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PubMed2003

Characterization of Nops, nodulation outer proteins, secreted via the type III secretion system of NGR234.

The nitrogen-fixing symbiotic bacterium Rhizobium species NGR234 secretes, via a type III secretion system (TTSS), proteins called Nops (nodulation outer proteins). Abolition of TTSS-dependent protein secretion has either no effect or leads to a change in the number of nodules on selected plants. More dramatically, Nops impair nodule development on Crotalaria juncea roots, resulting in the formation of nonfixing pseudonodules. A double mutation of nopX and nopL, which code for two previously identified secreted proteins, leads to a phenotype on Pachyrhizus tuberosus differing from that of a mutant in which the TTSS is not functional. Use of antibodies and a modification of the purification protocol revealed that NGR234 secretes additional proteins in a TTSS-dependent manner. One of them was identified as NopA, a small 7-kDa protein. Single mutations in nopX and nopL were also generated to assess the involvement of each Nop in protein secretion and nodule formation. Mutation of nopX had little effect on NopL and NopA secretion but greatly affected the interaction of NGR234 with many plant hosts tested. NopL was not necessary for the secretion of any Nops but was required for efficient nodulation of some plant species. NopL may thus act as an effector protein whose recognition is dependent upon the hosts' genetic background.

Amino Acid Sequence↗
PubMed1986

Alternative Path Mediated ATP Synthesis in Roots of Pisum sativum upon Nitrogen Supply.

Changes in the efficiency of root respiration were examined on intact plants of Pisum sativum L. cv Rondo after addition of nitrate or ammonium to the culture solutions. Nitrate was absorbed immediately after addition and elicited a respiratory rise (O(2)-uptake as well as CO(2)-production) to 160% at most. This occurred both in roots of plants fixing N(2) and in those of non-nodulated plants pregrown for 1 or 2 weeks on a nitrogen-free culture solution. In older plants, used after 2 weeks of N-free growth, the full capacity of the cytochrome path was engaged in root respiration. This was demonstrated by the absence of an effect of the uncoupler carbonylcyanide m-chlorophenylhydrazone in the presence of 25 millimolar salicylhydroxamate, an inhibitor of the alternative path. In these plants more than 90% of the nitrate-induced stimulation of root respiration was salicylhydroxamate-sensitive. In young plants, used after 1 week of N-free growth, the cytochrome path was not saturated. Its activity increased instantaneously at the expense of alternative path activity, which initially dropped to zero and subsequently increased to 160% of the control 7 hours after nitrate supply. The rate of photosynthesis rose to 120% of the control, but not before 1 hour after nitrate supply, suggesting that the stimulation of root respiration was not due to a higher rate of photosynthesis. Experiments with plants grown with a split-root system showed that respiration rate and alternative path activity only increased in the root halves exposed to nitrogen. Ammonium was equally effective as nitrate in stimulating root respiration. These results lead to the conclusion that alternative-path mediated root respiration contributes to synthesis of ATP during at least the first 24 hours following nitrogen supply.

Journal Article↗
PubMed1983

Phytohormones, Rhizobium Mutants, and Nodulation in Legumes : III. Auxin Metabolism in Effective and Ineffective Pea Root Nodules.

High specific activity [(3)H]indole-3-acetic acid (IAA) was applied to the apical bud of intact pea (Pisum sativum L. cv Greenfeast) plants. Radioactivity was detected in all tissues after 24 hours. More radioactivity accumulated in the nodules than in the parent root on a fresh weight basis and more in effective (nitrogen-fixing) nodules than in ineffective nodules (which do not fix nitrogen).For most samples, thin layer chromatography revealed major peaks of radioactivity at the R(F) values of IAA and indole-3-acetylaspartic acid (IAAsp) and further evidence of the identity of these compounds was obtained by chromatography in other systems. Disintegrations per minute due to IAA per unit fresh weight were significantly greater for root than for nodule tissue, but were not significantly different for effective and ineffective nodules. Radioactivity due to IAAsp, expressed both on a percentage basis and per unit fresh weight, was significantly greater for nodule than for root tissue and significantly greater for the effective nodules than for the ineffective nodules. When [(3)H]IAA was applied to effective nodules, IAAsp was the dominant metabolite in the nodule. The data suggest that metabolism of auxins may be important for the persistence of a functional root nodule.

Journal Article↗
PubMed1994

Flavone-enhanced accumulation and symbiosis-related biological activity of a diglycosyl diacylglycerol membrane glycolipid from Rhizobium leguminosarum biovar trifolii.

