Use of a New Tetrazolium-Based Assay to Study the Production of Superoxide Radicals by Tobacco Cell Cultures Challenged with Avirulent Zoospores of Phytophthora parasitica var nicotianae

The relationship between the production of reactive oxygen species and the hypersensitive response (HR) of tobacco (Nicotiana tabacum L.) toward an incompatible race of the Oomycete Phytophthora parasitica var nicotianae has been investigated. A new assay for superoxide radical (O₂⁻) production based on reduction of the tetrazolium dye sodium,3′-(1-[phenylamino-carbonyl]-3,4-tetrazolium)-bis(4-methoxy-6-nitro) benzene-sulfonic acid hydrate (XTT) has enabled the quantitative estimation of perhydroxyl/superoxide radical acid-base pair (HO₂^·/O₂⁻) production during the resistant response. Tobacco suspension cells were inoculated with zoospores from compatible or incompatible races of the pathogen. Subsequent HO₂^·/O₂⁻ production was monitored by following the formation of XTT formazan. In the incompatible interaction only, HO₂^·/O₂⁻ was produced in a minor burst between 0 and 2 h and then in a major burst between 8 and 10 h postinoculation. During this second burst, rates of XTT reduction equivalent to a radical flux of 9.9 × 10⁻¹⁵ mol min⁻¹ cell⁻¹ were observed. The HO₂^·/O₂⁻ scavengers O₂⁻ dismutase and Mn(III)desferal each inhibited dye reduction. An HR was observed in challenged, resistant cells immediately following the second burst of radical production. Both scavengers inhibited the HR when added prior to the occurrence of either radical burst, indicating that O₂⁻ production is a necessary precursor to the HR.