Recognition of phytotropins by the receptor for 1-N-naphthylphthalamic acid

The structure requirements for phytotropin activity and receptor binding are expressed in terms of a recognition site on the receptor with which phytotropins, including 1-N-naphthylphthalamic acid, interact. It is postulated that the site can be represented by a large region which accepts planar molecules, and is possibly electrophilic in nature. A second area is also postulated which may be lipophilic or electrophilic, together with a carboxyl acceptor. It is suggested that if the requirements of the carboxyl acceptor and adjacent area are met, then phytotropin activity will result if the candidate molecule has a configuration, or can adopt a configuration, such that a conjugated portion of the molecule can interact with the larger area. It is argued that the close relationship observed between receptor binding and effect on the gravitropic response implies that the receptors may be directly involved in the gravitropic response mechanism.