Genetic diversity and potential for promotion of plant growth detected in nodule endophytic bacteria of soybean grown in Heilongjiang province of China

A total of 98 non-symbiotic endophytic bacterial strains isolated from soybean root nodules were classified into eight rDNA types in ARDRA analysis and 21 BOX types in BOX-PCR. The phylogenetic analysis of 16S rDNA identified these strains as Pantoea, Serratia, Acinetobacter, Bacillus, Agrobacterium, and Burkholderia. Limited genetic diversity was revealed among these bacteria since most of the strains (85.7%) were found in three very similar rDNA types corresponding to Pantoea agglomerans, and many strains shared the same BOX-PCR patterns. The inoculation of nodule endophytes had no significant effects on the growth and nodulation of soybean, but most of the strains produced indoleacetic acid (IAA), could solubilize mineral phosphate, and could fix nitrogen, implying that they are a valuable pool for discovering plant growth promoting bacteria. Our results demonstrated that the nodule endophytes were common in soybean and their diversity was affected by the plant's character and the soil conditions. The 99% similarities found in the nifH genes of Bradyrhizobium japonicum and of the endophytic Bacillus strains strongly indicated that horizontal transfer of symbiotic genes happened between the symbiotic bacteria and the endophytes.     

Influence of growth-promoting bacteria on the growth of wheat in different soils and temperatures

Plant-growth-promoting bacteria isolated from the rhizosphere, phyllosphere and soil of the root zone in different climatic regions of Germany and Uzbekistan were analysed for plant-growth-promoting effects and nutrient uptake on winter wheat on different soils and under different temperature regimes. The investigations were carried out in pot experiments using loamy sand and sandy loam soils from Müncheberg, Germany and Calcisol soil from Tashkent, Uzbekistan. The temperature and soil types were found to influence growth-promoting effects. Inoculation with bacterial strains Pseudomonas fluorescens PsIA12, Pantoea agglomerans 050309 and Mycobacterium sp. 44 isolated from Müncheberg (semi-continental climate) was found to significantly increase the root and shoot growth of winter wheat at 16 °C compared to 26 °C in loamy sand. Mycobacterium phlei MbP18 and Mycoplana bullata MpB46 isolated from Tashkent (semi-arid climate) were found to significantly increase the root and shoot growth of winter wheat in nutrient-poor Calcisol at 38 °C as well as in nutrient-rich loamy sand at 16 °C. Bacterial inoculation also resulted in significantly higher N, P, and K contents of plant components. The bacteria isolates were able to survive in the rhizosphere and in the soil of winter wheat after root and shoot inoculation.     

Selenobacteria selected from the rhizosphere as a potential tool for Se biofortification of wheat crops

Cereal production in southern Chile is based on ash-derived volcanic Andisols, which present suboptimal levels of available selenium (Se). Strategies are needed to improve Se content in cereal crops and concomitantly improve the nutritional quality of grain. Here, we investigated the occurrence of Se-tolerant bacteria (STB) in Andisols and evaluated Se tolerance and accumulation in STB. The inoculation of wheat with STB and the contributions of these bacteria to Se content in plants were also evaluated under greenhouse conditions. The results showed that Se amendment of Andisols stimulated some bacterial groups (Paenibacillaceae and Brucellaceae) but inhibited others (Clostridia, Burkholderiales, Chitinophagaceae and Oxalobacteraceae), as revealed by denaturing gradient gel electrophoresis. Furthermore, we found four STB isolates that displayed 1-aminocyclopropane-1-carboxylate deaminase (ACC deaminase activity) and that carried the acdS gene as revealed by PCR. The selected STB were characterised as Stenotrophomonas, Bacillus, Enterobacter and Pseudomonas according to partial sequencing of the 16S rRNA gene. After 24 h of culture in nutrient broth, the selected STB showed the ability to grow in high Se concentrations (5 and 10 mM) and to accumulate elemental Se in micro- and nanospherical deposits, transforming 50–80 % of the Se initially added. Greenhouse experiments with wheat showed that Se associated with STB (micro- and nanospheres of elemental Se and other intracellular forms) can be translocated into leaves of wheat plantlets.     

