F. Le Tacon

The land flora: a phototroph-fungus partnership?

Numerous mutualistic associations between phototrophs and fungi exist in the extant land biota. Some are widespread, such as lichens and mycorrhizae, but some are less well known or restricted to special ecological conditions, such as endophytes in plants and algae. Recent molecular data and fossils suggest that associations arose repeatedly and that some of them are ancient, and even ancestral in the case of land plants. Mutualism, that provides various adaptations to terrestrial constraints, may have played a crucial role during terrestrialization and evolution of land phototrophs.

Genetic variability of phosphate solubilizing activity by monocaryotic and dicaryotic mycelia of the ectomycorrhizal fungus Laccaria bicolor (Maire) P.D. Orton

Homocaryotic mycelia obtained from spores of Laccaria bicolor S238 have been compared in vitro for their efficiency in solubilizing poorly soluble phosphates. This could lead to genetic selection according to such criteria. However, there is very little room for improvement as the wild strain was shown to be one of the most efficient solubilizers among the strains tested. Twenty dicaryotic strains obtained by crossing the compatible homocaryons have also been compared and no clear heritability of this character has been found. The four phosphate salts used are most probably solubilized by the same mechanism which is polygenetically controlled

Temporal persistence and spatial distribution of an American inoculant strain of the ectomycorrhizal basidiomycete Laccaria bicolor in a French forest plantation

Selected strains of ectomycorrhizal fungi, such as the basidiomycete Laccaria bicolor, are currently being used as inoculants in nurseries to improve growth of forest trees after outplanting. Information is needed on the survival of these introduced strains in forests and their impact on indigenous biodiversity. Dissemination and persistence of an American strain, L. bicolor S238N, were studied 10 years after outplanting in a Douglas fir plantation located at Saint‐Brisson (Morvan, France). About 430 Laccaria spp. sporophores were collected over 3 years. Inheritance of nuclear ribosomal DNA, as well as RAPD markers, was characterized in L. bicolor S238N, using a haploid progeny set of 91 monokaryons. More than 50 markers were identified (19 heterozygous and 33 homozygous or cytoplasmic markers), which unambiguously confirmed that the introduced strain was still present in the inoculated plots. Neither selfing (P < 0.0008) nor introgression with indigenous strains was detected although in vitro interfertility between the American strain and indigenous L. bicolor was identified. No ingress of the introduced genet into adjacent uninoculated plots colonized by various local Laccaria genets was detected. It is proposed that the spatial distributions identified have developed through mycelial propagation of the introduced strain and intraspecific competition with native genets. Although longer‐term data is still lacking, the stability of the inoculant strain and the limited disturbance to indigenous populations described support large‐scale nursery production of this host‐fungal combination.

Monitoring the persistence of Laccaria bicolor as an ectomycorrhizal symbiont of nursery-grown Douglas fir by PCR of the rDNA intergenic spacer.

The large‐scale inoculation of selected beneficial ectomycorrhizal fungi in forest nurseries has generated renewed interest in the ecology of these symbiotic fungi. However, information on the dissemination and persistence of introduced symbionts is scarce due to the limitation of the current identification methods. To identify ectomycorrhizal fungi on single root tips, we investigated the polymorphism of the PCR‐amplified ribosomal DNA intergenic spacer (IGS) from a wide range of ectomycorrhizal fungi. To investigate the reliability of this molecular approach in large‐scale surveys, the dissemination and persistence on Douglas fir seedlings of the introduced Laccaria bicolor S238N were assessed in a forest nursery in the Massif Central (France). Several hundred ectomycorrhizas and fruiting bodies were sampled from plots where control and L. bicolor inoculated‐Douglas fir seedlings were grown for 1.5 years. PCR typing of mycorrhizas indicated that trees inoculated with L. bicolor S238N remained exclusively colonized by that isolate (or sexually derived isolates) for the entire test period. In contrast, control seedlings were infected by indigenous isolates of Laccaria laccata and Thelephora terrestris. The molecular evidence for the persistence of the introduced mycobiont despite the competition from indigenous isolates of the same species provides further illustration of the potential of exotic species for large‐scale microbial application.

Effect of dose and formulation of Laccaria laccata inoculum on mycorrhizal infection and growth of Douglas fir in a nursery

Abstract A fumigated nursery bed on a sandy loam was inoculated with the ectomycorrhizal fungus Laccaria laccata and seeded with Douglas fir. Two types of inoculum were compared: mycelium grown in a vermiculite/peat mixture, and mycelium grown in liquid medium and entrapped in a calcium alginate gel with different quantities of mycelium. At the end of the first growing season, the alginate inoculum at the dose of 5 g mycelium (dry weight) per m 2 proved to be the most efficient. The top dry weight of the seedlings in this treatment was 2.3 fold that of the non-inoculated fumigated controls. This inoculation treatment also ensured nearly total mycorrhizal infection by L. laccata .

Role of Rhizosphere Microfungi in the Decline of Norway Spruce in Acidic Soils

In the late 1970s, Ulrich and collaborators hypothesized that acid deposits could initiate disturbances in soil mineral ion pool (Ulrich 1983a; Ulrich et al. 1980). In this scenario, the concentrations of potentially toxic ions such as Al3+, Mn2+ and H+ increase, while those of Ca2+ and Mg2+ decrease (Ulrich 1983b), possibly leading to root dysfunction. The rhizosphere could thus be affected by changes in root activity (exudates, respiration, mineral and water uptake). In addition, a prolonged water stress reducing the growth and metabolism of the trees might affect root symbionts and rhizosphere microorganisms, and so it was relevant to consider the interactions between roots and soil microbial communities when trying to elucidate the causes of spruce decline.

Ectomycorrhizas stress during forest trees transplantation ascertained by radiorespirometry and protein electrophoresis

Abstract When the plants are lifted from the nursery soil they loose a large proportion of their root system, especially the apical parts and the mycorrhizae. Furthermore, all the connections between the soil and the plant, through external mycelium and mycelium strands are destroyed. Then storage of the plants, modifies mycorrhizal metabolism. In this study, the stress encountered by ectomycorrhizae during storage has been investigated. Modification of polypeptides profiles as well as respiration rate have been recorded, and related to mycorrhizal fungi populations evolution during the first year after plantation. Different types of ectomycorrhizae responded differently to transplantation stress, leading to the conclusion that some ectomycorrhizal fungi could be selected according to this criteria for controlled inoculation in nursery.