Angiosperm Reproduction and Biotechnology

Chapter 38: Angiosperm Reproduction (Plant Sex!) and Biotechnology

I. Learning Outcomes

  • Gain understanding of the male and female components of a plant.
  • Understand the process of double fertilization and seed formation.
  • Discuss seed germination, seed structures, and maturation into a mature sporophyte.
  • Compare and contrast asexual and sexual reproduction in plants.
  • Understand genetic modification processes in plants and their potential benefits.

II. Angiosperms—Flowering Plants

  • Total estimated species: 250,000.
  • Key features:
    • Flowers
    • Double fertilization
    • Fruits
  • Life cycle begins with sporophyte (diploid) producing haploid spores via meiosis, which then undergo mitosis to form gametes (male and female).

III. Key Concepts and Terminology

  • Haploid (n): A cell that has a single set of chromosomes.
  • Diploid (2n): A cell containing two complete sets of chromosomes, one from each parent.
  • Fertilization: The fusion of gametes producing diploid zygotes, which develop into new sporophytes.

IV. Flower Structure and Function

A. Flower Components

  1. Flowers as reproductive shoots; they stop growing after flowers and fruits are formed.
  2. Four main organ types:
    • Sepals: Protect and enclose the floral bud before opening.
    • Petals: Attract pollinators through color.
    • Stamens: Male reproductive organs, consisting of:
      • Anther: Contains pollen.
      • Stalk.
    • Carpels: Female reproductive organs, consisting of:
      • Ovary: At the base.
      • Style: Neck structure.
      • Stigma: Sticky structure at the style's top.
  3. Complete Flowers: Contain all four organs.
  4. Incomplete Flowers: Lack at least one organ (e.g., grasses lack petals).

V. Angiosperm Life Cycle Overview

  • Simplified Cycle: Involves key stages of fertilization and seed formation:
    1. Pollen Development: Involves male gametophyte development in pollen grains.
    2. Fertilization: Direct delivery of sperm via pollen tube to the fertilization process leading to a zygote.
    3. Seed Formation: Development from ovule.

VI. Development of Male Gametophyte in Pollen Grains

  1. Each anther contains 4 pollen sacs producing haploid microspores through meiosis.
  2. Microspores undergo mitosis to create:
    • Generative Cell (n)
    • Tube Cell (n)
    • Air pollen grain formed by these components.
  3. Pollen grain lands on stigma, tube cell forms a pollen tube to deliver sperm to female gametophyte, growing at rates up to 1 cm/hr.

VII. Development of Female Gametophytes (Embryo Sacs)

  1. Initiated in ovule by megasporocyte undergoing meiosis to produce four megaspores; only one survives.
  2. Surviving megaspore undergoes mitosis to form an embryo sac with 8 haploid nuclei.
  3. Egg cell and two synergids (attract and guide pollen) at one end; three cells of unknown function at the opposite end; two polar nuclei share cytoplasm in center.

VIII. Pollination

  1. Definition: Transfer of pollen from anther to stigma.
  2. Methods of Pollination:
    • Wind Pollination: Approximately 20% of angiosperms; utilizes copious pollen for efficiency.
    • Insect Pollination: Approximately 65% of angiosperms; relies on various insects (e.g., honeybees for bright colors, moths/flies for odors).
    • Bird and Bat Pollination, with adaptations for various pollinators.

IX. Double Fertilization in Angiosperms

  1. Involves two sperm cells directed by GABA attracting through the pollen tube:
    • One sperm fertilizes the egg (forms zygote).
    • Other sperm combines with two polar nuclei to form endosperm (nutrient-storing tissue).
  2. Outcome: Formation of zygote blocks polyspermy through an increase in calcium ions in the egg.

X. Seed Development, Form, and Function

  1. Following fertilization, each ovule becomes a seed, and the ovary becomes the fruit.
  2. Endosperm Development: Occurs before embryo development:
    • Monocots store nutrients in endosperm.
    • Dicots transfer nutrients to cotyledon (example: coconut's meat/milk).
  3. Embryo Development:
    • Cotyledons are the first leaves, with differing numbers in monocots (1) vs. dicots (2).
    • Hypocotyl connects to the radicle/root; epicotyl connects to the first leaves.
  4. Seed Dormancy:
    • Requires environmental cues for germination, such as rainfall, light exposure, or passage through an animal's digestive tract.
  5. Germination Process:
    • Initiated by imbibition: uptake of water due to low water potential of dry seed.
    • Activation of enzyme activity for nutrient digestion and transfer to the embryo, leading to root radical emergence and shoot tip breakthroughs.
    • Epicotyl expands first foliage leaves to commence photosynthesis while cotyledons wither and fall away.