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. Polyamines are essential for cell proliferation and are implicated in cellular processes, ranging from DNA replication to apoptosis. H(+) + L-ornithine = CO2 + putrescine Inhibited by antizyme (AZ) in response to polyamine levels. AZ inhibits the assembly of the functional homodimer by binding to ODC monomers and targeting them for ubiquitin-independent proteolytic destruction by the 26S proteasome. 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. High expression in roots, shoot tips and whole flowers at anthesis. Lower expression in stems and the lowest in adult leaves. Transient induction at 8 days post-anthesis. ODC activity seems to be related to active cell division. Belongs to the Orn/Lys/Arg decarboxylase class-II family.