Arbuscular mycorrhizal fungi of Ammophila arenaria (L.) Link: Spore abundance and root colonisation in six locations of the European coast

Arbuscular mycorrhizal fungi (AMF) are common organisms in the rhizhosphere of plants in coastal sand dunes where they play a key role in the establishment, growth and survival of plants. This study presents a quantitative analysis of the AMF associated with Ammophila arenaria, the most important sand-fixing species in the foredunes of Europe, in six locations along the western European coast. Spore abundance and root colonisation by AMF were estimated in July 2003, October 2003 and April 2004. The number of spores varied significantly with time and location. A clear peak of sporulation in autumn was found for three of the northern sites, but no pattern was detected in the southern sites. Root colonisation showed no seasonal pattern, despite differences between sampling times. Both hyphal coils and arbuscules were observed inside the roots, indicating colonisation by more than one AMF species. No correlation was found between root colonisation and spore number, or between AMF abundance and soil fertility. We conclude that: (a) spore production is driven by climatic conditions in the studied northern sites and by plant phenology in the studied southern sites; and that (b) root colonisation is independent of climate, phenology and soil fertility in the studied locations.     

The role of the external mycelium in early colonization for three arbuscular mycorrhizal fungal species with different colonization strategies

Arbuscular mycorrhizal fungi (AMF) differ in their rate and extent of colonization of both plant roots and soil but the mechanism responsible for these differences is unclear. We compared the external mycelium of three AMF isolates (Glomus intraradices, Glomus etunicatum and Gigaspora gigantea) during early colonization of plant roots. We investigated whether an AMF with the most rapid colonization would have higher numbers of infective structures (i.e., infection hyphae and contact points), an AMF with extensive root colonization would have more infection units, and (3) AMF with extensive soil colonization would have large numbers of all external features (including absorptive hyphae, runner hyphae and hyphal bridges). Using specially designed soil and root observation chambers, we followed the development of the external mycelium for 7 weeks. We found that rapid colonization rate was due, in part, to the presence of more infective structures, in particular more infection hyphae and root contact points. Second, the extensive root colonizer had more, larger infection units. Third, data did not support the hypothesis that the extensive soil colonizer had more external structures. These results show that differences in the architecture of the external mycelium are responsible, in part, for variation in the colonization strategy of AMF.     

Arbuscular mycorrhizal fungi in northern Greece and influence of soil resources on their colonization

Abundance of arbuscular mycorrhizal fungi (AMF) in the roots of plant species was assessed in two areas in Greece in a 4-year study (2004–2007). The field experiment was conducted in a mountainous and herbaceous grassland in Greece in which both nitrogen (N)- and phosphorus (P)-limited plant community productivity. In 2006, data were also collected from a pot experiment in which 14 herbaceous plant species were grown as monocultures in P-limited soil. A factorial design of two levels of N and P was established in the mountainous field to test plant response to nutrient additions with respect to AMF colonization levels. Effects of fungicides were also investigated over year in the pot experiment and over three years in the field experiment. In addition, the effect of irrigation on AMF colonization was determined in a 1-year field study. Measurements included estimating the level of plant species specific hyphal colonization of roots according to the McGonicle et al. [McGonigle, T.P., Miller, M.H., Evans, D.G., Fairchild, D.L., Swan, J.A., 1990. A new method which gives an objective measure of colonization of roots by vesicular–arbuscular mycorrhizal fungi. New Phytol. 115, 495–501] method. AMF colonization was highest in the leguminous species, intermediate in the forbs and lowest in the grasses. AMF responses to N and P additions were not uniform. P addition in the field experiment increased the colonization level of the high P demanding annual forb (non-leguminous dicot) Galium lucidum, decreased hyphal abundance of the forb Plantago lanceolata and the grass Agrostis capillaris, and appeared to have a negligible effect on the forb Prunella vulgaris and on leguminous species. Effects of N addition were influenced by P addition and were only significant in plots not enriched with P where N addition increased the AMF colonization. Irrigation increased colonization of the tested species A. capillaris and P. lanceolata but only significantly increased that of P. lanceolata. There was interannual variation in the effects of fungicides on AMF colonization, which was partly due to differences in the active ingredient and formulation used. Among the tested species, A. capillaris was the most susceptible to fungicides.     

