Short notes on the Classification of Morphogenesis

It is the capacity exhibited by certain types of isolated, differentiated plant cells to regenerate whole plant. The phenomenon is thought to be evidence supporting the theory that all nucleated plant cells essentially have some complement of genes which are necessary for formation of a complete plant and inhibition of many genes causes differentiation.

Cellular Totipotemy:

Steward and his co-workers succeeded in demonstrating that even mature, fully differentiated tissues have the capability to grow in an organized fashion when excised from parent plant and grown in vitro. This capacity of plant cells is termed as cellular totipotency.

Steward excised-small pieces of tissues from the phloem region which were grown in special flasks containing liquid medium. This liquid nutrient medium was also containing coconut milk. These special flasks containing liquid medium and excised tissues of carrot were gently shaken by artificial automatic shaker.

Due to shaking of culture, cells and clumps of cells became separated and remained floated into the liquid medium. Some of the cells clusters developed roots. When these rooted clumps were transferred to a semi-solid medium in glass tubes, they formed shoots. These plantlets developed into whole plant that flowered and produced seeds. It was also found that phloem tissues excised from roots of these plants could be used to repeat the experiment.

Muir and co-workers (1958) performed a successful experiment to grow single cell in total isolation to demonstrate the cellular totipotency. Normally, single cell does not grow in isolation. They took a single cell from tobacco callus and placed it on a filter paper below which callus of tobacco was growing.

This technique though physically separated the single cell from rest of the callus, called nurse tissue, but provided the chance to pass necessary factor(s) from nurse tissue to the single cell. This single cell divided and formed a lump of tissue which on being transferred to the medium from the filter showed further growth. 1965, Vasil Hildebrondt also succeeded in growing whole plants of tobacco from a single cell of pith tissue callus.

They took a small piece of tissue explanted from pith of tobacco plant. From this pith tissue, a callus was raised in a test tube containing semi -solid nutrient medium. A small piece of this callus was then transferred to flasks containing liquid medium and by shaking. By mechanical shaking of these flasks single cells and clumps of these cells were obtained in the liquid medium. Callus tissue was dissociated into single cell and clumps of cells.

Now, they transferred a single cell to a drop of liquid medium placed on a glass slide and a small bridge was raised around the cell by putting three cover slips. This single cell when grown into small mass of cells by repeated division was transferred to semi-solid medium where it differentiated into plantlet consisting of roots, stem and leaf. These plantlets, when grown on soil developed into mature plants with flowers, fruits and seeds.

On the basis of above said work of Steward, Muir, Vasil and Hildebrandt, one can infer that every living cell of plant is totipotent, irrespective of location and age. Scientists have succeeded to grow whole plants from single cell in vitro culture at least in case of carrot, tobacco and several other plants. In spite of totipotency of a single cell, it cannot be compared with the single celled zygote.

As zygote undergoes a series of cell divisions and directly give rise to bipolar embryo (with plumule and radicle) in a predetermined manner while the isolated single cell in vitro culture first develops into irregular callus and then differentiate into roots and shoot buds.

Polarity:

Condition that results from establishment of definite orientation during differentiation as in cell, tissue or organ, evident in early growth of plants as is shown by separate development of root and shoot system from initially formed dipolar embryo.

The embryo lies dormant in the seed, but when the latter is supplied with moisture and the other necessary conditions it becomes active and tends to grow and develop into a small seedling. Germination may be defined as the process by which the dormant embryo wakes up, grows out of the seed coat and establishes itself as a seedling. For its growth the embryo utilizes the food stored up in the cotyledons or in the endosperm if that is present.