Lime pelleting inoculated serradella (Ornithopus spp.) increases nodulation and yield

Lime pelleting of the inoculated seed is recommended for most pasture legume species to improve survival of the rhizobia on the seed and to counter deleterious effects of soil or fertiliser acidity on rhizobial numbers. Except for New South Wales, lime pelleting is specifically not recommended for serradella (Ornithopus spp.). Our objectives were to evaluate effects of lime pelleting on bradyrhizobial numbers on seed, and nodulation and growth of the serradella plants. Three experiments are reported at two acid-soil sites in northern New South Wales involving four cultivars of yellow serradella (Ornithopus compressus) and Bradyrhizobium sp. (Lupinus) strains WSM471 (current inoculant strain) and WU425 and WSM480. Lime pelleting increased bradyrhizobial numbers on seed, 24 h after inoculation, by an average of 90%. Similarly, lime pelleting increased nodulation and shoot dry matter of the inoculated plants by an average of 57 and 28%, respectively. The three strains were similar in effects on plant growth. Relative values for shoot dry weight, averaged over sites, were 100 for WSM471 and 98 for both WU425 and WSM480. Our results confirmed previous research that lime pelleting inoculated serradella seed was not deleterious to survival of the bradyrhizobial inoculum, and showed that it could result in enhanced symbiotic activity of the inoculum in some instances. We recommend lime pelleting of serradella and that WSM471 remain the inoculant strain.     

Use of aliette,a basipetally translocated antimicrobial compound,to enhance rhizophere colonization and nodulation by root-nodule bacteria

Summary A method was developed to improve the colonizing ability of inoculated strains of root-nodule bacteria using aliette (aluminum tris-O-ethyl phosphonate), a basipetally translocated fungicide. Aliette applied to seeds of alfalfa inoculated with an aliette-resistant strain of Rhizobium meliloti increased the numbers of R. meliloti in the rhizosphere after 3 but not 37 days, increased the number of nodules, and with some seed treatments, increased the growth of alfalfa. The enhanced colonization by R. meliloti as a result of seed treatment with aliette lasted for at least 31 days for alfalfa, although plant weights did not increase, Colonization by R. meliloti was further enhanced if seeds and foliage were treated with the fungicide. Coating seeds or sparaying the foliage with aliette also increased the number and weight of nodules and nitrogenase activity in soybeans inoculated with an aliette-resistant strain of Bradyrhizobium japonicum. The stimulation of B. japonicum in the rhizosphere and of nodulation was evident with successive plantings of soybeans if the seeds for each planting were treated with the chemical, but aliette did not increase the yield of inoculated soybeans in the subsequent plantings. With only the seeds of the first planting of inoculated soybeans treated with aliette, the numbers of B. japonicum in the rhizosphere of subsequent plantings were only occasionally greater and the numbers of nodules on the later plantings were not increased. We suggest that root colonization, nodulation, and N₂ fixation by Rhizobium and Bradyrhizobium may be enhanced by the use of basipetally translocated antimicrobial compounds together with root-nodule bacteria that are resistant to those compounds.     

Enhanced bioremediation of soil containing 2,4-dinitrotoluene by a genetically modified Sinorhizobium meliloti

The symbiotic nitrogen-fixing soil bacterium, Sinorhizobium meliloti, is well known for its ability to interact with the leguminous plant Medicago sativa L. It has, however, not been reported that this species possesses the capability to degrade toxic nitroaromatic compounds, such as 2,4-dinitrotoluene (DNT) which is commonly associated with the degradation of the explosive trinitrotoluene (TNT). In this study, the pJS1 DNT-biodegradative plasmid was genetically transferred to S. meliloti strain USDA 1936, which was confirmed by plasmid profile analysis. Several standard analytical and chemical tests including high performance liquid chromatography (HPLC), nitrite (NO₂) release assays, rhizosphere population and plant greenhouse studies were conducted to test the ability of S. meliloti to degrade 2,4-DNT. The possible presence of 2,4-DNT remaining in the treated soil was tested, and no 2,4-DNT had been absorbed by the soil. The pJS1-carrying recombinant strain DHK1 produced ‘ARC’ alfalfa plants that were almost 2-fold higher in shoot dry weight than that produced by the parent strain on soil containing 0.14 mM 2,4-DNT. The transconjugant strain DHK1 reduced significantly one-third more 2,4-DNT in both 0.14 and 0.28 mM contaminated soil, and in 0.55 mM contaminated soil it degraded 94% of the 2,4-DNT present. In liquid cultures, however, only about 4% reduction in 2,4-DNT concentrations was obtained in 10 days. We interpret the results as clearly establishing that genetic modification was successfully used, for the first time, to improve the capability of the symbiotic nitrogen-fixing soil bacterium S. meliloti DHK1 to bioremediate in situ 2,4-DNT-contaminated soil in the presence of alfalfa plants.     

