Plant Growth Regulators - Auxins
Auxins were the first growth hormones to be discovered. Auxins are a class of plant hormones (or plant-growth regulators) derived from the amino acid tryptophan. Plants produce natural auxins such as Indole-3-acetic acid (IAA) and Indole butyric acid (IBA). Natural auxins are found in growing stems and roots from where they migrate to their site of action. Naphthalene acetic acid (NAA) and 2, 4-dichlorophenoxyacetic (2, 4-D) are examples of synthetic auxins.
Auxins are found in shoot and root tips and promote cell division, stem and root growth. They can also drastically affect plant orientation by promoting cell division to one side of the plant in response to sunlight and gravity, named phototropism. Auxins in the flower promote maturation of the ovary wall and promote steps in the full development of the fruit.