Biological roles of tryptamine and serotonin in fleshy fruits: the tomato TDC gene family

Serotonin (5-hydroxytryptamine) is a neurotransmitter indolamine that acts as a neuro-hormone by affecting mood, sleep and anxiety in humans and it is involved in the intestinal peristalsis regulation as well. In the recent past, several papers showed that plants are also able to produce indolamines, including serotonin. Exogenous administration of serotonin stimulated the germination of radish seeds, the growth of roots and the hook of oat coleoptiles. In Arabidopsis thaliana, higher doses of serotonin repressed lateral and primary root growth and root hair development, but stimulated adventitious root formation. In rice plants, serotonin might act as a physical defense and confer resistance against the fungus Bipolaris oryzae. Indeed, “Sekiguchi‐asahi” plants, which do not accumulate serotonin due to a mutation in the gene responsible for the synthesis of serotonin from tryptamine, show enhanced susceptibility towards this pathogen. Moreover, serotonin might reduce the biotic stress by acting as a ROS scavenger in rice plants infected by Magnaportea grisea. If the biological role of serotonin in plants is still poorly investigated, almost nothing is known about the role of tryptamine, with the only exception of the observed repelling activity against pests. The biosynthetic pathway of tryptamine and serotonin in plants has been elucidated and it results in the final production of melatonin. In rice, it consists of four steps. The first involves the tryptophan decarboxylase (TDC), which converts tryptophan in tryptamine. This last is turned into serotonin by the tryptamine-5-hydroxylase (T5H). Serotonin could be derivatized with hydroxycinnamate moieties and/or subsequently converted in N-acetylserotonin and finally melatonin by serotonin N-acetyltransferase (SNAT) and N-acetylserotonin methyltransferase (ASMT), respectively. During a previous project focused in the metabolome characterization of different fruits, we found high levels of tryptamine and serotonin in methanolic extracts of Actinidia spp. and Solanum lycorpesicum, rising the question about the biological role played by these indolamines in this plant organ. Therefore, the overall project aims to unravel the biological role of tryptamine and serotonin in a model plant species, i.e. tomato (Solanum lycopersicum L.) cv MicroTom; in this work the initial functional characterization of tomato TDC gene family is shown. Acknowledgements: this project was funded by Regione Veneto.