Competitive Ability and Efficiency in Nodule Formation of Strains of Bradyrhizobium japonicum

In the American Midwest, superior N₂-fixing inoculant strains of Bradyrhizobium japonicum consistently fail to produce the majority of nodules on the roots of field-grown soybean. Poor nodulation by inoculant strains is partly due to their inability to stay abreast of the expanding soybean root system in numbers sufficient for them to be competitive with indigenous bradyrhizobia. However, certain strains are noncompetitive even when numerical dominance is not a factor. In this study, we tested the hypothesis that the nodule occupancy achieved by strains is related to their nodule-forming efficiency. The nodulation characteristics and competitiveness of nine strains of B. japonicum were compared at both 20 and 30°C. The root tip marking technique was used, with the nodule-forming efficiency of each strain estimated from the average position of the uppermost nodule and the number of nodules formed above the root tip mark. The competitiveness of the nine strains relative to B. japonicum USDA 110 was determined by using immunofluorescence to identify nodule occupants. The strains differed significantly in competitiveness with USDA 110 and in nodulation characteristics, strains that were poor competitors usually proving to be inferior in both the average position of the uppermost root nodule and the number of nodules formed above the root tip mark. Thus, competitiveness was correlated with both the average position of the uppermost nodule (r = 0.5; P = 0.036) and the number of nodules formed above the root tip mark (r = 0.64; P = 0.005), while the position of the uppermost nodule was also correlated to the percentage of plants nodulated above the root tip mark (r = 0.81; P < 0.001) and the percentage of plants nodulated on the taproot (r = 0.67; P = 0.002).     

Competitiveness of indigenous populations of Bradyrhizobium japonicum serocluster 123 as determined using a root-tip marking procedure in growth pouches

Soil Bradyrhizobium populations limit nodule occupancy of soybean by symbiotically-superior inoculant strains throughout much of the American midwest. In this study, the competitiveness of indigenous populations of B. japonicum serocluster 123 from Waukegan and Webster soils was evaluated in growth pouches using a root-tip marking procedure. The native rhizobia were from soils incubated 0–8 h in soybean root exudate (SRE) or plant nutrient solution (PNS) prior to inoculation. Populations of serocluster 123 strains in soil and nodule occupancy by these strains were assessed using fluorescent antibodies prepared against B. japonicum USDA 123. There were no significant differences in populations that came from SRE or PNS incubated soils: both populations increased in number over the incubation period. Nodule occupancy by both populations in growth pouches was similar to that previously encountered in field studies with these two soils. With the Waukegan soil, the serocluster 123 population dominated nodulation forming 69 and 62% of taproot nodules above and below the root tip mark, respectively. However, for the more alkaline Webster soil, serocluster 123 strains were much less competitive, producing only 9 and 13%, respectively, of the nodules formed above and below the root tip mark. In growth pouches, soil populations of bradyrhizobia from the Webster soil produced significantly more nodules than those from the Waukegan soil, but both strains and a pure culture of USDA 110 had a similar distribution of nodules.     

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.     

Influence of Bradyrhizobium japonicum Location and Movement on Nodulation and Nitrogen Fixation in Soybeans

The influence of seed and soil inoculation on bradyrhizobial migration, nodulation, and N₂ fixation was examined by using two Bradyrhizobium japonicum strains of contrasting effectiveness in N₂ fixation. Seed-inoculated strains formed fewer nodules on soybeans (mostly restricted to the tap and crown roots within 0 to 5 cm from the stem base) than did bradyrhizobia distributed throughout the soil or inoculated at specific depths. Nodulation was greater below the depths at which bradyrhizobial cells were located rather than above, even though watering was done from below to minimize passive bradyrhizobial migration with percolating water. The most profuse nodulation occurred within approximately 5 cm below the point of placement and was generally negligible below 10 cm. These and other results suggest that bradyrhizobial migration from the initial point of placement was very limited. Nevertheless, the more competitive strain, effective strain THA 7, migrated into soil to a greater extent than the ineffective strain THA 1 did. Nitrogen fixation resulting from the dual-strain inoculations differed depending on the method of inoculation. For example, the amount of N₂ fixed when both strains were slurried together onto the seed was about half that obtained from mixing the effective strain into the soil with the ineffective strain on the seed. The results indicate the importance of rhizobial distribution or movement into soil for nodulation, nodule distribution, strain competitiveness, and N₂ fixation in soil-grown legumes.     

