The five main groups of natural plant growth regulators are: auxins, gibberellins, cytokinins, abscisic acid and ethylene.

Discovery of Auxins: The discovery began with observations by Charles Darwin and his son Francis Darwin, who noticed that coleoptiles of canary grass responded to unilateral illumination by growing towards the light source (phototropism). After a series of experiments, it was concluded that the tip of coleoptile was the site of transmittable influence that caused bending. Auxin was isolated by F.W. Went from tips of coleoptiles of oat seedlings. The term 'auxin' comes from Greek 'auxein' meaning 'to grow'. It was first isolated from human urine.

Physiological Functions: Auxins are generally produced by the growing apices of stems and roots, from where they migrate to regions of their action. They are involved in growth promoting activities such as cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting and seed formation. They are classified as plant growth promoters.

Agricultural/Horticultural Applications: Auxins like IAA (indole-3-acetic acid) and IBA (indole butyric acid) are natural, while NAA (naphthalene acetic acid) and 2,4-D are synthetic auxins. All these auxins have been used extensively in agricultural and horticultural practices, such as inducing rooting in twigs.