Mycorrhizas can produce negative crop responses when phosphorus availability is sufficient in agricultural soils because the fungi are of no benefit in nutrient acquisition yet continue to colonize roots and invoke parasitic costs. Benomyl fungicide was used to test this prediction in the field by limiting mycorrhizal colonization of 2-yr-old Valencia orange trees (Citrus sinensis (L.) Osbeck) on four rootstocks of varying mycorrhizal dependency in P-deficient soil fertilized with and without phosphate. No known fungal pathogens of citrus roots controlled by benomyl were present on the trees or in the field soil. Young trees with or without P fertilization and benomyl treatment remained sufficient in P ([ges ]0·10% leaf P) throughout the 27-month study. Root zone drenches of benomyl reduced mycorrhizal colonization and leaf P status of Valencia orange trees on the three slower-growing rootstocks, trifoliate orange (Poncirus trifoliata (L. ) Raf.), Swingle citrumelo (Citrus paradisi Macf.×P. trifoliata) and sour orange (Citrus aurantium L.), for the duration of three growing seasons. Benomyl affected root colonization and P status of trees on the faster-growing rootstock, Volkamer lemon (Citrus volkameriana Tan. and Pasq.), less than for trees on the slower-growing rootstocks and the effects were sustained for only two seasons. The shorter duration of benomyl effect for trees on Volkamer lemon rootstock compared with the slower-growing rootstocks was explained by the loss of inhibition of mycorrhizal activity when roots grew out of the drench zone and mycorrhizas were no longer in direct contact with the fungicide. Benomyl treatment increased growth rate of Valencia orange on the slow-growing rootstocks from 5 to 17% after three seasons, and from 2 to 9% on Volkamer lemon rootstock after two seasons compared with the non-benomyl treated trees. The benomyl effect was attributed to reduction of costs of root colonization over time, and consequently, a greater availability of carbon assimilate for shoot growth of trees. Since mycorrhizal fungi are ubiquitous in fertilized agricultural soils and obligate biotrophs on the roots of most crop species, these results indicate a need to further investigate whether negative growth responses of P-sufficient plants in the field occur because mycorrhizal fungi are no longer behaving as mutualists.

Seeds of the orchid species, Spiranthes sinensis (Pers.) Ames, were sterilized and germinated in vitro with the symbiotic fungus Ceratobasidium cornigerum (Bourdot) Rogers. Colonized embryos developed into protocorms and these were examined for changes in microtubule arrays, after initial invasion of fungal hyphae into embryos and during peloton formation and degradation. Methods utilized to detect microtubules included immunofluorescence combined with laser scanning confocal microscopy, conventional transmission electron microscopy combined with morphometric analysis, and immunogold labelling. Microtubules were regularly found in close association with intracellular hyphae and degraded hyphal masses. Cortical microtubules disappear during peloton formation but reappear in cells that show fungal lysis. With conventional transmission electron microscopy and immunogold labelling the microtubules associated with fungal hyphae and degenerated hyphal masses were located close to the perifungal membrane that separates fungal hyphae from protocorm cytoplasm.

The relationship between precipitation chemistry and the concentrations of nitrogen ([N]) and phosphorus ([P]) in the cushion-forming lichen Cladonia portentosa (Dufour) Coem. (=C. impexa (Harm)) was investigated. Samples of C. portentosa were collected from heathlands and upland moorlands close to 31 rural sampling stations in the UK Acid Deposition Monitoring Network, which provides data on wet deposition and NO2 concentrations in air. The [N] and [P] were measured in the top 5 mm of lichen thalli (thallus apices) and also in a horizontal stratum between 40–50 mm from the apices (thallus base). The [N] (per unit dry mass) was 0·08–1·82% and [P] was 0·04–0·17%, depending on collection site and lichen fraction analysed. Concentrations of both elements were c. 2–5 times greater in the apices than in the basal strata, and [N] and [P] values in each stratum were strongly positively correlated. Lichen [N] was positively correlated with N deposition: this relationship was stronger when using [N] values for thallus bases than for the apices. By contrast, thallus [N] was poorly correlated with [N] values in precipitation. When [NO2] in air was included together with NO3− deposition in a linear regression model explaining thallus base N, the model fit was significantly improved, whereas modelled values of NH3 deposition rate for the heathland sites did not correlate with lichen [N]. It is suggested that the proportionately greater enrichment of [N] in the thallus base might reflect a perturbation of internal recycling of thallus N at polluted sites. Thallus [P] was generally weakly linked to wet N deposition but positively correlated with [NO2] in air. It is not known whether the trend for increasing thallus [P] values indicates decreasing lichen growth rate and reduced growth dilution of P in polluted areas, or is due to regional variation in P deposition rate.

