Mutation or increased copy number of nodE has no effect on the spectrum of chitolipooligosaccharide nod factors made by Rhizobium leguminosarum bv. trifolii

Multimodal Spectroscopic Imaging of Pea Root Nodules to Assess the Nitrogen Fixation in the Presence of Biofertilizer Based on Nod-Factors

Multimodal spectroscopic imaging methods such as Matrix Assisted Laser Desorption/Ionization Mass Spectrometry Imaging (MALDI MSI), Fourier Transform Infrared spectroscopy (FT-IR) and Raman spectroscopy were used to monitor the changes in distribution and to determine semi quantitatively selected metabolites involved in nitrogen fixation in pea root nodules. These approaches were used to evaluate the effectiveness of nitrogen fixation by pea plants treated with biofertilizer preparations containing Nod factors. To assess the effectiveness of biofertilizer, the fresh and dry masses of plants were determined. The biofertilizer was shown to be effective in enhancing the growth of the pea plants. In case of metabolic changes, the biofertilizer caused a change in the apparent distribution of the leghaemoglobin from the edges of the nodule to its centre (the active zone of nodule). Moreover, the enhanced nitrogen fixation and presumably the accelerated maturation form of the nodules were observed with the use of a biofertilizer.

Nonspecific Symbiosis Between Sophora flavescens and Different Rhizobia

We explored the genetic basis of the promiscuous symbiosis of Sophora flavescens with diverse rhizobia. To determine the impact of Nod factors (NFs) on the symbiosis of S. flavescens, nodulation-related gene mutants of representative rhizobial strains were generated. Strains with mutations in common nodulation genes (nodC, nodM, and nodE) failed to nodulate S. flavescens, indicating that the promiscuous nodulation of this plant is strictly dependent on the basic NF structure. Mutations of the NF decoration genes nodH, nodS, nodZ, and noeI did not affect the nodulation of S. flavescens, but these mutations affected the nitrogen-fixation efficiency of nodules. Wild-type Bradyrhizobium diazoefficiens USDA110 cannot nodulate S. flavescens, but we obtained 14 Tn5 mutants of B. diazoefficiens that nodulated S. flavescens. This suggested that the mutations had disrupted a negative regulator that prevents nodulation of S. flavescens, leading to nonspecific nodulation. For Ensifer fredii CCBAU 45436 mutants, the minimal NF structure was sufficient for nodulation of soybean and S. flavescens. In summary, the mechanism of promiscuous symbiosis of S. flavescens with rhizobia might be related to its nonspecific recognition of NF structures, and the host specificity of rhizobia may also be controlled by currently unknown nodulation-related genes.

New insights into Nod factor biosynthesis: Analyses of chitooligomers and lipo-chitooligomers of Rhizobium sp. IRBG74 mutants

Soil-dwelling, nitrogen-fixing rhizobia signal their presence to legume hosts by secreting lipo-chitooligomers (LCOs) that are decorated with a variety of chemical substituents. It has long been assumed, but never empirically shown, that the LCO backbone is synthesized first by NodC, NodB, and NodA, followed by addition of one or more substituents by other Nod proteins. By analyzing a collection of in-frame deletion mutants of key nod genes in the bacterium Rhizobium sp. IRBG74 by mass spectrometry, we were able to shed light on the possible substitution order of LCO decorations, and we discovered that the prevailing view is probably erroneous. We found that most substituents could be transferred to a short chitin backbone prior to acylation by NodA, which is probably one of the last steps in LCO biosynthesis. The existence of substituted, short chitin oligomers offers new insights into symbiotic plant–microbe signaling.

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Rhizobia produce chemically substituted, short chitooligomers (COs).

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Deacetylation of the non-reducing GlcNAc is necessary for most substitutions.

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Acylation may be one of the last steps in the biosynthesis of rhizobial lipo-chitooligosaccharides (LCOs).

