Rhizobium Purine Metabolism and Its Role in Infection of Legumes

A. Rationale. Bacteria of the family Rhizobiaceae enter into a symbiotic relationship with legumes in which the bacterial partner reduces atmospheric nitrogen to ammonia that is supplied to the host plant in exchange for reduced carbon sources. This occurs following a complex series of developmental steps that culminate in the formation of a specialized plant organ, the root nodule. Purine auxotrophy in Rhizobium has consistently been found to cause symbiotic defects (Chen et al., 1985; Dickstein et al., 1991; Djordjevic et al., 1988; Fedorov and Zaretskaya, 1979; Kerppola and Kahn, 1988; Kim et al., 1988; Noel et al., 1988; Pain, 1979; Pankhurst and Schwinghamer, 1974; Scherrer and Denarie, 1971; VandenBosch et al., 1985). On bean, Rhizobium leguminosarum biovar phaseoli purine auxotrophs elicit uninfected, poorly developed nodules (Noel et al., 1988; VandenBosch et al., 1985). It has been reported that while purines support the growth of these mutants, purine addition to the root medium has no effect on the nodulation phenotype. However, infection is restored by supplementation with 5-aminoimidazole-4- carboxamide (AICA) riboside, the unphosphorylated form of the purine biosynthetic intermediate AICAR (Noel et al., 1988). The finding that infection can be promoted by AICA riboside, but not by purines, led to the hypothesis that AICAR is required for infection thread development While characterization of the Rhizobium purine auxotrophs indicated that all were blocked before the production of AICAR, several mutants had phenotypes that could not be explained by the current understanding of bacterial purine metabolism, which is based almost entirely on studies with Escherichia coli and Salmonella typhimurium. To detennine whether purine auxotrophy, and possibly also the symbiotic defect, of the mutants was a pleiotropic effect, it was desirable to identify the specific purine biosynthetic steps in which each of the auxotrophs was blocked. In addition, it was necessary to characterize AICA riboside transport and utilization to narrow down possible mechanisms by which AICA riboside could promote infection..