| Abstract |
Polyamine oxidases (PAOs) are enzymes that catalyze the oxidative deamination of higher polyamines (PAs), producing hydrogen peroxide (Η2Ο2) and aldehydes. Η2Ο2 possesses signaling roles, inducing either defence responses or cell destruction. In order to reveal the signaling role and the physiological role of the produced Η2Ο2 from the apoplastic PAO, transgenic tobacco plants were contructed, either overexpressing (S-PAO) or downregulating (A-PAO) a pao gene from maize. Transgenic plants showed decreased and increased PAs respectively, of the higher PAs Spermidine (Spd) and Spermine (Spm). Nevertheless, they do not exhibit significant alteration of the titers of the produced Η2Ο2 in comparison with WT plants in normal growth conditions. The previous is attributed to the induction of the antioxidant machinery, specifically in the transgenic S-PAO cells. On the contrary, when stressed S-PAO plants, cannot tolerate the increased Η2Ο2, thus inducing instead of additional responses PCD. In high salt conditions, S-PAO plants were significantly more sensitive in comparison with the corresponding WT plants, whereas A-PAO plants were more tolerant. Moreover, the mechanism by which PAs act during stress was elucidated. Thus, it was found that PA biosynthesis in being induced in salt stress conditions and Spd is transferred to the apoplast were is oxidized producing Η2Ο2, which ‘signature’ promotes either tolerance responses (WT and A-PAO; medium and low levels of Η2Ο2) or the PCD syndrome (S-PAO; High Η2Ο2 levels). Moreover, endogenous PAs seem to possess a significant role in these responses, since transgenic plants that suppress the samdc gene, which is involved in synthesis of higher PAs, were significantly more sensitive to salt stress, mimicking in that notion S-PAO plants. On the contrary, in biotic stress S-PAO plants were significantly more tolerant to bacteria, and oomycetes but not to viruses. The increased tolerance of these plants is due to two factors: To the induction of defense genes and to cell wall modifications found in these plants. Both of these alterations depend on the Η2Ο2 produced in S-PAO plants. During infection Η2Ο2is produced in the apoplast from Spm oxidation. Thus, S-PAO plants post-infection accumulate significantly higher levels of Η2Ο2. On the contrary, A-PAO plants are more sensitive to infection. Moreover, pao genes from Arabidopsis were also cloned and analyzed. Five genes were found that encode for proteins with domains found in PAO enzymes. These genes were expressed in heterologous systems and the five corresponding proteins were purified. Thus, four out of five (AtPAO1, AtPAO2, AtPAO3, AtPAO4, and AtPAO5) oxidize PAs, and more specifically Spd and Spm. In contrast to the previously characterized apoplastic PAOs, these genes are implicated to the interconversion of PAs, exhibiting strong resemblance to their mammalian counterparts. To elucidate the role of these genes, transgenic Arabidopsis plants overexpressing AtPAO1, AtPAO3 and AtPAO5 were constructed. The transgenic plants that overexpress AtPAO3 exhibited significantly increased Put. These plants showed increased osmotic tolerance, probably due to the increased levels of 3-aminopropanal that is produced by the increased AtPAO3 protein. 3-aminopropanal could be further converted to β-alanine-betaine which is a compatible osmolyte. Finally, the results presented here show that PA apoplastic oxidation confers sensitivity against abiotic stress and tolerance against biotic stress. On the other hand, non-apoplastic PA oxidation seems to have a more complicated role
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Ανάλυση του φυσιολογικού ρόλου των οξειδασών των πολυαμινών σε άγριου τύπου και γενετικά τροποποιημένα φυτά καπνού (Nicotiana tabacum L.) & arabidopsis (Arabidopsis thaliana)
