Tryptophan decarboxylase (TDC) in sweet pepper (Capsicum annuum L.): Gene expression analysis during fruit ripening and after nitric oxide exposure

Tryptophan decarboxylase (TDC) catalyzes the conversion of L-tryptophan (Trp) to tryptamine, a first step in the biosynthesis of serotonin and melatonin in plants. Pepper (Capsicum annuum L.) fruit, a globally popular horticultural product has great nutritional and economic values. In addition to that pepper fruit undergoes phenotypical changes during ripening, many other alterations also occur at the transcriptomic, proteomic, biochemical, and metabolic levels. However, little information is known on how many genes encoding for TDC in pepper plants and their expression levels during the ripening of sweet pepper fruit. In the current study, based on a data-mining approach on the pepper genome and transcriptome (RNA-seq), five putative CaTDC genes were identified. They are designated as 1 to 5 based on their localizations in chromosomes and also their previous biochemical data. Among them, CaTDC3 and CaTDC4 encode proteins with tryptophan decarboxylase activity; however, CaTDC1, CaTDC2 and CaTDC5 encode either tyrosine decarboxylase (TYDC) or aromatic aldehyde synthase (AAS), although CaTDC5 shares some degree TDC homology. Therefore, they are considered as the putative CaTDCs until their activity is corroborated. The CaTDC4 and putative CaTDC5 are expressed in pepper fruit. The time-course analysis of these genes during fruit ripening (green immature, breaking point, and red ripe) showed that they were differentially expressed, i.e., CaTDC4 was upregulated, and putative CaTDC5 was downregulated. CaTDC4 was positively modulated by two light-responsive elements, Box4 and TCT-motif, while CaTDC5 was influenced by GT1-motif and G-Box. The protein sequence analysis also allowed identifying the Trp-substrate-binding pocket which is a characteristic of the TDC proteins. Exogenous NO (a signaling molecule) treatment triggered the downregulation of CaTDC4 but not putative CaTDC5. These data provide a novel insight on the potential functions involved in the secondary metabolism of TDCs in fleshy fruits. In the identified three new CaTDC genes, two (CaTDC4 and CaTDC5) expressed in pepper fruits are modulated by exogenous NO treatment during ripening.