Estimation of root nodule development by <Emphasis Type="Italic">Rhizobium</Emphasis> sp. Using hydroponic culture

A nitrogen-fixing bacterium isolated from the root nodules of a cultivated leguminous plant, soybean (Glycine max L.), was cultivable and was identified as Rhizobium sp. Bacterial species isolated from root nodules of wild leguminous plants including -bush clover, white dutch clover, wisteria, and false acacia were identified as Burkholderia cepacia, Pseudomonas migulae, Pseudomonas putida, and Flavobacterium sp, respectively, all of which are heterotrophic bacteria that grow in the rhizosphere. Temperature gradient gel electrophoresis (TGGE) 16S-rDNA bands extracted directly from the bacterial population within the root nodules of the wild leguminous plants were identified as Rhizobium sp, Mesorhizobium sp, and Bradyrhizobium sp. none were cultivable. Rhizobium sp. isolated from soybean root nodule generated approximately 48 and 19 mg/L of ammonium in glucose- and starch-defined medium, respectively, during 8 days of growth. The growth rate of Rhizobium sp. was increased by the addition of yeast extract but not by the addition of ammonium. K _m and V _max for starch saccharification measured with the extracellular crude enzyme of Rhizobium sp. were 0.7556 mg/L and 0.1785 mg/L/min, respectively. The inoculation of Rhizobium sp. culture into a hydroponic soybean plant culture activated root nodule development and soybean plant growth. The inoculated Rhizobium sp. survived for at least 4 weeks, based on the TGGE pattern of 16S-rDNA. The 16S-rDNA of Rhizobium sp. isolated from newly developed root nodules was homologous with the inoculated species.     

Effects of disease resistance genes on Rhizobium symbiosis in an annual legume

Summary The evolution of disease resistance in plants may be constrained if genes conferring resistance to pathogens interfere with plant responses toward other, nonpathogenic organisms. To test for such effects, we compared symbiotic nitrogen fixation in Amphicarpaea bracteata plants that differed at a major locus controlling resistance to the pathogen Synchytrium decipiens. Both resistant and susceptible plant genotypes nodulated successfully and grew significantly better in the presence of Rhizobium, although growth enhancement by Rhizobium was altered by different levels of nitrate fertilization. Plants homozygous for disease resistance achieved 2% higher growth than susceptible homozygotes across all treatments, but this difference was not significant. Resistant and susceptible plant genotypes did not differ in the mean number of nodules formed per plant or in nodule diameter. However, there was highly significant variation among replicate families within each disease resistance category for both nodulation characteristics. These results imply that genetic variation exists among A. bracteata plants both for diease resistance and for traits affecting symbiotic nitrogen fixation. However, there were no evident pleiotropic effects of disease resistance genes on the plant-Rhizobium symbiosis.     

Biocontrol of wilt disease complex of pea using Pseudomonas fluorescens and Rhizobium sp.

Biocontrol of wilt disease complex of pea caused by the root-knot nematode Meloidogyne incognita and Fusarium oxysporum f. sp. pisi was studied on pea (Pisum sativum L.) using plant growth-promoting rhizobacterium Pseudomonas fluorescens and root nodule bacterium Rhizobium sp. Inoculation of M. incognita and F.oxysporum alone caused significant reductions in plant growth over un-inoculated control. Reduction in plant growth caused by M. incognita was statistically equal to that caused by F. oxysporum. Inoculation of M. incognita plus F. oxysporum together caused a greater reduction in plant growth than the sum of damage caused by these pathogens singly. Inoculation of P. fluorescens and Rhizobium sp. individually or both together increased plant growth in pathogen inoculated and un-inoculated plants. Inoculation of P. fluorescens to pathogen-inoculated plants caused a greater increase in plant growth than caused by Rhizobium sp. Application of Rhizobium plus P. fluorescens caused a greater increase in plant growth than caused by each of them singly. Inoculation of P.fluorescens caused higher reduction in galling and nematode multiplication than caused by Rhizobium sp. Use of Rhizobium plus P. fluorescens caused higher reduction in galling and nematode multiplication than their individual inoculation. Plants inoculated with both pathogens plus Rhizobium showed less nodulation than plants inoculated with single pathogen plus Rhizobium. Inoculation of Rhizobium plus P. fluorescens resulted in higher root-nodulation than inoculated only with Rhizobium. Wilting indices were 4 and 5, respectively, when plants were inoculated with F. oxysporum and F. oxysporum plus M. incognita. Wilting indices were reduced maximum to 1 and 2, respectively, when plants inoculated with F.oxysporum and plants with both pathogens were treated with P. fluorescens plus Rhizobium.     

