🌱 Rethinking Plant Tissue Culture: Why Species-Specific Conditions Matter
Plant tissue culture has come a long way.
From early experiments with isolated plant cells, it has developed into an important technology for plant micropropagation, breeding, conservation, biotechnology, and commercial plant production.
Yet after decades of research, one challenge remains surprisingly common:
Why do some plants still refuse to regenerate efficiently in vitro?
These plants are often described as “recalcitrant” species.
But is the plant really the problem?
A recent viewpoint published in the International Journal of Plant Biology takes a closer look at this question and challenges some long-standing assumptions about in vitro plant regeneration.
🔬 What Does “Recalcitrant” Really Mean?
Not all plants respond to tissue culture in the same way.
Some species readily produce shoots and roots under commonly used conditions. Others may produce excessive callus, weak shoots, poor roots, or fail to regenerate altogether.
These plants are commonly classified as recalcitrant.
However, being difficult to regenerate does not necessarily mean that a plant has little or no regenerative potential.
Plants have remarkable developmental plasticity. Under suitable conditions, certain cells can change their developmental state, divide, differentiate, and eventually contribute to the formation of a complete plant.
The important question may therefore be:
Is the plant truly incapable of regeneration, or have we simply not yet identified the conditions it requires?
This changes the way difficult tissue culture systems should be approached.
Instead of asking only:
“Which standard protocol should we use?”
it may be more useful to ask:
“What does this particular plant actually require for regeneration?”
🧬 Auxin Is More Complicated Than a Simple Hormone Ratio
The balance between auxin and cytokinin has played a fundamental role in plant tissue culture.
Classic research showed that changing the relative levels of these plant growth regulators could influence whether cultured tissues developed roots, shoots, or other forms of growth.
This principle remains extremely useful.
However, plant regeneration is considerably more complex than simply adjusting the concentration of hormones added to a culture medium.
Plants do not passively receive hormones from the medium.
They also:
- produce hormones internally
- transport them between cells
- modify and metabolize them
- regulate their concentration
- respond differently according to cell type and developmental stage
This is particularly important for auxin.
🧪 Why Endogenous Auxin Matters
Auxin plays a central role in plant development and regeneration.
But its biological effect depends not only on how much auxin is present, but also on:
where it is produced → how it is transported → where it accumulates → how the cell responds
Plants possess multiple pathways for endogenous auxin biosynthesis, and these pathways can contribute differently depending on the tissue, developmental stage, and biological process.
Auxin transport also helps establish local concentrations and gradients within plant tissues. These gradients can influence cell division, differentiation, root formation, and shoot development.
⚠️ More auxin does not automatically mean more regeneration.
Simply increasing the amount of auxin added to a culture medium may not produce the expected result.
The more fundamental questions are:
Where is auxin being produced?
How is it being transported?
How do individual cells interpret the auxin signal?
This represents an important shift from a simple auxin-to-cytokinin ratio toward a more complete understanding of plant regeneration.
🧬 A Plant Cell's Regenerative Ability Is Not Always “On”
Another important issue is the regenerative competence of individual cells.
Plants are often described as having cellular totipotency—the potential for a cell to give rise to an entire plant.
In practice, however, regenerative capacity is not equally expressed by every cell under every condition.
The biological state of the cell matters.
One important factor is epigenetic regulation.
Before a cell can follow a new developmental pathway, changes in gene expression must occur. These changes can involve chromatin remodeling and other forms of epigenetic reprogramming.
In simple terms:
A cell may contain all the genetic information required for regeneration without having the necessary genes activated at the right time.
This helps explain why different:
- explant types
- tissues
- developmental stages
- genotypes
- culture conditions
can respond very differently to exactly the same culture medium.
⚗️ MS Medium: Powerful, but Not Universal
Murashige and Skoog (MS) medium is one of the most widely used media in plant tissue culture.
Its importance is unquestionable. MS medium remains highly effective for many species and applications.
But widespread use does not mean that it is universally optimal.
A culture medium provides much more than basic minerals.
The balance of nutrients can influence:
- plant growth
- hormone metabolism
- cell differentiation
- callus formation
- shoot development
- root development
- regeneration efficiency
An inappropriate nutrient balance may encourage excessive vegetative growth or callus formation rather than efficient organogenesis.
🌱 More biomass is not necessarily better regeneration.
For commercial micropropagation, the objective is not simply to make tissue grow faster.
The goal is to produce:
healthy + uniform + physiologically competent plants
efficiently and consistently.
Therefore, optimizing the nutritional composition of the culture medium for a particular species can be just as important as adjusting plant growth regulators.
🌿 Growth Is Not the Same as Regeneration
This distinction is easy to overlook.
A culture producing large leaves and rapidly expanding tissues may appear successful.
But if the objective is multiplication, excessive vegetative growth may not be desirable.
Likewise, rapidly growing callus is not automatically a sign of successful regeneration.
The real question is:
Is the culture producing the type of development that we actually want?
Successful in vitro regeneration therefore depends on the interaction between several factors:
| Factor | Possible influence |
|---|---|
| 🧪 Nutrients | Growth and developmental balance |
| 🧬 Plant hormones | Cell division and organ formation |
| 🌱 Explant | Regenerative competence |
| 💡 Environment | Development and physiology |
| 🧫 Culture medium | Chemical and physical environment |
| 🧬 Cellular state | Gene expression and regeneration potential |
Plant regeneration is a system, not a single variable.
