EFFECT OF SPERMIDINE ON PHYSIOLOGY AND ANTIOXIDANTS UNDER SALINITY STRESS IN ONION SEEDLINGS

Abstract

In this study, protective role of spermidine (Spd) on NaCl induced oxidative damage in onion seedlings was investigated for better understanding the salinity tolerance mechanism. Onion seedlings were subjected to salinity stress of 16 dSm -1 and 100 µM of Spd were sprayed twice daily. Data were analyzed on Chlorophyll (Chl), Carotenoid (Car), Leaf water, Proline, Reactive Oxygen Species (ROS), Methylglyoxal (MG), lipid peroxidation as Malondialdehyde (MDA), Polyamines, Glutathione and Ascobate related antioxidant enzymes and Glyoxalase (Gly) systems. Salinity reduced the Chl, Car and Relative Water Content (RWC) of leaves where foliar spray of Spd reduced the loss of these contents. Contrary, Spd increased the proline contents in salinity stressed seedlings. The contents of O 2 •- , H 2 O 2, MDA were decreased in the Spd supplied leaves. However, Spd effectively maintained the ROS and MDA at first 3 day of stress, while Glutathion (GSH), Ascorbic acid (ASA) and their redox homeostasis were maintained higher throughout the study period. Higher increments of Polyamine Oxidase (PAO) and Diamine Oxidase (DAO) activities upto 3 day suggested its protective role via H 2 O 2 signal transduction pathway. In presence of Spd, higher activities of superoxide dismutase (SOD), Ascorbate Peroxidase (APX) and Monodehydroascorbate reductase (MDHAR) over salinity during the study period suggested its ROS scavenging role. However, higher activities of Glutathione Peroxidase (GPX) and Dehydroascorbatereductase (DHAR) by Spd indicated maintenance of ROS and ASA, respectively. Similarly, Spd increased activities of Gly-I and Gly-II upto 3 day of stress concurrently with lower MG suggested the improvement of MG detoxification system. Higher activity of Glutathione-S-Transferase (GST) by Spd in saline stress seedling might involve in protein stabilization and detoxification role in onion plant. Since GST activity increased strongly in presence or absence of Spd under salinity, attempt was taken to test its accumulation. Therefore, from different plant parts it was found that both root and bulb showed higher GST activity. However, onion bulb was chosen for easy availability. In chromatographic separation of onion bulb, three component GSTs were separated among which one GST eluted at 120 mM KCl was found to be accumulated under salinity in western blotting analysis. Taken together, in short term salinity, foliar spray of Spd might be useful to reduce the oxidative damage through increment of antioxidants and glyoxalase system.