ASSOCIATED BACTERIA INCREASE THE PHYTOEXTRACTION OF CADMIUM AND ZINC FROM A METAL-CONTAMINATED SOIL BY MYCORRHIZAL WILLOWS

In order to enhance phytoremediation efficiency, we investigated the effects of dual inoculation with ectomycorrhizal fungi and the ectomycorrhiza associated bacteria Micrococcus luteus and Sphingomonas sp. on the growth and metal accumulation of willows (Salix viminalis x caprea) on contaminated soil. The bacterial strains were previously collected from sporocarps of ectomycorrhizal fungi. The bacteria increased plant growth and the mycorrhizal dependency of willows colonized with the ectomycorrhizal fungus Hebeloma crustuliniforme. The total cadmium (Cd) and zinc (Zn) accumulation in the shoot biomass was increased after inoculation with the fungal strain Hebeloma crustuliniforme in combination with Micrococcus luteus up to 53% and in combination with Sphingomonas sp. up to 62%, respectively. The dual inoculation in combination with Laccaria laccata did not increase the accumulation of Cd and Zn in the willows. We conclude that associated bacteria can enhance the ectomyorrhiza formation and growth of willows and, thereby, the Cd and Zn accumulation in the plant biomass. The results suggest that bacterial support of root growth promoting ectomycorrhizal fungi may be a promising approach to improve the remediation of metal-contaminated soils by using willows.     

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.     

The survival and development of inoculant ectomycorrhizal fungi on roots of outplanted Eucalyptus globulus Labill

The survival and development of two inoculant ectomycorrhizal fungi (Hebeloma westraliense Bough. Tom. and Mal. and Setchelliogaster sp. nov.) on roots of outplanted Eucalyptus globulus Labill. was examined at two expasture field sites in the south-west of Western Australia. Site 1 was a gravelly yellow duplex soil, and Site 2 was a yellow sandy earth. Plants were grown in steamed or unsteamed soil, in root bags designed as field containers for young growing trees. Three, 6 and 12 months after outplanting, plants were removed from these bags and assessed for dry weights of shoots and ectomycorrhizal colonization of roots.The inoculant ectomycorrhizal fungi (identified on the basis of the colour and morphology of their mycorrhizas) survived on roots of E. globulus for at least 12 months after outplanting at both field sites. At Site 1, however, colonization of new fine roots by the inoculant fungi was low (less than 20% of fine root length). Inoculation had no effect on the growth of E. globulus at this site. In contrast, at Site 2 the inoculant ectomycorrhizal fungi colonized up to 30–50% of new fine root length during the first 6 months after outplanting. There was a corresponding growth response to ectomycorrhizal inoculation at this site, with a close relationship (r²=0.82^**) between plant growth at 12 months and root colonization at 3 months. Plant growth at 12 months was related less closely with root colonization at 6 or 12 months. Root colonization by resident ectomycorrhizal fungi increased with time at both field sites. At Site 2, this increase appeared to be at the expense of colonization by the inoculant fungi, which was reduced to less than 10% of fine root length at 12 months. Steaming the soil had little effect on colonization by the inoculant ectomycorrhizal fungi at either field site, but decreased colonization by the resident ectomycorrhizal fungi.     

Comparison of biomass allocation in ectomycorrhizal and nonmycorrhizal Douglas fir seedlings of similar nutrition and overall size

Biomass allocation in 6-month-old ectomycorrhizal Douglas fir seedlings was compared to that in nonmycorrhizal seedlings of the same age, nutrient status and total biomass. Seedlings colonized by Rhizopogon vinicolor had the same distribution of biomass between roots, stems and needles, but only 56% of the total length of roots (including mycorrhizal branches) compared to nonmycorrhizal seedlings. Laccaria laccata had no effect on distribution of biomass or root length of seedlings. The results for Rhizopogon provide direct evidence that the process of ectomycorrhizal colonization can significantly affect plant biomass allocation by one or more mechanisms not directly related to altered nutrition or overall plant size.     

