Floral Development and Organ Identity Notes

Flower Development and Organ Identity

Introduction to Flowering

  • Flowering is coordinated with day lengths to optimize seed production.
  • The lecture covers:
    • Conversion of the shoot apical meristem into a flower meristem.
    • Key signals: light spectrum, day length, and temperature.
    • Flower organ identity genes and their role in flower phenotypes.

Vegetative vs. Reproductive Development

  • Plants initially grow vegetatively with a shoot apical meristem, forming shoots and leaves for photosynthesis.
  • Flowering is induced later, often by environmental conditions like short/long days or cold treatment.
  • Arabidopsis: Vegetative rosette transitions to a stem with meristems after flowering induction.

Meristem Types

  • Primary inflorescence meristem: The initial shoot apical meristem.
  • Secondary inflorescence meristems: Branches of the main stem that convert into floral meristems.
  • Flowers are modified leaves, with leaf structures changing into flower structures under specific conditions.

Vernalization

  • Many plants in temperate climates require specific conditions to flower.
  • Vernalization: A cold treatment required to convert a shoot apical meristem into a flower meristem.
  • Example: Wheat requires a cold treatment during its life cycle to initiate flowering.
  • Inflorescence initiation in wheat/barley involves producing a reproductive apex, followed by flowers and seeds during the summer.

Day Length and Photoreceptors

  • Short-day plants (long-night plants) flower when they have a minimum night period.
  • Interrupting the night with a flash of light inhibits flowering in short-day plants.
  • Phytochrome photoreceptors perceive red light, interacting with phytochrome interacting factors (PIFs).
  • PIFs are transcription factors that activate developmental responses.
  • Long-day plants flower when the night is under a critical threshold.
  • Red and far-red light can induce or inhibit flowering, demonstrating phytochrome regulation.
  • Phytochrome converts to the active PFR form under red light and reverts to the inactive PR form under far-red light or long periods of darkness.

Cold Treatment and Vernalization

  • Flowering can also be induced through cold treatment (vernalization).
  • Arabidopsis, a winter annual, requires vernalization to transition from vegetative growth to flowering.

Significance of Flowering Signals

  • Understanding flowering signals is crucial for farmers to regulate crop flowering times.
  • Crops grown outside their native range may not have the right day length, necessitating induction or prevention of flowering to synchronize yield.

Florigen: The Flowering Inducer

  • Experiments were conducted to identify the signal that induces flowering, initially termed "Florigen."
  • Grafting experiments demonstrated that florigen is a long-distance, transmissible signal originating in the leaf
  • A leaf grown under short-day conditions was grafted onto a stem grown under long-day conditions, inducing the stem to flower.
  • The signal is often transmissible between different plant species via grafting.

Genetic Studies and Mutants

  • Mutants in Arabidopsis and other plants were studied to identify genes involved in flowering.
  • These studies aimed to find mutants that either couldn't flower or flowered at incorrect day lengths.
  • Key Genes and Transcription Factors:
    • Shoot apical meristem converts to a flower meristem via light quality, day length, and temperature.
    • Homeotic genes (transcription factors) that change organ identity in the flower must be activated.
    • Leafy and Apetela: Transcription factors that switch on floral homeotic genes.
    • SOC1 (Suppressor of Overexpression of Constans): A transcription factor that induces leafy.
    • Flowering Locus T (FT): Protein transported to the shoot apical meristem from the phloem; it is florigen.
    • Flowering Locus D (FD): Another transcription factor related to FT.
    • Flowering Locus C (FLC): A negative regulator inhibited by low temperature in plants requiring vernalization, thus promoting flowering via SOC1.

Florigen Induction

  • Florigen is a transcription factor induced in leaves and stems.
  • It is produced in the companion cells of the SIF elements in the phloem.
  • The florigen protein is loaded into the sieve elements and transported to the shoot apical meristem (a sink tissue).
  • In the shoot apical meristem, florigen interacts with other transcription factors to switch on flowering.
  • Phloem transport is sink-regulated and faster than active, polar transport.

Day Lengths and Flowering

  • Arabidopsis needs a long day to start flowering.
  • Under short days, it remains vegetative because phytochrome doesn't get a long enough light period to become active.
  • Active phytochrome is necessary to switch on fluorogen gene expression and another regulator called Constance.
  • Constance messenger RNA needs the florigen messenger RNA induction to make constance protein.
  • Long day conditions activate phytochrome and cryptochrome (blue light receptor) to switch on regulators, including constans and flowering locus T (fluorogen).

Simplified Sequence of Events (Arabidopsis)

  • Florigen and Apetala work together to induce flowering.
  • Constants, a transcription factor, switches on fluorogen.
  • Long day lengths activate phytochrome A and cryptochrome.
  • Constants activate fluorogen and Apetala.
  • The entire cascade is inhibited by FLC, which is removed through vernalization.

