Following fertilization, embryo and endosperm development within the embryo sac are tandem. The oospore, or zygote, forms the embryo while the primary endosperm nucleus forms the endosperm. The remaining nuclei or cells in the embryo sac (synergids, antipodal cells) disorganize sooner or later.
Endosperm and its development
The tissue that supplies all the necessary food materials for the growth of the embryo and in most cases, for the young seedling, is known as endosperm.
The primary endosperm nucleus develops either by the fusion of one haploid male gamete and one diploid secondary nucleus, a fusion product of two haploid polar nuclei, or by the fusion of three haploid nuclei, which comprise one male gamete belonging to male gametophyte and two polar nuclei belonging to female gametophyte.
Generally, the endosperm nucleus divides after the division of the oospore but, in most cases, the endosperm is already formed even before the first division of the oospore.
During triple fusion, the process is fusion only between the male nucleus with the polar nuclei while the male cytoplasm is not included, and the membrane of the primary endosperm nucleus is made of both the secondary nucleus as well as male nucleus.
Depending upon the mode of development, three types of endosperms have been recognized:
1. Nuclear endosperm
2. Cellular endosperm
3. Helobial endosperm
Among them, the nuclear endosperm is the most common, which occurs in about 56% of families of angiosperms, followed by cellular endosperm found in more than 25% of species, followed by helobial endosperm found in about 19% of species.
Nuclear endosperm
In this type, the division of the primary endosperm nucleus and following nuclear divisions go on without wall formation, i.e. free nuclear type, and later, the nucleus lies around the periphery due to the presence of vacuole at the center.
In the nuclear type of endosperm there may be variations:
(1) Free nuclear condition of endosperm may persist till it is all used by the developing embryo, as occurs in Floerkea, Limnanthes, and Oxyspora.
(2) Endosperm may develop into a cellular one at later stages by the formation of a wall around the nuclei. The wall formation is generally centripetal. It is mainly found in Arachis hypogea.
The endosperm may later develop into completely cellular as in the case of Acalypha indica.
In many cases, the chalazal region is free nuclear and elongates and can be considered as haustorium. Examples are Crotalaria, Grevillea robusta, etc.
Cocus nucifera is the classical example of the free nuclear endosperm.
Cellular endosperm
In this kind of endosperm, wall formation follows every first division of the primary endosperm nucleus and succeeding other divisions and thus it becomes cellular right from the onset. The primary walls are transversely placed while the others are placed irregularly.
This type of endosperm is found in Adoxa, Peperomia, Villarsia etc.
Helobial endosperm
This type, so named due to its appearance in the representatives of order Helobiales (monocotyledons), is typical of a widely scattered series of angiosperm genera. This type of endosperm, development is intermediate between the nuclear and the cellular types. Here the first division of the primary endosperm nucleus is accompanied by the formation of the transverse wall. This divides the embryo sac unequally into two compartments- a small chalazal chamber and a large micropylar chamber.
Unequal chambers form and then free nuclear division occurs in both chambers but there are relatively more free nuclear divisions in the micropylar chamber as compared to the chalzal one. The chalazal chamber often degenerates. The free nuclear divisions in the micropylar chamber are followed by wall formation and thus a cellular endosperm tissue is formed.
This type of endosperm is found in Eremurus himalaicus.
Reserve food material in Endosperm
Persistent endosperm at maturity ensures a rich store of reserve food material which is carbohydrates, fats, and proteins. The reserve food material is digested to provide nutrients to the germinating seed until the seedling has enough chlorophyll to carry out photosynthesis.
Embryo Development
After fertilization, some changes occur in the ovule and finally seed is formed. Simultaneously with the endosperm development, the zygote develops into an embryo after a period of rest. The process of development of a mature embryo from a zygote is known as embryogenesis. The embryo has the potential to develop into a complete plant.
In all angiosperms, embryogenesis begins with the transverse division of the zygote into two-celled proembryo. The large cell nearer to the micropyle is called the basal cell and the smaller cell is called the terminal cell or apical cell.
