Rhizobium meliloti Genes Encoding Catabolism of Trigonelline Are Induced under Symbiotic Conditions

Nitrogen fixation mutants of Medicago truncatula fail to support plant and bacterial symbiotic gene expression

The Rhizobium-legume symbiosis culminates in the exchange of nutrients in the root nodule. Bacteria within the nodule reduce molecular nitrogen for plant use and plants provide bacteria with carbon-containing compounds. Following the initial signaling events that lead to plant infection, little is known about the plant requirements for establishment and maintenance of the symbiosis. We screened 44,000 M2 plants from fast neutron-irradiated Medicago truncatula seeds and isolated eight independent mutant lines that are defective in nitrogen fixation. The eight mutants are monogenic and represent seven complementation groups. To monitor bacterial status in mutant nodules, we assayed Sinorhizobium meliloti symbiosis gene promoters (nodF, exoY, bacA, and nifH) in the defective in nitrogen fixation mutants. Additionally, we used an Affymetrix oligonucleotide microarray to monitor gene expression changes in wild-type and three mutant plants during the nodulation process. These analyses suggest the mutants can be separated into three classes: one class that supports little to no nitrogen fixation and minimal bacterial expression of nifH; another class that supports no nitrogen fixation and minimal bacterial expression of nodF, bacA, and nifH; and a final class that supports low levels of both nitrogen fixation and bacterial nifH expression.

Transcript analysis of early nodulation events in Medicago truncatula

Within the first 72 h of the interaction between rhizobia and their host plants, nodule primordium induction and infection occur. We predicted that transcription profiling of early stages of the symbiosis between Medicago truncatula roots and Sinorhizobium meliloti would identify regulated plant genes that likely condition key events in nodule initiation. Therefore, using a microarray with about 6,000 cDNAs, we compared transcripts from inoculated and uninoculated roots corresponding to defined stages between 1 and 72 h post inoculation (hpi). Hundreds of genes of both known and unknown function were significantly regulated at these time points. Four stages of the interaction were recognized based on gene expression profiles, and potential marker genes for these stages were identified. Some genes that were regulated differentially during stages I (1 hpi) and II (6-12 hpi) of the interaction belong to families encoding proteins involved in calcium transport and binding, reactive oxygen metabolism, and cytoskeleton and cell wall functions. Genes involved in cell proliferation were found to be up-regulated during stages III (24-48 hpi) and IV (72 hpi). Many genes that are homologs of defense response genes were up-regulated during stage I but down-regulated later, likely facilitating infection thread progression into the root cortex. Additionally, genes putatively involved in signal transduction and transcriptional regulation were found to be differentially regulated in the inoculated roots at each time point. The findings shed light on the complexity of coordinated gene regulation and will be useful for continued dissection of the early steps in symbiosis.

Grafting between model legumes demonstrates roles for roots and shoots in determining nodule type and host/rhizobia specificity

Previous grafting experiments have demonstrated that legume shoots play a critical role in symbiotic development of nitrogen-fixing root nodules by regulating nodule number. Here, reciprocal grafting experiments between the model legumes Lotus japonicus and Medicago truncatula were carried out to investigate the role of the shoot in the host-specificity of legume-rhizobia symbiosis and nodule type. Lotus japonicus is nodulated by Mesorhizobium loti and makes determinate nodules, whereas M. truncatula is nodulated by Sinorhizobium meliloti and makes indeterminate nodules. When inoculated with M. loti, L. japonicus roots grafted on M. truncatula shoots produced determinate nodules identical in appearance to those produced on L. japonicus self-grafted roots. Moreover, the hypernodulation phenotype of L. japonicus har1-1 roots grafted on wild-type M. truncatula shoots was restored to wild type when nodulated with M. loti. Thus, L. japonicus shoots appeared to be interchangeable with M. truncatula shoots in the L. japonicus root/M. loti symbiosis. However, M. truncatula roots grafted on L. japonicus shoots failed to induce nodules after inoculation with S. meliloti or a mixture of S. meliloti and M. loti. Instead, only early responses to S. meliloti such as root hair tip swelling and deformation, plus induction of the early nodulation reporter gene MtENOD11:GUS were observed. The results indicate that the L. japonicus shoot does not support normal symbiosis between the M. truncatula root and its microsymbiont S. meliloti, suggesting that an unidentified shoot-derived factor may be required for symbiotic progression in indeterminate nodules.

Detection of and response to signals involved in host-microbe interactions by plant-associated bacteria

Diverse interactions between hosts and microbes are initiated by the detection of host-released chemical signals. Detection of these signals leads to altered patterns of gene expression that culminate in specific and adaptive changes in bacterial physiology that are required for these associations. This concept was first demonstrated for the members of the family Rhizobiaceae and was later found to apply to many other plant-associated bacteria as well as to microbes that colonize human and animal hosts. The family Rhizobiaceae includes various genera of rhizobia as well as species of Agrobacterium. Rhizobia are symbionts of legumes, which fix nitrogen within root nodules, while Agrobacterium tumefaciens is a pathogen that causes crown gall tumors on a wide variety of plants. The plant-released signals that are recognized by these bacteria are low-molecular-weight, diffusible molecules and are detected by the bacteria through specific receptor proteins. Similar phenomena are observed with other plant pathogens, including Pseudomonas syringae, Ralstonia solanacearum, and Erwinia spp., although here the signals and signal receptors are not as well defined. In some cases, nutritional conditions such as iron limitation or the lack of nitrogen sources seem to provide a significant cue. While much has been learned about the process of host detection over the past 20 years, our knowledge is far from being complete. The complex nature of the plant-microbe interactions makes it extremely challenging to gain a comprehensive picture of host detection in natural environments, and thus many signals and signal recognition systems remain to be described.

Transgenic expression of the soybean apyrase in Lotus japonicus enhances nodulation

The soybean apyrase, GS52, was previously characterized as an early nodulin that is expressed in roots and localized to the plasma membrane. Transgenic Lotus japonicus plants were constructed constitutively expressing the GS52 apyrase. Segregation and Southern-blot analysis identified four single-copy sense lines, several double-copy sense lines, and one double-copy antisense line for further analysis. The single- and double-copy sense gs52 L. japonicus lines had enhanced nodulation that correlated with expression of the transgene. The sense transgenic lines were also found to have increased infection thread formation and enhanced infection zone length when infected by Mesorhizobium loti, the natural symbiont of L. japonicus. The data presented show that expression of the GS52 apyrase can enhance nodulation in L. japonicus and points to an important role for this group of enzymes in nodulation.

