Nucleotide sequence and putative regulatory elements of a nodule-development-specific member of the soybean (Glycine max) chalcone synthase multigene family, Gmchs 7.
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Acyltransferases↗
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Ubiquitin-mediated proteolysis. To be in the right place at the right moment during nodule development.
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Cell Cycle↗
Regulators and regulation of legume root nodule development.
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Carbohydrate Sequence↗
[Radiological case: a developing nodule of the right lower lung].
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Aged↗
Regulation of nodulation in the absence of N2 is different in actinorhizal plants with different infection pathways.
Root nodulation in actinorhizal plants, like Discaria trinervis and Alnus incana, is subject to feedback regulatory mechanisms that control infection by Frankia and nodule development. Nodule pattern in the root system is controlled by an autoregulatory process that is induced soon after inoculation with Frankia. The final number of nodules, as well as nodule biomass in relation to plant biomass, are both modulated by a second mechanism which seems to be related to the N status of the plant. Mature nodules are, in part, involved in the latter process, since nodule excision from the root system releases the inhibition of infection and nodule development. To study the effect of N(2) fixation in this process, nodulated D. trinervis and A. incana plants were incubated under a N(2)-free atmosphere. Discaria trinervis is an intercellularly infected species while A. incana is infected intracellularly, via root hairs. Both symbioses responded with an increment in nodule biomass, but with different strategies. Discaria trinervis increased the biomass of existing nodules without significant development of new nodules, while in A. incana nodule biomass increased due to the development of nodules from new infections, but also from the release of arrested infections. It appears that in D. trinervis nodules there is an additional source for inhibition of new infections and nodule development that is independent of N(2) fixation and nitrogen assimilation. It is proposed here that the intercellular Frankia filaments commonly present in the D. trinervis nodule apex, is the origin for the autoregulatory signals that sustain the blockage of initiated nodule primordia and prevent new roots from infections. When turning to A. incana plants, it seems likely that this signal is related to the early autoregulation of nodulation in A. incana seedlings and is no longer present in mature nodules. Thus, actinorhizal symbioses belonging to relatively distant phylogenetic groups and displaying different infection pathways, show different feedback regulatory processes that control root nodulation by Frankia.
Alnus↗
Identification of new protein species among 33 different small GTP-binding proteins encoded by cDNAs from Lotus japonicus, and expression of corresponding mRNAs in developing root nodules.
In this study, 266 cDNA clones were isolated from a cDNA library made from mRNA of three-week-old root nodules of Lotus japonicus, employing a degenerate oligonucleotide probe that corresponds to a conserved region of small GTP-binding (SMG) proteins. The clones were sorted into groups by cross hybridization and 3' sequencing, and 33 contigs were sequenced in an orderly fashion. Twenty-seven complete and six incomplete protein structures were deduced, which represent three subfamilies of the superfamily of signal transducing GTP-binding proteins. The 33 proteins are divided into nine subclasses, of which seven belong to the Ypt/Rab subfamily, one subclass represents the Rho/Rac subfamily, and one subclass represents the Ran subfamily of small GTP-binding proteins. The protein sequences were compared with related proteins from other plants, from mammals and other species, and discussed with respect to structure and function in different cellular processes. It is apparent that the number of genes encoding SMG proteins in plants must be quite large, since the large number of subclasses found in other eukaryotes is not fully represented in our analysis. Transcription patterns through root nodule development were analysed for 27 of the 33 cDNAs. Differential expression patterns may reflect whether the coded gene product is of importance for organ development. Most mRNAs appear to be constitutively expressed; however, a few unique mRNAs representing the subclasses Rab1, Rab2, Rab5, Rab7 and Rac show elevated levels in root nodules, and certain Rab7, Rab8 and Rab11 species are enriched in aerial parts of the plant. This suggests that most small GTPases have household functions, whereas a few may be required for specialized activities that are important for specialized cells.
Amino Acid Sequence↗
Patterns of pectin methylesterase transcripts in developing stem nodules of Sesbania rostrata.
