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:21232776). Catalyzes the first and rate-limiting step of polyamine biosynthesis that converts ornithine into putrescine, which is the precursor for the polyamines, spermidine and spermine (By similarity). Polyamines are essential for cell proliferation and are implicated in cellular processes, ranging from DNA replication to apoptosis (By similarity). H(+) + L-ornithine = CO2 + putrescine Alkaloid biosynthesis; nicotine biosynthesis. Amine and polyamine biosynthesis; putrescine biosynthesis via L-ornithine pathway; putrescine from L-ornithine: step 1/1. Homodimer. Only the dimer is catalytically active, as the active sites are constructed of residues from both monomers. Induced by wounding. Reduced nicotine but increased anatabine contents; these phenotypes are enhanced upon treatment with jasmonic acid (MeJA) and wounding (e.g. removal of apices). Belongs to the Orn/Lys/Arg decarboxylase class-II family.