Positive association between mycorrhiza and foliar endophytes in Poa bonariensis, a native grass

The interaction between mycorrhiza and leaf endophytes (Neotyphodium sp.) was studied in three Poa bonariensis populations, a native grass, differing significantly in endophyte infection. The association between endophytes and mycorrhizal fungi colonisation was assessed by analysing plant roots collected from the field. We found that roots from endophyte-infected populations showed a significantly higher frequency of colonisation by mycorrhizal fungi and that soil parameters were not related to endophyte infection or mycorrhiza colonization. In addition, we did not observe significant differences in the number of AM propagules in soils of the three populations sites. We also report the simultaneous development of Paris-type and Arum-type mycorrhiza morphology within the same root systems of P. bonariensis. The co-occurrence of both colonisation types in one and the same root system found in the three populations, which differed in Neotyphodium infection, suggests that foliar endophytes do not determine AM morphology. The percentage of root length colonised by different types of fungal structures (coils, arbuscules, longitudinal hyphae and vesicles) showed significant and positive differences in arbuscular frequency associated with endophyte infection, whereas the much smaller amounts of vesicles and hyphal coils did not differ significantly.     

Ploidy-specific interactions of three host plants with arbuscular mycorrhizal fungi: Does genome copy number matter?

? Premise of the study: Polyploidy has been shown to affect different plant traits and modulate interactions between plants and other organisms, such as pollinators and herbivores. However, no information is available on whether it can also shape the functioning of mycorrhizal symbiosis. ? Methods: The mycorrhizal growth response was assessed for three angiosperms with intraspecific ploidy variation. Different cytotypes of Aster amellus, Campanula gentilis, and Pimpinella saxifraga were either left uninoculated or were inoculated with arbuscular mycorrhizal (AM) fungi in a pot experiment. After 3 mo of cultivation in a greenhouse, plant growth, phosphorus concentration in the shoot biomass, and development of the AM symbiosis were evaluated. ? Key results: No significant ploidy-specific differences in AM development were recorded. The inoculation led to consistently greater phosphorus uptake; however, the effect on plant growth differed considerably among plant species, populations, ploidy levels, and AM species. A salient ploidy-specific response was observed in A. amellus. Whereas diploid plants benefited from AM inoculation, the hexaploids consistently showed negative or no-growth responses (depending on the AM species). In contrast to A. amellus, no interactions between inoculation and ploidy were observed in C. gentilis and P. saxifraga. ? Conclusions: The first evidence is provided of a ploidy-specific response of a mycotrophic plant to AM fungi. Our results demonstrate the complexity of interaction between plants and associated AM fungi, with the ploidy level of the host plant being one component that may modulate the functioning of the symbiosis.     

Increasing flavonoid concentrations in root exudates enhance associations between arbuscular mycorrhizal fungi and an invasive plant

Many invasive plants have enhanced mutualistic arbuscular mycorrhizal (AM) fungal associations, however, mechanisms underlying differences in AM fungal associations between introduced and native populations of invasive plants have not been explored. Here we test the hypothesis that variation in root exudate chemicals in invasive populations affects AM fungal colonization and then impacts plant performance. We examined flavonoids (quercetin and quercitrin) in root exudates of native and introduced populations of the invasive plant Triadica sebifera and tested their effects on AM fungi and plant performance. We found that plants from introduced populations had higher concentrations of quercetin in root exudates, greater AM fungal colonization and higher biomass. Applying root exudates more strongly increased AM fungal colonization of target plants and AM fungal spore germination when exudate donors were from introduced populations. The role of root exudate chemicals was further confirmed by decreased AM fungal colonization when activated charcoal was added into soil. Moreover, addition of quercetin into soil increased AM fungal colonization, indicating quercetin might be a key chemical signal stimulating AM fungal associations. Together these results suggest genetic differences in root exudate flavonoids play an important role in enhancing AM fungal associations and invasive plants’ performance, thus considering root exudate chemicals is critical to unveiling mechanisms governing shifting plant-soil microbe interactions during plant invasions.Subject terms: Population dynamics, Community ecology, Plant ecology     

