14 May 2024

Exploring Somatic Embryogenesis: Techniques and Applications

Anjali Singh, MS

As a content and community manager, I leverage my expertise in plant biotechnology, passion for tissue culture, and writing skills to create compelling articles, simplifying intricate scientific concepts, and address your inquiries. As a dedicated science communicator, I strive to spark curiosity and foster a love for science in my audience.

Table of Contents

What Is Somatic Embryogenesis?

Before we talk about somatic embryogenesis, do you know the difference between somatic cells and zygotic cells?

As you can see, typically, zygotic cells develop into embryos, and somatic cells perform the other essential roles in plants. 

However, somatic cells can be converted into embryos for plant development using the tissue culture technique. The process is known as somatic embryogenesis. 

You can define the process as:

 “Somatic embryogenesis is a process where a somatic cell transforms into a totipotent embryonic stem cell, capable of producing an embryo under right conditions.

This article further explores somatic embryogenesis, the steps involved, its applications, and the factors affecting the process. 

A Review on Somatic Embryogenesis

Somatic embryogenesis (SE) constitutes an in vitro developmental pathway where embryos with the potential to mature into whole plants are derived from somatic cells, bypassing gamete fusion. 

While natural SE events have been recorded, this process is typically induced using cultured plant cells and tissues. The success of SE induction hinges on the application of specific hormonal modulators and the imposition of controlled stress conditions.  

Following successful induction, the resulting somatic embryos restate the same morphogenetic program as zygotic embryos during their subsequent development.

These stages have distinct shapes, depending on the plant type. For example, dicotyledonous plants (plants with two seed leaves) go through globular, heart-shaped, torpedo-shaped, and cotyledonary stages. 

Monocotyledonous plants (plants with one seed leaf) have globular, scutellar, and coleoptile stages. Once somatic embryos reach the cotyledonary stage, they develop a shoot meristem, which is the starting point for shoot growth, and begin to grow like seedlings.

Types of Somatic Embryogenesis

Somatic embryogenesis (SE) manifests in two distinct developmental pathways within the plant kingdom:

Direct Embryogenesis

This pathway offers a fascinating example of cellular reprogramming. Initially, isolated somatic cells derived from leaves or stems are cultured under precisely controlled in vitro conditions, with specific hormonal and nutritional cues.

Under these defined conditions, the transdifferentiation of somatic cells can be triggered, letting them to regain a degree of totipotency. This newly found developmental plasticity allows the reprogrammed cells to initiate the embryogenic program, ultimately leading to the formation of a complete embryo.

While offering a more efficient route to SE, direct embryogenesis is unfortunately less frequent and achieving it can be highly dependent on the specific plant species and the chosen cell type.

Indirect Embryogenesis

This more commonly observed pathway involves a multi-stage process. The initial step involves the induction of somatic embryogenesis by prompting the isolated somatic cells to undergo a transformation into a callus. 

This callus represents a mass of undifferentiated, proliferating cells. Specific plant growth regulators and optimized culture conditions are crucial for stimulating this initial callus formation. 

Subsequently, under further manipulation of the in vitro environment, involving adjustments to hormonal composition and potentially exposure to stress factors, somatic embryos can then develop from this callus tissue. 

While offering a broader applicability across diverse plant species, indirect embryogenesis necessitates careful optimization of the culture conditions at each stage to ensure successful embryo formation.

Factors Affecting The Process Of Somatic Embryogenesis

Somatic embryogenesis, a three-step process for growing plant embryos in a lab, can be finicky.  Several factors significantly impact the final stage: embryo maturation.  Let's explore some of the key players:

Explant Choice: The starting material, called an explant, significantly influences success. For most plants, immature zygotic embryos (early embryos from seeds) work best. However, some species have exceptions. For example, soybean cotyledons (seed leaves) and alfalfa leaf sections are better starting tissues than immature embryos for those plants.

Growth Regulators: Specific plant hormones play a crucial role. Auxins, for example, are essential for initiating embryo development in most studied plants. However, some researchers have found success using high sucrose concentrations or varying the culture medium's pH to trigger embryo formation without auxins (like increased pH promoting carrot explant proliferation). Cytokinins, known for promoting cell division, often inhibit embryo formation. But in some cases, like coffee, cytokinins alone can trigger the process.

Plant Genetics: A plant's inherent genetic makeup (genotype) plays a role. Scientists suspect this is linked to hormone levels within the plant. For instance, most alfalfa cultivars have a low regeneration rate (around 10%), but the Rangelander variety boasts a much higher success rate. Similar variations exist in rice, where only 19 out of 500 tested varieties showed a strong response to somatic embryogenesis.

Polyamines: These are naturally occurring molecules that may influence the process. While research suggests polyamine concentrations are higher in plants that naturally produce multiple embryos (polyembryonic), a clear link between polyamine levels and triggering somatic embryogenesis hasn't been established yet.

Nitrogen Source: The type of nitrogen in the culture medium matters: carrots need reduced nitrogen sources like potassium nitrate, while alfalfa and orchardgrass require ammonia. Adding specific amino acids such as proline and serine/threonine can enhance the embryogenic response in many plants.

Applications of Somatic Embryogenesis

Given below are some well-known applications of somatic embryogenesis:

For Research Studies

In woody plants, the phenomenon of secondary somatic embryogenesis ensures that cultures retain their capacity to generate embryos for prolonged durations. This persistence of embryogenic competence not only sustains a continuous supply of valuable research material but also facilitates ongoing investigations into various aspects of plant development and biotechnology.

For Woody Plant Micropropagation

Somatic embryogenesis offers a valuable tool for propagating woody plants. These plants typically live for a long time and are difficult to obtain using traditional methods. In a lab setting (tissue cultures), somatic embryogenesis allows plants to regenerate through embryo formation. This method has several advantages over another technique called organogenesis.

Here's why somatic embryogenesis might be preferred:

• Single-cell origin: Somatic embryos can potentially come from a single cell, which can help maintain genetic consistency.
• Large-scale production: Somatic embryos can potentially be produced in large quantities using special containers called bioreactors, making it easier to grow many plants at once. These embryos can then be used for sowing directly in the field, similar to synthetic seeds.
• Direct plantlet development: Somatic embryos have both a shoot and a root, letting them to develop directly into plantlets without needing a separate rooting stage, unlike plants regenerated through organogenesis.
• Reduced risk of genetic variation: Since somatic embryos can potentially originate from a single cell, they might be less likely to have genetic variations compared to plants regenerated through other methods. This can be beneficial for ensuring consistent plant traits, especially when using genetic modification techniques.

Cryopreservation

Somatic embryos from plants can be frozen for long-term storage using a technique called cryopreservation. This method involves keeping them in liquid nitrogen at a very cold temperature, around -196°C.  Cryopreservation is particularly useful for preserving somatic embryos of woody plants, which tend to have longer life length.

Recently, scientists have developed new and improved cryopreservation methods. Additionally, techniques like encapsulation-dehydration and encapsulation-vitrification, similar to those used for creating synthetic seeds, have emerged. These progresses have enabled researchers to successfully cryopreserve somatic embryos from various woody plant species, including cork oak, citrus trees, grapevines, and cassava.

Figure: Schematic illustration of somatic embryogenesis in larch (Larix spp.) and the practical uses of somatic embryos.

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