PLASTICITY OF POLYAMINE METABOLISM ASSOCIATED WITH HIGH OSMOTIC-STRESS IN RAPE LEAF DISCS AND WITH ETHYLENE TREATMENT

In rape leaf discs the response to osmotic stress has been found to be associated with increases in putrescine and 1,3-diaminopropane (an ox idation product of spermidine and/or spermine) and decreases in spermi dine titers. In contrast, agmatine and spermine titers showed small ch anges while cadaverine accumulated massively. Similar results were obs erved in whole rape seedlings subjected to drought conditions. alpha-D L-difluoromethylarginine (DFMA), a specific irreversible inhibitor of arginine decarboxylase, strongly inhibited polyamine accumulation in u nstressed rape leaf discs, which suggested that the arginine decarboxy lase pathway is constitutively involved in putrescine biosynthesis. In leaf discs treated under high osmotic stress conditions, both DFMA an d DFMO (alpha-DL-difluoromethylomithine, a specific and irreversible i nhibitor of ornithine decarboxylase) inhibited the accumulation of pol yamines. Although the stressed discs treated with DFMA had a lower con centration of putrescine than those treated with DFMO, we propose that under osmotic stress the synthesis of putrescine might involve both e nzymes. DFMA, but not DFMO, was also found to inhibit cadaverine forma tion strongly in stressed explants. The effects on polyamine biosynthe sis and catabolism of cyclohexylamine, the spermidine synthase inhibit or, aminoguanidine, the diamine-oxidase inhibitor and gamma-aminobutyr ic acid, a product of putrescine oxidation via diamine oxidase or sper midine oxidation via polyamine oxidase were found to depend on environ mental osmotic challenges. Thus, it appears that high osmotic stress d id not block spermidine biosynthesis, but induced a stimulation of spe rmidine oxidation. We have also demonstrated that in stressed leaf dis cs, exogenous ethylene, applied in the form of (2-chloroethyl) phospho nic acid or ethephon, behaves as an inhibitor of polyamine synthesis w ith the exception of agmatine and diaminopropane. In addition, in stre ssed tissues, when ethylene synthesis was inhibited by aminooxyacetic acid or aminoethoxyvinylglycine, S-adenosylmethionine utilization in p olyamine synthesis was not promoted. The relationships between polyami ne and ethylene biosynthesis in unstressed and stressed tissues are di scussed.