Wetting of the upper leaf surface of Juglans regia L. and of model surfaces colonized by epiphytic micro-organisms was investigated by measuring contact angles of aqueous solutions buffered at different pH values. During June to October 1995, contact angles of aqueous solutions on the leaf surface of J. regia decreased by angles ranging from 12° (low pH values) to 25° at high pH values. At the end of this vegetation period, wetting was strongly dependent on pH showing significantly lower contact angles with alkaline solutions (pH 9·0) than with acidic solutions (pH 3·0). Contact angle titration measured angles on the leaf surface as a function of the pH of buffered aqueous solutions, covering a pH range from 3·0 to 11·0. Titration curves revealed inflection points around 7·5, indicating the existence of ionizable carboxylic groups at the interface of the phylloplane. Altered leaf-surface wetting properties observed on the intact leaf surface could be simulated in model experiments by measuring contact angles on artificial surfaces colonized by Pseudomonas fluorescens and by epiphytic micro-organisms isolated from the phylloplane of J. regia. Strong evidence is provided that interfacial carboxylic groups derive from epiphytic micro-organisms present on the phylloplane. Results suggest that the age-dependent increase in, and pH dependence of, wetting as leaves mature are related to the presence of epiphytic micro-organisms on the phylloplane. Ecological consequences of increased leaf-surface wetting, concerning the structure of the leaf surface as a microhabitat for epiphytic micro-organisms, are discussed.

Mycelial cord systems, up to 50-cm diameter, of the basidiomycete Phanerochaete velutina (DC.: Pers.) Parmasto, a common woodland saprotroph, grown on non-sterile soil in model laboratory microcosms were baited, after 27 d, with pairs of fresh beech wood blocks (baits), placed at 10 d intervals behind the foraging colony margin. System development was quantified by image analysis. Mean radial extent and hyphal cover increased linearly with time until day 21, but declined before the mycelial systems reached the edges of the laboratory microcosms. The mass (DBM) and border (DBS) fractal dimensions of the mycelial systems changed with time but the ratio DBM[ratio ]DBS became constant after 14 d. A separate central compartment containing the inoculum was supplied with 32P orthophosphate and its translocation to wood baits monitored non-destructively for 73 d. Whilst total 32P acquisition by wood baits increased linearly with time, the proportion of total allocated to baits varied significantly both temporally and according to the length of time that baits had been in contact with the mycelium. Most recently supplied wood baits were not the main sink for supplied phosphorus; rather, the rate of 32P acquisition was initially greatest in baits from which egress of the fungus had already occurred. The rate of 32P acquisition by the most recently added baits increased with time, supported by efflux from other wood baits, which had initially been the main sinks for translocated phosphorus. The results raise important questions about the ecological and functional significance of nutrient partitioning in cord systems and imply that ‘observed’ translocation, rather than being an absolute measure, indicates the degree to which phosphorus is loaded from a translocation stream in regions where it is being actively utilized and/or stored.

Symploca PCC 8002 Kützing is a filamentous cyanobacterium that lacks the specialized cells, known as heterocysts, that protect nitrogenase from O2 in most aerobic N2-fixing cyanobacteria. Nevertheless, Symploca is able to carry out N2 fixation in the light under aerobic conditions. When cultures were grown under light/dark cycles, nitrogenase activity commenced and increased in the light phase and declined towards zero in the dark. Immunolocalization of dinitrogenase reductase in sectioned Symploca trichomes showed that the enzyme was present only in 9% of the cells. These cells lacked any obvious mechanical protection against atmospheric O2 and their ultrastructural characteristics were similar to those of cells that did not contain any dinitrogenase reductase. The nitrogenase-containing cells possessed carboxysomes that were rich in ribulose-1,5-bisphosphate carboxylase/oxygenase and phycoerythrin, a light harvesting pigment of PS II. This indicates that these cells had a capacity for both N2 fixation and photosynthesis. The significance of the localization pattern for dinitrogenase reductase is discussed in the context of N2 fixation in Symploca PCC 8002.