Functional diversity of arbuscular mycorrhizal fungal isolates in relation to extraradical mycelial networks

We investigated the functional significance of extraradical mycorrhizal networks produced by geographically different isolates of the arbuscular mycorrhizal fungal (AMF) species Glomus mosseae and Glomus intraradices. A two-dimensional experimental system was used to visualize and quantify intact extraradical mycelium (ERM) spreading from Medicago sativa roots. Growth, phosphorus (P) and nitrogen (N) nutrition were assessed in M. sativa plants grown in microcosms. The AMF isolates were characterized by differences in extent and interconnectedness of ERM. Phenotypic fungal variables, such as total hyphal length, hyphal density, hyphal length per mm of total or colonized root length, were positively correlated with M. sativa growth response variables, such as total shoot biomass and plant P content. The utilization of an experimental system in which size, growth rate, viability and interconnectedness of ERM extending from mycorrhizal roots are easily quantified under realistic conditions allows the simultaneous evaluation of different isolates and provides data with a predictive value for selection of efficient AMF.     

INFLUENCE OF SUPPLEMENTAL INORGANIC NUTRIENTS ON GROWTH,SURVIVORSHIP, AND MYCORRHIZAL RELATIONSHIPS OF SCHIZACHYRIUM SCOPARIUM (POACEAE) GROWN IN FUMIGATED AND UNFUMIGATED SOIL

Little bluestem grass Schizachyrium scoparium ([Michx.] Nash) plants were grown under field conditions for 2 years in soils fumigated with methyl bromide and chloropicrin, or in unfumigated soil, and treated with supplemental inorganic nutrients (bases calcium and magnesium) phosphorus, nitrogen, and potassium. Most differences in measured plant responses were due to interactions between fumigation and nutrient treatments. These included biomass production, root mass per unit length (μg/cm), root lengths, flowering culm production, percent colonization, colonized root length, and spore production in rhizosphere soil. Plants generally responded to mycorrhizal fungal colonization by reducing total root length and producing thicker roots. Treatment of plants with bases appeared to profoundly affect the mycorrhizal association by reducing sporulation of vesicular-arbuscular mycorrhizal fungi and increasing colonization. When fumigated or unfumigated soils were considered separately, base-treated plants produced more biomass than other treatments. Base-treated plants grown on unfumigated soil had more flowering culms and longer colonized root lengths than all other plants. Percent colonization by mycorrhizal fungi and colonized root length were positively correlated with phosphorus/nitrogen ratios, but the ratio was not correlated with plant biomass production. This suggests that phosphorus is not a limiting nutrient in our soil and investment in a mycorrhizal association may not result in enhanced plant growth. The base-nutrient effects may indicate a need to reevaluate earlier studies of macro nutrient effects that did not take into account the role played by calcium and magnesium in assessing fungus-host plant interactions.     

Development of a model system to assess the impact of genetically modified corn and aubergine plants on arbuscular mycorrhizal fungi

We developed an experimental model system to monitor the impact of generically modified (GM) plants on arbuscular mycorrhizal (AM) fungi, a group of non-target soil microorganisms, fundamental for soil fertility and plant nutrition. The system allowed us to study the effects of root exudates of both commercial Bt corn and aubergine plants expressing Dm-AMP1 defensin on different stages of the life cycle of the AM fungal species G. mosseae. Root exudates of Bt 176 corn significantly reduced pre-symbiotic hyphal growth, compared to Bt 11 and non-transgenic plants. No differences were found in mycelial growth in the presence of Dm-AMP1 and control plant root exudates. Differential hyphal morphogenesis occurred irrespective of the plant line, suggesting that both exuded Bt toxin and defensin do not interfere with fungal host recognition mechanisms. Bt 176 affected the regular development of appressoria, 36% of which failed to produce viable infection pegs. Our experimental model system represents an easy assay for testing the impact of GM plants on non-target soil-borne AM fungi.     

Ability of native ectomycorrhizal fungi from northern Spain to colonize Douglas-fir and other introduced conifers

 Thirty-six isolates from 27 species of native ectomycorrhizal fungi collected in northern Spain were tested for ectomycorrhiza formation with Pseudotsuga menziesii seedlings in pure culture syntheses. Thirteen of those species were also tested for ectomycorrhiza formation with six other species of conifers (two native and four introduced) to compare their colonization potential. Twenty-three fungal isolates from 18 species formed ectomycorrhizas with Pseudotsuga menziesii. The colonization level of the root system varied markedly among the different fungal species. Eight fungi colonized over 50% of the short roots. Nine fungi did not form ectomycorrhizas even though some of them were collected in pure stands of Pseudotsuga menziesii. Laccaria laccata, Lyophyllum decastes, Pisolithus tinctorius, and Scleroderma citrinum formed abundant ectomycorrhizas on all the conifers tested. Lactarius deliciosus, Rhizopogon spp., and Suillus luteus showed the greatest host specificity. The success in the introduction of some exotic conifers for reforestation in northern Spain is discussed in relation to their compatibility with native ectomycorrhizal fungi. Accepted: 28 August 1995     

