Nodule initiation elicited by noninfective mutants of Rhizobium phaseoli

Role of core lipopolysaccharide biosynthetic genes in the infection and adsorption of broad-host-range bacteriophages of Rhizobium etli

In this study, we examined the role of the lipopolysaccharide (LPS) core of Rhizobium etli in facilitating the adsorption and infection of phages with broad host range. When the plasmid-encoded LPS biosynthesis genes, wreU and wreV, were disrupted, distinct and contrasting effects on phage infection were observed. The wreU mutant strains exhibited wild-type adsorption and infection properties, whereas the wreV mutant demonstrated resistance to phage infection, but retained the capacity to adsorb phages. Complementation of the wreV mutant strains with a recombinant plasmid containing the wreU and wreV, restored the susceptibility to the phages. However, the presence of this recombinant plasmid in a strain devoid of the native lps-encoding plasmid was insufficient to restore phage susceptibility. These results suggest that the absence of wreV impedes the proper assembly of the complete LPS core, potentially affecting the formation of UDP-KdgNAg or KDO precursors for the O-antigen. In addition, a protein not yet identified, but residing in the native lps-encoding plasmid, may be necessary for complete phage infection.

Lotus japonicus NOOT-BOP-COCH-LIKE1 is essential for nodule, nectary, leaf and flower development

The NOOT-BOP-COCH-LIKE (NBCL) genes are orthologs of Arabidopsis thaliana BLADE-ON-PETIOLE1/2. The NBCLs are developmental regulators essential for plant shaping, mainly through the regulation of organ boundaries, the promotion of lateral organ differentiation and the acquisition of organ identity. In addition to their roles in leaf, stipule and flower development, NBCLs are required for maintaining the identity of indeterminate nitrogen-fixing nodules with persistent meristems in legumes. In legumes forming determinate nodules, without persistent meristem, the roles of NBCL genes are not known. We thus investigated the role of Lotus japonicus NOOT-BOP-COCH-LIKE1 (LjNBCL1) in determinate nodule identity and studied its functions in aerial organ development using LORE1 insertional mutants and RNA interference-mediated silencing approaches. In Lotus, LjNBCL1 is involved in leaf patterning and participates in the regulation of axillary outgrowth. Wild-type Lotus leaves are composed of five leaflets and possess a pair of nectaries at the leaf axil. Legumes such as pea and Medicago have a pair of stipules, rather than nectaries, at the base of their leaves. In Ljnbcl1, nectary development is abolished, demonstrating that nectaries and stipules share a common evolutionary origin. In addition, ectopic roots arising from nodule vascular meristems and reorganization of the nodule vascular bundle vessels were observed on Ljnbcl1 nodules. This demonstrates that NBCL functions are conserved in both indeterminate and determinate nodules through the maintenance of nodule vascular bundle identity. In contrast to its role in floral patterning described in other plants, LjNBCL1 appears essential for the development of both secondary inflorescence meristem and floral meristem.

Lack of galactose or galacturonic acid in Bradyrhizobium japonicum USDA 110 exopolysaccharide leads to different symbiotic responses in soybean

Exopolysaccharide (EPS) and lipopolysaccharide (LPS) from Bradyrhizobium japonicum are important for infection and nodulation of soybean (Glycine max), although their roles are not completely understood. To better understand this, we constructed mutants in B. japonicum USDA 110 impaired in galactose or galacturonic acid incorporation into the EPS without affecting the LPS. The derivative LP 3010 had a deletion of lspL-ugdH and produced EPS without galacturonic acid whereas LP 3013, with an insertion in exoB, produced EPS without galactose. In addition, the strain LP 3017, with both mutations, had EPS devoid of both galactosides. The missing galactosides were not replaced by other sugars. The defects in EPS had different consequences. LP 3010 formed biofilms and nodulated but was defective in competitiveness for nodulation; and, inside nodules, the peribacteroid membranes tended to fuse, leading to the merging of symbiosomes. Meanwhile, LP 3013 and LP 3017 were unable to form biofilms and produced empty pseudonodules but exoB suppressor mutants were obtained when LP 3013 plant inoculation was supplemented with wild-type EPS. Similar phenotypes were observed with all these mutants in G. soja. Therefore, the lack of each galactoside in the EPS has a different functional effect on the B. japonicum-soybean symbiosis.

Rhizobium meliloti genes required for nodule development are related to chromosomal virulence genes in Agrobacterium tumefaciens

Symbiotically essential genes have been identified in Rhizobium meliloti that are structurally and functionally related to chromosomal virulence (chv) genes of Agrobacterium tumefaciens. Homologous sequences also exist in the genomes of other fast-growing rhizobia including Rhizobium trifolii, Rhizobium leguminosarum, and Rhizobium phaseoli. In Agrobacterium, the chvA and chvB loci are known to be essential for oncogenic transformation of dicotyledonous plants and for attachment to plant cells [Douglas, C. J., Staneloni, R. J., Rubin, R. A. & Nester, E. W. (1985) J. Bacteriol. 64, 102-106], and the chvB locus has been implicated in the production of (1-->2)-beta-glucan, a unique exopolysaccharide component [Puvanesarajah, V., Schell, F. M., Stacey, G., Douglas, C. J. & Nester, E. W. (1985) J. Bacteriol. 164, 102-106]. Site-directed transposon insertion mutants in the chvA and chvB-equivalent regions of R. meliloti are symbiotically defective. Mutants in the chvB-equivalent region have been examined in detail and have been found to induce the formation of nodule-like structures on alfalfa that are devoid of bacteroids, lack infection threads, and cannot fix nitrogen. Such mutants fluoresce normally in the presence of Calcofluor, a histochemical stain for beta-linked polysaccharides, and produce normal amounts of total exopolysaccharide. The Rhizobium loci have been designated ndv because of their requirement for nodule development.