The purpose of this work was to establish how the distribution of local curvatures changed during the mushroom stem gravitropic reaction and to suggest a suitable mathematical model based on these new data.

The gravitropic bending of base- and apex-pinned Coprinus cinereus (Fries) S. F. Gray stems was recorded on videotapes. The images were captured from the tapes after each 10 min, rotated by 45° and transformed into tables of changing co-ordinates of points for each stem. The non-linear regression of these points was performed using Legendre polynomials. From the resulting equations the patterns of changing local curvature for 50 subsections per stem during 400 min of gravitropic reaction were calculated.

It was observed that base-pinned stems first bent from the apex, but later the curvature of this part decreased, and in the late stages the apex became nearly completely straight again. Subsections, located about one third of stem length from the base determined the main part of the final curvature. The free basal part of the apex-pinned stems bent upward and after a certain bending time also began to straighten. However, this process started significantly later and was weaker. Bending of the subsections close to the pinned apex did not stop when they reached the vertical position, and the final angle of gravitropic curvature could exceed 180°.

Plotting various functions of local bending speed and its derivatives against each other and against local angle indicated that, if the hypothetical signal about reorientation arises in the apex, its propagation towards the base did not follow simple wave or simple diffusion laws. The importance of the local angle of all subsections both for signal origin and transmission was established and a signal transmission equation, involving local angle of each subsection, was derived. After creating a suitable program this partial differential equation was solved numerically. The generated shapes of the bending stem coincided in high degree with experimentally observed images.

Norway spruce (Picea abies (L.) Karst.) seedlings were inoculated with the ectomycorrhizal fungus Laccaria bicolor ((Marie) Orton), strain S238 N, in axenic conditions. The presence of the fungus slowed tap–root elongation by 26% during the first 15 d after inoculation and then stimulated it by 136%. In addition, it multiplied in vitro lateral root formation by 4.3, the epicotyl growth of the seedlings by 8.4 and the number of needles by 2. These effects were maintained when the fungus was separated from the roots by a cellophane membrane preventing symbiosis establishment, thus suggesting that the fungus acted by non-nutritional effects. We tested the hypothesis that IAA produced by L. bicolor S238 N would be responsible for the stimulation of fungal induced rhizogenesis. We showed in previous work that L. bicolor S238 N can synthesize IAA in pure culture. Exogenous IAA supplies (100 and 500 μm) reproduced the stimulating effect of the fungus on root branching but inhibited root elongation. The presence of 2,3,5-triiodobenzoic acid (TIBA) in the culture medium significantly depressed lateral root formation of inoculated seedlings. As TIBA had no significant effect on IAA released in the medium by L. bicolor S238 N, but counteracted the stimulation of lateral rhizogenesis induced by an exogenous supply of IAA, we suggest that TIBA inhibited the transport of fungal IAA in the root. Furthermore TIBA blocked the colonization of the main root cortex by L. bicolor S238 N and the formation of the Hartig net. These results specified the role of fungal IAA in the stimulation of lateral rhizogenesis and in ectomycorrhizal symbiosis establishment.

Eucalyptus coccifera Hook., a plant capable of forming both arbuscular mycorrhizas and ectomycorrhizas, was used to compare the effects of the two mycorrhizal types on phosphorus uptake and C allocation. Seedlings were grown in a P-deficient soil/sand mixture inoculated with peat/vermiculite spawn of Laccaria bicolor (Maire) Orton or Thelephora terrestris (Ehrh.) Fr.; or with 250-μm sievings from leek colonized by Glomus caledonium (Nicol. & Gerd.) Trappe & Gerde., Glomus sp. type E3 or Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or with autoclaved spawn (non-mycorrhizal control). Before the 89-d harvest, a subset of the harvested plants was labelled with 14C (45–60-min pulse, 202-h chase). Growth promotion and the increase in seedling P content was largest in the two ectomycorrhizal treatments. Production of fluorescein diacetate-stained external hyphae was three to seven times higher by ectomycorrhizal (ECM) fungi compared with arbuscular mycorrhizal (AM) fungi and was highly correlated with P uptake and shoot weight. Phosphorus inflow rates of ECM and AM seedlings were 3·8 times, and 2·0–2·7 times those of non-mycorrhizal seedlings. Phosphorus acquisition efficiencies were similar (11·2 and 10·0 μmol P mmol−1C for T. terrestris and Glomus E3 plants, respectively) for the two mycorrhizal types, and appeared to be greater than in uninoculated plants (7.2 μmol P mmol−1C) grown at the same P level.