The abundance and efficacy of Rhizobium leguminosarum bv. viciae in cultivated soils of the eastern Canadian prairie

For optimum production, the use of commercial rhizobial inoculant on pea (Pisum sativum L.) at seeding is necessary in the absence of compatible rhizobial strains or when rhizobial soil populations are low or symbiotically ineffective. Multiple site experiments were conducted to characterize the abundance and effectiveness of resident populations of Rhizobium leguminosarum bv. viciae (Rlv) in eastern Canadian prairie soils. A survey of 20 sites across a broad geographical range of southern Manitoba was carried out in 1998 and was followed by more intensive study of five of the sites in 1999 and 2000. Appreciable nodulation of uninoculated pea was observed at all sites which had previously grown inoculated pea. However, uninoculated pea grown at two sites, which had not previously grown pea, had negligible nodulation. Likewise, wild Lathyrus sp. and Vicia sp. plants collected from uncultivated areas adjacent to agricultural sites were poorly nodulated. In the more intensively studied sites, there was a tendency towards higher nodulation in pea plants receiving commercial inoculant containing Rlv strain PBC108 across all site-years (e.g., 4.7% in nodulation and 22% in nodule mass), but the effect was significant at only 2 of 10 site-years. Despite a relatively high range of soil pH (6-8), regression analysis indicated that decreasing soil pH resulted in lower nodulation rates. Likewise, electrical conductivity (EC) was correlated to nodulation levels, however the effect of EC was likely more indicative of the influence of soil texture and organic matter than salinity. As with nodulation, commercial inoculation tended to increase above-ground dry matter (DM) and fixed-N (estimated by the difference method) at the early pod-filling stage, but again the effects were significant at only 2 of 10 site-years. Specifically, above-ground DM and fixed-N levels were up to 29 and 51% greater, respectively, in inoculated compared to non-inoculated treatments at these sites. Addition of N-fertilizer at a rate of 100 kg N ha⁻¹ decreased nodulation at almost all site-years (by as much as 70% at one site), but rarely resulted in increases in above-ground DM compared to inoculated plots. The study indicates for the first time that populations of infective, and generally effective strains of Rlv occur broadly in agricultural soils across the eastern Canadian prairie, but that there is a tendency for increased symbiotic efficiency with the use of commercial inoculant.     

胶质芽孢杆菌与苜蓿根瘤菌双接种对排土场紫花苜蓿生长和土壤性质的影响

为了明确胶质芽孢杆菌和苜蓿根瘤菌在矿区排土场复垦中的协同作用,以矿区排土场土壤作为基质进行盆栽试验,设置两菌单接种处理、双接种处理及对照,其中胶质芽孢杆菌和苜蓿根瘤菌用量分别为1.50%和0.25%,测定紫花苜蓿产量指标、叶片生理指标、土壤速效养分含量和土壤微生物指标。结果表明,接种胶质芽孢杆菌和苜蓿根瘤菌的各处理均能显著提高紫花苜蓿的产量指标,其中以双接种处理效果最优,将生物量和结瘤量分别比对照显著提高110.27%和124.32%;在生理指标方面,双接种处理效果低于单接种处理,未表现出协同作用;在土壤养分和微生物性质方面,单接种胶质芽孢杆菌仅显著提高土壤速效钾含量,单接种苜蓿根瘤菌对大部分指标均有显著效果,而双接种处理效果最佳,土壤碱解氮、速效钾含量比对照分别显著提高94.14%和84.55%,土壤微生物指标MBC含量、呼吸强度、蔗糖酶、脲酶、磷酸酶、过氧化氢酶活性比对照分别显著提高204.02%,65.86%,212.32%,91.87%,30.57%,51.87%。双接种处理对紫花苜蓿产量指标、养分和微生物指标表现出显著的协同作用。因此,1.50%用量胶质芽孢杆菌和0.25%用量苜蓿根瘤菌双接种能够有效提高排土场土壤的复垦效果,具有协同作用,可以作为矿区排土场复垦的高效方法。     

