Antisense inhibition of a spermidine synthase gene highlights the role of polyamines for stress alleviation in pear shoots subjected to salinity and cadmium

Three transgenic European pear (Pyrus communis L.) lines with reduced spermidine synthase (SPDS) expression and spermidine (Spd) titers were developed using a construct containing an apple SPDS gene (MdSPDS1) in antisense orientation. After exposure to either salt or cadmium stress, growth inhibition was more severe in the antisense lines than in the wild-type (WT). The antioxidant system, as shown by glutathione (GSH) content, activity of glutathione reductase (GR) and superoxide dismutase (SOD), and proline accumulation, was not effectively induced under stress in the antisense lines as compared with the WT. The reduction in antioxidant system function in the antisense lines was accompanied by a greater accumulation of malondialdehyde (MDA), an indicator of lipid peroxidation. Growth inhibition, Spd level, and parameters indicative of the antioxidant system were significantly ameliorated by exogenous Spd application. Under either salt or cadmium stress, GSH content, GR and SOD activity, and proline accumulation were positively correlated with Spd, putrescine (Put), and total polyamine titers. Conversely, MDA level showed a significantly negative correlation with these polyamines under both stress conditions. Thus, the responses to stress treatments were first identified in the SPDS antisense European pears, and the results provide further evidence for the important role of polyamines in both salt and cadmium stress tolerance, in which the polyamines act, at least in part, by influencing the antioxidant system.