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Results for “Plant Root Nodulation”

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PubMed1993

Expression of antisense nodulin-35 RNA in Vigna aconitifolia transgenic root nodules retards peroxisome development and affects nitrogen availability to the plant.

A nodulin-35 (N-35) cDNA encoding nodule-specific uricase (EC 1.7.3.3.) was isolated from a Vigna aconitifolia (mothbean) root nodule cDNA library. Sequence analysis of Vigna uricase (VN-35) cDNA revealed 90% homology to that of soybean. The VN-35 cDNA was inserted in the antisense orientation downstream of the caMV-35S promoter, and transgenic hairy roots were formed on Vigna plants using Agrobacterium rhizogenes. Infection with Bradyrhizobium (cowpea) gave rise to root nodules on transgenic hairy roots supported by the wild-type shoot. Expression of antisense VN-35 RNA was detected in transgenic nodules on individual roots using polymerase chain reaction (PCR). The nodules expressing antisense VN-35 RNA were smaller in size and showed lower uricase activity than nodules formed on the hairy roots transformed with a binary vector containing beta-glucuronidase (GUS) gene (used as control), and the plants exhibited nitrogen deficiency symptoms. Ultrastructural analysis and immunogold labeling with antibody against soybean N-35 revealed that the growth of peroxisomes was retarded in transgenic nodules expressing antisense VN-35 RNA. These data suggest that a reduction in ureide biosynthesis limits the availability of symbiotically reduced nitrogen to the plant. The nodules of tropical legumes appear to be specialized in nitrogen assimilation and are developmentally controlled to produce and transport ureides.

Base Sequence↗

PubMed1982

Effect of Atmospheric CO(2) Enrichment on Growth, Nonstructural Carbohydrate Content, and Root Nodule Activity in Soybean.

The objective of this study was to determine whether the supply of current photosynthate was limiting root nodule activity. Both short-term (36 hours) and long-term (16 days) periods of CO(2) enrichment were imposed on vegetative, growth chamber-grown soybean plants (Glycine max. [L.] Merr. cv. ;Clay') to increase the supply of current photosynthate and to observe the effects on photosynthate partitioning in the plants, plant growth, and root nodule activity.Neither total nor specific nodule activities were increased during exposure to short-term (36 hours) CO(2) enrichment. Dry weight of the leaves increased after 12, 24, and 36 hours of CO(2) enrichment and dry weight of the stems plus petioles increased after 36 hours of CO(2) enrichment. Dry weights of the roots and nodules were not altered by short-term CO(2) enrichment. Short-term CO(2) enrichment increased the total nonstructural carbohydrates in the leaves and stems plus petioles, but not in the roots and nodules. Analyses of the separate pools of carbohydrate reserves indicated that the majority of the additional carbohydrate provided by short-term CO(2) enrichment was stored as leaf starch with relatively little being partitioned to the roots and nodules.Long-term CO(2) enrichment (16 days) did not enhance specific nodule activity. Shoot, root, and nodule dry weights were increased 109, 34%, and 56% respectively. Total nodule activity per plant was significantly enhanced only after 16 days of treatment and was related to increased nodule mass. These results indicate that the increased total nodule activity in response to CO(2) enrichment is a consequence of a general growth response of the plant.Results of both studies indicate that nodule activity was not directly limited by current photosynthesis but rather by the partitioning and utilization of photosynthate in the plant.

Journal Article↗

PubMed2005

Stress-induced changes in protease composition are determined by nitrogen supply in non-nodulating white clover.

