-day flowering plant (long days promote flowering, but are not essential).
  • Koornneef (1991) identified late flowering in single-gene mutants.
  • Molecular genetics (1995-onward) led to the isolation of key genes for floral signalling.
  • Mutants in the FT gene flower later in long days.

Florigen Identified as a Protein

  • In 2007, four papers were simultaneously published establishing that florigen is a protein, now called FT (FLOWERING LOCUS T).
  • FT encodes a small, mobile protein that integrates environmental and endogenous cues to regulate flowering time.
  • Day-length is sensed in the leaves, leading to the production of 'FT protein'.
  • FT moves to buds and induces floral regulator genes.

CONSTANS (CO)

  • CO is a nuclear zinc finger transcription factor.
  • Expressed in leaves.
  • In long days, CO activates the transcription of FT.

The Circadian Clock

  • All organisms perceive day and night cycles.
  • A mechanism based on genetic feedback loops has evolved to adapt and prepare for these cycles.
  • The circadian clock in plants affects many processes on a genetic level, involving an input, oscillator, and output.
  • Oscillations of Arabidopsis thaliana clock genes occur across the day.
  • Oscillations in output genes are accomplished through direct binding of rhythmically expressed clock proteins to promoters or transcription factors (TF).
  • Links into other pathways:
    • Metabolizing sugars produced through photosynthesis peaks early in the day.
    • Starch degradation peaks in the middle of the night.

Circadian Clock Genes

  • CCA1 and LHY1 are activated by RVE8 and REV8/LNKS, and they repress TOC1.
  • PRR9, PRR7, and PRR5 are involved in the circadian clock.
  • ELF3, ELF4, LUX, ZTL, and GI are also components of the clock.

CO and FT Interaction

  • CO protein is degraded in the dark.
  • CO protein is stabilized by a phytochrome signal in the light.
  • Stabilized CO induces the FT gene.
  • FT protein acts as the floral stimulus.
  • 'Constans (CO)' plays a role in photoperiodic control.
  • Day-length is sensed in leaves, leading to 'FT protein' production.
  • Short-day plants like rice have extra controls.

Vernalization

  • Vernalization is the flowering response to a transition from cold temperatures (winter) to warm temperatures (spring).
  • Vernalization effects on flowering are quantitative; longer cold exposures promote flowering more than shorter ones.
  • After vernalization, plants acquire the competence to flower but do not necessarily initiate flowering immediately.
  • Meristems “remember” their vernalized state.
  • This memory is stably maintained through mitosis via epigenetic mechanisms.

Climate Change Impact

  • Warming Temperatures: Warmer winters can shorten the period of cold needed for vernalization, potentially delaying or preventing flowering.
  • Altered Phenology: Changes in vernalization can lead to shifts in plant phenology, including flowering time and leaf growth, which can disrupt ecosystems and plant-pollinator interactions.
  • Crop Yields: Changes in vernalization can significantly impact the yields of crops that rely on this process for flowering and seed production, such as winter wheat and oilseed rape.

Virus-Induced Flowering

  • A 'disarmed' virus is engineered to express FT.
  • Infecting a crop with this virus introduces the virus and FT into the phloem, leading to flowering.

Floral Regulator Genes

  • FT moves to buds and induces floral regulator genes.
  • 'LEAFY' (LFY) is a master regulator of floral genes.
  • Mutant 'lfy' flowers have leaf-like organs instead of petals, stamens, or carpels.

Transgenic Experiments with Floral Regulator Genes

  • 'p35S:LFY' flowers at the 'rosette' stage without an inflorescence stem.
  • 'p35S:LFY' involves a wild-type LFY transgene expressed from a constitutive promoter (always on).

LFY Homologues

  • LFY has homologues in other species like Snapdragon, Arabidopsis, and Petunia.

The ABC Model

  • Once the meristem transitions from vegetative to floral, different genes are expressed around the meristem to induce different developmental patterns, generating the different floral organs.
  • These developmental patterns depend on different transcription factors.

Homeotic Genes

  • Depending on the homeotic genes expressed in each of the whorls, a different organ will develop from that region of the meristem.

Mutants and the ABC Model

  • Wild-type (WT) flower displays normal floral structures.
  • Single ap2 mutant flower: carpelloid sepals, stamens, stamens, and carpels.
  • pi mutant flower: sepals, sepals, carpels, and carpels.
  • ag flower: stamens transformed into petals, carpels replaced by another flower repeating the same pattern.
  • ap2 pi double mutant: flowers composed only of sepalloid carpels.
  • ap2 ag flowers: leaf-like organs in the first and fourth whorls, mosaic petal/stamen organs in the second and third whorls.
  • ap3 ag double mutants: flowers composed of repeated whorls of sepals.
  • ap2 pi ag mutant: leaf-like organs with some residual carpel properties.
  • AP refers to APETALA, PI to PISTILLATA and AG to AGAMOUS.

Class E Genes

  • Floral organ identity is controlled by multiple feedback loops.
  • Class E genes, identified as SEPALLATA genes (SEP1-4), function with the other homeotic genes in specifying floral organs.

Summary

  • Photoperiod is sensed in leaves, not buds.
  • Evidence supports the idea of a mobile 'florigen'.
  • 'FT protein' identified from Arabidopsis mutants.
  • FT protein from leaves induces floral regulator genes in buds.
  • Multiple transcription factors control flower formation.