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 forced to form embryos in plant tissue culture, these embryos are called somatic embryos.
The zygotic embryo is formed following double fertilization 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 embryos are formed from plant cells that are not normally involved in, or competent for the development of embryos. This process is used in plant tissue culture to generate plants from single cells called protoplasts, or from small pieces of plant tissue. Cells that have been cultured to form undifferentiated tissue called callus can be induced to form embryos with the addition of plant hormones, the hormone or ratio of different hormones required to induce embryo formation varies with the type of plant. No endosperm or seed is formed around a somatic embryo.
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 leathal for somatic embryos. From the eight cell stage (octant) the zygotic embyro, 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.