An inbreeding line of white clover has been identified which remains non-nodulated under appropriate physiological conditions and so the nitrogen concentration of the plant can be manipulated by altering the nitrate supply to the roots. Non-nodulating plants were used to test the hypothesis that acclimation to nitrogen limitation in white clover involves changes in protease activity and composition. These results indicate that acclimation to nitrogen limitation involves the realignment of constituent proteases without necessarily incurring significant changes in total protease activity. Plants grown at 2.5, 5.0, 7.5, and 10 mM nitrate showed a positive correlation between nitrate supply and foliar protein concentration. Protein profiles, revealed by Coomassie-stained SDS-PAGE, were unchanged between treatments for a given amount of protein. Serine, aspartate/metalloprotease, and two cysteine proteases were identified in the leaves. Although total protease activity per gram fresh weight was unchanged between treatments, the relative contributions of these four proteases was determined by nitrate supply. When plants were stressed further by withholding nitrate there was an increase in cysteine protease activity, but a senescence-related aspartate/metalloprotease was not visible. Hence, while protease expression in white clover leaves responded to the current and past nitrogen status of the plant, the proteases involved in remobilization during nutrient limitation were distinct from those involved during the main senescence period. It is suggested that nitrogen limitation induced an early, reversible stage of senescence in which perturbations in protease activity facilitated the degradation of non-essential proteins in order to increase the chances of plant survival or seed set.

Chlorophyll↗

PubMed1979

The Rhizobium--legume symbiosis.

The rhizobia are soil microorganisms that can interact with leguminous plants to form root nodules within which conditions are favourable for bacterial nitrogen fixation. Legumes allow the development of very large rhizobial populations in the vicinity of their roots. Infections and nodule formation require the specific recognition of host and Rhizobium, probably mediated by plant lectins. Penetration of the host by a compatible Rhizobium species usually provokes host root cell division to form the nodule, and a process of differentiation by both partners then ensues. In most cases the rhizobia alter morphologically to form bacteroids, which are usually larger than the free-living bacteria and have altered cell walls. At all stages during infection, the bacteria are bounded by host cell plasmalemma. The enzyme nitrogenase is synthesized by the bacteria and, if leghaemoglobin is present, nitrogen fixation will occur. Leghaemoglobin is a product of the symbiotic interaction, since the globin is produced by the plant while the haem is synthesized by the bacteria. In the intracellular habitat the bacteria are dependent upon the plant for supplies of energy and the bacteroids, in particular, appear to differentiate so that they are no longer able to utilize the nitrogen that they fix. Regulation of the supply of carbohydrate and the use of the fixed nitrogen thus appear to be largely governed by the host.

Leghemoglobin↗

PubMed2005

Ascending migration of endophytic rhizobia, from roots to leaves, inside rice plants and assessment of benefits to rice growth physiology.

Rhizobia, the root-nodule endosymbionts of leguminous plants, also form natural endophytic associations with roots of important cereal plants. Despite its widespread occurrence, much remains unknown about colonization of cereals by rhizobia. We examined the infection, dissemination, and colonization of healthy rice plant tissues by four species of gfp-tagged rhizobia and their influence on the growth physiology of rice. The results indicated a dynamic infection process beginning with surface colonization of the rhizoplane (especially at lateral root emergence), followed by endophytic colonization within roots, and then ascending endophytic migration into the stem base, leaf sheath, and leaves where they developed high populations. In situ CMEIAS image analysis indicated local endophytic population densities reaching as high as 9 x 10(10) rhizobia per cm3 of infected host tissues, whereas plating experiments indicated rapid, transient or persistent growth depending on the rhizobial strain and rice tissue examined. Rice plants inoculated with certain test strains of gfp-tagged rhizobia produced significantly higher root and shoot biomass; increased their photosynthetic rate, stomatal conductance, transpiration velocity, water utilization efficiency, and flag leaf area (considered to possess the highest photosynthetic activity); and accumulated higher levels of indoleacetic acid and gibberellin growth-regulating phytohormones. Considered collectively, the results indicate that this endophytic plant-bacterium association is far more inclusive, invasive, and dynamic than previously thought, including dissemination in both below-ground and above-ground tissues and enhancement of growth physiology by several rhizobial species, therefore heightening its interest and potential value as a biofertilizer strategy for sustainable agriculture to produce the world's most important cereal crops.

Colony Count, Microbial↗

PubMed1987

Nodule activity and allocation of photosynthate of soybean during recovery from water stress.

