Embryogenesis is the fascinating journey from a single cell to a complex plant . It involves intricate stages like 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 , and the accumulation of energy reserves. Hormones like 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
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 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 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 (, , 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
(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