Callus
Culture
Callus
culture is a fundamental technique in plant tissue culture, involving the
growth of undifferentiated plant cell masses in a controlled, artificial
environment. Here's a more in-depth look:
What is Callus?
A callus is an unorganized, proliferating mass of parenchyma cells. It forms when plant tissues are subjected to certain stimuli, particularly in response to wounding or when exposed to specific plant growth regulators. In essence, it's a mass of plant cells that have "dedifferentiated," meaning they've lost their specialized functions and reverted to a more basic, dividing state.
The
Callus Culture Process:
- Explant
Selection:
- The
process begins with selecting a suitable plant tissue, known as an
explant. This can be a piece of leaf, stem, root, or other plant part.
- Sterilization:
- Strict
sterilization is crucial to prevent contamination by microorganisms. The
explant is thoroughly sterilized to eliminate any bacteria or fungi.
- Culture
Medium:
- The
explant is placed on a nutrient-rich culture medium. This medium
typically contains:
- Essential mineral salts.
- Vitamins.
- Sugars (as an energy source).
- Plant growth regulators (hormones), such as auxins and cytokinins, which are critical for inducing callus formation.
- Incubation:
- The
cultures are incubated under controlled environmental conditions,
including temperature, light, and humidity.
- Callus
Formation:
- Over
time, the explant cells begin to divide and form a callus.
- Subculturing:
- To
maintain the callus culture, portions of the callus are periodically
transferred to fresh culture medium (subcultured). This provides a
continuous supply of nutrients and prevents the accumulation of waste
products.
- Regeneration
(Optional):
- Depending
on the desired outcome, the callus can be induced to regenerate whole
plants through:
- Organogenesis: The formation of organs (shoots and roots) from the callus
- Somatic embryogenesis: The formation of embryos from the callus cells.
Applications
of Callus Culture:
- Micropropagation:
Rapidly producing large numbers of genetically identical plants.
- Genetic
engineering: Introducing foreign genes into plant cells.
- Production
of secondary metabolites: Obtaining valuable compounds (e.g., pharmaceuticals)
from plant cells.
- Plant
breeding: Creating new plant varieties.
- Germplasm
preservation: Conserving rare or endangered plant species.
Subculture
of Callus
Subculturing refers to the transfer of callus
from an old medium to a fresh medium after a certain time (typically 3–4
weeks), to maintain active growth and prevent browning or senescence. It helps
in:
Purpose
of Subculturing:
- Replenishing
Nutrients:
- As
callus grows, it depletes the nutrients in the culture medium.
Subculturing involves transferring the callus to a fresh medium, ensuring
a continuous supply of essential nutrients for healthy growth.
- Preventing
Accumulation of Waste Products:
- Metabolic
byproducts can accumulate in the culture medium, potentially inhibiting
callus growth or causing toxicity. Subculturing helps to remove these
waste products.
- Maintaining
Callus Viability:
- Regular
subculturing helps to maintain the viability and vigor of the callus
culture, preventing it from senescence or death.
- Promoting
Continued Growth:
- By
providing fresh medium, subculturing stimulates continued cell division
and proliferation, ensuring a consistent supply of callus tissue.
- Controlling
Callus Characteristics:
- Sub culturing can also be used to influence the characteristics of the callus, for
example by changing the hormone composition of the media.
The Sub culturing Process:
- Preparation:
- A fresh batch of sterile culture medium is prepared.
- Sterile tools and a laminar flow hood are used to maintain aseptic conditions.
- Callus
Transfer:
- A portion of the actively growing callus is carefully excised from the existing culture.
- This callus tissue is then transferred to the fresh culture medium.
- Incubation:
- The
subcultured callus is incubated under controlled environmental
conditions, such as temperature, light, and humidity.
- Frequency:
- The
frequency of subculturing depends on the growth rate of the callus and
the specific requirements of the plant species.
- Maintaining the viability of callus
- Enhancing biomass
- Inducing differentiation (if required)
- Avoiding nutrient depletion and accumulation of toxic metabolites
Types
of Callus Based on Texture and Color
- Friable
Callus
In plant tissue culture, "friable callus" refers to a specific texture of callus tissue. Here's a breakdown:
Callus:
A callus is an undifferentiated mass of plant cells. It forms in response to wounding or when plant tissue (an explant) is placed on a culture medium containing plant growth regulators.
Friable Callus:
- This type of callus is characterized by its loose, crumbly, and easily separable texture. The cells are loosely attached and have a relatively high-water content.
- In contrast to a "compact
callus," which is dense and firm, a friable callus is soft and delicate.
- Friable callus is often desirable
for certain applications in plant biotechnology, particularly for:
- Cell
suspension cultures:
Friable callus can be easily broken up and transferred to liquid media
for cell suspension cultures, which are used for producing secondary
metabolites or for genetic transformation.
