The effect of micronutrient stress (either deficiency or toxicity) on the expression of different superoxide dismutase isoenzymes in plants is reviewed. The induction of Fe-SOD and Mn-SOD by different metals and the potential use of the metalloenzyme system SOD for the appraisal of the micronutrient status of plants, is examined. At subcellular level, evidence for the participation of peroxisomal SOD in the molecular mechanism of plant tolerance to Cu is presented, and the activated oxygen-dependent toxicity of a xenobiotic (clofibrate) in plant peroxisomes is examined.