7.2 Embryogenesis and seed development
7 min read•august 20, 2024
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
Key Terms to Review (18)
Asexual reproduction: Asexual reproduction is a process by which an organism can produce offspring without the involvement of gametes, resulting in genetically identical clones of the parent organism. This method of reproduction allows for rapid population growth and colonization of environments, as it does not require a mate. Asexual reproduction is common in many plants and can occur through various mechanisms, such as vegetative propagation, fragmentation, or budding.
Auxins: Auxins are a class of plant hormones that play a crucial role in regulating plant growth and development, particularly by influencing cell elongation, apical dominance, and responses to light and gravity. These hormones are essential for coordinating various physiological processes in plants, including growth patterns and developmental stages.
Dicot: A dicot, or dicotyledon, is a group of flowering plants characterized by having two cotyledons or seed leaves in their seeds. This trait plays a crucial role in the seed's structure and function, influencing its embryogenesis and development. Dicot seeds tend to have a more complex structure compared to monocots, which affects their germination process and the subsequent development of seedlings.
Embryo: An embryo is an early developmental stage of a plant, beginning after fertilization and lasting until the seed matures. It consists of the zygote that undergoes cell division and differentiation, leading to the formation of key structures such as the cotyledons, shoot, and root. Understanding embryos is crucial because they directly impact seed structure and function, as well as the processes involved in embryogenesis and seed development.
Genotype: Genotype refers to the genetic constitution of an organism, specifically the alleles that are present in its DNA. It serves as the underlying blueprint for the physical traits, or phenotype, that the organism exhibits. Understanding genotype is crucial in studying how traits are inherited and expressed during processes such as embryogenesis and seed development, where the genetic material influences the growth and characteristics of the resulting plants.
Germination: Germination is the process by which a seed develops into a new plant, starting with the growth of the embryo within the seed. This phase is crucial as it marks the beginning of a plant's life cycle, relying on specific environmental conditions such as moisture, temperature, and light. The success of germination is influenced by the seed's structure and function, as well as the events leading up to fertilization and embryogenesis, which ultimately determine how effectively a seed can sprout and thrive.
Gibberellins: Gibberellins are a class of plant hormones that play a crucial role in regulating various growth processes, including stem elongation, seed germination, and flowering. They stimulate cell division and elongation, which is essential for the development of stems and seeds, making them vital for overall plant growth and reproduction.
Light: Light is a form of electromagnetic radiation that is visible to the human eye and plays a crucial role in the processes of photosynthesis and plant growth. In the context of embryogenesis and seed development, light influences the formation of seedlings and the orientation of plant structures, as well as triggering specific developmental pathways through photoreceptors.
Maturation: Maturation refers to the process by which plant cells and tissues undergo changes that lead to the development of mature structures and functions. This process is critical in various stages of plant development, including the formation of specialized cells in tissues and the transformation of embryos into viable seeds. Through maturation, plants ensure that their cells are properly differentiated and functional, allowing for effective growth and reproduction.
Microscopy: Microscopy is the technique used to view small objects that are not visible to the naked eye, employing various types of microscopes to magnify specimens for detailed observation. This method is essential in many biological sciences as it allows for the examination of cellular structures, tissues, and even whole organisms at a microscopic level. By enhancing visibility, microscopy plays a vital role in understanding plant classification, cell wall composition, and developmental processes.
Monocot: Monocots, or monocotyledons, are a major group of flowering plants characterized by having a single cotyledon in their seeds. This distinctive feature influences various aspects of their structure and function, including leaf venation patterns, floral arrangements, and root systems. Monocots play crucial roles in ecosystems and agriculture, with significant implications for seed structure, embryogenesis, and germination processes.
Ovule: An ovule is a structure found in seed plants that contains the female gametophyte and develops into a seed after fertilization. It is essential in the reproduction process as it contains the egg cell that, upon fertilization, will give rise to the embryo, along with surrounding tissues that nourish the developing seed. The ovule’s development and its transformation into a seed are critical for the propagation of flowering plants and gymnosperms.
Phenotype: Phenotype refers to the observable physical and biochemical characteristics of an organism, determined by both genetic makeup and environmental influences. In the context of plant development, the phenotype can include traits such as seed size, shape, color, and overall plant architecture, which can be shaped by the interaction between genetics and the conditions during embryogenesis and seed development.
Seed Coat: The seed coat is the protective outer covering of a seed, formed from the outer layers of the ovule after fertilization. It serves as a barrier against physical damage and desiccation, ensuring the survival of the embryo inside. The seed coat also plays a crucial role in seed dormancy and germination, as it can control the passage of water and gases into the seed.
Sexual reproduction: Sexual reproduction is a biological process where two parent organisms contribute genetic material to produce offspring, resulting in genetic variation among the individuals. This process typically involves the fusion of male and female gametes during fertilization, leading to the formation of a zygote, which ultimately develops into a new organism. This form of reproduction is vital for the evolution and adaptability of species, as it enhances genetic diversity.
Temperature: Temperature refers to the measure of the warmth or coldness of an environment, impacting biological processes such as growth and development. In the context of plant life, temperature plays a vital role in seed formation, germination rates, and susceptibility to fungal diseases. It influences cellular activities, enzyme reactions, and overall plant health, which can affect everything from embryogenesis to the emergence of seedlings and the challenges posed by pathogens.
Tissue culture: Tissue culture is a technique used to grow and maintain plant cells, tissues, or organs in a controlled, sterile environment on a nutrient medium. This method allows for the propagation of plants without the need for seeds or cuttings, making it an invaluable tool in plant breeding, conservation, and research. Through this process, scientists can produce genetically identical plants and restore endangered species, while also studying embryogenesis and facilitating vegetative reproduction.
Zygote: A zygote is a single cell formed when two gametes, usually a sperm and an egg, fuse during fertilization. This initial cell is the first stage in the development of a new organism and marks the beginning of embryogenesis, leading to seed development in plants. The zygote contains a complete set of genetic material from both parents, setting the stage for growth and differentiation into various tissues and structures.