Embryogeny in Monocots: Structure, Differences, Examples

What Is Embryogeny?

Embryogenesis is the sequence of processes by which a fertilised egg, or zygote, develops into a mature embryo. During this intricate series of cell division events, differentiations, and morphogenesis, the basic structures of a plant are formed, including primarily the root, shoot, and seed leaves (cotyledons). Studying embryogeny in monocots is important to understand their unique patterns of development and adaptations; these plants include many key crops such as rice, wheat, and corn.

This kind of information from embryogeny may be useful in breeding programs aimed at improving crop yields and increasing resistance to diseases and environmental stresses. The other broad division of flowering plants is the monocot plants, otherwise known as monocotyledons, distinguished by the presence of a single cotyledon in its seed. In addition, they possess parallel leaf venation, fibrous root systems, and floral parts usually in multiples of three. The monocots include simple food crops and grasses—components constituting a crucial part of both ecosystems and human nutrition.

Structure And Characteristics Of Monocot Embryos

Monocot embryos include these parts: one cotyledon called the scutellum; an embryo sac, where fertilisation occurs; a zygote, which develops from fertilisation; and a suspensor that connects the embryo to nutritive tissue, holding it in place while it develops.

Basic Structure Of A Monocot Embryo

• Embryo Sac

The embryo sac is the female gametophyte within which the egg cell becomes fertilised by the male gamete. This shall contain seven cells, with a total of eight nuclei: the egg cell, two synergids, three antipodal cells, and one central cell having two polar nuclei.

• Zygote

The zygote is formed when the sperm cell from the pollen fertilises the egg cell in the embryo sac. From this single cell, which will undergo multiple rounds of division and differentiation, a mature embryo will develop.

• Suspensor

The suspensor is a structure that develops from the basal cell of the zygote. It pushes the embryo inside for the absorption of nutrients in the endosperm and thus helps in the initial stages of development by anchorage and support of the embryo.

Key Differences Between Monocot And Dicot Embryos

In contrast, monocots have only one cotyledon, with leaves showing parallel venation and scattered vascular bundles. Dicotyledons, on the other hand, possess two cotyledons, leaves with reticulate venation, and vascular bundles in a ring.

Stages Of Embryogeny In Monocots

Monocot embryogeny includes several defined stages. These stages include proembryo, globular, heart, and torpedo stages. Each of these stages marks the formation of important organs and tissues. The stages are given below:

Proembryo Stage

The proembryo stage begins when the zygote divides into two cells: an apical small cell that gives rise to the embryo proper and a large basal cell that grows to form the suspensor.

Globular Stage

At this stage, the developing embryo becomes globular in shape. Cellular differentiation matures, and the suspensor is extended to allow the embryo to grow further into the nutrient-containing tissue. Cells begin to differentiate into various tissue types.

This establishes the basis for future development of the plant organs and structures. The suspensor becomes larger and continues growing to deliver the nutrients ingested from the endosperm into the growing embryo.

Heart Stage

At this stage, the heart-shaped structure is developed in dicots. In monocots, a single cotyledon starts to develop, that is, the scutellum, with initiation of the root and shoot meristems. The scutellum develops from the apical part of the embryo and acts as a nutrient-absorbing organ, which in turn supports the development of seedlings after germination.

Meristems are regions of undifferentiated cells that will give rise to the root and shoot systems. The root apical meristem is established at the base end of the embryo and the shoot apical meristem forms at the opposite end.

Torpedo Stage

During the torpedo stage, the embryo becomes elongated as the tissues become even more specialized. The cotyledon becomes elongated and the shoot and root systems become more developed. The cells in the embryo start to elongate and further differentiate into definite tissues to tune the embryo for its future transition to the seedling stage.

By this stage, the monocot embryo is fully mature and has a very well-defined scutellum, root meristem and shoot meristem among other embryonic structures, ready to germinate.

Embryo Development Processes

Embryo development involves cell division and tissue differentiation, which are precisely regulated for organised plant growth. Apical meristems, along with hormones like auxins and cytokinin, influence the development stages. The details are given below:

Cell Division And Differentiation

In the process of embryogenesis, several rounds of cell division occur in the developing embryo to form specialised cells and tissues. This entire process is tightly regulated for proper growth and development.

Role Of Apical Meristems

Apical meristems are located at the shoot and root tips. They are the region of primary tissue growth and formation, responsible for the continuous growth and organ formation of the plant.

Hormonal Regulation In Embryo Development

Plant hormones, especially auxins, cytokinins, gibberellins, and abscisic acid, play a critical role in embryo development by regulating cell division, elongation, differentiation and the shift from dormancy to an active growing phase.

Types Of Monocot Embryogeny

• Classical Embryogeny

Classic embryogeny involves well-defined successive stages through which a typical monocot embryo is formed. This process is commonly observed in plants, as exemplified by maize. Example: Maize

• Non-Classical Embryogeny

Some monocots, among which a few orchids also are, stand for non-classical embryogeny with atypical sequences in the course of development and differentiation. The non-classical embryogeny includes alternative developmental pathways, leading to peculiar structures of the embryo—showing the diversity of embryonic development among monocots.

