Enhanced tolerance to sulfur dioxide and salt stress of transgenic Chinese cabbage plants expressing both superoxide dismutase and catalase in chloroplasts

To explore the possibility of overcoming the highly phytotoxic effect of SO2 and salt stress, we introduced the maize Cu/ZnSOD and/or CAT genes into chloroplasts of Chinese cabbage (Brassica campestris L. ssp. pekinensis cv. Tropical Pride) (referred to as SOD, CAT and SOD + CAT plants). SOD + CAT plants showed enhanced tolerance to 400 ppb SO2, and visible damage was one-sixth that of wild-type (CK) plants. In addition, when SOD + CAT plants were exposed to a high salt treatment of 200 mM NaCl for 4 weeks, the photosynthetic activity of the plants decreased by only 6%, whereas that of CK plants decreased by 72%. SOD plants had higher total APX and GR activities than CK plants. As expected, SOD plants showed levels of protection from SO2 and salt stress that were moderately improved compared to CK plants. However, CAT plants showed inhibition of APX activity and provided only limited improvements in plant stress tolerance. Moreover, SOD + CAT plants accumulated more K+, Ca2+ and Mg2+ and less Na+ in their leaves compared with those of CK plants. These results suggest that the expression of SOD and CAT simultaneously is suitable for the introduction of increased multiple stress protection.