ABA - Dormancy
Abscisic acid functions as an antagonist of all three promoters like IAA, GAS and Cytokinins. ABA nullifies the activity of GA, IAA and cytokinin right by
a) Repressing the genes
b) By inhibiting the synthesis of enzymes at translation level (of those enzymes to be promoted by IAA, GA, Cytokinins);
c) By promoting enzymes involved in abscission, dormancy etc.
Some seeds are shed before the embryo has developed sufficient size or maturity for it to start growing. In this case, the seed may be dormant for a period because of the immaturity of the embryo. All these dormant systems seem to share essentially the same mechanism of dormancy-short days somehow promote the synthesis of ABA, through the agency of a phytochrome-mediated system and long days prevent the onset of dormancy. In certain cases, it also seems likely that shortage of nutrients or changes in the balance of hormonal constituents can bring about dormancy. Leopold's group has pointed out that the dormant condition is considered to be one in which the metabolic machinery of the cells lies idle because of a repressed condition of the nucleic acid system.