Rhizobium leguminosarum bv. trifolii is the bacterial symbiont which induces nitrogen-fixing root nodules on the leguminous host, white clover (Trifolium repens L.). In this plant-microbe interaction, the host plant excretes a flavone, 4',7-dihydroxyflavone (DHF), which activates expression of modulation genes, enabling the bacterial symbiont to elicit various symbiosis-related morphological changes in its roots. We have investigated the accumulation of a diglycosyl diacylglycerol (BF-7) in wild-type R. leguminosarum bv. trifolii ANU843 when grown with DHF and the biological activities of this glycolipid bacterial factor on host and nonhost legumes. In vivo labeling studies indicated that wild-type ANU843 cells accumulate BF-7 in response to DHF, and this flavone-enhanced alteration in membrane glycolipid composition was suppressed in isogenic nodA::Tn5 and nodD::Tn5 mutant derivatives. Seedling bioassays performed under microbiologically controlled conditions indicated that subnanomolar concentrations of purified BF-7 elicit various symbiosis-related morphological responses on white clover roots, including thick short roots, root hair deformation, and foci of cortical cell divisions. Roots of the nonhost legumes alfalfa and vetch were much less responsive to BF-7 at these low concentrations. A structurally distinct diglycosyl diacylglycerol did not induce these responses on white clover, indicating structural constraints in the biological activity of BF-7 on this legume host. In bioassays using aminoethoxyvinylglycine to suppress plant production of ethylene, BF-7 elicited a meristematic rather than collaroid type of mitogenic response in the root cortex of white clover. These results indicate an involvement of flavone-activated nod expression in membrane accumulation of BF-7 and a potent ability of this diglycosyl diacylglycerol glycolipid to perform as a bacterial factor enabling R. leguminosarum bv. trifolii to activate segments of its host's symbiotic program during early development of the root nodule symbiosis.

Bacterial Proteins↗
PubMed1984

5-Phosphoribosylpyrophosphate amidotransferase from soybean root nodules: kinetic and regulatory properties.

Most of the nitrogen transported from the nodules of nitrogen-fixing soybean plants is in the form of the ureides allantoin and allantoic acid. Recent work has shown that ureides are formed in the plant fraction of the nodule from de novo purine biosynthesis and purine oxidation. 5-Phosphoribosylpyrophosphate amidotransferase (PRAT), which catalyzes the first committed step of purine biosynthesis, has been purified 1500-fold from soybean root nodules. The enzyme had an apparent Mr of 8 X 10(6), but this estimate may have been for an aggregation of several purine biosynthetic activities. PRAT showed a pH optimum of pH 8.0, and Km values were 18 and 0.4 mM for glutamine and 5-phosphoribosyl-1-pyrophosphate (PRPP), respectively. The reaction required Mg2+, and PRPPMg3- was shown to be the reactive molecular species of PRPP. Ammonia could replace glutamine as a substrate, and the Vm with ammonia was twice that obtained when glutamine was the substrate. The initial-rate kinetics showed sequential addition of substrates to the enzyme. Product inhibition data was consistent with the order of product release being phosphoribosylamine, PPi, and glutamate. The enzyme was subject to regulation by end products of the purine biosynthetic pathway. IMP and GMP inhibited competitively with PRPP and promoted cooperativity in the binding of this substrate; there was no cooperativity in the binding of IMP to the enzyme. XMP was a linear competitive inhibitor with PRPP. The results are discussed in terms of the key regulatory point occupied by PRAT in the pathway of ureide biogenesis.

Amidophosphoribosyltransferase↗
PubMedDate not supplied

Biological activity of Rhizobium sp. NGR234 Nod-factors on Macroptilium atropurpureum.

The broad host range of Rhizobium sp. NGR234 is based mainly on its ability to secrete a family of lipooligosaccharide Nod factors. To monitor Nod-factor purification, we used the small seeded legume Macroptilium atropurpureum, which responds evenly and consistently to Nod factors. At concentrations between approximately equal to 10(-11) M and 10(-9) M, this response takes the form of deformation of the root hairs. Higher concentrations (approximately equal to 10(-9) to 10(-7) M), provoked profound "shepherd's crook" type curling of the root hairs. Similar concentrations of Nod factors of Bradyrhizobium japonicum, Rhizobium leguminosarum, and R. meliloti also provoked marked curling of the root hairs, but the latter two species are unable to nodulate Macroptilium. On the other hand, plant hormones, hormone-like substances, inhibitors of hormone action, as well as substituents of Nod factors were without effect in this bioassay. We thus conclude that only Nod factors are capable of inducing shepherd's crook type curling of Macroptilium root hairs. Perturbations in the auxin-cytokinin balance induced "pseudo" nodulation on M. atropurpureum, as did NodNGR factors at concentrations between 10(-7) and 10(-6) M. Concomitant inoculation of Macroptilium with a NodABC- mutant of NGR234 and sulfated NodNGR factors (NodNGR[S]) gave rise to plants that slowly greened, showing that the NodNGR factors permitted entry of the Nod- mutant into the roots.

