UPTAKE COMPETITION BETWEEN POLYAMINES AND ANALOGS IN CARROT PROTOPLASTS

Uptake of different polyamines and a number of polyamine analogues, po tentially substrates for the putrescine and spermidine uptake system, were studied in protoplasts isolated from carrot (Daucus carota L. cv. ''Ingrid'') phloem parenchyma. Since calcium was shown to stimulate t he uptake of polyamines into carrot protoplasts, competition studies w ere performed both in the presence and absence of this cation in the i ncubation medium. Results suggest that, when calcium is absent, the tr ansport system for the uptake of putrescine and spermidine seems not t o be highly specific, as it is inhibited by other polyamines and by co mpounds structurally similar to them. Methylglyoxal-bis (guanylhydrazo ne) (MGBG), a competitive inhibitor of S-adenosylmethionine decarboxyl ase, seems to share the transport system for spermidine, by virtue of its structural similarity to this triamine. When calcium is present, t he polyamine transport system becomes much more complex, and spermine stimulates both putrescine and spermidine uptake; this stimulation is highly specific and energy-dependent. In the presence of calcium, sper mine uptake is not affected at all by putrescine and spermidine; this might suggest that spermine utilizes a distinct transport system.