Fine-scale distribution of pine ectomycorrhizas and their extramatrical mycelium

In order to clarify the functional role of individual ectomycorrhizal (EcM) fungal species in the field, we need to relate their abundance and distribution as mycorrhizas to their abundance and distribution as extramatrical mycelium (EMM). We divided each of four 20 cm x 20 cm x 2 cm slices of pine forest soil into 100 cubes of 2 cm x 2 cm. For each cube, ectomycorrhizas were identified and the presence of EMM of the EcM fungi recorded as ectomycorrhizas was determined by terminal restriction fragment length polymorphism (T-RFLP) analysis of ITS rDNA. Ectomycorrhizas and EMM of seven EcM species were mapped. Spatial segregation of mycorrhizas and EMM was evident and some species produced their EMM in different soil layers from their mycorrhizas. The spatial relationship between mycorrhizas and their EMM generally conformed to their reported exploration types, but EMM of smooth types (e.g. Lactarius rufus) was more frequent than expected. Different EcM fungi foraged at different spatial scales.     

Methods for inoculating field crops with mycorrhizal fungi

Four methods were compared for inoculating red clover with selected mycorrhizal fungi when sown in a field containing an indigenous mycorrhizal population. The largest amount of mycorrhizal infection (around 65% of root length infected) was obtained by placing inoculum with the seeds in furrows. The inoculum used was standard soil inoculum from stock plant cultures spread by hand or the same inoculum concentrated to about one seventh by wet-sieving, and then fluid-drilled. The effectiveness of multiseeded pellets (seeds stuck onto pellets of soil inoculum) applied broadcast was more variable, infection ranging widely around an average of 30%. Applying both soil inoculum and seeds broadcast produced just under 10% infection, similar to that in the controls given autoclaved inoculum. Seedling establishment, in contrast, was-better where seeds were applied broadcast than in furrows. It seemed therefore that multiseeded pellets might be the best compromise for achieving reasonable infection in most plants, but fluid drilling had the advantages of greatly reducing the amount of inoculum needed and of readily combining seeds and inoculum in a single carrier.     

Below-ground interactions with arbuscular mycorrhizal shrubs decrease the performance of pinyon pine and the abundance of its ectomycorrhizas

Few studies have examined how below-ground interactions among plants affect the abundance and community composition of symbiotic mycorrhizal fungi. Here, we combined observations during drought with a removal experiment to examine the effects of below-ground interactions with arbuscular mycorrhizal (AM) shrubs on the growth of pinyon pines (Pinus edulis), and the abundance and community composition of their ectomycorrhizal (EM) fungi. Shrub density was negatively correlated with pinyon above- and below-ground growth and explained 75% of the variation in EM colonization. Consistent with competitive release, pinyon fine-root biomass, shoot length and needle length increased with shrub removal. EM colonization also doubled following shrub removal. EM communities did not respond to shrub removal, perhaps because of their strikingly low diversity. These results suggest that below-ground competition with AM shrubs negatively impacted both pinyons and EM fungi. Similar competitive effects may be observed in other ecosystems given that drought frequency and severity are predicted to increase for many land interiors.     

The effect of ectomycorrhizas on the phosphorus and dry weight acquisition of Eucalyptus seedlings

The inoculation ofEucalyptus pilularis seedlings withPisolithus tinctorius and the subsequent development of ectomycorrhizas, led to an improved acquisition of phosphorus (P) from three different sources of P. These sources included insoluble phytate. Dry weight gain of seedlings was increased by inoculation in those treatments where growth was limited by P supply.     

Interactions between water-stress and different mycorrhizal inocula on plant growth and mycorrhizal development in maize and sorghum

Maize (Zea mays) and sorghum (Sorghum bicolor) were inoculated with a range of VAM fungi and grown under water-stressed and unstressed conditions. There was considerable variation amongst the inocula in their effects on plant growth. Inoculation with Glomus clarum produced the biggest plants in each host, with Glomus monosporum and Acaulospora sp. giving the least growth overall. Root infection produced by the different inocula also varied, but levels were not correlated with effects on plant growth. Water-stress reduced plant growth, with the effects not being altered by mycorrhizal infection. VAM infection levels were not affected by water-stress. Spore production from most inocula was reduced by water-stress, both in total spore numbers and in terms of spores per gram plant weight. Sporulation of G. clarum, G. epigeum and G. monosporum were affected less by stress than were the other inocula. Spore production was in general greater on sorghum than on maize, but the host effect varied amongst the inocula.     

