Aluminum tolerance in a spermidine synthase-overexpressing transgenic European pear is correlated with the enhanced level of spermidine via alleviating oxidative status

Aluminum (Al) stress is a major cause of poor crop yields, particularly in those countries where acid soil predominate. To verify whether polyamine can confer Al tolerance, in vitro shoots of a transgenic European pear (Pyrus communis L. ‘Ballad’) line #32 overexpressing apple spermidine synthase (MdSPDS1) and the wild type (WT) were subjected to long-term stress for 30 μM AlCl₃. Based on net increment of shoot height (SHI) or fresh weight (FWI) and morphological changes upon the stress, the performance of line #32 was much better than that of WT. Although SPDS expression levels and spermidine (Spd) titers in line #32 were higher than those in WT, firmly due to the transgene (MdSPDS1) expression, no further induction of SPDS expression was observed from the long-term Al stress trial in both lines. While, Spd titers were considerably increased in both lines after the stress. The activities of superoxide dismutase (SOD) or glutathione reductase (GR) and the accumulation of proline or malondialdehyde (MDA) altered upon this stress toward a more favorable status for survival in the transgenic line #32 than in WT. These antioxidant parameters were closely related to Spd titer. Concentrations of calcium (Ca) and some co-factor metals of SOD in line #32 were diversely higher than that in WT after the stress. These evidences indicate that Spd is implicated in elevating of Al stress tolerance of the transgenic line #32 chiefly via ameliorating oxidative status as well as by affecting mineral element balance.