The Role of Plant Size and Nutrient Concentrations in Associations between Medicago, and Rhizobium and/or Glomus

The aim of this research was to carry out a critical study of the method of obtaining size equivalence between non-symbiotic alfalfa and alfalfa associated with Glomus and/or Rhizobium by applying fixed addition rates of nutrients to the non-symbiotic controls. The experimental design included three nutrient response curves in which the levels of added phosphorus and/or nitrogen were constant during the whole plant growth process: 1) a phosphorus response curve, in order to compare the growth of double symbiotic plants with that of only-Rhizobium inoculated ones; 2) a nitrogen response curve, that consisted of a comparison between the growth of double symbiotic alfalfa and four treatments associated only with Glomus; 3) a phosphorus and nitrogen response curve, to compare the growth of non-inoculated alfalfa with that of double symbiotic plants. Although similar size was achieved among some treatments at harvest, shoot growth over time and nutrient concentrations in tissues differed, indicating that growth equivalence did not mean functional equivalence. A second experimental design was performed taking into account the establishment of microsymbionts for determining the adequate moment to add supplemental phosphorus and/or nitrogen. It included four treatments: a) double symbiotic plants (MR); b) plants inoculated with Rhizobium only (R); c) plants inoculated with Glomus only (M), and d) non-inoculated plants (N). Great similarity in terms of plant growth and nutrient contents in tissues were obtained. Moreover, symbiotic plants were able to produce similar dry matter than non-symbiotic ones under P and N limitations.     

Early seedling and seasonal N2-fixing symbiotic activity of two soybean [Glycine max (L.) Merr.] cultivars inoculated with Bradyrhizobium strains of diverse origin

In areas with a short growing season the poor adaptability of soybean [Glycine max Meer. (L.)] to cool soil conditions is considered the primary yield limiting factor. Soybean requires temperatures in the 25 to 30°C range for optimum N₂-fixation and yield. Field studies were conducted in 1990 and 1991 at Montreal, Quebec to determine whether adaptability to cool soil conditions, with respect to earlier symbiosis establishment and function, existed among either Bradyrhizobium strains or soybean genotypes. An early maturing isoline of the soybean cultivar Evans and the cultivar Maple Arrow were inoculated with one of four strains isolated from the cold soils of Hakkaido, northern Japan, or the commercially used strains 532C or USDA110, at two planting dates. Plot biomass and nodulation were assessed at seedling (V2), and flowering(R2) growth stages and harvest maturity. Soybean genotypes did not differ for pre-flowering nodulation or N₂-fixation in the cool spring conditions of the first year. Seasonal N₂-fixation rates were also determined at the final harvest by the N-balance and ¹⁵N-isotope dilution methods. Significantly higher symbiotic activity was found for two of the four Hakkaido strains and was reflected in higher final soybean seed yield and total N₂-fixation for the growing season, as compared to the two commercial strains. Planting 14 days earlier resulted in greater early vegetative and total seasonal N₂ fixation and yield in the second year when soil temperatures were warmer, emphasizing the need for the development of soybean-Bradyrhizobium combinations superior in nodule development and function under cool soil conditions.     