Hemichannels: permeants and their effect on development, physiology and death

Hemichannels, which are one half of the gap junction channels, have independent physiological roles. Although hemichannels consisting of connexins are more widely documented, hemichannels of pannexins, proteins homologous to invertebrate gap junction proteins also have been studied. There are at least 21 different connexin and three pannexin isotypes. This variety in isotypes results in tissue-specific hemichannels, which have been implicated in varied events ranging from development, cell survival, to cell death. Hemichannel function varies with its spatio-temporal opening, thus demanding a refined degree of regulation. This review discusses the activity of hemichannels and the molecules released in different physiological states and their impact on tissue functioning.     

培养容器容积对AM真菌生长发育的影响

研究宿主植物栽培容器对丛枝菌根(Arbuscularmycorrhizae,AM)真菌Glomusmosseae生长发育的影响。结果表明:小容积容器的根系密度相对较大,在菌根共生体建立初期,菌根真菌繁殖体与根接触的机会增大,对于菌根真菌的迅速侵染及共生体的迅速建立非常有利,同时还增大了根外菌丝二次侵染的机会,从而使菌根真菌生长发育形成了一个良性循环,最终有利于根外孢子的形成。容器对共生体的影响决不是简单的盆的体积问题,而与其面积和体积之比有关,也和种植密度有密切关系。     

Mycorrhizas and mycorrhizal fungal communities throughout ecosystem development

Background and scope

Plant communities and underlying soils undergo substantial, coordinated shifts throughout ecosystem development. However, shifts in the composition and function of mycorrhizal fungi remain poorly understood, despite their role as a major interface between plants and soil. We synthesise evidence for shifts among mycorrhizal types (i.e., ectomycorrhizas, arbuscular and ericoid mycorrhizas) and in fungal communities within mycorrhizal types along long-term chronosequences that include retrogressive stages. These systems represent strong, predictable patterns of increasing, then declining soil fertility during ecosystem development, and are associated with coordinated changes in plant and fungal functional traits and ecological processes.

Conclusions

Mycorrhizal types do not demonstrate consistent shifts through ecosystem development. Rather, most mycorrhizal types can dominate at any stage of ecosystem development, driven by biogeography (i.e., availability of mycorrhizal host species), plant community assembly, climate and other factors. In contrast to coordinated shifts in soil fertility, plant traits and ecological processes throughout ecosystem development, shifts in fungal communities within and among mycorrhizal types are weak or idiosyncratic. The consequences of these changes in mycorrhizal communities and their function for plant–soil feedbacks or control over long-term nutrient depletion remain poorly understood, but could be resolved through empirical analyses of long-term soil chronosequences.     

The effect of carbohydrate on the development of a Cattleya hybrid in association with its mycorrhizal fungus

To investigate beneficial effects of mycorrhizal fungi to advanced leafy orchids, growth studies on the development of symbiotic seedlings of the orchid Cattleya (aclandiae x schoeffeldiana) x aclandiae were conducted in vitro over a period of 18 months using split plates with minerals and carbohydrates on one side and water agar on the other. Mycorrhizal infection and shoot and root growth of seedlings on the nutrient side were compared to growth on the water agar side with nutrient uptake by the orchid only possible via external mycorrhizal hyphae. Seed germination was followed by mycorrhizal infection and rapid development of protocorms on both nutrient and non-nutrient sides of the plates. With 0.5% starch, development of protocorms was sustained for a least 12 weeks, compared to only 6 weeks with 0.1% starch. Advanced protocorms with two small leaves and a smoll root were transferred at week 22 to new fungal plates. When harvested at week 43, plantlets on 0.5% starch (both nutrient and water agar sides) had 2.7 times the dry weight of plantlets on 0.1% starch. Shoot-root ratios were higher on the lower level of carbon. In all plantlets, mycorrhizal infection involved less than 5% of the root length. With zero, 0.1% or 0.5% starch, the roots were re-infected on transfer to fresh fungal plates but young roots that developed following the transfer stayed free of infection, Plantlets on 0.5% starch (nutrient and water agar side) after 18 months had longer roots than plantlets grown in the absence of starch or on 0.1% starch. Shoots were small but significantly larger on the nutrient side than on the water agar side, independent of the carbohydrate level. The shoot-root ratio was highest on the nutrient side with no starch present. In this latter case, plantlet development was steady but plantlets on the non-nutrient side developed slowly; thus there was little evidence of nutrient translocation by the mycorrhizal fungus from the nutrient to the non-nutrient side in the absence of carbohydrates. Mycorrhizal infection is discussed as a mechanism for heterotrophic carbon assimilation. In advanced leafy orchids of Cattleya, external carbon resulted in increased root growth, decreased shoot/root ratio and sometimes yellowish-green plantlets.     