Nodulated soybean plants (Glycine max [L.] Merr. cv Ransom) in a growth-chamber study were subjected to a leaf water potential (psi w) of -2.0 megapascal during vegetative growth. Changes in nonstructural carbohydrate contents of leaves, stems, roots, and nodules, allocation of dry matter among plant parts, in situ specific nodule activity, and in situ canopy apparent photosynthetic rate were measured in stressed and nonstressed plants during a 7-day period following rewatering. Leaf and nodule psi w also were determined. At the time of maximum stress, concentration of nonstructural carbohydrates had declined in leaves of stressed, relative to nonstressed, plants, and the concentration of nonstructural carbohydrates had increased in stems, roots, and nodules. Sucrose concentrations in roots and nodules of stressed plants were 1.5 and 3 times greater, respectively, than those of nonstressed plants. Within 12 hours after rewatering, leaf and nodule psi w of stressed plants had returned to values of nonstressed plants. Canopy apparent photosynthesis and specific nodule activity of stressed plants recovered to levels for nonstressed plants within 2 days after rewatering. The elevated sucrose concentrations in roots and nodules of stressed plants also declined rapidly upon rehydration. The increase in sucrose concentration in nodules, as well as the increase of carbohydrates in roots and stems, during water stress and the rapid disappearance upon rewatering indicates that inhibition of carbohydrate utilization within the nodule may be associated with loss of nodule activity. Availability of carbohydrates within the nodules and from photosynthetic activity following rehydration of nodules may mediate the rate of recovery of N2-fixation activity.

Acetylene↗

PubMed1992

Molecular cloning of a cDNA encoding aspartate aminotransferase-P2 from lupin root nodules.

Two isoenzymic forms of aspartate aminotransferase are present in the plant fraction of developing lupin root nodules. One of these forms, aspartate aminotransferase-P2 (AAT-P2), increases dramatically with the onset of biological nitrogen fixation and is associated with the assimilation of ammonia by the plant in the Rhizobium-legume symbiosis. A day 18 lupin nodule cDNA library in the lambda ZapII vector was immunoscreened with a monoclonal antibody specific for AAT-P2 and yielded two near-full-length 1700 bp clones. These clones were sequenced. Amino acid sequences from three peptides derived from immunopurified AAT-P2 were aligned, and showed 100% homology with the amino acid sequence deduced from the cDNA clones. The DNA sequence showed 50% homology with AAT sequences from a range of animal sources. Conversion of the clones to the phagemid form allowed their expression in Escherichia coli where both exhibited enzyme activity that could be immunoprecipitated with AAT-P2-specific monoclonal antibodies. Western blot analysis revealed protein moieties with molecular masses of 39, 43, 45 and 55 kDa. The 5' end of the clones coded for a hydrophobic leader sequence of about 50 amino acids indicative of a targeting sequence and consistent with the plastid localisation of nodule AAT-P2.

Amino Acid Sequence↗

PubMed1988

Functioning haemoglobin genes in non-nodulating plants.

Haemoglobin has previously been recorded in plants only in the nitrogen-fixing nodules formed by symbiotic association between Rhizobium or Frankia and legume or non-legume hosts. Structural similarities amongst these and animal haemoglobins at the protein and gene level suggested a common evolutionary origin. This suggests that haemoglobin genes, inherited from an ancestor common to plants and animals, might be present in all plants. We report here the isolation of a haemoglobin gene from Trema tomentosa, a non-nodulating relative of Parasponia (Ulmaceae). The gene has three introns located at positions identical to those in the haemoglobin genes of nodulating plant species, strengthening the case for a common origin of all plant haemoglobin genes. The data argue strongly against horizontal haemoglobin gene transfer from animals to plants. The Trema gene has a tissue-specific pattern of transcription and translation, producing monomeric haemoglobin in Trema roots. We have also found that the Parasponia haemoglobin gene is transcribed in roots of non-nodulated plants. These results suggest that haemoglobin has a role in the respiratory metabolism of root cells of all plant species. We propose that its special role in nitrogen-fixing nodules has required adaptation of the haemoglobin-gene regulation pathway, to give high expression in the specialized environment of the nodule.

Amino Acid Sequence↗

PubMed2002

Strategies used by rhizobia to lower plant ethylene levels and increase nodulation.

