Torgny Unestam

Extramatrical structures of hydrophobic and hydrophilic ectomycorrhizal fungi

The extramatrical mycelia of Suillus bovinus, Rhizopogon luteolus and R. vinicolor, all examples of hydrophobic (ho), mat-forming mycorrhizal fungi, were examined while associated with their hosts in the unsterilized rhizoscope, and efforts were made to produce and examine similar structures in vitro. Comparisons were made with four hydrophilic (hi) mycorrhizal fungi, Thelephora terrestris, Cenococcum geophilum, Laccaria laccata and Hebeloma crustuliniforme. The ho fungi formed linear structures (coarse, rhizomorph-like cords, with vessels in the center) and fans, both in the rhizoscope and in vitro. The same was seen in mycorrhizal mycelia in forest soils. These cords did not themselves give rise to the fans peripherally, and were not proper rhizomorphs, but were created continuously from single exploring air hyphae in the preexisting fan. Thus the ho exploring hyphae aggregated into strands, which grew in thickness only when no suitable, exploitable substrate was found. The assembly of hyphae creating ho cords was seen in the air as well as on inert hydrophilic (glass) or hydrophobic (plastic) surfaces, but never in water. It is hypothesized that the ho cell wall surface glues hyphae together while cords are formed. Water disturbed strands and mantles already formed. The ho exploring hyphae could also create ho mycelial patches (as in a mat) at the water-air interface of a number of substrates. The periphery of these patches seemed to be composed of shorter exploiting hyphae penetrating different water-soaked substrates. Exploring, aerial hyphal tips of the ho fungi were shown to “excrete” water droplets from openings in the ho cell wall surface, both in vitro and in the rhizoscope. In the rhizoscope, droplet excretion was apparently directly governed by photosynthesis in the shoot of the seedling. It is proposed that the drop exudation represents a kidney-like function of the extramatrical hyphae and a bridge to drier soil particles to initiate nutrient uptake by the hyphae. The ecological function of the different extramatrical structures of ho fungi are discussed. The ho cords or hyphae may translocate water only in the vessels or symplastically and not in the cell walls. The ho property may be essential among the S-selected (stress-tolerant) factors in these forest fungi. The transfer from water-repelling exploring structures into more hi exploiting structures in water contact with surrounding soil debris is, therefore, of great importance. The hi fungi did not form rhizomorph-like strands, in most cases, but an extending hyphal mycelium, representing foraging, exploring and exploiting structures at the same time. In the field, short strands may be found. On the hi fungi droplets were also produced but readily fused into a water sheath around the hypha. The hyphae thus tended to wick water via the cell wall.

Exudation-reabsorption in a mycorrhizal fungus, the dynamic interface for interaction with soil and soil microorganisms

Abstract The mycelium of Suillus bovinus slowly absorbed [U-14C]glucose and other tracers from droplets placed on the cords, translocated them to the peripheral hyphae and exuded them into fluid drops on the hyphal tips. The exudate was characterized by 1H NMR spectroscopy and by sugar and amino acid analysis. The exuded compounds were mainly carbohydrates and peptides. Acetic acid and oxalic acid were also present in the exudate along with a number of unidentified compounds. Released ions (K, Na, Cl, P, Mg and Ca) were identified by X-ray microanalysis. The mycelium was shown to reabsorb up to 65% of the exuded 14C compounds in 2 days. Glucose, mannitol, glutamic acid (pH 3.2), and Rb+ (as well as other mineral ions) were all readily absorbed by the mycelium, while oxalic acid at pH 4.2 and glutamic acid at pH 6.5 were not. Exudation of fluid droplets on the surface of the hydrophobic mycorrhizal fungus S. bovinus may represent an ecophysiologically important function of the extramatrical hyphae, which provides an interface for interaction with the immediate hyphal environment and its other microorganisms where the peripheral hyphae exchange their photosynthetically derived products for nutrients to be used later by the pine host. We hypothesize that actively absorbed carbohydrates from the root are translocated to the peripheral hyphae along a concentration gradient of sugars and polyols by means of active translocation and diffusion in cell elements and by acropetal water transport in the cord vessels.

