Serotonin, a tryptophan-derived signal conserved in plants and animals, regulates root system architecture probably acting as a natural auxin inhibitor in Arabidopsis thaliana.

Serotonin (5-hydroxytryptamine) is a well-known neurotransmitter in mammals and is widely distributed in plants. This compound is synthesized from tryptophan and shares structural similarity with IAA. To date, little is known about the morphological, physiological and molecular responses of plants to serotonin.