Nitrogen input mediates the effect of free-air CO2 enrichment on mycorrhizal fungal abundance

Plots containing Lolium perenne L., Trifolium repens L. or a mixture of both plant species were exposed to elevated atmospheric CO₂ (eCO₂) for 10 consecutive seasons using free‐air CO₂ enrichment technology at ETH Zürich, Switzerland. The CO₂ treatment was crossed with a two‐level nitrogen (N) fertilization treatment. In the tenth year, soil samples were collected on three occasions through the growing season to assess the impact of eCO₂ and N fertilization on mycorrhizal fungal abundance. Soil moisture content, which varied with harvest date, was linked to the vegetation type and was higher under eCO₂. Root weight density was affected by vegetation type: lower for clover, higher for grass. Root weight density was stimulated by eCO₂ and decreased by high N fertilization. The percent root length colonized by mycorrhizal fungi was lowest in the clover plots and highest in the grass plots. High N significantly decreased root length colonized. There was no overall effect of eCO₂ on root length colonized; however, there was a significant eCO₂× N interaction: eCO₂ increased root length colonized at high N, but decreased root length colonized at low N. Extraradical mycorrhizal hyphal density was linked to soil moisture content. Extraradical mycorrhizal hyphal density was not affected by eCO₂ or high N individually, but as for root length colonized, there was a significant eCO₂× N interaction: eCO₂ increased extraradical mycorrhizal hyphal density at low N but not at high N. These environmental effects on root colonization and external mycorrhizal hyphae were independent of soil moisture content and root weight density. This field study demonstrated a significant mediating effect of N fertilization on the response of arbuscular mycorrhizal fungi to eCO₂ irrespective of any change in root biomass.     

The annual invasive plant Impatiens glandulifera reduces hyphal biomass of soil fungi in deciduous forests

Soil fungi play a crucial role in ecosystem functioning and there is increasing evidence that exotic plants invading forests can affect soil fungal communities. We examined potential effects of the invasive plant Impatiens glandulifera on hyphal biomass of ectomycorrhizal fungi, their genetic diversity and the diversity of other soil fungi in deciduous forests in Switzerland. We compared invaded patches with patches where I. glandulifera had been removed, by establishing pairs of 3-m long transect lines at the edge of seven areas of either type. Along the transects we assessed the length of ectomycorrhizal fungal hyphae using the ‘ingrowth mesh bag method’, and used terminal restriction fragment length polymorphism (T-RFLP) analysis to examine fungal genetic diversity. The invasive plant reduced fungal hyphal biomass by 30–80%: the reduction was largest in the centre of the patch. I. glandulifera did not alter fungal richness, but affected the composition of fungal communities. This is probably the result of a decrease of mycorrhizal fungi, coupled with an increase of saprotrophic fungi. Our findings demonstrate the adverse impacts of an annual invasive plant species on both fungal hyphal biomass and the composition of soil fungal communities. This may negatively affect forest nutrient and carbon cycling, soil stability and the functionality of the fungal community, with major consequences for forest ecosystem functioning.     

Rapid in vitro ectomycorrhizal infection onPinus densiflora roots byTricholoma matsutake

The root systems of 11-wk-oldPinus densiflora seedlings were inoculated with a hyphal suspension ofTricholoma matsutake and aseptically incubated for 4 wk in a forest soil without supplying exogenous carbohydrates. One week following inoculation, fungal hyphae had colonized the root surface and bound soil particles together establishing a root-substrate continuum. Fungal hyphae were visible within the main root cortex following clearing bleaching and staining. In the ensuing days, fungal colonization was observed within elongating lateral roots in which Hartig net formation was confirmed 4 wk after inoculation. This is the first report of rapid ectomycorrhizal infection ofP. densiflora seedings byT. matsutake.     

