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:32242247, PubMed:27126795). 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 (By similarity). Only the dimer is catalytically active, as the active sites are constructed of residues from both monomers (By similarity). Accumulates transiently in response to jasmonic acid (MeJA). Reduced alkaloids and nicotin levels associated with a lower putrescine production (PubMed:32242247). Occasionally, an early senescence and a lower viability of the older leaves is observed (PubMed:32242247). Plants lacking both ODC1A and ODC1B have lower concentrations of nicotine and nornicotine as well as of polyamines (putrescine, spermidine and spermine), tyramine and phenolamides (caffeoylputrescine and dicaffeoylspermidine), but significantly higher levels of anatabine and of amino acids ornithine, arginine, aspartate, glutamate and glutamine; these phenotypes are associated with reduced plant growth and vigor, and are exacerbated by wounding and shoot apex removal (PubMed:27126795). Belongs to the Orn/Lys/Arg decarboxylase class-II family.