Fine-scale structure of populations of the ectomycorrhizal fungus Hebeloma cylindrosporum in coastal sand dune forest ecosystems

The basidiomycete mushroom Hebeloma cylindrosporum is a frequently found pioneer ectomycorrhizal species naturally associated with Pinus pinaster trees growing in coastal sand dune ecosystems along the Atlantic south-west coast of France. The genotypic diversity and spatial structure of three populations of this fungal species have been studied. At each site the basidiocarps were mapped, sampled and propagated as pure mycelial cultures. For each of the isolates, we have studied polymorphisms in the mitochondrial genome, polymorphisms at two different nuclear loci and also fingerprints produced with a multicopy DNA probe. The comparison of the different polymorphisms obtained, with each of the four molecular methods used, allowed the identification of several of the different genets present in each site. In two of the studied sites most of the basidiocarps, which often occurred as dense patches of 10–30 in 1 m² or less, were of a unique genotype, suggesting the below-ground mycelia to be of a small size (from 50 cm² to approx. 7 m² for the larger mycelia) and that the root system of a single Pinus tree can host several genets of the same symbiotic fungus. In the two sites, which were studied again after a 3-year interval, none of the genotypes identified in the first year of sampling was re-identified 3 years later. These results contrast with those reported for other species of soilborne homobasidiomycete species, either ectomycorrhizal, parasitic or saprophytic, showing mostly large clones resulting from the vegetative growth and from persistence of below-ground mycelia. Sexual reproduction through meiospore dispersal seems to play a key role in the structuring of the populations of H. cylindrosporum. Mycelia associated with the root systems seem to be replaced after 1 or a few years, during which basidiocarp differentiation takes place. As opposed to the few other studied ectomycorrhizal species, H. cylindrosporum has the characteristics of ruderal species, with a short life-span adapted to pioneer situations, e.g. to nutrient-poor and unstable sandy soils of coastal sand dunes.     

Premiers résultats de recherches sur la compétitivité des champignons ectomycorhiziens

Resumé Les premiers résultats d'un modèle expérimental visant à quantifier le comportement de différents champignons ectomycorhiziens sur différents sols sont décrits. Ils mettent en évidence que l'intensité de l'inoculation par une souche introduite est un facteur décisif pour la mycorhization par cette souche. La photopériode et le temps sont également très importants à prendre en compte pour la définition de la capacité d'accueil d'un sol. Les deux sols testés ont des comportements très différents. La microflore du sol n'est pas toujours l'obstacle principal à l'infection mycorhizienne et à son extension.

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First results of research on the competitivity of ectomycorrhizal fungi

Summary The first results of an experimental model for quantifying the behaviour of different ectomycorrhizal fungi on different soils are described. They show that the inoculation intensity of the introduced strain is a decisive factor for the mycorrhization by this strain. Photoperiod and time are very important too for the definition of the soil receptivity. The two tested soils have very different behaviours. The soil microflora is not always the main obstacle to the mycorrhizal infection and to its extension.

     

Extracellular complexation of Cd in the Hartig net and cytosolic Zn sequestration in the fungal mantle of Picea abies – Hebeloma crustuliniforme ectomycorrhizas

Compartmentation of heavy metals on or within mycorrhizal fungi may serve as a protective function for the roots of forest trees growing in soils containing elevated concentrations of metals such as Cd and Zn. In this paper we present the first quantitative measurements by X‐ray microanalysis of heavy metals in high‐pressure frozen and cryosectioned ectomycorrhizal fungal hyphae. We used this technique to analyse the main sites of Cd and Zn in fungal cells of mantle and Hartig net hyphae and in cortical root cells of symbiotic Picea abies – Hebeloma crustuliniforme associations to gain new insights into the mechanisms of detoxification of these two metals in Norway spruce seedlings. The mycorrhizal seedlings were exposed in growth pouches to either 1 mM Cd or 2 mM Zn for 5 weeks. The microanalytical data revealed that two distinct Cd‐ and Zn‐binding mechanisms are involved in cellular compartmentation of Cd and Zn in the mycobiont. Whereas extracellular complexation of Cd occurred predominantly in the Hartig net hyphae, both extracellular complexation and cytosolic sequestration of Zn occurred in the fungal tissue. The vacuoles were presumed not to be a significant pool for Cd and Zn storage. Cadmium was almost exclusively localized in the cell walls of the Hartig net (up to 161 mmol kg ^{? 1} DW) compared with significantly lower concentrations in the cell walls of mantle hyphae (22 mmol kg ^{? 1} DW) and in the cell walls of cortical cells (15 mmol kg ^{? 1} DW). This suggests that the apoplast of the Hartig net is a primary accumulation site for Cd. Zinc accumulated mainly in the cell walls of the mantle hyphae (111 mmol kg ^{? 1} DW), the Hartig net hyphae (130 mmol kg ^{? 1} DW) and the cortical cells (152 mmol kg ^{? 1} DW). In addition, Zn occurred in high concentrations in the cytoplasm of the fungal mantle hyphae (up to 164 mmol kg ^{? 1} DW) suggesting that both the cell walls and the cytoplasm of fungal tissue are the main accumulation sites for Zn in P. abies resulting in decreased Zn transfer from the fungus to the root.     

Responses of Hebeloma radicosum fruit-bodies to light and gravity: negatively gravitropic and nonphototropic growth

Responses of the long-rooting agaric Hebeloma radicosum fruit-bodies to light and gravity were studied. In light from below or obliquely below, fruit-bodies grew straight downward with gills tilted and cap swollen and waved if they had emerged downward from the culture medium, or bent upward from the beginning if they had emerged obliquely downward. In light from above or obliquely above, they grew upward if they had emerged upward. Thus, they did not grow toward unilateral light from obliquely below or obliquely above, and hence their growth was nonphototropic and negatively gravitropic from the beginning of development. Even the straight downward growth seems to be latently negative-gravitropic. In the dark, fruit-bodies grew upward, forming pseudorhizas, but they remained immature; they matured only in the light. These characteristics may be related to the growth habits of the fungus colonizing deep in the ground, forming primordia there, and developing mature fruit-bodies on the ground. Received: March 26, 2001 / Accepted: July 12, 2001