Suitability of the microbial community composition and function in a semiarid mine soil for assessing phytomanagement practices based on mycorrhizal inoculation and amendment addition.

The recovery of species composition and functions of soil microbial community of degraded lands is crucial in order to guarantee the long-term self-sustainability of the ecosystems. A field experiment was carried out to test the influence of combining fermented sugar beet residue (SBR) addition and inoculation with the arbuscular mycorrhizal (AM) fungus Funneliformis mosseae on the plant growth parameters and microbial community composition and function in the rhizosphere of two autochthonous plant species (Dorycnium pentaphyllum L. and Asteriscus maritimus L.) growing in a semiarid soil contaminated by heavy metals. We analysed the phospholipid fatty acids (PLFAs), neutral lipids fatty acids (NLFAs) and enzyme activities to study the soil microbial community composition and function, respectively. The combined treatment was not effective for increasing plant growth. The SBR promoted the growth of both plant species, whilst the AM fungus was effective only for D. pentaphyllum. The effect of the treatments on plant growth was linked to shifts in the rhizosphere microbial community composition and function. The highest increase in dehydrogenase and ß-glucosidase activities was recorded in SBR-amended soil. The SBR increased the abundance of marker PLFAs for saprophytic fungi, Gram+ and Gram- bacteria and actinobacteria, whereas the AM fungus enhanced the abundance of AM fungi-related NLFA and marker PLFAs for Gram- bacteria. Measurement of the soil microbial community composition and function was useful to assess the success of phytomanagement technologies in a semiarid, contaminated soil.

Native plant growth promoting bacteria Bacillus thuringiensis and mixed or individual mycorrhizal species improved drought tolerance and oxidative metabolism in Lavandula dentata plants.

This study evaluates the responses of Lavandula dentata under drought conditions to the inoculation with single autochthonous arbuscular mycorrhizal (AM) fungus (five fungal strains) or with their mixture and the effects of these inocula with a native Bacillus thuringiensis (endophytic bacteria). These microorganisms were drought tolerant and in general, increased plant growth and nutrition. Particularly, the AM fungal mixture and B. thuringiensis maximized plant biomass and compensated drought stress as values of antioxidant activities [superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase APX)] shown. The AMF-bacteria interactions highly reduced the plant oxidative damage of lipids [malondialdehyde (MDA)] and increased the mycorrhizal development (mainly arbuscular formation representative of symbiotic functionality). These microbial interactions explain the highest potential of dually inoculated plants to tolerate drought stress. B. thuringiensis "in vitro" under osmotic stress does not reduce its PGPB (plant growth promoting bacteria) abilities as indole acetic acid (IAA) and ACC deaminase production and phosphate solubilization indicating its capacity to improve plant growth under stress conditions. Each one of the autochthonous fungal strains maintained their particular interaction with B. thuringiensis reflecting the diversity, intrinsic abilities and inherent compatibility of these microorganisms. In general, autochthonous AM fungal species and particularly their mixture with B. thuringiensis demonstrated their potential for protecting plants against drought and helping plants to thrive in semiarid ecosystems.

The combination of compost addition and arbuscular mycorrhizal inoculation produced positive and synergistic effects on the phytomanagement of a semiarid mine tailing.

A field experiment was carried out to assess the effectiveness of combining mycorrhizal inoculation with a native AM fungus (Glomus sp.) and the addition of an urban organic waste compost (OWC) applied at two rates (0.5 and 2.0% (w:w)), with regard to promoting the establishment of Anthyllis cytisoides L. seedlings in a heavy metal polluted mine tailing, as well as stimulating soil microbial functions. The results showed that the combined use of the highest dose of OWC and AM inoculation significantly increased shoot biomass - by 64% - compared to the control value. However, the separate use of each treatment had no effect on the shoot biomass of this shrub species. At the 2% rate, OWC enhanced root colonisation by the introduced fungus as well as soil nutrient content and soil dehydrogenase and ß-glucosidase activities. The combined treatment increased the uptake of Zn and Mn in shoots, although only Zn reached excessive or potentially toxic levels. This study demonstrates that the combination of organic amendment and an AM fungus is a suitable tool for the phytomanagement of degraded mine tailings, although its effectiveness is dependent on the dose of the amendment.