Effect of elevated atmospheric CO2 on mycorrhizal colonization, external mycorrhizal hyphal production and phosphorus inflow in Plantago lanceolata and Trifolium repens in association with the arbuscular mycorrhizal fungus Glomus mosseae

Plantago lanceolata and Trifolium repens were grown under ambient (400 μmol mol^–1) and elevated (650 μmol mol^–1) atmospheric CO₂ conditions. Plants were inoculated with the arbuscular mycorrhizal fungus Glomus mosseae and given a phosphorus supply in the form of bonemeal. Six sequential harvests were taken in order to determine whether the effect of elevated CO₂ on internal mycorrhizal colonization and external hyphal production was independent of the stimulatory effect of elevated CO₂ on plant growth. At a given time, elevated CO₂ increased the percentage of root length colonized (RLC), the total length of colonized root and the external mycorrhizal hyphal (EMH) density and decreased the ratio of EMH to total length of colonized root. When plant size was taken into account, the CO₂ effect on RLC and total length of colonized root was greatly reduced (and only apparent for early harvests in T. repens) and the effects on the EMH parameters disappeared. Root tissue P concentration was unchanged at elevated CO₂, but there was a decrease in shoot P at the later harvests. There was no direct effect of elevated CO₂ on P inflow for the earlier period (< 50 d) of the experiment. However, over the last period, there was a significant negative effect of elevated CO₂ on P inflow for both species, independent of plant size. It is concluded that elevated CO₂ had no direct effect on mycorrhizal colonization or external hyphal production, and that any observed effects on a time basis were due to faster growing plants at elevated CO₂. However, for older plants, elevated CO₂ had a direct negative effect on P inflow. This decrease in P inflow coincides with the observed decrease in shoot P concentration. This is discussed in terms of downregulation of photosynthesis often seen in elevated CO₂ grown plants, and the potential for mycorrhizas (via external hyphal turnover) to alleviate the phenomenon. The direction for future research is highlighted, especially in relation to carbon flow to and storage in the soil.     

The improvement of plant N acquisition from an ammonium-treated,drought-stressed soil by the fungal symbiont in arbuscular mycorrhizae

The ability of the external mycelium in arbuscular mycorrhiza for N uptake and transport was studied. The contribution of the fungal symbiont to N acquisition by plants was studied mainly under waterstressed conditions using ¹⁵N. Lettuce (Lactuca sativa L) was the host for two isolates of the arbuscular mycorrhizal fungi Glomus mosseae and G. fasciculatum. The experimental pots had two soil compartments separated by a fine mesh screen (60 m). The root system was restricted to one of these compartments, while the fungal mycelium was able to cross the screen and colonize the soil in the hyphal compartment. A trace amount of ¹⁵NH ₄ ⁺ was applied to the hyphal compartment 1 week before harvest. Under water-stressed conditions both endophytes increased the ¹⁵N enrichment of plant tissues; this was negligible in nonmycorrhizal control plants. This indicates a direct effect of arbuscular mycorrhizal fungi on N acquisition in relatively dry soils. G. mosseae had more effect on N uptake and G. fasciculatum on P uptake under the water-limited conditions tested, but both fungi improved plant biomass production relative to nonmycorrhizal plants to a similar extent.     

Soil Bacterial Diversity Responses to Root Colonization by an Ectomycorrhizal Fungus are not Root-Growth-Dependent

The hypothesis tested in this present study was that the ectomycorrhizosphere effect on the bacterial community was not root-growth-dependent. The impacts of ectomycorrhizal infection (Pisolithus albus COI007) and a chemical fertilization to reproduce the fungal effect on root growth were examined on (1) the structure of bacterial community and (2) fluorescent pseudomonad and actinomycete populations in the mycorrhizosphere of Acacia auriculiformis using both culture-independent and culture-dependent methods. A. auriculiformis plants were grown in disinfested soil in pots with or without addition of the ectomycorrhizal fungus or N/P/K fertilization (to reproduce the fungal effect on root growth) for 4 months and then transferred to 20-L pots filled with nondisinfested sandy soil. The fungal and fertilizer applications significantly improved the plant growth after 4-month culture in the disinfested soil. In the nondisinfested cultural substrate, these positive effects on plant growth were maintained. The total soil microbiota was significantly different within the treatments as revealed from DNA analysis [denaturing gradient gel electrophoresis (DGGE)]. The structure of fluorescent pseudomonad populations was also affected by fungal and fertilizer applications. In contrast, no qualitative effect was observed for the actinomycete communities within each treatment, but fungal inoculation significantly decreased the number of actinomycetes compared to the fertilizer application treatment. These results show that the mycorrhizosphere effect is not root-growth-dependent but is mainly due to the presence of the ectomycorrhizal fungus and more particularly to the extramatrical mycelium.     