Mycorrhizal fungi increase biocontrol potential of Pseudomonas fluorescens

Wheat roots are susceptible to colonisation by soil-borne pathogens, such as Gaeumannomyces graminis var. tritici (Ggt), which causes the globally important disease take-all, and mutualistic arbuscular mycorrhizal fungi (AMF). Certain rhizosphere fluorescent Pseudomonas strains have received much attention as potential biocontrol agents given their ability to produce antibiotics, such as 2,4-diacetylphloroglucinol (DAPG), that confer a measure of plant protection. Here we show that Pseudomonas fluorescens only produced DAPG in the presence of soluble carbon from soil containing either Ggt or AMF, and production increased by two orders of magnitude in response to both AMF and Ggt. Encouragement of mycorrhizal colonisation may therefore offer a sustainable strategy for protection against take-all.     

多胺对共生条件下丛枝菌根真菌及宿主植物生长发育的影响

为探讨多胺对共生条件下丛枝菌根真菌及其宿主植物生长发育的影响,本研究以丛枝菌根真菌(Gigaspora margarita)为试验材料,通过施用不同浓度的多胺(Polyamine,PA)及其生物合成抑制剂[Methylglyoxal bis(guanylhydrazone),MGBG]处理接种丛枝菌根真菌的葡萄微繁苗,研究共生培养条件下外源多胺及多胺合成抑制剂对丛枝菌根真菌孢子萌发、芽管菌丝及其宿主植物生长发育的影响.试验结果表明,共生培养条件下,一定浓度的外源PA对丛枝菌根真菌及其宿主植物的生长发育具显著促进作用,丛枝菌根真菌孢子数、菌丝长度、侵染率、丛枝丰富度及菌根化葡萄幼苗生长势均显著提高.MGBG则表现较强的抑制作用.且该抑制作用可被外源PA部分解除,证明外源多胺对菌根化葡萄微繁苗生长发育的促进作用是通过活化根系土壤中丛枝菌根真菌,促进微繁苗丛枝菌根共生体的良好发育,最大程度地发挥菌根化效应得以表现的.     

Host plant benefit from association with arbuscular mycorrhizal fungi: variation due to differences in size of mycelium

Plants differ in the benefit they receive from an association with arbuscular mycorrhizal fungi (AMF) but it is unclear what factors determine host response. We tested whether this difference is due in part to differences among AMF families (Glomaceae, Gigasporaceae, and Acaulosporaceae) in the size of their mycelium. For example, an AMF with a large mycelium might be better able to transfer nutrients to its host. Alternatively, the large mycelium might present a large carbon drain for the host. It might also be the location of AMF mycelium that is important: whether it occurs in the root or in the soil. In a greenhouse experiment, four different host plant species were inoculated with each of 21 AMF isolates representing different mycelial sizes and we measured both host plant biomass and foliar phosphorus concentration over 12 weeks. AMF family significantly affected host biomass with larger internal mycelia conferring greater host benefit. However, both intra-family variation in AMF effect and host identity also influenced host response. For foliar P, AMF family did not affect host response significantly. Our results show that differences in mycelial size among AMF families may contribute to variation in host responsiveness to AMF, but other factors are also important.     

The development of arbuscular mycorrhiza in two simulated stages of spoil-bank succession

