Study Notes on Angiosperm Reproduction and Biotechnology

Angiosperm (Flowering Plants) Life Cycle

  • Plant Life Cycles

    • Characterized by the alternation between sporophyte and gametophyte generations.

    • Sporophyte:

    • Multicellular diploid part of the life cycle.

    • Produces haploid spores by meiosis.

    • Gametophyte:

    • Multicellular haploid plants that produce gametes (sperm and eggs) by mitosis.

    • In angiosperms, the sporophyte is the dominant generation, being larger, more conspicuous, and longer-lived than gametophytes.

    • The angiosperm life cycle features three key components: flowers, double fertilization, and fruits.

Flower Structure and Function

  • Flowers are the reproductive shoots of the angiosperm sporophyte attached to a structure called the receptacle.

  • Flowers consist of four floral organs:

    • Carpels: Female reproductive structures.

    • Stamens: Male reproductive structures.

    • Petals: Attract pollinators.

    • Sepals: Protect unopened floral buds.

Female Flower Parts

  • Carpel Structure:

    • Has a long style topped with a sticky stigma that captures pollen.

    • Ovary: Located at the base of the style, contains one or more ovules.

    • Fertilization Outcome: Fertilized ovules develop into seeds.

    • A single carpel or groups of fused carpels are referred to as a pistil.

Male Flower Parts

  • Stamen Structure:

    • Composed of a filament topped by an anther.

    • Anther contains microsporangia (pollen sacs) that produce pollen.

Other Flower Parts

  • Sepals: Leaf-like structures that enclose and protect unopened floral buds.

  • Petals: Typically brightly colored to attract pollinators.

Types of Flowers

  • Complete Flowers: Contain all four floral organs (carpels, stamens, petals, sepals).

  • Incomplete Flowers: Lack one or more floral organs, e.g., petals or stamens.

  • Sterile Flowers: Lack both stamens and carpels.

  • Unisexual Flowers: Lack one of the reproductive organs.

  • Inflorescences: Clusters of flowers.

Methods of Pollination

  • Pollination: Transfer of pollen from anthers to stigma, must occur before fertilization in higher plants.

  • Can occur via:

    • Wind

    • Water

    • Animals

Abiotic Pollination by Wind

  • Approximately 20% of angiosperm species are pollinated by wind (e.g., many grasses and trees).

  • Wind-pollinated angiosperms produce small, inconspicuous flowers that lack nectar and scent, releasing large amounts of pollen.

Mutualisms

  • Interaction between two species where both benefit.

  • Most angiosperms attract insects using nectar or pollen, benefiting both the plant and the pollinator.

Pollination by Bees

  • About 65% of all angiosperms require insects for pollination, with bees being the most significant.

  • Bee-pollinated flowers:

    • Typically brightly colored.

    • Have sweet fragrances.

    • Contain nectar guides, which are ultraviolet markings that direct bees to nectar-producing glands.

Pollination by Moths and Butterflies

  • Flowers pollinated by moths and butterflies emit sweet fragrances.

  • Butterfly-pollinated flowers are usually brightly colored, while moth-pollinated flowers tend to be white or yellow.

Pollination by Bats

  • Bat-pollinated flowers tend to be light-colored and aromatic.

Pollination by Flies

  • Some fly-pollinated flowers mimic the appearance and scent of rotting meat to attract flies.

Corpse Flower (Amorphophallus titanum)

  • Native to the island of Sumatra, Indonesia.

  • Blooms once every 4-5 years.

  • Features the largest unbranched inflorescence in the Plant Kingdom.

  • Emits a strong stench resembling rotting meat to attract insect pollinators.

Flowers of Deceit

  • Insects assist angiosperms in reproducing with physically distant members of their species:

    • Rhizanthes flowers emit foul odors to attract female blowflies, which lay eggs on the flower and transfer pollen in the process, with no benefit to the fly.

    • Ophrys speculum attracts and mimics the appearance of wasps to ensure pollination.

Pollination by Birds

  • Bird-pollinated flowers are generally large, bright red or yellow, have little odor, and produce copious nectar.

  • Petals are often fused into a floral tube for better access by birds.

Coevolution

  • Coevolution refers to the joint evolution of interacting species due to mutual selection pressures.

  • The shapes and sizes of flowers commonly correspond with the pollen-transporting anatomy of their pollinators.

    • Notably demonstrated by Darwin’s prediction of a moth with a 28-cm long tongue aligning with a specific flower's morphology.

The Angiosperm Life Cycle: An Overview

  • Key components of the angiosperm life cycle include:

    • Gametophyte Development

    • Sperm Delivery by Pollen Tubes

    • Double Fertilization

    • Seed Development

Development of Female Gametophytes

  • Inside the ovules within the ovary, a diploid cell undergoes meiosis to create haploid spores, leading to the formation of the embryo sac, a multicellular (8-cell) female gametophyte.

Development of Male Gametophytes

  • Within pollen sacs in the anther, diploid cells undergo meiosis to generate haploid spores that continue mitosis, producing two cells: the generative cell and the tube cell.

  • A pollen grain thus comprises this two-celled male gametophyte along with the spore wall.

Sperm Delivery by Pollen Tubes

  • After adhering to a receptive stigma, a pollen grain forms a pollen tube that extends into the ovary and releases two sperm cells near the embryo sac.

Double Fertilization

  • In this process, one sperm fertilizes the egg, and the other fertilizes the two polar nuclei, forming a triploid endosperm (3n).

  • This process of double fertilization ensures that endosperm development occurs only in ovules with fertilized eggs.

Seed Development

  • Post double fertilization, each ovule matures into a seed, consisting of:

    • A dormant embryo.

    • Stored food (endosperm or in cotyledons).

    • Protective layers.

  • The ovary transforms into a fruit, aiding in seed dispersal by wind or animals.

  • Upon germination, the embryo develops into a new sporophyte.

Mechanisms to Prevent Self-Fertilization

  • Some flowers can achieve self-fertilization to maximize seed production, but this reduces genetic diversity among offspring.

  • Many species evolved mechanisms to avoid selfing:

    • Dioecious species: Have separate plants for staminate (male) and carpellate (female) flowers.

    • Other species have stamens and carpels that mature at different times or spatially arranged to prevent selfing.

Self-Incompatibility in Plants

  • The most prevalent method is self-incompatibility, which allows a plant to reject its own pollen.

  • Recognition of self-pollen is determined by S-genes, with some plants rejecting pollen matching the alleles in stigma cells.