We previously gave a picture of the homeostatic characteristics of polyamine (PA) biosynthesis and conjugation in tobacco (Nicotiana tabacum) plant organs during development. In this work, we present the sites and regulation of PA catabolism related to cell division/expansion, cell cycle progression, and vascular development in the tobacco plant. Diamine oxidase (DAO), PA oxidase (PAO), peroxidases (POXs), and putrescine N-methyltransferase expressions follow temporally and spatially discrete patterns in shoot apical cells, leaves (apical, peripheral, and central regions), acropetal and basipetal petiole regions, internodes, and young and old roots in developing plants. DAO and PAO produce hydrogen peroxide, a plant signal molecule and substrate for POXs. Gene expression and immunohistochemistry analyses reveal that amine oxidases in developing tobacco tissues precede and overlap with nascent nuclear DNA and also with POXs and lignification. In mature and old tissues, flow cytometry indicates that amine oxidase and POX activities, as well as pao gene and PAO protein levels, coincide with G2 nuclear phase and endoreduplication. In young versus the older roots, amine oxidases and POX expression decrease with parallel inhibition of G2 advance and endoreduplication, whereas putrescine N-methyltransferase dramatically increases. In both hypergeous and hypogeous tissues, DAO and PAO expression occurs in cells destined to undergo lignification, suggesting a different in situ localization. DNA synthesis early in development and the advance in cell cycle/endocycle are temporally and spatially related to PA catabolism and vascular development.Polyamines (PAs) are abundant DNA-and RNAbinding organic cations. They have been correlated with specific roles in embryonic development (Kusunoki and Yasumasu, 1978), carcinogenesis and immune system functions (Seiler et al., 1998), defense (Schroder et al., 1998), antibiotic potency (Petropoulos et al., 2004), cell proliferation and apoptosis (for review, see Wallace et al., 2003;Janne et al., 2004), and in many biotic and abiotic stresses (Jokela et al., 1997;Kurepa et al., 1998;Perez-Amador et al., 2002; for review, see Bouchereau et al., 1999). Investigations on PA physiological functions have been focused mainly on changes in their levels and spectra, leaving the biological significance to be determined. Although genetic analyses have been conducted at the gene level (Laurenzi et al., 1999;Sebela et al., 2001;Rea et al., 2002;Papadakis and Roubelakis-Angelakis, 2005). H 2 O 2 produced by the action of diamine oxidases (DAOs) and PA oxidases (PAOs) can be utilized by POX enzymes in wallstiffening reactions during cell growth and differentiation (Wisniewski et al., 2000). In alkaloid producing plants, such as tobacco (Nicotiana tabacum), putrescine (Put), apart from serving as substrate for spermidine (Spd) biosynthesis or for oxidation (Bagni and Tassoni, 2001), is N-methylated by Put N-methyltransferase (PMT), which catalyzes the first committed steps in the biosynthesis of n...