K regulates bacteroid-associated functions of Bradyrhizobium

Cowpea Bradyrhizobium 32H1 cells, when grown under 0.2% O(2), synthesize nitrogenase, as well as a methylammonium (ammonium) transport system and an electrogenic K(+)/H(+) antiporter. This effect was seen in growth medium containing 8-12 mM K(+) but not with 50 muM K(+). Addition of K(+) to cells growing under low O(2) tensions in low-K(+) medium led to various phenotypic properties associated with bacteroids, including the ability to reduce acetylene, induction of an ammonium transport carrier and the K(+)/H(+) antiporter, and increased synthesis of two heme-biosynthetic enzymes, delta-aminolevulinate synthase and delta-aminolevulinate dehydratase. K(+) addition caused the repression of glutamine synthetase and of capsular polysaccharide synthesis, functions related to the free-living state. A similar pattern of regulation was observed in Bradyrhizobium japonicum. In addition, K(+)-mediated depression in Bradyrhizobium 32H1 was inhibited by exudate of Vigna unguiculata, its host plant. We conclude that K(+) ions, in addition to low O(2) tension, are needed for the expression of several bacteroid-related functions in bradyrhizobia and thus are a major controlling influence in bacteroid development.

Lipopolysaccharides in Rhizobium-legume symbioses

The establishment of nitrogen-fixing symbiosis between a legume plant and its rhizobial symbiont requires that the bacterium adapt to changing conditions that occur with the host plant that both promotes and allows infection of the host root nodule cell, regulates and resists the host defense response, permits the exchange of metabolites, and contributes to the overall health of the host. This adaptive process involves changes to the bacterial cell surface and, therefore, structural modifications to the lipopolysaccharide (LPS). In this chapter, we describe the structures of the LPSs from symbiont members of the Rhizobiales, the genetics and mechanism of their biosynthesis, the modifications that occur during symbiosis, and their possible functions.

Cochleata: getting to the root of legume nodules

The homeotic mutant of Pisum sativum, cochleata, has stipules replaced by alternative leaf components, abnormal flowers and reduced fertility. Although the root system dry weight, root lengths and nodule numbers of cochleata are similar to those of its wild type, the nodulation phenotype of the mutant is unique. The nodules typically dichotomously branch and multiple callus and root structures emerge from their meristems. These nodule-roots incorporate a peripheral vascular bundle of the nodule into their own central vascular cylinder. Both the nodules and roots of the hybrid structures appear functional. Roles for COCHLEATA in development are discussed.

Development of ectopic roots from abortive nodule primordia

The symbiotic phenotype of five Tn5-induced mutants of Rhizobium etli affected in different anabolic pathways (namely, gluconeogenesis and biosynthesis of lysine, purine, or pyrimidine) was analyzed. These mutants induced, on the root of Phaseolus vulgaris, a normal early sequence of morphogenetics events, including root hair deformation and development of nodule primordia. Later on, however, from the resulting root outgrowths, instead of nodules, one or more ectopic roots (spaced closely related and agravitropic) emerged. Therefore, this group of mutant was collectively called "root inducer" (RIND). It was observed that the RIND-induced infection threads aborted early inside the invaded root hair, and that the resulting abortive nodules lack induction of late nodulin genes. Moreover, experiments performed using a conditional mutant (a methionine-requiring invader) revealed that bacterial invasion plays a key role in the maintenance of the program of nodule development and, in particular, in the differentiation of the most specific symbiotic tissue of globose nodules, the central tissue. These data indicate that, in P. vulgaris, the nodule primordium is a root-specified pro-meristematic tissue.

2-O-methylation of fucosyl residues of a rhizobial lipopolysaccharide is increased in response to host exudate and is eliminated in a symbiotically defective mutant

When Rhizobium etli CE3 was grown in the presence of Phaseolus vulgaris seed extracts containing anthocyanins, its lipopolysaccharide (LPS) sugar composition was changed in two ways: greatly decreased content of what is normally the terminal residue of the LPS, di-O-methylfucose, and a doubling of the 2-O-methylation of other fucose residues in the LPS O antigen. R. etli strain CE395 was isolated after Tn5 mutagenesis of strain CE3 by screening for mutant colonies that did not change antigenically in the presence of seed extract. The LPS of this strain completely lacked 2-O-methylfucose, regardless of whether anthocyanins were present during growth. The mutant gave only pseudonodules in association with P. vulgaris. Interpretation of this phenotype was complicated by a second LPS defect exhibited by the mutant: its LPS population had only about 50% of the normal amount of O-antigen-containing LPS (LPS I). The latter defect could be suppressed genetically such that the resulting strain (CE395 alpha 395) synthesized the normal amount of an LPS I that still lacked 2-O-methylfucose residues. Strain CE395 alpha 395 did not elicit pseudonodules but resulted in significantly slower nodule development, fewer nodules, and less nitrogenase activity than lps(+) strains. The relative symbiotic deficiency was more severe when seeds were planted and inoculated with bacteria before they germinated. These results support previous conclusions that the relative amount of LPS I on the bacterial surface is crucial in symbiosis, but LPS structural features, such as 2-O-methylation of fucose, also may facilitate symbiotic interactions.

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.