Isolation and characterization of plant growth promoting endophytic bacterial isolates from root nodule of Lespedeza sp.

Thirty-nine endophytic bacterial strains were isolated from the nodule of Lespedeza sp. grown in two different locations of South Korea. All strains were checked for their plant growth promoting (PGP) abilities under in vitro conditions. Most of the isolates showed multiple PGP activity, i.e., indole acetic acid production, ACC deaminase activity, siderophore production, and phosphate solubilization. The strains were identified by using 16S rRNA gene sequence analysis as belonging to Alphaproteobacteria, Betaproteobacteria, Actinobacteria, and Firmicutes phylum with nine different genera Arthrobacter, Bacillus, Bradyrhizobium, Burkholderia, Dyella, Methylobacterium, Microbacterium, Rhizobium, and Staphylococcus. Gene nodA amplification showed positive results only for strains from Bradyrhizobium and Rhizobium genera. The strains from Bradyrhizobium and Rhizobium genera enhanced plant growth, nodulation, and acetylene reduction activity when inoculated on Vigna unguiculata L. (cowpea), whereas other strains did not induce nodule formation but enhanced plant growth. Herbaceous legume Lespedeza sp. formed root nodules with diverse bacterial group, and probably, these bacteria can be used for stimulating plant growth.     

Synergistic effect of root-associated bacteria on plant growth and certain physiological parameters of banana plant (Musa acuminata)

Fifty strains of bacteria isolated from banana roots were studied for their plant growth promoting (PGP) activities. Indole -3- acetic acid (IAA) production by root-associated bacteria ranged from 20 to 302 µg ml^?1. Seventeen isolates (34%) were positive for siderophore production and 18 isolates (36%) showed phosphate solubilization. None of the isolates showed potassium solubilization. All the isolates showed growth on nitrogen free Jensen medium. Identification of the bacteria based on 16S rRNA gene sequencing revealed that the isolates belonged to genus Bacillus sp, Klebsiella sp, Microbacterium sp and Enterobacter sp. A pot experiment in a greenhouse was conducted to investigate the effect PGP bacteria on banana plant growth and enzyme activities. The results demonstrated a significant (P < 0.05) increase in plant growth, chlorophyll, total phenolics, proline, catalase and ascorbic acid oxidase in banana plants treated with PGP bacteria as compared to control. However, the plant-growth response was variable and dependent on the bacterial strains, enzyme activity, and growth parameter observed. The present study revealed that bacteria showing multiple PGP activity could be used as biostimulants in enhancing banana production.     

Inoculating wheat seedlings with exopolysaccharide-producing bacteria restricts sodium uptake and stimulates plant growth under salt stress

A pot experiment was conducted to elucidate the effects of inoculating five exopolysaccharide- (EPS-) producing bacterial strains on the dry matter yield and the uptake of K⁺, Na⁺, and Ca²⁺ by wheat seedlings grown in a moderately saline soil. The bacteria were isolated from the rhizosphere soil (RS) of wheat grown in a salt-affected soil and included Aeromonas hydrophila/caviae (strain MAS-765), Bacillus insolitus (strain MAS17), and Bacillus sp. (strains MAS617, MAS620 and MAS820). The inoculation substantially increased the dry matter yield of roots (149–527% increase) and shoots (85–281% increase), and the mass of RS (176–790% increase). All the strains, except MAS617, also increased the RS mass/root mass ratio as well as the population density of EPS bacteria on the rhizoplane, and both these parameters were significantly correlated with the content of water-insoluble saccharides in the RS. Inoculation restricted Na⁺ uptake by roots, which was not attributable to the binding of Na⁺ by the RS, or to the ameliorative effects of Ca²⁺ under salinity. The decreased Na⁺ uptake by roots of inoculated than uninoculated plants was probably caused by a reduced passive (apoplasmic) flow of Na⁺ into the stele due to the higher proportion of the root zones covered with soil sheaths in inoculated treatments. Among the strains tested, MAS820 was the most efficient in all respects, whereas MAS617 was the least effective. Results suggested that inoculating selected EPS-producing bacteria could serve as a useful tool for alleviating salinity stress in salt-sensitive plants.     

