Early Plant Development: Embryogenesis and Germination 8

Early Plant Development (Chapter 22)

• This chapter covers the foundational aspects of early plant development.

• Note: The spelling of "ynchama" is not critical for the midterm exam.

Monocot and Dicot Differences

• Monocot (e.g., Corn):

  • Seed: One cotyledon.

  • Root: Fibrous roots; primary root is short-lived, with root system developing from nodes via stem-borne roots later.

  • Vascular: Scattered vascular bundles in the stem.

  • Leaf: Parallel veins.

  • Flower: Flower parts typically in multiples of 3.

• Dicot:

  • Seed: Two cotyledons.

  • Root: Tap roots; the primary root persists and forms lateral roots.

  • Vascular: Vascular bundles arranged in a ring in the stem.

  • Leaf: Net-like veins.

  • Flower: Flower parts typically in multiples of 4 or 5.

Angiosperms and Embryogenesis

• Embryogenesis: The process of forming an embryo, encompassing the first two phases of seed development.

• Initiation: Begins with the division of a zygote within the embryo sac of an ovule.

• Zygote: The first cell of a new organism, formed when a sperm fertilizes an egg.

• First Division: Is asymmetrical and transverse relative to the long axis of the zygote. This crucial division establishes the apical-basal polarity of the embryo.

• Apical-Basal Polarity:

  • Upper (Chalazal) Pole: Contains the apical cell, which gives rise to most of the embryo.

  • Lower (Micropylar) Pole: Has a large basal cell, which produces a stalk-like suspensor. The suspensor anchors the embryo at the micropyle (the opening where the pollen tube enters to fertilize the egg).

Monocot Embryo Development

• Protoderm Establishment: The protoderm, which will form the epidermis of the plant, is established early.

• Apical Meristem Formation: As growth continues, a notch forms at the base of the cotyledon, indicating where the future apical meristem will develop.

• Cotyledon: This refers to the embryonic "leaf."

• Suspensor: The suspensor eventually disappears.

• Cotyledon Curvature: The cotyledon curves in a side-to-side manner.

• Vascular System Initiation: The procambium, the first step of the vascular system, begins to form and branches into the cotyledons.

Dicot Embryo Development

• Structural Axis: Once the structural axis is established.

• Embryo Proper Formation: Cell division occurs to form a nearly spherical embryo proper from the proembryo, which remains attached to the suspensor.

• Protoderm Formation: Initially undifferentiated, the protoderm forms from periclinal divisions (divisions parallel to the surface).

• Meristematic Distinction: Vertical divisions lead to the distinction between the ground meristem and the procambium.

  • Procambium: Will subsequently form the xylem and phloem, which constitute the vascular system.

• Apical Meristem Location: In dicots, the apical meristem is centrally located between the two cotyledons, unlike monocots where it is off to one side.

Stages of Embryo Development (Dicots Emphasis, Monocot Comparison)

• Globular Stage:

  • The embryo proper is spherical with a distinct protoderm (the outermost cell layer).

  • Development of the cotyledon(s) begins.

  • In both monocots and dicots, the apical-basal pattern develops during this stage, partitioning the embryo into distinct regions: shoot apical meristem, cotyledon(s), hypocotyl (the stem portion just below the cotyledon), embryonic root, and root apical meristem.

• Heart Stage:

  • As cotyledons develop, the globular embryo branches into two distinct lobes in dicots, giving it a heart shape.

  • In monocots, the single cotyledon becomes cylindrical.

• Torpedo Stage:

  • Cotyledons elongate significantly.

  • The primary meristem also extends along with the elongating cotyledons.

Cell Division in Embryonic Plants

• As the embryo develops, active cell division becomes restricted primarily to the apical meristems.

• Apical Meristems: These are found at the tips of all shoots and roots and consist of rapidly dividing, undifferentiated embryonic cells.

• Location by Type:

  • Dicots: The shoot apical meristem is located between the two cotyledons.

  • Monocots: The shoot apical meristem appears on one side of the single cotyledon.

Suspensors

• Metabolically Active: Suspensors are actively involved in metabolism.

• Function in Seedless Vascular Plants and Pinus spp.: In these plant groups, the suspensor's primary function is to push the embryo into the nutritive tissue.

• Function in Angiosperms: Suspensors provide essential nutrients and growth regulators, such as gibberellins, to the developing embryo.

• Connection to Embryo: Multiple plasmodesmata (cytoplasmic connections) link suspensor cells to the embryo proper, facilitating nutrient and signal transport.

• Lifespan: Suspensors are short-lived and typically undergo apoptosis (programmed cell death) during the torpedo stage of embryo development.

Maturing Embryos

• Epicotyl: The region above the cotyledons, containing one or two primordial leaves and the apical meristem.

• Plumule: The first embryonic bud, derived from the epicotyl, which will develop into the shoot.

• Hypocotyl: The stem-like area situated below the cotyledons but above the embryonic root.

• Radicle: The embryonic root located at the base of the hypocotyl. It possesses characteristics of a proper root, including an apical meristem and a rootcap. When these are present, the structure is called the hypocotyl-root axis.

• Scutellum: A large, specialized cotyledon found in grasses.

• Coleoptile: A protective sheath that covers the plumule in monocots (especially grasses).

