Jean Garbaye

Interactions between mycorrhizal fungi and other soil organisms

Mycorrhizal fungi interact with a wide range of other soil organisms, in the root, in the rhizosphere and in the bulk soil. These interactions may be inhibitory or stimulatory; some are clearly competitive, others may be mutualistic. Effects can be seen at all stages of the mycorrhizal fungal life-cycle, from spore population dynamics (predation, dispersal and germination) through root colonization to external hyphal growth. Two areas that seem likely to be of particular importance to the functioning of the symbiosis are the role of bacteria in promoting mycorrhiza formation and of soil animals in grazing the external mycelium. Mycorrhizal fungi also modify the interactions of plants with other soil organisms, both pathogens, such as root-inhabiting nematodes and fungi, and mutualists, notably nitrogen-fixing bacteria. These interactions are probably important both in natural ecosystems, where pathogens are increasingly recognized as playing controlling roles, and in agricultural systems, where mycorrhizas may be valuable in designing integrated systems of pest control and growth stimulation.

Year-round monitoring of diversity and potential metabolic activity of the ectomycorrhizal community in a beech (Fagus silvatica) forest subjected to two thinning regimes.

This work was aimed at understanding how the functional diversity of ectomycorrhizas (ECM) is driven by environmental factors and how it adapts to the structure of the forest stand. Superficial fine roots were sampled 21 times during an entire year in two adjacent plots (no thinning and strong thinning) of a mature beech (Fagus silvatica) forest. Individual ectomycorrhizal root tips were morphologically characterised and the symbiotic fungi were molecularly identified. ECM were also tested for dehydrogenase and acid phosphatase activities, and soil moisture and temperature were recorded. The results provide a description of ECM community dynamics over a whole year in the two stands. The main conclusions are threefold: (1) the species structure of the ECM community and metabolic activity of each morphotype change depending on the season, temperature and soil moisture, and a number of morphotypes are more abundant and active in winter than in summer, (2) the silviculture treatment (strong thinning) modifies the ectomycorrhizal community structure, and (3) the overall function of the ECM community results from the individual time pattern and specialisation of each morphotype.

Temporal Changes in the Ectomycorrhizal Community in Two Soil Horizons of a Temperate Oak Forest

ABSTRACT The species structure of an ectomycorrhizal (ECM) community was assessed monthly for 15 months in the two horizons (A1 and A2) of an oak temperate forest in northeastern France. Ectomycorrhizal species were identified each month by internal transcribed spacer sequencing. Seventy-five fungal symbionts were identified. The community was dominated by Tomentellaceae, Russulaceae, Cortinariaceae, and Boletales. Four species are abundant in the study site: Lactarius quietus, Tomentella sublilacina, Cenococcum geophilum, and Russula sp1. The relative abundance of each species varied depending on the soil horizon and over time. Some species, such as L. quietus, were present in the A1 and A2 horizons. C. geophilum was located particularly in the A2 horizon, whereas T. sublilacina was more abundant in A1. Some species, such as Clavulina sp., were detected in winter, while T. sublilacina and L. quietus were present all year long. Our results support the hypothesis that a rapid turnover of species composition of the ECM community occurs over the course of a month. The spatial and temporal unequal distribution of ECM species could be explained by their ecological preferences, driven by such factors as root longevity, competition for resources, and resistance to environmental variability.

Phylogenetic analysis, genomic organization, and expression analysis of multi‐copper oxidases in the ectomycorrhizal basidiomycete Laccaria bicolor

In forest soils, ectomycorrhizal and saprotrophic Agaricales differ in their strategies for carbon acquisition, but share common gene families encoding multi-copper oxidases (MCOs). These enzymes are involved in the oxidation of a variety of soil organic compounds. The MCO gene family of the ectomycorrhizal fungus Laccaria bicolor is composed of 11 genes divided into two distinct subfamilies corresponding to laccases (lcc) sensu stricto (lcc1 to lcc9), sharing a high sequence homology with the coprophilic Coprinopsis cinerea laccase genes, and to ferroxidases (lcc10 and lcc11) that are not present in C. cinerea. The fet3-like ferroxidase genes lcc10 and lcc11 in L. bicolor are each arranged in a mirrored tandem orientation with an ftr gene coding for an iron permease. Unlike C. cinerea, L. bicolor has no sid1/sidA gene for siderophore biosynthesis. Transcript profiling using whole-genome expression arrays and quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) revealed that some transcripts were very abundant in ectomycorrhizas (lcc3 and lcc8), in fruiting bodies (lcc7) or in the free-living mycelium grown on agar medium (lcc9 and lcc10), suggesting a specific function of these MCOs. The amino acid composition of the MCO substrate binding sites suggests that L. bicolor MCOs interact with substrates different from those of saprotrophic fungi.