The contribution of the extramatrical mycelium to N and P nutrition of mycorrhizal Norway spruce (Picea abies (L.) Karst.) was investigated. Seedlings either inoculated with Paxillus involutus (Batsch) Fr. or non-mycorrhizal were grown in a two compartment sand culture system where hyphae were separated from roots by a 45 μm nylon net. Nutrient solution of the hyphal compartment contained either 1.8 mm NH4+ and 0.18 mm H2PO4− or no N and P. Aluminium added to the hyphal compartment as a tracer of mass flow was not detected in the plant compartment, indicating that measurements of N and P transfer by the mycelium were not biased by solute movement across the nylon net.

The addition of N and P to the hyphal compartment markedly increased dry weight, N and P concentration and N and P content of mycorrhizal plants. Calculating uptake from the difference in input and output of nutrient in solution confirmed a hyphal contribution of 73% and 76% to total N and P uptake, respectively. Hyphal growth was increased at the site of nutrient solution input.

Dark septate root endophytes (DSE) are conidial or sterile fungi (Deuteromycotina, Fungi Imperfecti) likely to be ascomycetous and colonizing plant roots. They have been reported for nearly 600 plant species representing about 320 genera and 100 families. DSE fungi occur from the tropics to arctic and alpine habitats and comprise a heterogeneous group that functionally and ecologically overlaps with soil fungi, saprotrophic rhizoplane-inhabiting fungi, obligately and facultatively pathogenic fungi and mycorrhizal fungi. Numerous species of undescribed sterile and anamorphic taxa may also await discovery. Although DSE are abundant in washed root and soil samples from various habitats, and are easily isolated from surface-sterilized roots of ecto-, ectendo-, endo- and non-mycorrhizal host species, their ecological functions are little understood. Studies of DSE thus far have yielded inconsistent results and only poorly illustrate the role of DSE in their natural habitats. These inconsistencies are largely due to the uncertain taxonomic affinities of the strains of DSE used. In addition, because different strains of a single anamorph taxon seem to vary greatly in function, no clear generalizations on their ecological role have been drawn. This paper reviews the current literature on DSE and the ecology and discusses the need for and direction of future research.

Relatively little is known about the factors controlling ectomycorrhizal fungal communities. One possible factor is forest litter chemistry. In a series of experiments we demonstrated that the growth of ectomycorrhizal fungi able to colonize red pine (Pinus resinosa Ait.) are differentially affected by red pine needles and needle chemical components. For example, water extracts of pine needles stimulated the growth of Suillus intermedius (Smith & Thiers) Smith & Thiers and inhibited the growth of Amanita rubescens Pers. Catechin and epicatechin gallate, components of the water extract, acted similarly to the extract. The volatile compounds α- and β-pinene also had differential effects on the growth of the various species of ectomycorrhizal fungi. Our results suggest that forest litter chemistry has the potential differentially to affect the growth of ectomycorrhizal fungal species and so could affect the structure of ectomycorrhizal fungal communities.

To analyse the effect of arbuscular mycorrhizal (AM) colonization on tomato gene expression, two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) patterns of crude extracts, soluble and membrane proteins of tomato roots, either mycorrhizal and the AM fungus Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or non-mycorrhizal, have been compared. In the three fractions analysed, AM colonization induced up-regulation with down-regulation of the synthesis of polypeptides already present in tomato roots and induction of some new polypeptides. Separation of root extracts into soluble and membrane fractions allowed us to identify two soluble, and five membrane-bound, newly induced polypeptides in AM roots. Comparison of the protein patterns of AM roots with those of the external mycelium of G. mosseae showed that one of the newly induced polypeptides might correspond to a fungal polypeptide. By using this experimental approach, we have been able to detect 44 polypeptides that are differentially displayed in tomato roots as a consequence of the establishment of the AM symbiosis.

