Indolamine accumulation and TDC/T5H expression profiles reveal the complex and dynamic regulation of serotonin biosynthesis in tomato (Solanum lycopersicum L.)

Abstract

Tryptamine and serotonin are indolamines that fulfill diverse biological functions in all kingdoms of life. Plants convert l-tryptophan into tryptamine and then serotonin <i>via</i> consecutive decarboxylation and hydroxylation reactions catalyzed by the enzymes tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H). Tryptamine and serotonin accumulate to high levels in the edible fruits and seeds of many plant species, but their biological roles in reproductive organs remain unclear and the metabolic pathways have not been characterized in detail. We identified three <i>TDC</i> genes and a single <i>T5H</i> gene in tomato (<i>Solanum lycopersicum</i> L.) by homology-based screening and confirmed their activity by heterologous expression in <i>Nicotiana benthamiana</i>. The co-analysis of targeted metabolomics and gene expression data revealed complex spatiotemporal gene expression and metabolite accumulation patterns that suggest the involvement of the serotonin pathway in multiple biological processes. Our data support a model in which <i>SlTDC1</i> allows tryptamine to accumulate in fruits, <i>SlTDC2</i> causes serotonin to accumulate in aerial vegetative organs, and <i>SlTDC3</i> works with <i>SlT5H</i> to convert tryptamine into serotonin in the roots and fruits.