Gelling agents and their role in plant tissue culture

Gelling agents are an important ingredient in the media formulations for anyone working with plant tissue culture. Why is that? The plant tissue culture process is mostly carried out in a solid or semi-solid nutrient media. Acting like soil, a solid nutrient medium provides physical support for the culture to maintain contact with air for respiration.

However, the role of gelling agents does not finish here! They influence important characteristics of the culture medium such as gel strength. This parameter refers to the force that must be applied to cause a gel to break. It is key for plant differentiation and development. Each gelling agent has a different level of gel strength. For this reason, the type of gelling agent you use can influence the growth of the tissue in culture. For example, in Pinus species, high gel strength enhances somatic embryo development.

There is a wide range of gelling agents available for you in the market from xanthan gum to isubgol, and not all of them offer the same result.

If you want to know more about gelling agents, this article is for you!

What is a gelling agent?

Gelling agents (GA) are polysaccharides, which are large molecules made of simple sugars like glucose. They have the ability to form gels and as a result provide semi-solid or solid surface for plants to develop. You can also call them solidifiers. They help you provide a three-dimensional structural network for the growth of your plantlets.

What can go wrong during the gelling process?

Each gelling agent has a unique and efficient preparation protocol. Yet you may encounter two common problems during the gelling process. The first problem is ‘clumping’. Here the gelling agent particles form a clump that is hard to penetrate and does not mix uniformly with the medium. To avoid this problem, here are some alternatives for you to try:

1. Stir the gelling agent into the medium while using a sieve; and
2. Use a wetting agent such as glycerin to minimize clump formation. A wetting agent helps with the separation of the particles to prevent the clumps.

The second problem is ‘hyperhydricity’ (previously known as vitrification). This is a physiological disorder that results in excessive hydration, impaired stomatal function, and reduced mechanical strength of tissue cultured plants. The consequence is that the shoots and leaves become brittle.

You can reduce hyperhydricity by increasing the concentration of the gelling agent. GAs naturally reduce hyperhydricity by allowing contact of the cultured tissues with the air.

Why are gelling agents needed?

Do you always need to use a gelling agent while doing plant tissue culture? The answer is no.

You can decide whether it is wise to use a liquid or a solid medium depending on the type of experiment, the plant species, and the explant type. However, when you choose a liquid culture, you face the risk that your explants would submerge and die due to oxygen deprivation or hyperhydration.

Gelling agents make the medium-firm and influence some of its essential characteristics such as diffusion. The diffusion rate refers to the change in the amount of molecules/nutrients moving through the medium.

Why is the diffusion rate important? Because this parameter determines the availability of oxygen and water to the explants. You should note that the diffusion is determined by medium viscosity. This, in turn, depends on the physical and chemical characteristics as well as the concentration of the gelling agent. Besides this, you should also consider factors like pH, sterilizing method, age of the medium, and the storage conditions. These factors have a significant impact on the growth of your explants.

Types of gelling agents

Let us now discuss some characteristics of the commonly used gelling agents:  

Isubgol: This polysaccharide is derived from the husk of Plantago ovata seeds. It has the resistance for a plant’s enzymatic activity as well as good gelling ability. With isubgol, you do not need to worry about gel cracking, a frequent problem with agar and gellan gum. It is biodegradable, non-toxic, and economical.

Agar: This GA is obtained from the red algae family Rhodophyceae. It is the most commonly used GA due to its convenient gelling properties such as high stability, transparency, resistance to metabolic activities during culture, and non-toxicity. You can find different commercial agar products in the market

They differ on the basis of:

1. The level of impurities; and
2. Composition.

Carrageenan and alginate are also algae-derived gelling agents and are cheaper than agar. However, their gelling ability is dependent on the presence of specific ions, thus, their application is limited.

Also, agarose obtained from agar purification has superior gel strength and transparency. But it is only used when high gel strength is required (e.g., protoplast cultures) due to its high cost.

Gellan gum: It is produced by bacteria and commercialized under names such as ‘Gelrite’ and ‘Phytagel’. These products do not contain any contaminating substances. With this you can obtain a high-strength colorless gel by just using a small amount. As the medium is transparent, it is easier for you to identify contaminations.

Xanthan gum is also produced by bacteria and is popular for its viscous properties over a wide temperature range.

Other gelling agents are cellulose, carbomer, guar gum, and cassava starch.

What gelling agent to choose?

You may be wondering, among the different available GAs, which one to choose? In the case of gelling agents, there is no such thing as the “best” gelling agent. Only the one that best suits your specific requirements. Few aspects to keep in mind while choosing a gelling agent are:

1. Level of transparency;
2. Composition (it directly affects plant growth by reacting with media constituents);
3. Level of impurities and gel strength (may vary among manufacturers);
4. Ability to retain moisture;
5. Plant species; and
6. Tissue culture method you are planning to perform.

Also, it is interesting to know that:

1. The same GA at different concentrations has a profound influence on water retention and regulation of the moisture regime of the medium. For this, you need to use the correct concentration to meet the requirements of your plant tissue culture stage.
2. The solubility of the gelling agent also depends on the temperature of the medium. Therefore, it is beneficial to consider whether the GA of your choice dissolves better in hot or cold water.
3. Some gelling agents may require you to adjust the pH using a chemical or a neutralizer, e.g., carbomer.

We hope this article can serve you as a guide when choosing a gelling agent! If you are interested in acquiring a gelling agent, you can look for different options on our webshop.

By Valeria Franco Franklin | 13-December-2021

About the author

Valeria Franco is from Colombia, the land of orchids. She is a focused and passionate biologist who specializes in biotechnology and molecular biology. Valeria has prior laboratory and research experience. She is presently employed as a content creator at Lab Associates and is always looking for new challenges. Valeria is enthusiastic in plant science themes and reading as a tool for lifelong learning. Her hobbies include studying foreign languages, traveling, and archery.

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