Identification of seven polyamine oxidase genes in tomato (Solanum lycopersicum L.) and their expression profiles under physiological and various stress conditions

Polyamines (PAs) are implicated in developmental processes and stress responses of plants. Polyamine oxidases (PAOs), flavin adenine dinucleotide-dependent enzymes that function in PA catabolism, play a critical role. Even though PAO gene families of Arabidopsis and rice have been intensely characterized and their expression in response to developmental and environmental changes has been investigated, little is known about PAOs in tomato (Solarium lycopersicum). We found seven PAO genes in S. lycopersicum and named them SIPAO1 similar to 7. Plant PAOs form four clades in phylogenetic analysis, of which SIPAO1 belongs to Glade-I, SIPAO6 and SIPAO7 to Glade-III, and the residual four (SIPAO2 similar to 5) to clade-IV, while none belongs to Glade-II. All the clade-IV members in tomato also retain the putative peroxisomal-targeting signals in their carboxy termini, suggesting their peroxisome localization. SIPAO1 to SIPAO5 genes consist of 10 exons and 9 introns, while SIPAO6 and SIPAO7 are intronless genes. To address the individual roles of SIPAOs, we analyzed their expression in various tissues and during flowering and fruit development. The expression of SIPAO2 similar to 4 was constitutively high, while that of the other SIPAO members was relatively lower. We further analyzed the expressional changes of SIPAOs upon abiotic stresses, oxidative stresses, phytohormone application, and PA application. Based on the data obtained, we discuss the distinctive roles of SIPAOs.