The production of species-specific highly unsaturated fatty acyl-containing LCOs from Rhizobium leguminosarum bv. trifolii is stringently regulated by nodD and involves the nodRL genes

A proportion of the Nod factors of some Rhizobium leguminosarum bv. trifolii strains is characterized by the presence of highly unsaturated fatty acyl chains containing trans double bonds in conjugation with the carbonyl group of the glycan oligosaccharide backbone. These fatty acyl chains are C18:3, C20:3, C18:4, or C20:4 and have UV-absorption maxima at 303 and 330 nm. These Nod factors are presumed to be important for host-specific nodulation on clover species. However, in wild-type R. leguminosarum bv. trifolii ANU843, Nod factors with these characteristic acyl chains were not observed using standard growth conditions. They were observed only when nod genes were present in multiple copies or when transcription was artificially increased to higher levels by introduction of extra copies of the transcriptional regulator gene nodD. In a screen for the genetic requirements for production of the Nod factors with these characteristic structures, it was found that the region downstream of nodF and nodE is essential for the presence of highly unsaturated fatty acyl moieties. Mu-lacZ insertion in this region produced a mutant that did not produce detectable levels of the highly unsaturated fatty acyl-bearing Nod factors. The Mu-lacZ insertion was translationally fused to a putative new gene, designated nodR, in the nodE-nodL intergenic region; however, no predicted function for the putative NodR protein has been obtained from database homology searches. In a set of 12 wild-type strains of R. leguminosarum by. trifolii originating from various geographical regions that were analyzed for the presence of a nodR-like gene, it was found that seven strains carry a homologous NodR open reading frame. Taken together, our results suggest a tightly controlled regulation of nod genes, in which we propose that it is the balance of transcriptional levels of nodFE and the nodRL genes that is critical for determining the presence of highly unsaturated fatty acyl moieties in the Nod factors produced by R. leguminosarum bv. trifolii.

Promiscuity of hosting nitrogen fixation in rice: an overview from the legume perspective

The subject area of this review provides extraordinary challenges and opportunities. The challenges relate to the fact that the integration of various fields such as microbiology, biochemistry, plant physiology, eukaryotic as well as bacterial genetics, and applied plant sciences are required to assess the disposition of rice, an alien host, for establishing such a unique phenomenon as biological nitrogen fixation. The opportunities signify that, if successful, the breakthrough will have a significant impact on the global economy and will help improve the environment. This review highlights the literature related to the area of legume-rhizobia interactions, particularly those aspects whose understanding is of particular interest in the perspective of rice. This review also discusses the progress achieved so far in this area of rice research and the possibility of built-in nitrogen fixation in rice in the future. However, it is to be borne in mind that such research does not ensure any success at this point. It provides a unique opportunity to broaden our knowledge and understanding about many aspects of plant growth regulation in general.

Nod factor structures, responses, and perception during initiation of nodule development

The onset of nodule development, the result of rhizobia-legume symbioses, is determined by the exchange of chemical compounds between microsymbiont and leguminous host plant. Lipo-chitooligosaccharidic nodulation (Nod) factors, secreted by rhizobia, belong to these signal molecules. Nod factors consist of an acylated chitin oligomeric backbone with various substitutions at the (non)reducing-terminal and/or nonterminal residues. They induce the formation and deformation of root hairs, intra- and extracellular alkalinization, membrane potential depolarization, changes in ion fluxes, early nodulin gene expression, and formation of nodule primordia. Nod factors play a key role during nodule initiation and act at nano- to picomolar concentrations. A correct chemical structure is required for induction of a particular plant response, suggesting that Nod factor-receptor interaction(s) precede(s) a Nod factor-induced signal transduction cascade. Current data on Nod factor structures and Nod factor-induced responses are highlighted as well as recent advances in the characterization of proteins, possibly involved in recognition of Nod factors by the host plant.

