REGULATION OF RHIZOGENESIS BY POLYAMINES IN TOBACCO THIN-LAYERS

Tobacco thin layer explants were cultured on a rhizogenic medium for 2 1 days in the presence or absence of methylglyoxal-bis(guanylhydrazone ) (MGBG) or cyclohexylamine (CHA), competitive inhibitors of spermidin e synthesis through S-adenosylmethionine decarboxylase and spermidine synthase, respectively. On day 21, explants were transferred to hormon e-free medium of the same composition with or without MGBG or CHA, alo ne or in combination with labelled and unlabelled spermidine. The effe cts of these treatments on free and bound polyamine levels, on the put rescine biosynthetic activity (ornithine decarboxylase, ODC, and argin ine decarboxylase, ADC), on labelled spermidine incorporation and on t he rhizogenic response were studied. In MGBG-treated explants rhizogen esis was strongly inhibited while with CHA it was less affected. In th e former, the addition of spermidine caused a significant reversion of the rooting inhibition (considering both rooting percentage and the m ean number of roots per explant), while not in the latter. MGBG induce d strong putrescine and spermidine depletion both in the free and boun d forms; CHA induced strong spermidine depletion (especially on day 21 ), but putrescine accumulation. In both treatments, in the presence of labelled spermidine, labelled free and bound putrescine were also det ected. A general enhancement in ADC activity was observed in inhibitor -treated explants; the addition of spermidine to the inhibitors caused a further strong increase in ADC activity. These results are discusse d in relation to the different metabolic pathways affected by the inhi bitors.