Cell biological changes of outer cortical root cells in early determinate nodulation

In the symbiosis of leguminous plants and Rhizobium bacteria, nodule primordia develop in the root cortex. This can be either in the inner cortex (indeterminate-type of nodulation) or outer cortex (determinate-type of nodulation), depending upon the host plant. We studied and compared early nodulation stages in common bean (Phaseolus vulgaris) and Lotus japonicus, both known as determinate-type nodulation plants. Special attention was paid to the occurrence of cytoplasmic bridges, the influence of rhizobial Nod factors (lipochitin oligosaccharides [LCOs]) on this phenomenon, and sensitivity of the nodulation process to ethylene. Our results show that i) both plant species form initially broad, matrix-rich infection threads; ii) cytoplasmic bridges occur in L. japonicus but not in bean; iii) formation of these bridges is induced by rhizobial LCOs; iv) formation of primordia starts in L. japonicus in the middle root cortex and in bean in the outer root cortex; and v) in the presence of the ethylene-biosynthesis inhibitor aminoethoxyvinylglycine (AVG), nodulation of L. japonicus is stimulated when the roots are grown in the light, which is consistent with the role of cytoplasmic bridges during nodulation of L. japonicus.

A guaB mutant strain of Rhizobium tropici CIAT899 pleiotropically defective in thermal tolerance and symbiosis

Rhizobium tropici strain CIAT899 displays a high intrinsic thermal tolerance, and had been used in this work to study the molecular basis of bacterial responses to high temperature. We generated a collection of R. tropici CIAT899 mutants affected in thermal tolerance using TnS-luxAB mutagenesis and described the characterization of a mutant strain, CIAT899-10T, that fails to grow under conditions of high temperature. Strain CIAT899-10T carries a single transposon insertion in a gene showing a high degree of similarity with the guaB gene of Escherichia coli and other organisms, encoding the enzyme inosine monophosphate dehydrogenase. The guaB strain CIAT899-10T does not require guanine for growth due to an alternative pathway via xanthine dehydrogenase and, phenotypically, in addition to the thermal sensitivity, the mutant is also defective in symbiosis with beans, forming nodules that lack rhizobial content. Guanine and its precursors restore wild-type tolerance to grow at high temperature. Our data show that, in R. tropici, the production of guanine via inosine monophosphate dehydrogenase is essential for growth at extreme temperatures and for effective nodulation.

Varying the abundance of O antigen in Rhizobium etli and its effect on symbiosis with Phaseolus vulgaris

Judged by migration of its lipopolysaccharide (LPS) in gel electrophoresis, the O antigen of Rhizobium etli mutant strain CE166 was apparently of normal size. However, its LPS sugar composition and staining of the LPS bands after electrophoresis indicated that the proportion of its LPS molecules that possessed O antigen was only 40% of the wild-type value. Its LPS also differed from the wild type by lacking quinovosamine (2-amino-2,6-dideoxyglucose). Both of these defects were due to a single genetic locus carrying a Tn5 insertion. The deficiency in O-antigen amount, but not the absence of quinovosamine, was suppressed by transferring into this strain recombinant plasmids that shared a 7.8-kb stretch of the R. etli CE3 lps genetic region alpha, even though this suppressing DNA did not carry the genetic region mutated in strain CE166. Strain CE166 gave rise to pseudonodules on legume host Phaseolus vulgaris, whereas the mutant suppressed by DNA from lps region alpha elicited nitrogen-fixing nodules. However, the nodules in the latter case developed slowly and were widely dispersed. Two other R. etli mutants that had one-half or less of the normal amount of O antigen also gave rise to pseudonodules on P. vulgaris. The latter strains were mutated in lps region alpha and could be restored to normal LPS content and normal symbiosis by complementation with wild-type DNA from this region. Hence, the symbiotic role of LPS requires near-normal abundance of O antigen and may require a structural feature conferred by quinovosamine.

The Rhizobium etli metZ gene is essential for methionine biosynthesis and nodulation of Phaseolus vulgaris

A mutant strain (CTNUX23) of Rhizobium etli carrying Tn5 unable to grow with sulfate as the sole sulfur source was isolated and characterized. Sequence analysis showed that Tn5 is inserted into a metZ (O-succinylhomoserine sulfhydrylase)-homologous gene. The CTNUX23 mutant strain had a growth dependency for methionine, although cystathionine or homocysteine, but not homoserine or O-succinylhomoserine, allowed growth of the mutant. RNase protection assays showed that the metZ-like gene had a basal level of expression in methionine- or cysteine-grown cells, which was induced when sulfate or thiosulfate was used. The metZ gene was cloned from the parent wild-type strain, CE3, and the resulting plasmid pAR204 relieved, after transformation, the methionine auxotrophy of both strains CTNUX23 of R. etli and PAO503(metZ) of Pseudomonas aeruginosa. Unlike strain CE3 or CTNUX23 (pAR204), strain CTNUX23 showed undetectable levels of O-succinylhomoserine sulfhydrylase activity. Strain CTNUX23 was unable to produce flavonoid-inducible lipo-chitin oligosaccharides (Nod factors) or to induce nodules or nodulelike structures on the roots of Phaseolus vulgaris, unless methionine was added to the growth medium. These data and our previous results support the notion that cysteine or glutathione, but not methionine, is supplied by the root cells to bacteria growing inside the plant.

Genes essential for nod factor production and nodulation are located on a symbiotic amplicon (AMPRtrCFN299pc60) in Rhizobium tropici

Amplifiable DNA regions (amplicons) have been identified in the genome of Rhizobium etli. Here we report the isolation and molecular characterization of a symbiotic amplicon of Rhizobium tropici. To search for symbiotic amplicons, a cartridge containing a kanamycin resistance marker that responds to gene dosage and conditional origins of replication and transfer was inserted in the nodulation region of the symbiotic plasmid (pSym) of R. tropici CFN299. Derivatives harboring amplifications were selected by increasing the concentration of kanamycin in the cell culture. The amplified DNA region was mobilized into Escherichia coli and then into Agrobacterium tumefaciens. The 60-kb symbiotic amplicon, which we termed AMPRtrCFN299pc60, contains several nodulation and nitrogen fixation genes and is flanked by a novel insertion sequence ISRtr1. Amplification of AMPRtrCFN299pc60 through homologous recombination between ISRtr1 repeats increased the amount of Nod factors. Strikingly, the conjugal transfer of the amplicon into a plasmidless A. tumefaciens strain confers on the transconjugant the ability to produce R. tropici Nod factors and to nodulate Phaseolus vulgaris, indicating that R. tropici genes essential for the nodulation process are confined to an ampliable DNA region of the pSym.