Rapid identification and discrimination among Egyptian genotypes of Rhizobium leguminosarum bv. viciae and Sinorhizobium meliloti nodulating faba bean (Vicia faba L.) by analysis of nodC, ARDRA, and rDNA sequence analysis

Twenty-eight Rhizobium strains were isolated from the root nodules of faba bean (Vicia faba L.) collected from 11 governorates in Egypt. A majority of these strains (57%) were identified as Rhizobium leguminosarum bv. viciae (Rlv) based on analysis of a nodC gene fragment amplified using specific primers for these faba bean symbionts. The strains were characterized using a polyphasic approach, including nodulation pattern, tolerance to environmental stresses, and genetic diversity based on amplified ribosomal DNA-restriction analysis (ARDRA) of both 16S and 23S rDNA. Analysis of tolerance to environmental stresses revealed that some of these strains can survive in the presence of 1% NaCl and a majority of them survived well at 37 °C. ARDRA indicated that the strains could be divided into six 16S rDNA genotypes and five 23S rDNA genotypes. Sequence analysis of 16S rDNA indicated that 57% were Rlv, two strains were Rhizobium etli, one strain was taxonomically related to Rhizobium rubi, and a group of strains were most closely related to Sinorhizobium meliloti. Results of these studies indicate that genetically diverse rhizobial strains are capable of forming N₂-fixing symbiotic associations with faba bean and PCR done using nodC primers allows for the rapid identification of V. faba symbionts.     

Response of alfalfa (Medicago sativa L.) to single and mixed inoculation with phosphate-solubilizing bacteria and Sinorhizobium meliloti

The objectives of this work were to phenotypically and genetically characterize alfalfa rhizosphere bacteria and to evaluate the effect of single or mixed inoculation upon nodulation and biological nitrogen fixation. Thirty-two strains showed tricalcium phosphate solubilization ability, and two of them caused bigger or equal solubilization halos than the control strain P. putida SP22. The comparison of the 16S ribosomal DNA sequences indicated that these strains are phylogenetically related to Bacillus spp. and Pseudomonas spp. A beneficial effect of both isolates on alfalfa growth was observed in coinoculation assays. Pseudomonas sp. FM7d caused a significant increase in root and shoot dry weight, length, and surface area of roots, number, and symbiotic properties of alfalfa plants. The plants coinoculated with Sinorhizobium meliloti B399 and the Bacillus sp. M7c showed significant increases in the measured parameters. Our results indicating that strains Pseudomonas sp. FM7d and Bacillus sp. M7c can be considered for the formulation of new inoculants.     

Effect of sewage sludge on suppressiveness to soil-borne plant pathogens

The objective of this study was to evaluate the effect of sewage sludge on soil suppressiveness to the pathogens Fusarium oxysporum f. sp. lycopersici on tomato, Sclerotium rolfsii on bean, Sclerotinia sclerotiorum on tomato, Rhizoctonia solani on radish, Pythium spp. on cucumber, and Ralstonia solanacearum on tomato. Soil samples were collected from an experimental corn field in which sewage sludge had been incorporated once a year, since 1999. Sludge from two sewage treatment stations in Brazil (Franca and Barueri, SP) were applied at the rates of one (1N), two (2N), four (4N) and eight (8N) times the N recommended doses for the corn crop. Soil suppressiveness was evaluated by methods using indicator host plants, baits and mycelial growth. There was no effect of sewage sludge on soil suppressiveness to Fusarium oxysporum f. sp. lycopersici in tomato plants. For S. rolfsii, reduction of the disease in bean was inversely proportional to the dose of Franca sludge. The incidence of dead plants, caused by S. sclerotiorum, was directly proportional to sludge doses applied. For R. solani and R. solanacearum, there was a linear trend with reduction in plant death in soils treated with increasing amounts of sludge from Franca. There was an increase in the pathogen community of Pythium spp., proportional to the amounts of sewage applied. The effects of sewage sludge varied depending on the pathogen, methodology applied and on the time interval between the sewage sludge incorporation and soil sampling.     