In the earlier stages, there is no difference between monocotyledons and dicotyledons but when the initials of plumule and cotyledons are laid down, the differences appear. From the 2-celled stage, until the establishment of organs, the development is often referred to as proembryo.
In a two-celled proembryo having a large basal cell towards the micropyle and a small center facing the terminal cell or apical cell, the basal cell forms suspensor and may or may not take part in the rest activities therefore sometimes termed suspensor cell but the terminal cell is concerned further development of an embryo, it is hence also called an embryo cell.
Types of embryo development
Based on the plane of division of the terminal cell, also called apical or embryo cell in the 2-celled pro-embryo, and the contribution of the basal cell and terminal cell in the formation of the embryo proper, six types of embryogeny (embryo development) have been reported (Johansen 1950; Maheshwari 1950) in angiosperms
1. Onagrad or Crucifer type
2. Asterad type
3. Solanad type
4. Caryophyllad type
5. Chenopodiad type
6. Piperad type
Ongrad or crucifer type– In this type, the basal cell gives a transverse suspensor while the terminal cell forms the embryo proper. Example- Brassica.
Asterad type– In this type, the suspensor is less prominent, and the embryonic cells form through regular divisions of the apical cell. Example- Helianthus (Sunflower).
Solanad type– In this type, the terminal cell gives rise to both suspensor and embryo proper thus showing mixed orientation. Example- Solanum sp.
Caryophyllad type– In this type, the basal cell does not undergo further division and terminal cells divide irregularly, forming an asymmetrical structure. Example- Members of the Caryophyllaceae family.
Chenopodiad type– In this type, the basal cell divides to produce a large suspensor whereas the terminal cell produces an embryo proper through repeated divisions. Example- Chenopodium.
Piperad type– In this type, the entire embryo is formed from the basal cell while the terminal cell degenerates during the early stages of development. Example- Piper sp.
Development of dicot embryo
After fertilization, the zygote splits unevenly to become two cells, a bigger basal cell, and a smaller apical cell.
The basal cell develops into a suspensor. The suspensor holds the embryo and acts as a passage for nutrients and nutrients.
The apical cell further develops to give rise to the embryo proper. The apical cell splits into a ball-shaped mass of cells. This marks the development of the primary tissue system.
Cell divisions become localized, and now two cotyledons are present. This endows the embryo with a typical heart shape indicating the onset of organ differentiation.
The cotyledons lengthen, while the embryonic axis becomes conspicuous. The radicle and plumule are now obvious.
The embryo reaches its final form with well-defined cotyledons, a shoot apex (plumule), and a root apex (radicle). The seed tissues surrounding the embryo desiccate to prepare the seed for dormancy and eventual germination.
Development of monocot embryo
The apical cell continues to divide to form the embryo proper. The basal cell develops into the suspensor that allows for the transfer of nutrients to the developing embryo.
The embryo becomes spherical, and the primary tissue systems of protoderm, ground meristem, and procambium begin to differentiate.
The single cotyledon, or scutellum, starts to form. It is a structure specialized for the absorption of nutrients from the endosperm during germination, characteristic of monocot embryos.
Coleoptile and coleorhiza start to differentiate. Coleoptile covers and protects the plumule (embryonic shoot) while coleorhiza encloses and protects the radicle (embryonic root).
The embryonic axis elongates, with the radicle developing at one end and the plumule at the other. The scutellum remains attached to the axis, assisting in nutrient uptake.
The embryo matures within the seed, where it undergoes desiccation and enters a state of dormancy. A seed is normally rich in endosperm that acts as a reserve food material during germination.
Development of Fruit
After fertilization, the ovary undergoes a series of processes to develop into a fruit. Initially, the fertilized ovule develops into a seed, and then the ovary starts developing into different layers of fruit. These layers are the exocarp, the outermost, mesocarp, the fleshy edible part, and the endocarp, the inner layer.