Agriculture depends heavily on biologically fixed nitrogen from the symbiotic association between rhizobia and plants. Molecular nitrogen is fixed by differentiated forms of rhizobia in nodules located on plant roots. The phytohormone, ethylene, acts as a negative factor in the nodulation process. Recent discoveries suggest several strategies used by rhizobia to reduce the amount of ethylene synthesized by their legume symbionts, decreasing the negative effect of ethylene on nodulation. At least one strain of rhizobia produces rhizobitoxine, an inhibitor of ethylene synthesis. Active 1-aminocyclopropane-1-carboxylate (ACC) deaminase has been detected in a number of other rhizobial strains. This enzyme catalyzes the cleavage of ACC to alpha-ketobutyrate and ammonia. It has been shown that the inhibitory effect of ethylene on plant root elongation can be reduced by the activity of ACC deaminase.

Carbon-Carbon Lyases↗

PubMed2000

Casuarina glauca prenodule cells display the same differentiation as the corresponding nodule cells.

Recent phylogenetic studies have implied that all plants able to enter root nodule symbioses with nitrogen-fixing bacteria go back to a common ancestor (D.E. Soltis, P.S. Soltis, D.R. Morgan, S.M. Swensen, B.C. Mullin, J.M. Dowd, and P.G. Martin, Proc. Natl. Acad. Sci. USA, 92:2647-2651, 1995). However, nodules formed by plants from different groups are distinct in nodule organogenesis and structure. In most groups, nodule organogenesis involves the induction of cortical cell divisions. In legumes these divisions lead to the formation of a nodule primordium, while in non-legumes they lead to the formation of a so-called prenodule consisting of infected and uninfected cells. Nodule primordium formation does not involve prenodule cells, and the function of prenodules is not known. Here, we examine the differentiation of actinorhizal prenodule cells in comparison to nodule cells with regard to both symbionts. Our findings indicate that prenodules represent primitive symbiotic organs whose cell types display the same characteristics as their nodule counterparts. The results are discussed in the context of the evolution of root nodule symbioses.

Actinomycetales↗

PubMed2006

Iron bioavailability of hemoglobin from soy root nodules using a Caco-2 cell culture model.

Heme iron has been identified in many plant sources-most commonly in the root nodules of leguminous plants, such as soy. Our objective was to test the effectiveness of soy root nodule (SRN) and purified soy hemoglobin (LHb) in improving iron bioavailability using an in vitro Caco-2 cell model, with ferritin response as the bioavailability index. We assessed bioavailability of iron from LHb (either partially purified (LHbA) or purified (LHbD)) with and without food matrix and compared it with that from bovine hemoglobin (BHb), ferrous sulfate (FeSO4), or SRN. Bioavailability of each treatment was normalized to 100% of the FeSO4 treatment. When iron sources were tested alone (100 ug iron/mL), ferritin synthesis by LHbD and BHb were 19% (P > 0.05) and 113% (P < 0.001) higher than FeSO4, respectively. However, when iron sources were used for fortification of maize tortillas (50 ppm), LHbA and BHb showed similar bioavailability, being 27% (P < 0.05) and 33% (P < 0.05) higher than FeSO4. Heat treatment had no effect on heme iron but had a significant reduction on FeSO4 bioavailability. Adding heme (LHbA) iron with nonheme (FeSO4) had no enhancement on nonheme iron absorption. Our data suggest that heme iron from plant sources may be a novel value-added product that can provide highly bioavailable iron as a food fortificant.

Biological Availability↗

PubMed2006

[Analysis of DNA homology and 16S rDNA sequence of rhizobia, a new phenotypic subgroup, isolated from Xizang Autonomous Region of China].

