BIBC 140 - Lecture 12b

Arabidopsis Flower Structure

  • The Arabidopsis flower consists of four main organs arranged in concentric whorls:
    • Sepals (outermost whorl)
    • Petals
    • Stamens
    • Carpels (innermost whorl)

ABC Model of Flower Development

  • Floral organ identity is determined by the combinatorial activity of ABC genes.
  • The ABC model explains how these genes control the development of sepals, petals, stamens, and carpels.
    • A function alone: Specifies sepal development.
    • A + B functions: Specifies petal development.
    • B + C functions: Specifies stamen development.
    • C function alone: Specifies carpel development.

Gene Activity in Floral Whorls

  • Idealized pattern of gene activity in a normal flower (whorls 1-4):
    • Whorl 1: A genes
    • Whorl 2: A and B genes
    • Whorl 3: B and C genes
    • Whorl 4: C genes

Antagonistic Activities of A and C Genes

  • The A and C gene activities are mutually exclusive and do not overlap in normal flower development.
  • C Mutant: In a C mutant, A gene activity expands throughout the flower.
    • One function of the C gene is to prevent A gene expression in cells that would normally form stamens and carpels.
  • A Mutant: In an A mutant, C gene activity expands throughout the flower.
    • One function of the A gene is to prevent C gene expression in cells that would normally form sepals and petals.

Phenotypes of A, B, and C Mutants

  • Understanding the effects of A, B, and C gene mutations on floral organ identity.
  • A Mutant: Sepals are converted into carpels; petals are converted into stamens.
    • The normal order (sepal-petal-stamen-carpel) becomes (carpel-carpel-stamen-stamen).
  • B Mutant: Petals are replaced by sepals, and stamens are replaced by carpels.
    • The normal order (sepal-petal-stamen-carpel) becomes (sepal-sepal-carpel-carpel).
  • C Mutant: Stamens are replaced by petals, and carpels are replaced by a new flower.
    • The normal order (sepal-petal-stamen-carpel) becomes (sepal-petal-petal-petal).

SUPERMAN (SUP) Gene

  • The SUPERMAN gene product prevents B gene activity from expanding into the center of the flower.
  • In SUPERMAN mutants (sup), B activity expands into the center, converting carpels into stamens.

Double and Triple Mutants

  • Examining the phenotypes of double and triple mutants to understand gene interactions.
  • BC Double Mutant: All floral organs are converted into sepals.
  • ABC Triple Mutant: All floral organs are converted into leaves.

Goethe's Theory

  • Goethe (1790) proposed that floral organs are derived from leaves.
  • The ABC triple mutant phenotype supports Goethe's hypothesis, as all flower organs revert to leaves.
  • The ABC genes are necessary to convert leaves into each of the flower organs.

Sufficiency of A and B Genes

  • If the A and B genes were mis-expressed in leaves, would they be sufficient to convert normal leaves into petals?

D-Function Genes

  • D activity is required for petal, stamen, and carpel development.
  • The D gene is expressed in cells that will form petals, stamens, and carpels.
  • The D gene is required for the activity of the B and C genes.
  • In D mutants all organs are sepals.

Revised Model: ABCD Model

  • The ABCD genes act alone, or in combination, to specify the four types of flower organs
    • A = Specifies sepals.
    • A + B + D = Specifies petals.
    • C + D = Specifies carpels.
    • B + C + D = Specifies stamens.
  • The ABCD genes are sufficient to convert leaves into flower organs.
  • The ABCD model is applicable to all flowering plants.