Plant growth, leaf water potential, nitrogenase activity and nodule anatomy in Leucaena leucocephala as affected by water stress and nitrogen availability

 The effects of water stress and nitrogen availability on leaf water potential, nitrogenase activity, and growth was studied in a pot experiment with Leucaena leucocephala seedlings. Water stress was imposed on fertilized and unfertilized plants after inoculation with Rhizobium. Non-inoculated seedlings were used as control plants. Water stress lowered leaf water potential in all seedlings after 14 days of treatment. In inoculated seedlings, fertilized plants were more sensitive to water stress than unfertilized plants, as shown by a higher leaf water potential in plants of the latter treatment. Uninoculated and fertilized seedlings were most affected by water stress. This indicates that Rhizobium might increase stress tolerance in unfertilized seedlings at moderate water stress levels. The combined effects of water stress and applied fertilizers resulted in cessation of nitrogen fixation. Nitrogen fixation came to a complete stop after 22 days of water stress in fertilized seedlings. The different treatments were accompanied by anatomical changes of nodule structure. It is hypothesised that the leaf water potential may be used as an indicator to predict changes in nitrogen fixation in legume tree/shrub species during periods of water stress. Received: 21 October 1996 / Accepted: 12 May 1997     

Response of introduced Bradyrhizobium strains infectinga promiscuous soybean cultivar

Two field experiments were established to assess the competitiveness of foreign bradyrhizobia in infecting the promiscuous soybean cultivar TGX 536-02D. Seeds were inoculated with antibiotic mutants of the bradyrhizobia strains before planting after land preparation. Soybean plants were harvested at pre-determined days after planting for estimating nodule number, nodule dry weight, nodule occupancy, shoot dry weight and seed yield. Results show that nodule number and dry weight significantly increased and showed great variability at 84 days after planting (DAP), probably due to differences in the ability of inoculant bradyrhizobia to form nodules with the soybean cultivar TGX 536-02D. Increased shoot dry weight, %N, total N and seed yield were a result of increased nodulation by the effective and competitive inoculant Bradyrhizobium strains. Strain USDA 110 occupied the highest percentage of nodule sites because it was more competitive than the other Bradyrhizobium strains. These results show that there was high potential for increasing growth and seed yield of the promiscuous soybean cultivar TGX 536-02D by inoculation with foreign Bradyrhizobium strains.     

Variation in N2 fixation,N and P contents of mycorrhizalVigna unguiculata in relation to the progressive development of extraradical hyphae ofGlomus mosseae

Summary Vigna unguiculata cv. 58–185 grown in a sterile Dek soil was inoculated withRhizobium sp. orRhizobium sp. plusGlomus mosseae. Response of the host plant to the treatments was estimated by periodic measurements of shoot and nodule dry weights, N₂ fixation (C₂H₂ reduction activity) and N and P contents up to the 50th day of the growth cycle. It was only 45 days after planting that shoot dry weight of dually inoculated plants differed significantly from that of plants inoculated withRhizobium sp. alone. Nodule dry weight and N₂ fixation of dually inoculated plants were significantly higher than those of plants inoculated withRhizobium sp. alone from day 20 after planting, but there was no significant difference in N content (%). During the first 20 days, shoot P content (%) of both sets of plants decreased progressively, P content of dually inoculated plants being lower than that of the others. Later, P content of dually inoculated plants increased rapidly whereas P content of the other plants remained constant. Increase in nodule dry weight, N₂ fixation and P content of dually inoculated plants corresponded to the onset of the development of the extra-radical hyphae ofGlomus mosseae. In the rhizosphere.