The katA catalase gene is regulated by OxyR in both free-living and symbiotic Sinorhizobium meliloti

The characterization of an oxyR insertion mutant provides evidences that katA, which encodes the unique H2O2-inducible HPII catalase, is regulated by OxyR not only in free-living Sinorhizobium meliloti but also in symbiotic S. meliloti. Moreover, oxyR is expressed independently of exogenous H2O2 and downregulates its own expression in S. meliloti.

Visualization of nodulation gene activity on the early stages of Rhizobium leguminosarum bv. viciae symbiosis

A technique was optimized for the in situ detection of nodulation (nod) gene activity in Rhizobium leguminosarum bv. viciae symbiosis with compatible plant hosts Vicia tetrasperma (L.) SCHREB. and Pisum sativum L. The transcription of nodABC-lacZ fusion was visualized as beta-galactosidase (beta-Gal) activity after reaction with the chromogenic substrate X-Gal and subsequent light microscopy, while the background of the indigenous beta-Gal activity of rhizobia and the host plant was eliminated by glutaraldehyde treatment. V. tetrasperma was suggested as a suitable model plant for pea cross-inoculation group due to its advantages over the common model of V. hirsuta (L.) S.F. GRAY: compactness of the plant, extremely small seeds, fast development and stable nodulation under laboratory conditions. In the roots of both plants, a certain extent of nod gene activity was detectable in all rhizobia colonizing the rhizoplane. In pea 1 d after inoculation (d.a.i.), the maximum was localized in the region of emerging root hairs (RH) later (3 and 6 d.a.i.) shifting upwards from the root tip. Nodulation genes sustained full expression even in the infection threads inside the RH and the root cortex, independently of their association with nodule primordia. Comparison of two pea symbiotic mutant lines, Risnod25 and Risnod27, with the wild type did not reveal any differences in the RH formation, RH curling response and rhizoplane colonization. Both mutants appeared to be blocked at the infection thread initiation stage and in nodule initiation, consistent with the phenotype caused by other mutant alleles in the pea sym8 locus. Judging from the nod gene expression level and pattern in the rhizoplane, flavonoid response upon inoculation is preserved in both pea mutants, being independent of infection thread and nodule initiation.

nip, a symbiotic Medicago truncatula mutant that forms root nodules with aberrant infection threads and plant defense-like response

To investigate the legume-Rhizobium symbiosis, we isolated and studied a novel symbiotic mutant of the model legume Medicago truncatula, designated nip (numerous infections and polyphenolics). When grown on nitrogen-free media in the presence of the compatible bacterium Sinorhizobium meliloti, the nip mutant showed nitrogen deficiency symptoms. The mutant failed to form pink nitrogen-fixing nodules that occur in the wild-type symbiosis, but instead developed small bump-like nodules on its roots that were blocked at an early stage of development. Examination of the nip nodules by light microscopy after staining with X-Gal for S. meliloti expressing a constitutive GUS gene, by confocal microscopy following staining with SYTO-13, and by electron microscopy revealed that nip initiated symbiotic interactions and formed nodule primordia and infection threads. The infection threads in nip proliferated abnormally and very rarely deposited rhizobia into plant host cells; rhizobia failed to differentiate further in these cases. nip nodules contained autofluorescent cells and accumulated a brown pigment. Histochemical staining of nip nodules revealed this pigment to be polyphenolic accumulation. RNA blot analyses demonstrated that nip nodules expressed only a subset of genes associated with nodule organogenesis, as well as elevated expression of a host defense-associated phenylalanine ammonia lyase gene. nip plants were observed to have abnormal lateral roots. nip plant root growth and nodulation responded normally to ethylene inhibitors and precursors. Allelism tests showed that nip complements 14 other M. truncatula nodulation mutants but not latd, a mutant with a more severe nodulation phenotype as well as primary and lateral root defects. Thus, the nip mutant defines a new locus, NIP, required for appropriate infection thread development during invasion of the nascent nodule by rhizobia, normal lateral root elongation, and normal regulation of host defense-like responses during symbiotic interactions.

Involvement of exo5 in production of surface polysaccharides in Rhizobium leguminosarum and its role in nodulation of Vicia sativa subsp. nigra

Analysis of two exopolysaccharide-deficient mutants of Rhizobium leguminosarum, RBL5808 and RBL5812, revealed independent Tn5 transposon integrations in a single gene, designated exo5. As judged from structural and functional homology, this gene encodes a UDP-glucose dehydrogenase responsible for the oxidation of UDP-glucose to UDP-glucuronic acid. A mutation in exo5 affects all glucuronic acid-containing polysaccharides and, consequently, all galacturonic acid-containing polysaccharides. Exo5-deficient rhizobia do not produce extracellular polysaccharide (EPS) or capsular polysaccharide (CPS), both of which contain glucuronic acid. Carbohydrate composition analysis and nuclear magnetic resonance studies demonstrated that EPS and CPS from the parent strain have very similar structures. Lipopolysaccharide (LPS) molecules produced by the mutant strains are deficient in galacturonic acid, which is normally present in the core and lipid A portions of the LPS. The sensitivity of exo5 mutant rhizobia to hydrophobic compounds shows the involvement of the galacturonic acid residues in the outer membrane structure. Nodulation studies with Vicia sativa subsp. nigra showed that exo5 mutant rhizobia are impaired in successful infection thread colonization. This is caused by strong agglutination of EPS-deficient bacteria in the root hair curl. Root infection could be restored by simultaneous inoculation with a Nod factor-defective strain which retained the ability to produce EPS and CPS. However, in this case colonization of the nodule tissue was impaired.

Rapid analysis of legume root nodule development using confocal microscopy

•  A rapid method for detailed analysis of nodule formation has been developed. •  Inoculated root tissues were stained with SYTO 13, a cell-permeant fluorescent nucleic acid-binding dye, and visualized using confocal laser scanning microscopy (CLSM). Structures with high concentrations of DNA and RNA, such as plant cell nuclei and bacteria, labeled strongly. The autofluorescent properties of cell walls made it possible to use CLSM to visualize both plant and rhizobial structures and generate a three-dimensional reconstruction of the root and invading bacteria. •  This method allowed clear observation of stages and structures important in nodule formation, such as rhizobial attachment to root hairs, hair deformation, infection thread ramification, nodule primordium development and nodule cell invasion. Bacteroid structures were easily assessed without the need for fixation that might alter cellular integrity. Plant nodulation mutants with phenotypic differences in thread growth, cellular invasion and plant defense response were also documented. •  Multiple samples can be assessed using detailed microscopy without the need for extensive preparative work, labor-intensive analysis, or the generation of genetically modified samples.