亚热带典型树种根毛特征及其与共生真菌的关系

根毛和共生真菌增加了吸收面积,提高了植物获取磷等土壤资源的能力。由于野外原位观测根表微观结构较为困难,吸收细根、根毛、共生真菌如何相互作用并适应土壤资源供应,缺乏相应的数据和理论。该研究以受磷限制的亚热带森林为对象,选取了21种典型树种,定量了根毛存在情况、属性变异,分析了根毛形态特征与共生真菌侵染率、吸收细根功能属性之间的关系,探讨了根表结构对低磷土壤的响应和适应格局。结果表明:1)在亚热带森林根毛不是普遍存在的, 21个树种中仅发现7个树种存有根毛, 4个为丛枝菌根(AM)树种, 3个为外生菌根(ECM)树种。其中,马尾松(Pinus massoniana)根毛出现率最高,为86%;2)菌根类型是理解根-根毛-共生真菌关系的关键,AM树种根毛密度与共生真菌侵染率正相关,但ECM树种根毛直径与共生真菌侵染率负相关; 3) AM树种根毛长度和根毛直径、ECM树种根毛出现率与土壤有效磷含量呈负相关关系。该研究揭示了不同菌根类型树种根毛-共生真菌-根属性的格局及相互作用,为精细理解养分获取策略奠定了基础。     

The foliar endophytic fungal community composition in Cirsium arvense is affected by mycorrhizal colonization and soil nutrient content

Foliar fungal endophytes are ubiquitous, but understudied symbionts of most plant species; relatively little is known about the factors affecting their occurrence, diversity and abundance. We tested the effects of soil nutrient content and arbuscular mycorrhizal (AM) colonization on the occurrence of foliar endophytic fungi in Cirsium arvense in two field studies. In the first study, we assessed relationships between soil moisture, organic matter, carbon and nitrogen content and plant water, nitrogen and carbon content and AM colonization and the occurrence of foliar endophytic fungal species. In the second study, we manipulated soil nutrient content and AM colonization of potted seedlings and identified differences in endophytic fungal species composition of the leaves and stems. The results reveal that endophytes can occur either more or less frequently, depending on soil nutrient and plant water content and AM colonization. We propose that these patterns were the result of differences in fungal growth responses to nutrient availability in the leaves, which can be affected by resources obtained from the soil or symbiotic fungi in the roots.     

Availability and function of arbuscular mycorrhizal and ectomycorrhizal fungi during revegetation of dewatered reservoirs left after dam removal

Revegetation following dam removal projects may depend on recovery of arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungal communities, which perform valuable ecosystem functions. This study assessed the availability and function of AM and EM fungi for plants colonizing dewatered reservoirs following a dam removal project on the Elwha River, Olympic Peninsula, Washington, United States. Availability was assessed via AM fungal spore density in soils and EM root tip colonization of Salix sitchensis (Sitka willow) in an observational field study. The effect of mycorrhizal fungi from 4 sources (reservoir soils, commercial inoculum, and 2 mature plant community soils) on growth and nutrient status of S. sitchensis was quantified in a greenhouse study. AM fungal spores and EM root tips were present in all field samples. In the greenhouse, plants receiving reservoir soil inoculum had only incipient mantle formation, while plants receiving inoculum from mature plant communities had fully formed EM root tips. EM formation corresponded with alleviation of phosphorus stress in plants (lower shoot nitrogen:phosphorus). Thus, revegetating plants have access to AM and EM fungi following dam removal, and EM formation may be especially important for plant P uptake in reservoir soils. However, availability of mycorrhizal fungi declines with distance from established plant communities. Furthermore, EM fungal communities in recently dewatered reservoirs may not be as effective at forming beneficial mycorrhizae as those from mature plant communities. Whole soil inoculum from mature plant communities may be important for the success of revegetating plants and recovery of mycorrhizal fungal communities.     

