Overproduction of ascorbate peroxidase in the tobacco chloroplast does not provide protection against ozone

Transgenic tobacco (Nicotiana tabacum cv Bel W3) plants were used to test the hypothesis that protection from O-3 injury could be conferred by overproduction of ascorbate peroxidase (APX) in the chloroplast. The 10-fold increase in soluble APX activity in the chloroplast was expected to alleviate an implied increase in oxidative potential and prevent damage caused by O-3. Three different O-3 exposure experiments (one acute and two chronic) with two replicates each were conducted. APX activity in nontransgenic plants increased in response to chronic O-3 exposure. However, most responses to O-3 were similar between transgenic and nontransgenic plants. These included reductions in net photosynthesis and stomatal conductance, increases in ethylene emission and visible injury, and a decline in the level of the small subunit of ribulose-1,5-biphosphate carboxylase/oxygenase mRNA transcripts observed in response to the air pollutant in the acute and/or chronic experiments. No O-3-induced effect on ribulose-l,5-biphosphate carboxylase/oxygenase quantity was observed in the chronic experiments. O-3 did not induce acceleration of senescence, as expected from studies with most other species; rather, the tobacco plants rapidly developed necrotic lesions. Thus, overproduction of APX in the chloroplast did not protect this cultivar of tobacco from O-3.