Flowers and Arabidopsis thaliana Notes

Flowers: Function and Structure

  • Flowers attract pollinators using colorful petals, scent, nectar, and pollen.
  • Floral organs:
    • Carpel/pistil: female reproductive organs.
      • Stigma: where pollen sticks to.
      • Style: connects stigma to ovary.
      • Ovary: contains ovules and becomes the fruit.
      • Ovules: contain the female gametophyte and become the seed.
    • Stamen: male reproductive organ.
      • Anther: produces pollen.
      • Filament: holds the anther.
      • Pollen: immature male gametophyte.
    • Non-reproductive organs:
      • Petals: attract pollinators; the corolla is the whorl of petals.
      • Sepals: protect the unopened flower bud; the calyx is the whorl of sepals.
    • Other structures:
      • Receptacle
      • Peduncle

Overview of Floral Organs

  • Key structures include the receptacle, stigma, carpel, style, ovary, anther, ovule, stamen, filament, pedicel, sepal (calyx), and petal (corolla).

Reproductive Floral Organs: Ovule-Producing

  • Carpel or pistil (female reproductive organs):
    • Stigma: where pollen adheres.
    • Style: connects the stigma to the ovary.
    • Ovary: contains ovules and develops into fruit.
    • Ovules: contain the female gametophyte and become the seed.

Reproductive Floral Organs: Pollen-Producing

  • Stamen (male floral organ):
    • Anther: produces pollen.
    • Filament: supports the anther.
    • Pollen: immature male gametophyte.

Non-Reproductive Floral Organs

  • Petals: attract pollinators.
    • Corolla: whorl of petals.
  • Sepals: protect the flower bud.
    • Calyx: whorl of sepals.

Pollination

  • Pollination: transfer of pollen from anther to stigma.

Importance of Pollination

  • Sexual reproduction is important for evolution because it produces variable offspring, creating diversity for natural selection to occur.
  • Outbreeding is favored over inbreeding.

Sexual Reproduction

  • Animals: Separate male and female individuals facilitate sexual reproduction.
  • Flowering plants: Most have both male and female parts (perfect flowers), requiring special adaptations to ensure outbreeding and prevent inbreeding.

Complete vs. Incomplete Flowers

  • Complete flowers: Possess all four organs (sepals, petals, stamens, pistils).
  • Incomplete flowers: Lack one or more floral organs.

Strategies to Avoid Self-Pollination

  • Timing: Male and female structures mature at different times.
  • Morphological: Flower structure prevents self-pollination (imperfect flowers).
  • Biochemical: Chemical incompatibility prevents pollen tube germination on the same flower.

Morphological Strategies in Cannabis

  • Separate “male” (staminate) and “female” (pistillate) plants exist.

Pollen Self-Incompatibility

  • Pollen tube growth is blocked in the style if the pollen's S alleles match those of the stigma cells.

Pollination Methods

  • Wind pollination: Used by gymnosperms and some flowering plants (grasses, trees). Inefficient due to its chancy and wasteful nature.
  • Water pollination: Used by aquatic plants.

Coevolution

  • Coevolution: Interactions between species drive reciprocal adaptations, increasing interdependency.
  • Animal-flowering plant interactions: Classic example of coevolution, with plants evolving to attract pollinators and animals evolving specialized traits for pollination.

Animal Pollinators: Bees

  • Bees: Most important group of flower pollinators; live on nectar, feed larvae, and eat pollen.
  • Guided by sight and smell; see yellow, blue, and ultraviolet colors (but not red).
  • Flowers have “honey guides” and bee landing platforms.

Butterflies and Moths

  • Guided by sight and smell.
  • Butterflies: See red and orange flowers, often with long tubular shapes to match the insect’s proboscis.
  • Moths: Pollinate white or pale flowers with sweet, strong odors, facilitating night pollination.

Flies and Beetles

  • Flies: Attracted to flowers smelling like dung or rotten meat.
  • Beetles: Pollinate dull-colored flowers with strong odors.

Birds as Pollinators

  • Good sense of color (yellow and red), but poor sense of smell.
  • Flowers provide large quantities of fluid nectar.
  • Hummingbird-pollinated flowers: Usually have long, tubular corollas and large, sticky pollen.

Mammals: Bats and Mice

  • Bats: Pollinate white flowers at night.
  • Mice: Pollinate inconspicuous flowers that open at night.

Why Animals Pollinate Plants

  • Reward: Food in exchange for moving pollen.
    • Nectar: Sugary solution produced in nectaries; concentration matches pollinator’s energy requirements.
    • Pollen: High in protein; eaten by some bees and beetles.
  • Flowers produce normal and sterile pollen, with the latter being tasty for insects.

Getting the Pollinator’s Attention

  • Plants advertise rewards with:
    • Color: Bees see blue, yellow, UV; birds see red; bats favor white flowers.
    • Nectar or honey guides: Visual cues for pollinators.
    • Aromas: Attract insects; can mimic carrion or dung smells.

Plant Mimicry

  • Mimicry: Some plants mimic female insects to exploit the sex drive of male insects.
  • Orchids: Look and smell like female wasps, leading males to attempt mating and pollinate the plant.

Arabidopsis thaliana

  • Arabidopsis thaliana: A small plant in the mustard family, related to broccoli, cabbage, and cauliflower.

Introduction to Arabidopsis

  • Arabidopsis thaliana: Model system for plant biology research.
  • Important discoveries: Understanding plant growth and development through molecular genetics.

Arabidopsis Genome

  • Small genome: Organized into five chromosomes with approximately 25,000 genes.
  • Genome sequencing: Completed by 2000, enhancing its value as a model for plant biology.

Arabidopsis: A Simple Plant

  • Simple plant: Easy to grow, short (20 cm).
  • Short life cycle: 6-8 weeks from seed to seed.
  • Small genome.
  • Related to mustard, broccoli, etc.

Simple Morphology

  • Flowers have all four organs (sepals, petals, stamens, carpels) in four concentric rings.

ABC Model for Plant Development

  • Three genes (A, B, C) regulate flower organ formation.

ABC Flower Development Genes

  • Gene A: Rings 1 and 2.
  • Gene B: Rings 2 and 3.
  • Gene C: Rings 3 and 4.

ABC Gene Combinations

  • Explains the spatial arrangement of floral organs based on gene expression.

ABC Gene Expression

  • Ring 1: A only = Sepals.
  • Ring 2: A + B = Petals.
  • Ring 3: B + C = Stamens.
  • Ring 4: C only = Carpels.
  • Genes A and C mutually inhibit each other's expression.

Consequences of Missing Genes

  • Mutations in A, B, or C genes: Result in altered flower phenotypes.

Examples of Mutants

  • Wild type: Sepal, Petal, Stamen, Carpel.

Important Scientific Discoveries in Arabidopsis

  • Flower development.
  • Photoreceptors.
  • Phototropism, gravitropism.
  • Plant-pathogen interactions.
  • Mechanisms of plant hormone action.