The pathology and distribution of European beech trees bearing elongated bark lesions (strip-cankers) were investigated. Two types of canker were recognized: those on small trees (<40 cm diameter at breast height (dbh): 1·4 m above ground level) which bore fruit bodies of the xylariaceous ascomycete Biscogniauxia nummularia (Bull.) O. Kuntze, and those on larger specimens (>40 cm dbh) which were consistently associated with the diatrypaceous ascomycete Eutypa spinosa (Pers.) Tul. & C. Tul. All cankers were strictly annual, having formed during single growing seasons following periods of low water availability. The regional and local distribution of trees bearing lesions also appeared to be correlated with environmental conditions, being most severe where low rainfall or high temperatures had occurred. Population studies of the associated fungi, generally considered as saprotrophs, indicated the presence of unique genotypes within individual cankered trees and provided no evidence for the existence of pathotypes within either species. Within the decay columns which underlay canker surfaces, both B. nummularia and E. spinosa formed longitudinally extensive genets, implying non-mycelial spread in colonization. Suppression of both inter- and intraspecific incompatibility between fungi occurred in regions of canker decay columns with elevated water contents. The possible significance of coexistence between the ascomycetes B. nummularia and ‘Hypoxylon purpureum’ (sensu Sharland & Rayner, 1989b) and of the formation of heterokaryons by E. spinosa is discussed.

The involvement of the cytoskeleton in symbiotic interactions such as arbuscular mycorrhizas has received little attention. In this paper, we examine the organization of actin in tobacco mycorrhizal roots and compare actin and tubulin patterns within arbuscule-containing cells.

Our results show drastic reorganization of microfilaments and microtubules upon fungal infection and how those new cytoskeletal patterns relate to the host cytoplasm rearrangement and the intracellular fungal structures. Whereas in uninfected cells a network of cortical and perinuclear actin filaments was observed, in infected cells actin filaments closely follow the fungal branches and envelop the whole arbuscule in a dense coating network. Microtubules are less closely connected with the fungus surface. They run across the whole arbuscule mass, linking branches to each other and to the host cell cortex and nucleus.

These major differences between the two cytoskeletal components are used to advance some suggestions concerning their contribution to structural functions in the plant–fungus interactions during the mycorrhizal symbiosis.

To clarify the role of the fungal nitrate assimilation pathway in nitrate reduction by mycorrhizal plants, nitrate reductase (NR)-deficient (NR−) mutants of the ectomycorrhizal basidiomycete Hebeloma cylindrosporum Romagnesi have been selected. These mutants were produced by u.v. mutagenesis on protoplasts originating from homokaryotic mycelia belonging to complementary mating types of this heterothallic tetrapolar species. Chlorate-resistant mutants were first selected in the presence of different nitrogen (N) sources in the culture medium. Among 1495 chlorate resistant mycelia, 30 failed to grow on nitrate and lacked a detectable NR activity. Growth tests on different N sources suggested that the NR activity of all the different mutants is specifically impaired as a result of mutations in either the gene coding for NR apoprotein or genes controlling the synthesis of the molybdenum cofactor. Furthermore, restoration of NR activity in some of the dikaryons obtained after crosses between the different mutant mycelia suggested that not all the selected mutations mapped in the same gene. Utilization of N on a NH415NO3 medium was studied for two mutant strains and their corresponding wild-type homokaryons. None of the mutants could use nitrate whereas 15N enrichment values indicated that 13–27% of N present in 13-d-old wild-type mycelia originated from nitrate. Apparently, the mutant mycelia do not compensate their inability to use nitrate by a more efficient use of ammonium. These different NR mutants still form mycorrhizas with the habitual host plant, Pinus pinaster (Ait.), making them suitable for study of the contribution of the fungal nitrate assimilation pathway to nitrate assimilation by mycorrhizal plants.

One hundred and sixty-eight sterile endophytic mycelia were isolated from roots of four Woollsia pungens (Cav.) F. Muell. (Epacridaceae) plants collected from a field site in New South Wales, Australia. All isolates formed typical ericoid mycorrhizal structures when inoculated onto roots of Vaccinium macrocarpon Ait. (Ericaceae). Microsatellite-primed PCR fingerprints generated using the primers (GTG)5 and (GACA)4 indicated that considerable genetic diversity exists within the endophyte population. It was estimated that a minimum of 43 genetically distinct mycelial genets were present in the root systems of the sampled W. pungens population, with most genets confined to individual plants. Two genets, however, were present within the root systems of two adjacent plants. While most genets were represented by less than eight isolates, three genets contained up to 41 isolates, suggesting that root system colonization by some endophytic mycelia might be extensive.

