Flavoenzyme involved in polyamine back-conversion (PubMed:16778015, PubMed:20532512, PubMed:21081665, PubMed:26973665). Catalyzes the oxidation of the secondary amino group of polyamines, such as spermine and its acetyl derivatives (PubMed:16778015, PubMed:20532512, PubMed:21081665). Substrate preference is thermospermine > norspermine > spermine > N(1)-acetylspermine (PubMed:16778015, PubMed:21081665). No activity detected when putrescine, spermidine or N(1)-acetylspermidine are used as substrates (PubMed:16778015). Plays an important role in the regulation of polyamine intracellular concentration (Probable) (PubMed:26973665). Involved in the production of hydrogen peroxide in response to salt and cold stresses (PubMed:26973665). H2O + O2 + spermine = 3-aminopropanal + H2O2 + spermidine H2O + O2 + spermine = 3-aminopropanal + H2O2 + spermidine H2O + N(1)-acetylspermine + O2 = 3-acetamidopropanal + H2O2 + spermidine H2O + norspermine + O2 = 3-aminopropanal + H2O2 + norspermidine H2O + O2 + thermospermine = 3-aminopropanal + H2O2 + spermidine Binds 1 FAD per subunit. Inhibited by guazatine, N-prenylagmatine and 1,12-diaminododecane. Optimum pH is 8.0. Amine and polyamine degradation; spermine degradation. Expressed at very low levels in leaves, stems and inflorescences. No visible phenotype of seedlings under normal growth conditions (PubMed:26973665). The double mutants pao1 and pao5 exhibit enhanced tolerance to salt and drought stress (PubMed:26973665). Belongs to the flavin monoamine oxidase family.