Inhibition of Rhizobium etli Polysaccharide Mutants by Phaseolus vulgaris Root Compounds

Crude bean root extracts of Phaseolus vulgaris were tested for inhibition of the growth of several polysaccharide mutants of Rhizobium etli biovar phaseoli CE3. Mutants deficient only in exopolysaccharide and some mutants deficient only in the O-antigen of the lipopolysaccharide were no more sensitive than the wild-type strain to the extracts, whereas mutants defective in both lipopolysaccharide and exopolysaccharide were much more sensitive. The inhibitory activity was found at much higher levels in roots and nodules than in stems or leaves. Inoculation with either wild-type or polysaccharide-deficient R. etli did not appear to affect the level of activity. Sequential extractions of the crude root material with petroleum ether, ethyl acetate, methanol, and water partitioned inhibitory activity into each solvent except methanol. The major inhibitors in the petroleum ether and ethyl acetate extracts were purified by C 18 high-performance liquid chromatography. These compounds all migrated very similarly in both liquid and thin-layer chromatography but were distinguished by their mass spectra. Absorbance spectra and fluorescence properties suggested that they were coumestans, one of which had the mass spectrum and nuclear magnetic resonances of coumestrol. These results are discussed with regard to the hypothesis that one role of rhizobial polysaccharides is to protect against plant toxins encountered during nodule development.

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.

Molecular and symbiotic characterization of exopolysaccharide-deficient mutants of Rhizobium tropici strain CIAT899

We studied the symbiotic behaviour of 20 independent Tn5 mutants of Rhizobium tropici strain CIAT899 that were deficient in exopolysaccharide (EPS) production. The mutants produced non-mucoid colonies, were motile, grew in broth cultures at rates similar to those of the parent, and produced significantly less EPS than did CIAT899 in broth culture. A genomic library of strain CIAT899, constructed in pLA2917, was mobilized into all of the mutants, and cosmids that restored EPS production were identified. EcoRI restriction digests of the cosmids revealed nine unique inserts. Mutant complementation and hybridization analysis showed that the mutations affecting EPS production fell into six functional and physical linkage groups. On bean, the mutants were as efficient in nodulation and as effective in acetylene reduction as strain CIAT899, induced a severe interveinal chlorosis, and all but one were less competitive than CIAT899. On siratro, CIAT899 induced nodules that were ineffective in acetylene reduction, whereas the EPS-deficient mutants induced effective nodules. Microscopic examination of thin sections showed that nodules from both siratro and bean plants inoculated with either CIAT899 or an EPS-deficient mutant contained infected cells. These data indicate that EPS is not required for normal nodulation of bean by R. tropici, that it may contribute to competitiveness of R. tropici on bean, and that the loss of EPS production is accompanied by acquisition of the ability to reduce acetylene on siratro.

Dissection of Nodule Development by Supplementation of Rhizobium leguminosarum biovar phaseoli Purine Auxotrophs with 4-Aminoimidazole-5-Carboxamide Riboside

Purine auxotrophs of Rhizobium leguminosarum biovar phaseoli CFN42 elicit uninfected pseudonodules on bean (Phaseolus vulgaris L.). Addition of 4-aminoimidazole-5-carboxamide (AICA) riboside to the root medium during incubation of the plant with these mutants leads to enhanced nodule development, although nitrogenase activity is not detected. Nodules elicited in this manner had infection threads and anatomical features characteristic of normal nodules, such as peripheral vasculature rather than the central vasculature of the pseudonodules that were elicited without AICA riboside supplementation. Although 10(5) to 10(6) bacteria could be recovered from these nodules after full development, bacteria were not observed in the interior nodule cells. Instead, large cells with extensive internal membranes were present. Approximately 5% of the normal amount of leghemoglobin and 10% of the normal amount of uricase were detected in these nodules. To promote the development of true nodules rather than pseudonodules, AICA riboside was required no later than the second day through no more than the sixth day following inoculation. After this period, removal of AICA riboside from the root medium did not prevent the formation of true nodules. This observation suggests that there is a critical stage of infection, reached before nodule emergence, at which development becomes committed to forming a true nodule rather than a pseudonodule.

Rhizobium leguminosarum CFN42 lipopolysaccharide antigenic changes induced by environmental conditions

Four monoclonal antibodies were raised against the lipopolysaccharide of Rhizobium leguminosarum bv. phaseoli CFN42 grown in tryptone and yeast extract. Two of these antibodies reacted relatively weakly with the lipopolysaccharide of bacteroids of this strain isolated from bean nodules. Growth ex planta of strain CFN42 at low pH, high temperature, low phosphate, or low oxygen concentration also eliminated binding of one or both of these antibodies. Lipopolysaccharide mobility on gel electrophoresis and reaction with other monoclonal antibodies and polyclonal antiserum indicated that the antigenic changes detected by these two antibodies did not represent major changes in lipopolysaccharide structure. The antigenic changes at low pH were dependent on growth of the bacteria but were independent of nitrogen and carbon sources and the rich or minimal quality of the medium. The Sym plasmid of this strain was not required for the changes induced ex planta. Analysis of bacterial mutants inferred to have truncated O-polysaccharides indicated that part, but not all, of the lipopolysaccharide O-polysaccharide portion was required for binding of these two antibodies. In addition, this analysis suggested that O-polysaccharide structures more distal to lipid A than the epitopes themselves were required for the modifications at low pH that prevented antibody binding. Two mutants were antigenically abnormal, even though they had abundant lipopolysaccharides of apparently normal size. One of these two mutants was constitutively unreactive toward three of the antibodies but indistinguishable from the wild type in symbiotic behavior. The other, whose bacteroids retained an epitope normally greatly diminished in bacteroids, was somewhat impaired in nodulation frequency and nodule development.

