The Assimilation of Ureides in Shoot Tissues of Soybeans

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Abstract
The ureides, allantoin and allantoic acid, are major forms of N transported from nodules [inoculated with Rhizobium japonicum] to shoots in soybeans (Merr.). Little is known about the occurrence, localization or properties of the enzymes involved in the assimilation of ureides in shoot tissues. The capacity of the shoot tissues to assimilate allantoin via allantoinase (EC 3.5.2.5) during leaf and fruit development in nodulated soybeans was examined. Specific activity of allantoinase in leaves peaked during pod formation and early seed filling. In developing fruits allantoinase activity in the seeds was 2-4 times that in the pods when expressed on a fresh weight or organ basis. In seeds, the embryos contained the highest specific allantoinase activity. Stems and petioles had appreciable allantoinase activity. With development, peaks in the amounts of allantoic acid but not allantoin, were measured in both leaves and fruits suggesting that the assimilation of allantoic acid may be a limiting factor in ureide assimilation. Highest amounts of ureides were measured in the pith and xylem of stem tissues and in developing pod walls.