Phytohormone is an organic substance other than nutrients active in minute quantity which is formed in certain parts of a plant and is then usually translocated to some other parts or sites where it evokes physiological, biochemical and /or morphological responses.
Phytohormones are divided into five groups: Auxin, Cytokininis, Gibberllins, Abscisic acid (ABA), and ethylene.
Auxin is earliest known plant growth regulator. It promotes cell elongation especially in shoots. It obtained from plant are called natural auxin.
Gibberellin: Kurosawa first time noted the existence of gibbellins in a fungus Gibberella fuzikuroi of ascomycetes infected rice plant. This is popularly known as Bakanne disease of rice.
Auxins vs Gibberellin