Cytokinins play opposite roles in lateral root formation, and nematode and Rhizobial symbioses

We used the cytokinin-responsive Arabidopsis response regulator (ARR)5 gene promoter fused to a beta-glucuronidase (GUS) reporter gene, and cytokinin oxidase (CKX) genes from Arabidopsis thaliana (AtCKX3) and maize (ZmCKX1) to investigate the roles of cytokinins in lateral root formation and symbiosis in Lotus japonicus. ARR5 expression was undetectable in the dividing initial cells at early stages of lateral root formation, but later we observed high expression in the base of the lateral root primordium. The root tip continues to express ARR5 during subsequent development of the lateral root. These results suggest a dynamic role for cytokinin in lateral root development. We observed ARR5 expression in curled/deformed root hairs, and also in nodule primordia in response to Rhizobial inoculation. This expression declined once the nodule emerged from the parent root. Root penetration and migration of root-knot nematode (RKN) second-stage larvae (L2) did not elevate ARR5 expression, but a high level of expression was induced when L2 reached the differentiating vascular bundle and during early stages of the nematode-plant interaction. ARR5 expression was specifically absent in mature giant cells (GCs), although dividing cells around the GCs continued to express this reporter. The same pattern was observed using a green fluorescent protein (GFP) reporter driven by the ARR5 promoter in tomato. Overexpression of CKX genes rendered the transgenic hairy roots resistant to exogenous application of the cytokinin [N6-(Delta2 isopentenyl) adenine riboside] (iPR). CKX roots have significantly more lateral roots, but fewer nodules and nematode-induced root galls per plant, than control hairy roots.

Dual genetic pathways controlling nodule number in Medicago truncatula

We report the isolation and characterization of a new Medicago truncatula hyper-nodulation mutant, designated sunn (super numeric nodules). Similar to the previously described ethylene-insensitive mutant sickle, sunn exhibits a 10-fold increase in the number of nodules within the primary nodulation zone. Despite this general similarity, these two mutants are readily distinguished based on anatomical, genetic, physiological, and molecular criteria. In contrast to sickle, where insensitivity to ethylene is thought to be causal to the hyper-nodulation phenotype (R.V. Penmetsa, D.R. Cook [1997] Science 275: 527-530), nodulation in sunn is normally sensitive to ethylene. Nevertheless, sunn exhibits seedling root growth that is insensitive to ethylene, although other aspects of the ethylene triple response are normal; these observations suggest that hormonal responses might condition the sunn phenotype in a manner distinct from sickle. The two mutants also differ in the anatomy of the nodulation zone: Successful infection and nodule development in sunn occur predominantly opposite xylem poles, similar to wild type. In sickle, however, both infection and nodulation occur randomly throughout the circumference of the developing root. Genetic analysis indicates that sunn and sickle correspond to separate and unlinked loci, whereas the sunn/skl double mutant exhibits a novel and additive super-nodulation phenotype. Taken together, these results suggest a working hypothesis wherein sunn and sickle define distinct genetic pathways, with skl regulating the number and distribution of successful infection events, and sunn regulating nodule organogenesis.

Dual genetic pathways controlling nodule number in Medicago truncatula

We report the isolation and characterization of a new Medicago truncatula hyper-nodulation mutant, designated sunn (super numeric nodules). Similar to the previously described ethylene-insensitive mutant sickle, sunn exhibits a 10-fold increase in the number of nodules within the primary nodulation zone. Despite this general similarity, these two mutants are readily distinguished based on anatomical, genetic, physiological, and molecular criteria. In contrast to sickle, where insensitivity to ethylene is thought to be causal to the hyper-nodulation phenotype (R.V. Penmetsa, D.R. Cook [1997] Science 275: 527-530), nodulation in sunn is normally sensitive to ethylene. Nevertheless, sunn exhibits seedling root growth that is insensitive to ethylene, although other aspects of the ethylene triple response are normal; these observations suggest that hormonal responses might condition the sunn phenotype in a manner distinct from sickle. The two mutants also differ in the anatomy of the nodulation zone: Successful infection and nodule development in sunn occur predominantly opposite xylem poles, similar to wild type. In sickle, however, both infection and nodulation occur randomly throughout the circumference of the developing root. Genetic analysis indicates that sunn and sickle correspond to separate and unlinked loci, whereas the sunn/skl double mutant exhibits a novel and additive super-nodulation phenotype. Taken together, these results suggest a working hypothesis wherein sunn and sickle define distinct genetic pathways, with skl regulating the number and distribution of successful infection events, and sunn regulating nodule organogenesis.

A Sinorhizobium meliloti lipopolysaccharide mutant altered in cell surface sulfation

The Rhizobium-legume symbiosis involves the formation of a novel plant organ, the nodule, in which intracellular bacteria reduce molecular dinitrogen in exchange for plant photosynthates. Nodule development requires a bacterial signal referred to as Nod factor, which in Sinorhizobium meliloti is a beta-(1,4)-linked tetramer of N-acetylglucosamine containing N-acyl and O-acetyl modifications at the nonreducing end and a critical 6-O-sulfate at the reducing end. This sulfate modification requires the action of three gene products: nodH, which catalyzes the sulfonyl transfer, and nodPQ, which produce the activated form of sulfate, 3'-phosphoadenosine-5'-phosphosulfate. It was previously reported that S. meliloti cell surface polysaccharides are also covalently modified by sulfate in a reaction dependent on NodPQ. We have further characterized this unique form of bacterial carbohydrate modification. Our studies have determined that one of the nodPQ mutant strains used in the initial study of sulfation of cell surface harbored a second unlinked mutation. We cloned the gene affected by this mutation (referred to as lps-212) and found it to be an allele of lpsL, a gene previously predicted to encode a UDP-glucuronic acid epimerase. We demonstrated that lpsL encoded a UDP-glucuronic acid epimerase activity that was reduced in the lps-212 mutant. The lps-212 mutation resulted in an altered lipopolysaccharide structure that was reduced in sulfate modification in vitro and in vivo. Finally, we determined that the lps-212 mutation resulted in a reduced ability to elicit the formation of plant nodules and by altered infection thread structures that aborted prematurely.

