Totipotency is the ability of a single cell to divide and produce all the differentiated cells in an organism, including extraembryonic tissues. Totipotent cells formed during sexual and asexual reproduction include spores and zygotes. Zygotes are the products of the fusion of two gametes. In some organisms, cells can dedifferentiate and regain totipotency. For example, a plant cutting or callus can be used to grow an entire plant. Human development begins when a sperm fertilizes an egg and creates a single totipotent cell called zygote. In the first hours after fertilization, this cell divides into identical totipotent cells. Approximately four days after fertilization and after several cycles of cell division, these totipotent cells begin to specialize. Totipotent cells have total potential. They can specialize into pluripotent cells that can give rise to most, but not all, of the tissues necessary for fetal development. Pluripotent cells undergo further specialization into multipotent cells that are committed to give rise to cells that have a particular function. For example, multipotent blood stem cells give rise to the red cells, white cells and platelets in the blood. Importantly, totipotent1 cells must be able to differentiate not only into any cell in the organism, but also into the extraembryonic tissue associated with that organism. For example, human stem cells are considered totipotent only if they can develop into any cell in the body, or into placental cells that do not become part of the developing fetus. This fact is an important aspect of the stem cell controversy because the human embryonic stem cells used for research purposes are pluripotent; they are collected from human embryos that have developed past the totipotent cell stage. All human embryos used in stem cell experimentation are destroyed in the process. Rational-explant
The tissue obtained from the plant to culture is called an explant. Based on work with certain model systems, particularly tobacco, it has often been claimed that a totipotent explant can be grown from any part of the plant. However, this concept has been vitiated in practice. In many species explants of various organs vary in their rates of growth and regeneration, while some do not grow at all. The choice of explant material also determines if the plantlets developed via tissue culture are haploid or diploid. Also the risk of microbial contamination is increased with inappropriate explants. Thus it is very important that an appropriate choice of explant be made prior to tissue culture. 1 (http://en.wikipedia.org/wiki/Totipotency)
The specific differences in the regeneration potential of different organs and explants have various explanations. The significant factors include differences in the stage of the cells in the cell cycle, the availability of or ability to transport endogenous growth regulators, and the metabolic capabilities of the cells. The most commonly used tissue explants are the meristematic ends of the plants like the stem tip, auxiliary bud tip and root tip. These tissues have high rates of cell division and either concentrate or produce required growth regulating substances including auxins and cytokinins. Some explants, like the root tip, are hard to isolate and are contaminated with soil microflora that become problematic during the tissue culture process. Certain soil microflora can form tight associations with the root systems, or even grow within the root. Soil particles bound to roots are difficult to remove without injury to the roots that then allows microbial attack. These associated microflora will generally overgrow the tissue culture medium before there is significant growth of plant tissue. B. rapa is used as the explants in this experiment. It is a plant widely cultivated as a leaf vegetable, root vegetable and an oilseed. Seedlings of rapid-cycling B. rapa grow...