Methods To Alter the Recovery and Nodule Location of Bradyrhizobium japonicum Inoculant Strains on Field-Grown Soybeans

Three strains of Bradyrhizobium japonicum, I17, 110, and 61A76, were evaluated for their ability to form nodules on field-grown soybeans in soil with a highly competitive indigenous B. japonicum population. The predominant indigenous strain, 0336, in the field site used was unlike the more common isolates from Midwestern soils which belong to the 123 or 138 serogroups. This strain persisted in the soil for at least 30 years without any soybean crops. The three inoculant strains differed in their ability to compete with indigenous strains for nodule formation. Four different inoculation treatments were tested in three adjacent fields. When the amount of inoculum was increased, a higher proportion of nodules contained the inoculant strain. The most competitive inoculant strain was I17, a recent field isolate. Strain 61A76 was better than 110. There was no difference in recovery of the inoculant strains on the Hodgson or Corsoy soybean cultivars, nor was there a difference in recovery of the inoculant strains during the growing season. The vertical distribution of nodules containing the inoculant strains was affected by the method of adding the inoculant to the soil. Inoculant added to the seed furrow produced nodules mainly in the top region of the soybean root. Inoculant tilled into the soil produced nodules primarily in the bottom part of the root. The nodules that were produced in the bottom part of the root are younger and may contribute significant amounts of fixed nitrogen to the soybean during seed formation.     

Early Infection and Competition for Nodulation of Soybean by Bradyrhizobium japonicum 123 and 138

Interactions of soybean with Bradyrhizobium japonicum 123 (serogroup 123) and 138 (serogroup c1) were used to examine the relationship between early infection rates, competition for nodulation, and patterns of nodule occupancy. Both strains formed more infections in autoclaved soil (sterile soil) than in untreated soil (unsterile soil). Inoculation did not increase numbers of infection threads in unsterile soil-grown plants, where infection of proximal portions of primary roots was complete by 5 days after planting. Both strains infected and nodulated at similar rates in sterile soil. Nodules were always clustered on the upper root system, regardless of inoculation and soil treatment. Sixty-seven percent of the nodules of uninoculated plants grown in unsterile soil were occupied by rhizobia belonging to serogroups other than 123 or c1. Inoculation with strain 123 or 138 increased occupancy by that strain at the expense of residency by other rhizobia. Eighty-three percent of all nodules on plants dually inoculated with both strains in sterile soil contained strain 138. The corresponding value for plants inoculated in unsterile soil was 31%. Neither inoculum strain dominated occupancy of first-formed nodules in unsterile soil. It appears that north central Missouri soil may not have populations of highly competitive serogroup 123 and that early infection and nodulation rates do not contribute to the competitive success of strain 138.     

Nodulation of Glycine max by Six Bradyrhizobium japonicum Strains with Different Competitive Abilities

The root nodule locations of six Bradyrhizobium japonicum strains were examined to determine if there were any differences which might explain their varying competitiveness for nodule occupancy on Glycine max. When five strains were added to soybeans in plastic growth pouches in equal proportions with a reference strain (U.S. Department of Agriculture, strain 110), North Carolina strain 1028 and strain 110 were the most competitive for nodule occupancy, followed by U.S. Department of Agriculture strains 122, 76, and 31 and Brazil strain 587. Among all strains, nodule double occupancy was 17% at a high inoculum level (10⁷ CFU pouch⁻¹) and 2% at a low inoculum level (10⁴ CFU pouch⁻¹). The less competitive strains increased their nodule representation by an increase in the doubly occupied nodules at the high inoculum level. Among all strains, the number of taproot and lateral root nodules was inversely related at both the high and low inoculum levels (r = −0.62 and −0.69, respectively; P = 0.0001). This inverse relationship appeared to be a result of the plant host control of bacterial infection. Among each of the six strains, greater than 95% of the taproot nodules formed at the high inoculum density were located on 25% of the taproot length, the nodules centering on the position of the root tip at the time of inoculation. No differences among the six strains were observed in nodule initiation rates as measured by taproot nodule position. Taproot nodules were formed in the symbiosis before lateral root nodules. One of the poorly competitive strains (strain 76) occupied three times as many taproot nodules as lateral root nodules when competing with strain 110 (nodules were harvested from 4-week-old plants). Among these six wild-type strains of B. japonicum, competitive ability evidently is not related to nodule initiation rates.     