Antagonists of Gaeumannomyces graminis from the rhizoplane of wheat in soils fertilized with ammonium- or nitrate-nitrogen

Take-all of wheat caused by Gaeumannomyces graminis var. tritici was less when soils in glasshouse pots were fertilized with NH₄⁺-N than with NO₃^?-N. The form of N did not alter countable populations of microorganisms in the rhizosphere or rhizoplane, but altered the numbers of bacteria and streptomycetes that inhibited the pathogen's growth in vitro. The pH of the medium used to isolate these microorganisms, whether similar or dissimilar to the pH of the rhizosphere, had some influence both upon countable populations and upon the proportions of antagonists. Highest counts of the rhizoplane microflora were on agar media with a pH similar to that of the soil. Most antagonists were isolated from a soil that is physically and chemically conducive to parasitism of wheat roots by Gaeumannomyces, but which contains a microflora suppressive toward the parasitic colonization of the roots. Isolates of the general bacterial flora, of Pseudomonas spp. and of streptomycetes, but not of Bacillus spp. inhibited the in vitro growth of G. graminis.     

Response of the soil bacterial community to the addition of toluene and toluene-degrading bacteria

Changes in soil bacterial communities after toluene and/or toluene degrading bacteria were added were monitored by growth on various media, and by the culture-independent method of Reverse Sample Genome Probing (RSGP). A total of 397 isolates that were cultured from toluene-amended and non-amended soil were identified using fatty acid methyl ester (FAME) analysis. In 75% of the soil samples, gram-positive bacteria dominated, primarily Bacillus sp. and Cellulomonas sp. In contrast, RSGP revealed Proteobacteria (α, β, and γ subgroups) to be the dominant species, with Bacillus sp. dominant in only one soil sample.In toluene-treated soil, FAME and RSGP analyses indicated that by day 5 the major bacterial populations were gram-negative bacteria, in particular, Pseudomonas sp., Acinetobacter sp., and Alcaligenes sp. When toluene and Pseudomonas putida D8 were coincidentally introduced, the proportion of Pseudomonas sp. in the bacterial population recovered using non-selective medium increased from 16 to 62% and then rapidly decreased to about 9%. When we used selective medium to monitor the population of P. putida D8, a rapid initial increase followed by a gradual decline was also observed. In samples analyzed by RSGP, D8 was one of the major species of the bacterial community at day 2, but its signal intensity dropped to 9.5% by day 5.The influence of D8 addition on the bacterial profile was monitored in growth-based examinations. Bacillus sp. and Pseudomonas sp. were initially dominant. By day 5, Bacillus sp. decreased while the Proteobacteria, (including Acinetobacter sp., Agrobacterium sp., Alcaligenes sp., Burkholderia sp., Erwinia sp., and Pseudomonas sp.) increased. At the same time, D8 decreased to a level indistinguishable from background. Conversely, RSGP analysis revealed the population dominance of P. putida (including D8) and Rhizobium fredii at day 2. Populations shifted toward Agrobacterium tumefaciens, Bacillus subtilis, R. fredii, and D8 by day 5.P. putida D8 levels could be monitored using RSGP when cultivation failed. However, cultivation of Bacillus sp. was always successful, while the organism was only occasionally detected by RSGP. While cultivation and RSGP methods comparably detected the same major bacterial populations, the overall bacterial diversity was greater with RSGP than with growth-based testing.     