Based on the studies of numerical taxonomy, the seven rhizobial strains isolated from the root nodules of leguminous plants Trigonella spp. and Astragalus spp. growing in the Xizang Autonomous Region of China constituted a new phenotypic subgroup, where wide phenotypic and genotypic diversity among legume crops had been reported due to complex terrain and various climate. The new phenotypic subgroup were further identified to clarify its taxonomic position by DNA homology analysis and 16S rDNA gene sequencing. The mol% G + C ratio of the DNA among members of the new subgroup ranged from 59.5 to 63.3 mol% as determined by T (m) assay. The levels of DNA relatedness, determined by using the DNA liquid hybridization method, among the members of the new subgroup were between 74.3% and 92.3%, while level of DNA relatedness between the central strains XZ2-3 of the new subgroup and the type strains of known species of Rhizobium was less than 47.4%. These results indicated that the new phenotypic subgroup is a DNA homological group different from described species of Rhizobium. Therefore, this new phenotypic subgroup was supposed to be a new species in the genus of Rhizobium since the strains in the same species generally exhibit levels of DNA homology ranging from 70 to 100%. A systematic identification method-16S rDNA gene sequence comparison was carried out to determine the phylogenetic relationships of the new subgroup with the described species of Rhizobium. The GenBank accession number for the 16S rDNA sequence of the central strain XZ2-3 of the new subgroup is DQ099745. The full-length 16S rDNA gene sequence were sequenced by chain terminator techniques and analyzed with PHYLIP. The phylogenetic trees were constructed by using the programs DRAWTREE. The phylogenetic analysis indicated that new subgroup occupy a independent sub-branch in phylogenetic tree. The sequence similarities between the center strain XZ2-3 and the closest relatives, strain R. leguminosarum USDA2370T and R. etli CFN42T, were 96.55% and 96.62% respectively. Both the 16S rDNA sequence data and the DNA relatedness data suggested that the new phenotypic subgroup isolated from the root nodules of leguminous plants Trigonella spp. and Astragalus spp. obtained from the Xizang Autonomous Region of China represents a new rhizobial species in the genus of Rhizobium according to the minimal standards for the description of new genera and species of root-and stem-nodulating bacteria.

Base Sequence↗

PubMed1993

Characterization of nitrogenous solutes in tissues and xylem sap of Leucaena leucocephala.

Amino acid profiles of leaf, stem, and root tissues from nodulated and nonnodulated Leucaena leucocephala (Lam.) de Wit plants were determined by gas chromatography-mass spectrometry. High concentrations of mimosine and several other potentially toxic nonprotein amino acids, including pipecolic acid and two isomers of hydroxypipecolic acid, were identified in the tissues. Five metabolites remain unidentified. Of the foliar free amino acid nitrogen, 57-66% was associated with the potentially toxic amino acids. Major constituents in the leaf tissues of nonnodulated plants were mimosine and hydroxypipecolic acid (isomer 1). Mimosine was recovered in both the neutral plus basic and acidic amino acid fractions. Major differences between amino acid profiles of nodulated and nonnodulated roots were the low percentages of asparagine + aspartate (3.6% of the total pool compared to 33% in nodulated plants) and pipecolic acid in nonnodulated roots (1% of the total compared to 12.5% in nodulated plants). A novel plant betaine (dihydroxypipecolic acid betaine) was identified by fast-atom-bombardment mass spectrometry in leaf tissues, albeit at relatively low concentrations (< 1 micro mol per gram fresh weight). Analyses of the xylem sap collected from nodulated plants confirmed that Leucaena is an asparagine transporter, as suggested by the high concentrations of asparagine and the low concentrations of ureides in its root nodules. Amino acid profiles of xylem sap from nonnodulated plants showed extremely low concentrations of asparagine + aspartate (0.12 micro mol ml(-1)), whereas asparagine + aspartate was the major constituent (4.38 micro mol ml(-1)) in the xylem sap of nodulated plants. Two nonprotein amino acids, pipecolic acid and hydroxypipecolic acid, were major constituents of the xylem sap of nodulated and nonnodulated plants, respectively. Three unidentified compounds detected in xylem sap samples from both nodulated and nonnodulated plants did not correspond with any of the peaks characterized from tissue samples.

Journal Article↗

PubMed1995

A mutant Bradyrhizobium japonicum delta-aminolevulinic acid dehydratase with an altered metal requirement functions in situ for tetrapyrrole synthesis in soybean root nodules.

