Stress-dependent accumulation of spermidine and spermine in the halophyte Mesembryanthemum crystallinum under salinity conditions

We studied the effects of chloride salinity (300 and 500 mM NaCl) on the content of free polyamines (PAs) from putrescine (Put) family in Mesembryanthemum crystallinum L. leaves and roots. The contents of Put and spermidine (Spd) in leaves increased temporarily, achieving the highest values on the third day of salinity treatment; thereafter (by days 7–14), they dropped sharply. The content of spermine (Spm) increased gradually, and its high level was maintained until the end of experiment. The dynamics of Spm accumulation in leaves under salinity conditions resembled that of phosphoenolpyruvate carboxylase (PEPC), a key enzyme of the water-saving CAM pathway of photosynthesis. This indicates indirectly the involvement of Spm in the common ice plant adaptation to salinity. A decrease in the molar ratios of Spd to Spm in the leaves under salinity conditions could point to the acceleration of Spm biosynthesis (accumulation) during plant adaptation, whereas the levels of Spm precursors, Put and Spd, did not increase. This phenomenon could be explained by an accelerated conversion of Spd into Spm, an active liberation of free Spm from its conjugates, or changes in the rates of studied PA biosynthesis and degradation under salinity. At the same time, the intracellular concentration of ethylene rose under these conditions. It was supposed and then demonstrated, that the pathway of ethylene biosynthesis and that of the synthesis of Put family PAs compete under severe salinity conditions. This competition might be based on the disturbances in sulfur metabolism and a decrease in the methionine content, an immediate precursor of S-adenosyl-L-methionine.