Experimental Evidence: Vernalization and FLC

  • Arabidopsis plants were either cold-induced (vernalized) or grown under non-vernalized conditions.
  • FLC (a negative regulator) is expressed in non-vernalized plants but disappears when the plant is cold-treated.
  • Mutating FLC to prevent messenger RNA production also induces flowering.
  • Northern blot experiments showed that FLC messenger RNA is removed by cold treatment.

Flower Organ Identity

  • Arabidopsis flowers have four different organs: sepals, petals, carpels, and stamens.
  • These organs are arranged in whorls.
  • The lecture goes into detail on the various parts of the flower from the pedicle to the stigma.
  • Male and female flowers can occur in different arrangements across species.

Floral Organ Identity

  • During flower induction, leaf structures of the shoot apical meristem are reprogrammed.
  • Floral organ identities are switched on according to whorls:
    • Outermost: Sepals.
    • 2nd: Petals.
    • 3rd: Stamens.
    • Innermost: Carpels.
  • Developmental fields are determined by the expression of homeotic genes, described by the ABC model of flowering.

The ABC Model of Flowering

  • Different organ identity genes are expressed in different whorls.
    • Outermost whorl (sepals): A gene.
    • Between sepals and petals: A and B genes.
    • Between petals and stamens: B and C genes.
    • Innermost whorl (carpels): C gene.
  • Mutant studies in Arabidopsis, missing different flower organs, revealed these genes.
  • Examples of mutants:
    • Apatala two: No petals and no sepals.
    • Pistillata: No petals or stamens.
    • Agamis: No stamens and no carpels.
    • Mutating all flowering genes results in whorls of modified leaves.

Gene Expression and Organ Formation

  • Overlapping gradients of gene expression.
  • A gene: Sepals and petals.
  • C gene: Stamens and carpels.
  • B gene overexpression.
  • Sepals: A gene only.
  • Petals: A and B genes.
  • Stamens: B and C genes.
  • Carpels: C gene only.
  • Losing class C identity genes and expressing only A gene result in just sepals and petals.

Flower Identity Genes

  • People cloned genes and identified proteins.
  • A Genes: Apetala one and two
  • C gene: Agamis
  • B genes: Apetela three and epistillata.
  • These are transcription factors that bind to each other.
  • SAPILATA 14: Expressed everywhere.
  • They interact with APTLA1 to determine flower identity genes.
  • Metzbox proteins are common homeotic genes in animal development too.

Homeotic Genes and Animal Development

  • Homeotic genes in Drosophila switch the location of organs by mutating transcription factors.
  • Same principle for organ identity in flowers.
  • Transcription factors for organ identities are fairly conserved and act in combination.
  • In animal development (segmentation), overlapping combinations of transcription factors determine body plan segments.
  • This is the same principle as organ identity in flowers.

Summary of Flower Induction

  • The shoot apical meristem converts to a floral meristem under specific conditions, determined by day length and spectrum, as well as temperature.
  • Flowering is induced by a protein, initially called florigen, produced in leaves and transported to the shoot apical meristem under specific conditions.
  • Florigen induction depends on day lengths through phytochrome and constants.
  • Vernalization is required in some species, inhibiting flowering locus C (an inhibitor) that inhibits florigen expression.

Regulations Summary

  • Temperature and light regulation for vernalization.
  • Complex gene expressions to control transcription factors, as well as plant response.
  • The ABC model of flowering regulates flower organ identity, via combinations of key flower identity genes and homeotic genes.

ABC models and Genes

  • There are overlapping and more complicated functions between the A, B, and C genes.
  • The A gene and C gene depend on the plant, leading to male-only or female-only phenotypes.
    • This is because stamens also require the B gene in expression.

Dose Response for the ABC Model

  • An experiment looking at a dose response could include inducible promoters in a mutant.
  • An inducible promoter requires certain promoters that are inducible by external signals.
  • You can change the dose of the external signal to tune expression of the genes, while expressed in the right region.

Inducible promoter

  • Have to introduce a GM plant to get a promoter.
  • The GM plant in Australia would be legally a social issue.
  • Introducing a GM tomorrow would be easier than introducing a GM today.

Gene Editing and GM Crops

  • You can edit the genome of a plant, without introducing foreign DNA.
  • You can also make mutations in existing genes.
  • That will lead to more acceptable legal easier growing gene edited crops.

GM regulation issues

  • GM regulation in crops and in particular consumer acceptance is a major concern.

Fertility Genes

  • Fertility genes are controlled and affect the diversity of gene, causing the plants to be more separate from each other.
  • Hybrid breeding and infertile male plants are used to produce a fertile female.
    • This would lead to the transfer of pollen from a certain line that has certain characteristics.
    • High-yielding varieties of craft plants, and partially sterile plants are grown for the production of hybrids.