Expression of NtNEK2DD, a constitutively active mutant of NtMEK2, activates endogenous salicylic acid-induced protein kinase (SIPK) and wounding-induced protein kinase (WIPK), and leads to several stress/defense responses in tobacco. In this study, we used ACP (annealing control primer)-based differential display reverse transcription–PCR to isolate the downstream effectors mediated by the NtMEK2–SIPK/WIPK cascade. The arginine decarboxylase gene (ADC), which is involved in plant putrescine biosynthesis, was one of nine differentially expressed genes. When compared with NtMEK2KR plants, NtMEK2DD transgenic plants exhibited a significant increase in ADC and ODC (ornithine decarboxylase) transcript levels, as well as in putrescine and its catabolite, γ-aminobutyric acid, following SIPK/WIPK activation. Taken together, these results suggest that the NtMEK2–SIPK/WIPK cascade is involved in regulating polyamine synthesis, especially putrescine synthesis, through transcriptional regulation of the biosynthetic genes in tobacco. [ABSTRACT FROM AUTHOR]