Mycorrhizal Symbiosis and Local Adaptation in Aster amellus: A Field Transplant Experiment

Many plant populations have adapted to local soil conditions. However, the role of arbuscular mycorrhizal fungi is often overlooked in this context. Only a few studies have used reciprocal transplant experiments to study the relationships between soil conditions, mycorrhizal colonisation and plant growth. Furthermore, most of the studies were conducted under controlled greenhouse conditions. However, long-term field experiments can provide more realistic insights into this issue. We conducted a five-year field reciprocal transplant experiment to study the relationships between soil conditions, arbuscular mycorrhizal fungi and plant growth in the obligate mycotrophic herb Aster amellus. We conducted this study in two regions in the Czech Republic that differ significantly in their soil nutrient content, namely Czech Karst (region K) and Ceske Stredohori (region S). Plants that originated from region S had significantly higher mycorrhizal colonisation than plants from region K, indicating that the percentage of mycorrhizal colonisation has a genetic basis. We found no evidence of local adaptation in Aster amellus. Instead, plants from region S outperformed the plants from region K in both target regions. Similarly, plants from region S showed more mycorrhizal colonisation in all cases, which was likely driven by the lower nutrient content in the soil from that region. Thus, plant aboveground biomass and mycorrhizal colonisation exhibited corresponding differences between the two target regions and regions of origin. Higher mycorrhizal colonisation in the plants from region with lower soil nutrient content (region S) in both target regions indicates that mycorrhizal colonisation is an adaptive trait. However, lower aboveground biomass in the plants with lower mycorrhizal colonisation suggests that the plants from region K are in fact maladapted by their low inherent mycorrhizal colonization. We conclude that including mycorrhizal symbiosis in local adaptation studies may increase our understanding of the mechanisms by which plants adapt to their environment.     

Plant interspecific differences in arbuscular mycorrhizal colonization as a result of soil carbon addition

Soil nutrient availability and colonization by arbuscular mycorrhizal fungi are important and potentially interacting factors shaping vegetation composition and succession. We investigated the effect of carbon (C) addition, aimed at reducing soil nutrient availability, on arbuscular mycorrhizal colonization. Seedlings of 27 plant species with different sets of life-history traits (functional group affiliation, life history strategy and nitrophilic status) were grown in pots filled with soil from a nutrient-rich set-aside field and amended with different amounts of C. Mycorrhizal colonization was progressively reduced along the gradient of increasing C addition in 17 out of 27 species, but not in the remaining species. Grasses had lower colonization levels than forbs and legumes and the decline in AM fungal colonization was more pronounced in legumes than in other forbs and grasses. Mycorrhizal colonization did not differ between annual and perennial species, but decreased more rapidly along the gradient of increasing C addition in plants with high Ellenberg N values than in plants with low Ellenberg N values. Soil C addition not only limits plant growth through a reduction in available nutrients, but also reduces mycorrhizal colonization of plant roots. The effect of C addition on mycorrhizal colonization varies among plant functional groups, with legumes experiencing an overproportional reduction in AM fungal colonization along the gradient of increasing C addition. We therefore propose that for a better understanding of vegetation succession on set-aside fields one may consider the interrelationship between plant growth, soil nutrient availability and mycorrhizal colonization of plant roots.     

Investment in Fine Roots and Arbuscular Mycorrhizal Fungi Decrease During Succession in Three Brazilian Ecosystems