Erosion of root epidermal cell walls by Rhizobium polysaccharide-degrading enzymes as related to primary host infection in the Rhizobium-legume symbiosis

A central event of the infection process in the Rhizobium-legume symbiosis is the modification of the host cell wall barrier to form a portal of entry large enough for bacterial penetration. Transmission electron microscopy (TEM) indicates that rhizobia enter the legume root hair through a completely eroded hole that is slightly larger than the bacterial cell and is presumably created by localized enzymatic hydrolysis of the host cell wall. In this study, we have used microscopy and enzymology to further clarify how rhizobia modify root epidermal cell walls to shed new light on the mechanism of primary host infection in the Rhizobium-legume symbiosis. Quantitative scanning electron microscopy indicated that the incidence of highly localized, partially eroded pits on legume root epidermal walls that follow the contour of the rhizobial cell was higher in host than in nonhost legume combinations, was inhibited by high nitrate supply, and was not induced by immobilized wild-type chitolipooligosaccharide Nod factors reversibly adsorbed to latex beads. TEM examination of these partially eroded, epidermal pits indicated that the amorphous, noncrystalline portions of the wall were disrupted, whereas the crystalline portions remained ultrastructurally intact. Further studies using phase-contrast and polarized light microscopy indicated that (i) the structural integrity of clover root hair walls is dependent on wall polymers that are valid substrates for cell-bound polysaccharide-degrading enzymes from rhizobia, (ii) the major site where these rhizobial enzymes can completely erode the root hair wall is highly localized at the isotropic, noncrystalline apex of the root hair tip, and (iii) the degradability of clover root hair walls by rhizobial polysaccharide-degrading enzymes is enhanced by modifications induced during growth in the presence of chitolipooligosaccharide Nod factors from wild-type clover rhizobia. The results suggest a complementary role of rhizobial cell-bound glycanases and chitolipooligosaccharides in creating the localized portals of entry for successful primary host infection.

Growth temperature regulation of host-specific modifications of rhizobial lipo-chitin oligosaccharides: the function of nodX is temperature regulated

Lipo-chitin oligosaccharides (LCOs) are usually produced and isolated for structural analysis from bacteria cultured under laboratory rather than field conditions. We have studied the influence of bacterial growth temperature on the LCO structures produced by different Rhizobium leguminosarum strains, using thin-layer chromatographic, high-performance liquid chromatographic, and mass spectrometric analyses. Wild-type R. leguminosarum bv. viciae A1 was shown to produce larger relative amounts of nodX-mediated, acetylated LCOs at 12 degrees C than at 28 degrees C, indicating that the activity of nodX (a gene encoding an LCO O-acetyl transferase) is temperature dependent. Interestingly, symbiotic resistance genes sym1 and sym2 found in primitive pea cultivars are also temperature sensitive, only being active at low temperatures, at which they block nodulation by R. leguminosarum bv. viciae strains lacking nodX. We therefore propose that the gene-for-gene relationship between plant and bacterium has a temperature-sensitive mechanism as an adaptation to environmental conditions. An R. leguminosarum bv. trifolii strain was also shown to produce larger relative amounts of nodX-mediated, acetylated LCOs at 12 degrees C than at 28 degrees C. The major components synthesized by the two strains are produced at both temperatures but in different relative amounts, while some minor components are only produced at one of the two temperatures.

Carbohydrate determinants of Rhizobium-legume symbioses

Rhizobium is a genus of symbiotic nitrogen-fixing soil bacteria that induces the formation of root nodules on leguminous plants and, as such, has been the subject of considerable research attention. Much of this work was initiated in response to the question 'how does recognition occur between free living rhizobial bacteria in the soil and potential host legumes?' The answer to this question has been shown to involve both cell-surface carbohydrates on the external face of the bacteria and secreted extracellular signal oligosaccharides. This review will focus on the structure, function, and biosynthesis of two of these components--the host-specific nodule-promoting signals known as Nod(ulation) factors and the rhizobial lipopolysaccharides.