Distribution and genetic diversity of rhizobia nodulating natural populations of Medicago truncatula in tunisian soils

A collection of 299 isolates of rhizobia nodulating Medicago truncatula was isolated from 10 Tunisian soils and was characterized by restriction fragment length polymorphism analysis of polymerase chain reaction (PCR/RFLP) of 16S rRNA gene. Results showed that 227 and 72 isolates were assigned, respectively, to Sinorhizobium meliloti and Sinorhizobium medicae. In 9 out of 10 soils S. meliloti was detected, whereas S. medicae was recovered from only 5 out of 10 soils. The cross-nodulation of three populations of M. truncatula grown on Bulla Regia soil, which contained naturally the two Sinorhizobium species, showed that M. truncatula population collected from Amra site was selective to S. meliloti at least in soil conditions. Forty-eight isolates of each Sinorhizobium species trapped by M. truncatula populations collected from Bulla Regia, Soliman and Rhayet sites on Bulla Regia soil were characterized by repetitive extragenic palindromic-PCR (REP-PCR) and showed a clear distinction between the two Sinorhizobium species and a higher diversity for S. meliloti.     

Co-inoculation of Halomonas maura and Ensifer meliloti to improve alfalfa yield in saline soils

Salinity is the major environmental factor limiting crop production. Alfalfa is a legume with high nutritional value that establishes a symbiosis relation with Ensifer meliloti. Under saline conditions the alfalfa yield decreases and this symbiosis is affected. The aim of this work is to study the effect of the co-inoculation of alfalfa plants with Halomonas maura (a moderately halophile bacterium) and E. meliloti in saline soils to improve their productivity and growth under greenhouse and field conditions. Alfalfa plants were grown in Leonard jar under greenhouse conditions, using a N-free mineral solution to mimic the conditions of an Orthic Solonchak. Then alfalfa plants were grown in the field in the same soil type. Seeds were inoculated with E. meliloti, H. maura, co-inoculated with E. meliloti and H. Maura, or non-inoculated as a control in both experiments. In greenhouse experiments the co-inoculation of alfalfa plants increased significantly the shoot dry weight (0.64 ± 0.02 vs. 0.79 ± 0.02), the leghaemoglobin content (10.17 ± 0.03 vs. 11.25 ± 0.06) and water potential (−3.12 ± 0.02 vs. −2.79 ± 0.02) compared with the single inoculation with E. meliloti. In the field experiments, biomass of co-inoculated plants clearly outyielded those of plants inoculated with any inoculant. The co-inoculation of H. maura and E. meliloti enhances alfalfa productivity in saline soils, thus contributing to the agricultural exploitation of low productive areas. H. maura and E. meliloti could be considered in formulation of bioinoculants to contribute in the reduction of the overuse of chemical fertilizers and their environmental impacts.     

The effect of acidity on the production of signal molecules by Medicago roots and their recognition by Sinorhizobium

The Medicago sativa-Sinorhizobium symbiosis is challenged by acidity, resulting in generally poor nodulation and production. Medicago murex, however, can nodulate and grow at low pH. The effect of low pH on signal exchange in the Sinorhizobium-Medicago symbiosis was studied to gain a greater understanding of the basis for poor nodulation of M. sativa compared to M. murex. Root exudates from M. sativa and M. murex grown in buffered nutrient solution at pH 4.5, 5.8 and 7.0, were collected to measure the expression of nodB induction in Sinorhizobium. A nodB-gusA fusion was constructed and inserted into Sinorhizobium medicae strains WSM419 (acid tolerant) and CC169 (acid sensitive). We identified greater induction by root exudates from both Medicago spp. collected at pH 4.5 than at pH 5.8 and 7.0, less induction by M. murex than M. sativa and less induction of WSM419 than CC169. The same major inducing compounds, 4′,7-dihydroxyflavanone (liquiritigenin), 4′,7-dihydroxyflavone, and 2′,4′,4-trihydroxychalcone (isoliquiritigenin), were identified in exudates of M. murex and M. sativa at all pH values, although in increasing amounts at lower pH. Poor nodulation of M. sativa relative to M. murex under acid conditions is not the consequence of decreased induction of Sinorhizobium nodB by chemical inducers present in the root exudates of both species at low pH.     