The process of development of the ovary into fruit is influenced by hormones like auxin, gibberellins, and cytokinins. Auxins promote cell division and growth within the ovary wall. Gibberilins help in continuous growth and development and cytokinins promote cell division. During the ripening, the fruit experiences different biochemical changes preparing it for seed dispersion. Starches are generally stored within the mesocarp and are then converted into sugar, which contributes to the flavor of the fruits as being tasty and edible. Organic acids are broken, which contribute to the fruit’s sour flavor, resulting in a sweeter flavor. Different cell wall products, including pectin break down, which contributes to the softness of the fruits. Other pigments, such as carotenoids that produce the orange/yellow color, or anthocyanins responsible for the red/blue hue, accumulate when others like chlorophyll decompose.
Simultaneously, volatile compounds are emitted that add to the fruit’s aroma. These changes make it more attractive to animals, which may eat the fruit and spread its seeds, or it may mechanically release its seeds, depending on the species.
The type of fruit being formed also determines the transformation of the ovary into fruit. In simple fruits, the wall of the ovary forms the entire fruit. In aggregate fruits, multiple ovaries of a single flower form the fruit. In multiple fruits, the ovaries of multiple flowers are involved to form a single fruit.
Development of Seed
After embryo and endosperm formation, testa and tegmen, collectively known as seed coats are formed from integuments. The seed coat protects the developing embryo and endosperm from physical damage, water loss, and pathogen invasion. Seed coats can be thin and soft (in fruits like peaches) or thick and hard (in nuts or beans). In some seeds, the seed coat is also responsible for controlling water absorption during seed germination.
As the seed matures, the embryo and endosperm will grow and the seed coat becomes tough. The seed goes into desiccation in which the water is removed to survive during unfavorable conditions. The metabolism slows down and the seed reaches dormancy.
The time of dormancy is controlled by several environmental factors such as temperature, light, moisture, etc. Once the optimum temperature and moisture start to prevail, the seed germinates, the embryo will begin to grow and the seed coat will crack. Now the radicle and plumule come out and the seedling is formed which starts growing into a mature plant.
How do plants prepare for seed production and germination?
After fertilization, plants continue to prepare for seed production with a series of changes in the ovule as it develops to become a mature seed ready to germinate once the conditions become favorable for seed germination.
One of the vital steps in the seed preparation procedure is embryo development, where the cell division and differentiation of the fertilized zygote result in a mature embryo. The embryonic structures crucial for germination include the radicle, plumule, and cotyledons. At the same time, the endosperm or cotyledons undergo development to carry nutrients such as carbohydrates, proteins, and fats, which become the energy required for the growing seedling up to the period when it achieves self-sufficiency.
The seed coat, which forms from integuments of the ovule, develops for protection against physical damage, desiccation, and microbial attack. In some plants, seeds can go dormant, a form of suspended growth and metabolism, after which they can survive unfavorable environmental conditions. External conditions such as water, temperature, and light, as well as internal hormonal signals like gibberellin, break dormancy and induce germination. Through these preparatory steps, seeds become resistant and germinate well when environmental conditions are favorable, thus ensuring the continuity of plant species.
Importance of post-fertilization in plants
Post-fertilization in plants is key in plant reproduction because they can allow the movement from fertilized ovules into viable seeds and fruits. Events post-fertilization ensure embryo development, seed coat formation, endosperm development, as well as the development of fruits, all playing a role in seed dispersal, germination, and speciation.
The formation of the seed coat and fruit protects the developing seeds from predators, desiccation, and microbial infections. Fruits also help in seed dispersal, which spreads the species to new environments.
Besides, some post-fertilization events direct seed dormancy, letting seeds survive unfavorable conditions and germinate only when environmental conditions are great.
Without these events, seeds would lack the structural, nutritional, and protective adaptations necessary for successful germination and establishment. Post-fertilization processes are, therefore, indispensable for the reproductive success and survival of plants.
References
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