The exoD gene of Rhizobium meliloti encodes a novel function needed for alfalfa nodule invasion

During the symbiotic interaction between alfalfa and the nitrogen-fixing bacterium Rhizobium meliloti, the bacterium induces the formation of nodules on the plant roots and then invades these nodules. Among the bacterial genes required for nodule invasion are the exo genes, involved in production of an extracellular polysaccharide, and the ndv genes, needed for production of a periplasmic cyclic glucan. Mutations in the exoD gene result in altered exopolysaccharide production and in a nodule invasion defect. In this work we show that the stage of symbiotic arrest of exoD mutants is similar to that of other exo and ndv mutants. However, the effects of exoD mutations on exopolysaccharide production and growth on various media are different from the effects of other exo and ndv mutations. Finally, exoD mutations behave differently from other exo mutations in their ability to be suppressed or complemented extracellularly. The results suggest that exoD represents a new class of Rhizobium genes required for nodule invasion, distinct from the other exo genes and the ndv genes. We discuss models for the function of exoD.

Expression of Rhizobium leguminosarum CFN42 genes for lipopolysaccharide in strains derived from different R. leguminosarum soil isolates

Two mutant derivatives of Rhizobium leguminosarum ANU843 defective in lipopolysaccharide (LPS) were isolated. The LPS of both mutants lacked O antigen and some sugar residues of the LPS core oligosaccharides. Genetic regions previously cloned from another Rhizobium leguminosarum wild-type isolate, strain CFN42, were used to complement these mutants. One mutant was complemented to give LPS that was apparently identical to the LPS of strain ANU843 in antigenicity, electrophoretic mobility, and sugar composition. The other mutant was complemented by a second CFN42 lps genetic region. In this case the resulting LPS contained O-antigen sugars characteristic of donor strain CFN42 and reacted weakly with antiserum against CFN42 cells, but did not react detectably with antiserum against ANU843 cells. Therefore, one of the CFN42 lps genetic regions specifies a function that is conserved between the two R. leguminosarum wild-type isolates, whereas the other region, at least in part, specifies a strain-specific LPS structure. Transfer of these two genetic regions into wild-type strains derived from R. leguminosarum ANU843 and 128C53 gave results consistent with this conclusion. The mutants derived from strain ANU843 elicited incompletely developed clover nodules that exhibited low bacterial populations and very low nitrogenase activity. Both mutants elicited normally developed, nitrogen-fixing clover nodules when they carried CFN42 lps DNA that permitted synthesis of O-antigen-containing LPS, regardless of whether the O antigen was the one originally made by strain ANU843.

The occurrence of unusual laminated structures rich in β-1,4-glucans in plastids of Phaseolus vulgaris root-nodule cells infected by an ineffective C4-dicarboxylic-acid mutant of Rhizobium leguminosarum bv. phaseoli

Root nodules induced in Phaseolus vulgaris L. by the wild-type (WT) and a C4-dicarboxylic-acid mutant strain of Rhizobium leguminosarum biovar. phaseoli were compared on the basis of ultrastructure and cytochemistry of cellulose subunits. The mutant bacteroids failed to colonize infected host cells in a normal manner, and presented a premature degenerative appearance. Starch granules, rough endoplasmic reticulum and mitochondria were found to accumulate in the ineffective nodules. The most striking difference between effective and ineffective nodules was the presence of unusual spherical, laminated structures in plastids of mutant-infected host cells only. Cytochemical observations showed that these structures contain β-1,4-glucans. The presence of β-1,4-glucans within such structures may be caused by the activity of a cellulase which is produced by either the bacteroids or the host cell and is locally hydrolyzing the host cell-wall, thus releasing cellulose subunits into the cytoplasm. Another possibility is denovo synthesis of β-1,4-glucans in the host cell.

Rhizobium leguminosarum exopolysaccharide mutants: biochemical and genetic analyses and symbiotic behavior on three hosts

Ten independently generated mutants of Rhizobium leguminosarum biovar phaseoli CFN42 isolated after Tn5 mutagenesis formed nonmucoid colonies on all agar media tested and lacked detectable production of the normal acidic exopolysaccharide in liquid culture. The mutants were classified into three groups. Three mutants harbored Tn5 insertions on a 3.6-kilobase-pair EcoRI fragment and were complemented to have normal exopolysaccharide production by cosmids that shared an EcoRI fragment of this size from the CFN42 genome. The Tn5 inserts of five other mutants appeared to be located on a second, slightly smaller EcoRI fragment. Attempts to complement mutants of this second group with cloned DNA were unsuccessful. The mutations of the other two mutants were located in apparently adjacent EcoRI fragments carried on two cosmids that complemented those two mutants. The latter two mutants also lacked O-antigen-containing lipopolysaccharides and induced underdeveloped nodules that lacked nitrogenase activity on bean plants. The other eight mutants had normal lipopolysaccharides and wild-type symbiotic proficiencies on bean plants. Mutants in each of these groups were mated with R. leguminosarum strains that nodulated peas (R. leguminosarum biovar viciae) or clovers (R. leguminosarum biovar trifolii). Transfer of the Tn5 mutations resulted in exopolysaccharide-deficient R. leguminosarum biovar viciae or R. leguminosarum biovar trifolii transconjugants that were symbiotically deficient in all cases. These results support earlier suggestions that successful symbiosis with peas or clovers requires that rhizobia be capable of acidic exopolysaccharide production, whereas symbiosis with beans does not have this requirement.

