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). Required for the biosynthesis of putrescine (By similarity). Catalyzes the first step of polyamine (PA) biosynthesis to produce putrescine from arginine (By similarity). H(+) + L-arginine = agmatine + CO2 Alkaloid biosynthesis; nicotine biosynthesis. Amine and polyamine biosynthesis; agmatine biosynthesis; agmatine from L-arginine: step 1/1. Interacts, via its C-terminal internal region, with the tobacco mosaic virus (TMV) replicase helicase region. 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). Belongs to the Orn/Lys/Arg decarboxylase class-II family. SpeA subfamily.