Other Hormonal Interactions

The development of roots and shoots in both tissue culture and in girdled stems is also regulated by a combination of auxins and cytokinins. Experiments with living pith cells of tobacco plants have shown that such cells will enlarge when supplied with auxin and nutrients, but they will not divide unless small amounts of cytokinin are added. By varying the amounts of cytokinin, it is possible to stimulate the pith cells to differentiate into roots or into buds from which stems will develop.

Gibberellins and ABA are important in the regulation of germination of seeds. Gibberellins promote cell elongation, thereby enabling the developing root to break through the seed coat. After water has been imbibed by cereal seeds, gibberellins released by the embryo stimulate the secretion of enzymes that digest endosperm. ABA has been shown to inhibit the synthesis and release of these enzymes. The relative amounts of gibberellins and ABA in seeds other than cereal grains also influence whether the seeds germinate or remain dormant.

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