Comparison of characteristics of the nodX genes from various Rhizobium leguminosarum strains

We have analyzed the nucleotide sequences of the nodX genes from two strains of Rhizobium leguminosarum bv. viciae able to nodulate Afghan peas (strains A1 and Himalaya) and from two strains of R. leguminosarum bv. trifolii (ANU843 and CSF). The nodX genes of strains A1 and ANU843 were shown to be functional for the induction of nodules on Afghan peas. To analyze the cause of phenotypic differences of strain A1 and strain TOM we have studied the composition of the lipochitin-oligosaccharides (LCOs) produced by strain A1 after induction by the flavonoid naringenin or various pea root exudates. The structural analysis of the LCOs by mass spectrometry revealed that strain A1 synthesizes a family of at least 23 different LCOs. The use of exudates instead of naringenin resulted only in quantitative differences in the ratios of various LCOs produced.

Mass spectrometric analysis of lipo-chitin oligosaccharides--signal molecules mediating the host-specific legume-rhizobium symbiosis

Lipo-chitin oligosaccharides (LCOs) are novel bacterial glycolipid signal molecules that mediate the species--specific symbiosis between rhizobial bacteria and leguminous plants. Nodulation of the legume roots and nitrogen-fixation in the resulting nodules by Rhizobia is controlled by the bacterial nodulation genes that encode the LCO biosynthetic enzymes. The length of the LCO chitin backbone, the length and degree of unsaturation of the fatty acyl chain attached to it, and the combination of different chemical substituents on the reducing- and nonreducing-terminal residues all contribute to the species--specificity of the signal. LCOs are bioactive in the nanomolar and subnanomolar concentration range and are produced as heterogeneous mixtures, making determination of their structures a difficult task, most successfully approached by the application of modern mass spectrometric methods in combination with specific chemical treatments aimed at identifying specific chemical moieties. This review presents an overview of these methods as they are being used for the structural elucidation of LCOs, and discusses the role of structural diversity in mediating species-specificity.

New Rhizobium leguminosarum flavonoid-induced proteins revealed by proteome analysis of differentially displayed proteins

Proteome analysis was used to establish the first two-dimensional protein map of Rhizobium. R. leguminosarum bv. trifolii strain ANU843 was grown in defined medium in the presence and absence of the flavonoid 7,4'-dihydroxyflavone. Over 1,700 constitutive proteins were resolved, representing about 30% of the estimated genomic output. Proteome analysis of flavonoid-treated cells was done to reveal differentially displayed proteins. The results showed that while the global expression pattern of proteins was largely unaltered by the treatment, four inducible proteins were observed. The four inducible proteins and 20 constitutively expressed proteins were subjected to sequence analysis to provide internal standards for the construction of a two-dimensional Rhizobium protein data base. The identity of 12 proteins, including NodE and NodB, was established. NodE was present throughout the growth of the cells but was diminished in amount in stationary phase cells whereas NodB was not detected in the later stages of growth. Two of the induced proteins sequenced did not match any known nodulation gene product, with one of these being present in mid-late log and stationary phase cells and possessing four consecutive His residues at the N-terminal sequencing was successful with 100 to 200 fmol of protein. Proteome analysis provides a sensitive new tool to examine plant-microbe interactions.

Signalling events elicited in plants by defined oligosaccharide structures

Developments during the past year have confirmed that several classes of oligosaccharides are able to activate the plant cell machinery, leading either to defence reactions or to plant developmental processes. Both fungal and plant cell walls contain molecules that elicit plant defence reactions; however, most of the studies focus on the activities induced by lipochito-oligomers (LCOs, or Nod factors) produced by bacteria which trigger plant infection and nodule formation (organogenesis). LCOs can be described as growth regulators of plants in general as they also induce protoplast cell divisions of tobacco plant (a nonlegume) at femtomolar concentrations. Recognition of the appropriate symbiotic bacteria by legumes is mediated by the structure of Nod factors, but the structural determinants involved in this recognition process are not always clearly understood. Although specific substitutions of the oligochitin backbone by several chemical groups are involved, it seems that some host range variations of the bacteria can only be explained by small variations in the hydrophobic balance between both ends of the molecule.