Factors Influencing Embryogeny In Monocots

In general, environmental factors, such as temperature, light, and nutrient availability, are known to play a major role in embryonic development. The influencing factors include those that would affect the hormonal levels and change the rate of cell division and differentiation.

Genetic Factors

Different genes control the various phases of embryo development. Therefore, their mutations or variations disturb the normal process of embryogenesis. WUSCHEL, LEAFY COTYLEDON and FUSCA3 genes play a crucial role in controlling the development and differentiation of embryonic cells.

Environmental Factors

• Temperature

Optimal temperatures are needed for cell division and differentiation processes. Extreme temperatures result either in developmental abnormalities or dormancy.

• Light

The quality and duration of light may alter the hormonal balance in the developing embryo. All physiological processes, including germination and growth, are influenced by light.

• Nutrient Availability

Adequate nutrients are needed for the embryo's growth and development; nutrient deficiencies or imbalances lead to poor development or dormancy.

MCQ On Embryogenesis

Question : During embryo germination in a grass family, an absorptive organ that forms an interface between the embryo and the starchy endosperm tissue is called

Option 1 - Coleorhiza

Option 2 - Coleoptile

Option 3 - Scutellum

Option 4 - Mesocotyl

Solution - During embryo germination in the grass family, the absorptive organ that forms an interface between the embryo and the starchy endosperm tissue is called the scutellum.

The scutellum is a specialized structure found in the seeds of grasses (family Poaceae) and serves as the primary absorptive organ during germination. It is a shield-shaped structure located on one side of the embryo, specifically on the side facing the endosperm. The scutellum is rich in enzymes that break down complex carbohydrates into simpler forms, facilitating their absorption by the developing embryo.

As the seed germinates, the scutellum secretes enzymes such as amylases that hydrolyze the starch stored in the endosperm into sugars. These sugars are then transported to the growing embryo, providing it with a source of energy and nutrients for growth and development.

The scutellum plays a crucial role in the early stages of seed germination in grasses and is an essential adaptation for these plants to utilize the stored nutrients in the endosperm and ensure successful seedling establishment.

Hence, the correct answer is option 3) Scutellum.

Question: Directions: In the following questions, a statement of Assertion (A) is followed by a statement of reason (R).

Assertion – The aleurone layer separates scutellum and endosperm in monocotyledonous seeds.

Reason – The aleurone layer is triploid.

Mark the correct choice as:

Option 1 - If both assertion and reason are true and reason is correct explanation of assertion

Option 2 - If both assertion and reason are true but the reason is not the correct explanation of the assertion

Option 3 - If the assertion is true but the reason is false

Option 4 - If both assertion and reason are false

Solution - The aleurone layer is the outermost layer of the endosperm found in seeds, and it plays a crucial role in storing and mobilizing nutrients during germination. This layer is rich in proteins, which is why it is described as proteinaceous. It serves as a barrier that separates the cotyledon (in dicots) or the scutellum (a specialized cotyledon in monocots) from the main body of the endosperm.

While the aleurone layer is part of the endosperm, it has distinct functions that are important for seed development.

Hence, the correct answer is option 2) If both assertion and reason are true but the reason is not the correct explanation of the assertion.

Question: In mature Arabidopsis embryo, root apical meristem consists of cells derived from

Option 1 - Embryo and apical suspensor cell

Option 2 - Embryo only

Option 3 - Suspensor only

Option 4 - Hypophysis only

Solution - In a mature Arabidopsis embryo, the root apical meristem consists of cells derived from both the embryo and the apical suspensor cell.

During the early stages of embryogenesis in Arabidopsis, the developing embryo is attached to the maternal tissue through a structure called the suspensor. The suspensor plays a role in providing nutrients and support to the developing embryo.

Within the suspensor, there is a specialized cell known as the apical suspensor cell. This cell gives rise to the root apical meristem, which is responsible for the formation of the root system in the mature plant. The root apical meristem consists of cells that continuously divide and differentiate, leading to the growth and development of the root.

Therefore, the cells in the root apical meristem of a mature Arabidopsis embryo are derived from both the embryo and the apical suspensor cell. The embryo proper contributes to the development of the root apical meristem, while the apical suspensor cell provides the initial cells for its formation.

Hence, the correct answer is option 1) Embryo and apical suspensor cell.

Question: The process of formation and development of an embryo is called.

Option 1 - Fertilization

Option 2 - Embryo cleavage

Option 3 - Embryogeny

Option 4 - Sporulation

Solution - Embryogeny refers to the formation and development of an embryo. Embryogeny refers to the formation and development of an embryo from a fertilized egg or zygote. It involves a series of cell divisions, differentiation, and tissue formation to establish the body plan of the organism. The process typically includes key stages such as cleavage, blastula formation, gastrulation, and organogenesis. Embryogeny ensures the proper arrangement of cells and tissues necessary for the growth and functionality of the mature organism.

Hence, the correct answer is option 3) Embryogeny.

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