Rhizobium meliloti mutants unable to synthesize anthranilate display a novel symbiotic phenotype

Competition, Nodule Occupancy, and Persistence of Inoculant Strains: Key Factors in the Rhizobium-Legume Symbioses

Biological nitrogen fixation by Rhizobium-legume symbioses represents an environmentally friendly and inexpensive alternative to the use of chemical nitrogen fertilizers in legume crops. Rhizobial inoculants, applied frequently as biofertilizers, play an important role in sustainable agriculture. However, inoculants often fail to compete for nodule occupancy against native rhizobia with inferior nitrogen-fixing abilities, resulting in low yields. Strains with excellent performance under controlled conditions are typically selected as inoculants, but the rates of nodule occupancy compared to native strains are rarely investigated. Lack of persistence in the field after agricultural cycles, usually due to the transfer of symbiotic genes from the inoculant strain to naturalized populations, also limits the suitability of commercial inoculants. When rhizobial inoculants are based on native strains with a high nitrogen fixation ability, they often have superior performance in the field due to their genetic adaptations to the local environment. Therefore, knowledge from laboratory studies assessing competition and understanding how diverse strains of rhizobia behave, together with assays done under field conditions, may allow us to exploit the effectiveness of native populations selected as elite strains and to breed specific host cultivar-rhizobial strain combinations. Here, we review current knowledge at the molecular level on competition for nodulation and the advances in molecular tools for assessing competitiveness. We then describe ongoing approaches for inoculant development based on native strains and emphasize future perspectives and applications using a multidisciplinary approach to ensure optimal performance of both symbiotic partners.

Metabolomics and Dual RNA-Sequencing on Root Nodules Revealed New Cellular Functions Controlled by Paraburkholderia phymatum NifA

Paraburkholderia phymatum STM815 is a nitrogen-fixing endosymbiont that nodulate the agriculturally important Phaseolus vulgaris and several other host plants. We previously showed that the nodules induced by a STM815 mutant of the gene encoding the master regulator of nitrogen fixation NifA showed no nitrogenase activity (Fix⁻) and increased in number compared to P. vulgaris plants infected with the wild-type strain. To further investigate the role of NifA during symbiosis, nodules from P. phymatum wild-type and nifA mutants were collected and analyzed by metabolomics and dual RNA-Sequencing, allowing us to investigate both host and symbiont transcriptome. Using this approach, several metabolites’ changes could be assigned to bacterial or plant responses. While the amount of the C₄-dicarboxylic acid succinate and of several amino acids was lower in Fix⁻ nodules, the level of indole-acetamide (IAM) and brassinosteroids increased. Transcriptome analysis identified P. phymatum genes involved in transport of C₄-dicarboxylic acids, carbon metabolism, auxin metabolism and stress response to be differentially expressed in absence of NifA. Furthermore, P. vulgaris genes involved in autoregulation of nodulation (AON) are repressed in nodules in absence of NifA potentially explaining the hypernodulation phenotype of the nifA mutant. These results and additional validation experiments suggest that P. phymatum STM815 NifA is not only important to control expression of nitrogenase and related enzymes but is also involved in regulating its own auxin production and stress response. Finally, our data indicate that P. vulgaris does sanction the nifA nodules by depleting the local carbon allocation rather than by mounting a strong systemic immune response to the Fix⁻ rhizobia.

Characterization of Mutations That Affect the Nonoxidative Pentose Phosphate Pathway in Sinorhizobium meliloti

