UPTAKE COMPETITION BETWEEN POLYAMINES AND ANALOGS IN CARROT PROTOPLASTS

Uptake of different polyamines and a number of polyamine analogues, potentially 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 the uptake of polyamines into carrot protoplasts, competition studies were performed both in the presence and absence of this cation in the incubation medium. Results suggest that, when calcium is absent, the transport system for the uptake of putrescine and spermidine seems not to be highly specific, as it is inhibited by other polyamines and by compounds structurally similar to them. Methylglyoxal-bis (guanylhydrazone) (MGBG), a competitive inhibitor of S-adenosylmethionine decarboxylase, seems to share the transport system for spermidine, by virtue of its structural similarity to this triamine. When calcium is present, the 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, spermine uptake is not affected at all by putrescine and spermidine; this might suggest that spermine utilizes a distinct transport system.