---

Resumen Se cultivóVigna unguiculata cv. 58–185 en un suelo estéril tipo Dek, se inoculó conRhizobium sp. o conRhizobium sp. másGlomus mosseae. La respuesta de la planta huésped a los tratamientos se estudió midiendo periodicamente el peso seco de la parte aerea y de los nódulos, la fijación de N (actividad reductora de C₂H₂) y los contenidos de N y P hasta el 50° día del ciclo de crecimiento. La diferencia entre el peso seco de la parte aerea de las plantas con doble inoculación y aquellas inoculadas conRhizobium sp. unicamente, no fue significativa hasta 45 días despúés de la siembra. A los 20 días de la siembra tanto el peso seco de los nódulos como la fijación de nitrógeno de las plantas con doble inoculación eran significativamente superiores a los valores obtenidos para las plantas con soloRhizobium sp., aunque no se observaron diferencias en el contenido en N (%). Durante los primeros 20 días del ciclo el contenido en P (%) de ambos grupos de plantas disminuyó progresivamente, siendo los valores obtenidos por las plantas con doble inoculación inferiores a los de las demás. Más tarde el contenido en P de las plantas con doble inoculación aumentó rapidamente manteniéndose constante el de las demás. El incremento en el peso seco de los nódulos, en la fijación de N y en el contenido en P de las plantas con doble inoculación se correspondió con el inicio del desarrollo de las hifas extraradiculares deGlomus mosseae.

---

Résumé On a inoculéV. unguiculata poussant dans un sol Dek stérile avecRhizobium etRhizobium plusGlomus mosseae. On a recherché la réponse de la plante-hôte à ces deux traitements en estimant périodiquement les poids des nodules et des parties aériennes de la plante, la fixation d'azote (activité réductrice de C₂H₂), les teneurs en N et P jusqu'au 50^e jour du cycle de végétation. C'est seulement au 45^e jour après la plantation que le poids sec des parties aériennes des plantes inoculées avec deux symbiotes (plantes doublement inoculées) diffère significativement de celui des plantes inoculées avec Rhizobium seul. Le poids sec des nodules et la fixation N₂ des plantes doublement inoculées sont significativement plus élevés que ceux des plantes inoculées avecRhizobium seul au 20^e jour après la plantation mais il n'y a pas de différence significative pour la teneur en N (%). Pendant les 20 premiers jours, la teneur en P (%) des parties aériennes des deux catégories de plantes décroit progressivement; la teneur en P des plantes doublement inoculées est plus faible que celle des plantes inoculées seulement avecRhizobium. Plus tard, la teneur en P des plantes doublement inoculées augmente rapidement tandis que celle des autres plantes reste constante. L'accroissement du poids sec des nodules, de la fixation d'azote et de la teneur en P observé chez les plantes doublement inoculées correspond au démarrage du développement des hyphes extra-radicales deGlomus mosseae dans la rhizosphère.

     

Effect of dual inoculation ofRhizobium and vesicular arbuscular mycorrhizal fungi onPisum sativum

The interaction of two symbionts,Rhizobium (a bacterium) andGigaspora calospora (a VAM fungus) was studied inPisum sativum (a nodulating legume).Pisum sativum plants inoculated with VA mycorrhizal fungi andRhizobium singly and in combination responded favourably as compared to uninoculated control. Characteristically dual inoculation exerted a higher beneficial effect on shoot and root dry mass, phosphorus and nitrogen uptake, nodule formation and degree of mycorrhizal infection than either inoculum alone.     

Soybean dry matter and N accumulation responses to flooding stress, N sources and hypoxia

