Tissue Culture as an Important Tool for Good Quality Planting Material Production

Introduction

A plant is composed of different specialized organs like leaves as a site for photosynthesis, roots for absorption of nutrients from soil, vascular systems for carrying the water and nutrients, stems and branches for giving a plant its characteristic shape – just to name a few. Each organ, in turn, is composed of a multitude of specialized tissues and cells to carrying out specific physiological and metabolic tasks.

Differentiation

A plant starts its life as a single cell in the seed. That primordial single cell produces all the specialized cells and tissues as the plant grows. This process is called “Differentiation”. Plants have about a dozen basic cell types that are required for everyday functioning and survival. The fate of plant cells is often determined by their location in the growing plant rather than how they started out. Intriguingly, this suggests that plant cells recognize where they are and can alter gene activity in response to their location.

De-differentiation

If a plant suffers any injury, it repairs itself. If an entire branch is lost to a grazing animal, a new branch may grow from a lateral bud. In that lateral bud, a group of cells which had differentiated into a specific tissue may revert back to their original undifferentiated state and, subsequently, can get differenced into other types of tissues. A lateral bud, which actually looks like a small clump of leaf can flourish into a branch with leaves, vascular tissues, woody supporting structure, cuticle, etc. The process of conversion of fully differentiated cells into undefined undifferentiated cells with potential of conversion of other tissue types is called “De-differentiation”. Later, we will discuss about the process of De-differentiation of leaf tissue into undifferentiated “Callus” tissue and “Re-differentiation” of the callus tissue into a whole plant.

Callus

When a specific part is cut from a plant and it is incubated in sterile condition in a medium supplied with all nutrients and specific plant growth promoters, clumps of un-differentiated cells may grow from the cut surface. These types of undifferentiated clumps of cells may be maintained for indefinite period in growing condition by periodically transferring a part of the clump of cells in fresh sterile medium. This, essentially undifferentiated aggregation of cells is called a “Callus”. 

Totipotency

The undifferentiated cells of a callus can be induced to transform into a fully differentiated whole plant through proper plant hormone signaling. Thus, every cell of the callus has the potential of getting transformed into a whole plant. Totipotency is the genetic potential of a plant cell to produce the entire plant. In other words, totipotency is the cell characteristic in which the potential for forming all the cell types in the adult organism is retained.

Tissue Culture

Tissue culture (TC) is the cultivation of plant cells, tissues, or organs on specially formulated nutrient media. Under the right conditions, an entire plant can be regenerated from a single cell. Plant tissue culture relies on the fact that many plant cells have the ability to regenerate a whole plant (totipotency). Single cells, plant cells without cell walls (protoplasts), pieces of leaves, stems or roots can often be used to generate a new plant on culture media given the required nutrients and plant hormones.

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Different techniques in plant tissue culture may offer certain advantages over traditional methods of propagation, including:

  1. The production of exact copies of plants that have all the desirable traits.

  2. Quick production of plants in large numbers

  3. Avoidance of genetic heterogeneity in seed born plants due to cross pollination

  4. Regeneration of whole plants from plant cells that have been genetically modified.

  5. Production of disease-free plants

  6. Predictable multiplication of plants which have very low seed germination percentage

Micropropagation, which is a form of tissue culture, increases the amount of planting material to facilitate distribution and large scale planting. In this way, thousands of copies of a plant can be produced in a short time.

 

Micropropagated plants are observed to establish more quickly, grow more vigorously and are taller, have a shorter and more uniform production cycle.

 

Plant tissue culture processes were initially developed and practiced in university and government-based laboratories. In recent years, however, the process has moved beyond these research facilities to widespread use among commercial enterprises as a cost-effective tool for plant propagation, new variety introductions and research.

 

Meristem

The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Meristem tissues can be found in all actively growing part of the plant. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells continue to divide until a time when they get differentiated into any specific tissue or organ and then they lose the ability to divide.

Meristematic cells are undifferentiated or incompletely differentiated. Thus, they are totipotent and capable of getting differentiated into any organ. Division of meristematic cells provides new cells for expansion and differentiation of tissues and the initiation of new organs, providing the basic structure of the plant body. The cells are small, completely filled up with cellular sap (protoplasm) and are closely packed together. The cell walls of these cells are also very thin.

There are three types of meristematic tissues: apical (at the tips), intercalary (in the middle), and lateral (at the sides). Plant parts, which are rich in meristematic tissue are often used for regeneration of whole plants through the tissue culture process.

 

Explants

An explant is the part of a plant which has got the regeneration potential and is capable to give rise to the whole plant. There are various types of explants frequently used for regeneration purpose: nodal segments, apical meristems, roots, cotyledons (the embryonic leaf in seed), plant embryo, leaf disc, leaf blade, pedicle (flower stalk), petiole (leaf stalk), pollen etc.

 

Micropropagation

Micropropagation is an artificial method for rapid multiplication of plants in a short duration using the tissue or cell culture techniques in a controlled environment. Micropropagation has a number of advantages over traditional plant propagation techniques:

  1. The main advantage of micropropagation is the production of many plants that are clones of each other.

  2. Micropropagation can be used to produce disease-free plants.

  3. It can have an extraordinarily high fecundity rate, producing thousands of propagules while conventional techniques might only produce a fraction of this number.

  4. It is useful in multiplying plants which produce seeds in uneconomical amounts, or when plants are sterile and do not produce viable seeds or when seed cannot be stored

  5. Micropropagation often produces more robust plants, leading to accelerated growth compared to similar plants produced by conventional methods

  6. A greater number of plants can be produced per square meter and the propagules can be stored longer and in a smaller area.

 

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