A Sinorhizobium meliloti lipopolysaccharide mutant altered in cell surface sulfation

The Rhizobium-legume symbiosis involves the formation of a novel plant organ, the nodule, in which intracellular bacteria reduce molecular dinitrogen in exchange for plant photosynthates. Nodule development requires a bacterial signal referred to as Nod factor, which in Sinorhizobium meliloti is a beta-(1,4)-linked tetramer of N-acetylglucosamine containing N-acyl and O-acetyl modifications at the nonreducing end and a critical 6-O-sulfate at the reducing end. This sulfate modification requires the action of three gene products: nodH, which catalyzes the sulfonyl transfer, and nodPQ, which produce the activated form of sulfate, 3'-phosphoadenosine-5'-phosphosulfate. It was previously reported that S. meliloti cell surface polysaccharides are also covalently modified by sulfate in a reaction dependent on NodPQ. We have further characterized this unique form of bacterial carbohydrate modification. Our studies have determined that one of the nodPQ mutant strains used in the initial study of sulfation of cell surface harbored a second unlinked mutation. We cloned the gene affected by this mutation (referred to as lps-212) and found it to be an allele of lpsL, a gene previously predicted to encode a UDP-glucuronic acid epimerase. We demonstrated that lpsL encoded a UDP-glucuronic acid epimerase activity that was reduced in the lps-212 mutant. The lps-212 mutation resulted in an altered lipopolysaccharide structure that was reduced in sulfate modification in vitro and in vivo. Finally, we determined that the lps-212 mutation resulted in a reduced ability to elicit the formation of plant nodules and by altered infection thread structures that aborted prematurely.

Bacteriophytochrome controls photosystem synthesis in anoxygenic bacteria

Plants use a set of light sensors to control their growth and development in response to changes in ambient light. In particular, phytochromes exert their regulatory activity by switching between a biologically inactive red-light-absorbing form (Pr) and an active far-red-light absorbing form (Pfr). Recently, biochemical and genetic studies have demonstrated the occurrence of phytochrome-like proteins in photosynthetic and non-photosynthetic bacteria--but little is known about their functions. Here we report the discovery of a bacteriophytochrome located downstream from the photosynthesis gene cluster in a Bradyrhizobium strain symbiont of Aeschynomene. The synthesis of the complete photosynthetic apparatus is totally under the control of this bacteriophytochrome. A similar behaviour is observed for the closely related species Rhodopseudomonas palustris, but not for the more distant anoxygenic photosynthetic bacteria of the genus Rhodobacter, Rubrivivax or Rhodospirillum. Unlike other (bacterio)phytochromes, the carboxy-terminal domain of this bacteriophytochrome contains no histidine kinase features. This suggests a light signalling pathway involving direct protein-protein interaction with no phosphorelay cascade. This specific mechanism of regulation may represent an important ecological adaptation to optimize the plant-bacteria interaction.

pMH11, A tool for gene disruption and expression analysis in Azorhizobium caulinodans

Tools for mutagenesis and expression analyses are needed to study the role of bacterial genes. Here, we report the construction of pMH11, a small, mobilizable plasmid that replicates in Escherichia coli, but not in Azorhizobium caulinodans, a nodulating microsymbiont of Sesbania rostrata, and that contains a unique BamHI restriction site upstream of a promoterless lacZ gene. pMH11 and two derivatives with the multiple cloning site of pBluescript (KS(II)) are useful for mutagenesis by gene disruption and for expression analyses after selection for cointegration by kanamycin resistance. Weakly constitutive promoter activity from the vector allowed transcription of genes downstream of the integration site, so that no polar effects were caused by gene disruption.

Xylem colonization of tomato by Azorhizobium caulinodans ORS571

Tomato seedlings growing aseptically in Murashige and Skoog Medium were inoculated with Azorhizobium caulinodans ORS571 (pXLGD4), carrying the lacZ reporter gene. By microscopic analyses of inoculated tomato roots, it has been demonstrated that the xylem of tomato roots can be colonized by Azorhizobium. We discuss whether this colonization of the xylem of tomato roots by diazotrophic azorhizobia might provide a suitable niche for endophytic nitrogen fixation.

Construction of improved vectors and cassettes containing gusA and antibiotic resistance genes for studies of transcriptional activity and bacterial localization

Broad-host-range, conjugative vectors with a constitutively expressed gusA gene combined with different antibiotic resistance (tetracycline, gentamicin, kanamycin) genes have been constructed. These plasmids are designed for tracking Gram-negative bacterial strains without the risk of random mutagenesis. We also constructed a set of cassettes containing a promoterless gusA gene linked with different antibiotic resistance genes for making transcriptional fusions and for cassette mutagenesis. New plasmids and cassettes can be useful tools for studying gene expression, interaction of bacteria with plants and monitoring bacteria in the environment.

A functional myo-inositol dehydrogenase gene is required for efficient nitrogen fixation and competitiveness of Sinorhizobium fredii USDA191 to nodulate soybean (Glycine max [L.] Merr.)

Inositol derivative compounds provide a nutrient source for soil bacteria that possess the ability to degrade such compounds. Rhizobium strains that are capable of utilizing certain inositol derivatives are better colonizers of their host plants. We have cloned and determined the nucleotide sequence of the myo-inositol dehydrogenase gene (idhA) of Sinorhizobium fredii USDA191, the first enzyme responsible for inositol catabolism. The deduced IdhA protein has a molecular mass of 34,648 Da and shows significant sequence similarity with protein sequences of Sinorhizobium meliloti IdhA and MocA; Bacillus subtilis IolG, YrbE, and YucG; and Streptomyces griseus StrI. S. fredii USDA191 idhA mutants revealed no detectable myo-inositol dehydrogenase activity and failed to grow on myo-inositol as a sole carbon source. Northern blot analysis and idhA-lacZ fusion expression studies indicate that idhA is inducible by myo-inositol. S. fredii USDA191 idhA mutant was drastically affected in its ability to reduce nitrogen and revealed deteriorating bacteroids inside the nodules. The number of bacteria recovered from such nodules was about threefold lower than the number of bacteria isolated from nodules initiated by S. fredii USDA191. In addition, the idhA mutant was also severely affected in its ability to compete with the wild-type strain in nodulating soybean. Under competitive conditions, nodules induced on soybean roots were predominantly occupied by the parent strain, even when the idhA mutant was applied at a 10-fold numerical advantage. Thus, we conclude that a functional idhA gene is required for efficient nitrogen fixation and for competitive nodulation of soybeans by S. fredii USDA191.

Leafy gall formation by Rhodococcus fascians

Rhodococcus fascians infects a wide range of plants, initiating the formation of leafy galls that consist of centers of shoot amplification and shoot growth inhibition. R. fascians is an epiphyte but it also can establish endophytic populations. Bacterial signals involved in symptom development initiate de novo cell division and shoot meristem formation in differentiated tissues. The R. fascians signals exert activities that are distinct from mere cytokinin effects, and the evidence points to a process that adopted cytokinin biosynthetic enzymes to form derivatives with unique activity. Genes implicated in leafy gall formation are located on a linear plasmid and are subject to a highly controlling, complex regulatory network, integrating autoregulatory compounds and environmental signals. Leafy galls are considered as centers with specific metabolic features, a niche where populations of R. fascians experience a selective advantage. Such "metabolic habitat modification" might be universal for gall-inducing bacteria.

