Plant sulfite oxidase )SO( is a molybdo-enzyme responsible for the oxidation of excess SO₂/sulfite into non-toxic sulfate. The effect of toxic sulfite level on leaves and fruits was studied in tomato plants with different SO expression levels: wild-type (WT), SO overexpression (OE) and SO RNA interference (Ri) plants. Leaf discs and ripe fruit of plants lacking SO were more susceptible, whereas SO OE plants were more resistant as revealed by remaining chlorophyll content and tissue damage levels. Application of molybdenum further enhanced the tolerance of leaf discs to sulfite by enhancing SO activity in SO OE lines, but not in WT or Ri plants. Notably, incubation with tungsten, the molybdenum antagonist, overturned the effect of molybdenum spray in SO OE plants, revealed by remaining chlorophyll content and SO activity. The results indicate that SO determines, in tomato leaves and ripe fruits, the resistance to toxic sulfite and the application of molybdenum enhances sulfite resistance in OE plants by increasing SO activity. The results suggest that overexpressing SO mechanism can be employed in agriculture with or without molybdenum application, for the development of more tolerate crops and vegetables to higher concentrations of sulfite/SO₂ containing postharvest treatments.