Organogenesis - Plant Tissue and Organ Culture
Organogenesis refers to the differentiation of organs such as roots, shoots or flowers. Shoot bud differentiation may occur directly from the explant or from the callus. The stimulus for organogenesis may come from the medium, from the endogenous compounds produced by the cultured tissue or substances carried over from the original explant. Organogenesis is chemically controlled by growth regulators. Skoog while working with tobacco pith callus, observed that the addition of an auxin Indole Acetic Acid (IAA) enhanced formation of roots and suppressed shoot differentiation. He further observed that adenine sulphate, (Cytokinin) reversed the inhibition of auxin and promoted the formation of shoots. You should know that:
1) Organogenesis is controlled by a balance between cytokinin and auxin concentration i.e. it is their relative rather than the absolute concentration that determines the nature of differentiation.
2) A relatively high auxin: Cytokinin ration induces root formation, whereas a high cytokinin: auxin ratio favours shoot bud differentiation.
3) Differential response to exogenously applied growth regulators may be due to differences in the endogenous levels of the hormones within the tissue. Organogenesis is a complex process. Whereas in the cultured tissues of many species organogenesis can be demonstrated in this pattern, some plants, notably the monocots, are exceptions.