Differential display was applied to the early stages of the interaction between the tropical legume Sesbania rostrata and its microsymbiont Azorhizobium caulinodans ORS571. An upregulated clone that is similar to pectin methylesterase-encoding genes was isolated (Srpmel). The full-length sequence of Srpme1 was used to localize PME transcripts in situ during S. rostrata stem-nodule development. Several expression patterns were distinguished, hinting at general roles in vascular tissue development and cell division or expansion and at symbiosis-specific functions, such as uninfected cell differentiation.
Amino Acid Sequence↗
The Medicago species A2-type cyclin is auxin regulated and involved in meristem formation but dispensable for endoreduplication-associated developmental programs.
Phytohormones as well as temporal and spatial regulation of the cell cycle play a key role in plant development. Here, we investigated the function and regulation of an alfalfa (Medicago sativa) A2-type cyclin in three distinct root developmental programs: in primary and secondary root development, nodule development, and nematode-elicited gall formation. Using transgenic plants carrying the Medsa;cycA2;2 promoter-beta-glucuronidase gene fusion, in combination with other techniques, cycA2;2 expression was localized in meristems and proliferating cells in the lateral root and nodule primordia. Rapid induction of cycA2;2 by Nod factors demonstrated that this gene is implicated in cell cycle activation of differentiated cells developing to nodule primordia. Surprisingly, cycA2;2 was repressed in the endoreduplicating, division-arrested cells both during nodule development and formation of giant cells in nematode-induced galls, indicating that CycA2;2 was dispensable for S-phase in endoreduplication cycles. Overexpression of cycA2;2 in transgenic plants corresponded to wild type protein levels and had no apparent phenotype. In contrast, antisense expression of cycA2;2 halted regeneration of somatic embryos, suggesting a role for CycA2;2 in the formation or activity of apical meristems. Expression of cycA2;2 was up-regulated by auxins, as expected from the presence of auxin response elements in the promoter. Moreover, auxin also affected the spatial expression pattern of this cyclin by shifting the cycA2;2 expression from the phloem to the xylem poles.
Animals↗
A ClC chloride channel homolog and ornithine-containing membrane lipids of Rhizobium tropici CIAT899 are involved in symbiotic efficiency and acid tolerance.
Rhizobium tropici CIAT899 is highly tolerant to several environmental stresses and is a good competitor for nodule occupancy of common bean plants in acid soils. Random transposon mutagenesis was performed to identify novel genes of this strain involved in symbiosis and stress tolerance. Here, we present a genetic analysis of the locus disrupted by the Tn5 insertion in mutant 899-PV9, which lead to the discovery of sycA, a homolog of the ClC family of chloride channels and Cl-/H+ exchange transporters. A nonpolar deletion in this gene caused serious deficiencies in nodule development, nodulation competitiveness, and N2 fixation on Phaseolus vulgaris plants, probably due to its reduced ability to invade plant cells and to form stable symbiosomes, as judged by electron transmission microscopy. A second gene (olsC), found downstream of sycA, is homologous to aspartyl/asparaginyl beta-hydroxylases and modifies two species of ornithine-containing lipids in vivo, presumably by hydroxylation at a still-unknown position. A mutant carrying a nonpolar deletion in olsC is symbiotically defective, whereas overexpressed OlsC in the complemented strain provokes an acid-sensitive phenotype. This is the first report of a ClC homolog being essential for the establishment of a fully developed N2-fixing root nodule symbiosis and of a putative beta-hydroxylase that modifies ornithine-containing membrane lipids of R. tropici CIAT899, which, in turn, are contributing to symbiotic performance and acid tolerance.
Amino Acid Sequence↗
Early Events in the Infection of Soybean (Glycine max L. Merr) by Rhizobium japonicum: I. LOCALIZATION OF INFECTIBLE ROOT CELLS.