A greenhouse experiment based on a dual mode of mycorrhizal inoculation simulated the formation of mycorrhizal symbiosis at two different stages of plant succession on coalmine spoil banks. The model plants were inoculated either with propagules of the arbuscular mycorrhizal fungi (AMF) Glomus mosseae BEG95, which represented the initial stages of succession, or were provided with the pre-established extraradical mycelium (ERM) network of the same AMF isolate, which simulated later succession stages. The plant species used – non-mycotrophic Atriplex sagittata and Sisymbrium loeselii, and mycotrophic Tripleurospermum inodorum, Calamagrostis epigejos and Elytrigia repens – represented succession dominants at those sites. Even though the grasses were colonised in both mycorrhizal treatments, the presence of an established ERM network increased the intensity of their colonisation and arbuscular abundance. No trace of colonisation of non-mycotrophic plants was found in the treatment inoculated with propagules. Surprisingly, marked colonisation, including abundant arbuscules, was observed when non-mycotrophic plants were grown in the presence of a pre-established ERM network. In A. sagittata, arbuscules were found at maturity and senescence of the plants after 16 weeks of growth. In S. loeselii, however, the arbuscules were found at the vegetative stage of the leaf rosette after 8 weeks and then completely disappeared during the following weeks. When the ability of propagules and ERM to induce mycorrhizal colonisation is compared, it seems that the established mycelium probably has an enhanced potential to colonise roots of plants, even if the plants belong to species usually not hosting mycorrhizal fungi.     

Saprophytic survival of the take-all fungus and its antagonist, Gaeumannomyces graminis var. graminis under conventional and no-tillage

The saprophytic survival of the take-all fungus, Gaeumannomyces graminis var. tritici, and its antagonist, G. graminis var. graminis, was studied over 9-month periods in 1982 and 1983 under conventional and no-tillage cultivation. In 1982, when the rainfall for the 9-month period was 30.4% below the long-term average, the survival of the fungi was comparable under the two tillage systems. However, the survival of G. graminis var. graminis was significantly greater (P 0.05) than that of G. graminis var. tritici at each sampling time. In 1983, when the rainfall for the 9-month period was 25.1% above the long-term average, the survival of both fungi was significantly greater under no-tillage than under conventional tillage. The survival of the two isolates of G. graminis var. graminis after 3 months was significantly greater (P 0.05) than that of the two isolates of the pathogen. The implications for the greater survival of the fungal antagonist compared to the take-all fungus under no-tillage are discussed in relation to the biological control of take-all.     

低磷土壤接种菌根真菌和解磷细菌对大田玉米生长和磷吸收的影响

丛枝菌根真菌(Arbuscular mycorrhizal fungi,AMF)能与多数陆生植物共生,促进植物吸收养分尤其是磷。解磷细菌(Phosphate-solubilizing bacteria,PSB)可以活化土壤中难溶性无机磷和有机磷。本研究采用苯菌灵对田间低磷土壤中土著AM真菌进行灭菌,并接种外源AM真菌(Glomusversiforme,G.v)和PSB(Pseudomonassp.),研究AM真菌和PSB接种对不同生育期玉米生长、磷养分吸收和产量的影响。结果表明,施用苯菌灵能够有效地抑制土著AM真菌对玉米根系的侵染,未施用苯菌灵处理中土著AM真菌促进了玉米前期和收获期的生长,提高了玉米吸磷量;接种Pseudomonas sp.促进了玉米六叶期根系的生长;接种外源AM真菌G.v促进了玉米六叶期和收获期地上部的生长,但降低了玉米产量。双接种Pseudomonas sp.和G.v对玉米生长、吸磷量和产量未表现出显著的协同效应。     

The role of arbuscular mycorrhizal colonization in the carbon and nutrient economy of the tripartite symbiosis with nodulated Phaseolus vulgaris

In the tripartite symbiosis between nodulated legume roots and arbuscular mycorrhizal (AM) fungi, symbiont sink strength may depend upon developmental stage and the nutrient benefits to the host plant. The cost-benefits of the tripartite symbiosis were investigated in terms of C-economy and nutrition. Nodulated Phaseolus vulgaris seedlings, with and without AM, were hydroponically grown under high (2 mM) and low (1 μM) P conditions in an N-free Long Ashton nutrient solution. Plants were sequentially harvested at 17, 24 and 31 days after emergence. At each harvest, measurements for biomass, N₂-fixation, photosynthesis, root respiration, calculated C and nutritional economy were taken. Nodular growth was suppressed by the early development of AM colonization. This coincided with higher photosynthetic and respiratory rates in AM plants. These effects were most pronounced under low P when AM colonization peaked. Once AM levels reached the plateau phase, the efficiency of P nutrition increased. This was followed by improved nodular and host growth and enhanced N₂-fixation. This indicates that the AM was the dominant symbiont for host C in the tripartite symbiosis, due to its rapid development and subsequent role in supplying P more effectively to both host and nodules.     

