Read this article to learn about the types, nutritional requirements and applications of embryo culture.
Embryo Culture is of two types. They are: (1) Mature Embryo Culture and (2) Embryo Rescue.
Embryo culture deals with the sterile isolation and in vitro growth of a mature or an immature embryo with an ultimate objective of obtaining a viable plant. Conventionally, the term embryo culture refers to the sexually produced zygotic embryo culture. There are two types of embryo culture — mature embryo culture and immature embryo culture (embryo rescue).
Types of Embryo Culture:
1. Mature Embryo Culture:
Mature embryos are isolated from ripe seeds and cultured in vitro. Mature embryo cultures are carried out in the following conditions:
i. When the embryos remain dormant for long periods.
ii. Low survival of embryos in vivo.
iii. To avoid inhibition in the seed for germination.
iv. For converting sterile seeds to viable seedlings.
Seed dormancy in plant species is a common occurrence. This may be due to chemical inhibitors or mechanical resistance exerted by the structures covering the embryo. Seed dormancy can be successfully bypassed by culturing the embryos in vitro.
Embryo culture is relatively easy as they can be grown on a simple inorganic medium supplemented with energy source (usually sucrose). This is possible since the mature embryos excised from the developing seeds are autotrophic in nature.
2. Embryo Rescue:
Embryo rescue involves the culture of immature embryos to rescue them from unripe or hybrid seeds which fail to germinate. This approach is very useful to avoid embryo abortion and produce a viable plant. Wild hybridization involving crossing of two different species of plants from the same genus or different genera often results in failure. This is mainly because the normal development of zygote and seed is hindered due to genetic barriers.
Consequently, hybrid endosperm fails to develop leading the abortion of hybrid embryo. The endosperm may also produce toxins that ultimately kill the embryo. In the normal circumstances, endosperm first develops and supports embryo development nutritionally. Thus, majority of embryo abortions are due to failure in endosperm development. Embryo abortion can be avoided by isolating and culturing the hybrid embryos prior to abortion. The most important application of embryo rescue is the production of interspecific and inter-generic hybrids from wild plant species.
Culture Technique for Embryo Rescue:
The isolation of immature embryos often poses some difficulty. The aseptically isolated embryos can be grown in a suitable medium under optimal conditions. In general, a complex nutrient medium is required for culture methods involving embryo rescue. For adequate nutritional support of immature embryos, embryo-endosperm transplant is used.
The endosperm transplant technique used for culturing immature embryos is given in Fig. 47.7, and briefly described below.
The hybrid embryo from the ovule in which endosperm development has failed is taken out by excision. Another normally developed ovule with endosperm enclosing an embryo is chosen. This ovule is dissected and the normal embryo is pressed out. This leaves a normal endosperm with an exit hole.
Now, the hybrid embryo can be inserted into the normal endosperm through exit hole. This results in embryo-endosperm transplant which can be cultured in a suitable medium. By using embryo-endosperm transplant, many interspecific and inter-generic plants have been raised e.g., hybrid plants of legumes.
Nutritional Requirements of Embryo Cultures:
There are two phases in the embryo development and the nutritional requirement varies accordingly:
1. Heterotrophic phase:
This is an early phase and the embryo is mostly dependent on the endosperm and maternal tissues for nutrient supply.
2. Autotrophic phase:
This phase is characterized by the metabolic capability of the embryo to synthesize substances required for its growth which slowly makes it independent. The critical stage is the intervening phase between the heterotrophic and autotrophic phases. The nutrient supply is highly variable at this phase which mostly depends on the plant species.
In general, the composition of the medium for culturing immature embryos is more complex than that required by mature embryos which can grow on a simple inorganic medium. Further, the transfer of embryos from one medium to another is frequently needed in order to achieve full development of embryos.
Composition of the Medium:
Some salient features of medium and culture conditions are listed below:
i. Inorganic constituents of MS, B5 or White’s media are adequate.
ii. Sucrose is most commonly used energy source.
iii. Ammonium nitrate is the preferred source of nitrogen.
iv. Casein hydrolysate, rich in various amino acids is frequently used.
v. Certain natural plant extracts with embryo factor promote embryo cultures e.g. liquid endosperm of coconut milk. The embryo factor is believed to supply certain amino acids, sugars, growth regulators etc.
vi. In general, growth regulators are not required, as they induce callus formation.
vii. Embryos grow well in the pH range of 5-7.5.
viii. An incubation temperature of 24-26°C is ideal.
ix. Better growth of embryos is observed in darkness which are then transferred to light for germination.
During the culture conditions, the embryos are grown into plantlets, and then transferred to sterile soil for full-pledged growth to maturity.
Applications of Embryo Culture:
1. Prevention of Embryo Abortion:
Incompatibility barriers in interspecific and inter-generic hybridization programmes leading to embryo abortion can be successfully overcome by embryo rescue. In fact, many distant hybrids have been obtained through embryo rescue techniques. A selected list of distant plant species crossed and the resistance traits developed by employing embryo rescue technique is given in Table 47.2.
2. Overcoming Seed Dormancy:
Seed dormancy is caused by several factors—endogenous inhibitors, embryo immaturity, specific light and temperature requirements, dry storage requirements etc. Further, in some plants the natural period of seed dormancy itself is too long. Embryo culture is successfully applied to overcome seed dormancy, and to produce viable seedlings in these plant species.
3. Shortening of Breeding Cycle:
Some of the plants in their natural state have long breeding cycles. This is mostly due to seed dormancy attributed to seed coat and/or endosperm. The embryos can be excised and cultured in vitro to develop into plants within a short period. For instance, Hollies, a Christmas decoration plant can be grown in 2-3 weeks time through embryo cultures in contrast to 3 years period required through seed germination.
4. Production of Haploids:
Embryo culture has been successfully used for the production of haploid (or monoploid) plants e.g. barley.
5. Overcoming Seed Sterility:
Certain plant species produce sterile seeds that do not germinate e.g. early ripening varieties of cherry, apricot, and plum. Seed sterility is mostly associated with incomplete embryo development which leads to the death of the germinating embryo. Using embryo cultures, it is possible to raise seedlings from sterile seeds of early ripening fruits e.g. apricot, plum.
6. Clonal Propagation:
Embryos are ideally suited for in vitro clonal propagation. This is due to the fact that embryos are juvenile in nature with high regenerative potential. Further, it is possible to induce organogenesis and somatic embryogenesis from embryonic tissues.