The infectible cells of soybean roots appear to be located at any given time just above the zone of root elongation and just below the position of the smallest emergent root hairs. The location of infectible cells on the primary root at the time of inoculation was inferred from the position of subsequent nodule development, correcting for displacement of epidermal cells due to root elongation. Marks were made on the seedling growth pouches at the time of inoculation to indicate the position of the root tip and the zones of root hair development. Virtually all of the seedlings developed nodules on the primary root above the marks made at the root tips at the time of inoculation. None of the plants formed nodules on the root where mature root hairs were present at the time of inoculation. These results and profiles of nodulation frequency indicate that the location of infectible cells is developmentally restricted. When inoculations were delayed for intervals of 1 to 4 hours after marking the positions of the root tips, progressively fewer nodules were formed above the root tip marks, and the uppermost of these nodules were formed at progressively shorter distances above the marks. These results indicate that the infectibility of given host cells is a transient property that appears and then is lost within a few hours. The results also indicate that host responses leading to infection and nodulation are triggered or initiated in less than 2 hours after inoculation. The extent of nodulation above the root tip mark increased in proportion to the logarithm of the number of bacteria in the inoculum.
Journal Article↗
Nodulation inhibition by Rhizobium leguminosarum multicopy nodABC genes and analysis of early stages of plant infection.
During analysis of early events in the infection and nodulation of Vicia hirsuta roots inoculated with normal and mutant strains of Rhizobium leguminosarum and strains containing cloned nodulation (nod) genes, a number of novel observations were made. (i) Alternating zones of curled and straight root hairs were seen on roots of V. hirsuta inoculated with the wild-type strain of R. leguminosarum. This phasing of root hair curling was not seen if plants were grown under continuous light or continuous dark conditions. (ii) Reduced nodulation and delayed nodule initiation was observed with a strain carrying a Tn5 mutation in the nodE gene. In addition the phased root hair curling was absent, and root hair curling was observed along the length of the root. (iii) The nodABC genes cloned on a multicopy plasmid in a wild-type strain inhibited nodulation but induced a continuous root hair curling response. Those few nodules that eventually formed were found to contain bacteria which had lost the plasmid carrying the nodABC genes. (iv) With a strain of Rhizobium cured of its indigenous symbiotic plasmid, but containing the cloned nodABCDEF genes, continuous root hair curling on V. hirsuta was observed. However, no infection threads were observed, and surprisingly, it did appear that initial stages of nodule development occurred. Observations of thin sections of these early developing nodules indicated that early nodule meristematic divisions may have occurred but that no bacteria were found within the nodules and no infection threads were observed either within the nodule bumps or within any of the root hairs. It was concluded that for normal infections to occur, precise regulation of the nod genes is required and that overexpression of the root hair curling genes inhibits the normal infection process.
Bacterial Proteins↗
Five Nodulation Mutants of White Sweetclover (Melilotus alba Desr.) Exhibit Distinct Phenotypes Blocked at Root Hair Curling, Infection Thread Development, and Nodule Organogenesis.
In an effort to obtain a developmental sequence of mutations in the Rhizobium-legume interaction within a single legume species, we have characterized the early events of nodule development in 10 nodulation mutants of sweetclover, Melilotus alba Desr. cv U389, representing five genetic loci. Both seed and root exudates from all of the sweetclover mutants induced expression of the nod genes of Rhizobium meliloti. Mutants in three loci were blocked in the early stages of root hair curling. Of these, a mutant in the sym-3 locus exhibited root hair deformations in response to inoculation with R. meliloti but produced no nodules or emerging nodule primordia, suggesting a blockage in the signal transduction events leading to nodule organogenesis. In contrast, mutants in both the sym-1 and sym-5 loci formed ineffective nodules in response to inoculation but differed slightly in the type of root hair response observed. None of these three early mutants formed infection threads. Infection threads were observed in mutant sym-2 as well as in ineffective nodules. Mutant sym-4 also formed infection threads but lacked nodules. The phenotypes observed for mutants from these five loci suggest that a secondary receptor or signal produced by the plant is required for nodule development.
Journal Article↗
Rhizobial exopolysaccharides: genetic control and symbiotic functions.