The functional groups of plants that characterize different phases of succession are expected to show differences in root distribution, fine‐root traits and degrees of association with arbuscular mycorrhizal (AM) fungi. The relationship involving fine‐root traits and AM fungi that regulate the nutrient acquisition potential among different plant functional groups are still not well understood. We assessed fine‐root morphology, AM fungal variables and soil fertility in grassland, secondary forest and mature forest in Atlantic, Araucaria and Pantanal ecosystems in Brazil. Soil cores were collected at 0–10 and 10–20 cm depths. Fine roots were extracted from soil by sieving and root morphological traits and AM colonization were determined. The AM spores were extracted from soil and counted. In all ecosystems, soil fertility, fine‐root mass and root diameter increased with the succession, while root length, specific root length, root‐hair length, root‐hair incidence, AM colonization and AM spore density decreased. These results suggest that plant species from early stages of tropical succession with inherent rapid growth invest in fine roots and maintain a high degree of AM colonization in order to increase the capacity for nutrient acquisition. Conversely, fine root morphological characteristics and low degree of AM colonization exhibited by plants of the later stages of succession lead toward a low nutrient uptake capacity that combine with their typical low growth rates. Abstract in Portuguese is available at http://www.blackwell‐synergy.com/loi/btp .     

Differences in above- and below-ground responses to ozone between two populations of a perennial grass

Our study examined the influence of elevated ozone levels on the growth and mycorrhizal colonization of two populations of Elymus glaucus L. (blue wildrye). We hypothesized that ozone would reduce carbon availability to the plants, particularly below ground, and would affect mycorrhizal colonization. Because of the wide geographic range of E. glaucus, two populations of plants were selected from areas of contrasting ozone histories to examine intraspecies variation in response to ozone. Two populations of E. glaucus (southern California versus northern California) exposed in a factorial experiment involving ozone, mycorrhizal inoculation with Glomus intraradices Schenck and Smith, and plant source population. Ozone had a subtle effect on leaf area and number of tillers but did not affect overall root:shoot ratio in either population. The impact of ozone on above-ground growth characteristics was most pronounced in the southern population that came from a high-ozone environment, while below-ground responses such as reduced arbuscular colonization was most pronounced in the northern population which originated in a low-ozone environment. Further analysis of soil characteristics from the northern population of plants revealed a significant reduction in active soil bacterial biomass and an increase in total fungi per gram dry weight soil, suggesting a possible role for ozone in altering soil processes. Whether or not population differences in response to ozone were due to genetic shifts resulting from prior ozone remains to be determined. However, these subtle but important differences in population response to ozone above- and below-ground have significant implications in any attempt to generalize plant response, even within a species. Future research efforts need to include better characterization of intraspecific variation in response to ozone as well as possible adaptive strategies that may result from chronic ozone exposure.     

How do Mycorrhizas Affect C and N Relationships in Flooded Aster tripolium Plants?

The mycorrhizal associations established between plants and fungi have multiple effects on plant growth, directly affecting stress tolerance. This work aimed to explore arbuscular mycorrhizal (AM) effects on carbon and nitrogen relationships of Aster tripolium L. and consequently on its flooding tolerance. Mycorrhizal and non-mycorrhizal juvenile plants were submitted to non-flooding and tidal flooding conditions for 56 d. Tidal flooding reduced biomass, but the presence of mycorrhiza had an ameliorating effect. The AM symbioses seem to have, like flooding, a stressful effect on A. tripolium at an early stage of plant development. However, once the plant was established, an improvement of growth performance of plants with mycorrhiza under flooding conditions was observed. The better tolerance of AM plants to flooding was mediated through an improvement of the osmotic adjustment of the plant tissues (higher concentrations of soluble sugars and proline) and through the increment of nitrogen acquisition in tidal-flooded plants.     

An arbuscular mycorrhizal inoculum enhances root proliferation in, but not nitrogen capture from, nutrient-rich patches in soil