Contribution of arbuscular mycorrhizal fungi and/or bacteria to enhancing plant drought tolerance under natural soil conditions: effectiveness of autochthonous or allochthonous strains.

Autochthonous microorganisms [a consortium of arbuscular-mycorrhizal (AM) fungi and Bacillus thuringiensis (Bt)] were assayed and compared to Rhizophagus intraradices (Ri), Bacillus megaterium (Bm) or Pseudomonas putida (Psp) and non-inoculation on Trifolium repens in a natural arid soil under drought conditions. The autochthonous bacteria Bt and the allochthonous bacteria Psp increased nutrients and the relative water content and decreased stomatal conductance, electrolyte leakage, proline and APX activity, indicating their abilities to alleviate the drought stress. Mycorrhizal inoculation significantly enhanced plant growth, nutrient uptake and the relative water content, particularly when associated with specific bacteria minimizing drought stress-imposed effects. Specific combinations of autochthonous or allochthonous inoculants also contributed to plant drought tolerance by changing proline and antioxidative activities. However, non-inoculated plants had low relative water and nutrients contents, shoot proline accumulation and glutathione reductase activity, but the highest superoxide dismutase activity, stomatal conductance and electrolyte leakage. Microbial activities irrespective of the microbial origin seem to be coordinately functioning in the plant as an adaptive response to modulated water stress tolerance and minimizing the stress damage. The autochthonous AM fungi with Bt or Psp and those allochthonous Ri with Bm or Psp inoculants increased water stress alleviation. The autochthonous Bt showed the greatest ability to survive under high osmotic stress compared to the allochthonous strains, but when single inoculated or associated with Ri or AM fungi were similarly efficient in terms of physiological and nutritional status and in increasing plant drought tolerance, attenuating and compensating for the detrimental effect of water limitation.

Effects of water stress, organic amendment and mycorrhizal inoculation on soil microbial community structure and activity during the establishment of two heavy metal-tolerant native plant species.

Our aim was to examine the effect of water stress on plant growth and development of two native plant species (Tetraclinis articulata and Crithmum maritimum) and on microbial community composition and activity in the rhizosphere soil, following the addition of an organic amendment, namely sugar beet residue (SBR), and/or the inoculation with an arbuscular mycorrhizal (AM) fungus, namely Glomus mosseae, in a non-sterile heavy metal-polluted soil. The AM inoculation did not have any significant effect on plant growth of both species. In T. articulata, SBR increased shoot growth, foliar P, total phospholipid fatty acids (PLFA), fungi-related PLFA, AM fungi-related neutral lipid fatty acid, bacterial gram-positive/gram-negative PLFA ratio and the ß-glucosidase and dehydrogenase activities. SBR and AM inoculation increased phosphatase activity in T. articulata plants grown under drought conditions. In both plants, there was a synergistic effect between AM inoculation and SBR on mycorrhizal colonisation under drought conditions. In C. maritimum, the increase produced by the SBR on total amounts of PLFA, bacterial gram-positive-related PLFA and bacterial gram-negative-related PLFA was considerably higher under drought conditions. Our results suggest that the effectiveness of the amendment with regard to stimulating microbial communities and plant growth was largely limited by drought, particularly for plant species with a low degree of mycorrhizal colonisation.

Comparative effects of native filamentous and arbuscular mycorrhizal fungi in the establishment of an autochthonous, leguminous shrub growing in a metal-contaminated soil.