Variation in symbiotic characters of alfalfa cultivars inoculated with Sinorhizobium meliloti strains

In this paper the interactive effects of different Sinorhizobium meliloti strains and alfalfa cultivars on the host plant growth and nodulation were evaluated. In the experiments, six alfalfa cultivars (Giza-1, Ismailia-94, Kometa, Radius, Luzella and Legend) as well as five S. meliloti strains (ARC-1, ARC-2, A₂, TAL and L5-30) were included. The alfalfa cultivars, Giza-1 and Ismailia-94, exhibited the highest average dry forage weight with all studied strains. Generally, plants inoculated with the S. meliliti ARC-2 strain produced significantly higher shoot and root dry weights over all evaluated cultivars. The strain L5-30 of S. meliloti produced the highest nodule number with all the studied alfalfa cultivars whereas, among tested cultivars, N₂-fixing Luzella showed the highest nodulation with the majority of rhizobial strains. Analysis of correlation coefficients for the studied alfalfa-rhizobium associations revealed positive and significant relationships of shoot dry weight with root dry weight. In contrast, nodule number per plant correlated negatively with forage dry weight of plants. It was also found that nodule number per plant is not a promising trait with which to select the most effective N₂-fixing rhizobium-legume systems; selection of such symbiotic associations should be done on the basis of shoot and root dry weights.     

Phosphate-solubilizing Pseudomonas putida can influence the rhizobia-legume symbiosis

Allfalfa and soybean are the most important leguminous plants in the agricultural system of the semiarid pampas of Argentina. The possible action of phosphate solubilizing bacteria on the leguminous-rhizobia symbiosis was studied since in this region the available phosphorus distribution is not uniform. The strains used were Sinorhizobium meliloti 3DOh13, a good solubilizer of iron and phosphorus for alfalfa, Bradyrhizobium japonicum TIIIB for soybean and two phosphorus-solubilizing strains of Pseudomonas putida (SP21 and SP22) for growth promotion treatments. Modification of shoot and root system dry weights occured in soybean but not in alfalfa in presence of Pseudomonas strains.     

Restoring forest herb communities through landscape-level deer herd reductions: Is recovery limited by legacy effects?

White-tailed deer (Odocoileus virginianus) overbrowsing has altered plant species diversity throughout deciduous forest understories in eastern North America. Here we report on a landscape-level (306 km²) project in Pennsylvania, USA that tracked the herbaceous community response to deer herd reductions. From 2001 to 2007, we estimated deer densities, browse impact on woody seedlings, and censused the herbaceous flora in permanent plots throughout the area. We assessed herb layer species richness, abundance, and dominance and measured three known phytoindicators of deer impact: Trillium spp., Maianthemum canadense, and Medeola virginiana. We predicted that browse-sensitive taxa would increase in abundance, size, and flowering as would overall species diversity following deer culls and browse impact that declined by an order of magnitude by 2007. Following intensified deer harvests, we observed a limited recovery of the herbaceous community. Trillium spp. abundance, height, and flowering; M. canadense cover; and M. virginiana height all increased following herd reductions. Similarly, forb and shrub cover increased by 130% and 300%, respectively. Nevertheless, species diversity (i.e., richness and dominance) did not vary. Our work demonstrates that reducing deer densities can provide rapid morphological and population-level benefits to palatable species without a concomitant recovery in diversity. We suggest that decreasing deer populations alone may not promote plant diversity in overbrowsed, depauperate forests without additional restoration strategies to mitigate a browse-legacy layer dominated by browse-resistant species.     

Population responses of oribatids and enchytraeids to ectomycorrhizal and saprotrophic fungi in plant-soil microcosms

The oribatid mites Oppiella nova, Tectocepheus velatus and Nothrus silvestris and the enchytraeid worm Cognettia sphagnetorum are four common animal species in boreal forest soils. According to the literature, they respond differently to clear-cutting of forest stands. O. nova responds with population decreases, T. velatus and N. silvestris with small changes and C. sphagnetorum with population increases. We hypothesised that the presence/absence of ectomycorrhizal (EM) fungi is a major factor in explaining these reactions. The population responses of these soil animals to inoculation of five species of EM fungi growing in symbiosis with their host tree (Pinus sylvestris L.) and one saprotrophic fungus, Hypholoma capnoides, growing on wood were tested in pot microcosms with artificial soil (peat and vermiculite) for 70-84 days. Additionally, plants without inoculation of EM fungi, plants growing in forest soil (FS) and plant-free peat and vermiculite (PV) were included. O. nova increased significantly in abundance in the treatments with the EM fungi Suillus variegatus and Paxillus involutus, but not in the other treatments. T. velatus increased significantly in abundance in FS, but declined in most of the other treatments, and N. silvestris showed a similar, albeit not significant, response. C. sphagnetorum did not increase in abundance in any of the treatments with EM fungi but increased its abundance four times in both FS and PV. The results show that the fungivore O. nova preferentially feeds on certain EM fungi, especially S. variegatus, whereas the EM fungus Piloderma fallax and the saprotrophic H. capnoides are not preferred. In contrast, C. sphagnetorum is restricted in its population growth by the EM fungi studied, and this dominant microbi-detritivore is clearly favoured by the absence of EM fungi.     