Rhizobium leguminosarum bv. trifolii produces lipo-chitin oligosaccharides with nodE-dependent highly unsaturated fatty acyl moieties. An electrospray ionization and collision-induced dissociation tandem mass spectrometric study

The lipo-chitin oligosaccharides (LCO) or nodulation factors synthesized by Rhizobium leguminosarum bv. trifolii were analyzed using positive mode fast atom bombardment and positive and negative mode electrospray ionization mass spectrometry. From their mass spectrometric behavior it is clearly possible to distinguish between the [M + Na]+ pseudomolecular ion of the nodE-independent molecule IV(C18:1,Ac) and the [M + H]+ pseudomolecular ion of the nodE-dependent molecule IV(C20:4,Ac), although they both have the same mass value. The results unequivocally show that the bacterial strain investigated produces nodE-dependent LCOs with highly unsaturated fatty acyl moieties. We further demonstrate that the interpretation of the mass spectrometric data by Philip-Hollingsworth et al. (Philip-Hollingsworth, S., Orgambide, G. G., Bradford, J. J., Smith, D. K., Hollingsworth, R. I., and Dazzo, F. B. (1995) J. Biol. Chem. 270, 20968) is incorrect and that their data do not contradict our hypothesis that the nodE gene determines the host specificity of R. leguminosarum bv. trifolii.

Modulation of development, growth dynamics, wall crystallinity, and infection sites in white clover root hairs by membrane chitolipooligosaccharides from Rhizobium leguminosarum biovar trifolii

We used bright-field, time-lapse video, cross-polarized, phase-contrast, and fluorescence microscopies to examine the influence of isolated chitolipooligosaccharides (CLOSs) from wild-type Rhizobium leguminosarum bv. trifolii on development of white clover root hairs, and the role of these bioactive glycolipids in primary host infection. CLOS action caused a threefold increase in the differentiation of root epidermal cells into root hairs. At maturity, root hairs were significantly longer because of an extended period of active elongation without a change in the elongation rate itself. Time-series image analysis showed that the morphological basis of CLOS-induced root hair deformation is a redirection of tip growth displaced from the medial axis as previously predicted. Further studies showed several newly described infection-related root hair responses to CLOSs, including the localized disruption of the normal crystallinity in cell wall architecture and the induction of new infection sites. The application of CLOS also enabled a NodC- mutant of R. leguminosarum bv. trifolii to progress further in the infection process by inducing bright refractile spot modifications of the deformed root hair walls. However, CLOSs did not rescue the ability of the NodC- mutant to induce marked curlings or infection threads within root hairs. These results indicate that CLOS Nod factors elicit several host responses that modulate the growth dynamics and symbiont infectibility of white clover root hairs but that CLOSs alone are not sufficient to permit successful entry of the bacteria into root hairs during primary host infection in the Rhizobium-clover symbiosis.

Glycoconjugate and lipid components ofRhizobium"hedysari" IS123, a root-nodule symbiont of the stress-tolerant legumeHedysarum coronarium

We have examined the diversity of glycoconjugates and cellular lipids of Rhizobium "hedysari" IS123, a bacterial symbiont that specifically nodulates the drought-tolerant forage legume Hedysarum coronarium. IS123 develops a complete capsule consisting of a loose fibrillar network of ruthenium-red-staining acidic polymers and produces two different exopolysaccharides (EPS). EPS-A contains glucose, galactose, mannose, and a noncarbohydrate substitution tentatively identified as a lactyl ester. The composition of EPS-B, which includes glucose and galactose, as well as O-acetyl, pyruvyl, and succinyl substituents, is very similar to that of the EPS-II described in Rhizobium meliloti. IS123 also makes an O-acetylated heterooligosaccharide and unsubstituted β-1,2-glucans. The cellular fatty acid composition of IS123 is dominated by 18:1 and also includes 14:0, 16:0, 16:1, 3OH-16:0, 17:0Δ, 18:0, 3OH-18:0, and 19:0Δ. Phospholipids of IS123 include phosphatidylethanolamine, N-methyl phosphatidylethanolamine, N,N-dimethyl phosphatidylethanolamine, phosphatidylcholine, and phosphatidylglycerol.Key words: Rhizobium, capsule, exopolysaccharides, oligosaccharides, lipids.