Exopolysaccharide-Deficient Mutants of Rhizobium fredii HH303 Which Are Symbiotically Effective

Nineteen Tn 5 -induced mutants of Rhizobium fredii HH303 defective in acidic exopolysaccharide synthesis were isolated by screening for lack of Calcofluor fluorescence. They were grouped by complementation analysis by using Rhizobium meliloti cosmids carrying exo genes. All of the 19 mutants were symbiotically effective or partially effective, indicating that the major bacterial acidic exopolysaccharide of this strain of R. fredii may not be required for symbiotic development in the soybean.

Characterization of Rhizobium phaseoli Sym plasmid regions involved in nodule morphogenesis and host-range specificity

Two nodulation regions from the symbiotic plasmid (pSym) of Rhizobium phaseoli CE-3 were identified. The two regions were contained in overlapping cosmids pSM927 and pSM991. These cosmids, in a R. phaseoli pSym-cured strain background, induced ineffective nodules on Phaseolus vulgaris roots. Transconjugants of Rhizobium meliloti harbouring pSM991 induced nodule-like structures on bean roots, suggesting that this cosmid contains host-range determinants. Analysis of deletions and insertional mutations in the sequences of pSM991 indicated that the genes responsible for the induction and development of nodules in P. vulgaris are organized in two regions 20 kb apart. One region, located in a 6.8 kb EcoRI fragment, includes the common nodABC genes. The other region, located in a 3.5 kb EcoRI fragment, contains information required for host-range determination.

Which steps are essential for the formation of functional legume nodules?

Nodulation is reviewed in terms of the phenotypes proposed by Vincent (1980). Individual legumes may be infectible by one or more of the three bacterial genera (collectively known as rhizobia) Rhizobium, Bradyrhizobium, or Azorhizobium. The type of infection process by which rhizobia gain entry is largely governed by the host genotype. In addition to the widely studied root-hair pathway, infections may be associated with lateral root emergence or occur between root epidermal cells. The exact chemical and physical nature of the root hair/epidermal cell wall is likely to be a critical factor in determining whether infections can proceed. In addition to differing with species, wall composition may be influenced by soil chemical (e.g. Ca²⁺ ) and biotic factors (e.g. bacteria). Rhizobial features essential for infection include particular surface polysaccharides and the induction of nodulation genes by plant root exudates. Neither of these is likely to be a major barrier to the extension of nodulation to new hosts. Dissemination of rhizobia within developing nodules may be intercellular, via infection threads or by division of a small number of infected cells. All functional symbioses eventually have 'intracellular' bacteria, in the sense that rhizobia are geographically located within the boundary of the host cell walls. However, they remain extracellular in the sense that they are always confined by a membrane which is largely of host cell origin. In some genera they are also surrounded by infection thread walls, probably modified forms of 'invasive' infection thread walls, which allow differentiation of rhizobia into the nitrogen-fixing form. Thus, natural, functional, symbioses may (a) never involve a stage in which bacteria are confined within tubular infection threads or (b) never release bacteria from infection threads. These features are determined by host genotype. The one feature of legume nodules so far found never to vary is the stem-like character of a peripheral vascular system. This contrasts with the central vascular system of actinorhizas and the rhizobial-induced nodules on the Ulmaceous genus Parasponia. Although of great intrinsic interest, this character is unlikely to present an insurmountable barrier to the extension of nodulation to new species. Other features, such as the ability to produce haemoglobin are now known to the in the genetic makeup of many higher plants. The discovery of the wide range of nodule structures occurring in nature, together with work on mutant rhizobia which may bypass critical stages in the nodulation process, suggest various ways in which the extension of nodulation to non-nodulated legumes and to other (initially at least, dicotyledonous) plants may be engineered. CONTENTS Summary 129 I. Introduction 130 II. The symbionts 130 III. Stages in nodulation 132 IV. Stems and nodules 143 V. Prospects for finding/making new symbioses 144 VI. Conclusions 145 Acknowledgements 147 References 147.

Rhizobium leguminosarum CFN42 genetic regions encoding lipopolysaccharide structures essential for complete nodule development on bean plants

Eight symbiotic mutants defective in lipopolysaccharide (LPS) synthesis were isolated from Rhizobium leguminosarum biovar phaseoli CFN42. These eight strains elicited small white nodules lacking infected cells when inoculated onto bean plants. The mutants had undetectable or greatly diminished amounts of the complete LPS (LPS I), whereas amounts of an LPS lacking the O antigen (LPS II) greatly increased. Apparent LPS bands that migrated between LPS I and LPS II on sodium dodecyl sulfate-polyacrylamide gels were detected in extracts of some of the mutants. The mutant strains were complemented to wild-type LPS I content and antigenicity by DNA from a cosmid library of the wild-type genome. Most of the mutations were clustered in two genetic regions; one mutation was located in a third region. Strains complemented by DNA from two of these regions produced healthy nitrogen-fixing nodules. Strains complemented to wild-type LPS content by the other genetic region induced nodules that exhibited little or no nitrogenase activity, although nodule development was obviously enhanced by the presence of this DNA. The results support the idea that complete LPS structures, in normal amounts, are necessary for infection thread development in bean plants.