The aim of this study was to assess the effectiveness of inoculation with a native arbuscular mycorrhizal (AM) fungus, Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe, or a filamentous fungus, Penicillium aurantiogriseum Dierckx 1901, on the establishment of Coronilla juncea L. seedlings grown in a polluted, semiarid soil. For that, root and shoot biomass, nutrient uptake, mycorrhizal colonisation and nitrate reductase (NR) and phosphatase activities were analysed. Six months after planting, the shoot biomass of C. juncea was increased only by the inoculation with G. mosseae (by about 62% compared with non-mycorrhizal plants). The shoot NR and root acid phosphatase activities were increased more by inoculation with G. mosseae than with P. aurantiogriseum inoculation. The root NR activity and foliar nutrient contents were increased only by the inoculation with the AM fungus. The root Zn and Cu decreased with the AM fungus. In conclusion, the autochthonous AM fungus was an effective inoculant with regard to stimulating growth and alleviating heavy metal toxicity for plants growing on a soil contaminated by multiple heavy metals. Inoculation with an autochthonous, filamentous fungus does not seem to be a good strategy for phytoremediation of such problematic sites.

The effectiveness of arbuscular-mycorrhizal fungi and Aspergillus niger or Phanerochaete chrysosporium treated organic amendments from olive residues upon plant growth in a semi-arid degraded soil.

Arbuscular mycorrhizal (AM) fungi and a residue from dry olive cake (DOC) supplemented with rock phosphate (RP) and treated with either Aspergillus niger (DOC-A) or Phanerochaete chrysosporium (DOC-P), were assayed in a natural, semi-arid soil using Trifolium repens or Dorycnium pentaphyllum plants. The effects of the AM fungi and/or DOC-A were compared with P-fertilisation (P) over eleven successive harvests to evaluate the persistence of the effectiveness of the treatments. The biomass of dually-treated plants after four successive harvests was greater than that obtained for non-treated plants or those receiving the AM inoculum or DOC-A treatments after eleven yields. The AM inoculation was critical for obtaining plant growth benefit from the application of fermented DOC-A residue. The abilities of the treatments to prevent plant drought stress were also assayed. Drought-alleviating effects were evaluated in terms of plant growth, proline and total sugars concentration under alternative drought and re-watering conditions (8th and 9th harvests). The concentrations of both compounds in plant biomass increased under drought when DOC-A amendment and AM inoculation were employed together: they reinforced the plant drought-avoidance capabilities and anti-oxidative defence. Water stress was less compensated in P-fertilised than in DOC-A-treated plants. DOC-P increased D. pentaphyllum biomass, shoot P content, nodule number and AM colonisation, indicating the greater DOC-transforming ability of P. chrysosporium compared to A. niger. The lack of AM colonisation and nodulation in this soil was compensated by the application of DOC-P, particularly with AM inoculum. The management of natural resources (organic amendments and soil microorganisms) represents an important strategy that assured the growth, nutrition and plant establishment in arid, degraded soils, preventing the damage that arises from limited water and nutrient supply.

Addition of microbially-treated sugar beet residue and a native bacterium increases structural stability in heavy metal-contaminated Mediterranean soils.

A mesocosm experiment was conducted to investigate the effect of the addition of Aspergillus niger-treated sugar beet waste, in the presence of rock phosphate, and inoculation with a native, metal-tolerant bacterium, Bacillus thuringiensis, on the stabilisation of soil aggregates of two mine tailings, with differing pH values, from a semiarid Mediterranean area and on the stimulation of growth of Piptatherum miliaceum. Bacterium combined with organic amendment enhanced structural stability (38% in acidic soil and 106% in neutral soil compared with their corresponding controls). Only the organic amendment increased pH, electrical conductivity, water-soluble C, water-soluble carbohydrates and plant growth, in both soils. While in neutral soil both organic amendment and bacterium increased dehydrogenase activity, only organic amendment had a significant effect in acidic soil. This study demonstrates that the use of P. miliaceum in combination with organic amendment and bacterium is a suitable tool for the stabilisation of the soil structure of degraded mine tailings, although its effectiveness is dependent on soil pH.