Root-associated nitrogen-fixing bacteria and their role in the nitrogen nutrition of wheat estimated by 15N isotope dilution

The ability of two cultivars of spring wheat, Cadet and Rescue, and their reciprocal chromosome substitution lines, C-R5D and R-C5D, to obtain significant quantities of N from atmospheric N₂ was investigated in glasshouse experiments using ¹²⁵N dilution. The wheat was inoculated with N₂-fixing bacteria, including Azotohacter, Azospirillum, Klebsiella and Bacillus spp, in pure and mixed culture, at N concentrations ranging from 1 to 56 mg N plant^?1 (14–168 μg N ml^?1), in sand culture and in three soils of differing N content. Root-ussociutcd N₂-fixalion was negligible unless carbohydrate was added to the rooting medium. Atmospheric N₂ was incorporated into wheat roots and translocated to the tops, when plants inoculated with Azotobacter beijerinckii or Azospirillum brasilense sp. 107 were amended with glucose and malate respectively, under monoxenic conditions in sand culture.     

Isolation and characterization of rhizosphere auxin producing Bacilli and evaluation of their potency on wheat growth improvement

Enhancement of plant growth by Bacillus is well documented and several mechanisms have been suggested for the phytostimulatory activity of this group of plant growth-promoting rhizobacteria (PGPR). In the present work, the PGP potential of plant associated Bacillus spp. and their growth-promoting effect on wheat were studied. Six out of 35 strains were chosen based on seed germination assay, plant growth-promoting abilities, enzymatic function, and auxin production. All tested strains were subjected to pot experiments and their phenotypic and molecular assays were also done. Two Bacillus strains including WhIr-15 and WhIr-12 produce maximum amount of auxin (16.2 and 14 µg ml^?1, respectively). Strain WhIr-15 had just the ability to produce indo-3-acetic acid (IAA), lipase, and protease enzymes. Strain WhIr-12 was also recorded positive for siderophore, auxin production, and phosphorus (P) solubilization. Bacterial IAA production positively correlated with root length (r = 0.875; p ≤ 0.05). Significant enhancement in root weight (71% and 53%) and in panicle weight (91% and 77%) was recorded in WhIr-15 and WhIr-12, respectively, over untreated controls. Based on phenotypic and 16S rDNA sequencing, these two strains belong to Bacillus sp. Based on our results, phytohormone-producing Bacillus sp. can be applied at field level to improve wheat productivity.     

Molecular and functional characterization of bacteria isolated from straw and goat manure based vermicompost

A total of 193 vermicompost bacteria that exhibit antagonistic and biofertilizing potential were isolated from straw and goat manure based vermicompost produced by earthworm Eisenia foetida and taxonomically designated on the basis of their 16S rRNA sequence homology and subsequent molecular phylogeny analysis. Bacteria belonged to three major genera viz., Pseudomonas (15%), Bacillus (57%), Microbacterium (12%) and remaining bacteria comprised of genus Acinetobacter (5%), Chryseobacterium (3%), Arthrobacter, Pseudoxanthomonas, Stenotrophomonas, Paenibacillus, Rhodococcus, Enterobacter, Rheinheimera and Cellulomonas. Of 193 bacteria, 96 strains (49%) showed antagonistic potential against phytopathogenic fungus. Functional characterization of the bacteria was assessed by the production of protease, cellulase, lipase, xylanase, chitinase, amylase, gelatinase, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, indole-3-acetic acid (IAA), phosphate solubilization, nitrate reduction, and assimilation of different carbon sources. The bacterial strains showed a varying degree of carbon utilization profiles and majority of them were known to utilize citrate, malonate. A total of 86 strains produced protease (44%) and 99 strains (51%) produced cellulase. 51 strains (26%) produced IAA, 99 strains (51%) produced siderophores and 124 strains (64%) produced ACC deaminase. 52 strains (27%) showed phosphate solubilization while 24 strains (12%) produced HCN. 43 strains (22%) showed antibacterial activity against human pathogens under in vitro conditions. None of the strains tested positive for lipase and chitinase. A total of 31 strains (16%) produced DNase, 80 strains (41%) produced xylanase and 93 strains (48%) produced amylase. Among the vermicompost bacteria reported in the present study, Bacillus was found to be the predominant bacteria followed by Pseudomonas and Microbacterium. Present study, reports the molecular and functional characterization of vermicompost bacteria.     