Molecular evidence for shifts in polysaccharide composition associated with adaptation of soybean Bradyrhizobium strains to the Brazilian Cerrado soils

Pyrolysis mass spectrometry (PyMS) and DNA fingerprinting (RAPD and RSα hybridization) were used to characterize soybean inoculant strains and root nodule isolates of bradyrhizobia from the Brazilian Cerrado soils. Most isolates were shown to be derived from the inoculant strains on the basis of genotype comparisons by DNA fingerprinting. Phenotypic analysis (using PyMS) of the strains and separately of the polysaccharides derived from them showed that the nodule isolates differed from the parental strains, suggesting adaptation to the Cerrado soil environment. The extent of the differences between the derivatives and inoculant strains was similar for comparisons made on the basis of whole-cell preparations or from the isolated polysaccharides, indicating that the adaptation was caused by changes in the composition of the polysaccharides produced.     

用发光酶基因(luxAB)标记法研究慢生花生根瘤菌的竞争结瘤能力

luxAB基因标记是一种新型基因标记技术,在很多研究领域都有着良好的应用前景。研究通过三亲本杂交将luxAB基因成功地向慢生型花生根瘤菌进行了转移,并获得了一株带LuxAB基因标记的菌株Cspr7-1。对Cspr7-1进行性状、标记基因的遗传稳定性检测,结果表明,LuxAB基因不仅能有效表达,而且性状稳定。在无氮水培条件下进行标记菌株与土著根瘤菌的竞争结瘤试验。结果证实,Cspr7-1在植物根系上的占瘤率平均达到61.3％,比土著根瘤菌的竞争结瘤能力强,而且Cspr7-1在主根上的侵染能力远较侧根上的强,平均高出22.3％-39.6％。     

Effect of soil bradyrhizobia on the success of soybean inoculant strain CB 1809

Four decades of soybean [Glycine max (L.) Merr.] cultivation in South Africa has resulted in the establishment of populations of bradyrhizobia against which the recently introduced inoculant strain CB 1809 must compete. Serological and DNA fingerprinting methods were used to study the diversity of nodule isolates from soils at Bergville, Koedoeskop and Morgenzon. Dominant serogroups included Bradyrhizobium elkanii serotype 76 at Bergville (67%), Bradyrhizobium japonicum serotype 123 at Morgenzon (81%) and B. japonicum serotype 135 at Koedoeskop (100%). Their origin is unknown as they do not correspond in serotype to strains used in previous inoculants. A small percentage of isolates from Bergville (13%) and Morgenzon (16%) were serologically homologous to strain WB 1 (serotype 31/76), applied for two decades before CB 1809 (serotype 122). Nitrogen-fixing effectiveness of CB 1809 was superior to 60% of the isolates tested from Bergville and Morgenzon, but similar to 73% of the Koedoeskop isolates. Seed and liquid-in-furrow application methods increased CB 1809 nodule occupancy at least three-fold above background levels at Bergville (pH 5.16) and Morgenzon (pH 6.33). Inoculation did not, however, increase CB 1809 nodule occupancy at Koedoeskop (pH 7.76), possibly because alkaline soil conditions favoured the serotype 135 population predominant at this site.     

