The tomato serotonin pathway: unravelling the puzzling biological roles of plant indolamines

Tryptamine and serotonin are specialized metabolites belonging to the group of tryptophan-derived indolamines that have been demonstrated to be widespread among all the living kingdoms, in which evolution shaped very different distributions and functional versatility. First discovered in humans, these metabolites were later detected in plants in which, despite their wide occurrence in several plant families, the study of their biological roles has been largely neglected. Tryptamine, due to its central position as a precursor of many plant specialized metabolites, including serotonin, has long been considered a mere metabolic intermediate; on the other hand, the increasing awareness of the many medical issues of serotonin (e.g. neurotransmission and hormonal activity), triggered the botanical research towards the elucidation of its biosynthetic pathway and functions also in plants, leading to a huge number of experimental evidences that, yet often controversial, suggest its putative involvement in many different plant physiological processes (e.g. development, stress response and reproduction). This PhD thesis proposed to shed a light on the biological roles of plant tryptamine and serotonin, with a particular focus on an aspect that has never been investigated in plants, i.e. the high level of accumulation of these metabolites within the reproductive organs, such as the fruit, observed in many edible species, which, given the high costs to plant metabolism, can be reasonably hypothesized to reflect an important plant physiological function. To fulfil this aim, this PhD project relied on the use of Solanum lycopersicum, a tryptamine and serotonin accumulator that is a model plant for fruit-bearing crops. The first step consisted in the genetic characterization of the tomato tryptamine and serotonin biosynthetic pathway: a three-member gene family and one single gene codifying for the enzymes of the 2-step pathway that leads to the production of serotonin from tryptophan via tryptamine (i.e. tryptophan decarboxylase, TDC and tryptamine-5-hydroxylase, T5H) were respectively identified and functionally characterized as bona-fide SlTDCs and SlT5H. The expression analysis of these genes and the investigation of tryptamine and serotonin distribution revealed organ and developmental-specific expression and accumulation patterns in tomato, confirming the complementary but not redundant activity of the three SlTDC genes in the plant and the presence of notable amounts of the two indolamines in the fruit, which accumulated with a characteristic trend during development and ripening. Moreover, it was revealed the fruit-specific nature of the SlTDC1 gene that, as a preliminary point in the elucidation of the biological roles of plant tryptamine and serotonin, was targeted by a metabolic engineering approach in order to look for the effects resulting from altered levels of these metabolites on the plant phenotype. Transgenic plants overexpressing this gene resulted in deep modifications of plant metabolome presenting in one case altered morphology of younger leaves. This evidence, together with the observation along the main axis of the wild type plant of complex expression and accumulation gradients of SlTDCs/SlT5H genes and related products, i.e. tryptamine and serotonin, leads to hypothesize the possible interference with the hormonal cross-talk. On the other hand, SlTDC1 knock-out fruits did non exhibit obvious phenotype but further characterization of their metabolome are needed to speculate on the biological roles of tryptamine and serotonin in this organ. In summary, this work provided useful information and details to the biosynthesis, regulation and putative biological roles of plant indolamines in the model plant of tomato and highlighted the putative involvement of the actors of the plant serotonin pathway in important physiological functions, which deserve, thus, future deeper investigation.