Suppression of wheat take-all and ophiobolus patch by fluorescent pseudomonads from a fusarium-suppressive soil

Fluorescent pseudomonads isolated from a soil suppressing Fusarium wilt significantly reduced take-all (Gaeumannomyces graminis var. tritici) in wheat and Ophiobolus patch (G. graminis var. avenae) in Agrostis turfgrass. The bacteria were mixed into a conducive soil at a concentration of 10⁷ colony-forming units (cfu)g^?1 soil at sowing. There were significantly fewer (P ? 0.05) diseased wheat roots in the treatments with the bacteria and pathogen than in those with the pathogen alone. Dry weights of the tops of wheat and Agrostis turfgrass were significantly greater (P ? 0.01) in treatments inoculated with the bacteria in the presence of the pathogens compared to controls with the pathogens alone. Dry weights of the tops of plants from treatments inoculated with the bacteria alone were not significantly different to those of healthy wheat non-inoculated with the bacteria, showing that the fluorescent pseudomonads did not stimulate plant growth. At the end of the experiments, the bacterial isolates (genetically-marked with rifampicin resistance) were recovered from wheat roots and rhizosphere soil at concentrations of 10⁵–10⁷cfu g^?1 fresh weight of roots or oven-dried rhizosphere soil.Many of the fluorescent pseudomonads and some non-fluorescent pseudomonads showed in vitro antibiosis on quarter-strength potato dextrose agar (QPDA) against the pathogens. However, there was no correlation between in vitro antibiosis on agar plates and suppression of disease in pot experiments. Further, while some isolates of G. graminis var. tritici and var. avenae were inhibited by certain bacterial isolates, other isolates of the same fungus were not similarly inhibited by the same isolates of bacteria. Most of the fluorescent pseudomonads that produced inhibition zones (>5mm) against G. graminis var. tritici on QPDA did not do so on King's medium B, where fluorescent siderophores were formed. In vitro antibiosis is, therefore, a poor criterion for selecting effective bacterial antagonists of the wheat take-all fungus. All of the fluorescent pseudomonads tested produced siderophores in low-Fe media while a non-fluorescent pseudomonad and the fungal pathogens did not produce siderophores of comparable activity. The addition of 500 μg FeEDTA g^?1 with a lower stability constant did not. The evidence suggests that iron competition at the rhizoplane or in the rhizosphere is one mechanism of suppression.     

Cross-protection against the wheat and oat take-all fungi by Gaeumannomyces graminis var. graminis

Glasshouse experiments have shown that the prior colonisation of wheat roots by Gaeumannomyces graminis var. graminis, a fungus closely related to the wheat and oat take-all fungi but non-pathogenic to temperate cereals, reduced take-all infection along the roots. Cross-protected wheat plants produced grain yields significantly greater than those of unprotected plants but not significantly different to those of healthy wheat plants. A Phialophora-like fungus from grass roots did not confer the same degree of protection. There is some evidence that the cross-protection mechanism may be a specific host response nduced by var. graminis. The possible use of var. graminis in the biological control of take-all is discussed.     

Haplotype analysis of molecular markers linked to stem rust resistance genes in Ethiopian improved durum wheat varieties and tetraploid wheat landraces