Soybean (Glycine max [L.] Merr.) is generally considered sensitive to flooding stress. Data on relative sensitivities of biomass accumulation and N2 fixation to flooding stress, however, are limited. Additionally, it is not clear why plants dependent on N2 fixation appear to be more flood-sensitive than plants supplemented with inorganic N. This study evaluated the response to flooding and N source of biomass and N accumulation in various soybean genotypes. Soybean plants were grown in a potting mixture in a greenhouse and flooded for 21 d in degassed nutrient solution. An additional experiment evaluated root hypoxia by exposing roots of plants to a gas mixture supplying 1.5 kPa pO2. Dry matter and N were determined at various times following the initiation of flood or low O2 treatment. In all experiments, N2 fixation was more sensitive to flooding than was biomass accumulation. The decrease in N2 fixation occurred faster (within 7 d of flooding) than the decrease in biomass (within 14-21 d), and the decrease in N2 fixation was more pronounced than the decrease in biomass. Addition of nitrate decreased flood sensitivity relative to plants dependent on N2 fixation. Plant response to hypoxia was similar to flooding. Biomass of plants with roots exposed to 1.5 kPa pO2 was decreased by 34% when dependent on N2 fixation and 12% when supplemented with nitrate. Collectively, the data indicate that decreased soybean growth under flooding is a result of decreased N2 fixation and that supplementation of soybean plants with nitrate may improve their tolerance to flooding relative to those relying on N2 fixation.Keywords: Soybean, Glycine max, flooding stress, hypoxia, N source, nitrogen fixation.      

Production of Exo-polysaccharide by <Emphasis Type="Italic">Rhizobium</Emphasis> sp.

Two fold increase in the yield of glucose and maltose containing exo-polysaccharide (EPS) by Rhizobium sp. was observed during its growth in modified YEMB. EPS production, plant growth promotion activity and root colonization of Rhizobium sp. studies showed enhanced EPS synthesis, more seed germination and over all improvement in plant growth over control and R. meliloti treatment. Groundnut seeds bacterized with Rhizobium sp. resulted in 69.75% more root length, 49.51% more shoot height, 13.75% more number of branches and 13.60% more number of pods over the control and R. meliloti treatment. Bacterization of wheat seeds increased the dry matter yield of roots (1.7-fold), and roots adhering soil (RAS) (1.5) and shoot mass (1.9-fold). Rhizobium sp. inoculation also increased the population density of EPS-producing bacteria on the rhizoplane. Roots of plants inoculated with Rhizobium sp. maintained a higher K⁺/Na⁺ ratio and K⁺–Na⁺ selectivity.     

Nitrate Effect on Nitrogen Fixation (Acetylene Reduction): ACTIVITIES OF LEGUME ROOT NODULES INDUCED BY RHIZOBIA WITH VARIED NITRATE REDUCTASE ACTIVITIES

The effect of nitrate on symbiotic nitrogen fixation by root nodules of cowpea (Vigna unguiculata L., Walp., cv. California Blackeye) and lupine (Lupinus augustifolius L., cv. Frost) plants inoculated with nitrate reductase-expressing and nitrate reductase-nonexpressing Rhizobium strains were examined. Nitrate reductase of Rhizobium bacteroids in the nodules of cowpea and lupine reduced nitrate to nitrite. Both cowpea and lupine nodules accumulated nitrite when grown in the presence of 15 millimolar nitrate and induced by Rhizobium strains which express nitrate reductase activity (Rhizobium sp. 32H1 and 127E15). The nitrogen fixation (acetylene reduction) activities of cowpea and lupine nodules were inhibited by nitrate whether the nodules were induced by Rhizobium strains that express (Rhizobium sp. 32H1 and 127E15) or do not express (Rhizobium sp. 127E14 and R. lupini ATCC 10318) nitrate reductase activity. These findings indicate that nitrite, the product of bacteroid nitrate reductase, may not play a role in the inhibitory effect of nitrate on nitrogen fixation activities of legume root nodules. However, the degree of inhibition on the fixation activity by nitrate varied in different legume-Rhizobium combinations.     