Induction of pathogenic-like responses in the legume Macroptilium atropurpureum by a transposon-induced mutant of the fast-growing, broad-host-range Rhizobium strain NGR234

Mutant strain ANU2861, a transposon Tn5 mutant of the fast-growing, broad-host-range Rhizobium strain ANU280 (NGR234 Smr Rfr) overproduces polysaccharide, is an ade auxotroph, and induces poorly developed nodules on Leucaena leucocephala and Lablab purpureus (H.C. Chen, M. Batley, J.W. Redmond, and B.G. Rolfe, J. Plant Physiol. 120:331-349, 1985). Strain ANU2861 cannot form nodules on Macroptilium atropurpureum Urb. (siratro) or on Desmodium intortum and D. uncinatum and the nonlegume Parasponia. The parent strain, ANU280, effectively nodulates all these legume species except Parasponia, on which it forms ineffective nodules. Ultrastructural examination of infection sites on the legume siratro showed that mutant strain ANU2861 caused root hair curling (Hac+ phenotype), some cortical cell division (Noi+), but no infection threads (Inf-). Localized cellular responses, known to occur in phytopathological interactions, were observed in electron micrographs of the epidermal tissue at or near the infection zone after inoculation with strain ANU2861 but not the wild-type parental strain. These include (i) the rapid (within 20 h) accumulation of osmiophilic droplets attached to membranes at potential sites of strain ANU2861 penetration and (after 48 h) in the epidermal cells in the immediate region of the curled root hairs, and (ii) localized cell death of the epidermal cells. In addition, strain ANU2861 can initiate a systemic response in split-root siratro plants which prevents the successful nodulation of strain ANU280. A 6.3-kilobase fragment of wild-type genomic DNA, which includes the site of Tn5 insertion in strain ANU2861, was cloned and introduced to strain ANU2861. All the phenotypic defects of the mutant strain were corrected by the introduction of this DNA fragment. This indicates that the original Tn5 insertion is responsible for the phenotype.

Rhizobium japonicum USDA 191 has two nodD genes that differ in primary structure and function

Several Rhizobium genes (designated nod genes) are involved in early steps in nodule formation. Here we present the results of DNA sequence and functional analysis of two nodD genes from the symbiotic plasmid of USDA 191, a fast-growing strain that forms nitrogen-fixing nodules on soybeans. Both genes encoded full-length nodD-related polypeptides, which were 69% homologous to each other. One of these genes, nodD1, complemented a Rhizobium trifolii nodD::Tn5 mutant for clover nodulation; the other gene, nodD2, did not. The nodD1 coding region was preceded by a conserved DNA sequence previously noted in other rhizobia, but no such sequence was found in front of nodD2. Plants inoculated with a nodD1 insertion mutant appeared to be nitrogen starved and had a greatly reduced nodule number. Plants inoculated with a nodD2 mutant had a partially nitrogen-starved appearance and normal nodule number, were slightly delayed in nodule formation, and formed nodules that contained reduced levels of nodulin-35 and had fewer bacteroids per infected plant cell. Thus, both of these genes are involved in symbiosis. USDA 191 carrying extra copies of nodD2 on a plasmid vector had an altered colony morphology that suggested inhibition of exopolysaccharide synthesis. The predicted gene products of nodD1 and nodD2 both showed homology to LysR, an E. coli regulatory protein. We conclude that nodD1 probably has the same function as nodD in temperate rhizobia, namely, activation of nodABC transcription in the presence of plant signals. nodD2 may be involved in regulation of exopolysaccharide synthetic genes.

Cellular compartmentation of ureide biogenesis in root nodules of cowpea (Vigna unguiculata (L.) Walp.)

Cowpea (Vigna unguiculata (L.) Walp.) nodules have been investigated by means of cytochemical and immunocytochemical procedures at the ultrastructural level in order to assess the role of the uninfected cells in ureide biogenesis. Uricase activity in the nodules was shown by cytochemical methods to be localized exclusively in the numberous large peroxisomes confined to the uninfected cells; the small peroxisomes in the infected cells did not stain for uricase. Uricase was also localized in the peroxisomes of uninfected cells by immunogold techniques employing polyclonal antibodies against nodule-specific uricase of soybean. There was no labeling above background of any structures in the infected cells. The results indicate that the uninfected cells are essential for ureide biogenesis in cowpea. Although tubular endoplasmic reticulum, the presumptive site of allantoinase, increases greatly in the uninfected cells during nodule development, it virtually disappears as the nodules mature. The inconsistency between the disappearance of the tubular endoplasmic reticulum from older nodules and the high allantoinase activity reported for older plants remains to be explained.

Isolation and characterization of symbiotic mutants of bradyrhizobium sp. (Arachis) strain NC92: mutants with host-specific defects in nodulation and nitrogen fixation

Random transposon Tn5 mutagenesis of Bradyrhizobium sp. (Arachis) strain NC92, a member of the cowpea cross-inoculation group, was carried out, and kanamycin-resistant transconjugants were tested for their symbiotic phenotype on three host plants: groundnut, siratro, and pigeonpea. Two nodulation (Nod- phenotype) mutants were isolated. One is unable to nodulate all three hosts and appears to contain an insertion in one of the common nodulation genes (nodABCD); the other is a host-specific nodulation mutant that fails to nodulate pigeonpea, elicits uninvaded nodules on siratro, and elicits normal, nitrogen-fixing nodules on groundnut. In addition, nine mutants defective in nitrogen fixation (Fix- phenotype) were isolated. Three fail to supply symbiotically fixed nitrogen to all three host plants. Surprisingly, nodules elicited by one of these mutants exhibit high levels of acetylene reduction activity, demonstrating the presence of the enzyme nitrogenase. Three more mutants have partially effective phenotypes (Fix +/-) in symbiosis with all three host plants. The remaining three mutants fail to supply fixed nitrogen to one of the host plants tested while remaining partially or fully effective on the other two hosts; two of these mutants are Fix- in pigeonpea and Fix +/- on groundnut and on siratro, whereas the other one is Fix- on groundnut but Fix+ on siratro and on pigeonpea. These latter mutants also retain significant nodule acetylene reduction activity, even in the ineffective symbioses. Such bacterial host-specific fixation (Hsf) mutants have not previously been reported.

