The physiological network regulating sulfite detoxification in plants

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Previously, we have discovered the enzyme sulfite oxidase (SO) in plants, cloned its gene, and provided knowledge about its biochemistry (atomic structure, spectroscopy, and reaction mechanism), localization and function. The substrate of SO - sulfite - is highly reactive and highly toxic for the plant cell. Sulfite can be produced endogenously during sulfur metabolism and can also derive from external sources like the air pollutant sulfur dioxide. We showed that SO is essential for detoxifying excessive amounts of sulfite (safety-valve function) in Arabidopsis and poplar. The aim of the present project is to analyze (a) the transcriptional and posttranslational regulation of SO-activity and (b) the interaction with an additional sulfite detoxification mechanism. For transcriptional regulation specific transcription factors for activation/inactivation of the SO promoter are of particular importance: specific DNA-binding-sites strongly suggest a control by RAV1. Moreover, posttranslational modifications of SO protein (SUMOylation, ubiquitinilation and phosphorylation) will be studied. Moreover, mRNA deep-sequencing revealed a second sulfite detoxification mechanism in plants: Apoplastic peroxidase, also known from work of Pfanz and colleagues (1990). Induction of the apoplastic peroxidases by SO2 in SO-knock out plants indicates independent apoplastic and symplastic detoxification mechanisms. We have done first localization experiments and have first preliminary data of the enzymatic reaction with the substrates guajacol and sulfite. Now, we will to investigate the apoplastic peroxidases on molecular, cell-biological and biochemical level (reaction mechanism for sulfite detoxification) and plan to study the importance of these apoplastic peroxidases and their co-regulation with peroxisomal SO. In addition, we intend to study plant adaptations to increased levels of SO2 both, in natural habitats and in controlled fumigation experiments. We plan to collect plant material at Volcano Island (Aeolian Archipelago, Italy) known for SO2-concentration in the µL L-1-level. We want to answer the question, how do plants react to higher SO2-concentrations in natural environments. Do we find a long-term adaptation of plants to SO2 achieved in nature by higher SO protein amounts (transcriptional or post-transcriptional modification) or by an adapted post-translational regulation?