Summary

Agar plays an important role in plant tissue culture, but choosing the right one can be confusing. This blog explains what agar is, how it works in tissue culture, and how it differs from the complete culture medium. We’ll also look at the different grades of agar, what makes tissue-culture-grade agar suitable, and the key factors to consider when choosing one. Finally, we’ll cover agar concentration and alternatives such as gellan gum.

Introduction

Move over hydroponics; the new star of the scene is tissue culture. Whether being carried out at home with DIY tissue culture sets or in large-scale grows, tissue culture is becoming a household technique amongst growing enthusiasts.

Also known as micropropagation, the tissue culture process is a technique that can help you grow uniform plants in quick succession. Although it was once primarily used amongst scientists in white coats at fluorescent labs, the tissue culture process is now readily used amongst the everyday avid cultivator. 

It is a basic technique and can be used for indoor grows as long as sufficient light is provided, along with a sterile environment and the right materials (such as nutrient media, Agar, and hormones).

In this article, we will discuss different types and grades of agar used in the tissue culture process, what makes tissue-culture-grade agar different, and what to look for when choosing agar for micropropagation.

What Is Agar and Why Is It Used in Tissue Culture? 

If you are seeking out the best agar, then you are most likely already aware that the tissue culture process is an umbrella term for a diverse range of techniques that rely on the same principle: to use a minute part of an existing plant (whether cells or tissues) to propagate other, new plants in a sterile and tightly controlled environment.

Most methods will need the following:

• A sterile container

• Controlled environment

• The correct medium

So, where does agar fit into the process?

Agar is a natural, jelly-like substance derived primarily from certain species of red algae. It is commonly used as a gelling agent, meaning it can turn a liquid culture medium into a firm or semi-solid gel.

In plant tissue culture, agar is added to the culture medium to provide physical support for the plant tissue or explant. Instead of growing in a completely liquid medium, the plant material can grow on or within a solidified medium while still receiving the nutrients and other components it needs.

This distinction is important: agar is the gelling agent, while the culture medium is the complete mixture used to grow the plant tissue.

The culture medium may contain:

• Mineral nutrients

• A carbon source such as sucrose

• Vitamins

• Plant growth regulators, such as auxins and cytokinins

• Water

• A gelling agent such as agar, when a solid medium is required

Auxins and cytokinins can influence processes such as rooting and shoot development, but they are components of the culture medium rather than ingredients that agar itself generally provides.

Combined, these components create a suitable environment for the plant tissue to develop, while agar provides the physical structure needed when a solid or semi-solid medium is used.

 

What Are the Different Types of Agar?

There are several different grades of agar, and they are produced for different applications. This is important because agar intended for food, microbiology, or other laboratory applications is not necessarily the same as agar selected for plant tissue culture.

Common grades include:

• Technical-grade agar

• Food-grade agar

• Tissue-culture-grade agar

• Bacteriological or microbiological-grade agar

• Pharmaceutical-grade agar

Technical-Grade Agar

Technical-grade agar is generally intended for industrial or technical applications where highly controlled biological culture conditions are not the primary requirement.

It may be suitable for applications such as thickening, binding, or other industrial processes, depending on the product specifications.

For plant tissue culture, however, the important question is not simply whether a product can form a gel. The agar also needs to provide suitable and consistent characteristics for culture media.

Food-Grade Agar

Food-grade agar is commonly used as a thickener and gelling agent in food products.

It can form a firm gel, but food-grade agar is manufactured for food applications rather than specifically for plant tissue culture.

In some laboratory applications, food-grade agar has been explored as a lower-cost alternative to laboratory-grade agar. However, differences in clarity and impurities can matter when working with cultures.

For serious plant tissue culture work, it is therefore worth looking specifically for a product intended for tissue culture rather than choosing an agar simply because it is inexpensive or readily available.

Tissue-Culture-Grade Agar

Tissue-culture-grade agar is specifically selected for use as a gelling agent in plant culture media.

This is generally the type of agar you should look for when preparing solid or semi-solid media for micropropagation.

The characteristics that matter include:

• Consistent gel strength

• Suitable purity

• Good clarity

• Consistent performance between batches

• Stability during preparation and sterilization

• Predictable performance at the recommended concentration

The goal is not simply to create a gel. The gel needs to provide a stable and predictable physical environment for the developing plant tissue.

Bacteriological or Microbiological-Grade Agar

Bacteriological or microbiological-grade agar is formulated primarily for growing and maintaining microorganisms in laboratory culture media.

It can have useful properties such as good clarity and consistent gelling, but its intended application is different from plant tissue culture.

That does not automatically make it unsuitable for every plant culture application, but if your goal is plant micropropagation, a product specifically intended for plant tissue culture is the more relevant choice.

Pharmaceutical-Grade Agar

Pharmaceutical-grade agar is produced for pharmaceutical and related applications where specific quality and purity requirements apply.

