Genome-wide characterization of melatonin biosynthetic pathway genes in carnation (Dianthus caryophyllus L.) and their expression analysis in response to exogenous melatonin

Carnation (Dianthus caryophyllus L.) is a prominent floricultural crop valued for its diverse colors, offering significant economic and ornamental value globally. However, the global demand for its flowers makes flower yield an important attribute that relies on the quantity of lateral branches in the crop. Melatonin as a multi-regulatory phytohormone play vital functions in governing plant growth and development. It is synthesized from tryptophan via four key enzymes. Tryptophan decarboxylase (TDC), Tryptamine 5-hydroxylase (T5H), Serotonin N-acetyltransferase (SNAT), and N-Acetylserotonin O-methyltransferase (ASMT). Although the significance of melatonin is recognized, its impact on the growth and development of carnation remains understudied. In the current study, we investigated the effect of exogenous melatonin at different concentrations, on growth pattern of carnation, followed by genome-wide characterization, in-silico analysis and expression profiling of melatonin biosynthetic pathway genes. Results showed increased branching and reduced height with increased melatonin concentrations up to a point. In-silico analysis identified ten genes in the melatonin biosynthetic pathway, including two TDC, two T5H, one SNAT, and five ASMT members. Domain analysis confirmed the presence of characteristic domains such as pyridoxal-dependent decarboxylase, cytochrome P450, Acetyltransferase_1, and O-methyltransferase. Physiochemical properties, gene structure, conserved motifs, promoter regions, gene ontology, synteny, and evolutionary relationships through phylogeny were also analysed. Sub-cellular localization predictions showed distribution of melatonin biosynthetic enzymes across various cellular compartments. Expression analysis of these genes under different exogenous melatonin concentrations (100, 200, 300, 400, 500, and 1000 µM) revealed significant upregulation at 100 µM and 500 µM, while no change was observed at 1000 µM. These findings suggest that optimal exogenous melatonin concentrations enhance the expression of biosynthetic pathway genes ultimately led to increased branching in carnation due to increased endogenous melatonin levels. This study establishes a basis for future functional characterization of melatonin biosynthetic pathway genes to elucidate their roles in carnation growth and development.