Plant embryogenesis

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

An embryo is made up of actively growing cells and the term is normally used to describe the early formation of tissue in the first stages of growth. It can refer to different stages of the sporophyte and gametophyte plant.

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 apogamy) 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 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.

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Zygotic embryogenesis

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

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 from the callus. The ratio of different plant growth regulators required to induce callus or embryo formation varies with the type of plant.^[1]

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) in the zygotic embryo, 'embryo patterning' is apparent; however, somatic embryos at this stage may be quite variable, therefore zygotic and somatic embryos become most comparable from the globular stage.

In the globular stage, the embryo develops radial patterning through a series of cell divisions, eith the outer layer of cells differentiating into the 'protoderm.' The globular embryo can be thought 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; provascular cells will also differentiate at this stage. In the subsequent torpedo and cotyledonary stages of embryogenesis, the embryo completes its growth by elongating and enlarging.

References

Bibliography

• Dodeman V.L., Ducreux G., and Kreis M. Zygotic embryogenesis versus somatic embryogenesis. 1997 Journal of Experimental Botany. 48(313): 1493-1509
• Brady, M. Embryogenesis in Arabidopsis thaliana
• C R Ashburner, M G Faure, D R Tomlinson & W K Thompson Guide to the Zygotic Embryo Culture of Coconut Palms (Cocos nucifera L.), 1995, ISBN: 1 86320 155 6