A metabolic engineering approach to investigate the serotonin metabolic pathway of Solanum lycopersicum

Tryptamine (TAM) and serotonin (SER; 5-hydroxytryptamine, 5-HT) are secondary metabolites belonging to the class of tryptophan-derived compounds known as indole-alkaloids or, more specifically, as indolamines. In the last years, a large body of scientific literature provided evidence that plant indolamines are probably involved in the regulation of many processes, including plant architecture and morphogenesis, protection against reactive oxygen species (ROS), biotic and abiotic stress responses, chronoregulation, and developmental events such as flowering and fruit ripening. Although TAM and SER accumulate to high levels (μg/g fw) in edible fruits and seeds of many plant species, their biological functions in reproductive organs remain unclear and the metabolic pathways have not yet been characterized in detail. In plants, TAM and SER are generally produced from tryptophan via consecutive decarboxylation and hydroxylation reactions catalysed by the enzymes tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H). We recently functionally characterized a 3-member TDC gene family and a T5H gene involved in the biosynthesis of TAM and SER in the model species Solanum lycopersicum. By coupling metabolomics and expression data, we revealed a complex spatiotemporal gene expression and metabolite accumulation pattern that suggested a model in which SlTDC1 allows TAM to accumulate in fruits, SlTDC2 and SlTDC3 work together with SlT5H to convert TAM to SER, respectively, in aerial vegetative organs or in roots and fruits. In order to modify TAM and SER levels in flowers, fruits and seeds, the traditional transgenesis as well as the CRISPR/Cas9 mediated gene knockout methods were both employed to produce several transgenic and edited tomato lines in which the normal function of the TAM and SER biosynthetic genes is altered.