A small heat shock protein,GmHSP17.9, from nodule confers symbiotic nitrogen fixation and seed yield in soybean

Legume–rhizobia symbiosis enables biological nitrogen fixation to improve crop production for sustainable agriculture. Small heat shock proteins (sHSPs) are involved in multiple environmental stresses and plant development processes. However, the role of sHSPs in nodule development in soybean remains largely unknown. In the present study, we identified a nodule-localized sHSP, called GmHSP17.9, in soybean, which was markedly up-regulated during nodule development. GmHSP17.9 was specifically expressed in the infected regions of the nodules. GmHSP17.9 overexpression and RNAi in transgenic composite plants and loss of function in CRISPR-Cas9 gene-editing mutant plants in soybean resulted in remarkable alterations in nodule number, nodule fresh weight, nitrogenase activity, contents of poly β-hydroxybutyrate bodies (PHBs), ureide and total nitrogen content, which caused significant changes in plant growth and seed yield. GmHSP17.9 was also found to act as a chaperone for its interacting partner, GmNOD100, a sucrose synthase in soybean nodules which was also preferentially expressed in the infected zone of nodules, similar to GmHSP17.9. Functional analysis of GmNOD100 in composite transgenic plants revealed that GmNOD100 played an essential role in soybean nodulation. The hsp17.9 lines showed markedly more reduced sucrose synthase activity, lower contents of UDP-glucose and acetyl coenzyme A (acetyl-CoA), and decreased activity of succinic dehydrogenase (SDH) in the tricarboxylic acid (TCA) cycle in nodules due to the missing interaction with GmNOD100. Our findings reveal an important role and an unprecedented molecular mechanism of sHSPs in nodule development and nitrogen fixation in soybean.     

Studies on the Relationship Between Photosynthesis and Nitrogen Fixation in the Symbiotic System of Soybean and Nodule Bacteria( Rhizobium)

The relationship between photosynthesis of soybean and nitrogen fixation of the nodules by symbiotic Rhizobium was studied. The contents of total nitrogen and chlorophyll, the net photosynthetic rate and seed yield of soybean were much higher in either hydroponically cultivated or field-grown plants inoculated with Rhizobium B16–11C (or Clark nodulating strain) than in control without inoculation (or Clark non-nodulating strain). These results show that the symbiotic nitrogen fixation has a beneficial effect on photosynthesis. However, the effect was indirect and slow so that there was no change in the net photosynthetic rate of the soybean leaves until three clays after removing nodules from the soybean roots. On the other hand, decreasing the photosynthate supply to nodule by shade, defoliation or shoot removal of the soybean, the nodule activity declined significantly. It seems that the supply of photosynthate to root nodule is a limiting factor for symbiotic nitrogen fixation. However, the diurnal variation of the nodule activity could not be explained by change neither in the contents of sucrose and starch of the root nodules nor in the ambient temperature. The factor controlling the diurnal variation deserves further study.     

大豆-根瘤菌共生体系光合与固氮关系研究

水培大豆和田间生长的大豆,接种根瘤菌 Rhizobium B16-11C 后植株全氮含量、叶片叶绿素含量和净光合速率及种子产量都明显增加。比较 Clark 大豆的结瘤品系和不结瘤品系获类似结果。摘除根瘤后3天内叶片净光合速率无明显变化。大豆植株遮阴、去叶或切掉地上部导致根瘤活性明显下降。但去豆荚不能提高根瘤固氮的比活性。根瘤活性的日变化不能用根瘤蔗糖、淀粉含量或周围温度的变化来解释,其控制因子尚待深入研究。     

Effect of Rhizobium inoculation methodologies on nodulation and growth of Leucaena leucocephala

The aim of this study was to evaluate the effect of five methods of Rhizobium inoculum application on nodulation and nitrogen fixation in Leucaena leucocephala seedlings cultivated for 6 months in the greenhouse. Plants inoculated with alginate beads were significantly more developed and more nodulated than plants inoculated with the other methodologies used.     

