Spermidine and Spermine Converted from Putrescine Improve the Resistance of Wheat Seedlings to Osmotic Stress

Polyamines play an important role in the plant resistance to drought and osmotic stress (OS). However, the precise function of putrescine conversion to spermidine and spermine is not clear in wheat seedling leaves under OS. Changes in the levels of three major free polyamines, putrescine, spermidine and spermine, were investigated in the seedlings of two wheat cultivars Triticum aestivum L. Jinmai 98 (drought-resistant) and Wenmai 6 (drought-sensitive) under PEG 6000 OS. Furthermore, the activity of arginine decarboxylase and S-adenosylmethionine decarboxylase was determined. In addition, experiments with exogenous polyamines and the polyamine biosynthesis inhibitors were also implemented to supply more evidence. Under OS, spermidine and spermine levels increased more markedly (P < 0.05) in drought-resistant Jinmai 98 than in drought-sensitive Wenmai 6, suggesting that free spermidine and spermine, which were converted from free putrescine, were possibly involved in the resistance of seedlings to OS. Treatment with exogenous spermidine treatment enhanced the OS-induced increase in endogenous spermidine and spermine content in drought-sensitive Wenmai 6, accompanied with an increase in resistance, as judged by a decrease in the relative permeability of plasma membrane of seedling leaf and an increase in the relative water content of seedling leaves. The suggestion was further testified by treatment with methylglyoxal-bis guanylhydrazone and o-phenanthrolin. In sum, it could be inferred that spermidine and spermine converted from free putrescine in leaves functioned in an increase in resistance of wheat seedlings to OS.