Specific complex interactions between soil bacteria belonging to Rhizobium, Sinorhizobium, Mesorhizobium, Phylorhizobium, Bradyrhizobium and Azorhizobium commonly known as rhizobia, and their host leguminous plants result in development of root nodules. Nodules are new organs that consist mainly of plant cells infected with bacteroids that provide the host plant with fixed nitrogen. Proper nodule development requires the synthesis and perception of signal molecules such as lipochitooligosaccharides, called Nod factors that are important for induction of nodule development. Bacterial surface polysaccharides are also crucial for establishment of successful symbiosis with legumes. Sugar polymers of rhizobia are composed of a number of different polysaccharides, such as lipopolysaccharides (LPS), capsular polysaccharides (CPS or K-antigens), neutral beta-1, 2-glucans and acidic extracellular polysaccharides (EPS). Despite extensive research, the molecular function of the surface polysaccharides in symbiosis remains unclear. This review focuses on exopolysaccharides that are especially important for the invasion that leads to formation of indetermined (with persistent meristem) type of nodules on legumes such as clover, vetch, peas or alfalfa. The significance of EPS synthesis in symbiotic interactions of Rhizobium leguminosarum with clover is especially noticed. Accumulating data suggest that exopolysaccharides may be involved in invasion and nodule development, bacterial release from infection threads, bacteroid development, suppression of plant defense response and protection against plant antimicrobial compounds. Rhizobial exopolysaccharides are species-specific heteropolysaccharide polymers composed of common sugars that are substituted with non-carbohydrate residues. Synthesis of repeating units of exopolysaccharide, their modification, polymerization and export to the cell surface is controlled by clusters of genes, named exo/exs, exp or pss that are localized on rhizobial megaplasmids or chromosome. The function of these genes was identified by isolation and characterization of several mutants disabled in exopolysaccharide synthesis. The effect of exopolysaccharide deficiency on nodule development has been extensively studied. Production of exopolysaccharides is influenced by a complex network of environmental factors such as phosphate, nitrogen or sulphur. There is a strong suggestion that production of a variety of symbiotically active polysaccharides may allow rhizobial strains to adapt to changing environmental conditions and interact efficiently with legumes.
Journal Article↗
Flavonoids induce temporal shifts in gene-expression of nod-box controlled loci in Rhizobium sp. NGR234.
Rhizobia, soil bacteria of the Rhizobiales, enter the roots of homologous legumes, where they induce the formation of nitrogen-fixing nodules. Signals emanating from both symbiotic partners control nodule development. Efficient nodulation requires precise, temporal regulation of symbiotic genes. Roots continuously release flavonoids that interact with transcriptional activators of the LysR family. NodD proteins, which are members of this family, act both as sensors of the environment and modulate the expression of genes preceded by conserved promoter sequences called nod-boxes. The symbiotic plasmid of the broad host-range Rhizobium sp. NGR234 caries 19 nod-boxes (NB1 to NB19), all of which were cloned upstream of a lacZ-reporter gene. A flavonoid, daidzein was able to induce 18 of the 19 nod-boxes in a NodD1-dependent manner. Interestingly, induction of four nod-boxes (NB6, NB15, NB16 and NB17) is highly dependent on NodD2 and was delayed in comparison with the others. In turn, NodD2 is involved in the repression of the NB8 nodABCIJnolOnoeI operon. Activation of transcription of nodD2 is also dependent on flavonoids despite the absence of a nod-box like sequence in the upstream promoter region. Mutational analysis showed that syrM 2 (another member of the LysR family), which is controlled by NB19, is also necessary for expression of nodD 2. Thus, NodD1, NodD2 and SyrM2 co-modulate a flavonoid-inducible regulatory cascade that coordinates the expression of symbiotic genes with nodule development.
Base Sequence↗
Rhizobium leguminosarum has two glucosamine synthases, GlmS and NodM, required for nodulation and development of nitrogen-fixing nodules.