Soil acidity determines the effectiveness of an organic amendment and a native bacterium for increasing soil stabilisation in semiarid mine tailings.

Unstable mine tailings are vulnerable to water and air erosion, so it is important to promote their surface stabilisation in order to avoid the spread of heavy metals. In a greenhouse experiment, we assessed the effect of the addition of Aspergillus niger-treated sugar beet waste and inoculation with a native bacterium, Bacillus cereus, on the stabilisation of soil aggregates of two acidic, semiarid mine tailings, with different acidity degree, during watering and drying periods. Organic amendment raised the pH of both the moderately and highly acidic tailings, whereas the bacterial inoculation increased this parameter in the former. Only the amendment addition increased soil water-soluble carbon in both tailings compared with their controls, under either watering or drying conditions. Both the amendment and B. cereus enhanced water-soluble carbohydrates. Both treatments increased dehydrogenase activity and aggregate stability, particularly in the moderately acidic tailing under drying conditions. After soil drying, aggregate stability was increased by the amendment (about 66% higher than the control soil) and by the bacterium (about 45% higher than the control soil) in the moderately acidic tailing. The effectiveness of these treatments as structure-stabilisation methods for degraded, semiarid mine ecosystems appears to be restricted to tailings of moderate acidity.

Drought tolerance and antioxidant activities in lavender plants colonized by native drought-tolerant or drought-sensitive Glomus Species.

This study compared the effectiveness of four arbuscular mycorrhizal (AM) fungal isolates (two autochthonous presumably drought-tolerant Glomus sp and two allochthonous presumably drought-sensitive strains) on a drought-adapted plant (Lavandula spica) growing under drought conditions. The autochthonous AM fungal strains produced a higher lavender biomass, specially root biomass, and a more efficient N and K absorption than with the inoculation of similar allochthonous strains under drought conditions. The autochthonous strains of Glomus intraradices and Glomus mosseae increased root growth by 35% and 100%, respectively, when compared to similar allochthonous strains. These effects were concomitant with an increase in water content and a decline in antioxidant compounds: 25% glutathione, 7% ascorbate and 15% H(2)O(2) by G. intraradices, and 108% glutathione, 26% ascorbate and 43% H(2)O(2) by G. mosseae. Glutathione and ascorbate have an important role in plant protection and metabolic function under water deficit; the low cell accumulation of these compounds in plants colonized by autochthonous AM fungal strains is an indication of high drought tolerance. Non-significant differences between antioxidant activities such as glutathione reductase (GR), catalase (CAT) and superoxide dismutase (SOD) in colonized plants were found. Thus, these results do not allow the generalization that GR, CAT and SOD were correlated with the symbiotic efficiency of these AM fungi on lavender drought tolerance. Plants colonized by allochthonous G. mosseae (the less efficient strain under drought conditions) had less N and K content than those colonized by similar autochthonous strain. These ions play a key role in osmoregulation. The AM symbiosis by autochthonous adapted strains also produced the highest intraradical and arbuscular development and extraradical mycelial having the greatest fungal SDH and ALP-ase activities in the root systems. Inoculation of autochthonous drought tolerant fungal strains is an important strategy that assured the greatest tolerance water stress contributing to the best lavender growth under drought.

An indigenous drought-tolerant strain of Glomus intraradices associated with a native bacterium improves water transport and root development in Retama sphaerocarpa.

