Polyamines conjugated to plasma membrane functioned in enhancing the tolerance of cucumber seedlings to osmotic stress via elevating H⁺-ATPase activity

Polyamine (PA), one of the important plant growth regulators, is closely associated with drought stress. However, the function of conjugated PA is not still clear in the roots of cucumber seedlings under polyethylene glycol (PEG) osmotic stress. Therefore, in this study the relationship between the levels of conjugated polyamines and the activity of H⁺-ATPase in plasma membrane was elucidated with the roots of two cucumber (Cucumis sativus L.) cultivars, which were different in drought tolerance, as experimental materials. Furthermore, the contents of free PAs and the activities of S-adenosylmethionine decarboxylase (SAMDC) and transglutaminase (TGase), which were closely related to the levels of conjugated polyamines, were also determined. Results showed that under osmotic stress, the increases of the levels of non-covalently conjugated (non-CC) spermidine (Spd) and spermine (Spm), covalently conjugated (CC) putrescine (Put) and Spd in plasma membrane of drought-tolerant Tangshan 5 were more obvious than those of drought-sensitive Jinyou 1. Furthermore, the conjugated PAs mentioned above were closely correlated with increase rate of seedling dry weight, plasma membrane permeability, water content and H⁺-ATPase activity in plasma membrane. Results of the additional tests, in which exogenous Spd, Spm and two inhibitors, MGBG and phenanthrolin were used, were complementary to the results above. From these results, it could be concluded that non-CC Spd and Spm, CC Put and Spd in plasma membrane functioned in enhancing the tolerance of cucumber seedlings to osmotic stress via elevating H⁺-ATPase activity.