Relationship between total nodulation and nodulation at the root crown of peanut, soybean and common bean plants

The objective of this study was to simplify the procedure for evaluation of legume nodulation, by examining if there is a relationship between the nodulation of the whole root system and at the crown region. Roots of peanut, soybean and common bean plants growing in soils were split in two parts (crown and bottom) and assessed for nodulation (nodule number and dry weight). In general, most nodules were concentrated at the crown, and crown nodulation was generally positively correlated with total nodulation of all three legume crops. The results are highly applicable in studies such as strain selection for inoculants and assessment of inoculation technologies, among others, and is an important contribution to help reduce the time and labor required for the evaluation of nodulation parameters.     

Effect of successive cauliflower plantings and Rhizoctonia solani AG 2-1 inoculations on disease suppressiveness of a suppressive and a conducive soil

Disease suppressiveness against Rhizoctonia solani AG 2-1 in cauliflower was studied in two marine clay soils with a sandy loam texture. The soils had a different cropping history. One soil had a long-term (40 years) cauliflower history and was suppressive, the other soil was conducive and came from a pear orchard not having a cauliflower crop for at least 40 years. These two soils were subjected to five successive cropping cycles with cauliflower or remaining fallow in a greenhouse experiment. Soils were inoculated with R. solani AG 2-1 only once or before every crop. Disease decline occurred in all treatments cropped with cauliflower, either because of a decreased pathogen population or increased suppressiveness of the soil. Disease suppressiveness tests indicated that the conducive soil became suppressive after five subsequent cauliflower crops inoculated each cycle with R. solani AG 2-1. Suppressiveness of all treatments was measured in a seed germination test (pre-emergence damping-off) as well as by measuring the spread of R. solani symptoms in young plants (post-emergence damping-off). Results showed that suppressiveness was significantly stimulated by the successive R. solani inoculations; presence of the cauliflower crop had less effect. Suppressiveness was of biological origin, since it disappeared after sterilization of the soil. Moreover, suppressiveness could be translocated by adding 10% suppressive soil into sterilized soil. The suppressive soil contained higher numbers of culturable filamentous actinomycetes than the conducive soil, but treatments enhancing suppressiveness did not show an increased actinomycetes population. The suppressiveness of the soil samples did also not correlate with the number of pseudomonads. Moreover, no correlation was found with the presence of different mycoparasitic fungi, i.e. Volutella spp., Gliocladium roseum, Verticillium biguttatum and Trichoderma spp. The suppressive soil contained a high percentage of bacteria with a strong in vitro inhibition of R. solani. These bacteria were identified as Lysobacter (56%), Streptomyces (23%) and Pseudomonas (21%) spp. A potential role of Lysobacter in soil suppressiveness was confirmed by quantitative PCR detection (TaqMan), since a larger Lysobacter population was present in suppressive cauliflower soil than in conducive pear orchard soil. Our experiments showed that successive cauliflower plantings can cause a decline of the damage caused by R. solani AG 2-1, and that natural disease suppressiveness was most pronounced after subsequent inoculations with the pathogen. The mode of action of the decline is not yet understood, but antagonistic Lysobacter spp. are potential key organisms.     

Burr medic (Medicago polymorpha L.) selections for improved N2 fixation with naturalised soil rhizobia