💧 Hyperhydricity: When “More Water” Is Not Better
Another persistent problem in plant tissue culture is hyperhydricity, also known as vitrification.
Hyperhydric tissues typically contain excessive water and may become:
- translucent
- fragile
- unusually soft
- poorly adapted to external conditions
This can reduce plant quality and make acclimatization more difficult.
Hyperhydricity is another example of why faster growth is not necessarily better growth.
The objective of an effective in vitro system is not simply to maximize biomass.
It is to create conditions that support:
appropriate morphology + physiological development + successful acclimatization
🧫 The Physical Culture Environment Also Matters
When discussing plant tissue culture, most attention naturally goes to nutrients and plant growth regulators.
But there is another part of the system that deserves attention:
The physical environment created by the culture medium.
In semi-solid culture systems, the gelling agent creates the matrix in which explants, shoots, and roots develop.
Agar has traditionally been one of the most widely used gelling agents.
Gellan gum is another option used in plant tissue culture media.
Gellan gum can form a clear, firm gel and provide a consistent physical matrix for in vitro culture.
This makes it relevant when researchers are considering not only the chemical composition of a culture medium, but also its physical characteristics.
⚠️ Gellan gum is one component of the overall culture system, and its role should be considered alongside other factors that influence plant regeneration.
Changing the gelling agent alone cannot solve problems caused by:
- unsuitable nutrients
- inappropriate hormone conditions
- poor explant selection
- unsuitable environmental conditions
- limited cellular competence
Instead, gellan gum should be viewed as one component of the overall culture system.
As plant tissue culture moves toward more precise optimization, the physical properties of the medium—including gel structure and consistency—can be considered alongside its chemical composition.
🔄 From Standard Recipes to Species-Specific Systems
The message is not that traditional plant tissue culture methods are no longer useful.
They are.
MS medium, plant growth regulators, agar-based media, and established micropropagation protocols have supported decades of successful research and commercial production.
The problem arises when these tools are treated as universal recipes.
A protocol that works well for one species may perform poorly in another.
Even within the same species, responses can vary according to:
- genotype
- explant type
- developmental stage
- physiological condition
- culture environment
The future of plant tissue culture is therefore likely to become increasingly species-specific and data-driven.
🔎 What needs closer investigation?
Researchers may need to consider a much wider range of factors, including:
Endogenous auxin biosynthesis
Understanding where and how auxin is produced.
Hormone transport and distribution
Understanding how signals move through tissues.
Epigenetic regulation
Understanding how cells become competent for regeneration.
Nutrient requirements
Developing media according to the needs of individual species.
Explant competence
Selecting tissues with the appropriate regenerative potential.
Physical culture conditions
Considering the structure and properties of the culture environment.
Hyperhydricity
Understanding and controlling abnormal water accumulation and tissue development.
🚀 Where Is Plant Tissue Culture Heading?
The long history of plant tissue culture has produced an enormous collection of protocols.
But having more protocols does not necessarily mean that we fully understand why they work.
The next stage may require a shift from:
Protocol-driven experimentation
to
Mechanism-driven optimization
Instead of assuming that a plant is recalcitrant because a standard medium does not work, researchers can investigate the biological reasons behind the response.
Instead of focusing only on the amount of auxin and cytokinin added to the medium, endogenous hormone production and transport can also be considered.
Instead of assuming that MS medium is optimal for every application, nutrient requirements can be investigated according to species and developmental objectives.
And instead of viewing a gelling agent simply as something that makes a medium solid, its contribution to the physical culture environment can also be considered.
Modern technologies such as single-cell RNA sequencing and spatial transcriptomics may further improve our understanding of how individual cells respond during regeneration.
These approaches could help researchers identify which cells are capable of regeneration and what molecular processes allow them to change their developmental fate.
🌱 Conclusion: Rethinking “Difficult” Plants
Plant tissue culture has become an essential technology for modern plant biotechnology and commercial micropropagation.
Yet many of its long-standing challenges remain.
The key lesson is that plant regeneration is more complex than simply adjusting hormone concentrations in a standard culture medium.
Endogenous auxin biosynthesis, hormone transport, epigenetic regulation, nutrient balance, cellular competence, environmental conditions, and the physical culture environment can all influence the final result.
This means the concept of a “recalcitrant” plant deserves closer examination.
In some cases, poor regeneration may reflect the limitations of the culture system rather than an absolute limitation of the plant itself.
The future of in vitro plant tissue culture may not be about finding one better universal recipe.
It may be about understanding what each plant—and each cell—actually needs.
Within this broader approach, the physical properties of the culture medium also deserve consideration.
Gellan gum is one option for providing a defined gel matrix in plant tissue culture, and its characteristics can be evaluated alongside nutrients, plant growth regulators, explant selection, and other culture conditions when developing a consistent and reproducible in vitro system.
📚 Reference
Pasternak, T.; Steinmacher, D. Plant Tissue Culture In Vitro: A Long Journey with Lingering Challenges. International Journal of Plant Biology, 2025, 16(3), 97. DOI: 10.3390/ijpb16030097.
