STRESS-INDUCED CHANGES IN POLYAMINE AND TYRAMINE LEVELS CAN REGULATE PROLINE ACCUMULATION IN TOMATO LEAF DISCS TREATED WITH SODIUM-CHLORIDE

The effect of salt stress on proline (Pro) accumulation and its relati onship with the changes occurring at the level of polyamine (PA) metab olism and tyramine were investigated in leaf discs of tomato (Lycopers icon esculentum). The rate of accumulation of Pro, FA and tyramine was higher in the salt-sensitive than in the salt-tolerant cultivar. In t he salt-sensitive cultivar, Pro started to accumulate 4 h after the on set of the NaCl treatment, its maximum level being reached 27 h later. The lag phase was associated with a rapid decrease in putrescine (Put ) and spermidine (Spd) and some increase in 1,3-diaminopropane (Dap), a product of Spd and/or spermine (Spm) oxidation. This was followed by an increase in agmatine (Agm), cadaverine (Cad), Spm and tyramine. al pha-DL-difluoromethylarginine (DFMA), an inhibitor of arginine decarbo xylase (ADC, EC 4.1.1.19), induced a decrease in the Put level in both control and stressed discs, while alpha-DL-difluoromethylornithine (D FMO), an inhibitor of ornithine decarboxylase (ODC, EC 4.1.1.17), caus ed a decrease in Spd and Spm levels only in salinized discs. These dat a suggest that ADC is operating under both control and stress conditio ns, whereas ODC activity is promoted only in response to salt stress. DFMA also depressed the salt-induced Pro accumulation while DFMO did n ot inhibit this response. In salt-stressed leaf discs, the decrease in Spd level in response to methylglyoxal-bis-(guanylhydrazone) (MGBG) o r cyclohexylammonium (CHA) treatment suggests that salt stress did not block SAM decarboxylase or Spd synthase activities. However, the incr eased level of Dap reflected a salt stress-promoted oxidation of PA. C HA and MGBG had no effect on Pro accumulation. Putrescine, Dap and esp ecially tyramine supplied at low concentrations stimulated the Pro res ponse which was, however, suppressed by application of Spm. Treatment with aminoguanidine, an inhibitor of diamine oxidases, also strongly i nhibited Pro accumulation. These data suggest that salt-induced Pro ac cumulation in tomato leaf discs is closely related to changes in their PA metabolism, either via substrate-product relationships or regulato ry effects at target(s) which remain to be characterized.