The Rhizobium leguminosarum nodM gene product shows strong homology to the Escherichia coli glmS gene product that catalyses the formation of glucosamine 6-P from fructose 6-P and glutamine. DNA hybridization with nodM indicated that, in addition to nodM on the symbiotic plasmid, another homologous gene was present elsewhere in the R. leguminosarum genome. A glucosamine-requiring mutant was isolated and its auxotrophy could be corrected by two different genetic loci. It could grow without glucosamine when the nodM gene on the symbiotic plasmid was induced or if the cloned nodM gene was expressed from a vector promoter. Alternatively, it could be complemented by a second fragment of R. leguminosarum DNA that carries a region homologous to E. coli glmS. Biochemical assays of glucosamine 6-P formation confirmed that the two R. leguminosarum genes nodM and glmS have interchangeable functions. No nodulation of peas or vetch was observed with a double nodM glmS mutant, and this block occurred at a very early stage since no root-hair deformation or infection threads were seen. Nodulation and root-hair deformation did occur with either the nodM or the glmS mutant, showing that the gene products of either of these genes can be involved in the formation of the lipo-oligosaccharide nodulation signal. However, the glmS mutant formed nodules that had greatly reduced nitrogen fixation. Constitutive expression of nodM restored nitrogen fixation to the glmS mutant. Therefore the reduced nitrogen fixation probably occurs because glmS is absent and nodM is not normally expressed in nodules and, in the absence of glucosamine precursors, normal bacteroid maturation is blocked.
Cloning, Molecular↗
Recovery of development and functionality of nodules and plant growth in salt-stressed Pisum sativum--Rhizobium leguminosarum symbiosis by boron and calcium.
Nodules developed in Pisum sativum L. cv. Argona inoculated with Rhizobium leguminosarum bv. viciae 3841 and growing under saline conditions (75 mmol/L NaCl) are non functional and had abnormal structure. The infected cells contained a low amount of endophytic bacteria, compared to treatments without salt. Addition of B (up to 55.8 micromol/L) and Ca2+ (up to 2.72 mmol/L) increased bacterial population of host plant cells in salt-stressed nodules. Furthermore, symbiosomes developed inside the nodules from salt treated plants presented a degraded peribacteroid membrane. This effect was also prevented by combined addition of B and Ca2+. Given the importance of both nutrients in cell wall structure, the pectin fraction was studied by electron microscopy and immunological methods. Salt stress produced cells with walls dramatically altered or even degraded in several zones. Pectin polysaccharides, detected by JIM 5 monoclonal antibody, increased in cells under salinity. These effects resembled typical effects of B-deficiency reactions in cell walls, and the increase of both Ca2+ and especially B also prevented these alterations.
Boron↗
HAR1 mediates systemic regulation of symbiotic organ development.
Symbiotic root nodules are beneficial to leguminous host plants; however, excessive nodulation damages the host because it interferes with the distribution of nutrients in the plant. To keep a steady balance, the nodulation programme is regulated systemically in leguminous hosts. Leguminous mutants that have lost this ability display a hypernodulating phenotype. Through the use of reciprocal and self-grafting studies using Lotus japonicus hypernodulating mutants, har1 (also known as sym78), we show that the shoot genotype is responsible for the negative regulation of nodule development. A map-based cloning strategy revealed that HAR1 encodes a protein with a relative molecular mass of 108,000, which contains 21 leucine-rich repeats, a single transmembrane domain and serine/threonine kinase domains. The har1 mutant phenotype was rescued by transfection of the HAR1 gene. In a comparison of Arabidopsis receptor-like kinases, HAR1 showed the highest level of similarity with CLAVATA1 (CLV1). CLV1 negatively regulates formation of the shoot and floral meristems through cell-cell communication involving the CLV3 peptide. Identification of hypernodulation genes thus indicates that genes in leguminous plants bearing a close resemblance to CLV1 regulate nodule development systemically, by means of organ-organ communication.
Arabidopsis Proteins↗
The Rhizobium-legume symbiosis: plant morphogenesis in a nodule.
Development of the legume root nodule can be divided conceptually into two parallel processes. On the one hand, there is the induction of a nodule meristem and the progressive differentiation of specialised cells and tissues. On the other hand, there is cell and tissue invasion by Rhizobium, which leads ultimately to the differentiation of intracellular bacteroids as specialised nitrogen-fixing endosymbionts. The early stages of plant-microbe communication seem to be mostly mediated by the exchange of soluble, diffusible signal molecules: flavonoid compounds are secreted by plant roots, and chitin-like lipooligosaccharides are secreted by rhizobia. Further development of the nodule structure may involve the interplay of fairly conventional plant growth regulators. Direct physical contact between the cell surfaces of the symbionts also plays a prominent role in the process of tissue and cell invasion.
Extracellular Matrix↗