7.2 Embryogenesis and seed development

Embryogenesis is the fascinating journey from a single cell to a complex plant embryo. It involves intricate stages like zygote formation, globular and heart stages, and the development of crucial structures such as the shoot and root meristems.

Seed development encompasses more than just the embryo. It includes the formation of nutritive endosperm, the protective seed coat, and the accumulation of energy reserves. Hormones like auxins and abscisic acid play key roles in regulating this process.

Embryogenesis stages

• Embryogenesis is the process of embryo development from a single-celled zygote to a mature embryo within the seed
• Involves a series of coordinated cell divisions, differentiation, and patterning events that establish the basic body plan of the plant

Zygote formation

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• Occurs after successful fertilization of the egg cell by a sperm cell
• The resulting diploid cell, called the zygote, undergoes asymmetric cell division to form a small apical cell and a larger basal cell
• The apical cell gives rise to the embryo proper, while the basal cell forms the suspensor

Proembryo development

• The apical cell undergoes a series of cell divisions to form a proembryo
• The proembryo stage is characterized by the formation of a spherical mass of cells called the globular embryo
• The suspensor continues to elongate, pushing the developing embryo deeper into the endosperm

Globular stage

• The globular embryo undergoes rapid cell divisions and begins to differentiate into distinct regions
• The outer layer of cells, called the protoderm, gives rise to the future epidermis
• The inner cells differentiate into the ground meristem and procambium, which will form the ground tissue and vascular tissue, respectively

Heart stage

• The globular embryo undergoes a transition to the heart stage, characterized by the formation of cotyledon primordia
• The cotyledon primordia are the first visible signs of the future seed leaves (dicots) or single seed leaf (monocots)
• The embryonic axis, consisting of the shoot apical meristem and root apical meristem, becomes evident

Torpedo stage

• The heart-shaped embryo elongates and takes on a torpedo-like appearance
• The cotyledons continue to grow and expand, while the embryonic axis becomes more defined
• The procambium differentiates into the primary vascular tissue, connecting the root and shoot systems

Cotyledon stage

• The torpedo-shaped embryo enters the cotyledon stage, where the cotyledons expand and accumulate storage reserves
• In dicots, the two cotyledons grow to occupy most of the seed volume, while in monocots, the single cotyledon (scutellum) remains small
• The shoot apical meristem is located between the cotyledons, and the root apical meristem is at the opposite end of the embryonic axis

Mature embryo

• The embryo reaches maturity, having developed all the essential structures of a young plant
• The mature embryo consists of the embryonic axis (hypocotyl and epicotyl), cotyledon(s), and the apical meristems
• The embryo enters a state of dormancy, awaiting favorable conditions for germination

Endosperm development

• The endosperm is a nutritive tissue that supports embryo growth and development
• Endosperm formation begins after fertilization of the central cell by a sperm cell, resulting in a triploid (3n) tissue

Nuclear endosperm

• In nuclear endosperm development, the primary endosperm nucleus undergoes repeated free-nuclear divisions without cell wall formation
• This results in a syncytium, a multinucleate cell with a large central vacuole
• Examples of plants with nuclear endosperm include coconut and Arabidopsis

Cellular endosperm

• In cellular endosperm development, cell wall formation occurs after each nuclear division
• This results in a solid mass of endosperm cells filling the embryo sac
• Cellular endosperm is common in cereals such as maize, wheat, and rice

Helobial endosperm

• Helobial endosperm development is an intermediate type between nuclear and cellular endosperm
• The first division of the primary endosperm nucleus is followed by cell wall formation, creating a small chalazal chamber and a large micropylar chamber
• The micropylar chamber undergoes free-nuclear divisions, while the chalazal chamber divides cellularly
• Helobial endosperm is found in some members of the Asparagaceae family (Agave)

Suspensor roles

• The suspensor is a temporary structure that plays crucial roles in early embryo development

Embryo positioning

• The suspensor anchors the developing embryo to the micropylar end of the embryo sac
• It positions the embryo deep within the endosperm, ensuring access to nutrients
• The suspensor degenerates later in embryogenesis as the embryo matures

Nutrient transfer

• The suspensor acts as a conduit for nutrient transfer from the endosperm to the developing embryo
• It facilitates the uptake and transport of sugars, amino acids, and other essential nutrients
• The suspensor cells may also synthesize growth regulators (auxins) that support embryo development

Seed coat formation

• The seed coat, or testa, is a protective layer that surrounds the embryo and endosperm
• It is derived from the integuments of the ovule and provides physical and chemical barriers against environmental stresses

