Spermidine (Spd) is one of the most important polyamines (PAs) showing important roles in growth, development and stress responses in plants. The role of Spd in regulation of a dominant glutathione S-transferase (GST, E.C.126.96.36.199) and ionic balance of Na+ and K+ in onion leaves were examined. Onion GSTs were separated from fresh bulb tissue using DEAE-cellulose chromatography. Three GSTs were eluted at 56, 120 and 169 mM of KCl. Among them, GST2 containing >60% of total activity was termed as dominant GST and it was subsequently purified using affinity chromatography S-hexylglutathione-agarose. This purified protein was used for western blotting analysis of the GST in leaves of two months onion seedlings imposed on NaCl induced 16 dSm-1 salinity with or without foliar spray of 100 µM Spd on 1st, 3rd, 5th and 7th days of stress. The results suggested that dominant GST accumulated by salinity in onion seedling with or without Spd spray. However, at early saline stress, accumulation was significantly stronger in leaves without Spd than in those with Spd. At 7th day, GST band was increased and apparently seemed to be intensified in Spd sprayed leaves than those without spray. The values of Na+/K+ suggested that Spd maintained ionic balance better at early stage of stress than late stage.
Key words: Onion glutathione S-transferases (GST), ionic balance, salt stress, spermidine.
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