"Arabidopsis amiR:ADC-L2 is a non-lethal line with several developmental defects, it is characterized by a drastic reduction in free polyamine content. Herein, we found that catalase application had growth-promoting effects in amiR:ADC-L2 and parental Ws seedlings. Differences in ROS content between amiR:ADC-L2 and Ws seedlings were detected. Increased H2O2 levels were found in the amiR:ADC-L2, as well as low AtCAT2 gene expression and reduced catalase activity. Estimation of polyamine oxidase activity in amiR:ADC-L2 line indicated that the over-accumulation of H2O2 is independent of polyamine catabolism. However, increments in NADPH oxidase activity and O2•? content could be associated to the higher H2O2 levels in the amiR:ADC-L2 line. Our data suggest that low polyamine levels in Arabidopsis seedlings are responsible for the accumulation of ROS, by altering the activities of enzymes involved in ROS production and detoxification."