Required for the biosynthesis of putrescine. Catalyzes the first step of polyamine (PA) biosynthesis to produce putrescine from arginine (PubMed:14684170, PubMed:15733873, PubMed:16045477, PubMed:25409942). Is a major contributor to basal arginine decarboxylase (ADC) activity and putrescine biosynthesis (PubMed:25409942). Accumulation of putrescine plays a positive role in salt stress tolerance (PubMed:14684170). Accumulation of putrescine plays a positive role in freezing tolerance (PubMed:18701673). Production of PA 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 and heterodimer with ADC1. Localizes mainly in the chloroplast. Induced by wounding, methyl jasmonate and abscisic acid (ABA) (PubMed:12428010). Induced by salt stress (PubMed:14684170). Induced by osmotic stress (PubMed:10481069). Induced by freezing (PubMed:18701673). Induced by the bacterial pathogen Pseudomonas viridiflava (PubMed:25409942). Increased levels of lateral root branching. Plants over-expressing ADC2 accumulate very high levels of putrescine and exhibit dwarfism and late-flowering phenotypes. Belongs to the Orn/Lys/Arg decarboxylase class-II family. SpeA subfamily.