Histochemistry(1982) 74: 309 318 Histochemistry 9 Springer-Verlag 1982 Ultrastructural Localization of Urate Oxidase in Nodules of Sesbania exaltata, Giycine max, and Medicago sativa K.C. Vaughn 1 *, S.O. Duke 1, S.H. Duke 2, and C.A. Henson 2 1 Southern Weed ScienceLaboratory, USDA-ARS, P.O. Box 225, Stoneville,MS 38776, USA 2 Department of Agronomy, Universityof Wisconsin, Madison, WI 53706, USA Summary. The localization of urate oxidase (=uricase, E.C. 1.7.3.3) was determined cytochemically in nodules of Sesbania exaltata (Raf.) Cory, soy- bean (Glycine max [L.] Merr.) and alfalfa (Medicago sativa [L.]), using the precipitation of peroxide (produced during the oxidation of urate) by cerium chloride. Cerium perhydroxide reaction product was noted only in the micro- bodies, a localization consistent with biochemical fractionation studies on urate oxidase. Urate oxidase was present not only in the uninfected cells of the cortical tissue, but also in both infected and interstitial cells in the central tissue, suggesting that at least this enzyme of ureide metabolism is not confined to interstitial cells. Urate oxidase cytochemistry of nodules from alfalfa (Medicago sativa L.), an amide producer, also resulted in micro- body staining but the microbodies were infrequently noted in cell profiles. Introduction Nodules of leguminous plants are among the most complicated plant structures, consisting of distinct zones of tissue; one is comprised of only root cells, the cortical and epidermal tissue, while the other zone, the central tissue, is occupied by a mixture of infected and uninfected cells (Tu 1974). Each of the cell types within this tissue has a unique ultrastructure (Tu 1974; Newcomb and Tandon 1981). Because of this tissue and cellular heterogeneity, biochemical studies that differentiate between "bacteroid" and "cytosol" fractions give only gross ideas as to the localization of the enzymes within the nodule (e.g. Duke and Ham 1976; Triplett et al. 1980). Newcomb and Tandon (1981) have attempted to correlate biochemical stud- ies on the subcellular localization of enzymes of ureide metabolism in soybean nodules with the ultrastructure of this tissue. These authors observed that micro- bodies and smooth endoplasmic reticula (proposed sites of localization of the ureide metabolism enzymes urate oxidase and xanthine dehydrogenase, Hanks * To whom offprint requests should be sent 0301-5564/82/0074/0309/$02.00