The effects of interactions between Bacillus thuringiensis, a drought-adapted bacterium, and two isolates of Glomus intraradices, an arbuscular mycorrhizal (AM) fungus, on Retama sphaerocarpa, a drought-adapted legume, were investigated. The fungal isolates were an indigenous drought-tolerant and a nonindigenous drought-sensitive isolate. Shoot length and root growth, symbiotic parameters, water transport (in terms of percent relative plant water uptake), and volumetric soil moisture and soil enzymatic activities in response to microbial inoculations were evaluated. Retama plants colonized by G. intraradices plus Bacillus possessed similar shoot length after 30 days from sowing compared with noninoculated Retama plants after 150 days. Inoculation with drought-adapted bacterium increased root growth by 201%, but maximum root development was obtained by co-inoculation of B. thuringiensis and the indigenous G. intraradices. Nodules were formed only in plants colonized by autochthonous AM fungi. Relative water uptake was higher in inoculated than in noninoculated Retama plants, and these inoculants depleted soil water content concomitantly. G. intraradices-colonized Retama reached similar shoot length irrespective of the fungal origin, but there were strong differences in relative water uptake by plants colonized by each one of the fungi. Indigenous G. intraradices-colonized roots (evaluated as functional alkaline phosphatase staining) showed the highest intensity and arbuscule richness when associated with B. thuringiensis. The interactive microbial effects on Retama plants were more relevant when indigenous microorganisms were involved. Co-inoculation of autochthonous microorganisms reduced by 42% the water required to produce 1 mg of shoot biomass. This is the first evidence of the effectiveness of rhizosphere bacterium, singly or associated with AM fungus, in increasing plant water uptake, which represents a positive microbial effect on plants grown under drought environments.

Effectiveness of autochthonous bacterium and mycorrhizal fungus on Trifolium growth, symbiotic development and soil enzymatic activities in Zn contaminated soil.

AIMS: This study investigates how autochthonous micro-organisms [bacterium and/or arbuscular mycorrhizal (AM) fungi] affected plant tolerance to Zn contamination. METHODS AND RESULTS: Zinc-adapted and -nonadapted Glomus mosseae strains protected the host plant against the detrimental effect of Zn (600 microg g(-1)). Zn-adapted bacteria increased root growth and N, P nutrition in plants colonized by adapted G. mosseae and decreased the specific absorption rate (SAR) of Cd, Cu, Mo or Fe in plants colonized by Zn-nonadapted G. mosseae. Symbiotic structures (nodule number and extraradical mycelium) were best developed in plants colonized by those Zn-adapted isolates that were the most effective in increasing plant Zn tolerance. The bacterium also increased the quantity and quality (metabolic characteristics) of mycorrhizal colonization, with the highest improvement for arbuscular vitality and activity. Inocula also enhanced soil enzymatic activities (dehydrogenase, beta-glucosidase and phosphatase) and indol acetic acid (IAA) accumulation, particularly in the rhizosphere of plants inoculated with Zn-adapted isolates. CONCLUSIONS: Glomus mosseae strains have a different inherent potential for improving plant growth and nutrition in Zn-contaminated soil. The bacterium increased the potential of mycorrhizal mycelium as inoculum. SIGNIFICANCE AND IMPACT OF THE STUDY: Mycorrhizal performance, particularly that of the autochthonous strain, was increased by the bacterium and both contributed to better plant growth and establishment in Zn-contaminated soils.

Two bacterial strains isolated from a Zn-polluted soil enhance plant growth and mycorrhizal efficiency under Zn-toxicity.

