CELLULAR DIFFERENTIATION

Cellular differentiation is the process by which an unspecialised cell becomes altered to have a specific structure and function as a result of expressing genes characteristic for a particular type of cell. Once a cell becomes differentiated it only expresses the genes that produce the proteins characteristic for that type of cell. During cell differentiation, the changes may include cell shape, cell size, membrane potential, metabolic activities, and responsiveness. These changes are brought about by modifications in gene expressions.

Differentiation in plant cells

Within plants, meristems are regions of unspecialised cells that are capable of cell division. Meristems are found at the tips of shoots and roots (known as apical meristems, which contribute to growth in height) and in between the xylem and phloem tissues (known as lateral meristems, which contribute to growth in width).

Differentiation, Dedifferentiation and Redifferentiation

Differentiation:

The cells derived from root apical meristem (RAM) and shoot apical meristem (SAM) and cambium differentiate, mature to perform specific functions. This act leading to maturation is termed differentiation. They, undergo a few or major structural changes both in their cell walls and protoplasm.

Cells undergo structural changes during differentiation.

Changes takes place both in their cell wall and protoplasm.

For example, to form, a tracheary element the cells lose its protoplasm but develops a very strong, elastic, lignocellulosic secondary cell wall is best suited to carry water to long distances even under extreme tension.

Differentiation of xylem, phloem and parenchyma cells

Trichome differentiation

Dedifferentiation

Plant regeneration from differentiated cells is generally preceded by the cells becoming meristematic, followed by divisions to form an unorganized callus. The phenomenon of mature cells reverting back to meristematic state is termed dedifferentiation. In plants, the living differentiated cells can regain the capacity to divide mitotically under certain conditions. The sum of events, that bestow this capacity to divide once again, are termed dedifferentiation. A dedifferentiated tissue can act as meristem (e.g., interfascicular vascular cambium, wound meristem, cork cambium).

Mature cell (Differentiated) Meristematic cell (Undifferentiated)

Redifferentiation

The product of dedifferentiated cells/tissue which lose the ability to divide are called redifferentiate cells/tissues and the event, redifferentiation.

However, the growth in plants is open, and even differentiation in plants is open, because, e.g., the same apical meristem cells give rise to xylem phloem, fibres, etc., cells/tissues arising out of same meristem have different structures at maturity. The final structure at maturity of a cell/ tissue arising out of same meristem is determined by the location of the cell within.

For example, cells positioned distal to root apical meristem (RAM) differentiate as root cap cells, while those pushed to periphery mature as epidermis. However we do not know for as well as determined is called commitment. Terms determination and commitment are used as synonyms.

Transdifferentiation

Differentiation of one organ directly from another organ, such as shoot from root explants of Arabidopsis, is referred to as transdifferentiation.

Regeneration;

Generation of entire plant from cultured explants directly or via callus indirectly.

TOTIPOTENCY

Totipotency is the ability of a living cell to express all of its genes to generate a whole new individual. The potentiality of differentiated and specialized cells to form complete plants like the zygote is referred to as Cellular Totipotency. The term was probably coined by T.H. Morgan (1901). However, it was the famous German plant physiologist, Gottlieb Haberlandt, who in his famous address to the German Academy in 1902 introduced the concept of cellular totipotency.

It has been a routine horticultural practice to use leaf, stem, and root cuttings as source material to regenerate new individuals for vegetative propagation of some plant species.

Plant tissue culture has considerably enlarged the scope of regeneration of plants from highly differentiated and structurally and functionally specialized cells of leaves, roots, stem, floral parts, and endosperm.

In vitro regeneration of plants is also possible from isolated gametic cells (microspores, unfertilized egg or synergids).

A totipotent cell give rise to whole plant

In tissue cultures, cellular totipotency may be expressed via organogenesis (shoot differentiation) or embryogenesis (adventive embryony).

In more than 75 % of the plant species for which plant regeneration from protoplasts has been achieved via organogenesis.

Features of shoot bud and adventive embryo

(i) Shoot is monopolar the embryo is bipolar

(ii) The basal end of shoot is open and the basal end (radicularend) of an embryo is closed.

(iii) Provascular strands of shoot establish connection with the pre-existing vascular tissue dispersed in the callus or the cultured explant, and therefore if it is removed from the parent tissue the basal end is injured. Embryo does not have vascular connection with the parent tissue and can be removed from the parent tissue without causing injury.

Embryo Shoot bud

The basal end of the embryo is closed, whereas that of the shoot bud is open

Factors affecting cellular totipotency

Culture medium

Root-shoot differentiation in tissue cultures is a function of relative concentrations of auxin and cytokinin; relatively higher concentration of auxin favors root formation and relatively higher concentration of cytokinin promotes shoot bud differentiation.

Genotype

Plant regeneration was once thought to be primarily dependent on the concentration of phytohormones in the medium (Skoog and Miller, 1957). However, it is now well established that for in vitro differentiation the genotype of the plant plays an equally, if not more, critical role as the growth regulators.

Among the three monogenomic species of Brassica, B. oleracea is most regenerative and B. campestris the least, B. nigra being intermediate.

B. oleracea > B. nigra > B. campestris

Regenerative capacity

B. napus (amphidiploid of B. oleracea and B. campestris) is highly regenerative but less than B. oleracea because of the presence of B. campestris genome.

Similarly, the regenerative potential of B. carinata (an amphidiploid of B. oleracea and B. nigra) is less than B. oleracea because of the presence of genetic component from B. nigra.

Explant

The third most important factor, after the composition of the medium and the genotype, is the explant used to initiate cultures. Right choice of explant is of great significance for successful organogenesis and regeneration. Explant response in culture medium depends upon

The source of the explant,

The age of the tissue and the plant from which it is derived,

Preparation of the explant,

Orientation on the medium and

Inoculum density.

Source:

In Crotolaria juncea and Glycine- For shoot formation the hypocotyl shows higher potentiality than root segments.

In Lactuca sativa and B. juncea- Cotyledon.

In cereals and tree species- The regenerative cultures are mostly derived from immature embryos or young seedlings.

Age:

Five-day-old seedlings of B. juncea provided most regenerative cotyledons and those from older than10 days did not form shoots at all.

The cotyledons of Pinus radiata lose the potential to form adventitious shoot buds 3 days after germination.

In P. gerardiana, the cotyledons derived from ungerminated seeds show high potential to form shoot buds.

Orientation:

In cotyledon cultures of B. juncea planting the cotyledons with their abaxial side in contact with the medium and the petiolar cut end embedded in the medium gave better response.

In Cunninghamia lanceolata the explants placed horizontally on the medium produced three times more shoots than those planted vertically.

Electrical Stimulation

Organogenic and embryogenic differentiation in tissue cultures can be enhanced by the application of weak electric current.

The callus derived from mature embryos of wheat, which only formed roots, was induced to form several shoots by exposure to electrical treatments

Importance of totipotency