The mycorrhizal dependence of subordinates determines the effect of arbuscular mycorrhizal fungi on plant diversity

It is widely recognized that arbuscular mycorrhizal fungi (AMF) influence plant community structure. Several studies in the past few years have focused on the effect of AMF on plant diversity, showing contradictory results. On the basis of studies carried out in grasslands, it has been proposed that these results may not be in conflict but rather depend on mycorrhizal dependence of dominant and subordinate species in the plant community. Here, we propose a simple conceptual model to explain the effect of AMF on plant diversity and dominance hierarchy. Our model suggests that the effect of AMF on plant diversity can be explained mainly by mycorrhizal dependence of subordinates. It also gives predictions for a wider range of community types, and helps in identifying the systems in which empirical data are most critically needed.     

A quick and precise technique for identifying ectomycorrhizas by PCR

A rapid procedure was developed to amplify ITS fragments directly from Tuber ectomycorrhizas either synthesized in a greenhouse or collected from the field. The addition of Bovine Serum Albumin (BSA) to the reaction mixtures overcame the presence of reaction inhibitors present in fungal and root cells, and enabled the amplification of the ITS regions directly from ectomycorrhizal tissues. This method is cheaper and less time-consuming than conventional procedures, and reduces the time required from 1-4 h to a few minutes. It is also much more sensitive, allowing the identification of just a small fragment of a mycorrhizal root tip. Because of this it is possible to select only the target fungal tissue and hence minimise the risk of contamination by saprobic or other mycorrhizal species. The method also avoids the use of toxic or hazardous substances. This method could have a wider application in other areas of applied mycology.     

Diversity of culturable bacterial populations associated to Tuber borchii ectomycorrhizas and their activity on T. borchii mycelial growth

Isolation and physiological and molecular characterisation of culturable bacterial strains belonging to actinomycetes, pseudomonads and aerobic spore-forming bacteria were carried out on mycorrhizal root tips of Quercus robur var. peduncolata infected by Tuber borchii. Cellular density of the three bacterial groups in ectomycorrhizal root tips was estimated to be 1.3+/-0.11 x 10(6) cfu g(-1) dry weight for total heterotrophic bacteria and 1.08+/-0.6 x 10(5) (mean+/-S.E.), 1.3+/-0.3 x 10(5) and 1.4+/-0.2 x 10(5) cfu g(-1) dry weight for pseudomonads, actinomycetes and spore-forming bacteria respectively. Identification of pseudomonads by the Biolog system indicated, besides the most represented species Pseudomonas fluorescens (biotypes B, F and G), the occurrence of strains belonging to Pseudomonas corrugata. Amplified ribosomal DNA restriction analysis of actinomycetes and spore formers revealed at least three and six different groups of patterns, respectively. Many bacterial isolates were able to induce variations in growth rates of T. borchii mycelium; among these, 101 strains showed antifungal activity, whereas 17 isolates, belonging to spore formers, were able to increase mycelial growth up to 78% when compared to uninoculated mycelial growth. The potential role of these populations in the development and establishment of mycorrhizas is discussed.     

Interactions between arbuscular mycorrhizal fungi and bacteria and their potential for stimulating plant growth

Arbuscular mycorrhizal (AM) fungi and bacteria can interact synergistically to stimulate plant growth through a range of mechanisms that include improved nutrient acquisition and inhibition of fungal plant pathogens. These interactions may be of crucial importance within sustainable, low-input agricultural cropping systems that rely on biological processes rather than agrochemicals to maintain soil fertility and plant health. Although there are many studies concerning interactions between AM fungi and bacteria, the underlying mechanisms behind these associations are in general not very well understood, and their functional properties still require further experimental confirmation. Future mycorrhizal research should therefore strive towards an improved understanding of the functional mechanisms behind such microbial interactions, so that optimized combinations of microorganisms can be applied as effective inoculants within sustainable crop production systems. In this context, the present article seeks to review and discuss the current knowledge concerning interactions between AM fungi and plant growth-promoting rhizobacteria, the physical interactions between AM fungi and bacteria, enhancement of phosphorus and nitrogen bioavailability through such interactions, and finally the associations between AM fungi and their bacterial endosymbionts. Overall, this review summarizes what is known to date within the present field, and attempts to identify promising lines of future research.     

菌根真菌对土壤呼吸以及凋落物分解的影响

土壤呼吸是植物固定的碳由陆地生态系统进入大气的主要途径之一; 凋落物分解是养分循环的重要环节。陆地植物的90%以上可同菌根真菌形成共生关系, 菌根真菌对于植物获取环境中的养分具有重要的作用。然而, 其对土壤呼吸和凋落物分解的影响却经常在生态系统对环境变化的响应研究中被忽视。本文系统地综述了国内外相关研究进展, 对菌根真菌如何影响土壤呼吸和凋落物分解这两个过程及这种影响如何受到环境变化的制约做了全面的分析, 并对以往研究中存在的问题以及未来的研究方向提出了展望。