Functional diversity in an Amazonian rainforest of French Guyana: a dual isotope approach (δ15N and δ13C)

Abstract Functional aspects of biodiversity were investigated in a lowland tropical rainforest in French Guyana (5°2′N, annual precipitation 2200 mm). We assessed leaf δ15N as a presumptive indicator of symbiotic N2 fixation, and leaf and wood cellulose δ13C as an indicator of leaf intrinsic water-use efficiency (CO2 assimilation rate/leaf conductance for water vapour) in dominant trees of 21 species selected for their representativeness in the forest cover, their ecological strategy (pioneers or late successional stage species, shade tolerance) or their potential ability for N2 fixation. Similar measurements were made in trees of native species growing in a nearby plantation after severe perturbation (clear cutting, mechanical soil disturbance). Bulk soil δ15N was spatially quite uniform in the forest (range 3–5‰), whereas average leaf δ15N ranged from −0.3‰ to 3.5‰ in the different species. Three species only, Diplotropis purpurea, Recordoxylon speciosum (Fabaceae), and Sclerolobium melinonii (Caesalpiniaceae), had root bacterial nodules, which was also associated with leaf N concentrations higher than 20 mg g−1. Although nodulated trees displayed significantly lower leaf δ15N values than non-nodulated trees, leaf δ15N did not prove a straightforward indicator of symbiotic fixation, since there was a clear overlap of δ15N values for nodulated and non-nodulated species at the lower end of the δ15N range. Perturbation did not markedly affect the difference δ15Nsoil − δ15Nleaf, and thus the isotopic data provide no evidence of an alteration in the different N acquisition patterns. Extremely large interspecific differences in sunlit leaf δ13C were observed in the forest (average values from −31.4 to −26.7‰), corresponding to intrinsic water-use efficiencies (ratio CO2 assimilation rate/leaf conductance for water vapour) varying over a threefold range. Wood cellulose δ13C was positively related to total leaf δ13C, the former values being 2–3‰ higher than the latter ones. Leaf δ13C was not related to leaf δ15N at either intraspecific or interspecific levels. δ13C of sunlit leaves was highest in shade hemitolerant emergent species and was lower in heliophilic, but also in shade-tolerant species. For a given species, leaf δ13C did not differ between the pristine forest and the disturbed plantation conditions. Our results are not in accord with the concept of existence of functional types of species characterized by common suites of traits underlying niche differentiation; rather, they support the hypothesis that each trait leads to a separate grouping of species.

Growth response of eucalypts in the Congo to ectomycorrhizal inoculation

Abstract Seedlings of hybrid eucalypts ( Eucalyptus urophylla × E. kirtoniana ) were grown in the nursery in polythene bags filled with a formalin-fumigated sandy soil and inoculated with mycelial cultures of five ectomycorrhizal fungi: Pisolithus tinctorius, Scleroderma aurantium, S. texense, S. dictyosporum and Hebeloma cylindrosporum . The plants were then outplanted in a nutrient-poor acidic sandy savanna soil, to compare with control plants grown in the same conditions, but not inoculated and infected by a native type of ectomycorrhiza. Three of the introduced fungi formed mycorrhizas and stimulated the growth of the trees; the most efficient ( P. tinctorius ) increased volume production by 30% at 50 months. The fact that introduced mycorrhizas had disappeared by that time indicates that inoculation with fungal strains better adapted to the site could tremendously increase eucalypt production in the Cargo.

Leaf natural 15N abundance and total N concentration as potential indicators of plant N nutrition in legumes and pioneer species in a rain forest of French Guiana

Abstract The suitability of the natural 15N abundance and of total N concentration of leaves as indicators of the type of plant N nutrition in a rain forest of French Guiana were tested. Leaf samples from primary legume species, non-legumes (pioneer species) and from the non-N2-fixing species Dicorynia guianensis were analyzed. Both δ15N and total leaf N varied widely (−1 ?δ15N (‰) ? 7 and 1 ? leaf N(%) ? 3.2) suggesting possible distinctions between diazotrophic and non-fixing plants. The δ15N also revealed two statistically distinct groups of non-N2-fixing species (δ15N = 5.14 ± 0.3 vs δ15N = 1.65 ± 0.17) related to the different ecological behaviors of these species in the successional processes. We conclude that the δ15N signature of plant leaves combined with their total N concentration may be relevant indicators for identifying functional groups within the community of non-N2-fixing species, as well as for detecting diazotrophy. Despite the variability in the δ15N of the non-N2-fixing species, N2-fixing groups can still be identified, provided that plants are simultaneously classified taxonomically, by their leaf δ15N and total N concentration and by the presence or absence of nodules. The variability in the δ15N of the non-fixing species is discussed.

