Polyamines (PAs), spermidine (Spd), spermine (Spm) and their diamine obligate precursor putrescine (Put), are small aliphatic amines that are ubiquitous in all plant cells. They have been proposed to be closely associated with development, morphogenesis and responses to diverse stresses. In comparison with large and detailed investigations focused on the regulation of free PA levels and biosynthetic enzyme activities, little attention was paid to the physiological roles of PA catabolism and its catabolic pr...