Sinorhizobium meliloti is a Gram-negative alphaproteobacterium that can enter into a symbiotic relationship with Medicago sativa and Medicago truncatula Previous work determined that a mutation in the tkt2 gene, which encodes a putative transketolase, could prevent medium acidification associated with a mutant strain unable to metabolize galactose. Since the pentose phosphate pathway in S. meliloti is not well studied, strains carrying mutations in either tkt2 and tal, which encodes a putative transaldolase, were characterized. Carbon metabolism phenotypes revealed that both mutants were impaired in growth on erythritol and ribose. This phenotype was more pronounced for the tkt2 mutant strain, which also displayed auxotrophy for aromatic amino acids. Changes in pentose phosphate pathway metabolite concentrations were also consistent with a mutation in either tkt2 or tal The concentrations of metabolites in central carbon metabolism were also found to shift dramatically in strains carrying a tkt2 mutation. While the concentrations of proteins involved in central carbon metabolism did not change significantly under any conditions, the levels of those associated with iron acquisition increased in the wild-type strain with erythritol induction. These proteins were not detected in either mutant, resulting in less observable rhizobactin production in the tkt2 mutant. While both mutants were impaired in succinoglycan synthesis, only the tkt2 mutant strain was unable to establish symbiosis with alfalfa. These results suggest that tkt2 and tal play central roles in regulating the carbon flow necessary for carbon metabolism and the establishment of symbiosis.IMPORTANCESinorhizobium meliloti is a model organism for the study of plant-microbe interactions and metabolism, especially because it effects nitrogen fixation. The ability to derive the energy necessary for nitrogen fixation is dependent on an organism's ability to metabolize carbon efficiently. The pentose phosphate pathway is central in the interconversion of hexoses and pentoses. This study characterizes the key enzymes of the nonoxidative branch of the pentose phosphate pathway by using defined genetic mutations and shows the effects the mutations have on the metabolite profile and on physiological processes such as the biosynthesis of exopolysaccharide, as well as the ability to regulate iron acquisition.

Key roles of microsymbiont amino acid metabolism in rhizobia-legume interactions

Rhizobia are bacteria in the α-proteobacterial genera Rhizobium, Sinorhizobium, Mesorhizobium, Azorhizobium and Bradyrhizobium that reduce (fix) atmospheric nitrogen in symbiotic association with a compatible host plant. In free-living and/or symbiotically associated rhizobia, amino acids may, in addition to their incorporation into proteins, serve as carbon, nitrogen or sulfur sources, signals of cellular nitrogen status and precursors of important metabolites. Depending on the rhizobia-host plant combination, microsymbiont amino acid metabolism (biosynthesis, transport and/or degradation) is often crucial to the establishment and maintenance of an effective nitrogen-fixing symbiosis and is intimately interconnected with the metabolism of the plant. This review summarizes past findings and current research directions in rhizobial amino acid metabolism and evaluates the genetic, biochemical and genome expression studies from which these are derived. Specific sections deal with the regulation of rhizobial amino acid metabolism, amino acid transport, and finally the symbiotic roles of individual amino acids in different plant-rhizobia combinations.

Engineering rhizobial bioinoculants: a strategy to improve iron nutrition

Under field conditions, inoculated rhizobial strains are at a survival disadvantage as compared to indigenous strains. In order to out-compete native rhizobia it is not only important to develop strong nodulation efficiency but also increase their competence in the soil and rhizosphere. Competitive survival of the inoculated strain may be improved by employing strain selection and by genetic engineering of superior nitrogen fixing strains. Iron sufficiency is an important factor determining the survival and nodulation by rhizobia in soil. Siderophores, a class of ferric specific ligands that are involved in receptor specific iron transport into bacteria, constitute an important part of iron acquisition systems in rhizobia and have been shown to play a role in symbiosis as well as in saprophytic survival. Soils predominantly have iron bound to hydroxamate siderophores, a pool that is largely unavailable to catecholate-utilizing rhizobia. Outer membrane receptors for uptake of ferric hydroxamates include FhuA and FegA which are specific for ferrichrome siderophore. Increase in nodule occupancy and enhanced plant growth of the fegA and fhuA expressing engineered bioinoculants rhizobial strain have been reported. Engineering rhizobia for developing effective bioinoculants with improved ability to utilize heterologous siderophores could provide them with better iron acquisition ability and consequently, rhizospheric stability.

Identification of genes relevant to symbiosis and competitiveness in Sinorhizobium meliloti using signature-tagged mutants

Sinorhizobium meliloti enters an endosymbiosis with alfalfa plants through the formation of nitrogen-fixing nodules. In order to identify S. meliloti genes required for symbiosis and competitiveness, a method of signature-tagged mutagenesis was used. Two sets, each consisting of 378 signature-tagged mutants with a known transposon insertion site, were used in an experiment in planta. As a result, 67 mutants showing attenuated symbiotic phenotypes were identified, including most of the exo, fix, and nif mutants in the sets. For 38 mutants in genes previously not described to be involved in competitiveness or symbiosis in S. meliloti, attenuated competitiveness phenotypes were tested individually. A large part of these phenotypes was confirmed. Moreover, additional symbiotic defects were observed for mutants in several novel genes such as infection deficiency phenotypes (ilvI and ilvD2 mutants) or delayed nodulation (pyrE, metA, thiC, thiO, and thiD mutants).

