Functions of ethylene:

Ethylene has various physiological responses which are listed below:

Ethylene plays vital role in fruit ripening: The changes which typically takes place due to the stimulating effect of ethylene includes softening of the fruit due to the enzymatic breakdown of the cell walls, sugar accumulation, starch hydrolysis and disappearance of organic acids and phenolic compounds including tannins.  Ethylene has the ability to initiate germination in certain seeds, such as cereals and break the dormancy.  Ethylene increases the rate of seed germination of several species. Ethylene can also break bud dormancy and ethylene treatment is used to promote bud sprouting in tubers such as potatoes etc. It stimulates shoot and root growth along with differentiation. Ethylene induces abscission. Abscission takes place in specific layers of cells, called abscission layers, which become morphologically and biochemically differentiated during organ development. Weakening of the cell walls at the abscission layer depends on cell wall–degrading enzymes such as polygalacturonase and cellulase. Ethylene induces flowering in Bromeliaceae family plants which includes pineapple and its relatives. Flowering of other species of plants, such as mango, is also initiated by ethylene. Ethylene may change the sex of developing flowers on monoecious plants (plants which have separate male and female flowers).
The femaleness of diocious flowers in plants is stimulated by the production of ethylene. The promotion of female flower formation in cucumber is one example of this effect.Flower and leaf senescence stimulation is caused by ethylene.  Exogenous applications of ethylene accelerate leaf and flower senescence. Enhanced ethylene production in plants is associated with the loss of chlorophyll and the fading of colours.

Role of plant hormones in tissue culture:

The culture of plant tissues or plant cells in a synthetic culture medium under controlled aseptic conditions is known as plant tissue culture. Plant tissue culture is the aseptic culture of plant protoplasts, cells, tissues or organs under conditions which lead to cell multiplication or regeneration of organs or whole plants. Tissue culture produces clones, in which all product cells have the same genotype. The culture medium provides all minerals and growth hormones necessary for the growth of cells. The controlled conditions give the culture a suitable microenvironment for the cell growth, proliferation and morphogenesis. During plant tissue culture, cells of small segments of plant tissue undergo repeated divisions to form masses of cells called calli. Plant tissue culture techniques are central to pioneering areas of applied plant science, including plant biotechnology and agriculture. Selected plants can be cloned and cultured as suspended cells from which plant products can be harvested. The management of genetically engineered cells to form transgenic whole plants requires tissue culture procedures; tissue culture methods are also required in the formation of somatic haploid embryos from which homozygous plants can be generated.

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 protoplasts, pieces of leaves, or roots can often be used to generate a new plant on culture media given the required nutrients and plant hormones. The composition of the medium, particularly the plant hormones and the nitrogen source has profound effects on the morphology of the tissues that grow from the initial explant. Thus, plant hormones are one of the most essential components of the medium used in plant tissue culture. For example, an excess of auxin will often result in a proliferation of roots, while an excess of cytokinin may yield shoots. A balance of both auxin and cytokinin will often produce an unorganized growth of cells or callus. Effect of different auxin and cytokinin concentration on tissue development is shown in Figure 11. The ratio of these two hormones can determine plant development: