2 STRUCTURALLY SIMILAR MAIZE CYTOSOLIC SUPEROXIDE-DISMUTASE GENES, SOD4 AND SOD4A, RESPOND DIFFERENTIALLY TO ABSCISIC-ACID AND HIGH OSMOTICUM

The maize (Zea mays) superoxide dismutase genes Sod4 and Sod4A are hig hly similar in structure but each responds differentially to environme ntal signals. We examined the effects of the hormone abscisic acid (AB A) on the developmental response of Sod4 and Sod4A. Although both Sod4 and Sod4A transcripts accumulate during late embryogenesis, only Sod4 is up-regulated by ABA and osmotic stress. Accumulation of Sod4 trans cript in response to osmotic stress is a consequence of increased endo genous ABA levers in developing embryos. Sod4 mRNA is up-regulated by ABA in viviparous-1 mutant embryos. Sod4 transcript increases within 4 h with ABA not only in developing embryos but also in mature embryos and in young leaves. Sod4A transcript is up-regulated by ABA only in y oung leaves, but neither Sod4 nor Sod4A transcripts changed in respons e to osmotic stress. Our data suggest that in leaves Sod4 and Sod4A ma y respond to ABA and osmotic stress via alternate pathways. Since the Sod genes have a known function, we hypothesize that the increase in S od mRNA in response to ABA is due in part to ABA-mediated metabolic ch anges leading to changes in oxygen free radical levels, which in turn lead to the induction of the antioxidant defense system.