ABSTRACT
Plants possess the enzymatic machinery for the biosynthesis of the neurotransmitters serotonin and melatonin, in which serotonin is a precursor of melatonin. Pharmacological application and manipulation of their endogenous levels via overexpression of biosynthetic enzymes in different plant species have unraveled the important roles of these emerging signals in the configuration of root architecture. Although both compounds show similar effects in repressing primary root growth and promoting lateral or adventitious roots, which are shared with auxins, detailed bioactivity analysis indicates that they may interfere with auxin biosynthesis and/or response or act via crosstalk with reactive oxygen species, nitric oxide, ethylene, and jasmonic acid signaling pathways. Since serotonin and melatonin cannot be considered as auxins, they may rather bind to their own, yet undiscovered receptor (s) to modulate plant growth, immunity, and stress tolerance.