Beech nuts (Fagus sylvatica L.) were germinated and grown in soil inoculated with the ectomycorrhizal fungus Laccaria laccata or Paxillus involutus for 18–20 wk. The success of mycorrhizal infection was monitored by measuring the ergosterol contents of the mycorrhizas. Ergosterol levels ranged from 122±23 μg g−1 d. wt (Laccaria mycorrhizas) to 94±36 μg g−1 d. wt (Paxillus mycorrhizas), indicating that ectomycorrhizal symbiosis was established. In root incubation chambers, rates of sulphate uptake and the xylem loading of sulphate of excised mycorrhizas were investigated. Both types of mycorrhizas showed saturation kinetics in external sulphate concentrations from 2·5–1000 μmol l−1. Linearization of these kinetics revealed two phases with low apparent Km (Laccaria mycorrhizas: 15±3 μmol l−1; Paxillus mycorrhizas: 13±3 μmol l−1) and Vmax (Laccaria mycorrhizas: 19±3 nmol h−1 g−1 f. wt; Paxillus mycorrhizas: 25±4 nmol h−1 g−1 f. wt) at low external sulphate concentrations and significantly higher kinetic constants at higher sulphate supplies. Relative xylem loading, i.e. the portion of sulphate loaded into the xylem that was taken up, remained constant over the entire concentration range investigated (c. 4–7% of the sulphate taken up). If trees were supplied for 72 h with different N and sulphur concentrations, both uptake of sulphate and relative xylem loading were unaffected by sulphur availability, but modulated by N supply. Nitrogen depletion diminished the rates of sulphate uptake in Laccaria and Paxillus mycorrhizas. In response to higher N availability combined with sulphur depletion, sulphate uptake of Laccaria mycorrhizas, but not of Paxillus mycorrhizas, increased. Organic compounds considered to be possible signals for the regulation of sulphate uptake were fed to excised mycorrhizas. l-Cysteine but not l-methionine and glutathione (γ-Glu-Cys-Gly) inhibited sulphate uptake of the two mycorrhizas and xylem loading of sulphate was stimulated rather than inhibited by l-Cys in both types. In Paxillus mycorrhizas glutathione had a similar effect. O-Acetyl-l-serine (OAS), a precursor of l-cysteine, stimulated sulphate uptake, but did not affect xylem loading. Apparently, OAS, generated in N metabolism, and l-cysteine, a product of assimilatory sulphate reduction, act as antagonists, together mediating regulation of sulphate uptake.

Contrary to the long-held belief that, internal to the cortical sterome, the central region of Sphagnum stems comprises unspecialized parenchyma, the present light- and electron-microscope study has revealed that these cells in fact have a highly specialized cytoplasmic organization. Their key features are: (a) the absence of large central vacuoles; (b) a spindle-shaped nucleus positioned internally; (c) a prominent axial system of endoplasmic microtubules associated with the nucleus, mitochondria, pleomorphic vacuoles, and membrane-bounded tubules and vesicles; (d) a distinct cytoplasmic polarization, with the cellular region near the capitulum being richer in organelles than the basal region; and (e) a high frequency of plasmodesmata in the cross walls with an enlarged median region containing no discernible desmotubule. Such a distinctive combination of cytological features has been hitherto only described for putative food-conducting cells in bryoid mosses. The results introduce a major new character common to Sphagnum and bryoid mosses and strongly suggest that this cytological organization underlines cellular specialization in symplasmic transport.

Mitochondrial and nuclear genes have different inheritance, thus studies of fungal populations should use both mitochondrial and nuclear markers. Using nuclear markers, the S238N strain of the ectomycorrhizal basidiomycete Laccaria bicolor ((Maire) Orton) has been previously shown to persist for at least 10 yr after outplanting in a plantation of Douglas fir (Pseudotsuga menziesii (Mir.) Franco) inoculated with this strain. In the present study, we have sampled 539 sporophores of Laccaria spp. from this plantation, some of which had the S238N nuclear genotype, to study mitochondrial DNA polymorphism and persistence of the inoculated S238N mitochondrial genome. Length polymorphism in fragments of the large subunit of mitochondrial ribosomal DNA (LrDNA) allowed distinction of the haplotypes present in the plantation at the species level. In addition, heteroduplex analysis and sequencing revealed intraspecific polymorphism of LrDNA among the L. bicolor sporophores and enabled specific identification of S238N LrDNA. This haplotype was only retained in sporophores carrying the S238N nuclear genome, confirming the survival of this introduced strain in a natural population.