The role of polyamines in the regulation of the plasma membrane and the tonoplast proton pumps under salt stress

Polyamine content (PAs) often changes in response to abiotic stresses. It was shown that the accumulation of PAs decreased in roots treated for 24 h with 200 mM NaCl. The role of polyamines (putrescine – PUT, spermidine – SPD and spermine – SPM) in the modification of the plasma membrane(PM) H+-ATPase (EC 3.6.3.6) and the vacuolar(V) H+-ATPase (EC 3.6.3.14) activities in cucumber roots treated with NaCl was investigated. 24 h treatment of seedlings with 50 μM PUT, SPD or SPM lowered the activities of proton pumps in both membranes. The decreased H+-ATPase activity in plasma membranes isolated from the PA-treated roots was positively correlated with a lower level of PM-H+-ATPase CsHA3 transcript. However, transcript levels of PM-H+-ATPase CsHA2 and V-ATPase subunit A and c in roots treated with 50 μM PAs were similar to those in the control. Additionally, treatment of plants with salt markedly increased the activity of the PM- and V-H+-ATPases. However, exposure of plants to 20% PEG had no effect on these activities. These data suggest that, under salt stress conditions, the increase in H+-ATPase activities is caused mainly by the ionic component of salt stress. It seems that the main role of the PAs in the 24 h salt-treated cucumber plants could be a result of their cationic character. The PA levels decreased when concentration of Na+ increased, so action of PAs contributes to ionic equilibrium. Moreover, the decrease in the concentration of polyamines, which inhibit the PM-H+-ATPase and the V-H+-ATPase, at least under the studied conditions, seems to be beneficial. Thus, plants can increase salinity tolerance by modifying the biosynthesis of polyamines.