- Somatic
embryogenesis:
Some friable callus types are more conducive to the development of
somatic embryos, which can then be grown into whole plants.
- Loose, crumbly, and soft texture
- Useful
for suspension culture
- Usually light in color (whitish or yellowish)
- Compact
Callus
When discussing callus culture in
plant tissue culture, "compact callus" refers to a specific type of
callus with distinct characteristics. Here's a breakdown:
Characteristics of Compact Callus:
- Dense
and Firm:
- Unlike
friable callus, which is loose and crumbly, compact callus is
characterized by its tightly packed cells, resulting in a firm and dense
texture.
- Tightly
Aggregated Cells:
- The
cells within a compact callus are closely adhered to one another,
contributing to its solid appearance.
- Often
Green:
- Compact
callus may often have a greenish coloration, especially if it's derived
from tissues with chloroplasts.
- Relatively
Dry:
- Compared
to a friable callus, a compact callus will usually contain less water.
Contrasting with Friable Callus:
- It's important to differentiate compact callus from friable callus. Friable callus has a loose, crumbly texture, making it more suitable for cell suspension cultures.
- Compact callus is denser.
- The texture of the callus is very dependent on the hormone balances within the growth medium.
Factors Influencing Callus Type:
- Plant
Growth Regulators:
- The
ratio of auxins to cytokinins in the culture medium significantly
influences callus texture. Different ratios can favor the formation of
either compact or friable callus.
- Plant
Species and Genotype:
- Different
plant species and even different cultivars within a species exhibit
variations in callus texture.
- Culture
Conditions:
- Environmental
factors like light, temperature, and the composition of the basal medium
also play a role.
Significance:
- While friable callus is often preferred for cell suspension cultures, compact callus can be suitable for other applications in plant tissue culture.
- It is a form of callus that is produced, and then can be manipulated to produce different plant parts, through the changing of the hormone balances in the growth medium.
- Hard, dense, and tightly packed cells
- Often greenish or cream-colored
- Less
suitable for suspension cultures
- Embryogenic
Callus
Embryogenic callus is a specialized
form of callus that holds significant importance in plant tissue culture.
Here's a detailed explanation:
Key Characteristics:
- Potential
for Somatic Embryogenesis:
- The
defining characteristic of embryogenic callus is its ability to give rise
to somatic embryos. Somatic embryos are embryos that develop from plant
cells other than zygotes (fertilized eggs).
- Organized
Structure:
- Unlike
undifferentiated callus, embryogenic callus often exhibits a degree of
organization, with cells that are predisposed to develop into embryos.
- Distinct
Cellular Features:
- Embryogenic
cells tend to be small, densely cytoplasmic, and have prominent nuclei.
- High
Regeneration Capacity:
- This
type of callus has a high capacity for regeneration, meaning it can
efficiently produce whole plants.
Significance in Plant Tissue
Culture:
- Efficient
Plant Regeneration:
- Embryogenic
callus is highly valuable for regenerating large numbers of plants,
especially in species that are difficult to propagate through
conventional methods.
- Genetic
Transformation:
- It
serves as an excellent target for genetic transformation techniques,
allowing for the introduction of desired genes into plants. The resulting
somatic embryos can then develop into transgenic plants.
- Clonal
Propagation:
- Embryogenic
callus allows for the clonal propagation of plants, ensuring that all
regenerated plants are genetically identical to the parent plant.
- Plant
Breeding:
- It's
utilized in plant breeding programs to generate new plant varieties.
Factors Influencing Embryogenic
Callus Formation:
- Plant
Growth Regulators:
- The
type and concentration of plant growth regulators, particularly auxins,
play a crucial role in inducing embryogenic callus formation.
- Genotype:
- The
genetic makeup of the plant species or cultivar significantly influences
its ability to produce embryogenic callus.
- Explant
Source:
- The
type of explant used (e.g., leaf, stem, root) can affect the formation of
embryogenic callus.
- Culture
Medium:
- The
composition of the culture medium, including nutrients and other
additives, is essential for optimal embryogenic callus development.
- Capable of forming somatic embryos
- Usually yellowish-white or translucent
- Totipotent and can regenerate into whole plants
- Non-embryogenic
Callus
"Non-embryogenic callus"
refers to callus tissue that lacks the capacity to produce somatic embryos.
This distinguishes it from embryogenic callus, which is specifically
characterized by its ability to develop into embryos. Here's a breakdown:
Key Characteristics:
- Lack
of Somatic Embryogenesis:
- The
defining feature of non-embryogenic callus is its inability to form
somatic embryos, even when exposed to conditions that would induce
embryogenesis in suitable callus tissue.
- Variable
Morphology:
- Non-embryogenic
callus can exhibit a range of textures, including friable (loose and
crumbly) or compact (dense and firm).
- Different
Cellular Structure:
- Compared
to embryogenic callus, which often has cells with dense cytoplasm and
prominent nuclei, non-embryogenic callus may have cells with larger
vacuoles and a less organized structure.