The recent emergence of wheat stem rust race Ug99 (TTKSK) and related strains threaten Ethiopian as well as world wheat production because they overcome widely used resistance genes that had been effective for many years. The major cause which aggravates the ineffectiveness of Ethiopian wheat varieties against stem rust is the narrow genetic base on which the breeding for resistance has been founded, however, little is known about the resistance genotypes of Ethiopian durum wheat varieties and tetraploid wheat landraces. The objective of the study was to identify stem rust resistance genes that are present in the Ethiopian tetraploid wheat landraces and improved durum wheat varieties using molecular markers and assess which genes are effective for current Ethiopian stem rust races of Puccinia graminis f. sp. tritici including Ug99. The investigated 58 tetraploid wheat accessions consisted of 32 (Triticum durum s.l. incl. Triticum aethiopicum Jakubz., Triticum polonicum) landraces and 22 registered T. durum varieties released in Ethiopia between 1966 and 2009 and four T. durum varieties from ICARDA. A total of 17 molecular markers (SSR, EST and InDel) linked or diagnostic for stem rust resistance genes Sr2, Sr13, Sr22 and Sr35 were used for genotyping. Haplotype analysis indicated that only few of the Ethiopian durum wheat varieties carried Sr13. The resistant variety ‘Sebatel’ showed a haplotype for Sr2 and Sr22 and variety ‘Boohai’ for Sr22, however further evaluation is needed for the diagnostic value of these haplotypes. This study is the first report on the presence of stem rust resistance (Sr) genes in Ethiopian durum wheat varieties and tetraploid wheat landraces based on linked or associated molecular markers. Thus it might help in the identification of varieties carrying resistant alleles that provide valuable genetic material for the development of new improved varieties in further breeding programmes.     

Application of plant growth-promoting endophytes (PGPE) isolated from Solanum nigrum L. for phytoextraction of Cd-polluted soils

Many plant growth-promoting endophytes (PGPE) can assist their host plants cope with contaminant-induced stress responses, which can improve plant growth. In this study, four heavy metals resistant endophytic bacteria, Serratia nematodiphila LRE07, Enterobacter aerogenes LRE17, Enterobacter sp. LSE04 and Acinetobacter sp. LSE06, were isolated from Cd-hyperaccumulator Solanum nigrum L. grown in metal-polluted soil. Their plant growth promoting properties such as production of 1-aminocyclopropane-1-carboxylic (ACC) deaminase, indole-3-acetic acid (IAA), siderophores and phosphate solubilizing activity were characterized in vitro. When added to the Cd-amended soils, all of these four bacteria significantly increased Cd extraction from the soils. Subsequently, a pot experiment was conducted to elucidate the effects of inoculating of these PGPE on the plant growth and Cd uptake by S. nigrum L. grown in three different levels of Cd-contaminated soils. Results showed that the inoculation with these PGPE not only stimulated the growth of host plant, but also influenced the accumulation of Cd in the root, stem and leaf tissue of S. nigrum L. All four strains could colonize the rhizosphere soil and even some can be found in plant interior tissues. The present observations demonstrated that PGPE were valuable microorganism resource which can be exploited to improve the efficiency of phytoextraction.     

Effect of halotolerant endophytic bacteria isolated from Salicornia europaea L. on the growth of fodder beet (Beta vulgaris L.) under salt stress

The aim of this study was to determine the influence of selected halotolerant endophytic bacteria isolated from the roots of Salicornia europaea on the growth parameters of Beta vulgaris under different concentrations of salinity. Two endophytic strains were selected as inocula for the pot experiment: Pseudomonas sp. ISE-12 (B1) and Xanthomonadales sp. CSE-34 (B2). Surface-sterilised seeds were incubated in the bacterial inoculation suspensions before sowing and cultivated in a sterile mixture of sand and vermiculite (1:1). Six salinity treatments were taken into account: 0, 50, 100, 150, 200 and 300 mM NaCl. Inoculation of seeds with B1 and B2 positively affected germination percentage and germination index and shortened mean germination time, which led to a quickening of the growth stages of seedlings. After 42 days inoculated plants had, in general, a greater root length, higher dry biomass, lower tissue water content and lower specific leaf area compared with the control. While the positive effect of B2 bacteria was visible only at low salinity, strain B1 stimulated plant growth at higher salinities (200 and 300 mM NaCl). We suggest that the superior growth promotion observed for B1 may be related to the higher metabolic activity of these bacteria.     

