Required for the biosynthesis of putrescine. Catalyzes the first step of polyamine (PA) biosynthesis to produce putrescine from arginine (PubMed:11576438, PubMed:15733873, PubMed:25409942). Is a minor contributor to basal arginine decarboxylase (ADC) activity and putrescine biosynthesis (PubMed:25409942). Accumulation of putrescine plays a positive role in freezing tolerance (PubMed:18701673). Production of polyamines is essential for normal seed development (PubMed:15733873). Controls PA homeostasis which is crucial for normal plant growth and development (PubMed:27014322). H(+) + L-arginine = agmatine + CO2 Amine and polyamine biosynthesis; agmatine biosynthesis; agmatine from L-arginine: step 1/1. Homodimer. Only the dimer is catalytically active, as the active sites are constructed of residues from both monomers. May form a head-to-tail homodimer (PubMed:11576438). Homodimer and heterodimer with ADC2 (PubMed:28109885). Localizes mainly in the chloroplast. Induced by freezing (PubMed:18701673). Induced by the bacterial pathogen Pseudomonas viridiflava (PubMed:25409942). Increased levels of lateral root branching. Belongs to the Orn/Lys/Arg decarboxylase class-II family. SpeA subfamily.