Time-course study and partial characterization of a protein on hyphae of arbuscular mycorrhizal fungi during active colonization of roots

Material on the surface of hyphal walls of arbuscular mycorrhizal fungi (AMF) during active colonization of plant roots was detected by a monoclonal antibody. Pot-cultured isolates of Glomus, Acaulospora, Gigaspora, Scutellospora, and Entrophospora had immunofluorescent material (IM) on younger, thinner, intact hyphae, but IM was scant to absent on thicker, melanized or lysing hyphae. Colonization of corn (Zea mays L.), Sudangrass (Sorghum sudanense (Piper) Staph.) or red clover (Trifolium pratense L.) was examined during 5 months of plant growth by removing cores and performing an indirect immunoassay on roots with attached hyphae. Fresh spores of some Glomus spp. had IM on the outer layer of the spore wall. Abundant IM was seen on root hairs of plants colonized by some isolates, and some IM was detected on root surfaces of all plants examined even during early colonization. After cultures were dried, hyphae, roots and spores had little to no IM. Uninoculated control roots had very rare, small patches of IM. An immunoreactive protein was extracted from hyphae of Gigaspora and Glomus isolates by using 20mM citrate (pH 7.0) at 121°C for 90 min. Gel electrophoresis profiles indicated that all isolates tested had the same banding patterns. Lectin-binding of extracted protein is suggestive of a glycoprotein. The immunofluorescence assay can be used to examine root sections for active colonization by AMF, and the potential use of the protein to quantify AMF activity in soil is discussed.     

Arbuscular mycorrhizal fungi and plant symbiosis in a saline-sodic soil

The seasonality of arbuscular mycorrhizal (AM) fungi–plant symbiosis in Lotus glaber Mill. and Stenotaphrum secundatum (Walt.) O.K. and the association with phosphorus (P) plant nutrition were studied in a saline-sodic soil at the four seasons during a year. Plant roots of both species were densely colonized by AM fungi (90 and 73%, respectively in L. glaber and S. secundatum) at high values of soil pH (9.2) and exchangeable sodium percentage (65%). The percentage of colonized root length differed between species and showed seasonality. The morphology of root colonization had a similar pattern in both species. The arbuscular colonization fraction increased at the beginning of the growing season and was positively associated with increased P concentration in both shoot and root tissue. The vesicular colonization fraction was high in summer when plants suffer from stress imposed by high temperatures and drought periods, and negatively associated with P in plant tissue. Spore and hyphal densities in soil were not associated with AM root colonization and did not show seasonality. Our results suggest that AM fungi can survive and colonize L. glaber and S. secundatum roots adapted to extreme saline-sodic soil condition. The symbiosis responds to seasonality and P uptake by the host altering the morphology of root colonization.     

Utilization of organic nitrogen at two different substrate pH by different ectomycorrhizal fungi growing in symbiosis with Pinus sylvestris seedlings

The aim of this study was to test the potential of four isolates of ectomycorrhizal (EM) fungi to utilize organic nitrogen (N) at two different substrate pHs. The organic N source (¹⁵N labelled lyophilised fungal mycelium) was mixed with either untreated peat/sand mixture (pH 4.9) or peat/sand mixture limed to a pH of 5.9 and put in cylindrical containers added to each pot. The content of the containers was separated from the roots of Pinus sylvestris seedlings by a nylon mesh and a 2 mm air gap to reduce diffusion of labelled N to the roots. The mycorrhizal plants (except those colonized by Suillus variegatus 2) took up significantly more ¹⁵N from the labelled mycelium than uncolonized seedlings. Liming significantly reduced the uptake of ¹⁵N by one of the EM fungi (unidentified) but not the other tested species (Paxillus involutus and two isolates of S. variegatus). The EM fungal isolates differed in their influence on the bacterial activity of the soil. This was reduced with P. involutus at both pH levels and increased with one of the two S. variegatus isolates at the high pH and with the other S. variegatus isolate at the low pH level. Liming the soil generally increased bacterial activity. The influence of liming on the proportion of organic N uptake in relation to inorganic N uptake by ectomycorrhizal trees is discussed.     