Auxotrophy in rhizobia revisited

Among the various types of mutations studied in rhizobia, the auxotrophic mutations (which confer on the mutants the inability to synthesize certain essential substances such as amino acids, vitamins and nucleic acids), are the most favoured ones as these can be used as suitable markers for genetic analysis. An important property of rhizobia is their effectiveness i.e. their ability to fix atmospheric nitrogen into ammonia within the nodule. Special interest in this category of mutations by rhizobial geneticists is due to the fact that there is a strong correlation between the metabolic defects and the ineffectiveness (Nod(-) and/or Fix(-)) of the rhizobial strains. Auxotrophic mutants of various species of rhizobia with defects in the synthesis of nucleic bases, vitamins and amino acids have been obtained by mutagenising with physical, chemical and Tn5 mutagens. These mutants have been used in mapping studies as well as in establishing a correlation between its metabolic requirement and symbiotic relationship with the host plant. The present review deals with the isolation of auxotrophs, and their genetic, biochemical and symbiotic characterization. The review also encompasses the studies on the elucidation of biosynthetic pathways of nutritional substances in rhizobia.

Effects of AiiA-mediated quorum quenching in Sinorhizobium meliloti on quorum-sensing signals, proteome patterns, and symbiotic interactions

Many behaviors in bacteria, including behaviors important to pathogenic and symbiotic interactions with eukaryotic hosts, are regulated by a mechanism called quorum sensing (QS). A "quorum-quenching" approach was used here to identify QS-regulated behaviors in the N-fixing bacterial symbiont Sinorhizobium meliloti. The AiiA lactonase from Bacillus produced in S. meliloti was shown to enzymatically inactivate S. meliloti's N-acyl homoserine lactone (AHL) QS signals, thereby disrupting normal QS regulation. Sixty proteins were differentially accumulated in the AiiA-producing strain versus the control in early log or early stationary phase cultures. Fifty-two of these QS-regulated proteins, with putative functions that include cell division, protein processing and translation, metabolite transport, oxidative stress, and amino acid metabolism, were identified by peptide mass fingerprinting. Transcription of representative genes was reduced significantly in the AiiA-producing strain, although the effects of AiiA on protein accumulation did not always correspond to effects on transcription. The QS signal-deficient strain was reduced significantly in nodule initiation during the first 12 h after inoculation onto Medicago truncatula host plants. The AiiA lactonase also was found to substantially inactivate two of the AHL mimic compounds secreted by M. truncatula. This suggests some structural similarity between bacterial AHLs and these mimic compounds. It also indicates that quorum quenching could be useful in identifying Sinorhizobium genes that are affected by such host QS mimics in planta.

Organogenesis of legume root nodules

The N(2)-fixing nodules elicited by rhizobia on legume roots represent a useful model for studying plant development. Nodule formation implies a complex progression of temporally and spatially regulated events of cell differentiation/dedifferentiation involving several root tissues. In this review we describe the morphogenetic events leading to the development of these histologically well-structured organs. These events include (1) root hair deformation, (2) development and growth of infection threads, (3) induction of the nodule primordium, and (4) induction, activity, and persistence of the nodular meristem and/or of foci of meristematic activities. Particular attention is given to specific aspects of the symbiosis, such as the early stages of intracellular invasion and to differentiation of the intracellular form of rhizobia, called symbiosomes. These developmental aspects were correlated with (1) the regulatory signals exchanged, (2) the plant genes expressed in specific cell types, and (3) the staining procedures that allow the recognition of some cell types. When strictly linked with morphogenesis, the nodulation phenotypes of plant and bacterial mutants such as the developmental consequence of the treatment with metabolic inhibitors, metabolic intermediates, or the variation of physical parameters are described. Finally, some aspects of nodule senescence and of regulation of nodulation are discussed.

