The construction of a full embryo from a zygote; seed germination; the elaboration of a mature vegetative plant from the embryo; the formation of flowers, fruits, and seeds; and many of the plant’s responses to its environment are all covered under the umbrella phrase “plant development.” The growth and differentiation of cells, tissues, organs, and organ systems are all part of plant development. Plant development is similar to animal development in many ways, but because plants are nonmotile photosynthetic creatures, they require several novel developmental processes in addition to the standard ones.
Development of Embryos and Seeds
One of the most spectacular and well-studied elements of plant growth is embryogenesis, the production of a multicellular embryo from a single-celled zygote. Embryogenesis involves four important developmental steps. The zygote, for starters, has apical-basal polarity, which means that the apical and basal ends of the zygote cell are physically and biochemically distinct. When a zygote divides asymmetrically, it produces a small apical cell with dense cytoplasm and a big basal cell with watery cytoplasm. Despite the fact that these two cells have similar nuclei, their fates are vastly different. The embryo is formed by the apical cell, whereas the basal cell produces a short-lived structure termed a suspensor and the root system’s apex. The globular-stage embryo is formed when the progeny of the apical cell expand and divide to form a spherical mass of cells. Second, differential growth inside the globular embryo produces the “heart” stage embryo, which is the earliest stage at which the precursors of cotyledons, root, and stem may be seen. Organogenesis is the name for this crucial embryogenic process. Histogenesis is the process by which cells within embryonic cotyledons, roots, and stems acquire diverse morphologies, generating the precursors of plant tissue systems. At the apical and basal ends of the embryo, the apical meristems of the shoot and root systems are created.
At the cellular level, developmental processes continue after an embryo has attained full size. Embryonic cells, especially those in the cotyledons, begin to produce and store the proteins, lipids, and starch that will provide the energy and basic building blocks for seedling germination and growth. The embryo then begins to desiccate, losing up to 80% of its former water content, and enters a dormant phase. Seeds harbouring latent embryos can live for many years (even centuries) and tolerate high temperatures and drought since their development and metabolism are halted.
Plant hormones have a crucial role in embryogenesis and seed dormancy. During embryogenesis, the hormones auxin, gibberellic acid, and cytokinin all drive growth and are present in the embryo. These hormones are destroyed as the embryo develops, and abscisic acid is generated by the embryo. Abscisic acid acts as a developmental signal to the embryo, causing it to begin synthesis of storage compounds and to desiccate. Abscisic acid is found in latent seeds and is hypothesised to play a function in seed dormancy maintenance.
Seedling Growth and Germination
After seed germination, embryo growth and metabolism resume. The dried seed begins to take up water and the embryo begins to grow and metabolise again when the appropriate mix of water availability, temperatures, and light is present. Some species have unique germination needs; for example, many temperate zone tree species need temperatures of 4 degrees Celsius (39.2 degrees Fahrenheit) or less for several weeks in order to germinate. In order to germinate, other species require low levels of light. Once germination has begun, the embryo develops in a predictable fashion. The embryonic root elongates first in many plants, forcing its way out of the seed coat and into the soil. The embryonic stem then elongates, usually below the attachment of the cotyledons (the hypocotyl). The cotyledons swell after the hypocotyl has taken them into the light, offering a large area for photosynthesis.
Seedling development is influenced by environmental influences and their translation into hormone signals. For example, dark germination triggers developmental events that aid the seedling’s push through the soil and into the light. The hypocotyl swiftly elongates and forms a “hook” near its tip to protect the cotyledons and shoot apical meristem region. Cotyledon expansion is slowed so they don’t get injured while being pushed through the earth. When the same seeds germinate in the light, however, the hypocotyl barely elongates and does not form a hook, while the cotyledons rapidly develop. Gibberellic acid is a hormone that is involved in seed germination and early seedling growth. Gibberellic acid stimulates the production of enzymes involved in the digestion of stored nutrients, supplying energy to seedlings. Gibberellic acid also promotes cell division and expansion in dark-grown hypocotyls, allowing them to continue to grow quickly in the soil.
Apical Meristems and Their Role in Development
The enlargement of the root, hypocotyl, and cotyledons that were produced in the embryo are all that is required in the early phases of germination. The apical meristems, on the other hand, are the focus of postembryonic development. All of the leaves, stems, and their component cells are generated in the shoot apical meristem over the plant’s existence. A tiny population of permanently embryonic (meristematic) cells make up the meristem. These cells divide and develop, producing new cells, but they never mature. As a result, there is always a supply of new cells near the shoot’s tip. A similar number of meristematic cells exists at the root tip, which gives rise to all root tissues. Both of these meristems have an indeterminate growth pattern, which means that it is not finite but might, in theory, continue throughout the plant’s existence.
Apical meristems play a role in a number of developmental stages. Meristems are the sites of cell proliferation in the shoot and root systems, and thus the source of all new cells. The areas below the meristems are active growth zones, as new shoot and root tissue expands fast. Organogenesis, the production of new leaves and axillary buds in a specific spatial arrangement, is aided by the shoot apical meristem. The root apical meristem, on the other hand, is not engaged in organogenesis; lateral roots are initiated by pericycle cells, which are produced from the meristem and are located several centimetres away from it. The apical meristems also play a role in histogenesis, as they give rise to cells that differentiate in diverse ways to generate the specialised tissue types of the shoot and root. While the embryo produces the precursors of dermal, ground, and vascular tissues (protoderm, ground meristem, and procambium, respectively), the apical meristems continue to produce these tissue precursors, which are the early stages of cell and tissue differentiation.
Conclusion
We conclude that plants are necessary for all life on the planet. Plants are vital because they absorb CO2 from the atmosphere and create oxygen. Plants also form the foundation of the food web by making their own food with the help of light, water, carbon dioxide, and other substances.