Water repellency, mat formation, and leaf-stimulated growth of some ectomycorrhizal fungi

SummaryEctomycorrhizal short roots, mycelia, rhizomorphs and mats from conifer soil were examined in relation to their hydrophobic properties. In some cases connected fruit bodies were included in the study. Mycorrhizal soils gathered from the forest and/or colonized in a laboratory rhizoscope were studied, as were mycelia in pure culture. Most forest-derived species were hydrophobic. The drought-resistant Cenococcum geophilum and the more ruderal and moisture-dependent Thelephora terrestris were both strongly hydrophilic. The hydrophobic mycelium seemed solely responsible for the water repellence properties, and adjacent soil and plant debris remained unaffected and hydrophilic. In hydrophobic fungi, mat formation was induced in the rhizoscope by hyphal contact with alder litter leaves. This stimulating effect was not found when the leaves were covered by water or when fresh, green alder leaves were used. Thelephora terrestris did not form such mats in vitro and spread sparsely in air pockets as well as in the adjacent water film. The possibility is discussed that many mycorrhizal fungi in the forest may partly control their soil environment via aeration created by their hydrophobia.

Involvement of Cylindrocarpon destructans in root death of Pinus sylvestris seedlings: Pathogenic behaviour and predisposing factors

The common rhizospheric fungus Cylindrocarpon destructans was investigated in relation to its role in root death of Pinus sylvestris in Nordic nurseries and plantations. Laboratory methods were developed for studying similar root problems as well as the early effects of phytotoxicity and fungal infection. Seedlings grown under standardized optimal conditions were exposed to controlled stress (known to occur in nurseries), with or without C. destructans in the rhizosphere. Low light conditions, anaerobic root environment, and fungicide treatment were each found to predispose pine seedlings to invasion by the pathogen. The pathogen was very sensitive to competition as well as antagonism on the root, and fungicide‐induced inhibition of antagonists such as Trichoderma spp. also increased the severity of attack by the pathogen. To compete successfully, the pathogen would have to invade and dominate weakened roots prior to the arrival of saprophytes. Toxic metabolites produced by the pathogen weakened or killed...

A method for observing and manipulating roots and root associated fungi on plants growing in nonsterile substrates

Standard 14‐cm plastic‐petri dishes were used as root substrate chambers allowing controlled inoculation, regular inspection under the dissecting microscope, and manipulation of root infections with pathogens and mycorrhizal fungi under nonsterile conditions. Roots of young pine seedlings were placed on the root substrate and, especially those growing on the substrate surface under the lid, were very easy to manipulate. Mycorrhizal species and inoculation methods were compared and the infection process followed continuously under various conditions. The air pockets formed between the lid and the substrate favoured mycorrhiza formation. Brief, periodic flooding prevented formation of mycorrhizas in two mycorrhizal species having hydrophobic hyphae but did not prevent it in less hydrophobic species. IAA treatment of roots led to heavy production of mycorrhizas. Fruiting body formation in Lacearia laccata was induced and followed in the dish cultures. Pure mycorrhizal stock cultures were able to be revitaliz...

Comparative studies of ectomycorrhiza formation in Alnus glutinosa and Pinus resinosa with Paxillus involutus

Abstract Mycorrhiza ontogeny and details of Hartig net and mantle structure were compared in ectomycorrhizas synthesized in growth pouches between the broad host range fungus Paxillus involutus and the tree species European black alder (Alnus glutinosa) and red pine (Pinus resinosa). In Alnus glutinosa, a paraepidermal Hartig net was restricted to the proximal (basal) portion of first-order laterals; the hypodermal layer appeared to be a barrier to fungal penetration. Phi-thickenings were present in some cortical cells but these were not related to lack of fungal ingress into the cortex. The mantle was often present close to the root apex but in many roots it was loosely organized and patchy. In several instances, the mantle formed around the root apex was only temporary; renewed root growth occurred without the formation of a mantle. In Pinus resinosa, the Hartig net developed between cortical cell layers of monopodial and dichotomously branched first–order laterals. Fungal hyphae in the Hartig net exhibited a complex labyrinthine mode of growth. The mantle had a pseudoparenchymatous structure and covered the root, including apices of dichotomously branched roots. The Paxillus–Pinus resinosa interaction had all the characteristics of a compatible ectomycorrhizal association. The Paxillus–Alnus glutinosa interaction, however, showed only aspects of superficial ectomycorrhizas, including the presence of a minimal (sometimes absent) and mostly proximal Hartig net and variable mantle development. Sclerotia were produced in the extraradical mycelium of Paxillus involutus when associated with either Alnus glutinosa or Pinus resinosa.

Gremmeniella abietina in Norway Spruce, Latent Infection, Sudden Outbreaks, Acid Rain, Predisposition

The pathogen Gremmeniella abietina often grows endophytically in twigs without causing symptoms. Stressing conditions may predispose the tree to infection and/or break the latent infection. Inoculation of spruce plants, kept in a climate chamber, resulted in typical disease symptoms. Experimental treatment with acid rain increased leakage of ions, enhanced the germination of conidia and changed the phylloplane flora. Detergents can be used to simulate the effects of acid rain treatment.