Essential for biosynthesis of the polyamines spermidine and spermine. Essential for polyamine homeostasis, and normal plant embryogenesis, growth and development. H(+) + S-adenosyl-L-methionine = CO2 + S-adenosyl 3-(methylsulfanyl)propylamine Binds 1 pyruvoyl group covalently per subunit. Optimum pH is 6.8-7.2. Amine and polyamine biosynthesis; S-adenosylmethioninamine biosynthesis; S-adenosylmethioninamine from S-adenosyl-L-methionine: step 1/1. Down-regulated by auxin. Is synthesized initially as an inactive proenzyme. Formation of the active enzyme involves a self-maturation process in which the active site pyruvoyl group is generated from an internal serine residue via an autocatalytic post-translational modification. Two non-identical subunits are generated from the proenzyme in this reaction, and the pyruvate is formed at the N-terminus of the alpha chain, which is derived from the carboxyl end of the proenzyme. The post-translation cleavage follows an unusual pathway, termed non-hydrolytic serinolysis, in which the side chain hydroxyl group of the serine supplies its oxygen atom to form the C-terminus of the beta chain, while the remainder of the serine residue undergoes an oxidative deamination to produce ammonia and the pyruvoyl group blocking the N-terminus of the alpha chain. Reduction in the length of stem internodes. Increased thickness of veins in leaves and inflorescence stems. Altered morphology of xylem vessel elements. The double mutants of bud2-1 and samdc1-1 are embryonic lethal. Belongs to the eukaryotic AdoMetDC family.