Effects of Rhizobium meliloti nif and fix mutants on alfalfa root nodule development

Ineffective alfalfa nodules were examined at the light and electron microscope level after inoculation with Rhizobium meliloti strains with mutations in nif and fix genes. All the mutant strains induced nodules that contained elongated bacteroids within the host cells, but the bacteroids quickly senesced. The nodules were small and numerous, and the host cells also exhibited symptoms of an ineffective symbiosis. nifB, fixA, and fixB bacteroids appeared to be completely differentiated (by ultrastructural criteria), i.e., as bacteroids developed, they increased in diameter and length and their cytoplasm underwent a change from homogeneous and electron dense to heterogeneous and electron transparent after enlargement. In contrast, nifA bacteroids rarely matured to this state. The bacteroids degenerated at an earlier stage of development and did not become electron transparent.

Nodule-specific glutamine synthetase is expressed before the onset of nitrogen fixation in Phaseolus vulgaris L

Glutamine synthetase expression was studied in developing root-nodules of common bean with regard to the time-course of specific activity, antigen accumulation, polypeptide composition and in vitro translation products. This analysis shows that the nodule-specific GS polypeptide (GS-gamma) is detected prior to the nitrogenase acetylene-reducing activity, and that its accumulation together with that of the GS-alpha and GS-beta polypeptides vary with nodule age. GS-gamma is present in ineffective nodules, although in a lower ratio to GS-beta than in wild-type nodules. Comparisons of in vitro translated and in vivo synthesized GS polypeptides suggest no post-translational modifications. The possible factors and mechanisms involved in the regulation of expression of GS polypeptides are discussed.

Mutations in Rhizobium phaseoli that lead to arrested development of infection threads

Two Rhizobium phaseoli mutants, isolated previously by Tn5 mutagenesis, elicited infection threads which ceased development prematurely, usually within root hairs. These infection threads were wide, globular, and otherwise altered in morphology, compared with normal infection threads. Anatomy and division of the root cortical cells during initial stages of nodule morphogenesis appeared normal. However, later nodule differentiation deviated considerably from normal development, and release of bacteria from infection threads was not observed. In tryptone-yeast extract medium the mutants sedimented during growth in shaken cultures and formed rough colonies on agar. Electrophoresis of washed cultures solubilized in dodecyl sulfate revealed that the major carbohydrate band was absent from the mutants. The behavior of this carbohydrate in phenol-water extraction and gel chromatography, its apparent ketodeoxyoctonate content, and its susceptibility to mild acid hydrolysis suggested that it was a lipopolysaccharide. From the results of genetic crosses or reversion analysis, the defect in synthesizing this carbohydrate material and the defect in infection could be attributed to a single mutation in each mutant.

Second symbiotic megaplasmid in Rhizobium meliloti carrying exopolysaccharide and thiamine synthesis genes

Using physical and genetic data, we have demonstrated that Rhizobium meliloti SU47 has a symbiotic megaplasmid, pRmeSU47b, in addition to the previously described nod-nif megaplasmid pRmeSU47a. This plasmid includes four loci involved in exopolysaccharide (exo) synthesis as well as two loci involved in thiamine biosynthesis. Mutations at the exo loci have previously been shown to result in the formation of nodules which lack infection threads (Inf-) and fail to fix nitrogen (Fix-). Thus, both megaplasmids contain genes involved in the formation of nitrogen-fixing root nodules. Mutations at two other exo loci were not located on either megaplasmid. To mobilize the megaplasmids, the oriT of plasmid RK2 was inserted into them. On alfalfa, Agrobacterium tumefaciens strains containing pRmeSU47a induced marked root hair curling with no infection threads and Fix- nodules, as reported by others. This plant phenotype was not observed to change with A. tumefaciens strains containing both pRmeSU47a and pRmeSU47b megaplasmids, and strains containing pRmeSU47b alone failed to curl root hairs or form nodules.

Genetic locus in Rhizobium japonicum (fredii) affecting soybean root nodule differentiation

A genetic locus in fast-growing Rhizobium japonicum (fredii) USDA 191 (Fix+ on several contemporary soybean cultivars) was identified by random Tn5 mutagenesis as affecting the development and differentiation of root nodules. This mutant (MU042) is prototrophic and shows no apparent alterations in its surface properties. It induces aberrant nodules, arrested at the same early level of differentiation, on all its host plants. An 8.1-kilobase EcoRI fragment containing Tn5 was cloned from MU042. In USDA 191 as well as another fast-growing strain, USDA 201, the affected locus was found to be unlinked to the large symbiotic plasmid and appears to be chromosomal. An analogous sequence has been shown to be present in Bradyrhizobium japonicum (J. Stanley, G.G. Brown, and D.P.S. Verma, J. Bacteriol. 163:148-154, 1985) as well as in R. trifolii and R. meliloti. MU042 was complemented for effective nodulation of soybean by a cosmid clone from USDA 201, and the complementing locus was delimited to a 6-kilobase EcoRI subfragment. An R. trifolii strain (MU225), whose indigenous symbiotic plasmid was replaced by that of strain USDA 191, induced more highly differentiated nodules on soybean than did MU042. This suggests that the mutation in MU042 can be functionally substituted by similar loci of other fast-growing rhizobia. Leghemoglobin and nodulin-35 (uricase II) were present in the differentiated Fix- nodules induced by MU225, whereas both were absent in MU042-induced pseudonodule structures.