In this study we investigated the interactions among plant, rhizosphere microorganisms and Zn pollution. We tested the influence of two bacterial strains isolated from a Zn-polluted soil on plant growth and on the symbiotic efficiency of native arbuscular mycorrhizal fungi (AMF) under Zn toxicity. The two bacterial strains exhibited Zn tolerance when cultivated under increasing Zn levels in the medium. However, strain B-I showed a higher Zn tolerance than strain B-II at the two highest Zn levels in the medium (75 and 100 mg l(-1) Zn). Molecular identification placed the strain B-I within the genus Brevibacillus. Our results showed that bacterial strain B-I consistently enhanced plant growth, N and P accumulation, as well as nodule number and mycorrhizal infection which demonstrated its plant-growth promoting (PGP) activity. This strain B-I has been shown to produce IAA (3.95 microg ml) and to accumulate 5.6% of Zn from the growing medium. The enhanced growth and nutrition of plants dually inoculated with the AMF and bacterium B-I was observed at three Zn levels assayed. This effect can be related to the stimulation of symbiotic structures (nodules and AMF colonization) and a decreased Zn concentration in plant tissues. The amount of Zn acquired per root weight unit was reduced by each one of these bacterial strains or AMF and particularly by the mixed bacterium-AMF inocula. These mechanisms explain the alleviation of Zn toxicity by selected microorganisms and indicate that metal-adapted bacteria and AMF play a key role enhancing plant growth under soil Zn contamination.

Brevibacillus brevis isolated from cadmium- or zinc-contaminated soils improves in vitro spore germination and growth of Glomus mosseae under high Cd or Zn concentrations.

In this study we investigated the saprophyte growth of two arbuscular-mycorrhizal fungi (Glomus mosseae isolate) under increasing Cd or Zn levels and the influence of a selected bacterial strain of Brevibacillus brevis. Microorganisms here assayed were isolated from Cd or Zn polluted soils. B. brevis increased the presymbiotic growth (germination rate growth and mycelial development) of Glomus mosseae. Spore germination and mycelial development of both G. mosseae isolate were reduced as much as the amount of Cd or Zn increased in the growth medium. In medium supplemented with 20 microg Cd mL(-1), the spore germination was only 12% after 20 days of incubation, but the coinoculation with B. brevis increased this value to 40% after only 15 days. The addition of 20 microg Cd mL(-1) to the growth medium drastically inhibited hyphal development, but the presence of the bacterium increased hyphal growth of G. mosseae from 195% (without Cd) until 254% (with 20 microg Cd mL(-1)). The corresponding bacterial effect increasing micelial growth ranged from 125% (without Zn) to 232% (200 microg Zn mL(-1)) in the case of G. mosseae isolated from Zn-polluted soil. Mycelial growth under 5 microg Cd mL(-1) (without bacterium) was similarly reduced from that produced at 15 microg Cd mL(-1) in the presence of the bacteria. As well, 50 microg Zn mL(-1) (without bacterium) reduced hyphal growth as much as 200 microg Zn mL(-1) did in the presence of B. brevis. The bacterial effect on the saprophytic growth of G. mosseae in absence of metal may be due to the involvement of indole acetic acid (IAA) produced by these bacteria. The Cd bioaccumulation ability exhibited (76%) by Cd-adapted B. brevis reduced the Cd damage on G. mosseae in Cd-contaminated medium. These capabilities of B. brevis isolates partially alleviate the inhibitory effects of Cd or Zn on the axenic growth of G. mosseae.

Application of Aspergillus niger-treated agrowaste residue and Glomus mosseae for improving growth and nutrition of Trifolium repens in a Cd-contaminated soil.

The microbial transformation of sugar beet (SB) agrowaste with or without rock-phosphate (RP) has utility for the improvement of plant growth in a Cd (5 microg g-1) artificially contaminated soil, particularly when the soil is co-inoculated with arbuscular mycorrhizal (AM) fungus Glomus mosseae isolated from a Cd-polluted area. Under such Cd-polluted conditions, the limited growth, mineral nutrition, symbiotic developments (nodulation and AM-colonization) and soil enzymatic activities were stimulated using SB or SB+RP as soil amendments and G. mosseae as inoculant. G. mosseae enhanced plant establishment in a higher extent in amended soil; it is probably due to the interactive effect increasing the potential fertility of such compounds and its ability for decreasing Cd transfer from soil to plant. The amount of Cd transferred from soil solution to biomass of AM-colonized plants ranged from 0.09 microg Cd g-1 (in SB+RP-amended soil) to 0.6 microg Cd g-1 (in non-amended soil). Nodule formation was more sensitive to Cd than AM-colonization, and both symbioses were stimulated in amended soils. Not only AM-colonization but also amendments were critical for plant growth and nutrition in Cd-polluted soil. The high effectiveness of AM inoculum increasing nutrients and decreasing Cd in amended soil indicated the positive interaction of these treatments in increasing plant tolerance to Cd contamination.