Bacterial Proteins↗
PubMed1982

Nodule protein synthesis and nitrogenase activity of soybeans exposed to fixed nitrogen.

Nitrate or ammonium was added to soybean (Glycine max L. Merrill cv Corsoy) plants grown in plastic pouches 10 days after nodules first appeared. By the third day of treatment with 10 millimolar nitrate, nitrogenase specific activity (per unit nodule weight) had decreased to 15% to 25% of that of untreated plants. Longer incubations and higher concentrations of nitrate had no greater effect. In addition, exogenous nitrate or ammonium resulted in slower nodule growth and decreased total protein synthesis in both the bacterial and the plant portion of the nodule (as measured by incorporation of (35)S). Two-dimensional gel electrophoresis revealed that the nitrogenase components were not repressed or degraded relative to other bacteroid proteins. In the presence of an optimal carbon source, the nitrogenase specific activity of nodules detached from nitrate-treated plants was equivalent to that of nodules from untreated plants. These results are consistent with models that propose decreased availability or utilization of photosynthate in root nodules when legumes are exposed to fixed nitrogen.

Journal Article↗
PubMed2004

Effects of elevated carbon dioxide concentration on growth and N2 fixation of young Robinia pseudoacacia.

Effects of elevated CO2 concentration ([CO2]) on carbon (C) and nitrogen (N) uptake and N source partitioning (N2 fixation versus mineral soil N uptake) of 1-year-old Robinia pseudoacacia were determined in a dual 13C and 15N continuous labeling experiment. Seedlings were grown for 16 weeks in ambient (350 ppm) or elevated [CO2] (700 ppm) with 15NH4 15NO3 as the only mineral nitrogen source. Elevated [CO2] increased the fraction of new C in total C, but it did not alter C partitioning among plant compartments. Elevated [CO2] also increased the fraction of new N in total N and this was coupled with a shift in N source partitioning toward N2 fixation. Soil N uptake was unaffected by elevated [CO2], whereas N2 fixation was markedly increased by the elevated [CO2] treatment, mainly because of increased specific fixation (mg N mg(-1) nodule). As a result of increased N2 fixation, the C/N ratio of tree biomass tended to decrease in the elevated [CO2] treatment. Partitioning of N uptake among plant compartments was unaffected by elevated [CO2]. Total dry mass of root nodules doubled in response to elevated [CO2], but this effect was not significant because of the great variability of root nodule formation. Our results show that, in the N2-fixing R. pseudoacacia, increased C uptake in response to increased [CO2] is matched by increased N2 fixation, indicating that enhanced growth in elevated [CO2] might not be restricted by N limitations.

Carbon Dioxide↗
PubMed2003

Cysteine proteases in nodulation and nitrogen fixation.

The cysteine proteinases or cysteine endopeptidases (EC 3.4.22) are known to occur widely in plant cells. They are involved in almost all aspects of plant growth and development including germination, circadian rhythms, senescence and programmed cell death. They are also involved in mediating plant cell responses to environmental stress (such as water stress, salinity, low temperature, wounding, ethylene, and oxidative conditions) and plant-microbe interactions (including nodulation). In the development and function of legume root nodules, cysteine proteases could be involved in several important processes:-(i) a defence response to root invasion by microorganisms; (ii) protein turnover required during the formation of new tissue; (iii) cellular homeostasis and metabolism; (iv) adaptation of host cells to physiological stresses; (v) control of nodule senescence. Because of their central importance to plant physiology, cysteine proteases could serve as important targets for the study of nodule development and functioning at the molecular level. Because of their widespread occurrence in nodulating plants they could also serve as candidate genes for targeted plant breeding programmes.

Cysteine Endopeptidases↗
PubMed1979

Effect of a bacteriophage on colonisation and nodulation of clover roots by paired strains of Rhizobium trifolii.

The effect of a virulent bacteriophage on the competitive behaviour of paired strains of Rhizobium trifolii in the root zone of clover plants was examined. The presence of bacteriophage reduced the population density of susceptible strains on the root surface and, in nodulation, favoured resistant or even partially resistant strains which were otherwise less able to form nodules. These effects occurred with different growth temperatures, host species, and ratios of strains in the inoculum.

Antibiosis↗
PubMed1992

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

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

Journal Article↗
PubMed2004

Structural characterization of extracellular polysaccharides of Azorhizobium caulinodans and importance for nodule initiation on Sesbania rostrata.

