Role of Growth Regulators in Organogenesis
Of the many factors that influence organogenesis in vitro, the most important single factor seems to be the phytohormones. Skoog and Miller ( 1957) found that when the concentration of cytokinins are high relative to auxin, shoots are induced, when the concentrations of cytokinins are low relative to auxin, roots are induced, and at intermediate concentration the tissues grow as unorganised callus. This basic concept has been used to regenerate a wide variety of dicotyledonous plants. In general monocotyledonous plants do not show a pronounced response to cytokinins and need high concentration of auxin such as 2,4-D to obtained changes in development of cultured tissue. Other plant hormones particularly abscisic acid and gibberellins have some dramatic action on in vitro organogenesis. Endogenous ethylene retards organ initiation during early stages of culture but in later stages it helps shoot initiation.
Phytohormones are regarded as primary morphogens. According to this, hypothesis responding cells or group of cells are competent to react to the hormones but are not committed to a particular development fate. When the cells are treated with hormones, the cells start to move a specific development pathway. Alternatively, hormones the cells start to move a specific developmental pathway. Alternatively, hormone responsive cells are already determined and that hormones stimulate the expression of the committed state.
Increased levels of phosphate (PO4) in the medium is reported to countered the inhibitory effect of auxin and promote bud initiation in absence of cytokinins. Casein
Hydrolysate or tyrosine also induce kinetin type bud formation even in presence of higher level of IAA in medium.
Similarly, polyamines, BAP, 2-iP, 6-Tetrahydropyrane-adenine and zeatin are found to induce shoot bud initiation.