- Expression of a heterologous S-adenosylmethionine decarboxylase cDNA in plants demonstrates that changes in S-adenosyl-L-methionine decarboxylase activity determine levels of the higher polyamines spermidine and spermine. Thu-Hang, P., Bassie, L., Safwat, G., Trung-Nghia, P., Christou, P., Capell, T. Plant Physiol. (2002)