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) (PubMed:8038607, PubMed:31276744, PubMed:34095742). Methyltransferase that mediates the conversion of putrescine to N-methylputrescine (PubMed:8038607, PubMed:9620262). Promotes leaves ripening (PubMed:34095742). putrescine + S-adenosyl-L-methionine = H(+) + N-methylputrescine + S-adenosyl-L-homocysteine Alkaloid biosynthesis; nicotine biosynthesis. Predominantly expressed in roots. Down-regulated by auxin (PubMed:8038607). Accumulates upon the removal of flower heads and young leaves (PubMed:8038607, PubMed:10598105). Triggered by jasmonic acid (MeJA) (PubMed:10965939, PubMed:15604714, PubMed:9869416). Accumulation of anatabine at the expense of nicotine (PubMed:14756309). Plants suppressed for PMT1, PMT2, PMT3 and PMT4 exhibit strongly reduced nicotine levels but accumulate polyamines in roots, and have an impaired leaf maturation phenotype at harvest (PubMed:34095742). Belongs to the class I-like SAM-binding methyltransferase superfamily. Putrescine methyltransferase family.