INFECTION OF SOYBEAN AND PEA NODULES BY RHIZOBIUM SPP PURINE AUXOTROPHS IN THE PRESENCE OF 5-AMINOIMIDAZOLE-4-CARBOXAMIDE RIBOSIDE

Purine auxotrophs of various Rhizobium species are symbiotically defec tive, usually unable to initiate or complete the infection process. Ea rlier studies demonstrated that, in the Rhizobium etli-bean symbiosis, infection by purine auxotrophs is partially restored by supplementati on of the plant medium with 5-amino-imidazole-4-carboxamide (AICA) rib oside, the unphosphorylated form of the purine biosynthetic intermedia te AICAR. The addition of purine to the root environment does not have this effect. In this study, purine auxotrophs of Rhizobium fredii HH3 03 and Rhizobium leguminosarum 128C56 (bv. viciae) were examined. Nutr itional and genetic characterization indicated that each mutant was bl ocked in purine biosynthesis prior to the production of AICAR. R. fred ii HH303 and R. leguminosarum 128C56 appeared to be deficient in AICA riboside transport and/or conversion into AICAR, and the auxotrophs de rived from them grew very poorly with AICA riboside as a purine source . All of the auxotrophs elicited poorly developed, uninfected nodules on their appropriate hosts. On peas, addition of AICA riboside or puri ne to the root environment led to enhanced nodulation; however, infect ion threads were observed only in the presence of AICA riboside. On so ybeans, only AICA riboside was effective in enhancing nodulation and p romoting infection. Although AICA riboside supplementation of the auxo trophs led to infection thread development on both hosts, the numbers of bacteria recovered from the nodules were still 2 or more orders of magnitude lower than in fully developed nodules populated by wild-type bacteria. The ability of AICA riboside to promote infection by purine auxotrophs, despite serving as a very poor purine source for these st rains, supports the hypothesis that AICAR plays a role in infection ot her than merely promoting bacterial growth.