During lateral root base nodulation, the microsymbiont Azorhizobium caulinodans enters its host plant, Sesbania rostrata, via the formation of outer cortical infection pockets, a process that is characterized by a massive production of H(2)O(2). Infection threads guide bacteria from infection pockets towards nodule primordia. Previously, two mutants were constructed that produce lipopolysaccharides (LPSs) similar to one another but different from the wild-type LPS, and that are affected in extracellular polysaccharide (EPS) production. Mutant ORS571-X15 was blocked at the infection pocket stage and unable to produce EPS. The other mutant, ORS571-oac2, was impaired in the release from infection threads and was surrounded by a thin layer of EPS in comparison to the wild-type strain that produced massive amounts of EPS. Structural characterization revealed that EPS purified from cultured and nodule bacteria was a linear homopolysaccharide of alpha-1,3-linked 4,6-O-(1-carboxyethylidene)-D-galactosyl residues. In situ H(2)O(2) localization demonstrated that increased EPS production during early stages of invasion prevented the incorporation of H(2)O(2) inside the bacteria, suggesting a role for EPS in protecting the microsymbiont against H(2)O(2). In addition, ex planta assays confirmed a positive correlation between increased EPS production and enhanced protection against H(2)O(2).

Azorhizobium caulinodans↗
PubMed1997

Effect of nitrate on nitrogen fixation and nitrate assimilation in Azorhizobium-Sesbania rostrata symbiotic system.

Root nodule formation was inhibited by 30% and 50% respectively at low concentration of 1 mM and 2 mM nitrate, while stem nodule formation was enhanced by 50% only at 1 mM nitrate. The nodule specific nitrogenase activity decreased with the increasing concentration of nitrate. At 1 mM nitrate nitrogenase activity per plant stem nodule was not affected, but it was less than 50% in the root nodules as compared to control. Increasing concentration of nitrate increased in vivo activity of nitrate reductase (NR) significantly in stem, root nodules and leaves. Nodule cytosolic NR utilized both NADH and succinate as electron donor, but not reduced MV. However bacteroidal NR utilised reduced MV as reductant more efficiently than succinate.

Fabaceae↗
PubMed2006

Identification of a Sed5-like SNARE gene LjSYP32-1 that contributes to nodule tissue formation of Lotus japonicus.

We identified a Sed5-like clone LjSYP32-1 which contributes to nodule tissue formation and plant growth in Lotus japonicus. In the L. japonicus expressed sequence tag (EST) clone databases of Kazusa DNA Research Institute, another syntaxin-related clone (LjSYP32-2) was also detected, and the nucleotide and amino acid sequences of these two clone are very similar to each other. Real-time PCR and promoter analysis indicated that expression of LjSYP32-1 was dominant compared with LjSYP32-2 in the various plant organs. Promoter analysis and in situ hybridization revealed that LjSYP32-1 was expressed significantly in the inner cortex cell layer surrounding the infected zone of young nodules and in the meristem area of developing lateral root. To explore the function and physiological role of LjSYP32-1 in nodules and other plant organs, stable transformation lines of L. japonicus expressing either sense or antisense LjSYP32-1 were prepared. The antisense plants showed a significantly retarded plant growth phenotype, suggesting a role for LjSYP32-1 in supporting plant growth. In the same transgenic lines, the plants were capable of forming nodules, but the acetylene reduction activity was reduced by around 50% per plant. The nodules were much smaller and some nodules were fused to each other by sharing the inner cortex. The rate of occurrence of such irregular nodules was twice that observed in wild-type plants. The data suggest that LjSYP32-1 contributes to the support of plant growth and normal nodule tissue differentiation.

Amino Acid Sequence↗
PubMed2001

Yellow lupine cyclophilin transcripts are highly accumulated in the nodule meristem zone.

Cyclophilin (CyP) is one of the enzymes that act as peptidylprolyl cis-trans isomerases (EC 5.2.1.8). The cDNA and an intronless gene coding for cytosolic CyP have been isolated from yellow lupine. The deduced amino acid sequence of the characterized open reading frame shows approximately 80% homology with cytosolic CyP from other organisms. Southern blots of genomic DNA indicate that there is a small family of genes for CyP-related genes in the yellow lupine genome. RNA blot analyses demonstrate that CyP genes are expressed in all plant organs. The amount of CyP transcripts is dramatically increased in root nodules. In situ hybridization experiments indicate that CyP transcripts are localized mainly in meristematic tissues, with the highest level observed in the nodule meristem zone. The promoter of the sequenced gene contains 5' AAAGAT 3' and AT-rich motifs that are characteristic for some nodulin promoters.

Base Sequence↗