Selective consumption and digestion of litter microbes by Porcellio scaber (Isopoda: Oniscidea)

In feeding preference tests with artificial diets consisting of food sources inoculated with different types of litter microbes, Porcellio scaber was capable of discriminating between different microbe species. Generally, microbial colonisation increased the attractiveness of a given food source, in particular, when the food source was of low quality (cellulose) and when food sources were inoculated with single species of actinomycetes (Streptomyces celluloflavus or Pseudonocardia autotrophica). In most cases, actinomycetes (Gram-positive bacteria) were preferred over both Gram-negative bacteria and fungi, whether or not these microbes exhibited cellulolytic activity. Enzymatic in vitro digestion of both the Gram-positive S. celluloflavus and P. autotrophica was significantly greater than that of Gram-negative bacteria (Pseudomonas fluorescens and Myxococcus xanthus) or fungi (Chaetomium globosum and Fusarium ventricosum). S. celluloflavus biomass was more effectively incorporated into isopod biomass than that of fungi and Gram-negative bacteria; P. autotrophica was more effectively incorporated into isopod biomass than that of Gram-negative bacteria. Based on these results, we hypothesise that P. scaber preferentially feeds on those microbes that it can readily digest. Whether this holds true for other Gram-positive bacteria or for other detritivores awaits investigation.     

Species-dependent partitioning of C and N stable isotopes between arbuscular mycorrhizal fungi and their C3 and C4 hosts

Natural ¹³C and ¹⁵N abundances of mycorrhizal fungi are increasingly used in ecology but reference data on arbuscular mycorrhizal fungi (AMF) are scarce. In experiments with nine phylogenetically dispersed AMF strains inoculated on leek (C3 plant) and sorghum (C4) in pot cultures, we measured the ¹³C/¹²C and ¹⁵N/¹⁴N ratios in shoots, roots, AMF spores as well as carbon isotope signature of the C16:1ω5 fatty acid (FA), which is diagnostic for AMF. Spore δ¹³C values varied among AMF strains on any given host. They were significantly lower than shoot δ¹³C for sorghum (−2.5‰ on average) while for leek, no clear C isotope partitioning between spores and host shoots was observed. The FA C16:1ω5 fatty acids were more ¹³C-depleted than spores, without correlation with spore δ¹³C values. For both, sorghum and leek, spore δ¹⁵N was higher (+1–2‰ on average) than for shoots. We found no evidence that isotopic partitioning between the partners drives ¹³C and ¹⁵N abundances in plant–AMF symbiosis. Mycorrhizal roots displayed relatively high δ¹³C typical for heterotrophic organs, and not a mix between AMF and plant signatures. Interestingly, inoculation slightly decreased shoot δ¹³C on leek but not on sorghum, as compared with non-mycorrhizal plants, suggesting that AMF improved the plant's water status, a parameter affecting the δ¹³C of C3 but not C4 plants. Phylogenetically closer AMF displayed more similar spore δ¹³C and induced similar ¹³C and ¹⁵N abundances on leek shoots, but this observation was not confirmed for sorghum. Plant and AMF isotopic abundances hardly correlated with other parameters related to plant development, mineral nutrition or root mycorrhizal colonisation, and these correlations were never consistent between sorghum and leek. Thus, isotopic abundances in plant–AMF symbiosis were rather constrained by each AMF/plant interaction. Nevertheless, our data provide a valuable reference for future investigations of AMF communities and AM symbiosis in situ.     