Influence of Location, Host Cultivar, and Inoculation on the Composition of Naturalized Populations of Rhizobium meliloti in Medicago sativa Nodules

A phage typing system was used to evaluate the composition of indigenous populations of Rhizobium meliloti inhabiting nodules of Medicago sativa cultivars grown with and without inoculation at two field sites during 1983 and 1984. Soil at both locations contained established populations of R. meliloti at planting. Analysis of 1,920 nodule isolates revealed 55 unique phage types of indigenous R. meliloti at one site and 65 indigenous types at the other location. The distributions of phage types differed markedly between locations. At one site, the nodule population was dominated by two phage types; seven others occurred consistently but at lower frequency, and the remainder were encountered infrequently. No indigenous types predominated at the other location, although nine occurred more frequently than the remaining types. Indigenous R. meliloti predominated in nodules from inoculated plots at both sites, with inoculant recovery varying between 10 and 38% in each of two years. The frequency of occurrence of particular phage types at one location was significantly influenced by both M. sativa cultivar and inoculation. At this location, the interaction of cultivar and inoculation on the incidence of phage types suggests that the presence of an inoculant strain differentially affected nodule occupancy of M. sativa cultivars by members of the indigenous R. meliloti population. At both sites, the frequency of specific phage types differed between years. The data emphasize the importance of understanding the ecology and characteristics of indigenous Rhizobium populations as a prerequisite for elucidating problems of inoculant establishment and persistence in competitive situations.     

Developmental changes in peanut root structure during root growth and root-structure modification by nodulation

Background and Aims

Basic information about the root and root nodule structure of leguminous crop plants is incomplete, with many aspects remaining unresolved. Peanut (Arachis hypogaea) forms root nodules in a unique process. Structures of various peanut root types were studied with emphasis on insufficiently characterized lateral roots, changes in roots during their ontogenesis and root modification by nodule formation.

Methods

Peanut plants were grown in the field, in vermiculite or in filter paper. The taproot, first-order and second-order lateral roots and root nodules were analysed using bright-field and fluorescence microscopy with hand sections and resin sections.

Key Results

Three root categories were recognized. The primary seminal root was thick, exhibiting early and intensive secondary thickening mainly on its base. It was tetrarch and contained broad pith. First-order lateral roots were long and thin, with limited secondary thickening; they contained no pith. Particularly different were second- and higher-order lateral roots, which were anatomically simple and thin, with little or no secondary growth. Unusual wall ingrowths were visible in the cells of the central part of the cortex in the first-order and second-order lateral roots. The nodule body was formed at the junction of the primary and lateral roots by the activity of proliferating cells derived originally from the pericycle.

Conclusions

Two morphologically and anatomically distinct types of lateral roots were recognized: long, first-order lateral roots, forming the skeleton of the root system, and thin and short second- and higher-order lateral roots, with an incomplete second state of endodermal development, which might be classified as peanut ‘feeder roots’. Formation of root nodules at the base of the lateral roots was the result of proliferating cell divisions derived originally from the pericycle.Key words: Endodermis, lateral root structure, nodule structure, peanut, Arachis hypogaea, primary root structure     

Effect of Microbial Inoculants on the Indigenous Actinobacterial Endophyte Population in the Roots of Wheat as Determined by Terminal Restriction Fragment Length Polymorphism

The effect of single actinobacterial endophyte seed inoculants and a mixed microbial soil inoculant on the indigenous endophytic actinobacterial population in wheat roots was investigated by using the molecular technique terminal restriction fragment length polymorphism (T-RFLP). Wheat was cultivated either from seeds coated with the spores of single pure actinobacterial endophytes of Microbispora sp. strain EN2, Streptomyces sp. strain EN27, and Nocardioides albus EN46 or from untreated seeds sown in soil with and without a commercial mixed microbial soil inoculant. The endophytic actinobacterial population within the roots of 6-week-old wheat plants was assessed by T-RFLP. Colonization of the wheat roots by the inoculated actinobacterial endophytes was detected by T-RFLP, as were 28 to 42 indigenous actinobacterial genera present in the inoculated and uninoculated plants. The presence of the commercial mixed inoculant in the soil reduced the endophytic actinobacterial diversity from 40 genera to 21 genera and reduced the detectable root colonization by approximately half. The results indicate that the addition of a nonadapted microbial inoculum to the soil disrupted the natural actinobacterial endophyte population, reducing diversity and colonization levels. This was in contrast to the addition of a single actinobacterial endophyte to the wheat plant, where the increase in colonization level could be confirmed even though the indigenous endophyte population was not adversely affected.     