The response of two Glomus mycorrhizal fungi and a fine endophyte to elevated atmospheric CO2, soil warming and drought

Plantago lanceolata plants were grown under various environmental conditions in association with the mycorrhizal fungi Glomus mosseae, G. caledonium and a fine endophyte either individually or all together. Using a time‐course approach, we investigated the effects of elevated atmospheric CO₂ (eCO₂), soil warming and drought and their interactions on root length colonized (RLC) by mycorrhizal fungi and extraradical mycorrhizal hyphal (EMH) production. Plant growth responded as would be expected to the environmental manipulations. There was no plant growth‐independent effect of eCO₂ on mycorrhizal colonization; however, EMH production was stimulated by eCO₂, i.e. there was increased partitioning of below‐ground carbon to the EMH. Soil warming directly stimulated both percent RLC by the Glomus species and EMH density; soil warming did not affect RLC by the fine endophyte. Drought decreased percent RLC for the fine endophyte, but not for the Glomus species. The presence of one mycorrhizal fungus did not affect the response of another to the environmental variables. There was no evidence of any interactive effects of the environmental variables on RLC, but there were significant environmental interactions on EMH production. In particular, the stimulatory effects of eCO₂ and soil warming on EMH density were not additive. The results are discussed in terms of the soil carbon cycle, highlighting some crucial gaps in our knowledge. If future environmental changes affect mycorrhizal fungal turnover and respiration, then this could have important implications for the terrestrial carbon cycle.     

Growth profile of ectomycorrhizal fungal mycelium: emphasis on substrate pH influence

A knowledge of behaviour of ectomycorrhizal fungal isolates to substrate pH would help in identifying candidate fungi for plantation programs. Ectomycorrhizal fungus isolates were studied to determine the pH optima for growth, substrate acidification by the culture and the effect of substrate acidification on culture growth. Of the isolates tested, the members of Agaricales (except Laccaria laccata) and Aphyllophorales favored neutral to near neutral pH, while members of the order Sclerodermatales strictly favored acidic pH. The change in substrate pH (ΔpH) was maximum at the optimum growth pH for acidophilic isolates but minimum for neutro/basophilic isolates. The experiments indicate that the substrate pH would not only determine the growth rate of the fungus but also limits further proliferation of the fungus in medium. This gives vital information for determining the subculture frequency and for designing substrate parameters for nursery/plantation programs. This revised version was published online in June 2006 with corrections to the Cover Date.     

Arbuscular mycorrhizal fungi reduce growth and infect roots of the non‐host plant Arabidopsis thaliana

The arbuscular mycorrhizal (AM) symbiosis is widespread throughout the plant kingdom and important for plant nutrition and ecosystem functioning. Nonetheless, most terrestrial ecosystems also contain a considerable number of non‐mycorrhizal plants. The interaction of such non‐host plants with AM fungi (AMF) is still poorly understood. Here, in three complementary experiments, we investigated whether the non‐mycorrhizal plant Arabidopsis thaliana, the model organism for plant molecular biology and genetics, interacts with AMF. We grew A. thaliana alone or together with a mycorrhizal host species (either Trifolium pratense or Lolium multiflorum) in the presence or absence of the AMF Rhizophagus irregularis. Plants were grown in a dual‐compartment system with a hyphal mesh separating roots of A. thaliana from roots of the host species, avoiding direct root competition. The host plants in the system ensured the presence of an active AM fungal network. AM fungal networks caused growth depressions in A. thaliana of more than 50% which were not observed in the absence of host plants. Microscopy analyses revealed that R. irregularis supported by a host plant was capable of infecting A. thaliana root tissues (up to 43% of root length colonized), but no arbuscules were observed. The results reveal high susceptibility of A. thaliana to R. irregularis, suggesting that A. thaliana is a suitable model plant to study non‐host/AMF interactions and the biological basis of AM incompatibility.