Effect of Bradyrhizobium photosynthesis on stem nodulation of Aeschynomene sensitiva

Some leguminous species of the genus Aeschynomene are specifically stem-nodulated by photosynthetic bradyrhizobia. To study the effect of bacterial photosynthesis during symbiosis, we generated a photosynthesis-negative mutant of the Bradyrhizobium sp. strain ORS278 symbiont of Aeschynomene sensitiva. The presence of a functional photosynthetic unit in bacteroids and the high expression of the photosynthetic genes observed in stem nodules demonstrate that the bacteria are photosynthetically active during stem symbiosis. Stem inoculation by the photosynthetic mutant gave a 50% decrease in stem-nodule number, which reduced nitrogen fixation activity and plant growth in the same proportion. These results indicate an important role of bacterial photosynthesis in the efficiency of stem nodulation.

Photosynthetic bradyrhizobia are natural endophytes of the African wild rice Oryza breviligulata

We investigated the presence of endophytic rhizobia within the roots of the wetland wild rice Oryza breviligulata, which is the ancestor of the African cultivated rice Oryza glaberrima. This primitive rice species grows in the same wetland sites as Aeschynomene sensitiva, an aquatic stem-nodulated legume associated with photosynthetic strains of Bradyrhizobium. Twenty endophytic and aquatic isolates were obtained at three different sites in West Africa (Senegal and Guinea) from nodal roots of O. breviligulata and surrounding water by using A. sensitiva as a trap legume. Most endophytic and aquatic isolates were photosynthetic and belonged to the same phylogenetic Bradyrhizobium/Blastobacter subgroup as the typical photosynthetic Bradyrhizobium strains previously isolated from Aeschynomene stem nodules. Nitrogen-fixing activity, measured by acetylene reduction, was detected in rice plants inoculated with endophytic isolates. A 20% increase in the shoot growth and grain yield of O. breviligulata grown in a greenhouse was also observed upon inoculation with one endophytic strain and one Aeschynomene photosynthetic strain. The photosynthetic Bradyrhizobium sp. strain ORS278 extensively colonized the root surface, followed by intercellular, and rarely intracellular, bacterial invasion of the rice roots, which was determined with a lacZ-tagged mutant of ORS278. The discovery that photosynthetic Bradyrhizobium strains, which are usually known to induce nitrogen-fixing nodules on stems of the legume Aeschynomene, are also natural true endophytes of the primitive rice O. breviligulata could significantly enhance cultivated rice production.

Critical protective role of bacterial superoxide dismutase in rhizobium-legume symbiosis

In nitrogen-poor soils, rhizobia elicit nodule formation on legume roots, within which they differentiate into bacteroids that fix atmospheric nitrogen. Protection against reactive oxygen species (ROS) was anticipated to play an important role in Rhizobium-legume symbiosis because nitrogenase is extremely oxygen sensitive. We deleted the sodA gene encoding the sole cytoplasmic superoxide dismutase (SOD) of Sinorhizobium meliloti. The resulting mutant, deficient in superoxide dismutase, grew almost normally and was only moderately sensitive to oxidative stress when free living. In contrast, its symbiotic properties in alfalfa were drastically affected. Nitrogen-fixing ability was severely impaired. More strikingly, most SOD-deficient bacteria did not reach the differentiation stage of nitrogen-fixing bacteroids. The SOD-deficient mutant nodulated poorly and displayed abnormal infection. After release into plant cells, a large number of bacteria failed to differentiate into bacteroids and rapidly underwent senescence. Thus, bacterial SOD plays a key protective role in the symbiotic process.

Megaplasmid pRme2011a of Sinorhizobium meliloti is not required for viability

We report the curing of the 1,360-kb megaplasmid pRme2011a from Sinorhizobium meliloti strain Rm2011. With a positive selection strategy that utilized Tn5B12-S containing the sacB gene, we were able to cure this replicon by successive rounds of selecting for deletion formation in vivo. Subsequent Southern blot, Eckhardt gel, and pulsed-field gel electrophoresis analyses were consistent with the hypothesis that the resultant strain was indeed missing pRme2011a. The cured derivative grew as well as the wild-type strain in both complex and defined media but was unable to use a number of substrates as a sole source of carbon on defined media.

Effects of glucosinolates and flavonoids on colonization of the roots of Brassica napus by Azorhizobium caulinodans ORS571

Plants of Brassica napus were assessed quantitatively for their susceptibility to lateral root crack colonization by Azorhizobium caulinodans ORS571(pXLGD4) (a rhizobial strain carrying the lacZ reporter gene) and for the concentration of glucosinolates in their roots by high-pressure liquid chromatography (HPLC). High- and low-glucosinolate-seed (HGS and LGS) varieties exhibited a relatively low and high percentage of colonized lateral roots, respectively. HPLC showed that roots of HGS plants contained a higher concentration of glucosinolates than roots of LGS plants. One LGS variety showing fewer colonized lateral roots than other LGS varieties contained a higher concentration of glucosinolates than other LGS plants. Inoculated HGS plants treated with the flavonoid naringenin showed significantly more colonization than untreated HGS plants. This increase was not mediated by a naringenin-induced lowering of the glucosinolate content of HGS plant roots, nor did naringenin induce bacterial resistance to glucosinolates or increase the growth of bacteria. The erucic acid content of seed did not appear to influence colonization by azorhizobia. Frequently, leaf assays are used to study glucosinolates and plant defense; this study provides data on glucosinolates and bacterial colonization in roots and describes a bacterial reporter gene assay tailored easily to the study of ecologically important phytochemicals that influence bacterial colonization. These data also form a basis for future assessments of the benefits to oilseed rape plants of interaction with plant growth-promoting bacteria, especially diazotrophic bacteria potentially able to extend the benefits of nitrogen fixation to nonlegumes.

A Krüppel-like zinc finger protein is involved in nitrogen-fixing root nodule organogenesis

Mechanisms regulating plant host differentiation of the nitrogen-fixing root nodules remain mostly unknown. Sinorhizobium meliloti induces this process in Medicago sativa in which the Mszpt2-1 gene is expressed in vascular bundles of roots and nodules. This gene codes for a Krüppel-like zinc finger protein, a class of transcription factors involved in many animal developmental processes. Expression of Mszpt2-1 in yeast cells conferred osmotic tolerance. Antisense plants grew normally but developed nonfunctional nodules, in which differentiation of the nitrogen-fixing zone and bacterial invasion were arrested. Hence, a vascular bundle-associated Krüppel-like gene is required for the formation of the central nitrogen-fixing zone of the root nodule.