• Coleorhiza: A protective sheath that covers the radicle within the seed, below the coleoptile, in monocots.

The Seed Coat

• Origin: All seeds possess a protective coating derived from the integument(s) of the ovule.

• Function: The primary role of the seed coat is to protect the embryo. It is typically highly impermeable to water, which helps regulate germination.

• Micropyle: A small pore often visible on the seed coat. It is the remnant of the opening through which the pollen tube entered the ovule.

• Hilum: A scar located next to the micropyle, marking where the seed separated from its stalk (the funiculus) on the parent plant.

Seed Maturation

• Nutrient Flow: During maturation, there is a continuous and substantial flow of nutrients from the parental tissue to the ovule.

• Storage Accumulation: Massive amounts of storage compounds—starch, storage proteins, and oils—accumulate in dedicated storage tissues such as the endosperm, perisperm, or cotyledons.

• Desiccation: The seed undergoes significant desiccation (drying out), which hardens the seed coat.

• Metabolic Slowdown: Desiccation effectively slows down metabolic activity to an almost imperceptible level.

• States: Seeds may then enter a quiescent state ("resting") or become dormant.

Seed Germination

• Definition: The resumption of embryonic growth, leading to the development of a seedling.

• Factors: Germination is dependent on both internal and external factors.

• External Factors: Key external requirements include adequate water, oxygen, and an appropriate temperature regime. Some seeds, particularly smaller seeds and "weed" species, may also require exposure to light.

• Metabolism Resumption: Metabolism resumes almost immediately once the seed imbibes water (takes up water).

• Enzyme Activity: Existing enzymes are activated, and new ones are synthesized to digest the stored food reserves.

• Pressure Buildup: As the embryo grows and water accumulates within the seed, internal pressure builds.

• Seed Coat Rupture: This pressure eventually causes the seed coat to rupture, allowing for the switch from anaerobic respiration to more efficient aerobic respiration as oxygen becomes readily available.

Dormant Seeds

• Causes: Dormancy can occur for various reasons, attributable to both internal and external factors.

• Coat-Imposed Dormancy: The seed coat itself can impose dormancy by being:

  • Impermeable to water and/or oxygen, preventing imbibition or respiration.

  • Too tough for the emerging root (radicle) to penetrate.

• Growth Inhibitors: The presence of chemical growth inhibitors within the seed can prevent germination.

• Plant Hormone Ratios: The balance of plant hormones, such as Abscisic
  acid (ABA) and Gibberellic
  acid (GA), plays a critical role. The ratio of ABA : GA can determine dormancy depth and duration, often influenced by environmental cues.

From Embryo to Adult

• Root Emergence: The root is typically the first structure to emerge from the seed.

  • Function: It anchors the plant and is responsible for water absorption.

  • Primary Root/Taproot: This initial root is called the primary root or taproot.

  • Lateral Roots: As the primary root grows, it branches to form lateral roots.

  • Monocot Roots: In monocots, the primary root is generally short-lived. The mature root system develops from nodes on the stem via stem-borne (adventitious) roots.

• Shoot Emergence: The shoot may emerge from the seed in different ways:

  • Epigeous Germination: Cotyledons are pulled aboveground.

  • Hypogeous Germination: Cotyledons remain belowground.

Epigeous Germination in Dicots

• Hypocotyl Elongation: The hypocotyl elongates and forms a distinctive hypocotyl hook.

• Shoot Tip Protection: This hook, as it pushes through the soil, protects the delicate shoot tip (plumule) from mechanical damage.

• Emergence of Cotyledons and Plumule: The elongating hypocotyl pulls the cotyledons and the plumule (embryonic shoot) into the air, above the soil surface.

• Cotyledon Fate: Once aboveground, the remaining stored nutrients in the cotyledons are consumed, after which they wither and drop off.

Hypogeous Germination in Dicots

• Epicotyl Elongation: The epicotyl elongates, forming an epicotyl hook.

• Shoot Tip Protection: This hook protects the shoot tip and young leaves as they push through the soil.

• Plumule Emergence: As the epicotyl straightens, the plumule is raised aboveground.

• Cotyledon Fate: In hypogeous germination, the cotyledons remain in the soil, utilizing their stored nutrients for the growth of the seedling before decomposing.

Epigeous Germination in Monocots

• Simpler Process: This type of germination in monocots is generally simpler than in dicots.

• Cotyledon Extension: The singular cotyledon extends, often becoming hooked, and as it straightens, it carries the seed coat with the endosperm aboveground.

• Photosynthetic Cotyledon: In many epigeous monocots, the cotyledon becomes photosynthetic.

• Plumule Emergence: Later, the plumule emerges, elongates, and develops into foliage leaves.

Hypogeous Germination in Monocots

• Coleorhiza Emergence: The coleorhiza (protective sheath of the radicle) emerges first through the pericarp (fruit wall).

• Radicle Growth: The radicle (embryonic root) begins rapid growth, anchoring the plant.

• Coleoptile Ascent: The coleoptile (protective sheath of the plumule) is pushed upward by the elongation of the first internode, known as the mesocotyl. The mesocotyl thus protects the plumule.

• Leaf Emergence: The true leaves emerge through the tip of the coleoptile.

• Stem-borne Roots: Concurrently, stem-borne roots begin to develop, further anchoring the plant.

Germination Types Summary