In Cantharanthus roseus transformed roots, the application of methylglyoxal bis(guanylhydrazone) (MGBG), an inhibitor of S-adenosylmethionine decarboxylase (SAMDC; EC 4.1.1.50), inhibited the root growth in a dose-dependent manner with a DL50 of about 300 μm. Spermidine and spermine (Spm) levels and SAMDC and phospholipase C (PLC; EC 3.1.4.3) activities were reduced in the presence of the inhibitor. The inhibition was reversed by the addition of Spm. Radioactivity from [14C[Spm was detected in an immunoprecipitated fraction with an antibody anti-PLC-δ. To our knowledge, this is the first direct evidence that demonstrates an interaction of Spm with the signal transduction cascade phosphoinositide-Ca 2+.