A Krüppel-like zinc finger protein is involved in nitrogen-fixing root nodule organogenesis

Mechanisms regulating plant host differentiation of the nitrogen-fixing root nodules remain mostly unknown. Sinorhizobium meliloti induces this process in Medicago sativa in which the Mszpt2-1 gene is expressed in vascular bundles of roots and nodules. This gene codes for a Krüppel-like zinc finger protein, a class of transcription factors involved in many animal developmental processes. Expression of Mszpt2-1 in yeast cells conferred osmotic tolerance. Antisense plants grew normally but developed nonfunctional nodules, in which differentiation of the nitrogen-fixing zone and bacterial invasion were arrested. Hence, a vascular bundle-associated Krüppel-like gene is required for the formation of the central nitrogen-fixing zone of the root nodule.

The xylem of rice (Oryza sativa) is colonized by Azorhizobium caulinodans

Following inoculation with Azorhizobium caulinodans ORS571 (pXLGD4), lateral root development of rice and colonization of lateral root cracks by bacteria were shown to be stimulated by the flavonoid naringenin. Rice seedlings growing aseptically in the presence of naringenin were inoculated with ORS571 (pXLGD4), carrying the lacZ reporter gene. By microscopic analysis of sections of inoculated rice roots, it has been demonstrated that the xylem of rice roots can be colonized by Azorhizobium caulinodans. We discuss whether this colonization of the xylem of rice roots by azorhizobia could provide a suitable niche for endophytic nitrogen fixation.

A plant regulator controlling development of symbiotic root nodules

Symbiotic nitrogen-fixing root nodules on legumes are founded by root cortical cells that de-differentiate and restart cell division to establish nodule primordia. Bacterial microsymbionts invade these primordia through infection threads laid down by the plant and, after endocytosis, membrane-enclosed bacteroids occupy cells in the nitrogen-fixing tissue of functional nodules. The bacteria excrete lipochitin oligosaccharides, triggering a developmental process that is controlled by the plant and can be suppressed. Nodule inception initially relies on cell competence in a narrow infection zone located just behind the growing root tip. Older nodules then regulate the number of nodules on a root system by suppressing the development of nodule primordia. To identify the regulatory components that act early in nodule induction, we characterized a transposon-tagged Lotus japonicus mutant, nin (for nodule inception), arrested at the stage of bacterial recognition. We show that nin is required for the formation of infection threads and the initiation of primordia. NIN protein has regional similarity to transcription factors, and the predicted DNA-binding/dimerization domain identifies and typifies a consensus motif conserved in plant proteins with a function in nitrogen-controlled development.

Ethylene-mediated phenotypic plasticity in root nodule development on Sesbania rostrata

Leguminous plants in symbiosis with rhizobia form either indeterminate nodules with a persistent meristem or determinate nodules with a transient meristematic region. Sesbania rostrata was thought to possess determinate stem and root nodules. However, the nature of nodule development is hybrid, and the early stages resemble those of indeterminate nodules. Here we show that, depending on the environmental conditions, mature root nodules can be of the indeterminate type. In situ hybridizations with molecular markers for plant cell division, as well as the patterns of bacterial nod and nif gene expression, confirmed the indeterminate nature of 30-day-old functional root nodules. Experimental data provide evidence that the switch in nodule type is mediated by the plant hormone ethylene.

Determination of plasmid-encoded functions in Rhizobium leguminosarum biovar trifolii using proteome analysis of plasmid-cured derivatives

We have used proteome analysis of derivatives of R. leguminosarum biovar trifolii strain ANU843, cured of indigenous plasmids by a direct selection system, to investigate plasmid-encoded functions. Under the conditions used, the plasmid-encoded gene products contributed to only a small proportion of the 2000 proteins visualised in the two-dimensional (2-D) protein map of strain ANU843. The level of synthesis of thirty-nine proteins was affected after curing of either plasmid a, c or e. The differences observed upon plasmid curing included: protein loss, up/down-regulation of specific proteins and novel synthesis of some proteins. This suggests that a complex interplay between the cured plasmid and the remaining replicons is occurring. Twenty-two proteins appeared to be absent in the cured strains and these presumably are encoded by plasmid genes. Of these, a small heat shock protein, a cold shock protein, a hypothetical YTFG-29.7 kDa protein, and the alpha and beta subunits of the electron transfer flavoprotein were identified by N-terminal microsequencing and predicted to be encoded by plasmid e. Four of the sequenced proteins putatively encoded on plasmid e and two encoded on plasmid c were novel. In addition, curing of plasmid e and c consistently decreased the levels of 3-isopropylmalate dehydratase and malate dehydrogenase, respectively, suggesting that levels of these proteins may be influenced by plasmid-encoded functions. A protein with homology to 4-oxalocrotonate tautomerase, which is involved in the biodegradation of phenolic compounds, was found to be newly synthesised in the strain cured of plasmid e. Proteome analysis provides a sensitive tool to examine the functional organisation of the Rhizobium genome and the global gene interactions which occur between the different replicons.

Lotus corniculatus nodulation specificity is changed by the presence of a soybean lectin gene

Plant lectins have been implicated as playing an important role in mediating recognition and specificity in the Rhizobium-legume nitrogen-fixing symbiosis. To test this hypothesis, we introduced the soybean lectin gene Le1 either behind its own promoter or behind the cauliflower mosaic virus 35S promoter into Lotus corniculatus, which is nodulated by R. loti. We found that nodulelike outgrowths developed on transgenic L. corniculatus plant roots in response to Bradyrhizobium japonicum, which nodulates soybean and not Lotus spp. Soybean lectin was properly targeted to L. corniculatus root hairs, and although infection threads formed, they aborted in epidermal or hypodermal cells. Mutation of the lectin sugar binding site abolished infection thread formation and nodulation. Incubation of bradyrhizobia in the nodulation (nod) gene-inducing flavonoid genistein increased the number of nodulelike outgrowths on transgenic L. corniculatus roots. Studies of bacterial mutants, however, suggest that a component of the exopolysaccharide surface of B. japonicum, rather than Nod factor, is required for extension of host range to the transgenic L. corniculatus plants.

Sym2 of Pea Is Involved in a Nodulation Factor-Perception Mechanism That Controls the Infection Process in the Epidermis

In pea (Pisum sativum) up to 50 nodulation mutants are known, several of which are affected in the early steps of the symbiotic interaction with Rhizobium sp. bacteria. Here we describe the role of the sym2 gene in nodulation (Nod) factor perception. Our experiments show that the sym2A allele from the wild pea variety Afghanistan confers an arrest in infection-thread growth if the Rhizobium leguminosarum bv viciae strain does not produce Nod factors with a NodX-mediated acetylation at their reducing end. Since the induction of the early nodulin gene ENOD12 in the epidermis and the formation of a nodule primordium in the inner cortex were not affected, we conclude that more than one Nod factor-perception mechanism is active. Furthermore, we show that sym2A-mediated control of infection-thread growth was affected by the bacterial nodulation gene nodO.

