Expression of both CuZnSOD and APX in chloroplasts enhances tolerance to sulfur dioxide in transgenic sweet potato plants

We have previously reported that transgenic sweet potato (Ipomoea batatas) plants overexpressing both CuZn superoxide dismutase (CuZnSOD) and ascorbate peroxidase (APX) under the control of a stress-inducible SWPA2 promoter in chloroplasts (referred to as SSA plants) showed increased resistance to methyl viologen-mediated oxidative stress and chilling. To investigate whether SSA plants show enhanced tolerance to air pollutants, they were exposed to 500 ppb of sulfur dioxide (SO2). SO2 caused visible damage to the leaves of sweet potato, but damage in the leaves of non-transgenic (NT) plants was more severe than in those of SSA plants. The photosynthetic activity (Fv/Fm) of the SSA plants decreased by only 7% on the 5th day after the treatment, whereas that of NT plants severely decreased by 63% after 5 days of recovery. Moreover, the chlorophyll content in the oldest leaf of NT plants decreased by 69%, whereas that of SSA plants remained at a high level. APX activity in NT plants increased about three times under an SO2 stress, and in SSA plants about five times compared to the case with no stress conditions. These results suggest that the overexpression of both CuZnSOD and APX in chloroplasts reduces the oxidative stress derived from SO2.