Exogenous Ethylene Inhibits Nodulation of Pisum sativum L. cv Sparkle

Exogenous ethylene inhibited nodulation on the primary and lateral roots of pea, Pisum sativum L. cv Sparkle. Ethylene was more inhibitory to nodule formation than to root growth; nodule number was reduced by half with only 0.07 μL/L ethylene applied continually to the roots for 3 weeks. The inhibition was overcome by treating roots with 1 μm Ag⁺, an inhibitor of ethylene action. Exogenous ethylene also inhibited nodulation on sweet clover (Melilotus alba) and on pea mutants that are hypernodulating or have ineffective nodules. Exogenous ethylene did not decrease the number of infections per centimeter of lateral pea root, but nearly all of the infections were blocked when the infection thread was in the basal epidermal cell or in the outer cortical cells.     

Overcoming non-selective nodulation of Lessertia by soil-borne rhizobium in the presence of inoculant mesorhizobium

Background and aims

Legumes of the South African genus Lessertia, along with their microsymbionts, were introduced into the Western Australia wheatbelt. They achieved poor establishment followed by weak summer survival. This was caused in part by low levels of nodulation with the inoculant strains, and by ineffective nodulation with naturalized strains –an example of non-selective nodulation. The aims of this work were to assess Lessertia spp. symbiotic promiscuity, to study the effect of increased doses of an effective inoculant strain (WSM3565) with L. herbacea, and to study the competitive ability and symbiotic performance of different Mesorhizobium strains nodulating L. diffusa.

Methods

A glasshouse experiment was set up to evaluate the ability of L. diffusa, L. capitata, L. herbacea and L. excisa to nodulate with inoculants under current use in Western Australia. To assess competitive ability two field experiments were set up at Karridale, Western Australia. L. herbacea was inoculated with the strain WSM3565 at different doses and L. diffusa was inoculated with ten different Mesorhizobium strains. Rhizobia were re-isolated from nodules and their identity confirmed through PCR fingerprinting and sequencing of their partial dnaK.

Results

There were differences in promiscuity between different Lessertia spp., where L. herbacea proved to be highly promiscuous under controlled conditions. Increasing the inoculation dose of L. herbacea with WSM3565 did not improve establishment and survival of the legume in the field. Although WSM3565 nodule occupancy improved from 28 to 54 % with higher doses of inoculation, none of the treatments increased L. herbacea yield over the inoculated control. The inoculation of L. diffusa with the strains WSM3598, 3636, 3626 and 3565 resulted in greater biomass production than the uninoculated control. These strains were able to outcompete resident rhizobia and to occupy a high (>60 %) proportion of lateral root nodules. The naturalised strains that achieved nodulation were identified as R. leguminosarum.

Conclusion

The high numbers of resident rhizobia and their ability to rapidly nodulate Lessertia spp. are likely to be the main reasons for the low nodule occupancy achieved by some effective inoculant strains with L. diffusa and L. herbacea. Strains WSM 3636 and 3598 were very competitive on nodule occupancy and together with WSM 3565, WSM 3612 and WSM3626, effective on nodule formation and plant growth of L. diffusa despite the high numbers of resident soil rhizobia. These strains and L. diffusa have potential to be introduced as exotic legumes species and rhizobia strains to Western Australia.     

Local and Systemic Regulation of Plant Root System Architecture and Symbiotic Nodulation by a Receptor-Like Kinase

In plants, root system architecture is determined by the activity of root apical meristems, which control the root growth rate, and by the formation of lateral roots. In legumes, an additional root lateral organ can develop: the symbiotic nitrogen-fixing nodule. We identified in Medicago truncatula ten allelic mutants showing a compact root architecture phenotype (cra2) independent of any major shoot phenotype, and that consisted of shorter roots, an increased number of lateral roots, and a reduced number of nodules. The CRA2 gene encodes a Leucine-Rich Repeat Receptor-Like Kinase (LRR-RLK) that primarily negatively regulates lateral root formation and positively regulates symbiotic nodulation. Grafting experiments revealed that CRA2 acts through different pathways to regulate these lateral organs originating from the roots, locally controlling the lateral root development and nodule formation systemically from the shoots. The CRA2 LRR-RLK therefore integrates short- and long-distance regulations to control root system architecture under non-symbiotic and symbiotic conditions.     

