REGULATION OF RHIZOGENESIS BY POLYAMINES IN TOBACCO THIN-LAYERS

Tobacco thin layer explants were cultured on a rhizogenic medium for 21 days in the presence or absence of methylglyoxal-bis(guanylhydrazone) (MGBG) or cyclohexylamine (CHA), competitive inhibitors of spermidine synthesis through S-adenosylmethionine decarboxylase and spermidine synthase, respectively. On day 21, explants were transferred to hormone-free medium of the same composition with or without MGBG or CHA, alone or in combination with labelled and unlabelled spermidine. The effects of these treatments on free and bound polyamine levels, on the putrescine biosynthetic activity (ornithine decarboxylase, ODC, and arginine decarboxylase, ADC), on labelled spermidine incorporation and on the rhizogenic response were studied. In MGBG-treated explants rhizogenesis was strongly inhibited while with CHA it was less affected. In the former, the addition of spermidine caused a significant reversion of the rooting inhibition (considering both rooting percentage and the mean number of roots per explant), while not in the latter. MGBG induced strong putrescine and spermidine depletion both in the free and bound 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 detected. 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 discussed in relation to the different metabolic pathways affected by the inhibitors.