Facilitation of seedling growth and nutrient uptake by indigenous arbuscular mycorrhizal fungi in intensive agroecosytems

A modified in-growth core technique was employed to determine the contribution of indigenous arbuscular mycorrhizal fungi (AMF) to plant growth and nutrient uptake in intensive agroecosystems at two Experimental field sites at Shangzhuang (Experiment I) and Quzhou (Experiment II) in North China. The growth cores (26.5 cm depth, 5 cm diameter for maize plants, and 4 cm for alfalfa and tomato plants) were covered with 40-μm nylon mesh (restriction of hyphal growth) and buried in the soil. They either remained static (static mesh) or were regularly rotated (rotated mesh) to disrupt hyphal penetration into the cores. A non-rotated 0.45-μm mesh (block mesh, inhibition of hyphal growth) treatment which remained static was also included to compare with the rotated mesh treatment (Experiment I). Growth cores from the two experimental sites had different soil types and two contrasting low P levels. The soil in the growth cores was sieved and sterilized before being placed into the growth core. Three plant species, namely maize, tomato and alfalfa were selected. The growth periods for maize plants were 35 days (Experiment I) and 39 days (Experiment II), respectively, and the corresponding growth periods for tomato and alfalfa were 67 days (Experiment I) and 53 days (Experiment II). At harvest the AMF species inside and outside the in-growth cores were identified by polymerase chain reaction (PCR), cloning and sequencing. Irrespective of plant species or genotype (maize), root colonization rates and hyphal length density (Experiment I) were generally suppressed in the rotated mesh treatment. The inhibition of hyphal growth by block mesh was comparable to that by the rotated mesh treatment. The growth of all three plant species in static mesh at the two Experimental sites, at both low (Experiment I) and sub-optimal soil P supply levels (Experiment II), was significantly higher than in the rotated (or block mesh) treatment. Root colonization rates of three maize genotypes were positively correlated with plant P concentration (Experiment II). Uptake efficiencies of P and N were significantly higher in static mesh than in the rotated (or block) treatment. AMF species detected (Experiment I) were all Glomerales, including the genera Glomus and Rhizophagus. One identified species of Rhizophagus intraradices and one Glomus viscosum-like phylotype were the dominant species. We conclude that the indigenous AM are crucial for early seedling growth, particularly for plants with small seeds and low P reserves and when seedlings exhibit P deficiency. The facilitation effect is highly relevant to enhanced root P (and possibly N) uptake and P delivery by the fungal mycelium. Our results have implications for the importance of maintenance of intact hyphal networks in intensive agroecosystems.     

Biological control of Phytophthora parasitica var. Nicotianae on tobacco seedlings with non-pathogenic binucleate Rhizoctonia fungi

Nonpathogenic binucleate Rhizoctonia fungi (BNR) controlled black shank caused by Phytophthora parasitica var. nicotianae on greenhouse-grown tobacco seedlings in styrofoam float trays. Three BNR isolates were incorporated into a soil-less mix on colonized, pulverized, sifted rice particles; colonized whole rice grains; or on pelleted tobacco seeds coated with 0.5% methyl cellulose. Five-wk-old seedlings were inoculated with zoospores of P. parasitica var. nicotianae and disease rated over 10 d. The level of protection varied with method of BNR application, ranging from 40 to 70%. Overall, control was better when BNR isolates were applied on rice inocula rather than on BNR-colonized tobacco seeds. From 15% to 80% of individual roots from seedlings grown in soil-less mix amended with BNR-colonized rice grains were colonized while only 0–20% of roots from seedlings grown from BNR-colonized tobacco seeds were colonized. Likewise, 37–100% of soil-less mix amended with BNR-colonized rice grains contained BNR's while less than 3% of soil-less mix was colonized when seedlings emerged from BNR-colonized tobacco seeds. This is the first demonstration of biocontrol of Phytophthora by BNR fungi.     

Temporal variation outweighs effects of biosolids applications in shaping arbuscular mycorrhizal fungi communities on plants grown in pasture and arable soils

Landspreading of biosolids (treated sewage sludge) in agroecosystems is a common waste management practice worldwide. Evidence suggests biosolids may be detrimental to arbuscular mycorrhizal fungi (AMF); however, previous studies focused on arable systems and often unrealistically high biosolids application levels. We investigated the effects of biosolids on AMF communities in grassland and arable agroecosystems, in the context of the natural seasonal dynamics of AMF community composition and diversity. A pasture and arable system under commercial farming management were amended annually with two different types of biosolids, applied at levels meeting current European Union regulations, in a factorial, replicated field-scale plot experiment. AMF root colonisation and community composition were measured in Lolium perenne roots from the pasture and Trifolium repens roots growing in arable soil across the seasons of two years. AMF community compositions were assessed by terminal-restriction fragment length polymorphism analyses. Biosolids had no significant effect on AMF root colonisation or community composition in either agroecosystem. Soil chemical analyses indicated several changes in the top 0–5 cm layer of the pasture soil, including small increases in heavy metal concentrations in biosolids relative to control plots. Temporal AMF dynamics were detected in soils from both agroecosystem indicating that the effect of seasonality outweighed that of biosolids application.     

