Experimental Evidence and In Silico Identification of Tryptophan Decarboxylase in Citrus Genus.

Luigi De Masi,Domenico Pignone,Domenico Castaldo,Angelo Facchiano,Luigi Servillo

doi:10.3390/molecules22020272

Abstract

Plant tryptophan decarboxylase (TDC) converts tryptophan into tryptamine, precursor of indolealkylamine alkaloids. The recent finding of tryptamine metabolites in Citrus plants leads to hypothesize the existence of TDC activity in this genus. Here, we report for the first time that, in Citrus x limon seedlings, deuterium labeled tryptophan is decarboxylated into tryptamine, from which successively deuterated N,N,N-trimethyltryptamine is formed. These results give an evidence of the occurrence of the TDC activity and the successive methylation pathway of the tryptamine produced from the tryptophan decarboxylation. In addition, with the aim to identify the genetic basis for the presence of TDC, we carried out a sequence similarity search for TDC in the Citrus genomes using as a probe the TDC sequence reported for the plant Catharanthus roseus. We analyzed the genomes of both Citrus clementina and Citrus sinensis, available in public database, and identified putative protein sequences of aromatic l-amino acid decarboxylase. Similarly, 42 aromatic l-amino acid decarboxylase sequences from 23 plant species were extracted from public databases. Potential sequence signatures for functional TDC were then identified. With this research, we propose for the first time a putative protein sequence for TDC in the genus Citrus.

Highlights

Plants are able to biosynthesize a multitude of chemicals acting as secondary metabolites, i.e., without an apparent essential function for the plant cell
We report for the first time that, in Citrus x limon seedlings, deuterium labeled tryptophan is decarboxylated into tryptamine, from which successively deuterated N,N,N-trimethyltryptamine is formed
With the aim to identify the genetic basis for the presence of tryptophan decarboxylase (TDC), we carried out a sequence similarity search for TDC in the Citrus genomes using as a probe the TDC sequence reported for the plant Catharanthus roseus

Summary

Introduction

Plants are able to biosynthesize a multitude of chemicals acting as secondary metabolites, i.e., without an apparent essential function for the plant cell. In the course of a pathogenic attack, plants rapidly respond with the production of multiple defense compounds resulting from complex metabolic pathways. Aromatic L-amino acids are important precursors for the production of secondary metabolites aiming at controlling pathogens. Decarboxylation of aromatic L-amino acids by specific decarboxylases leads to the production of biogenic amines, such as tryptamine and tyramine, which are the starter compounds for the biosynthesis of biologically active secondary metabolites involved in Molecules 2017, 22, 272; doi:10.3390/molecules22020272 www.mdpi.com/journal/molecules. Aromatic L-amino acid decarboxylases of plants and animals share high amino acid sequence similarity, but have remarkable differences in substrate specificities [1,2]. Animal aromatic L-amino acid decarboxylases accept a broad range of aromatic L-amino acids as substrate, without distinction between indole and phenol side chains, such as 5-hydroxy-L-tryptophan (5-HTP)

Methods

Results

Conclusion