To understand the physiological functions of polyamine oxidases (PAOs) in plants, we analyzed the effects of exogenous polyamines during seed germination and early seedling development, using Arabidopsis thaliana lines independently harboring T-DNA insertions in each PAO gene. Spermine caused seedling growth inhibition but did not affect the germination in all lines including wild-type Col-0. However, an AtPAO2 knockout mutant, -pao2, could not germinate under excess spermidine (Spd) conditions. The root growth rates of post-germination -pao2 seedlings were also strongly inhibited by the Spd treatment compared with the wild-type plants. AtPAO2 has a conserved peroxisome-targeting signal sequence at its C-terminus. We prepared two types of AtPAO2 expression plants in a -pao2 background. In -pao2/PAO2 plants a 5.8-kbp genomic fragment containing the complete coding sequence of AtPAO2 was introduced, while in -pao2/PAO2(Delta C) plants the same fragment lacking the peroxisome-targeting signal was introduced. The Spd-sensitive phenotypes observed in -pao2 were completely recovered in both of the transgenic complementation lines. Thus, AtPAO2 appears to be involved in excess Spd catabolism during seed germination and early seedling development irrespective of subcellular localization. (C) 2019 Elsevier Inc. All rights reserved.