Specific flavonoids promote intercellular root colonization of Arabidopsis thaliana by Azorhizobium caulinodans ORS571

The ability of Azorhizobium caulinodans ORS571 and other diazotrophic bacteria to internally colonize roots of Arabidopsis thaliana has been studied. Strains tagged with lacZ or gusA reporter genes were used, and the principal colonization sites were found to be the points of emergence of lateral roots, lateral root cracks (LRCs). High frequencies of colonization were found; 63 to 100% of plants were colonized by ORS571, and 100% of plants were colonized by Herbaspirillum seropedicae. After LRCs were colonized, bacteria moved into intercellular spaces between the cortical and endodermal cell layers. Specific flavonoids, naringenin and daidzein, at 5 x 10(-5) M, significantly promoted colonization by ORS571. By using a nodC and a nodD mutant of ORS571, it was shown that neither Nod factors nor NodD are involved in colonization or flavonoid stimulation of colonization. Flavonoids did not appear to be stimulating LRC colonization by their activity as nutritional factors. LRC and intercellular colonization by H. seropedicae was stimulated by naringenin and daidzein at the same concentration that stimulated colonization by ORS571.

The Rhizobium meliloti PII protein, which controls bacterial nitrogen metabolism, affects alfalfa nodule development

Symbiotic nitrogen fixation involves the development of specialized organs called nodules within which plant photosynthates are exchanged for combined nitrogen of bacterial origin. To determine the importance of bacterial nitrogen metabolism in symbiosis, we have characterized a key regulator of this metabolism in Rhizobium meliloti, the uridylylatable P(II) protein encoded by glnB. We have constructed both a glnB null mutant and a point mutant making nonuridylylatable P(II). In free-living conditions, P(II) is required for expression of the ntrC-dependent gene glnII and for adenylylation of glutamine synthetase I. P(II) is also required for efficient infection of alfalfa but not for expression of nitrogenase. However alfalfa plants inoculated with either glnB mutant are nitrogen-starved in the absence of added combined nitrogen. We hypothesize that P(II) controls expression or activity of a bacteroid ammonium transporter required for a functional nitrogen-fixing symbiosis. Therefore, the P(II) protein affects both Rhizobium nitrogen metabolism and alfalfa nodule development.

Expression of the Arabidopsis HMG2 gene, encoding 3-hydroxy-3-methylglutaryl coenzyme A reductase, is restricted to meristematic and floral tissues

The synthesis of mevalonate, which is considered the first rate-limiting step in isoprenoid biosynthesis, is catalyzed by the enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR; EC 1.1.1.34). In Arabidopsis, HMGR is encoded by two differentially expressed genes (HMG1 and HMG2). The transcriptional activity of the HMG2 gene was studied after fusing different regions of its 5' flanking region to the beta-glucuronidase (GUS) reporter gene and transforming the resulting constructs into tobacco plants. The spatial and temporal expression directed by the HMG2 promoter in the transgenic plants is consistent with the expression pattern previously established by RNA analysis using an HMG2-specific probe. HMG2 expression is restricted to meristematic (root tip and shoot apex) and floral (secretory zone of the stigma, mature pollen grains, gynoecium vascular tissue, and fertilized ovules) tissues. Deletion analysis of the HMG2 5' flanking region was conducted in transgenic plants and transfected protoplasts. The region containing nucleotides -857 to +64 of the HMG2 gene was sufficient to confer high levels of expression in both floral and meristematic tissues, although deletion to nucleotide -503 resulted in almost complete loss of expression. Sequences contained within the 5' transcribed, untranslated region are also important for gene expression. The biological significance of the restricted pattern of expression of HMG2 is also discussed.

Oxygen as a key developmental regulator of Rhizobium meliloti N2-fixation gene expression within the alfalfa root nodule

The symbiotic pattern of expression of Rhizobium meliloti N2-fixation genes is tightly coupled with the histological organization of the alfalfa root nodule and thus is under developmental control. N2-fixation gene expression is induced very sharply at a particular zone of the nodule called interzone II-III that precedes the zone where N2 fixation takes place. We show here that this coupling can be disrupted, hereby resulting in ectopic expression of N2-fixation genes in the prefixing zone II of the nodule. Uncoupling was obtained either by using a R. meliloti strain in which a mutation rendered N2-fixation gene expression constitutive with respect to oxygen in free-living bacterial cultures or by placing nodules induced by a wild-type R. meliloti strain in a microoxic environment. These results implicate oxygen as a key determinant of the symbiotic pattern of N2-fixation gene expression.

NolR controls expression of the Rhizobium meliloti nodulation genes involved in the core Nod factor synthesis

The synthesis of Rhizobium meliloti Nod signal molecules, encoded by the nod gene products, is finely regulated. A negative control of plasmid-borne nod gene expression is provided by the NolR repressor encoded by the chromosomal nolR gene. NolR was previously shown to downregulate the expression of the activator nodD1 gene and the common nodABC operon by binding to an overlapping region of the two promoters adjacent to the n1 nod-box (Kondorosi et al., 1989). We demonstrate here that NolR also controls the expression of two additional genes, nodD2 and nodM, but does not directly regulate the expression of the host-specific nod genes located downstream of the n2, n3 and n5 nod-boxes. Thus, the nod genes are differentially regulated by NolR and only those providing common nodulation functions, by determining the synthesis of the core Nod factor structure, are subjected to this negative regulation. Furthermore, NolR has a strong negative effect on the production of Nod metabolites, the level of which may serve as a fine-tuning mechanism for optimal nodulation, specific to host-plant genotypes. In addition, it elicits preferential synthesis of Nod factors carrying unsaturated C16 fatty acids. Expression of nolR was high both in the free-living bacterium and in the bacteroid and it was downregulated by its own product and by the nod gene inducer luteolin.

TE7, An Inefficient Symbiotic Mutant of Medicago truncatula Gaertn. cv Jemalong

A mutagenesis program using ethylmethane sulfonate on Medicago truncatula Gaertn cv Jemalong, an annual, autogamous and diploid lucerne, permitted the isolation of a mutant (TE7) unable to establish an effective nitrogen-fixing symbiosis, [Nod+Fix-], with Rhizobium meliloti wild-type strains. The mutant phenotype is characterized by an altered infection process that leads to the formation of two kinds of inefficient nodules on the same root system. A certain proportion of the nodules are small, round, and uninfected, with infection threads limited to the outer root cortical cells. Others develop to a normal elongated shape and are infected; bacterial release occurs but the bacteria do not differentiate into bacteroids. The ratio of invaded to uninvaded nodules depends on the bacterial strain used. Throughout the infection process, certain events correlated with the plant defense response against pathogens can be observed: (a) the presence of polyphenolic compounds associated with the walls of infected cells and also with some parts of infection threads in the root cortex; (b) appositions on infection thread walls during the early stage of infection and also within the central tissue of infected nodules; and (c) autophagy of the plant cells that contain released bacteria. Genetic data suggest that the phenotype of TE7 is under monogenic and recessive control; this gene has been designated Mtsym1.

