Factors affecting somatic embryogenesis

Factors affecting somatic embryogenesis

Somatic embryogenesis is a useful technique to develop plants. But, did you know about the factors that affect the success of this method?

Somatic embryogenesis is a process in which embryo-like structures are produced using somatic tissues. These embryo-like structures then develop into a whole plant using different culture mediums.

If you are new to this topic, then it might be interesting for you to know more about somatic embryogenesis. We have discussed it briefly in our article on "Somatic embryogenesis for plant tissue culture".

In plant tissue culture most methods perform well for most plant species, however, certain plant species react better to specific methods. Have you ever wondered why this happens? Or what are the factors that actually influence the success rate of a specific method?

Let us discuss today different factors that affect somatic embryogenesis:

Plant growth regulators

Auxin is an important PGR for this method. Auxin generally promotes cell enlargement, adventitious bud formation, and root initiation. They are also responsible for apical dominance and suppression of lateral bud formation. When it is present in the media for somatic embryogenesis, it promotes callus proliferation and inhibits the differentiation of callus cells into specific plant organs.

However, for obtaining mature embryos, auxin is not added to the media. The concentration of auxin decides the pace at which callus is going to develop further. According to scientists, auxin initiates somatic embryogenesis by inducing stress response in plant cells.

Cytokinin is an important growth regulator as well for tissue culture. It induces cell division in the explants. It also helps in delaying the aging process (also called 'senescence') and influences the transport of auxins. The most commonly used cytokinin for inducing somatic embryogenesis is 'TDZ (thidiazuron)' as it can help with in vitro shoot regeneration during somatic embryogenesis.

Nature of explant, explant genotype and culture conditions

The key point here is that several explants can be used for somatic embryogenesis. However, the correct development stage in which an explant is derived from the mother plant determines the progress in the initiation of embryogenic callus. According to scientists, it is always good to choose young or juvenile explants as they yield more somatic embryos than older explants.

Why is the type of explant tissue important? It is because different explant tissues from the same mother plant produce embryogenic cells at different frequencies. They would even require different concentrations of growth regulators for the induction of somatic embryos. Thus, the type and age of the explant play an important role in somatic embryogenesis.

Apart from these, one of the important factors to keep in mind is the genotype. Not all genotypes react to somatic embryogenesis. For example, in the case of hybrid tea roses, somatic embryogenesis could be induced only in two cultivars out of five.

Light and activated charcoal

For efficient somatic embryogenesis, it has been recommended to store calluses in reduced light intensity or in darkness. You need to do this to reduce the production of inhibitory compounds such as phenolic acids, etc. from tissues in the culture medium. These compounds lead to the oxidation of tissues in a culture which in turn causes tissue browning. When the tissues turn brown, they inhibit the activities of various important proteins, slowing down or inhibiting somatic embryo development. Thus continuous lighting is harmful to develop healthy plantlets.

However, activated charcoal can be used in this case to make the embryo development process more efficient. It has the potential to absorb the embryogenesis inhibitors and efficiently remove them from the culture medium.

Other biochemical factors

Certain biochemicals such as amino acids, glutamine, proline, and tryptophan show profound effect on somatic embryogenesis. We can also call them enhancers of somatic embryogenesis. They play specific roles in cellular developments during the development of somatic embryos which leads to better results.

Some scientists also suggest that organic extracts like coconut water, potato extract, and so on, are essential for somatic embryogenesis in some plant species. For instance in papaya, the use of potato extract contributes to the development of more stable somatic embryos.

Embryo maturation

It is believed that the plant recovery rate using mature embryos is high. Thus, maturation is a crucial stage of somatic embryogenesis. For obtaining somatic embryos, we need to follow sequential steps such as embryogenic callus initiation from vegetative cells, maintenance and development of embryogenic cell lines, somatic embryo formation, maturation, and finally conversion/germination of somatic embryos into viable plants.

Accumulation of carbohydrates, lipids, and protein reserves as well as a decrease in cellular respiration and embryo dehydration leads to maturation of the embryo. This stage is also crucial for effective germination which will further lead to healthy plantlets.

What are the uses of somatic embryogenesis?

Let us briefly discuss some important applications/uses of somatic embryogenesis:

  • This method is useful for the large-scale regeneration of plants using single selected cells.
  • This can play a key role in developing a somatic hybrid using breeding techniques along with tissue culture.
  • This method is useful for germplasm conservation. Germplasm can be stored in callus form for longer durations.
  • Using this method, we can obtain virus-free plants.
  • This technology is highly recommendable for extracting/producing secondary metabolites in important plant species. For instance, caffeine and theobromine from somatic embryos of cacao.
  • It is useful for homozygous or polyploid line production. Somatic embryogenesis has been used for the regeneration of triploid plant species like citrus, prunus, etc. using endosperm as explants for the plant.

We hope this article gives you a better view of different aspects of somatic embryogenesis. It is a method that goes from tissue to embryo stage and then transfers back to form a whole plant. Therefore, it is important to understand the importance of different factors that play a crucial role in it.

For more informational posts on different methods of plant tissue culture, keep checking this space!

By Nancy Bhatia | 13-July-2021

References

  • Bajaj, Y. P. S. (1995). Somatic Embryogenesis and Its Applications for Crop Improvement. Somatic Embryogenesis and Synthetic Seed I, 105–125. DOI:10.1007/978-3-662-03091-2_8.
  • Bhojwani S. S. and Razdan M. K. (1983). Plant Tissue Culture: Theory and Practice. Elsevier publications.
  • Hussein, S., Ibrahim, R., Ling Pick Kiong, A. (2006). Somatic Embryogenesis: An Alternative Method for in vitro Micropropagation. Iranian Journal of Biotechnology, 4(3), 156-161.
  • Zimmerman Lynn J. (1993). Somatic Embryogenesis: A Model for Early Development in Higher Plants. The Plant Cell, 5, 1411-1423.