Sulfite oxidase protects plants against sulfur dioxide toxicity

The gaseous pollutant SO₂ readily reacts with water to form sulfite that impacts deleteriously on animal and plant health. By modulating the level of sulfite oxidase (SO) that catalyzes the transformation of sulfites to the non-toxic sulfate, we show that Arabidopsis and tomato plants can be rendered resistant or susceptible to SO₂/sulfite. Plants in which sulfite oxidase expression was abrogated by RNA interference (RNAi) accumulated relatively less sulfate after SO₂ application and showed enhanced induction of senescence and wounding-associated transcripts, leaf necrosis and chlorophyll bleaching. In contrast, SO overexpression lines accumulated relatively more sulfate and showed little or no necrosis after SO₂ application. The transcript of sulfite reductase, a chloroplast-localized enzyme that reduces sulfites to sulfides, was shown to be rapidly induced by SO ₂ in a sulfite oxidase-dependent manner. Transcripts of other sulfite-requiring enzymatic activities such as mercaptopyruvate sulfur transferases and UDP-sulfoquinovose synthase 1 were induced later and to a lesser extent, whereas SO was constitutively expressed and was not significantly induced by SO₂. The results imply that plants can utilize sulfite oxidase in a sulfite oxidative pathway to cope with sulfite overflow.