Key role of bacterial NH(4)(+) metabolism in Rhizobium-plant symbiosis

Symbiotic nitrogen fixation is carried out in specialized organs, the nodules, whose formation is induced on leguminous host plants by bacteria belonging to the family Rhizobiaceae: Nodule development is a complex multistep process, which requires continued interaction between the two partners and thus the exchange of different signals and metabolites. NH(4)(+) is not only the primary product but also the main regulator of the symbiosis: either as ammonium and after conversion into organic compounds, it regulates most stages of the interaction, from the production of nodule inducers to the growth, function, and maintenance of nodules. This review examines the adaptation of bacterial NH(4)(+) metabolism to the variable environment generated by the plant, which actively controls and restricts bacterial growth by affecting oxygen and nutrient availability, thereby allowing a proficient interaction and at the same time preventing parasitic invasion. We describe the regulatory circuitry responsible for the downregulation of bacterial genes involved in NH(4)(+) assimilation occurring early during nodule invasion. This is a key and necessary step for the differentiation of N(2)-fixing bacteroids (the endocellular symbiotic form of rhizobia) and for the development of efficient nodules.

Auxotrophic mutant strains of Rhizobium etli reveal new nodule development phenotypes

We report here the isolation and characterization of amino acid-requiring mutant strains of Rhizobium etli. We observe that the phenotype of most mutations, even when causing a strict auxotrophy, is overcome by cross-feeding from the host plant Phaseolus vulgaris, thereby allowing bacterial production of Nod factors and, consequently, nodule induction. Conversely, light and electron microscopy analysis reveals that the nodules induced by all mutants, including those with normal external morphology, are halted or strongly altered at intermediate or late stages of development. Moreover, some mutants induce nodules that display novel symbiotic phenotypes, such as specific alterations of the invaded cells or the presence of a reduced number of abnormally shaped uninvaded cells. Other mutants induce nodules showing an early and vast necrosis of the central tissue, a phenotype not previously observed in bean nodules, not even in nodules induced by a Fix- mutant. These observations indicate that amino acid auxotrophs represent a powerful tool to study the development of globose determinate-type nodules and emphasize the importance of establishing their histology and cytology before considerations of metabolic exchange are made.

Metals and the rhizobial-legume symbiosis--uptake, utilization and signalling

In this review, we consider how the nitrogen-fixing root nodule bacteria, the 'rhizobia', acquire various metals, paying particular attention to the uptake of iron. We also review the literature pertaining to the roles of molybdenum and nickel in the symbiosis with legumes. We highlight some gaps in our knowledge, for example the lack of information on how rhizobia acquire molybdenum. We examine the means whereby different metals affect rhizobial physiology and the role of metals as signals for gene regulation. We describe the ways in which genetics has shown (or not) if, and how, particular metal uptake and/or metal-mediated signalling pathways are required for the symbiotic interaction with legumes.

Genetic organization of the region encoding regulation, biosynthesis, and transport of rhizobactin 1021, a siderophore produced by Sinorhizobium meliloti

Eight genes have been identified that function in the regulation, biosynthesis, and transport of rhizobactin 1021, a hydroxamate siderophore produced under iron stress by Sinorhizobium meliloti. The genes were sequenced, and transposon insertion mutants were constructed for phenotypic analysis. Six of the genes, named rhbABCDEF, function in the biosynthesis of the siderophore and were shown to constitute an operon that is repressed under iron-replete conditions. Another gene in the cluster, named rhtA, encodes the outer membrane receptor protein for rhizobactin 1021. It was shown to be regulated by iron and to encode a product having 61% similarity to IutA, the outer membrane receptor for aerobactin. Transcription of both the rhbABCDEF operon and the rhtA gene was found to be positively regulated by the product of the eighth gene in the cluster, named rhrA, which has characteristics of an AraC-type transcriptional activator. The six genes in the rhbABCDEF operon have interesting gene junctions with short base overlaps existing between the genes. Similarities between the protein products of the biosynthesis genes and other proteins suggest that rhizobactin 1021 is synthesized by the formation of a novel siderophore precursor, 1,3-diaminopropane, which is then modified and attached to citrate in steps resembling those of the aerobactin biosynthetic pathway. The cluster of genes is located on the pSyma megaplasmid of S. meliloti 2011. Reverse transcription-PCR with RNA isolated from mature alfalfa nodules yielded no products for rhbF or rhtA at a time when the nifH gene was strongly expressed, indicating that siderophore biosynthesis and transport genes are not strongly expressed when nitrogenase is being formed in root nodules. Mutants having transposon insertions in the biosynthesis or transport genes induced effective nitrogen-fixing nodules on alfalfa plants.