Survival in the soil of the ectomycorrhizal fungus Laccaria bicolor and the effects of a mycorrhiza helper Pseudomonas fluorescens

In disinfected forest nursery soils, inoculating Douglas fir (Pseudotsuga menziesii) seedlings with the ectomycorrhizal fungal strain Laccaria bicolor S238N significantly increases tree growth after outplantating. However, the success of the inoculation depends on survival of the fungal inoculum in the soil during the pre-symbiotic life of the fungus. We followed the survival of L. bicolor S238N in autoclaved nursery soil in the glasshouse, and under gnotobiotic conditions in autoclaved or γ-irradiated nursery soil. We also studied the effect of the mycorrhiza helper bacterium Pseudomonas fluorescens BBc6R8, which promotes the Douglas fir-L. bicolor S238N symbiosis, on fungal viability. In the glasshouse, fungal viability was assessed by trapping with Douglas fir seedlings. We showed that the fungus retained its viability in a pre-symbiotic state in the soil at least for 23 weeks, which is much longer than that reported in the literature for other ectomycorrhizal fungi. The bacterium did not significantly modify the survival of the fungus. In the gnotobiotic experiments, ergosterol, a specific fungal membrane component, was used to quantify fungal biomass. Fungal behaviour differed with the disinfection technique used, which modified the chemical characteristics of the initial soil. There was no fungal growth in the autoclaved soil but there was a rapid increase of fungal biomass in the irradiated soil. The effect of the bacterium on fungal biomass also varied with a significant stimulation in the autoclaved soil vs. a significant inhibition in the irradiated soil. Our results show that the beneficial effect of the bacterium on the fungus depends on the condition of the fungus, i.e. the greatest benefit occurs when the fungus is growing under unfavourable conditions.

Respiration activity of ectomycorrhizas from Cenococcum geophilum and Lactarius sp. in relation to soil water potential in five beech forests

Forest trees are involved in root symbioses with hundreds of species of ectomycorrhizal fungi which constitute functional guilds able to improve the water and mineral nutrition of host trees. In temperate ecosystems, water shortage is a main factor limiting tree vitality. To assess how soil water conditions affected the physiological state of beech (Fagus silvatica L.) ectomycorrhizal roots, we monitored glucose respiration of two ectomycorrhizal types (Lactarius sp. and Cenococcum geophilum) during two complete growing seasons. Five stands of contrasting soil conditions were chosen in north-eastern France. The top soil horizons were equipped with micropsychrometers for measuring water potential and temperature. Glucose respiration on individual ectomycorrhizas was measured in vitro by trapping [14C]-CO2 from radiolabelled glucose. For soil water potential <-0.2 MPa, the potential respiration activity of C. geophilumectomycorrhizas was significantly less altered than that of Lactariussp. ectomycorrhizas, indicating that C. geophilumis more likely than Lactariussp. to maintain the physiological integrity of beech roots facing drought stress.

Release of complexing organic acids by rhizosphere fungi as a factor in Norway spruce yellowing in acidic soils

A field experiment in the Vosges (a low mountain range in north-eastern France) had shown that a CaMg amendment of an acidic forest soil reduced the total number of fungal propagules per gramme dry soil, while the number of species considerably increased. Some rhizosphere fungi isolated from the amended and/or control plots were grown in three media with ammonium or nitrate as the sole source of nitrogen, and with or without aluminium. Complexing organic acids released by the fungi and likely to interfere with tree nutrition were analyzed in the culture medium after fourteen days incubation. The largest amounts of complexing organic acids are released by fungi isolated from the acid control plot. These rhizospheric fungi take part in soil acidification, which results in the long-term decrease of soil mineral reserves in a forest soil subjected to permanent leaching. Therefore, we conclude from this work that fungi might contribute to the deficient mineral nutrition of the Norway spruce on acidic soils, by impoverishment of the root environment in essential cations. However, among the causes of the Norway spruce yellowing, rhizosphere deleterious fungi act as secondary agents or contributing factors rather than primary causes of decline.