Peter Schweiger

Flavonoids and Strigolactones in Root Exudates as Signals in Symbiotic and Pathogenic Plant-Fungus Interactions

Secondary plant compounds are important signals in several symbiotic and pathogenic plant-microbe interactions. The present review is limited to two groups of secondary plant compounds, flavonoids and strigolactones, which have been reported in root exudates. Data on flavonoids as signaling compounds are available from several symbiotic and pathogenic plant-microbe interactions, whereas only recently initial data on the role of strigolactones as plant signals in the arbuscular mycorrhizal symbiosis have been reported. Data from other plant-microbe interactions and strigolactones are not available yet. In the present article we are focusing on flavonoids in plant-fungal interactions such as the arbuscular mycorrhizal (AM) association and the signaling between different Fusarium species and plants. Moreover the role of strigolactones in the AM association is discussed and new data on the effect of strigolactones on fungi, apart from arbuscular mycorrhizal fungi (AMF), are provided.

Belowground carbon allocation by trees drives seasonal patterns of extracellular enzyme activities by altering microbial community composition in a beech forest soil.

Plant seasonal cycles alter carbon (C) and nitrogen (N) availability for soil microbes, which may affect microbial community composition and thus feed back on microbial decomposition of soil organic material and plant N availability. The temporal dynamics of these plant–soil interactions are, however, unclear. Here, we experimentally manipulated the C and N availability in a beech forest through N fertilization or tree girdling and conducted a detailed analysis of the seasonal pattern of microbial community composition and decomposition processes over 2 yr. We found a strong relationship between microbial community composition and enzyme activities over the seasonal course. Phenoloxidase and peroxidase activities were highest during late summer, whereas cellulase and protease peaked in late autumn. Girdling, and thus loss of mycorrhiza, resulted in an increase in soil organic matter-degrading enzymes and a decrease in cellulase and protease activity. Temporal changes in enzyme activities suggest a switch of the main substrate for decomposition between summer (soil organic matter) and autumn (plant litter). Our results indicate that ectomycorrhizal fungi are possibly involved in autumn cellulase and protease activity. Our study shows that, through belowground C allocation, trees significantly alter soil microbial communities, which may affect seasonal patterns of decomposition processes.

Ectomycorrhizal communities associated with Populus tremula growing on a heavy metal contaminated site

European aspen is one of the most widely distributed trees in Central Europe and is a typical early colonizer of poor and disturbed soils. However, little is known about ectomycorrhizal (ECM) fungi in these ecosystems. We examined the ECM community of European aspen growing on a heavily contaminated site in southern Austria by analysing ECM roots, sorting them into morphotypes, subjecting them to DNA extraction, PCR, and DNA sequencing. ECM root symbionts were sampled two times in 2004. During this time, the below-ground community structure was relatively stable; we found no evidence of taxa adapted to summer or autumn conditions and only two species varied widely in occurrence between soil horizons. The ECM fungal community was diverse (54 species), rich in Basidiomycota (43 species), and dominated by Cenococcum geophilum and fungi with corticoid basidiomes (e.g. Thelephoraceae).

Impact of growth and uptake patterns of arbuscular mycorrhizal fungi on plant phosphorus uptake—a modelling study

In this paper we present a mathematical model for estimating external mycelium growth of arbuscular mycorrhizal fungi and its effect on root uptake of phosphate (P). The model describes P transport in soil and P uptake by both root and fungi on the single root scale. We investigate differences in soil P depletion and overall P influx into a mycorrhizal root by assuming that different spatial regions of mycelia are active in P uptake. When all external hyphae contribute to P uptake, overall uptake is dominated by the fungus and the most effective growth pattern appears to be the one using a high level of anastomosis. The same is true when only the proportion of external hyphae assumed to be active contributes to uptake. When uptake is restricted to the tips, hyphal contribution to overall P uptake is less dominant; the most effective growth pattern appears to be the one characterised by nonlinear branching where branching stops at a given maximal hyphal tip density. Comparison to measured P depletion in the literature suggests that the scenario where active hyphae are contributing to P uptake is likely to fit the data best. These quantitative predictions promote our understanding of the mycorrhizal symbiosis and its role in plant P nutrition.

Growth model for arbuscular mycorrhizal fungi

In order to quantify the contribution of arbuscular mycorrhizal (AM) fungi to plant phosphorus nutrition, the development and extent of the external fungal mycelium and its nutrient uptake capacity are of particular importance. We develop and analyse a model of the growth of AM fungi associated with plant roots, suitable for describing mechanistically the effects of the fungi on solute uptake by plants. The model describes the development and distribution of the fungal mycelium in soil in terms of the creation and death of hyphae, tip–tip and tip–hypha anastomosis, and the nature of the root–fungus interface. It is calibrated and corroborated using published experimental data for hyphal length densities at different distances away from root surfaces. A good agreement between measured and simulated values was found for three fungal species with different morphologies: Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders; Glomus sp.; and Acaulospora laevis Gerdemann & Trappe associated with Trifolium subterraneum L. The model and findings are expected to contribute to the quantification of the role of AM fungi in plant mineral nutrition and the interpretation of different foraging strategies among fungal species.

Zinc accumulation potential and toxicity threshold determined for a metal-accumulating Populus canescens clone in a dose-response study.

