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The zygote is formed inside the seed after an egg cell is fertilised by a sperm cell. The development process goes through histodifferentiation, cell growth, and maturation drying in the order described above.
It is characterised by the differentiation of the endosperm and the embryo at the stage of histodifferentiation (sometimes referred to as embryogenesis).
This mechanism is similar for monocots, dicots, and gymnosperms, but it is distinct for each of them.
Embryogeny in Monocots
The zygote or oospore elongates and then splits transversely to generate basal and terminal cells, which are the final stages of development. This cell is produced by the basal cell (which is located toward the micropylar end) and is big, thick, and swollen. It has the potential to serve as a haustorium. The terminal cell divides by a second transverse wall, resulting in the formation of two cells.
Following a series of divisions, the top cell develops into a plumule and a single cotyledon. The terminal plumule is pushed to one side by the rapid growth of a cotyledon known as the scutellum. The plumule eventually falls into a state of despair.
After many divisions, the central cell develops into a hypocotyl and a radicle. It also increases the number of cells in the suspensor. In some cereals, sheaths generated from the scutellum cover both the plumule and the radicle, which are referred to as coleoptile and coleorhiza, respectively.
Structure of Monocot Embryo
The presence of only one cotyledon characterises monocotyledon embryos. The scutellum is the name given to this cotyledon in the grass family (Gramineae). It is located on the lateral side of the embryonal axis, close to the midline. This axis’s radicle and root cap are enclosed in a sheath termed coleorhiza at the lower end of the axis.
The epicotyl is the portion of the axis that extends above the level of attachment of the scutellum. In addition, it has several leaf primordium that is enclosed in a hollow leaf structure known as a coleoptile, which is the shoot’s tip. The epiblast indicates the beginnings of the second cotyledon.
Proembryo stage
When compared to dicots, monocots have a more complicated embryo structure in the mature seed, yet their early embryo development is very comparable to that of dicots.
In monocots, the stages of embryogenesis include the proembryo, globular, scutellar, and coleoptile phases, among others.
Following fertilisation, the first cell division in corn is asymmetrical, resulting in the formation of an apical and basal cell (Zea mays). Eventually, the apical cell will overtake the basal cell in terms of division rate and will form the embryo.
Globular stage
During the globular stage, the proembryo is identical to that of dicots, with the exception that the suspensor does not consist of a single or double row of cells and is less differentiated.
The outer epidermal layer is visible in the late globular stage, and a group of cells on one side of the proembryo divides at a faster rate than the rest of the proembryo.
The embryo axis will be formed as a result of these events.
Scutellar stage
During the scutellar stage of development, the remnants of the cotyledon can still be observed.
Monocots have reduced the pair of cotyledons represented in dicot embryos to a single modified cotyledon known as the scutellum, which is present in all dicot embryos today.
It serves as a conduit between the endosperm and the embryonic axis, allowing nutrients to pass through.
Coleoptilar stage
The embryo axis divides into two parts: the plumule (shoot) and the radicle (root).
In monocots, the embryo axis is also surrounded by specialised tissue that aids in the emergence of the shoot and root tissue during the germination process.
The coleoptile and coleorhiza are the two structures in question.
Development of Dicot Embryo
The zygote divides, resulting in the formation of two uneven cells. The suspensor cell or hypobasal cell is a larger cell located towards the micropylar end of the micropylar end. The smaller cell, which is located at the chalazal end and is known as an embryonal cell or an epibasal cell, is located near the chalazal end. The transverse wall of the suspensor cell divides, whereas the longitudinal wall of the epibasal cell divides.
It is now in the four-cell stage, which is referred to as the pro-embryo stage. The suspensor divides by recurrent divisions until it reaches a stage of 6 to 19 cells. The hypophysis is a swelling suspensor cell located towards the micropylar end of the suspensor cell. This serves the purpose of a haustorium. The suspensor is responsible for pushing the pro-embryo into the endosperm.
The second embryonal cell divides longitudinally at a right angle to the first one, forming two new cells. This is followed by transverse division, which results in the formation of an 8 celled stage. The plumule and two cotyledons are formed by four cells that are located away from the suspensor and four cells that are located near the suspensor. The hypocotyls and stele of the radicle are formed by the other four basal cells, which are located near the suspensor.
Conclusion
This critical stage of the plant’s life cycle begins with the fertilisation of the zygote and ends with the development of a fully grown embryo, which is the culmination of the entire process. The procedure is carried out by predetermined patterns of cell division, which results in the correct creation of the body plan. The establishment of a functional shoot meristem during embryogenesis is a well-described and crucial step in the development of the plant’s leaves, stems, and floral structures during postembryonic growth and development. In Arabidopsis, a model system for embryogenesis, genetic investigations have revealed that the creation of the body plan is guided by a complex gene network that enables the proper execution of morphogenic events during the development of the plant.
