Plant embryogenesis

Plant embryogenesis is a sexual or asexual reproductive process that forms new plants. The process of embryogenesis may occur naturally in the plant as a result of sexual fertilization or asexual processes, these embryos are called zygotic embryos and develop into seeds, which germinate giving rise to seedlings. Plant cells can also be induced to form embryos in plant tissue culture; these embryos are called somatic embryos.

Embryonic is defined as tissue that is made up of actively growing cells. Normally the term is used to describe the early formation of tissue in the first stages of growth. In botany it can both refer to different stages of the sporophyte and gametophyte plant.

In seed plants, both gymnosperms and angiosperms, the young plant contained in the seed, which is a developing egg-cell formed after fertilization (sometimes with out fertilization in a process called apogamy) is an embryo in an embryonic state. In plants also, an embryonic condition 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 overwinter or when conditions are dry, then start growth when conditions are suitable, the buds before they start growing into stem, leaves, or flowers are said to be in an embryonic state.

Zygotic embryogenesis
The zygotic embryo is formed following double fertilisation of the ovule, forming the embryo and the endosperm which go on to develop into a seed. Seeds may also develop without fertilization though pathways referred to as apomixis.

Somatic embryogenesis
Somatic embryos are formed from plant cells that are not normally involved in the development of embryos. 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 a disorganized/dedifferentiated mass of cells called callus. Plant growth regulators in the tissue culture medium can be manipulated to induce callus formation and subsequently changed to induce embryos from the callus. The ratio of different plant growth regulators required to induce callus or embryo formation varies with the type of plant.

The stages of embryogenesis
Zygotic and somatic embryos share a number of characteristic developmental stages, however the very early steps in their development are not well correlated.

Following fertilization the zygote undergoes an asymmetrical cell division that gives rise to a small apical cell that becomes the embryo and a large basal cell called the suspensor that functions to provide nutrients from the endosperm to the growing embryo. 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. From the eight cell stage (octant) the zygotic embryo, embryo patterning is apparent, somatic embryos at this stage may be quite variable, therefore zygotic and somatic embryo most comparable from the globular stage.

The globular stage the embryo develops radial patterning through a series of cell divisions, the outer layer of cells differentiates into the protoderm. The globular embryo can be though of as two layers of inner cells with distinct developmental fates, the apical layer will go on to produce cotyledons and shoot meristem, while the lower layer produces the hypocotyl and root meristem. Bilateral symmetry is apparent from the heart stage, provasular cells are also differentiate at this stage. In the subsequent torpedo and cotyledonary stages of embryogenesis, the embryo completes its growth by elongating and enlarging.