Establishment of two ectomycorrhizal shrub species in a semiarid site after in situ amendment with sugar beet, rock phosphate, and Aspergillus niger.

A field experiment was carried out to assess the effectiveness of the addition of sugar beet, rock phosphate, and Aspergillus niger directly into the planting hole, and the mycorrhizal inoculation of seedlings with Scleroderma verrucosum, for promotion of plant growth of Cistus albidus L. and Quercus coccifera L. and enhancement of soil physicochemical, biochemical, and biological properties, in a degraded semiarid Mediterranean area. One year after planting, the available phosphorus content in the amended soils of both species was about fourfold higher than in the nonamended soil. Amendment addition increased the aggregate stability of the rhizosphere of C. albidus (by 56% with respect to control soil) while the mycorrhizal inoculation increased only the aggregate stability of the rhizosphere of Q. coccifera (by 13% with respect to control soil). Biomass C content and enzyme activities (dehydrogenase, urease, protease-BAA, acid phosphatase, and beta-glucosidase) of the rhizosphere of C. albidus were increased by amendment addition but not by mycorrhizal inoculation. Both treatments increased enzyme activities of the rhizosphere of Q. coccifera. The mycorrhizal inoculation of the seedlings with S. verrucosum was the most effective treatment for stimulating the growth of C. albidus (by 469% with respect to control plants) and Q. coccifera (by 74% with respect to control plants). The combined treatment, involving mycorrhizal inoculation of seedlings and addition of the amendment directly into soil, had no additive effect on the growth of either shrub species.

Interactive effect of Brevibacillus brevis and Glomus mosseae, both isolated from Cd contaminated soil, on plant growth, physiological mycorrhizal fungal characteristics and soil enzymatic activities in Cd polluted soil.

The interaction between two autochthonous microorganisms (Brevibacillus brevis and Glomus mosseae) isolated from Cd amended soil increased plant growth, arbuscular mycorrhizal (AM) colonization and physiological characteristics of the AM infection (measured as SDH or ALP activities). The enhanced plant Cd tolerance after coinoculation with native microorganisms seemed to be a consequence of increased P and K acquisition and, simultaneously, of decreased concentration of Cd, Cr, Mn, Cu, Mo, Fe and Ni in plant tissue. Autochthonous microbial strains were more efficient for nutrient uptake, to immobilize metals and decrease their translocation to the shoot than reference G. mosseae (with or without bacteria). Indole acetic acid produced by B. brevis may be related to its ability for improving root growth, nodule production and AM fungal intra and extraradical development. Dehydrogenase, phosphatase and beta-glucosidase activities, indicative of microbial metabolism and soil fertility, were maximized by the coinoculation of autochthonous microorganisms in cadmium polluted conditions. As a consequence, the use of native microorganisms may result very efficient in bioremediation.

Improvement of soil characteristics and growth of Dorycnium pentaphyllum by amendment with agrowastes and inoculation with AM fungi and/or the yeast Yarowia lipolytica.

