Genome-wide identification and expression profiling of copper-containing amine oxidase genes in sweet orange (Citrus sinensis)

Copper-containing amine oxidases (CuAOs) play important roles in polyamine catabolism and function in plant development and abiotic stresses response. In this study, eight putative CuAO genes (CsCuAO1-CsCuAO8), distributed on three chromosomes, were identified in sweet orange (Citrus sinensis Osbeck) genome. They were divided into two major groups according to the phylogenetic analysis. The CsCuAOs possess typically conserved 33 amino acid residues and contain either a C-terminal peroxisomal targeting peptide or an N-terminal signal. Transcript levels of the CsCuAOs were detected in leaf, stem cotyledon, and root, but tissue-specific expression pattern was observed. Many putative cis-elements involved in hormone and stress response were predicted in a 1.5-kb promoter region of the CsCuAOs. The CsCuAOs were significantly induced by exogenous polyamines, including putrescine, spermidine, and spermine. ABA and various abiotic stresses (cold, salt, and osmotic shock) down-regulated most of the CsCuAOs, whereas CsCuAO3 and CsCuAO7 were up-regulated. Transient expression assay demonstrate that CsCuAO2 was involved in putrescine catabolism. Taken together, these findings provide valuable knowledge for better understanding of the potential biological functions of CuAO genes in polyamine catabolism of sweet orange.