Analysis of the Rhizobium leguminosarum siderophore-uptake gene fhuA: differential expression in free-living bacteria and nitrogen-fixing bacteroids and distribution of an fhuA pseudogene in different strains

A mutation was isolated in the Rhizobium leguminosarum gene fhuA, which appears to specify the outer-membrane receptor for the siderophore vicibactin. The mutant was defective in iron uptake and accumulated the siderophore vicibactin in the extracellular medium. Expression of fhuA was regulated by Fe3+, transcription being higher in iron-depleted cells. Transcription of fhuA was independent of a functional copy of rpol, a neighbouring gene that specifies a putative ECF sigma factor of RNA polymerase and which is involved in siderophore production in Rhizobium. Mutations in fhuA did not detectably affect symbiotic N2 fixation on peas. An fhuA::gus fusion was expressed by bacteria in the meristematic zone of pea nodules but not in mature bacteroids. Some other strains of R. leguminosarum also contain a pseudogene version of fhuA. The sequences of some of these and the 'real' fhuA genes were determined.

The Rhizobium etli trpB gene is essential for an effective symbiotic interaction with Phaseolus vulgaris

A mutant strain (CTNUX4) of Rhizobium etli carrying Tn5 unable to grow with ammonium as the sole nitrogen source was isolated and characterized. Sequence analysis showed that Tn5 is inserted into a trpB (tryptophan synthase)-homologous gene. When tested on the roots of Phaseolus vulgaris, strain CTNUX4 was able to induce only small, slightly pink, ineffective (Fix-) nodules. However, under free-living conditions, strain CTNUX4 was unable to produce flavonoid-inducible lipo-chitin oligosaccharides (Nod factors) unless tryptophan was added to the growth medium. These data and histological observations indicate that the lack of tryptophan biosynthesis affects the symbiotic behavior of R. etli.

Bacterial genes induced within the nodule during the Rhizobium-legume symbiosis

During the symbiosis between the bacterium Rhizobium meliloti and plants such as alfalfa, the bacteria elicit the formation of nodules on the roots of host plants. The bacteria infect the nodule, enter the cytoplasm of plant cells and differentiate into a distinct cell type called a bacteroid, which is capable of fixing atmospheric nitrogen. To discover bacterial genes involved in the infection and differentiation stages of symbiosis, we obtained genes expressed at the appropriate time and place in the nodule by identifying promoters that are able to direct expression of the bacA gene, which is required for bacteroid differentiation. We identified 230 fusions that are expressed predominantly in the nodule. Analysis of 23 sequences indicated that only three encode proteins known to be involved in the Rhizobium-legume symbiosis, six encode proteins with homology to proteins not previously associated with symbiosis, and 14 have no significant similarity to proteins of known function. Disruption of a locus that encodes a protein with homology to a cell adhesion molecule led to a defect in the formation of nitrogen-fixing nodules, resulting in an increased number of nitrogen-starved plants. Our isolation of a large number of nodule-expressed genes will help to open the intermediate stages of nodulation to molecular analysis.

METABOLITE TRANSPORT ACROSS SYMBIOTIC MEMBRANES OF LEGUME NODULES

Infection of legume roots or stems with soil bacteria of the Rhizobiaceae results in the formation of nodules that become symbiotic nitrogen-fixing organs. Within the infected cells of these nodules, bacteria are enveloped in a membrane of plant origin, called the peribacteroid membrane (PBM), and divide and differentiate to form nitrogen-fixing bacteroids. The organelle-like structure comprised of PBM and bacteroids is termed the symbiosome, and is the basic nitrogen-fixing unit of the nodule. The major exchange of nutrients between the symbiotic partners is reduced carbon from the plant, to fuel nitrogenase activity in the bacteroid, and fixed nitrogen from the bacteroid, which is assimilated in the plant cytoplasm. However, many other metabolites are also exchanged. The metabolic interaction between the plant and the bacteroids is regulated by a series of transporters and channels on the PBM and the bacteroid membrane, and these form the focus of this review.

