Putrescine enhances salt tolerance of cucumber seedlings by regulating ion homeostasis

Appropriate amounts of polyamines (PM) alleviating salt-induced damage have been widely recognized. Maintaining ion homeostasis is one of the key mechanisms to improve salt tolerance. The present study aimed to explore the specific mechanism of putrescine (Put) regulating ion balance in NaCl-stressed cucumber (Cucumis sativus) seedlings. Biochemical analysis combined with microscope imaging techniques revealed that 0.8 mM Put effectively inhibited the accumulation of Na+ and the loss of K+ in cucumber plants exposed to 75 mM NaCl. More Na+ tended to accumulate in stem. Moreover, exogenous Put further increased K+ concentration but decreased Na+ concentration in xylem sap of cucumber seedlings stressed by NaCl. Put increasing expression levels of SOS/ and NHX was involved in the Na+ exclusion. The differential expression of HKT1 in roots and leaves caused by Put might regulate Na+ distribution. In addition, the application of N,N'-dimethylthiourea (DMTU) and aminoguanidine (AG) decreased H2O2 levels, accompanied by the increase of Na+ in Put-treated seedlings, suggesting that polyamines oxidation-generated H2O2 mediated the Put-regulated ion homeostasis to some extent. These results indicated that exogenous Put could interact with ion channels as well as induce the generation of H2O2 to maintain Na+/K+ homeostasis under NaCl stress conditions.