Flavoenzyme involved in polyamine back-conversion (PubMed:24550437, PubMed:24906355, PubMed:26973665, PubMed:28199662). Catalyzes the oxidation of the secondary amino group of polyamines, such as spermine and its acetyl derivatives (PubMed:24550437, PubMed:24906355, PubMed:28199662). Substrate preference is spermine > N(1)-acetylspermine > thermospermine > norspermine (PubMed:24550437). Plays an important role in the regulation of polyamine intracellular concentration (PubMed:24550437, PubMed:26973665, PubMed:28199662). Involved in xylem differentiation by controlling thermospermine homeostasis, and participating in the tightly controlled interplay between auxin and cytokinin that is necessary for proper xylem differentiation (PubMed:28199662). Involved in the production of hydrogen peroxide in response to salt and cold stresses (PubMed:26973665). 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. Optimum pH is 6.5 with thermospermine as substrate (PubMed:24906355). Optimum pH is 7.5 with spermine as substrate (PubMed:24906355). Optimum temperature is 40-45 degrees Celsius with thermospermine as substrate (PubMed:24906355). Optimum temperature is 35-45 degrees Celsius with spermine as substrate (PubMed:24906355). Amine and polyamine degradation; spermine degradation. Expressed in root vasculature, leaves and stems. Induced by salicylic acid (PubMed:18583528). Down-regulated upon treatment with flagellin 22, a pathogen elicitor (PubMed:18583528). Induced by auxin, cytokinin and thermospermine in roots (PubMed:28199662). Induced by spermine, thermospermine, N-acetylspermine and spermidine in roots (PubMed:24550437). 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). Increased length and thickness of floral stems (PubMed:28199662). Increased length of roots (PubMed:28199662). Delayed transition from vegetative to reproductive stage (PubMed:24906355). Increased levels of thermospermine (PubMed:24906355). Belongs to the flavin monoamine oxidase family.