Genetic analysis of the Rhizobium meliloti bacA gene: functional interchangeability with the Escherichia coli sbmA gene and phenotypes of mutants

The Rhizobium meliloti bacA gene encodes a function that is essential for bacterial differentiation into bacteroids within plant cells in the symbiosis between R. meliloti and alfalfa. An Escherichia coli homolog of BacA, SbmA, is implicated in the uptake of microcin B17, microcin J25 (formerly microcin 25), and bleomycin. When expressed in E. coli with the lacZ promoter, the R. meliloti bacA gene was found to suppress all the known defects of E. coli sbmA mutants, namely, increased resistance to microcin B17, microcin J25, and bleomycin, demonstrating the functional similarity between the two proteins. The R. meliloti bacA386::Tn(pho)A mutant, as well as a newly constructed bacA deletion mutant, was found to show increased resistance to bleomycin. However, it also showed increased resistance to certain aminoglycosides and increased sensitivity to ethanol and detergents, suggesting that the loss of bacA function causes some defect in membrane integrity. The E. coli sbmA gene suppressed all these bacA mutant phenotypes as well as the Fix- phenotype when placed under control of the bacA promoter. Taken together, these results strongly suggest that the BacA and SbmA proteins are functionally similar and thus provide support for our previous hypothesis that BacA may be required for uptake of some compound that plays an important role in bacteroid development. However, the additional phenotypes of bacA mutants identified in this study suggest the alternative possibility that BacA may be needed for membrane integrity, which is likely to be critically important during the early stages of bacterial differentiation within plant cells.

Iron Uptake by Symbiosomes from Soybean Root Nodules

To identify possible iron sources for bacteroids in planta, soybean (Glycine max L. Merr.) symbiosomes (consisting of the bacteroid-containing peribacteroid space enclosed by the peribacteroid membrane [PBM]) and bacteroids were assayed for the ability to transport iron supplied as various ferric [Fe(III)]-chelates. Iron presented as a number of Fe(III)-chelates was transported at much higher rates across the PBM than across the bacteroid membranes, suggesting the presence of an iron storage pool in the peribacteroid space. Pulse-chase experiments confirmed the presence of such an iron storage pool. Because the PBM is derived from the plant plasma membrane, we reasoned that it may possess a ferric-chelate reductase activity similar to that present in plant plasma membrane. We detected ferric-chelate reductase activity associated with the PBM and suggest that reduction of Fe(III) to ferrous [Fe(II)] plays a role in the movement of iron into soybean symbiosomes.

Genetic analysis of Rhizobium meliloti bacA-phoA fusion results in identification of degP: two loci required for symbiosis are closely linked to degP

The function of the Rhizobium meliloti bacA gene, which is a homolog of the Escherichia coli sbmA gene, is required for an intermediate step in nodule development. A strain carrying the bacA386::TnphoA fusion was mutagenized with N-methyl-N'-nitro-N-nitrosoguanidine, and three mutants that had higher levels of alkaline phosphatase activity were identified. The mutations in these strains were recessive and mapped to the same genetic locus. The gene affected by these mutations was identified and sequenced and was found to be a homolog of the E. coli degP gene, which encodes a periplasmic endopeptidase. Although degP function is important for the virulence of certain intracellular pathogens of mammals, it is not required for the R. meliloti-alfalfa symbiosis. The genetic analyses involving degP were complicated by the presence of a locus immediately upstream of depP that was lethal when present in multiple copies in a DegP- background. R. meliloti derivatives carrying insertion mutations in this locus displayed an N,N,N',N'-tetramethyl-p-phenylenediamine oxidase-negative phenotype, elicited the formation of white cylindrical nodules that did not fix nitrogen, and grew slowly in rich medium, suggesting that the locus was a cyc gene encoding a protein involved in the biosynthesis of a component or components of a respiratory chain. The previously identified fix-382::TnphoA, which similarly causes the formation of white cylindrical nodules that do not fix nitrogen, was shown to affect a gene that is separate from this cyc gene but extremely closely linked to it.

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 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.