Physiological role of sulfite oxidase in plants

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Previously, we have discovered the occurrence of the enzyme sulfite oxidase (SO) in plants. We have cloned its gene and provided a detailed knowledge about its biochemistry (atomic structure, spectroscopy, reaction mechanism, subcellular localization). The substrate of SO, sulfite, is highly reactive and potentially harmful for the cell; we showed that SO is essential for detoxifying excessive amounts of sulfite (safety-valve function). Sulfite is produced endogenously during sulfur assimilation and can also derive from external sources like the air pollutant sulfur dioxide. We also established that all plants (herbaceous plants, trees, cereals, mosses) posses a highly conserved SO. Hence SO must fulfill a basic metabolic function beyond its safety-valve role. It is the aim of this project to identify this basic function and to study the importance of SO for sulfur metabolism. We will clarify the function of SO for the catabolic degradation of reduced sulfur compounds. We will unveil whether SO plays a role during senescence, either directly in S-metabolism or via its production of H2O2. And we will provide molecular and biochemical details for the assumed regulation of SO at the posttranslational level.