丛枝菌根真菌对镉胁迫下芦竹生长、光合特性和矿质营养的影响

为揭示丛枝菌根真菌（Arbuscular mycorrhizal fungi，AMF）对芦竹耐镉（Cd）胁迫的作用及其机理，采用大棚盆栽试验，利用丛枝菌根真菌（AMF）摩西管柄囊霉（Funneliformis mosseae，FM）、根内根孢囊霉（Rhizophagus intraradices，RI）、地表球囊霉（Glomus versiforme，GV）进行接种试验，研究了在Cd胁迫下接种AMF对芦竹生长、光合、矿质营养的影响。结果表明：AMF能够显著改善Cd胁迫下芦竹的生长状况，与对照相比，接种处理芦竹的株高增加19.09%~27.98%，叶长增加12.18%~31.06%，叶绿素相对含量SPAD值增加8.55%~9.36%，地上和根系生物量分别增加20.08%~31.41%、12.24%~24.12%，最大净光合速率增加7.08%~32.12%，芦竹根系全P含量增加30.26%~46.05%。接种处理后芦竹地上Cd含量介于68~105.97 mg/kg之间，显著高于对照处理（42.20 mg/kg），根系Cd含量介于113.07~221.47 mg/kg之间，显著高于对照处理（46.47 mg/kg），且根系Cd含量显著高于地上部。Cd胁迫下不同AMF菌种对芦竹产生的效应有差异，其中，RI处理对芦竹株高、叶长促进效应最好，经GV处理的芦竹全N、全P、全K含量以及Cd含量最高。Cd胁迫下接种AMF能促进芦竹的生长，增强其光合作用，提高全N、全P、全K吸收量，同时增强了芦竹对Cd的吸收。该研究表明芦竹丛枝菌根共生体对重金属Cd具有较强的固持作用，在Cd污染土壤修复中具有潜在应用价值。     

Diversity of halophytes and identification of arbuscular mycorrhizal fungi colonising their roots in an abandoned and sustained part of Sečovlje salterns

Sečovlje salterns are an important protected area of biotic diversity in the Mediterranean. They represent an extreme environment with high salinity and drought that severely influence the growth of organisms. In the present study, diversity of plant halophytes and their mycorrhizal status were screened at eight different locations, which were mostly dikes and salt ponds, and which were deliberately selected for their distinct properties (e.g. soil salinity ranging from 105 to 2627 μS cm⁻¹, vegetation type and management practice of the salterns).Twelve different halophytic plant species were recorded, of which eleven are designated as vulnerable. With few exceptions, they were found at the abandoned (Fontanigge) and sustained (Lera) locations of the Sečovlje salterns, distributed according to their tolerance to the salinity and waterlogging. The highest diversity of halophytes was listed at Fontanigge, in the abandoned, periodically flooded and gradually overgrown salt ponds. All of the examined species were colonised with either arbuscular mycorrhizal fungi (AMF) and/or dark septate endophytes (DSEs). High levels of colonisation were however detected only for species belonging to the Asteraceae and Plantaginaceae families. Higher root colonisation frequencies were generally seen for plants growing in the abandoned parts, when compared to the managed parts, whereas there was little correlation of the colonisation parameters with physicochemical parameters of rhizospheric soil properties.Molecular analysis by temporal temperature gradient gel electrophoresis (TTGE) of roots of halophytic plant species with confirmed AMF colonisation (arbuscules present) revealed the occurrence of at least six different AMF species, related to Glomus geosporum, Glomus caledonium and Glomus intraradices, and to different Glomus sp. clades and the Diversispora clade. This is to the best of our knowledge the first report of AMF and DSE mycorrhizal status of most of the halophyte plant species examined and of the brother scale identification of AMF species based on molecular analyses of roots of diverse halophytes from high saline environments.     

