4.6.1: Embryogenesis
Learning Objectives
- Explain the steps of embryogensis in eudicots.
- Compare embryogenesis in eudicots versus monocots.
The Seed Plants chapter discussed the fertilization of the egg within the ovule. The zygote ultimately divides to produce the mature embryo, the ovule develops into a seed, and the ovary that contained one or more ovules develops into a fruit. A typical seed contains a seed coat, cotyledons, endosperm, and a single embryo. The development of the embryo occurs through a process called embryogenesis .
Embryogenesis in Eudicots
After fertilization in eudicots, the zygote (Figure \(\PageIndex{1-I}\)) divides to form two cells: the upper apical cell and the lower basal cell . The division of the basal cell gives rise to the suspensor , which attaches the embryo to the micropyle (the pore through which the pollen tube original entered). The suspensor provides a route for nutrition to be transported from the mother plant to the growing embryo. The apical cell also divides, initially producing a proembryo (figure \(\PageIndex{1-II}\)).
As the proembryo continues to divide, it takes a spherical form, called the globular stage (Figures \(\PageIndex{1-III}\) and \(\PageIndex{2a}\)). The globular is the first stage that is considered the embryo proper . Next, cotyledons arise from the embryo proper, forming the heart stage (Figures \(\PageIndex{1-IV}\) and \(\PageIndex{2-4}\)). Cotyledons are embryonic leaf-life structures that function in food storage, food absorption and/or photosynthesis. As the cotyledons elongate, and the base of the embryo thickens, it results a torpedo. (This stage is called the torpedo stage; Figures \(\PageIndex{1-V}\) and \(\PageIndex{2c}\)). Cell division is concentrated at the shoot apical meristem , located at the shoot tip in between the cotyledons, and the root apical meristem at the most basal (bottom) part of the embryo. Most of the suspensor deteriorates during the torpedo stage.
The final stage of embryogenesis results in the mature embryo (Figures \(\PageIndex{1-VI}\) and \(\PageIndex{2d}\)). The mature embryo includes an embryonic root called the radicle . The embryo becomes dormant at this point, halting metabolic activity and cell division. At this point, the seed is ready for dispersal. Growth resumes when the seed germinates and the embryo develops into a seedling.
In some eudicots, the endosperm (triploid tissue that was formed when a sperm fertilized the two polar nuclei) cells divide, and endosperm fills a substantial portion of the mature seed. The endosperm stores nutrients. In other (non-endospermic) eudicots, such as Capsella bursa-pastoris , the endosperm develops initially, but is then digested, and the nutrients are moved into the two cotyledons (Figure \(\PageIndex{2}\)). After germination, the developing seedling relies on these food reserves stored in the endosperm or cotyledons until the first set of leaves begin photosynthesis.
Embryogenesis in Monocots
The process of embryogenesis in monocots is similar to that of eudicots, but as there is only a single cotyledon, no heart stage occurs. Instead, the embryo proper of the monocot becomes cylindrical at this point in development. The shoot apical meristem, while still present at the shoot tip, is not in between cotyledons in the monocot (because there is only a single cotyledon).
Attribution
Curated and authored by Melissa Ha using 32.2 Pollination and Fertilization from Biology 2e by OpenStax (licensed CC-BY ). Access for free at openstax.org .