RESUMEN
In the present study, the significance of sulfite oxidase (SO) for sulfite detoxification and sulfur assimilation was investigated. In response to sulfur dioxide (SO(2)) exposure, a remarkable expansion of sulfate and a significant increase of GSH pool were observed in wild-type and SO-overexpressing Arabidopsis. These metabolic changes were connected with a negative feedback inhibition of adenosine 5'-phosphosulfate reductase (APR), but no alterations in gas exchange parameters or visible symptoms of injury. However, Arabidopsis SO-KO mutants were consistently negatively affected upon 600 nL L(-1) SO(2) exposure for 60 h and showed phenotypical symptoms of injury with small necrotic spots on the leaves. The mean g(H2O) was reduced by about 60% over the fumigation period, accompanied by a reduction of net CO(2) assimilation and SO(2) uptake of about 50 and 35%. Moreover, sulfur metabolism was completely distorted. Whereas sulfate pool was kept constant, thiol-levels strongly increased. This demonstrates that SO should be the only protagonist for back-oxidizing and detoxification of sulfite. Based on these results, it is suggested that co-regulation of SO and APR controls sulfate assimilation pathway and stabilizes sulfite distribution into organic sulfur compounds. In conclusion, a sulfate-sulfite cycle driven by APR and SO can be postulated for fine-tuning of sulfur distribution that is additionally used for sulfite detoxification, when plants are exposed to atmospheric SO(2).