, IAA (Indole-3-acetic acid) and IBA (Indole butyric acid) have been isolated from plants and are generally considered natural auxins. NAA (Naphthalene acetic acid) and 2,4-D (2, 4-dichlorophenoxyacetic acid) are mentioned specifically as synthetic auxins. GA refers to Gibberellic acid, which belongs to a different class of plant growth regulators called Gibberellins.