Infection of soybean and pea nodules by Rhizobium spp. purine auxotrophs in the presence of 5-aminoimidazole-4-carboxamide riboside

Abstract

Purine auxotrophs of various Rhizobium species are symbiotically defective, usually unable to initiate or complete the infection process. Earlier studies demonstrated that, in the Rhizobium etli-bean symbiosis, infection by purine auxotrophs is partially restored by supplementation of the plant medium with 5-aminoimidazole-4-carboxamide (AICA) riboside, the unphosphorylated form of the purine biosynthetic intermediate AICAR. The addition of purine to the root environment does not have this effect. In this study, purine auxotrophs of Rhizobium fredii HH303 and Rhizobium keguminosarum 128C56 (bv. viciae) were examined.Nutritional and genetic characterization indicated that each mutant was blocked in purine biosynthesis prior to the production of AICAR. R. fredii HH303 and R. leguminosarum 128C56 appeared to be deficient in AICA riboside transport and/or conversion into AICAR, and the auxotrophs derived 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 purine to the root environment led to enhanced nodulation; however, infection threads were observed only in the presence of AICA riboside. On soybeans, only AICA riboside was effective in enhancing nodulation and promoting infection. Although AICA riboside supplementation of the auxotrophs 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 strains, supports the hypothesis that AICAR plays a role in infection other than merely promoting bacterial growth.Root nodules of leguminous plants develop from cell division foci triggered within young root cortical layers by signals from bacteria of the genera Rhizobium, Bradyrhizobium, and Azorhizobium. The bacteria penetrate from the root epidermis to the interior of the developing nodule by means of an infection thread, composed of a growing linear bacterial colony and the surrounding tubular cell wall structure produced by the plant. Eventually, numerous plant cells in the nodule interior become infected by endocytosis of the bacteria from unwalled tips of the infection thread (17). These released bacteria then differentiate into bacteroids that commence nitrogen fixation.The process of infection thread formation remains obscure, especially at the molecular level. Very little is known about the contribution of the microsymbiont, although it is clear that certain deficiencies of the bacteria can prevent or severely limit infection. Mutations in crucial bacterial nod genes prevent infection, but since they also prevent (5,7,12,22,25,32) and auxotrophs (2,6,9,10,16,18,23,(26)(27)(28)30). Perhaps the most universally deleterious such defect is purine auxotrophy.Purine auxotrophs (Pur-) of Rhiz...

Key Role of Bacterial NH 4 + Metabolism in Rhizobium-Plant Symbiosis

“…N-containing compounds such as purines and pyrimidines are not provided by the corresponding plant partners to B. japonicum, R. etli, Rhizobium fredii, and R. leguminosarum, even in the ITs, since auxotrophic strains induce only empty pseudonodules (128,130). Mutants of S. meliloti induce Fix Ϫ nodules, but a histological analysis was not performed (85).…”

The Mesorhizobium loti purB Gene Is Involved in Infection Thread Formation and Nodule Development in Lotus japonicus

“…RL106 and RL110, purF and purY mutants of Rhizobium leguminosarum bv. viciae 128C56, induce small bumps lacking bacteria on Pisum sativum (16). purQ and purL mutants of Sinorhizobium (basonym, Rhizobium) fredii HH103 induce small bumps or pseudonodules on Glycine max (4,16).…”

“…viciae 128C56, induce small bumps lacking bacteria on Pisum sativum (16). purQ and purL mutants of Sinorhizobium (basonym, Rhizobium) fredii HH103 induce small bumps or pseudonodules on Glycine max (4,16). ANU2861 (purM::Tn5) and ANU2866 (purY:: Tn5), mutants of the broad-host-range Rhizobium strain NGR234, are defective in nodule formation on a wide variety of legumes.…”

Plasticity in the Purine–Thiamine Metabolic Network of Salmonella

“…In S. enterica, the thiamine requirement of a purH mutant (Yura 1956;Newell and Tucker 1968;Downs 1987) and the inability of the same mutant to grow on glycerol (Burton 1971;Dougherty et al 2006) have been attributed to the accumulation of AICAR. In Rhizobium etli, genetic studies showed that AICAR accumulation affected cellular processes such as the production of the cytochrome terminal oxidase cbb 3 (Soberon et al 1997) and the infection stages of symbiosis (Newman et al 1994). In mammals, AICAR has been shown to activate AMP-protein kinase (Sullivan et al 1994), an enzyme that maintains energy homeostasis by increasing fatty acid oxidation and inhibiting triglyceride synthesis (Karp et al 2002).…”