Discovery of Plant Hormones

To date, five major classes of plant hormones have been discovered namely auxins, gibberellins, cytokinins, abscisic acid and ethylene. It is possible that many other growth regulators, present in plants, may be classified as plant hormone in the future. Not all plant hormones fit the general definition of a hormone which is a chemical synthesised in one part of an organism that stimulates or inhibits a specific response in a target tissue elsewhere in the organism. Most animal hormones trigger highly specific responses in the target tissue. But the- responses of plant hormones are quite different compared to that of animal hormones.