Serotonin N-acetyltransferase (SNAT) is a key enzyme in the biosynthesis of melatonin, and plays an important role in the regulation of melatonin synthesis. The study of SNAT is of great significance to understand the function of melatonin. In this study, we analyzed the structural characteristics, phylogenetic relationship, gene structure, expression pattern, evolutionary relationship and stress response of the members of the SNAT gene family in upland cotton through bioinformatics. A putative Serotonin n-acetyltransferase gene GhSNAT3D was identified, and preliminarily function of GhSNAT3D was verified by virus-induced gene silencing. We identified a total of 52 SNAT genes in the whole genome of G. hirsutum, and part of the GhSNATs were regulated by exogenous melatonin. The content of melatonin, antioxidant enzyme activity and Ca2+ content of GhSNAT3D gene silenced plants decreased, and the salt tolerance of GhSNAT3D gene silenced plants was reduced. Exogenous melatonin supplementation restored the salt tolerance of GhSNAT3D gene silenced plants. GhSNAT3D may interact with GhSNAT25D and ASMT to regulate melatonin synthesis. This study provided an important basis for further study on the regulation of melatonin in cotton against abiotic stress.

Glutamate decarboxylase (GAD) mainly regulated the biosynthesis of γ-aminobutyric acid (GABA) and played an important role in plant growth and stress resistance. To explore the potential function of GAD in cotton growth, the genome-wide identification, structure, and expression analysis of GAD genes were performed in this study. There were 10, 9, 5, and 5 GAD genes identified in G. hirsutum, G. barbadense, G. arboreum, and G. raimondii, respectively. GAD was divided into four clades according to the protein motif composition, gene structure, and phylogenetic relationship. The segmental duplication was the main way of the GAD gene family evolution. Most GhGADs respond to abiotic stress. Clade Ⅲ GAD was induced by Cd2+ stress, especially GhGAD6, and silencing GhGAD6 would lead to more serious Cd2+ poisoning in cotton. The oxidative damage caused by Cd2+ stress was relieved by increasing the GABA content. It was speculated that the decreased expression of GhGAD6 reduced the content of GABA in vivo and caused the accumulation of ROS. This study will further expand our understanding of the relationship between the evolution and function of the GhGAD gene family and provide new genetic resources for cotton breeding under environmental stress and phytoremediation.