Ureide biosynthesis (Oryza sativa)

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Based on original gramene pathway.

Nitrogen is frequently the limiting nutrient for growth of many crop plants. Legumes solved this problem by establishing a symbiotic relationship with bacteria from the genus Bradyrhizobium, which contain the enzyme nitrogenase. Nitrogenase catalyzes the conversion of atmospheric dinitrogen to ammonia, which can be used to meet the plant's metabolic nitrogen requirements.

Before the fixed nitrogen can be used by the plant, it must be converted into organic forms that can be transported throughout the plant and further metabolized. There are two major forms of organic nitrogen compounds that can be transported - amides (such as L-asparagine and L-glutamine ) and ureides (such as allantoin and allantoate ). Temperate-region legumes, such as pea, are amide exporters, while legumes of tropical origin, such as soybean, kidney bean, and peanut, export nitrogen in the forms of ureides. Isotope-labeling studies performed with tropical legumes demonstrate that ammonia produced from dinitrogen reduction is rapidly converted into allantoin and allantoate [ Ohyama78 ], and that up to 95% of the nitrogen in the xylem sap in nodulated soybeans is in the form of ureides [ Schubert81 ].

The ammonia generated by the bacteria is first converted into the amino acid L-glutamine and then incorporated into inosine-5'-phosphate (IMP), a purine synthesized de novo during nitrogen fixation by roots (see purine nucleotides de novo biosynthesis II ).