The tetrapyrrole synthesis enzyme delta-aminolevulinic acid (ALA) dehydratase requires Mg2+ for catalytic activity in photosynthetic organisms and in Bradyrhizobium japonicum, a bacterium that can reside symbiotically within plant cells of soybean root nodules or as a free-living organism. ALA dehydratase from animals and other non-photosynthetic organisms is a Zn(2+)-dependent enzyme. A modified B. japonicum ALA dehydratase, ALAD*, was constructed by site-directed mutagenesis of hemB in which three proximal amino acids conserved in plant dehydratases were changed to cysteine residues as is found in the Zn(2+)-dependent enzyme of animals. These substitutions resulted in an enzyme that required Zn2+ rather than Mg2+ for catalytic activity, and therefore a region of the ALA dehydratase from B. japonicum, and probably from plants, was identified that is involved in Mg2+ dependence. In addition, the data show that a change in only a few residues is sufficient to change a Mg(2+)-dependent ALA dehydratase to a Zn(2+)-dependent one. B. japonicum strains were constructed that contained a single copy of either hemB or the altered gene hemB* integrated into the genome of a hemB- mutant. Cultures of the hemB* strain KPZn3 had Zn(2+)-dependent ALA dehydratase activity that functioned in vivo as discerned by its heme prototrophy and expression of wild type levels of cellular hemes. Strain KPZn3 elicited root nodules on soybean that contained viable bacteria and exhibited traits of normally developed nodules, and the symbiotic bacteria expressed nearly wild type levels of cellular hemes. We conclude that the Zn(2+)-dependent ALAD* can function and support bacterial tetrapyrrole synthesis within the plant milieu of root nodules.

Amino Acid Sequence↗

PubMed2001

Phosphatidylcholine levels in Bradyrhizobium japonicum membranes are critical for an efficient symbiosis with the soybean host plant.

Phosphatidylcholine (PC), the major membrane phospholipid in eukaryotes, is found in only some bacteria including members of the family Rhizobiaceae. For this reason, it has long been speculated that rhizobial PC might be required for a successful interaction of rhizobia with their legume host plants in order to allow the formation of nitrogen-fixing root nodules. A major pathway for PC formation in prokaryotes involves a threefold methylation of the precursor phosphatidylethanolamine (PE). Here, we report on the isolation of a Bradyrhizobium japonicum gene (pmtA) encoding the phospholipid N-methyltransferase PmtA. Upon expression of the bradyrhizobial pmtA gene in Escherichia coli, predominantly monomethylphosphatidylethanolamine was formed from PE. PmtA-deficient B. japonicum mutants still produced low levels of PC by a second methylation pathway. The amount of PC formed in such mutants (6% of total phospholipids) was greatly decreased compared with the wild type (52% of total phospholipids). Root nodules of soybean plants infected with B. japonicum pmtA mutants showed a nitrogen fixation activity of only 18% of the wild-type level. The interior colour of the nodules was beige instead of red, suggesting decreased amounts of leghaemoglobin. Moreover, ultrastructure analysis of these nodules demonstrated a greatly reduced number of bacteroids within infected plant cells. These data suggest that the biosynthesis of wild-type amounts of PC are required to allow for an efficient symbiotic interaction of B. japonicum with its soybean host plant.

Bacterial Proteins↗

PubMed1986

Role of plant root exudate and Sym plasmid-localized nodulation genes in the synthesis by Rhizobium leguminosarum of Tsr factor, which causes thick and short roots on common vetch.

