Synergy and Antagonism

Plant growth regulators rarely act alone. The response of a tissue usually depends on the balance of several regulators, which can interact in two ways.SynergyTwo regulators act synergistically when their combined effect is greater than the sum of their separate effects.Example: auxin and gibberellin both stimulate stem elongation. Suppose that in an experiment on stem segments, auxin alone increases length by 2 mm and gibberellin alone by 3 mm. If the two together increase length by 10 mm, this is more than 2 + 3 = 5 mm, so they are acting synergistically.Auxin and cytokinin also act synergistically to stimulate cell division.AntagonismTwo regulators act antagonistically when they have opposite effects, so the response depends on the ratio of the two.- Auxin and cytokinin in lateral buds: auxin from the shoot tip inhibits lateral bud growth, while cytokinin promotes it (apical dominance).
- Gibberellin and abscisic acid (ABA) in seeds: gibberellin promotes germination, while ABA maintains dormancy. When the ABA level falls relative to gibberellin, the seed germinates.
- Auxin and cytokinin in tissue culture: a high auxin to cytokinin ratio produces roots, while a high cytokinin to auxin ratio produces shoots.Why this mattersHaving several interacting regulators lets a plant fine-tune its responses. A small change in the concentration of one regulator relative to another can switch a tissue from one type of growth to another, without needing a separate signal for every response.