Involved in the biosynthesis of pyridine alkaloid natural products, leading mainly to the production of anabasine, anatabine, nicotine and nornicotine, effective deterrents against herbivores with antiparasitic and pesticide properties (neurotoxins); nornicotine serves as the precursor in the synthesis of the carcinogen compound N'-nitrosonornicotine (NNN) (By similarity). Methyltransferase that mediates the conversion of putrescine to N-methylputrescine (By similarity). putrescine + S-adenosyl-L-methionine = H(+) + N-methylputrescine + S-adenosyl-L-homocysteine Alkaloid biosynthesis; nicotine biosynthesis. Mainly expressed in roots. Triggered by jasmonic acid (MeJA); this induction is repressed by 2-chloroethylphosphonic acid (ethephon), an ethylene precursor, that can by alleviated by 1-methylcyclopropene (1-MCP), a competitive inhibitor of ethylene receptors (PubMed:11299398). Accumulates upon wounding and feeding by the specialist herbivore Manduca sexta; these induction is dramatically amplified by a pretreatment with 1-MCP (PubMed:11299398). Belongs to the class I-like SAM-binding methyltransferase superfamily. Spermidine/spermine synthase family.