""\\"Glutamate plays a central role in the flow of nitrogen to various regulatory molecules, such as Polyamines (PAs). PAs are low MW nitrogenous molecules. The most common PAs are the diamine Putrescine (Put), the triamine Spermidine (Spd) and the tetramine Spermine (Spm). In plants, Put and Spd are the obligatory precursors of Spd and Spm, respectively. PAs have been implicated in plant growth and development, abiotic and biotic stress responses (1,2,3,4,5,6). PA oxidation is catalyzed by Di- and PA oxidases (DAO and PAOs, respectively) in the apoplast [terminal catabolism; (3)],or in the cytoplasm and the peroxisomes [PA back-conversion; (4)]. In both cases, H2O2 is generated. Recently, PA catabolism-derived H2O2 was shown to significantly participate in the physiological functions of PAs. Stress induces PA exodus into the apoplast, where it is oxidized by apoplastic PAO leading to increase of H2O2 load; this in turn participates in signalling events, inducing differential gene expression, which results to either PA homeostatic responses or the execution of programmed cell death [PCD; (4,6)]. Thus, the level of apoplastic H2O2 determines whether stress-tolerant mechanisms or the PCD syndrome will be induced. Engineering the PA catabolic pathway led to increased tolerance to biotic but sensitivity to abiotic stress (5,6). The pathway is partially controlled by the phytohormone abscissic acid (ABA), which is involved in stress-dependent signalling cascades, including those controlling stomatal closure (7). ABA induces expression of AtPAO3, a peroxisomal Arabidopsis PAO and GUS reporter analysis, revealed that cis-regulatory elements in the promoter region control this induction (8,9). Interestingly, GUS activity post-treatment with ABA was specifically localized to guard cells, implying a direct role of PAO-derived H2O2 in stomatal closure. Moreover, the complete identification and analysis of AtPAOs in Arabidopsis revealed that all four PAOs (AtPAO1, AtPAO2, AtPAO3 and AtPAO4) are back-converting Spm to Spd and additionally AtPAO2 and AtPAO3 back-convert Spd to Put (8,9). Thus, Arabidopsis seems to lack PAOs involved in terminal catabolism of PAs in contrast to maize, in which the until now characterized PAOs produce 1,3-diaminopropane and 4-aminobutanal or N-(3-aminopropyl)-4-aminobutanal from Spd or Spm oxidation, respectively. H1-NMR studies revealed that AtPAOs, in addition to H2O2 and Spd or Put, produce 3-aminopropanal, which can be further converted to the osmoprotectant molecule β-alanine and pantothenate in a pairwise reaction. The AtPAOs are differentially expressed, as revealed by GUS reporter assays, implying functional diversity inside the AtPAOs family (8,9). Furthermore, Spd Oxidase-derived H2O2 regulates pollen plasma membrane hyperpolarization-activated Ca2+-permeable channels, pollen tube growth and number of seeds (10). Also, PAO-derived H2O2 acts as a signal for deposition of secondary wall and as a mediator of developmental PCD in plant tissues resulting to suppressed primary maize root elongation by inhibiting cell growth and altering cell cycle progression (11). To sum up, all these results along with the recent finding of the involvement of AtPAOs in catabolism of thermospermine, a Spm isomer involved in vascular differentiation and stress adaptation, reveal novel aspects of the PA catabolic pathway and its genetic engineering may result to significant phenotypical and stress-tolerant genotypes. . . 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Moschou PN, Skopelitis DS, Sanmartin M, Velanis C, Fincato P, Tisi A, et al. (2011). Nitrogen flow into regulatory molecules: The case of Polyamines and Polyamine oxidases.. In Environment workshop; Nitrogen use efficiency in plants: toward models of sustainable agriculture.