The effect of NaCl on the growth and activity of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) were investigated in the seedlings of four potato cultivars (Agria, Kennebec; relatively salt tolerant, Diamant and Ajax; relatively salt sensitive). The shoot fresh mass of Agria and Kennebec did not changed at 50 mM NaCl, whereas in Diamant and Ajax it decreased to 50 % of that in the controls. In Agria and Kennebec, SOD activity increased at 50 mM NaCl, but no significant changes observed in Diamant and Ajax. At higher NaCl concentration, SOD activity reduced in all cultivars. CAT and POD activities increased in all cultivars under salt stress. Unlike the other cultivars, in Ajax seedlings, APX activity increased in response to NaCl stress. We also observed new POD and SOD isoenzyme activities and changes in isoenzyme compositions under salt stress. These results suggest that salt-tolerant potato cultivars may have a better protection against reactive oxygen species (ROS) by increasing the activity of antioxidant enzymes (especially SOD) under salt stress.
1 Ag ri cul tural Bio tech nol ogy Re search In sti tute of Iran, Seed and Plant Im prove ment In sti tute Cam pus,The ef fects of salt stress were stud ied on the ac cu mu la tion and me tab o lism of proline and its cor re la tion with Na + and K + content in shoots and cal lus tis sue of four po tato cultivars, viz., Agria, Kennebec (rel a tively salt tol er ant), Diamant and Ajax (rel a tively salt sen si tive). Na + and proline con tents in creased in all cultivars un der salt stress. How ever, K + and pro tein contents de creased in re sponse to NaCl treat ments. The ac tiv i ties of en zymes in volved in proline me tab o lism, ∆ 1 -pyr ro line -5 --carboxylate synthetase (P5CS) and proline dehydrogenase (ProDH) in creased and de creased, re spec tively, in re sponse to el e vated NaCl con cen tra tions. The changes of P5CS and ProDH ac tiv i ties in more salt sen si tive cultivars (Diamant, Ajax) were more than those in the tol er ant ones. Then the stimu la tion of syn the sis in com bi na tion with a par tially in crease of pro tein pro te ol y sis, a de crease in proline uti li za tion and in hi bition of ox i da tion re sulted in high proline con tents in seed lings and calli un der salt stress. In cal lus tis sue, re duced growth and cell size may be par tially re spon si ble for high proline ac cu mula tion in re sponse to high NaCl lev els. How ever, al though the ba sic proline con tents in the seed lings of more salt tol er ant cultivars were higher than the sen si tive ones, a clear re la tionship was not gen er ally ob served be tween ac cu mu la tion of proline and salt tol er ance in po tato. In tro duc tionSa lin ity is one of the ma jor abiotic stresses af fecting plant growth, de vel op ment and pro duc tiv ity. Plants ex posed to salt stress must un dergo changes in their me tab o lism in or der to sur vive the del e te rious ef fects of the stress. They have to cope with the low er ing of the wa ter po ten tial in their en vi ronment, the pres ence of toxic ions, mainly Na + , and the dif fi cul ties that salt ex erts on the up take of essen tial nu tri ents. Tol er ance to salt re sults from cellu lar as well as co or di nated re sponse be tween the var i ous or gans (Gueta-Dahan et al. 1997, Ben-Hay yim et al. 1993).Proline is a ma jor or ganic mol e cule that ac cu mulates in many plants ex posed to en vi ron men tal stres ses such as drought and sa lin ity (Nanjo et al. 1999, Kuznetsov andShevyakova 1997). Un der os motic stress con di tions, proline acts as a me di ator of os motic ad just ment, as a pro tec tor of macromo le cules such as pro teins and mem branes, as a sink for en ergy, as a scav en ger of free rad i cals, and even as a stress-re lated sig nal (Hasegawa et al. 2000, Werner and Finkelstein 1995, Nanjo et al. 1999. In spite of many stud ies in plants, the roles of proline in osmotolerance in plants re main contro ver sial and there were few con crete dem on stra -263 ACTA PHYSIOLOGIAE
Bacterial mannitol 1-phosphate dehydrogenase (mtlD) gene was introduced into potato (Solanum tuberosum L.) by Agrobacterium tumefaciens-mediated transformation. Transgenic plants were selected on a medium containing 100 mg l -1 kanamycin and confirmed by polymerase chain reaction (PCR), Southern blotting, and RT-PCR analyses. All of the selected transformants accumulated mannitol, a sugar alcohol that is not found in wildtype potato. Experiments designed for testing salt tolerance revealed that there was enhanced NaCl tolerance of the transgenic lines both in vitro and in hydroponic culture. Compared to 0 mM NaCl, the shoot fresh weight of wildtype plants was reduced by 76.5% at 100 mM NaCl under hydroponic conditions. However, under the same condition, the shoot fresh weight of transgenic plants was reduced only by 17.3%, compared to 0 mM NaCl treatment. The improved tolerance of this transgenic line may be attributed to the induction and progressive accumulation of mannitol in the roots and shoots of the plants. In contrast to in vitro experiments, the mannitol content in the transgenic roots and shoots increased at 50 mM NaCl and decreased slightly at 75 and 100 mM NaCl, respectively. Overall, the amount of accumulated mannitol in the transgenic lines was too small to act as an osmolyte; thus, it might act as an osmoprotectant. However, the results demonstrated that mannitol had more contribution to osmotic adjustment in the roots (but not in shoots). Finally, we concluded that mtlD expression in transgenic potato plants can significantly increase the mannitol accumulation that contributes to the enhanced tolerance to NaCl stress. Furthermore, although this enhanced tolerance resulted mainly from an osmoprotectant action, an osmoregulatory effect could not be ruled out.
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