To test the feasibility of altering polyamine levels by influencing their catabolic pathway, we obtained transgenic tobacco (Nicotiana tabacum) plants constitutively expressing either maize (Zea mays) polyamine oxidase (MPAO) or pea (Pisum sativum) copper amine oxidase (PCuAO), two extracellular and H 2 O 2 -producing enzymes. Despite the high expression levels of the transgenes in the extracellular space, the amount of free polyamines in the homozygous transgenic plants was similar to that in the wild-type ones, suggesting either a tight regulation of polyamine levels or a different compartmentalization of the two recombinant proteins and the bulk amount of endogenous polyamines. Furthermore, no change in lignification levels and plant morphology was observed in the transgenic plants compared to untransformed plants, while a small but significant change in reactive oxygen species-scavenging capacity was verified. Both the MPAO and the PCuAO tobacco transgenic plants produced high amounts of H 2 O 2 only in the presence of exogenously added enzyme substrates. These observations provided evidence for the limiting amount of freely available polyamines in the extracellular space in tobacco plants under physiological conditions, which was further confirmed for untransformed maize and pea plants. The amount of H 2 O 2 produced by exogenously added polyamines in cell suspensions from the MPAO transgenic plants was sufficient to induce programmed cell death, which was sensitive to catalase treatment and required gene expression and caspase-like activity. The MPAO and PCuAO transgenic plants represent excellent tools to study polyamine secretion and conjugation in the extracellular space, as well as to determine when and how polyamine catabolism actually intervenes both in cell wall development and in response to stress.The polyamines putrescine (Put), spermidine (Spd), and spermine (Spm) are low M r metabolites naturally found in eukaryotic and prokaryotic cells (Cohen, 1998). Because of their polycationic nature at physiological pH, polyamines can bind strongly to negative charges in cellular components such as nucleic acids, various types of proteins, and acidic phospholipids (Cohen, 1998). In plant cells, polyamines can also be conjugated via an amide bond to hydroxycinnamic acids (Martin-Tanguy, 1997) and serve as precursors for secondary metabolites like nicotine. The function of the conjugated forms of polyamines is not known. However, they have long been associated with flowering (Martin-Tanguy, 1997) and plant-microbe interactions (Torrigiani et al., 1997;Mackintosh et al., 2001;Cowley and Walters, 2002).Polyamines play important roles in DNA stabilization, RNA and protein synthesis, membrane stabilization, modulation of ion channels, and protection against oxygen radicals, and they are essential for cell homeostasis, cell growth, and tumorigenesis (Wallace et al., 2003). In particular, plant polyamines have been implicated in a variety of plant growth and developmental processes involving cell proliferation an...