The effect of increasing soil Zn concentrations on growth and Zn tissue concentrations of a metal-accumulating aspen clone was examined in a dose-response study. Plants were grown in a soil with a low native Zn content which was spiked with Zn salt solutions and subsequently aged. Plant growth was not affected by NH(4)NO(3)-extractable soil Zn concentrations up to 60 microg Zn g(-1) soil, but it was completely inhibited at extractable concentrations above 90 microg Zn g(-1) soil. From these data an effective concentration of 68.5 microg extractable Zn g(-1) soil was calculated at which plant growth was reduced by 50%. The obtained information on toxicity threshold concentrations, and the relation between plant Zn accumulation and extractable soil Zn concentrations may be used to assess the suitability of the investigated Populus canescens clone for various phytoremediation strategies. The potential risk of metal transfer into food webs associated with P. canescens stands on Zn-polluted sites may also be estimated.

Bioconcentration of zinc and cadmium in ectomycorrhizal fungi and associated aspen trees as affected by level of pollution.

Concentrations of Zn and Cd were measured in fruitbodies of ectomycorrhizal (ECM) fungi and leaves of co-occurring accumulator aspen. Samples were taken on three metal-polluted sites and one control site. Fungal bioconcentration factors (BCF = fruitbody concentration: soil concentration) were calculated on the basis of total metal concentrations in surface soil horizons (BCF(tot)) and NH(4)NO(3)-extractable metal concentrations in mineral soil (BCF(lab)). When plotted on log-log scale, values of BCF decreased linearly with increasing soil metal concentrations. BCF(lab) for both Zn and Cd described the data more closely than BCF(tot). Fungal genera differed in ZnBCF but not in CdBCF. The information on differences between fungi with respect to their predominant occurrence in different soil horizons did not improve relations of BCF with soil metal concentrations. Aspen trees accumulated Zn and Cd to similar concentrations as the ECM fungi. Apparently, the fungi did not act as an effective barrier against aspen metal uptake by retaining the metals.

Ectomycorrhizal impact on Zn accumulation of Populus tremula L. grown in metalliferous soil with increasing levels of Zn concentration

AimsOur study aimed at characterizing the Zn phytoextraction potential of a metal tolerant Populus tremula accession in symbiosis with a community of ectomycorrhizal fungi from metal-contaminated soil that is naturally forming mycorrhizae with the experimental plant. Effects of the fungal community on P. tremula development, metal translocation and accumulation properties were tested under variable Zn bioavailability.MethodsIn a pot experiment, P. tremula seedlings were grown for 88 days in a substrate composed of metalliferous soil from an aspen stand and non-contaminated agricultural soil spiked with ZnSO4 to yield total Zn additions from 0 to 80 mg kg-1 substrate. The substrate contained the inherent mycorrhizal community of P. tremula (Nat-Myc) or was γ-irradiated to eliminate living microbial propagules (Irr-NM treatment).Resultsγ-Irradiation efficiently inhibited the formation of functional ectomycorrhizae in the control treatments. It increased dissolved organic carbon (DOC) in the substrate and increased the extractability of Zn and Cd by 1M NH4NO3. We found three times larger biomass and more than four times increased root lengths in the Irr-NM compared to the Nat-Myc treatments which may be explained by the doubled DOC concentrations and related Fe mobilization due to formation of labile complexes in the irradiation treatment and the absence of microbial competitors for (nutrient) resources. Our results indicate an imbalance of the normally mutualistic symbiosis between mycorrhizal fungi and the host at early growth stage, possibly further exacerbated by the high fragility and low nutrient reserves of the P. tremula seedlings obtained from a contaminated site. Foliar Zn concentrations were generally larger in the Nat-Myc treatments and exceeded those reported for numerous Salix and Populus species. While the Zn concentrations increased with increasing Zn additions, Zn translocation to shoots was inhibited at high Zn levels in the Nat-Myc treatments, indicating a barrier function of the mycorrhizal community.ConclusionsThe observed barrier properties in the mycorrhizal treatments suggest that mycorrhizal inoculation of P. tremula may be a promising strategy to enhance revegetation and phytostabilization of metal-polluted sites. However, early-stage growth of P. tremula may be limited by imbalances between the fungal and plant partner in such nutrient-deficient, toxic environments.

Media formulation influences in vitro ectomycorrhizal synthesis on the European aspen Populus tremula L.

The effect of various media formulations on in vitro ectomycorrhizal synthesis of identified fungal strains with European aspen (Populus tremula L.) was tested in Petri dishes. Pre-grown seedlings were transferred to various nutrient media and inoculated with Paxillus involutus isolates using modified sandwich techniques. Mycorrhiza formation was evaluated macroscopically and further confirmed by microscopic examination of semi-thin sections for anatomical features of the mantle and the Hartig net. Standard media formulations did not support successful ectomycorrhiza formation because of either very poor plant survival (below 20%) or impaired fungal growth. The inclusion of micronutrients and vitamins in a Melin Norkrans (MMN)-based medium increased plant survival rate to above 60% and supported successful mycorrhizal synthesis. P. involutus isolates formed mycorrhizas with a characteristic Hartig net restricted to the epidermis. Mantle density and thickness varied depending on the isolate. In a follow-up experiment, the adapted medium supported successful ectomycorrhiza formation by various Laccaria and Hebeloma isolates. Our results show that an exogenous supply of vitamins and micronutrients in the medium was a prerequisite for successful mycorrhization of P. tremula in vitro in Petri dishes.

Visualisation of ectomycorrhizal rhizomorph structure using laser scanning confocal microscopy

A method for the observation of the three-dimensional structure of intact ectomycorrhizal rhizomorphs is described. The method is based on a combination of clearing the material with KOH followed by staining with congo red and subsequent imaging under a laser scanning confocal microscope (LSCM). The images obtained are of a much higher three-dimensional resolution than those obtained previously by use of conventional light microscopical techniques. The structure of highly differentiated and undifferentiated rhizomorphs is described. Applications of the method are briefly discussed.