Infection of soybean and pea nodules by Rhizobium spp. purine auxotrophs in the presence of 5-aminoimidazole-4-carboxamide riboside
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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-amino-imidazole-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 leguminosarum 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 to 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.Keywords:
Rhizobium leguminosarum
Riboside
Auxotrophy
Purine metabolism
Accumulation of ribotide of 5-amino-4-imidazole carboxamide (AICAR), excreted in the riboside form by a purine-requiring mutant of Escherichia coli, was inhibited by a number of structural analogs of purines. The most potent inhibitors were 6-thioguanine, 6-mercaptopurine, and 2,6-diaminopurine. The type of inhibition obtained suggests that the antimetabolites sufficiently resemble natural purines in structure to act as feedback inhibitors in biosynthetic control.
Purine metabolism
Riboside
Ribonucleotide
Purine analogue
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Auxotrophs were isolated from four effective strains of Rhizobium leguminosarum and R. trifolii for a study of the relationship between metabolic defects and loss of ability for symbiosis. Most auxotrophs were isolated indirectly by prior isolation of mutants for resistance to metabolic inhibitors, especially D-alanine and D-histidine. The likelihood of some nutritional requirement was greatly increased among such resistant mutants, and was very high for derivatives of the R. trifolii strains. The complexity of growth factor requirement varied greatly between auxotrophs, and few had simple requirements. The most common requirement was for vitamins, notably thiamine. Partial or full restoration of effectiveness in symbiosis often accompanied reversion to prototrophy in some ineffective auxotrophs. The frequency of some degree of restoration among prototrophs varied from 0% to 100%, depending on the auxotrophic mutant involved. In one ineffective auxotroph of R. trifolii strain T1 the level of reversion (partial to complete) in vitro varied considerably and generally paralleled the degree of restoration of effectiveness. Biochemical deficiency appeared to be meaningfully related to impaired symbiosis in some auxotrophs but the relationship was probably incidental in most others. These auxotroph–prototroph studies are considered from the standpoint of relationship to several areas of research on Rhizobium.
Auxotrophy
Rhizobium leguminosarum
Reversion
Strain (injury)
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Rhizobium leguminosarum
Glycosidic bond
Biovar
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Rhizobium leguminosarum
Glycosidic bond
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Purine metabolism
Auxotrophy
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The DNA‐binding HU‐type proteins from several species of Rhizobiaceae including Rhizobium meliloti , two strains of Rhizobium leguminosarum with highly different phenotypic characters and Agrobacterium tumefaciens , were characterized and their amino acid sequences were determined. HU‐type proteins isolated from R. leguminosarum L 18 and A. tumefaciens are identical and show slight differences with the R. meliloti HU‐type protein. On the other hand the R. leguminosarum L 53 HU‐type protein is quite different from the proteins cited above; several amino acid substitutions encountered in this protein result in significant changes in the folding of the polypeptide chain. The biochemical characteristics of these proteins are in good agreement with the respective position of these bacteria in the phylogeny determined by numerical taxonomy.
Rhizobium leguminosarum
Sinorhizobium meliloti
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The research was aimed to study changes in peroxidase (PO) activity in the course of interaction with bacteria of various genera of Rhizobiaceae family: Rhizobium leguminosarum pea symbiont causing formation of root nodules and Agrobacterium rhizogеnes phytopathogen causing the syndrome of “hairy root”. Peroxidase activity in response to inoculation with Rh. leguminosarum virtually did not differ from control, whereas in the roots inoculated with A. rhizogines рeroxidase activity had a well-pronounced tendency to increase. It is concluded that the direction of enzyme activity change in pea seedlings interacting with Rh. leguminosarum promotes rhizobia penetration, and in the case with A. rhizogines pathogen it acts as an indicator of plant stress status.
Rhizobium leguminosarum
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Resistance to 2,6-diaminopurine in Salmonella typhimurium resulted in 2 phenotypically distinguishable mutants. The mutant dap-r-3 differed from the mutant dap-r-6 in its cross-resistance to other purine analogues and also in its ability to allow the satellite growth of the parent sensitive strain LT-2 when plated in presence of inhibitory concentrations of adenine or DAP. The satellite phenomenon could be explained by the fact that adenine and uracil, which were excreted by the mutant dap-r-3, prevented the inhibition of the wild type by DAP and adenine respectively. Excretions of these compounds were not directly responsible for the resistance to DAP, since the dap-r-3 type mutation could also be demonstrated in auxotrophic strains incapable of synthesizing adenine or uracil de novo. The mutation to a dap-r-3 type in a purine requiring auxotroph resulted in an additional defect characterized by poor growth response to all the purines.
Auxotrophy
Purine metabolism
Uracil
Strain (injury)
Wild type
Purine analogue
Hypoxanthine
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Rhizobium leguminosarum
Bradyrhizobium japonicum
Laminarin
Bradyrhizobium
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