14)Differentiation and dedifferentiation in plant cell culture. The obtaining callus mass and cultivation of callus tissue .

A plant grows by increasing its cell population while the cells specialize their functions. Increasing cell population is done by cell division (also called mitosis). Before a mother cell divides into two daughter cells, it makes an exact copy of its genome first. As a result, the two daughter cells usually have exactly the same genetic makeup as their mother cell. Therefore, every living cell of a plant should contain all the genes the plant has and thus has the capacity to grow back to a full plant. This is called cell totipotency. The process of specializing cells’ functions is called cell differentiation. It is accompanied by morphogenesis, the change of the cells’ morphology. Differentiation is done by turning on certain genes and turning off some others at a certain time. Therefore, for a highly differentiated cell to grow into a full plant, the differentiation process has to be reversed (called de-differentiation) and repeated again (called re-differentiation).

Organogenesis in plant tissue culture involves two distinct phases: dedifferentiation and redifferentiation. Dedifferentiation begins shortly after the isolation of the explant tissues with an acceleration of cell division and a consequent formation of a mass of undifferentiated cells (called callus). Redifferentiation, also called budding in plant tissue culture, may begin any time after the first callus cell forms. In this process of tissue called organ primordia is differentiated from a single or a group of callus cells. The organ primordia give rise to small meristems with cells densely filled with protoplasm and strikingly large nuclei. The development (or growth) of an organ is monopolar . Polarity of the longitudinal axis of the organizing growing points of the organs can be seen some time after the formation of meristem tissues. Different types of specialized cells again differentiate. The vascular system is formed connecting the new organs to their parent explants or callus mass.

In plant cell biology a callus is a mass of undifferentiated cells  (usually cultivated on gel media that develops during tissue culture of plant parts) induced byhormone treatment. A callus cell culture is usually sustained on gel medium. They are grown in adequate media consisting of water, agar and of a defined mix of macronutrients and micronutrients for the given cell type, much in the same manner as bacteria. Callus induction medium consists of agar and a mixture of macronutrients and micronutrients for the given cell type. There are several types of basal salt mixtures used in plant tissue culture, but most notably modified Murashige and Skoog medium,White's medium, and woody plant medium. Plant hormones are used to initiate callus growth. Specific auxin to cytokinin ratios in plant tissue culture medium give rise to an unorganized growing and dividing mass of callus cells.Vitamins are also provided to enhance growth such as Gamborg B5 vitamins.For plant cells, enrichment with Nitrogen, Phosphorus, and Potassium is especially important. A plant cell callus consists of somatic undifferentiated cells (usually thin-walled parenchyma cells) from an adult subject plant. The callus culture is a technique of tissue culture, it is usually carried out on solidified gel medium in the presence of growth regulators and initiated byinoculation of small explants or sections from established organ or other cultures (the inocula).