- Different
biochemical properties:
- There
are differences in the protein and enzyme production in non-embryogenic
callus, when compared to embryogenic callus.
Significance:
- Understanding
Developmental Pathways:
- Studying
non-embryogenic callus helps researchers understand the factors that
control somatic embryogenesis. By comparing it with embryogenic callus,
they can identify the genes and proteins involved in embryo development.
- Optimizing
Tissue Culture Protocols:
- Understanding
the conditions that lead to non-embryogenic callus formation is essential
for optimizing tissue culture protocols and maximizing the production of
embryogenic callus.
- Basic
research:
- It is used in many basic research projects, to better understand plant cellular biology.
- Cannot form somatic embryos
- Used mainly for metabolite production or transformation
- Organogenic
Callus
Organogenic callus is a type of
callus in plant tissue culture that has the capacity to develop into organized
plant organs, such as shoots or roots. This is a key distinction from
undifferentiated callus, which lacks this organizational potential. Here's a
more detailed explanation:
Key Characteristics:
- Organ
Formation:
- The
defining characteristic is the ability to differentiate into specific
plant organs. This process, known as organogenesis, can lead to the
formation of shoots, roots, or even flowers.
- Organized
Development:
- Unlike
the random cell proliferation in undifferentiated callus, organogenic
callus exhibits a degree of organized development, with cells forming
distinct structures.
- Response
to Hormones:
- The
development of organs from organogenic callus is heavily influenced by
the balance of plant growth regulators, particularly auxins and
cytokinins, in the culture medium.
- Potential
for Plant Regeneration:
- Organogenic
callus is a valuable source for regenerating whole plants, particularly
through the formation of shoots and roots.
Significance in Plant Tissue
Culture:
- Plant
Regeneration:
- It
is a crucial tool for regenerating plants, especially in species that are
difficult to propagate through other means.
- Micropropagation:
- Organogenic
callus is used in micropropagation to produce large numbers of
genetically identical plants.
- Genetic
Transformation:
- It
can serve as a target for genetic transformation, allowing for the
introduction of desired genes into plants.
- Plant
Breeding:
- It plays a role in plant breeding programs, facilitating the development of new plant varieties.
- Has potential to form organs like roots or shoots
- Often forms in response to specific PGR ratios
- Pigmented
Callus
Pigmented callus refers to callus
tissue in plant tissue culture that exhibits coloration due to the production
of various pigments. This coloration can vary significantly depending on the
plant species, the specific pigments produced, and the culture conditions.
Here's a more detailed explanation:
Causes of Pigmentation:
- Secondary
Metabolites:
- Many plant species produce secondary metabolites, such as flavonoids, anthocyanins, carotenoids, and betalains, which can impart color to the callus.
- These compounds often serve protective functions in plants, such as UV protection or defense against pathogens.
- Chlorophyll:
- If
the callus develops from tissues containing chloroplasts (e.g., leaves),
it may exhibit a green coloration due to the presence of chlorophyll.
- Accumulation
of Compounds:
- Sometimes
the plant will accumulate different compounds within the callus, in
response to the growth medium, or environmental conditions in the growth
chamber.
Significance:
- Production
of Valuable Compounds:
- Pigmented callus can be a valuable source for producing commercially important pigments or other secondary metabolites.
- For example, callus cultures of certain plants can be used to produce anthocyanins, which are used as natural food colorants and antioxidants.
- Visual
Marker:
- Pigmentation can serve as a visual marker for specific developmental stages or the production of certain compounds within the callus.
- This is useful for research purposes.
- Research
Tool:
- The
study of pigmented callus can provide insights into the biosynthesis of
plant pigments and other secondary metabolites.
- Genetic
Studies:
- Pigmentation
can also be used as a visual marker for genetic studies.
Examples:
- Callus cultures of certain species may exhibit red, purple, or blue coloration due to the accumulation of anthocyanins.
- Carotenoids can impart yellow, orange, or red coloration to callus tissue.
- Green callus is often observed when it originates from leaf tissue.
- May appear red, brown, purple due to pigment or stress
- Could indicate phenolic compound accumulation or senescence
Callus
Characteristics to Observe
Type of Callus |
Characteristics |
Friable Callus |
Soft, crumbly, loosely arranged; good for cell suspension culture |
Compact Callus |
Firm, dense, tightly packed; ideal for shoot/root regeneration |
Embryogenic Callus |
Appears granular; capable of forming somatic embryos and regenerating
plants |
Non-embryogenic Callus |
Undifferentiated; lacks potential for embryo or organ development |
Organogenic Callus |
Shows early signs of shoot/root initiation; capable of forming organs |
Pigmented Callus |
Contains pigments (green, red, brown); may indicate specific metabolic
activity |
Mucilaginous Callus |
Sticky, jelly-like due to mucilage secretion; common in species with
high polysaccharide |