INHIBITION OF POLYAMINE METABOLISM IN PHYTOPHTHORA SPECIES

The effects of inhibitors of polyamine metabolism on the growth, ornithine decarboxylase (ODC) and polyamine concentration of Phytophthora infestans have been investigated, and comparisons made with Phytophthora cactorum and Phytophthora nicotianae. The enzyme-activated inhibitor of ODC, difluoromethylornithine (DFMO) at 0.1 mm inhibited the radial growth of P. infestans after 21 d to ca 30% of the water control. Using-a homologous series of diamines, (NH2(CH2)xNH2.2HCl), full reversal of growth inhibition due to DFMO was achieved when x = 4, 5 and 10; and no reversal was found when x = 3 or 7. Spermidine, spermine and agmatine also fully reversed the inhibition due to DFMO. The inhibitor of arginine decarboxylase, monofluoromethyldehydroarginine, was almost as effective as DFMO as a growth inhibitor. The growth of P. cactorum and P. nicotianae was relatively insensitive to DFMO. The in vitro activity of the ODC was reduced by 0.1 mm DFMO, monofluoromethyldehydroornithine or methylacetylenic putrescine to less than 3% and 5% of the control in P. infestans and P. cactorum, respectively. In P. infestans (grown with or without 1 mm DFMO), putrescine, spermidine or spermine were all less than 10 nmol g-1 fresh weight. However in P. cactorum, spermidine was present at 1.4 mumol.g-1 fresh weight, and was reduced to 38% by 1 mm DFMO. The results suggest that the greater growth inhibition in P. infestans caused by the inhibitors of polyamine metabolism may be related to the inherently smaller polyamine concentration, which probably results from the relatively low ODC activity found in this fungus.