Somatic embryogenesis for plant tissue culture

Did you know that you can develop artificial or synthetic plant seeds using tissue culture?

The process where you can actually start the preparation for developing such seeds is 'somatic embryogenesis'.

There are different methods available for developing uniform plants using plant tissue culture. You can read more about them in our article on "7 methods of plant tissue culture". Most methods should ideally perform well for most plant species, however, certain plant species react well only to specific methods. This also depends on other factors like the type of explant and at what stage that explant was extracted. For many plant species like coffee, tea, eucalyptus, etc. somatic embryogenesis provides better plants as compared to other methods.

Before we delve deeper into this method, let us first discuss the basic terms that you will notice here:

What is an embryo?

The embryo is a part inside a plant seed that contains the undeveloped forms of that plant's roots, stem, and leaves. The embryo develops after the process of fertilization takes place in the flowers. The seed is the starting point for the plant as the embryo activates in the right conditions and eventually becomes a seedling.

What is fertilization?

Fertilization is a process where the pollen (male part of the flower) fuses with the ovule (female part of the flower) to form an embryo. This is an important step in plant sexual reproduction. When fertilization happens, the fused cells form a 'zygote'. This zygote undergoes constant cell division and finally forms an embryo. We call this process of developing an embryo 'embryogenesis'.

What is parthenogenesis?

When we talk about somatic embryogenesis, then we should also be aware of the process called 'parthenogenesis'. It is a natural process occurring for some plants such as pineapples, bananas, etc. Here the fruit is formed without going through fertilization. The female part of the flower directly converts into a fruit. As a result, the fruit developed is seedless.

In different researches, it has been observed that tissues that are part of the embryo sac, or those surrounding it, can also develop into an embryo. However, particular environmental conditions, which exist inside the embryo, are necessary for the development. This idea led to the development of the somatic embryogenesis method using the same principle.

What is somatic embryogenesis (SE)?

We can define SE as 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. The key to remember for you is that this method's success depends on the composition of the culture medium.

Four important steps in SE:

• Induction of pre-embryogenic callus;
• Maintenance of pre-embryogenic callus;
• Development of embryogenic cultures; and
• Regeneration of embryogenic cultures.

Plant regeneration via SE occurs in five steps:

• Initiation of embryogenic cultures;
• The proliferation of embryogenic cultures;
• Prematuration of somatic embryos;
• Maturation of somatic embryos; and
• Plant development on nonspecific media.

Initiation and proliferation occur on a medium rich in auxin, which induces differentiation of meristematic cells. For this purpose, 2,4-D is the common auxin in use. Once transferred to a medium with low or no auxin, these cells can then develop into mature embryos. Germination of the somatic embryo can only occur when it is mature enough to have functional root and shoot apices 

What are somatic tissues?

By now, you might be wondering, what are somatic tissues?

Somatic tissues are all the tissues other than the anthers (male reproductive cell) and the ovules (female reproductive part) in a plant. In SE, we use either cells from different parts of a plant or cells surrounding anther/ovule such as styles, nucellus, etc. which are not directly involved in embryo formation in nature. The embryos developed using these tissues do not possess seed coats. We can use different gelling agents to create a similar seed coat. This process leads to the formation of synthetic seeds. We will discuss more on this in upcoming articles!

Two ways to induce SE

Direct somatic embryogenesis: In this process, the embryo is developed directly from a cell or small group of cells such as nucellus, styles, or pollen without any intermediate callus stage. This method is rarely used as the survival rate of the produced plants is low.

Indirect somatic embryogenesis: In this process, the development of an embryo occurs with an intermediate callus stage. So, it is a multistep process. Firstly, callus is formed from explant and then these callus tissues are used for producing embryos. These embryos later form whole plantlets. This method is followed for many plant species such as tea, coffee, etc. which are generally difficult to propagate via other plant tissue culture methods.

Advantages of SE

Let us now see why this method is becoming popular:

• This method can lead to higher propagation and maximizing output;
• It is suitable in suspension culture and also for large scale productions involving bioreactors;
• Production of artificial seeds;
• This method is useful for germplasm conservation. Germplasm can be stored in callus form for longer durations;
• Seedlings produced from this method are virus-free; and
• This method permits easy scale-up and subculture with low labor inputs.

Limitations of SE

There are certain limitations to this method as well:

• Response of SE is tissue specific. It means the success depends on the type of explant you choose to work with. Only certain explants would produce expected results;
• There is a high chance of low frequency of embryo production;
• Many times there will be incomplete embryo production;
• There is a high probability that this method leads to unwanted genetic variation;
• In some plant species, this method is unable to generate large numbers of normal, free-living plantlets; and
• In some plant species, the plants developed by this method might be weaker.

We hope you got a glimpse of what somatic embryogenesis is and why it is one of the interesting methods for plant regeneration in the plant tissue culture industry. For more informational posts on different methods of plant tissue culture, keep checking this space!

By Nancy Bhatia | 7-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.
• https://sciencing.com/what-plant-embryo-4601843.html