Role of soil mesofauna in dispersal of Coniothyrium minitans: transmission to sclerotia of Sclerotinia sclerotiorum

Coniothyrium minitans Campbell is a mycoparasite with proven biocontrol activity against Sclerotinia sclerotiorum (Lib.) de Bary in the field and glasshouse. It is known to spread from sites of application but the mechanisms of dispersal are unclear. As C. minitans has been recovered from collembolans collected during glasshouse trials, and numerous mites and insects are often associated with decaying S. sclerotiorum-infected plant material in the glasshouse, the mite Acarus siro L. and the collembolan Folsomia candida Willem were used to investigate the potential of soil mesofauna to disperse C. minitans. In an initial investigation, A. siro was found to transmit the mycoparasite from infected to uninfected sclerotia of S. sclerotiorum in moist sterile sand and non-sterile soil. Subsequently, a simple assay system to monitor transfer of C. minitans from colonised wheat grains to uninfected sclerotia of S. sclerotiorum was developed. Both A. siro and F. candida transmitted C. minitans at least 55 mm to sclerotia in soil at water potentials ranging from saturation to −3.6 MPa. Transmission by A. siro was greater in drier conditions (−0.25 to −3.6 MPa) as mites survived poorly in saturated soil. However, water potentials between saturation to −3.6 MPa had no effect on transmission by F. candida, although collembolans died after 18 d at water potentials of −5.4 MPa or drier. Generally, maximum dispersal occurred within 2 weeks. In soil lacking added arthropods, negligible spread of the mycoparasite was observed. These results suggest that soil mesofauna may be important in the dissemination of C. minitans.     

Influence of soil and host plant crop on the genetic diversity of Azospirillum amazonense isolates

The genetic structure of Azospirillum amazonense populations isolated from the rhizosphere soil and washed and surface-sterilised roots of rice, maize and sorghum plants, cropped simultaneously in two different soils (clay loam and sandy loam) was characterised. Genetic diversity was measured by restriction fragment length polymorphism of the amplified 16S–23S rDNA intergenic spacer region (RISA-RFLP) and cluster analysis. Four genetically distinct clusters of isolates were observed with 78% similarity, suggesting that the A. amazonense population was heterogeneous at the strain level regardless of the soil type or host plant. Analysis of molecular variance (AMOVA) demonstrated that the host plant had a highly significant selective effect on the genetic structure of this species, especially on those isolates intimately associated with them, but also to a lesser extent on isolates from the rhizosphere and washed roots. The soil type also had a highly significant selective effect on A. amazonense genetic diversity, especially for those isolates from the rhizosphere soil. The selective effect of the soil type combined with that of the host plant suggests that environmental factors, such as soil texture and composition of exudates provided by C₃ or C₄ plants, play major roles in the overall genetic structure of A. amazonense populations associated with these cereals.     

Local diversity of native arbuscular mycorrhizal symbionts differentially affects growth and nutrition of three crop plant species

Intact whole native AMF communities occurring across a 100-m-long field were used for the evaluation of plant performance, as determined by the actual fungal species colonizing host roots. The soil from distinct plots within a “hot spot” field was collected to set up 54 experimental units where three different plant species were grown, in order to test whether the whole native AMF communities were able to differentially affect plant growth, to assess the genetic identity of the AMF actually colonizing the tested plants and to analyse their community composition in the different hosts. Molecular analyses revealed that plant growth and nutrition of the crop plants were differentially affected by the diverse native arbuscular mycorrhizal communities colonizing the roots of the three plants, whose performance varied depending on the identity of plant hosts and fungal symbionts, more than on a rich and diversified AMF community. Such results, improving our understanding of AMF distribution at the local scale, represent a starting point allowing the selection, isolation and characterization of the most efficient AMF assemblages to be used as inoculants in sustainable food production systems.