Plant embryonic development, also plant embryogenesis, is a process that occurs after the fertilization of an ovule to produce a fully developed plant embryo. This is a pertinent stage in the plant life cycle that is followed by dormancy and germination. The zygote produced after fertilization must undergo various cellular divisions and differentiations to become a mature embryo. However, both plants and animals including humans, pass through a phylotypic stage that evolved independently and that causes a developmental constraint limiting morphological diversification.
== Morphogenesis in eudicots == <!--yeah, so what about monocots, gymnosperms, and all other plants, hmm?-->
Embryogenesis in eudicot angiosperms occurs naturally as a result of single, or double fertilization, of the ovule, giving rise to two distinct structures: the plant embryo and the endosperm which go on to develop into a seed. The zygote undergoes a series of cellular differentiations and divisions to produce a mature embryo. These morphogenic events form the basic cellular pattern necessary for the development of the shoot-root axis and the primary tissue layers. They also initiates the formation of meristematic regions.
[[File:Seed Development Cycle.svg|thumb|309x309px|Six moments in embryogenesis
{|
|-
|
||
|}
]]
thumb|309px|Closer look at the early embryo
Plant
Following fertilization, the zygote and endosperm are present within the ovule, as shown in stage I of the illustration on this page. The zygote subsequently undergoes an asymmetric transverse cell division, producing to two distinct cells - a small apical cell positioned above a large basal cell.
These cells differ in structure and function and give rise to distinct embryonic components thereby establishing polarity in the developing embryo.
;apical cell:The apical cell, at the top, retains most of the cytoplasm from the original zygote. It gives rise to the hypocotyl, shoot apical meristem, and cotyledons. Stage II in the illustration above shows the embryo at this eight cell stage. According to Laux et al., four distinct domains are present at this stage. The first two domains contribute to the embryo proper. The apical embryo domain, gives rise to the shoot apical meristem and cotyledons. The second domain, the central embryo domain, gives rise to the hypocotyl, root apical meristem, and parts of the cotyledons. The basal embryo domain form the third domain and contains the hypophysis. Which will later give rise to the radicle and the root cap. The final domain, the suspensor, is located at the base of the embryo and connect it to the endosperm, facilitating nutrient transfer.
Heart stage
thumb|Cotyledon location
According to Evert and Eichhorn, the heart stage is a transition period where the cotyledons finally start to form and elongate.. Upon reaching this stage, the symmetry of the embryo shifts from radial to bilateral. However, in the torpedo stage of development, parts of the suspensor complex must be terminated.
Maturation
The second phase, or post-embryonic development, in higher seed plants, involves the maturation of cells, which involves cell growth and the storage of macromolecules (such as oils, starches and proteins) required as a 'food and energy supply' during germination and seedling growth. These are especially seen in plants that do not store large resources in their endosperm, and are chiefly responsible for the substance of the embryo. Lower forms do not have a discrete distinction between these stages, but transition continuously between embryonic and post-embryonic development. The appearance of a mature embryo is seen in Stage VI, in the illustration above.
Dormancy
The end of embryogenesis is defined by an arrested development phase, or stop in growth. This phase usually coincides with a necessary component of growth called dormancy. Dormancy is a period in which a seed cannot germinate, even under optimal environmental conditions, until a specific requirement is met. Breaking dormancy, or finding the specific requirement of the seed, can be rather difficult. For example, a seed coat can be extremely thick. According to Evert and Eichhorn, very thick seed coats must undergo a process called scarification, in order to deteriorate the coating.
The role of auxin
Auxin is a hormone related to the elongation and regulation of plants. It also plays an important role in the establishment polarity with the plant embryo. Research has shown that the hypocotyl from both gymnosperms and angiosperms show auxin transport to the root end of the embryo. They hypothesized that the embryonic pattern is regulated by the auxin transport mechanism and the polar positioning of cells within the ovule. The importance of auxin was shown, in their research, when carrot embryos, at different stages, were subjected to auxin transport inhibitors. The inhibitors that these carrots were subjected to made them unable to progress to later stages of embryogenesis. During the globular stage of embryogenesis, the embryos continued spherical expansion. In addition, oblong embryos continued axial growth, without the introduction of cotyledons. During the heart embryo stage of development, there were additional growth axes on hypocotyls. Further auxin transport inhibition research, conducted on Brassica juncea, shows that after germination, the cotyledons were fused and not two separate structures.
Alternative forms of embryogenesis
Somatic embryogenesis
Somatic embryos are formed from plant cells that are not normally involved in the development of embryos, i.e. ordinary plant tissue. No endosperm or seed coat is formed around a somatic embryo. Applications of this process include: clonal propagation of genetically uniform plant material; elimination of viruses; provision of source tissue for genetic transformation; generation of whole plants from single cells called protoplasts; development of synthetic seed technology. Cells derived from competent source tissue are cultured to form an undifferentiated mass of cells called a callus. Plant growth regulators in the tissue culture medium can be manipulated to induce callus formation and subsequently changed to induce embryos to form the callus. The ratio of different plant growth regulators required to induce callus or embryo formation varies with the type of plant. Asymmetrical cell division also seems to be important in the development of somatic embryos, and while failure to form the suspensor cell is lethal to zygotic embryos, it is not lethal for somatic embryos. Androgenesis usually occurs under stressful conditions. which are in a persistently embryonic state, to the growth of new buds on stems.
In both gymnosperms and angiosperms, the young plant contained in the seed, begins as a developing egg-cell formed after fertilization (sometimes without fertilization in a process called apomixis) and becomes a plant embryo.
This embryonic condition also occurs in the buds that form on stems. The buds have tissue that has differentiated but not grown into complete structures. They can be in a resting state, lying dormant over winter or when conditions are dry, and then commence growth when conditions become suitable. Before they start growing into stem, leaves, or flowers, the buds are said to be in an embryonic state.