Spermine Pre-Treatment Improves Some Physiochemical Parameters and Sodium Transporter Gene Expression of Pumpkin Seedlings under Salt Stress

It is known that polyamines (PAs) including spermine (Spm) enhance the abiotic stress tolerance of crops. Here, the effects of hydroponic Spm pre-treatment on the amelioration of the adverse effects of salinity were investigated in pumpkin (Cucurbita pepo L.) that is an economically important horticultural crop and sensitive to salinity, especially at the establishment stage. For this purpose, 10-day-old, uniform-sized seedlings were transplanted to plastic containers containing Hoagland nutrient solution. Spm was added at 0.1 and 1 mM to the hydroponic medium for 5 days before stress. The plants were treated with 40 and 80 mM NaCl for inducing salinity stress. Salt stress reduced the plant growth and potassium content in roots, and these detrimental effects were alleviated when plants were pre-treated with Spm. Salinity stress caused a significant increase in sodium and gamma-amino butyric acid (GABA) content when compared with controls. Spm pre-treatment ameliorated these salinity stress effects by increasing sodium content of root and leaves GABA content. Expression analysis of two sodium transporter genes, salt overly sensitive1 (SOS1) and Na+/H+ exchanger (NHX1) revealed that their expression was differentially induced in roots of plants treated either with salinity or Spm. These results suggest that Spm via the overexpression of the NHX1 gene substantially increased the tolerance to stress by sequestering excess Na+ into the vacuoles and sustaining a better cellular environment. Moreover, Spm has potential to scavenge directly free radical and to alleviate growth inhibition and promote the activity of antioxidant system enzymes in pumpkin seedlings under salt stress.