Burr medic (Medicago polymorpha L.) is an annual pasture legume that is widely distributed in southern Australian farming systems. Burr medic is nodulated by rhizobia (Sinorhizobium meliloti and Sinorhizobium medicae) that reside in many Australian soils, but the symbioses that develop are often sub-optimal in their rate of N₂ fixation. We attempted to identify burr medic lines, which are able to form effective symbioses with the naturalised Sinorhizobium in Australian field soils, as potential parents for a breeding program. There were three glasshouse experiments. Initially, 222 lines (including the M. polymorpha cvv. Santiago, Serena and Circle Valley) were inoculated with extracts of two soils that had been collected near Waikerie (soil S109) and Lochiel (soil S142) in South Australia. These soils were used because they contained numerically large communities of naturalised Sinorhizobium spp. that produced sub-optimal rates of N₂ fixation with cv. Santiago. None of the 222 lines of burr medic were able to form an effective symbiosis with the rhizobia from soil S109. However, when nodulated by the rhizobia from soil S142, some lines (e.g. SA8194) formed a very effective symbiosis, producing up to double the shoot dry matter (DM) of Santiago and eight times the DM of uninoculated plants. Seven promising lines were selected for further testing (with extracts of nine soils). Subsequently, two lines (SA20056 and SA8194) were selected and their symbiotic performance compared with that of Santiago, using extracts from 28 soils. While soil treatment had a major effect on mean shoot DM (soil N103=120 mg, soil N105=17 mg), the three medic lines performed similarly. Santiago, SA20056 and SA8914 all formed ineffective symbioses with the rhizobia in at least half of the 28 soils, even though >95% of the plants were nodulated. These experiments confirm that ineffective symbioses are common between burr medics and the rhizobia that have become naturalised in many Australian soils. Although some lines of burr medic were identified that were able to form more effective symbioses with the rhizobia in individual soils, none were able to form effective symbioses with a wide range of soil rhizobia. If a plant breeding approach is to be used to improve symbiotic performance of burr medic we propose that its hybridisation with other medic species, that have less specific rhizobial needs, will be required.     

The tripartite symbiosis between legumes, rhizobia and indigenous mycorrhizal fungi is more efficient in undisturbed soil

We investigated how the rate of colonization by indigenous arbuscular mycorrhizal fungi (AMF) affects the interaction between AMF, Sinorrhizobium meliloti and Medicago truncatula Gaertn. To generate a differential inoculum potential of indigenous AMF, five cycles of wheat, each of 1 month, were grown in sieved or undisturbed soil before M. truncatula was sown. The early colonization of M. truncatula roots by indigenous AMF was faster in undisturbed soil compared with sieved soil, but by pod-fill the frequency of hyphae, arbuscules and vesicles was similar in both treatments. At this latter stage, M. truncatula grown in undisturbed soil had accumulated a greater biomass in aboveground tissues, had a greater P concentration and derived more N from the atmosphere than plants grown in disturbed soil, although soil compaction resulted in plants having a smaller root system than those from disturbed soil. The difference in plant P content could not be explained by modifications in hydrolytic soil enzymes related to the P cycle as the activity of acid phosphatase was greater in sieved than in undisturbed soil, and the activity of alkaline phosphatase was unaffected by the treatment. Thus, the results observed were a consequence of the different rates of AMF colonization caused by soil disturbance. Together with earlier results for soybean, this study confirms that soil disturbance modifies the interaction between indigenous AMF, rhizobia and legumes leading to a reduced efficacy of the bacterial symbiont.     

Brassica napus seed meal soil amendment modifies microbial community structure, nitric oxide production and incidence of Rhizoctonia root rot

A low glucosinolate content (21.8 μmol g⁻¹) Brassica napus seed meal (RSM) applied to orchard soils altered communities of both pathogenic and saprophytic soil micro-organisms. RSM amendment reduced infection by native and introduced isolates of Rhizoctonia spp. and recovery of Pratylenchus spp. from apple roots. Root infection by Rhizoctonia solani AG-5 was also suppressed in split-root assays where a portion of the root system was cultivated in RSM-amended soils and the remainder grown in the presence of the pathogen but lacking RSM. R. solani hyphal growth was not inhibited by RSM amendment. Suppression of Pratylenchus was attained to an equivalent extent by amending soils with either RSM or soybean meal (SM) when applied to provide a similar N content. Thus, glucosinolate hydrolysis products did not appear to have a significant role in the suppression of Rhizoctonia spp. or Pratylenchus spp. obtained via RSM amendment. RSM amendment elevated populations of Pythium spp. and of ammonia-oxidizing bacteria that release nitric oxide but suppressed fluorescent pseudomonad numbers. Streptomyces spp. soil populations increased significantly in response to RSM but not SM amendment. The vast majority of Streptomyces spp. recovered from the apple rhizosphere produced nitric oxide and possessed a nitric oxide synthase homolog. We propose that transformations in the bacterial community structure are associated with the observed control of Rhizoctonia root rot, with NO production by soil bacteria potentially having a role in the induction of plant systemic resistance.