The effectiveness of two microbiologically treated agrowastes [dry olive cake (DOC) and/or sugar beet (SB)] on plant growth, soil enzymatic activities and other soil characteristics was determined in a natural soil from a desertified area. Dorycnium pentaphyllum, a legume plant adapted to stress situations, was the test plant to evaluate the effect of inoculation of native arbuscular mycorrhizal (AM) fungi and/or Yarowia lipolytica (a dry soil adapted yeast) on amended and non-amended soils. Plant growth and nutrition, symbiotic developments and soil enzymatic activities were limited in non-amended soil where microbial inoculations did not improve plant development. The lack of nodules formation and AM colonization can explain the limited plant growth in this natural soil. The effectiveness and performance of inocula applied was only evident in amended soils. AM colonization and spores number in natural soil were increased by amendments and the inoculation with Y. lipolytica promoted this value. The effect of the inoculations on plant N-acquisition was only important in AM-inoculated plants growing in SB medium. Enzymatic activities as urease and protease activities were particularly increased in DOC amended soil meanwhile dehydrogenase activity was greatest in treatments inoculated with Y. lipolytica in SB added soil. The biological activities in rhizosphere of agrowaste amended soil, used as indices of changes in soil properties and fertility, were affected not only by the nature of amendments but also by the inoculant applied. All these results show that the lignocellulosic agrowastes treated with a selected microorganism and its further interaction with beneficial microbial groups (native AM fungi and/or Y. lipolytica) is a useful tool to modify soil physico-chemical, biological and fertility properties that enhance the plant performance probably by making nutrients more available to plants.

Influence of bacterial strains isolated from lead-polluted soil and their interactions with arbuscular mycorrhizae on the growth of Trifolium pratense L. under lead toxicity.

We isolated two bacterial strains from an experimentally lead (Pb)-polluted soil in Hungary, 10 years after soil contamination. These strains represented the two most abundant cultivable bacterial groups in such soil, and we tested their influence on Trifolium pratense L. growth and on the functioning of native mycorrhizal fungi under Pb toxicity in a second Pb-spiked soil. Our results showed that bacterial strain A enhanced plant growth, nitrogen and phosphorus accumulations, nodule formation, and mycorrhizal infection, demonstrating its plant-growth-promoting activity. In addition, strain A decreased the amount of Pb absorbed by plants, when expressed on a root weight basis, because of increased root biomass due to the production of indoleacetic acid. The positive effect of strain A was not only evident after a single inoculation but also in dual inoculation with arbuscular mycorrhizal fungi. Strain A also exhibited higher tolerance than strain B when cultivated under increasing Pb levels in the spiked soil. Molecular identification unambiguously placed strain A within the genus Brevibacillus. We showed that it is important to select the most tolerant and efficient bacterial strain for co-inoculation with arbuscular mycorrhizal fungi to promote effective symbiosis and thus stimulate plant growth under adverse environmental conditions, such as heavy-metal contamination.

Symbiotic efficiency of autochthonous arbuscular mycorrhizal fungus (G. mosseae) and Brevibacillus sp. isolated from cadmium polluted soil under increasing cadmium levels.

The effect of inoculation with indigenous naturally occurring microorganisms (an arbuscular mycorrhizal (AM) fungus and rhizosphere bacteria) isolated from a Cd polluted soil was assayed on Trifolium repens growing in soil contaminated with a range of Cd. One of the bacterial isolate showed a marked PGPR effect and was identified as a Brevibacillus sp. Mycorrhizal colonization also enhanced Trifolium growth and N, P, Zn and Ni content and the dually inoculated (AM fungus plus Brevibacillus sp.) plants achieved further growth and nutrition and less Cd concentration, particularly at the highest Cd level. Increasing Cd level in the soil decreased Zn and Pb shoot accumulation. Coinoculation of Brevibacillus sp. and AM fungus increased shoot biomass over single mycorrhizal plants by 18% (at 13.6 mg Cd kg(-1)), 26% (at 33.0 mg Cd kg(-1)) and 35% (at 85.1 mg Cd (kg(1)). In contrast, Cd transfer from soil to plants was substantially reduced and at the highest Cd level Brevibacillus sp. lowered this value by 37.5% in AM plants. Increasing Cd level highly reduced plant mycorrhization and nodulation. Strong positive effect of the bacterium on inocula, are important in plant Cd tolerance and development in Cd polluted soils.