Its suitability for plant tissue culture should not be assumed simply because it has a high-grade designation. The important question is whether the product specifications and intended use make it appropriate for plant culture media.

What Makes Tissue-Culture-Grade Agar a Good Choice?

Agar has been used as an agent in tissue culture for several years due to its strength as a gel, its minimal mineral content, and its transparency. Developing plants in vitro requires that you follow a strict series of steps, including providing the developing culture with a suitable growing medium.

With the ever-increasing interest in cultivating plants, fruits, and flowers, the ease of the tissue culture process is being enjoyed by more and more at home growers and small-scale cultivators. And with the growing interest in the tissue culture process comes the rising curiosity in agar and DIY tissue culture kits.

Choosing agar for plant tissue culture is about more than finding a product that forms a gel.

Gel Strength

One of the most important properties is gel strength.

The agar needs to produce a medium firm enough to support the explant or developing plant tissue. The required firmness can vary depending on the plant species, culture stage, medium formulation, and protocol.

The concentration of agar also affects the firmness of the resulting medium. Higher concentrations generally produce firmer gels.

This means that a stronger gel is not automatically a better gel. What matters is whether the resulting medium has the appropriate physical properties for your particular culture.

Clarity

Agar is also valued for its clarity.

A relatively clear medium makes it easier to observe the developing culture, including roots, shoots, callus, and possible signs of contamination.

Clarity is not necessarily an indicator of better plant growth by itself, but it can make cultures easier to monitor.

Purity

The purity of agar can also matter.

Commercial agar products can vary in quality and purity, and these differences can affect the physical characteristics of the culture medium and, in some cases, plant growth.

This is one reason tissue-culture-grade agar is generally preferred over simply using whatever agar is easiest to purchase.

Consistency

For anyone doing repeated experiments or producing plants on a larger scale, batch-to-batch consistency matters.

If the physical properties of your agar change from one batch to another, the resulting culture medium can also behave differently.

A consistent product helps reduce one source of variation in your tissue culture process.

Performance During Sterilization

Culture media are commonly sterilized before the plant material is introduced.

The agar therefore needs to maintain its useful gelling properties through the preparation and sterilization process.

Agar is widely used in tissue culture partly because of its stability and suitable gelling characteristics.

Agar Concentration Also Matters

Even the right type of agar can produce different results depending on how much is used.

The concentration required depends on the agar itself and the culture protocol. Agar is commonly used around 0.6–0.8% (w/v) in plant tissue culture, although protocols vary.

Too little gelling agent can produce a soft medium that does not provide sufficient support. Increasing the concentration generally increases gel firmness, but a firmer medium is not automatically better for every plant or culture stage.

For this reason, it is best to follow the manufacturer's recommended concentration and the requirements of the specific tissue culture protocol.

Agar vs. Other Gelling Agents

Agar is not the only option available for solidifying plant tissue culture media.

Gellan gum is another commonly used gelling agent in plant tissue culture. It can produce clear, firm gels at lower concentrations than agar and has been used as an alternative in a range of plant culture systems.

Other alternatives have also been investigated, including different starches and plant-derived materials, particularly where reducing media costs is important.

The choice between agar and another gelling agent depends on the plant, culture system, medium formulation, desired gel characteristics, and protocol.

Where does Agar come from?

Agar comes from seaweed or red algae, and is not only used as a gelling agent in the tissue culture process, but also throughout other industries too. This dextrous substance has been utilized in Biotechnology, pharmaceuticals, dentistry, microbiology, and even cosmetics due to the following characteristics:

• Thickener
• Stabilizer
• Gelling agent

As we already mentioned, there are several different types, or grades, of Agar. These grades are:

• Technical
• Food
• Tissue culture
• Bacterial
• Pharmaceutical

As the name implies, you will need to use Tissue Culture grade Agar for your micropropagation.

Why is Agar so popular?

Agar is a favorite growing medium due to the following characteristics:

• It is currently the strongest known natural gelling agent
• It can withstand the temperatures required for sterilization purposes
• It is a thermo-reversible gel
• When it is mixed with H2O, it melts at temperatures from 60 - 100 Celsius
• It becomes solid at 45 degrees Celsius
• It remains a stable gel at incubation temperatures
• It remains unchanged or unaffected by other constituents in the process
• Plant enzymes do not digest agar

If you are curious about tissue culture, why not give it a go with a basic DIY tissue culture set?

By using a kit from a trusted company like Plant Cell Technology, you can get a kickstart as you begin to develop your culturing skills. Some packages will have everything included - beakers, flasks, and other equipment, while others will provide specific supplies of like tissue culture media such as agar.

Explore the full catalogue of Plant Cell Technology's tissue culture products here!

Resources

• https://www.intechopen.com/books/recent-advances-i...
• https://www.researchgate.net/post/What_ts_differen...