Plant Reproduction Notes

Plant Reproduction Notes

Life Cycle of an Angiosperm

• Angiosperms are characterized by the production of flowers and fruits.
• Emerged around 200 million years ago during the Jurassic Era.
• Undergo developmental changes leading to reproductive maturity, referred to as phase change.

Flower Production

• Four genetically regulated pathways are involved in flowering:
  1. Light-Dependent Pathway
  2. Temperature-Dependent Pathway
  3. Gibberellin-Dependent Pathway
  4. Autonomous Pathway
• While plants may rely on one primary pathway, all four can be present simultaneously.

Light-Dependent Pathway

• Also known as the photoperiodic pathway.
• Influenced by the ratio of light to dark in a 24-hour cycle (day length).

Day Length Effects on Flowering

• Long-Day Plants flower when daylight exceeds a certain critical length.
• Short-Day Plants flower when daylight is shorter than a critical length.
• Day-Neutral Plants flower regardless of day length when mature.

Darkness as a Flowering Signal

• The duration of uninterrupted darkness is crucial for triggering flowering.
• If a long nighttime is interrupted by light, plants behave as long-day plants.

Manipulation of Photoperiod

• Plants utilize light cues to time flowering for optimal abiotic conditions.
• Greenhouses manipulate photoperiods to induce flowering in short-day plants, like poinsettias, for holiday timing.

Temperature-Dependent Pathway

• Some plants require chilling periods for flowering (known as vernalization).
• Example: Winter wheat needs a chilling period to flower; can be chilled and planted in spring.

Gibberellin-Dependent Pathway

• Gibberellin is a critical plant hormone.
• Reduced gibberellin levels can delay flowering.
• Gibberellin promotes the expression of the LFY gene, which is key for initiating flowering.

Autonomous Pathway

• This pathway does not rely on external cues but on basic nutritional status.
• It may delay flowering through a balance of floral promoting and inhibiting signals.

Flowering Pathways Integration

• The four pathways converge to convert an adult meristem into a floral meristem.
• They activate or inhibit floral meristem identity genes (e.g., LFY and AP1), which trigger floral organ development.

Floral Organ Development

• The floral meristem identity genes define the four whorls of a flower:
  1. Sepals
  2. Petals
  3. Stamens (Androecium)
  4. Carpels (Gynoecium)

Structure and Evolution of Flowers

Flower Morphology: Four Whorls

• Calyx - Flattened sepals.
• Corolla - Petals.
• Androecium - All stamens (includes filament and anther).
• Gynoecium - All carpels (includes ovary, style, stigma); ovules are produced in the ovary.
• An incomplete flower lacks one or more of these whorls.

Trends in Floral Specialization

• Two major trends:
  1. Floral parts grouping together.
  2. Reduction or loss of floral parts associated with specific pollination mechanisms.

Trends in Floral Symmetry

• Primitive flowers are radially symmetrical.
• Advanced flowers exhibit bilateral symmetry, enhancing pollination efficiency.

Embryo Development

Double Fertilization

• The pollen tube enters the embryo sac and releases two sperm cells.
  • One sperm fertilizes the central cell, initiating endosperm development.
  • The other sperm fertilizes the egg, forming a zygote.

Developmental Stages of the Embryo

• The first zygote division is asymmetrical; the smaller cell forms the embryo, and the larger cell forms the suspensor for nutrient transport.
• The root-shoot axis forms early, designating future roots and shoots.
• Initial cell divisions lead to a globular proembryo stage, developing into a heart-shaped embryo with cotyledons (two in eudicots, one in monocots).

Tissue System Development

• Three fundamental tissue systems arise:
  1. Protoderm - Dermal tissue.
  2. Ground Meristem - Ground tissue.
  3. Procambium - Vascular tissue.
• Tissues are organized radially around the root-shoot axis.

Critical Developmental Events in Embryogenesis

1. Food Supply Development: Formation of endosperm.
2. Seed Coat Development: Differentiation of ovule tissue.
3. Fruit Development: Results from the carpel wall surrounding the ovule.

Endosperm Variation

• Different plants have varying types of endosperm (e.g., liquid "milk" in coconuts, solid in corn).
• In peas and beans, nutrients are stored in thick, fleshy cotyledons, consumed during embryogenesis.

Seeds

• Development of the embryo often halts after meristems and cotyledons form.
• Integuments become a seed coat enclosing the dormant embryo and stored food.

Fruits

• Defined as mature ovaries (carpels).
• Ovaries develop into fruits during seed formation; some develop without seed formation (asexual reproduction, e.g. bananas).

Fruit Dispersal Methods

• Various dispersal strategies include:
  • Ingestion and transport by animals.
  • Attachment to animals through spines.
  • Burial by herbivores.
  • Wind dispersal.
  • Water drift.

Germination Process

Initiation of Germination

• Germination marks the emergence of the radicle from the seed coat.
• Requires environmental signals: light, warmth, time, and moisture.
• Stratification may be necessary for some seeds, involving low-temperature periods pre-germination.
• Water is the primary trigger, splitting the seed coat and allowing oxygen to reach the embryo.

Nutrient Utilization Post-Germination

• Germination and early growth rely on stored metabolic reserves (starch and proteins).
• Reserves vary among seed types; e.g., stored in the embryo or endosperm.
• Hormones, particularly gibberellic acid, regulate germination events.

Specific Germination Examples

• In Eudicot Germination (e.g., common bean):

  • Hypocotyl bends to protect the delicate shoot apex.
  • Roots grow downward, and shoots grow upward, becoming photosynthetic.

• In cereal grain kernels, the cotyledon modifies into a scutellum to transfer nutrients from the endosperm.

Summary

• Angiosperms have complex reproductive cycles involving specific paths towards flowering, embryo development, and seed germination. Understanding these processes is critical for horticulture and agriculture, influencing practices such as greenhouse management and crop cultivation.