Integuments

• Integuments are the outer layers of the ovule that give rise to the seed coat
• Most angiosperms have two integuments (bitegmic ovules), while some have only one (unitegmic ovules)
• The integuments undergo cell division, expansion, and differentiation during seed development

Testa vs tegmen

• In bitegmic ovules, the outer integument forms the testa, and the inner integument forms the tegmen
• The testa is usually thicker and more lignified than the tegmen, providing mechanical strength and protection
• In unitegmic ovules, the single integument directly forms the testa

Embryo differentiation

• During embryogenesis, the embryo undergoes differentiation to establish the basic body plan of the plant

Shoot apical meristem

• The shoot apical meristem (SAM) is a group of undifferentiated cells located at the apex of the embryonic axis
• It gives rise to the above-ground organs of the plant, such as leaves, stems, and flowers
• The SAM is responsible for the continuous growth and development of the shoot system throughout the plant's life

Root apical meristem

• The root apical meristem (RAM) is located at the opposite end of the embryonic axis from the SAM
• It gives rise to the root system of the plant, including primary and lateral roots
• The RAM is responsible for the continuous growth and development of the root system, facilitating nutrient and water uptake

Cotyledons

• Cotyledons are the first leaves of the embryo and serve as storage organs for nutrients
• In dicots, there are typically two cotyledons that expand and become photosynthetic upon germination (Phaseolus)
• In monocots, there is a single cotyledon called the scutellum, which remains within the seed and functions in nutrient transfer (Zea mays)

Hypocotyl vs epicotyl

• The hypocotyl is the region of the embryonic axis between the cotyledons and the root apical meristem
• It gives rise to the stem of the seedling and is responsible for pushing the cotyledons above the soil surface during germination
• The epicotyl is the region of the embryonic axis above the cotyledons and below the shoot apical meristem
• It gives rise to the first true leaves and the subsequent aerial parts of the plant

Seed maturation

• Seed maturation is the final stage of seed development, preparing the seed for dispersal and germination

Accumulation of reserves

• During maturation, the embryo and endosperm accumulate storage reserves such as proteins, lipids, and carbohydrates
• These reserves provide energy and nutrients for the germinating seedling until it becomes autotrophic
• The composition and quantity of storage reserves vary among species, depending on their ecological and evolutionary adaptations

Acquisition of desiccation tolerance

• As the seed matures, it undergoes a programmed desiccation process, losing most of its water content
• The embryo and endosperm cells synthesize protective molecules such as late embryogenesis abundant (LEA) proteins and sugars (raffinose) to prevent damage during desiccation
• Desiccation tolerance allows the seed to survive in a dry state until favorable conditions for germination occur

Induction of dormancy

• Seed dormancy is an adaptive trait that prevents premature germination and ensures survival under adverse conditions
• During maturation, the seed may develop various types of dormancy, such as physical (hard seed coat), physiological (abscisic acid-mediated), or morphological (underdeveloped embryo)
• Dormancy is broken by specific environmental cues (light, temperature, moisture) or after-ripening, allowing the seed to germinate when conditions are favorable

Hormonal regulation

• Plant hormones play crucial roles in regulating embryogenesis, seed development, and germination

Auxins

• Auxins, such as indole-3-acetic acid (IAA), are involved in the establishment of the embryo polarity and patterning
• They promote cell division and expansion in the embryo and endosperm
• Auxins also play a role in the differentiation of the vascular tissue and the formation of the suspensor

Cytokinins

• Cytokinins, such as zeatin, promote cell division and differentiation in the embryo and endosperm
• They are involved in the regulation of the shoot apical meristem and the formation of cotyledons
• Cytokinins also delay senescence and promote nutrient mobilization to the developing seed

Gibberellins

• Gibberellins (GAs) are involved in the regulation of seed germination and the mobilization of storage reserves
• They promote the synthesis and secretion of hydrolytic enzymes (α-amylase) that break down starch in the endosperm
• GAs also stimulate the elongation of the hypocotyl and the emergence of the radicle during germination

Abscisic acid

• Abscisic acid (ABA) is a key regulator of seed maturation and dormancy
• It promotes the accumulation of storage reserves and the acquisition of desiccation tolerance in the embryo and endosperm
• ABA also induces dormancy by inhibiting germination and preventing precocious growth of the embryo
• The balance between ABA and GAs determines the timing of seed germination, with ABA favoring dormancy and GAs promoting germination