Seed Bacterization with Fluorescent Pseudomonas spp. Enhances Nutrient Uptake and Growth of Cajanus cajan L.

Among plant-growth-promoting rhizobacteria (PGPR), fluorescent Pseudomonas spp. are an important group affecting plant growth. Pigeon pea is an important pulse crop and most of the studies were aimed at using Pseudomonas spp. for pest management in pigeon pea. Seventy-five fluorescent Pseudomonas spp. were isolated from diverse agroecosystems of India and evaluated for their plant-growth-promoting ability, primarily by the paper cup method. Seventeen selected isolates were further evaluated by short-term pot assay for plant growth promotion. Seeds treated with bacteria showed greater nutrient concentration and growth than the control. Isolate P17 showed significant growth promotion in terms of root length (54.5 cm), dry mass (323 mg), chlorophyll (24 spad units), carbohydrates (21.2 percent), nitrogen (2.45 percent), calcium (1.82 percent), iron (984 ppm), and manganese (564 ppm). Pseudomonas sp. P17 strain was identified as a potential PGPR for nutrient uptake and plant growth promotion in pigeon pea, and this finding paves a way for integrated plant nutrient management in rainfed agroecosystems.     

Some effects of montmorillonite on the growth of mixed microbial cultures

A defined starch urea-mineral-salts medium was used to study the effect of montmorillonite on the growth of pairs of microorganisms. The pairs of microorganisms used were Bacillus 3B-Klebsiella U34 and Bacillus 3B-Candida N2. When Bacillus 3B and Klebsiella U34 were grown together between montmorillonite concentrations of 0.4 mg·ml^?1 and 10 mg·ml^?1, linear relationships existed between logio (montmorillonite concentration) and the maximum yield of both Bacillus 3B and Klebsiella U34 and the final $\text{3B}\text{U34}$ ratio. At montmorillonite concentrations of greater than 10 mg·ml^?1 growth of both Bacillus 3B and Klebsiella U34 was reduced, possibly due to complete binding of starch to montmorillonite. Montmorillonite reduced the growth of Bacillus 3B and Candida N2 when they were grown together, possibly due to reduced urea hydrolysis.In these well defined systems, the analysis of the effects of clay minerals on microbial growth was complex. This complexity was due to the large number of points in the system at which the mineral could affect the system. The use of mathematical tools for analysis helped to quantify the exact relationships between Bacillus 3B, Klebsiella U34 and montmorillonite.     

Effect of manganese-reducing rhizosphere bacteria on the growth of Gaeumannomyces graminis var. tritici and on manganese uptake by wheat (Triticum aestivum L.)

Summary Five bacterial strains capable of Mn reduction were isolated from the rhizosphere of plants growing in different South Australian soils. They differed in their Mn-reducing capacity. The antagonism of these strains compared to the imported strain 2–79 (from the United States) against Gaeumannomyces graminis var. tritici was tested in agar and in a soil sandwich experiment at different Mn²⁺ concentrations in the soil. In addition, wheat seeds were coated with the different strains and with MnSO₄ or with MnSO₄ only in order to investigate their effect on plant growth and Mn uptake. With one exception, all strains inhibited the growth of G. graminis in agar, but to different degrees. In contrast, only two strains significantly inhibited the growth of the fungus in the soil. The hyphal density was decreased more than the hyphal length. The Mn²⁺ concentration in the soil also had a marked effect on fungal growth; low Mn concentrations slightly increased while high Mn concentrations strongly decreased the fungal growth. Seed treatment with MnSO₄ only (+Mn) increased Mn uptake above that of the control (no seed treatment). Only the weakest Mn reducer on agar significantly increased plant growth and Mn uptake from soil in comparison with the Mn treatment. One strain was tested as seed coating without adding MnSO₄; it increased the plant growth to an extent similar to the Mn treatment. Increasing the Mn uptake by plants may be one of the growth-promoting effects exerted by rhizosphere bacteria.