Translocation of nitrogen and expression of nodule‐specific uricase (nodulin‐35) in Robinia pseudoacacia

Abstract

Uricase (urate oxidase, EC 1.7.3.3) activity and nodule‐specific uricase II (nodulin‐35) were detected in the nodules from a number of legume: Rhizobium symbioses (Vigna unguiculata (L.) Walp., Phaseolus vulgaris L., and Kennedia coccinea Vent.) in the Phaseoleae, as well as in those of Robinia pseudoacacia L. which belongs to the tribe Robineae. Neither uricase activity nor nodulin‐35 was detected in nodules from Lupinus angustifolius L., an amide‐forming symbiosis of the tribe Genisteae. Nodules of R. pseudoacacia also showed high levels of allantoinase (EC 3.5.2.5) activity but activity of enzymes earlier in the pathway of ureide synthesis (xanthine dehydrogenase, EC 1.2.1.37; inosine monophosphate dehydrogenase, EC 1.2.1.14; and xanthosine nucleosidase, EC 3.2.2.1) could not be detected. Analysis of transport fluids (xylem, phloem and nodule exudates) from R. pseudoacacia found that asparagine, and, to a lesser extent, glutamine were the major translocated nitrogenous solutes. Ureides accounted for, at most, 2.6% of the N in transport fluids (tracheal xylem sap) and in nodule exudate, 0.1%. In common with nodules of the ureide‐forming symbioses, those of R. pseudoacacia contained a high proportion of uninfected interstitial cells (53.7 ± 2.3%) in the central N2‐fixing tissue whereas in L. angustifolius only 2.5 ± 0.4% of cells in this tissue were uninfected. These data have been interpreted to indicate that expression of nodule‐specific uricase is related to the differentiation of uninfected interstitial cells in nodules and not to the synthesis of ureides.