Plant tryptamine and serotonin: in search of their biological role in the fruit

About a hundred of plant species, that include some common fruits and vegetables, are able to produce and accumulate the two neuroactive indolamines tryptamine and serotonin (1). Although these molecules have been well studied in animal systems in which they regulate various physiological functions (2), still more information is required to clear their biological role in plants. In some species, tryptamine - beyond being the precursor of serotonin - is also an intermediate in the biosynthesis of IAA (3) and indole alkaloids (4), whereas serotonin can be converted to melatonin (5) and various phenolic derivatives (6). Recent works, that suggest the involvement of serotonin in many plant physiological processes (1) (flowering, senescence (7), plant architecture, adaptation to environment, stress response (8), did not focus on its production in reproductive organs such as the fruit and the seeds where the highest levels of this substance have been reported (9) (Fig.1). This project aims to unravel the role of tryptamine and serotonin in the model fruit of tomato (Solanum lycopersicum cv. Microtom), a natural producer of these indolamines, by using a metabolic engineering approach. The strategy is to induce the depletion and a further accumulation of tryptamine and serotonin in the fruit by targeting a fruit-specific tryptophan decarboxylase (TDC), responsible for the conversion of tryptophan into tryptamine. Three tomato genes have been identified and characterized as TDCs through in-vivo functional assay in Nicotiana benthamiana . SlTDC1, whose expression pattern is fruit specific, has been chosen as the target to generate knock-out and overexpressing mutants that will be subjected to phenotypical analysis along the whole ripening process and tested for their ability to respond to abiotic and biotic stress. Moreover, the analysis of the accumulation pattern of tryptamine and serotonin by HPLC-ESI-MS in different organs and developmental stages of wild-type tomato plants, added further details to the comprehension of their functions in plants.