Characterization of polyamine oxidase genes in cucumber and roles of CsPAO3 in response to salt stress.

Polyamine oxidases (PAOs) are flavin adenine dinucleotide-dependent enzymes that catalyze oxidation of polyamines (PAs), and play an important roles in growth and development and stress tolerance of plant. In this study, we identified four PAO genes (CsPAO1 - CsPAO4) in cucumber (Cucumis sativus L), and classified them into three clades (Ⅰ, Ⅱ and Ⅲ). CsPAO1 and CsPAO3 were located in cell membrane or nucleus while CsPAO2 and CsPAO4 were likely localized in peroxisome. CsPAO1 - 2 and CsPAO4 consisted of ten exons and nine introns, but no intron was observed in CsPAO3. The CsPAOs showed higher expression level in root, with the exception of CsPAO1, which was mainly expressed in flower, and they had various responses to hormone and stress treatments. Stress treatments induced more obvious change of gene expression, and CsPAO2–4 responded to NaCl stress earlier both in root and leaf. Overexpression of CsPAO3 in Arabidopsis improved seed germination and root growth on medium containing NaCl. Furthermore, under salt stress, electrolyte leakage (EL), malonaldehyde (MDA) and hydrogen peroxide (H 2 O 2) content were decreased, but peroxidase (POD) and catalase (CAT) activities were increased in transgenic Arabidopsis lines as compared with wild type (WT), contributing to alleviating the salt stress-induced growth suppression. The OE15 line, on the other hand, had the highest expression level of CsPAO3 , exhibited identical phenotype to WT, suggesting that excess H 2 O 2 produced by polyamine oxidation might be harmful to plants. Taken together, our results indicated that CsPAO family members are involved in plant development and stress responses, and a proper level of CsPAO3 overexpression enhances salt tolerance of plants. • There are four PAO family members in cucumber genome. • CsPAO3 was a candidate gene enhancing plants' tolerance to salt. • Higher overexpression level of CsPAO3 couldn't lead to stronger salt resistance of plants, and excess H 2 O 2 produced by polyamine oxidation may be harmful to plants. [ABSTRACT FROM AUTHOR]