THE EFFECT OF METHIONINE, ETHYLENE AND POLYAMINE CATABOLIC INTERMEDIATES ON POLYAMINE ACCUMULATION IN DETACHED SOYBEAN LEAVES

In the present study we determined the effects of methionine, intermed iates of polyamine catabolic pathways and inhibitors of either ethylen e biosynthetic or polyamine catabolic pathways on polyamine accumulati on in soybean leaves. Inhibitors to SAM decarboxylase and spermidine s ynthase, methylglyloxal-bis-(guanylhydrazone) and cyclohexylamine, res pectively, suggest that methionine may provide aminopropyl groups for the synthesis of polyamine via S-adenosylmethionine (SAM). Results fro m experiments that utilized a combination of compounds which altered e ither ethylene or polyamine biosynthesis, namely, aminoethoxyvinyl gly cine, CoSO4, 2,5-norbornadiene, and CuSO4, suggest the two pathways co mpete for a common precursor. However, exogenous addition of ethylene (via ethephon treatments) had little or no effect on polyamine biosynt hesis. Likewise, polyamine treatments had little or no effect on ethyl ene biosynthesis. These data suggest that there are few or no inhibito ry effects from the end products of one pathway on the synthesis of th e other. Data from leaves treated with metabolic intermediates in the catabolic pathway of polyamines and inhibitors of enzymes in the catab olic pathway, i.e. aminoguanidine, hydroxyethyldrazine and gabaculine, suggest that the observed increases in polyamine titers were not due to decreased catabolism of the polyamines. One catabolic intermediate, gamma-aminobutyric acid (GABA), elevated putrescine, spermidine and s permine by 12-, 1.4-, and 2-fold, respectively. Ethylene levels decrea sed (25%) in GABA-treated leaves. This small decrease in ethylene coul d not account for such large increase in putrescine titers. Further an alysis demonstrated that the GABA-mediated polyamine accumulation was inhibited by difluoromethylarginine, an inhibitor of arginine decarbox ylase, but not by difluoromethylornithine, an inhibitor of ornithine d ecarboxylase. These data suggest that GABA directly or indirectly affe cts the biosynthesis of polyamines via arginine decarboxylase.