In a previous paper it was shown that cocultivation of Rhizobium leguminosarum with the plant Vicia sativa subsp. nigra on solid medium causes a changed mode of growth of the plant roots, resulting in thick and short roots (Tsr). The Sym plasmid present in the bacterium appeared to be essential for causing Tsr (A. A. N. van Brussel, T. Tak, A. Wetselaar, E. Pees, and C. A. Wijffelman, Plant Sci. Lett. 27:317-325, 1982). In the present paper, we show that a role in causing Tsr is general for Sym plasmids of R. leguminosarum and Rhizobium trifolii. Moreover, mutants with transposon insertions in the Sym plasmid-localized nodulation genes nodA, B, C, and D are unable to cause Tsr, in contrast to nodulation mutants localized in other parts of the Sym plasmid. The observation that Tsr could also be brought about in liquid medium enabled us to show that Tsr is caused by a soluble factor. Experiments in which plants and bacteria were grown separately in the sterile supernatant fluids of each other resulted in establishing the following sequence of events. (i) The plant produces a factor, designated as factor A. (ii) Factor A causes the Sym plasmid-harboring bacteria to produce Tsr factor. (iii) Growth of young plants in the presence of Tsr factor results in the Tsr phenotype. Models explaining this example of molecular signalling between bacteria and plants are discussed.

Phenotype↗

PubMed1982

Root and Nodule Enzymes of Ammonia Assimilation in Two Plant-Conditioned Symbiotically Ineffective Genotypes of Alfalfa (Medicago sativa L.).

Biochemical and physiological parameters associated with nitrogen metabolism were measured in nodules and roots of glasshouse-grown clones of two symbiotically ineffective alfalfa (Medicago sativa L.) genotypes supplied with either NO(3) (-) or NH(4) (+). Significant differences were observed between genotypes for nodule soluble protein concentrations and glutamine synthetase (GS) and glutamate synthase (GOGAT) specific activities, both in untreated controls and in response to applied N. Nodule soluble protein of both genotypes declined in response to applied N, while nodule GS, GOGAT, and glutamate dehydrogenase (GDH) specific activities either decreased or remained relatively constant. In contrast, no genotype differences were observed in roots for soluble protein concentrations and GS, GOGAT, and GDH specific activities, either in untreated controls or in response to applied N. Root soluble protein levels and GS and GOGAT specific activities of N-treated plants increased 2- to 4-fold within 4 days and then decreased between days 13 and 24. Root GDH specific activity of NH(4) (+)-treated plants increased steadily throughout the experiment and was 50 times greater than root GS or GOGAT specific activities by day 24.Enzymological data indicate that nodules of these ineffective alfalfa genotypes are uniquely differentiated plant organs. Decreasing or constant plant GS and GOGAT specific activities in ineffective nodules in response to applied N suggest that factors in addition to N supply are involved in the induction of high levels of plant ammonia-assimilating enzymes in nodules. Genotype differences observed for nodule enzyme specific activities support the concept that ineffectiveness may be expressed in different ways within the nodule. Senescence was evident in ineffective nodules of N-treated plants of both genotypes, indicating that nodule senescence induced by applied N may not be closely linked to symbiotic effectiveness in alfalfa. Data for ammonia-assimilating enzymes in roots suggest the GS/GOGAT pathway operates only at low levels of soil N and that GDH functions to detoxify high levels of soil NH(4) (+).

Journal Article↗

PubMed1998

Rearrangement of actin microfilaments in plant root hairs responding to rhizobium etli nodulation signals

The response of the actin cytoskeleton to nodulation (Nod) factors secreted by Rhizobium etli has been studied in living root hairs of bean (Phaseolus vulgaris) that were microinjected with fluorescein isothiocyanate-phalloidin. In untreated control cells or cells treated with the inactive chitin oligomer, the actin cytoskeleton was organized into long bundles that were oriented parallel to the long axis of the root hair and extended into the apical zone. Upon exposure to R. etli Nod factors, the filamentous actin became fragmented, as indicated by the appearance of prominent masses of diffuse fluorescence in the apical region of the root hair. These changes in the actin cytoskeleton were rapid, observed as soon as 5 to 10 min after application of the Nod factors. It was interesting that the filamentous actin partially recovered in the continued presence of the Nod factor: by 1 h, long bundles had reformed. However, these cells still contained a significant amount of diffuse fluorescence in the apical zone and in the nuclear area, presumably indicating the presence of short actin filaments. These results indicate that Nod factors alter the organization of actin microfilaments in root hair cells, and this could be a prelude for the formation of infection threads.

Journal Article↗