Assimilation of Symbiotically-Reduced Nitrogen in Tropical Legumes: Regulation of Induction of de novo Purine Biosynthesis and Peroxisome Proliferation Leading to Ureide Production

Tropical legumes, such as soybean and Vigna, transport fixed nitrogen as ureides. We have earlier demonstrated that glutamine produced in the host cell cytoplasm as a result of ammonia assimilation is not transported out of the infection zone and is funneled in to plastids of the infected cells where it is converted in to purines. These de novo synthesized purines are then exported to the uninfected cells of the infection zone and inner cortex of the nodule where they are oxidized into allantoin and allantoic acids (ureides) and transported to the shoot. This pathway of nitrogen assimilation is apparently more energy efficient in comparison to amides that are normally transported in temperate nodules. To understand the regulation of assimilation of reduced nitrogen through ureide synthesis, we cloned phosphoribosylpyrophosphate amidotransferase (PRPP-AT) gene from soybean and showed that its expression is regulated by glutamine. This was accomplished by fusing the promoter of this gene with GUS reporter and transfer to hairy roots of soybean. The transgenic hairy roots showed glutamine-induced expression of GUS. Since PRPP-AT gene controls the entire de novo purine biosynthesis understanding the mechanism of its regulation is important for our ability to modify nitrogen assimilation process. Purines also induce peroxisome proliferation in the uninfected cells and the synthesis of enzymes necessary for oxidation of purines.