Specialization for ureide biogenesis in the root nodules of black locust (Robinia pseudoacacia L.), an amide exporter

Summary

Black locust (Robinia pseudoacacia L.), a member of the legume tribe Robinieae, has indeterminate root nodules and is primarily an amide exporter. However, in greenhouse-grown seedlings inoculated with rhizobia, ureide-N comprises approximately 8% of the total soluble-N in the xylem sap. Ultrastructurally, young interstitial cells (i.e., uninfected cells in the infected region near the nodule meristem) develop enlarged peroxisomes and abundant tubular ER, properties heretofore found to be characteristic only of members of the Phaseoleae, which have determinate nodules and are exporters principally of ureides. Many of the peroxisomes in the interstitial cells of black locust nodules react strongly for uricase (EC 1.7.3.3) activity in a cytochemical test employing diaminobenzidine, but some of the peroxisomes, particularly those farther back from the meristem, react weakly or not at all, even though enlarged. The ultrastructural specializations of the interstitial cells seem disproportionately large in comparison to the relatively low levels of ureides in the xylem transport stream. We suggest that similar specialization of some of the interstitial cells might be found in other legumes that export low amounts of ureides.