Transgenic tobacco plants expressing antisense RNA for cytosolic ascorbate peroxidase show increased susceptibility to ozone injury

Ascorbate peroxidase (APX), as a participant in the ascorbate-glutathione cycle, has been suggested to be a particularly important antioxidant enzyme in helping plants survive oxidative stress, but direct evidence for this has not been reported. We demonstrate that expressing antisense RNA comprising 45% of the 3'-coding region of the tobacco cytosolic APX, can reduce significantly both the endogenous APX mRNA levels and the APX catalytic activity in transgenic tobacco plants. Those transgenic plants showing a reduction in both endogenous APX mRNA levels and extractable APX activity display a significant increase in ozone injury following high level ozone exposure. Lower-level ozone exposure reveals even more drastic differences between the antisense and control plants, suggesting that even a partial loss of APX function in oxidative defence cannot be fully compensated for by other antioxidant measures.