Rhizobium meliloti lipooligosaccharide nodulation factors: different structural requirements for bacterial entry into target root hair cells and induction of plant symbiotic developmental responses

Rhizobium meliloti produces lipochitooligosaccharide nodulation NodRm factors that are required for nodulation of legume hosts. NodRm factors are O-acetylated and N-acylated by specific C16-unsaturated fatty acids. nodL mutants produce non-O-acetylated factors, and nodFE mutants produce factors with modified acyl substituents. Both mutants exhibited a significantly reduced capacity to elicit infection thread (IT) formation in alfalfa. However, once initiated, ITs developed and allowed the formation of nitrogen-fixing nodules. In contrast, double nodF/nodL mutants were unable to penetrate into legume hosts and to form ITs. Nevertheless, these mutants induced widespread cell wall tip growth in trichoblasts and other epidermal cells and were also able to elicit cortical cell activation at a distance. NodRm factor structural requirements are thus clearly more stringent for bacterial entry than for the elicitation of developmental plant responses.

Characterization of genes for synthesis and catabolism of a new rhizopine induced in nodules by Rhizobium meliloti Rm220-3: extension of the rhizopine concept

Rhizopines are selective growth substrates synthesized in nodules only by strains of rhizobia capable of their catabolism. We report the isolation and study of genes for the synthesis and catabolism of a new rhizopine, scyllo-inosamine (sIa), from alfalfa nodules induced by Rhizobium meliloti Rm220-3. This compound is similar in structure to the previously described rhizopine 3-O-methyl-scyllo-inosamine from R. meliloti L5-30 (P.J. Murphy, N. Heycke, Z. Banfalvi, M.E. Tate, F.J. de Bruijn, A. Kondorosi, J. Tempé, and J. Schell, Proc. Natl. Acad. Sci. USA 84:493-497, 1987). The synthesis (mos) and catabolism (moc) genes for the Rm220-3 rhizopine are closely linked and located on the nod-nif Sym plasmid. The mos genes are directly controlled by the NifA/NtrA regulatory system. A comparison of the sequence of the 5' regions of the two mos loci shows very extensive conservation of sequence as well as strong homology to the nifH coding region. Restriction mapping and hybridization to DNA from the four open reading frames (ORFs) of the L5-30 mos locus indicate the absence of mosA and presence of the other three ORFs (ORF1 and mosB and -C) in Rm220-3. We suggest that the L5-30 mosA gene product is involved in the conversion of scyllo-inosamine to 3-O-methyl-scyllo-inosamine. Restriction fragment length polymorphism analysis of the moc regions of both strains shows that they are very similar. Regulation studies indicate that the moc region is not controlled by the common regulatory gene nifA, ntrA, and ntrC. We discuss the striking similarities in gene structure, location, and regulation between these two rhizopine loci in relation to the rhizopine concept.

Symbiotic nitrogen fixation by a nifA deletion mutant of Rhizobium meliloti: the role of an unusual ntrC allele

In the N2-fixing alfalfa symbiont Rhizobium meliloti, the three sigma 54 (NTRA)-dependent positively acting regulatory proteins NIFA, NTRC, and DCTD are required for activation of promoters involved in N2 fixation (pnifHDKE and pfixABCX), nitrogen assimilation (pglnII), and C4-dicarboxylate transport (pdctA), respectively. Here, we describe an allele of ntrC which results in the constitutive activation of the above NTRC-, NIFA-, and DCTD-regulated promoters. The expression and activation of wild-type NTRC occur in response to nitrogen availability, whereas in cells carrying the ntrC283 allele, the NTRC283 protein appears constitutively active and is constitutively expressed. The ntrC283 allele was shown to carry a single mutation resulting in the replacement of an Asp by a Tyr residue in the helix-turn-helix motif of ntrC283. Introduction of the ntrC283 allele into a nifA deletion mutant restores the N2-fixation ability to 70 to 80% of the wild-type level. Thus, the nifA gene is dispensable for symbiotic N2 fixation.

Rhizobium meliloti elicits transient expression of the early nodulin gene ENOD12 in the differentiating root epidermis of transgenic alfalfa

To study the molecular responses of the host legume during early stages of the symbiotic interaction with Rhizobium, we have cloned and characterized the infection-related early nodulin gene MtENOD12 from Medicago truncatula. In situ hybridization experiments have shown that, within the indeterminate Medicago nodule, transcription of the MtENOD12 gene begins in cell layers of meristematic origin that lie ahead of the infection zone, suggesting that these cells are undergoing preparation for bacterial infection. Histochemical analysis of transgenic alfalfa plants that express an MtENOD12 promoter-beta-glucuronidase gene fusion has confirmed this result and further revealed that MtENOD12 gene transcription occurs as early as 3 to 6 hr following inoculation with R. meliloti in a zone of differentiating root epidermal cells which lies close to the growing root tip. It is likely that this transient, nodulation (nod) gene-dependent activation of the ENOD12 gene also corresponds to the preparation of the plant for bacterial infection. We anticipate that this extremely precocious response to Rhizobium will provide a valuable molecular marker for studying early signal exchange between the two symbiotic organisms.

Genetic analysis of a region of the Rhizobium meliloti pSym plasmid specifying catabolism of trigonelline, a secondary metabolite present in legumes

Genes controlling the catabolism of trigonelline, a secondary metabolite that is often present in legumes, are located on the pSym megaplasmid of Rhizobium meliloti. To investigate the role of bacterial trigonelline catabolism in the Rhizobium-legume symbiosis, we identified and characterized the R. meliloti RCR2011 genetic loci (trc) controlling trigonelline catabolism. Tn5-B20 mutagenesis showed that the trc region is a continuous DNA segment of 9 kb located 4 kb downstream of the nifAB and fdxN genes. Trc mutants fell into two classes according to their phenotype and location: (i) mutants carrying Tn5-B20 insertions in the right-hand part of the trc region were incapable of growing on trigonelline as the sole carbon and/or nitrogen source, and (ii) insertions in the left-hand part of the trc region resulted in delayed growth on trigonelline as the sole carbon and/or nitrogen source. No significant defect in nodule formation or nitrogen fixation was detected for mutants of either class. Screening of a set of R. meliloti strains from various geographical origins showed that all of these strains are able to catabolize trigonelline and show sequence homology between their megaplasmids and a trc probe.