Diaphorina citri Genome Possesses a Complete Melatonin Biosynthesis Pathway Differentially Expressed under the Influence of the Phytopathogenic Bacterium, Candidatus Liberibacter asiaticus

SIMPLE SUMMARY: The indole-like compound, melatonin, is a tryptophan-derivative that is secreted by the pineal gland. Melatonin is ubiquitously distributed in both prokaryotes and eukaryotes including animals and plants. In animals, melatonin plays pleiotropic regulatory roles in several biological and physiological functions including sleep, circadian rhythm, oxidative stress, immune response, aging, apoptosis, and autophagy. Moreover, it might have anti-inflammatory, anti-tumor, and anti-cancer activities. Although most, if not all, of these genes were cloned and characterized previously in numerous animal species, none of them have been identified from the Asian citrus psyllid, Diaphorina citri, in the vector of Huanglongbing yet. In the current study, we performed a genome-wide analysis and introduces a shortlist of six putative melatonin biosynthesis-related genes included two putative tryptophan 5-hydroxylase (DcT5H-1 and DcT5H-2), a putative aromatic amino acid decarboxylase (DcAADC), two putative arylalkylamine N-acetyltransferase (DcAANAT-1 and DcAANAT-2), and putative N-acetylserotonin O-methyltransferase (DcASMT), which could indicate sites of functional or structural constraint. All these genes were differentially expressed under the influence of the phytopathogenic bacterium, Candidatus Liberibacter asiaticus, and after melatonin supplementation. Our findings could be a further step for optimization and cloning of melatonin biosynthesis genes of Diaphorina citri. ABSTRACT: Melatonin is synthesized from the amino acid (L)-tryptophan via the shikimic acid pathway and ubiquitously distributed in both prokaryotes and eukaryotes. Although most of melatonin biosynthesis genes were characterized in several plants and animal species including the insect model, Drosophila melanogaster, none of these enzymes have been identified from the Asian citrus psyllid, Diaphorina citri. We used comprehensive in silico analysis and gene expression techniques to identify the melatonin biosynthesis-related genes of D. citri and to evaluate the expression patterns of these genes within the adults of D. citri with gradient infection rates (0, 28, 34, 50, 58, and 70%) of the phytopathogenic bacterium Candidatus Liberibacter asiaticus and after the treatment with exogenous melatonin. We showed that the D. citri genome possesses six putative melatonin biosynthesis-related genes including two putative tryptophan 5-hydroxylase (DcT5H-1 and DcT5H-2), a putative aromatic amino acid decarboxylase (DcAADC), two putative arylalkylamine N-acetyltransferase (DcAANAT-1 and DcAANAT-2), and putative N-acetylserotonin O-methyltransferase (DcASMT). The infection with Ca. L. asiaticus decreased the transcript levels of all predicted genes in the adults of D. citri. Moreover, melatonin supplementation induced their expression levels in both healthy and Ca. L. asiaticus-infected psyllids. These findings confirm the association of these genes with the melatonin biosynthesis pathway.