Dry matter yield,root traits,and nodule occupancy of lucerne and Caucasian clover when grown in acidic soil with high aluminium concentrations

Aims and methods

Lucerne and Caucasian clover dry matter were measured in response to recommended lime and capital P inputs for six years in an acidic soil in the New Zealand high country. The initial three years of the field experiment indicated successful establishment and persistence of both legumes. Lucerne dry matter (DM) yield was up to 4 t/ha/yr in this period and higher than Caucasian clover yields. However, a lack of persistence of lucerne was apparent from this point forward compared with Caucasian clover which produced 7.7 t DM/ha in Year 6. An experiment using tubes of soil was used to investigate whether differences in root traits, nodulation and nodule occupancy were responsible for the differences observed in field persistence over time.

Results

These showed that when rhizobia inoculant was added, the fine root length of Caucasian clover was unaffected (R² = 0.14) by aluminium (Al) content of the soil. In contrast, fine root growth of lucerne was suppressed (R² = 0.79) by the soil Al content. Nodulation of Caucasian clover was unaffected by soil pH or Al when the rhizobia inoculant was provided which suggests the viability of the commercial genotype ICC148 in this soil with a pH of 5.5 and Al ca. 7 mg/kg soil. For lucerne, the maximum nodulation score of 7.3 occurred with 2 t/ha of lime added (soil pH ca.6, Al ca. 0.3 mg/kg) plus inoculant.

Conclusions

This suggests an Al toxic threshold of <1.0 mg Al/kg soil for effective lucerne nodulation. From the lucerne nodules, eight naturalized strains of Ensifer meliloti were identified. In contrast, only one R. leguminosarum strain was detected in the Caucasian clover nodules. The competition between those rhizobia genotypes may negatively affect the efficiency of biological nitrogen fixation in lucerne. Therefore, the lack of genetic diversity of R. leguminosarum bv. trifolii in New Zealand soils might be an advantage especially if the commercial strain is acid soil tolerant.

     

Steep,cheap and deep: an ideotype to optimize water and N acquisition by maize root systems

Background

A hypothetical ideotype is presented to optimize water and N acquisition by maize root systems. The overall premise is that soil resource acquisition is optimized by the coincidence of root foraging and resource availability in time and space. Since water and nitrate enter deeper soil strata over time and are initially depleted in surface soil strata, root systems with rapid exploitation of deep soil would optimize water and N capture in most maize production environments.• The ideotype Specific phenes that may contribute to rooting depth in maize include (a) a large diameter primary root with few but long laterals and tolerance of cold soil temperatures, (b) many seminal roots with shallow growth angles, small diameter, many laterals, and long root hairs, or as an alternative, an intermediate number of seminal roots with steep growth angles, large diameter, and few laterals coupled with abundant lateral branching of the initial crown roots, (c) an intermediate number of crown roots with steep growth angles, and few but long laterals, (d) one whorl of brace roots of high occupancy, having a growth angle that is slightly shallower than the growth angle for crown roots, with few but long laterals, (e) low cortical respiratory burden created by abundant cortical aerenchyma, large cortical cell size, an optimal number of cells per cortical file, and accelerated cortical senescence, (f) unresponsiveness of lateral branching to localized resource availability, and (g) low K_m and high V_max for nitrate uptake. Some elements of this ideotype have experimental support, others are hypothetical. Despite differences in N distribution between low-input and commercial maize production, this ideotype is applicable to low-input systems because of the importance of deep rooting for water acquisition. Many features of this ideotype are relevant to other cereal root systems and more generally to root systems of dicotyledonous crops.     

Identification of Bradyrhizobium japonicum Nodule Isolates from Wisconsin Soybean Farms

One-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis was a more discriminating method than serotyping for identifying strains of Bradyrhizobium japonicum. Analysis of 543 nodule isolates from southeastern Wisconsin soybean farms revealed that none of the isolates were formed by any of the inoculant strains supplied by either of two inoculant companies. Twenty-nine indigenous strains and six inoculant strains were identified. Strain 61A76, the most competitive indigenous strain, formed 21% of the nodules. Indigenous strains 3030, 3058, 0336, and 3052 formed 15, 11, 9, and 9% of the nodules, respectively. These predominant strains were not associated with a particular soybean cultivar, soil type, or farm location.