The present invention relates to isolated plant-derived polyamine metabolism-related enzyme genes characterized in that their level of expression changes when exposed to environmental stress; antisense DNA or antisense RNA to such genes; plants and their progeny having improved environmental stress tolerance, in which such genes are retained in a stable manner; methods for producing such plants, and methods for producing calli of such plants; method for selecting transformed plants which grow improved more than plants lacking an exogenous polyamine metabolism-related enzyme gene; and microorganims and plants which are transformed by such genes