A metabolic engineering approach to study the biological role of tryptamine and serotonin in the model species Solanum lycopersicum

Tryptamine (TAM) and serotonin (SER) are two tryptophan-derived compounds belonging to a widespread class of bioactive molecules known as indolamines or indole alkaloids. In plants, TAM and SER are primarily recognized as intermediates in the biosynthesis of melatonin, a well-studied molecule involved in key biological processes such as biotic and abiotic stress responses, reactive oxygen species (ROS) scavenging, embryo development, and plant morphogenesis. Although TAM and SER have been detected at very high concentrations (μg/g fresh weight) in the edible fruits and seeds of numerous plant species, their biological functions in reproductive organs remain unclear, and their metabolic pathways have yet to be fully elucidated. In plants, the biosynthesis of TAM and SER typically involves consecutive decarboxylation and hydroxylation reactions of tryptophan, catalyzed by tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H) enzymes, respectively. Our recent research has focused on the functional characterization of a three-member TDC gene family and a single T5H gene involved in TAM and SER biosynthesis in the model species Solanum lycopersicum. Our findings support a model in which SlTDC1 promotes TAM accumulation in fruits, SlTDC2 mediates TAM production in aerial vegetative organs, SlTDC3 drives TAM synthesis in roots and seeds, and SlT5H catalyzes the conversion of TAM to SER throughout the entire plant (Commisso et al., 2022). Our current research aims to unravel the biological functions of these two indolamines in various organs and tissues of the tomato plant. We employed a metabolic engineering approach, using both traditional transgenesis and CRISPR/Cas9-mediated gene knockout, to generate different tomato genotypes characterized by altered levels of TAM and SER. The results obtained from phenotypic and molecular characterization of these engineered genotypes indicate a potential involvement of TAM and/or SER in vegetative and reproductive development. SlTDC1 overexpression elevated SER levels in roots and leaves, accelerating the transition to the reproductive phase and anthesis, whereas SlTDC1 knockout depleted TAM and SER in fruits and seeds, resulting in altered seed coat pigmentation and reduced germinability. SlT5H knockout caused TAM accumulation and SER depletion, accompanied by shortened stem length, while SlT5H overexpression anticipated floral bud emergence but also reduced stem length. These findings, together with the collection of generated Micro-Tom genotypes, provide a solid foundation for further investigation into the biological roles of TAM and SER in Solanum lycopersicum.