Plants having improved tolerance to various types of environmental stress, their production, and polyamine metabolism-related enzyme genes

A method for producing a transgenic plant, which method comprises:

• (a) transforming cells of a plant with an exogenous spermidine synthase coding sequence under the control of a promoter capable of functioning in plants, wherein the exogenous spermidine synthase coding sequence comprises a base sequence encoding an amino acid sequence comprising at least 82% homology to SEQ ID NO:
• 2,
• (b) expressing the exogenous spermidine synthase coding sequence in the transformed cells,
• (c) generating plants from the transformed cells, and
• (d) selecting a transgenic plant from the step (c), wherein the selected transgenic plant has improved environmental stress tolerance against at least two environmental stresses selected from drought stress, herbicidal stress, oxidation stress, cold stress, osmotic stress, and salt stress as compared to a plant of the same species lacking the exogenous spermidine synthase coding sequence, and wherein the selected transgenic plant has 1.12 to 2.29 fold more free putrescine, 1.38 to 1.68 fold more free spermidine, and 1.12 to 1.98 fold more free spermine than a plant of the same species lacking the exogenous spermidine synthase coding sequence.