Rhizobium Strain Effects on Nitrogen Transport and Distribution in Soybeans

Neves, M. C. P, Didonet, A. D., Duque, F. F. and Dobereiner,J. 1985. Rhizobium strain effects on nitrogen transport anddistribution in soybeans.—J. exp. Bot. 36: 1179–1192. The role of six Rhizobium strains in the nitrogen metabolismof soybeans (Glycine max Merril) was studied under glasshouseand field conditions. The strains could be divided into twogroups, group I which produced a large nodule mass with relativelylow efficiency and group II which produced less nodule massbut which fixed the same amount of nitrogen. Plants inoculatedwith group I strains remobilized nitrogen faster from leavesbut also lost more nitrogen in senesced leaves. Although thetotal nitrogen transported in the xylem was similar for allstrains, plants inoculated with group I strains contained lessnitrogen in ureides in the xylem sap during the whole growthcycle. This difference was reflected in the nitrogen partitioningwithin the shoot, and smaller nitrogen harvest indexes wereobserved in these plants than in those inoculated with strainsof group II. The role of ureides in the nitrogen partitioningand grain yield was confirmed by the significant correlationbetween mean ureide content in xylem sap and nitrogen partitioningor yield. Further, nodules formed with group I strains evolvedmore hydrogen than those formed with group II strains and thepossible significance of this is discussed. Key words: Ureides, hydrogen evolution, grain yield, grain yield, harvest index, nitrogen fixation     

Enhanced competitiveness of a Bradyrhizobium japonicum mutant strain improved for nodulation and nitrogen fixation

Bradyrhizobium japonicum strain TA-11NOD⁺, with altered indole biosynthesis, exhibited enhanced nodulation and nitrogen fixation on soybean in previous greenhouse studies. In this study, field experiments were conducted at Upper Marlboro, Maryland, in the summers of 1988 and 1993. In 1988, the site used was essentially free of soybean-nodulating bacteria and seed yield in plots inoculated with either I-110ARS or TA-11NOD⁺ was significantly higher by 12 or 20%, respectively, than that of the uninoculated controls. The 1993 site had an indigenous soil population (about 10⁴ cells g^-1) of symbiotically ineffective soybean-nodulating bacteria. Nevertheless, six-week-old Morgan soybean plants inoculated with strain TA-11NOD⁺ had 44% more nodules and exhibited 50% more nitrogen fixation by acetylene reduction when compared with plants that received the parental strain I-110ARS. Nodule occupancy, as determined using genetic markers for rifampicin and streptomycin resistance, was significantly higher for strain TA-11NOD⁺ than for strain I-110ARS. Overall, for the two years and the two soybean genotypes, the yield obtained with TA-11NOD⁺ was 6% higher than that obtained with I-110ARS. Competition experiments were conducted in the greenhouse and strain TA-11NOD⁺ was significantly more competitive than strain I-110ARS in competition with strains USDA 6 or USDA 438.     

Some USDA studies on the soybean-Rhizobium symbiosis

Summary The ecology, strain evaluation, genetics of host strain interactions and physiology of nitrogen fixation ofRhizobium japonicum in association with the soybean,Glycine max, were studied. Results of inoculation experiments with selected strains ofRhizobium japonicum indicated that indigenous strains occupied most of the nodules of soybeans grown in highRhizobium japonicum populated soils. Nodule sampling indicated that inoculation did not result in quicker nodulation or a higher incidence of root nodules (primary or secondary) than uninoculated checks. Rhizosphere studies indicated that colonization by introduced strains did occur but did not compete successfully with field strains for nodule sites. Recovery of specific serological types from nodules was influenced by planting intervals. The distribution of the serotypes varied with the time of planting and the age of the plant. Temperature studies indicated that the distribution of serotypes recovered from the nodules was influenced by temperature. Field studies showed the selectivity of soybean genotypes on strains ofRhizobium japonicum. Some strains were more common in the nodules of some varieties than in others. Closely related varieties had similar populations in their nodules. Three genes which control nodule response in soybeans are reported. Nitrogen fixation profiles were determined for some variety-strain interactions. Combinations previously classified as inefficient showed some nitrogenase activity as measured by the acetylene reduction technique. Research Microbiologist; Research Agronomist; Research Plant Physiologist, Soybean Investigations, Crops Research Division, Beltsville, Md. (USDA, ARS); and Plant Pathologist currently located at Michigan State University, East Lansing, Michigan.