Most work on root proliferation to a localized nutrient supply has ignored the possible role of mycorrhizal fungi, despite their key role in nutrient acquisition. Interactions between roots of Plantago lanceolata , an added arbuscular mycorrhiza (AM) inoculum and nitrogen capture from an organic patch ( Lolium perenne shoot material) dual-labelled with ¹⁵N and ¹³C were investigated, to determine whether root proliferation and nitrogen (N) capture was affected by the presence of AM fungi. Decomposition of the organic patch in the presence and absence of roots peaked in all treatments at day 3, as shown by the amounts of ¹³CO₂ detected in the soil atmosphere. Plant N concentrations were higher in the treatments with added inoculum 10 d after patch addition, but thereafter did not differ among treatments. Plant phosphorus concentrations at the end of the experiment were depressed by the addition of the organic residue in the absence of mycorrhizal inoculum. Although uninoculated plants were also colonized by mycorrhizal fungi, colonization was enhanced at all times by the added inoculum. Addition of the AM inoculum increased root production, observed in situ by the use of minirhizotron tubes, most pronouncedly within the organic patch zone. Patch N capture by the end of the experiment was c . 7.5% and was not significantly different as a result of adding an AM inoculum. Furthermore, no ¹³C enrichments were detected in the plant material in any of the treatments showing that intact organic compounds were not taken up. Thus, although the added AM fungal inoculum benefited P. lanceolata seedlings in terms of P concentrations of tissues it did not increase total N capture or affect the form in which N was captured by P. lanceolata roots.     

Mycorrhizal colonization of Palafoxia feayi (Asteraceae) in a pyrogenic ecosystem

Although a number of factors have predictable effects on mycorrhizal colonization, determining generalized patterns for some variables have remained elusive. In particular, fire has been identified as a major event that may influence plant–mycorrhiza interactions, yet efforts to date have yielded contradictory results. Here, we assess the impact of fire on mycorrhizal colonization in Palafoxia feayi, a plant commonly found in the fireswept, nutrient-poor scrub community of central Florida. We determined soil nutrient conditions and percent colonization patterns for plants growing in replicate plots that were burned 1 to 15 years previously. The results showed a negative relationship between mycorrhizal colonization and time since fire, but there was no effect of fire return interval (lapsed time between successive fires). Soil nutrient analyses corroborated previous studies and showed no change in soil nutrients following fire. In contrast to previous studies of mycorrhizal colonization in Florida scrub, we conclude that fire can affect arbuscular mycorrhizal fungi colonization and we speculate that this is mediated by light availability.     

Arbuscular mycorrhizas in cycads of southern India

Root and soil samples of three potted or ground-grown cycads ( Cycas circinalis, C. revoluta, Zamiasp.) were collected between November 1999 and June 2000 and surveyed for arbuscular mycorrhizal (AM) colonization and spore populations. AM fungi were associated with all root systems and rhizosphere samples examined. Root colonization was of a typical Arum type and AM colonization levels differed significantly between species and between potted and ground-grown cycads. Mycorrhizal colonization levels were inversely related to root hair number and length. Spores of nine morphotypes belonging to three genera ( Acaulospora, Glomus, Scutellospora) were extracted from soil. The percentage root length colonized by AM fungi was not related to soil factors, but total AM fungal spore numbers in the rhizosphere soil were inversely related to soil nitrogen and phosphorus levels. AM fungal spore numbers in the soil were linearly related to root length colonized. The co-occurrence of septate non-mycorrhizal fungi was recorded for the first time in cycads. These observations and the relationship between plant mycorrhizal status and soil nutrients are discussed.     

Changes in arbuscular mycorrhizal associations and fine root traits in sites under different plant successional phases in southern Brazil

Fine root morphological traits and distribution, arbuscular mycorrhizal (AM) fungi, soil fertility, and nutrient concentration in fine root tissue were compared in sites under different successional phases: grass plants, secondary forest, and mature forest in Londrina county, Paraná state, southern Brazil. Soil cores were collected randomly at the 0-10- and 10-20-cm depths in three quadrants (50 m2) in each site. Plants from the different successional stages displayed high differences in fine root distribution, fine root traits, and mycorrhizal root colonization. There were increases in the concentration of nutrients both in soil and fine roots and decrease of bulk soil density along the succession. The fine root biomass and diameter increased with the succession progress. The total fine root length, specific root length, root hair length, and root hair incidence decreased with the succession advance. Similarly, the mycorrhizal root colonization and the density of AM fungi spores in the soil decreased along the succession. Mycorrhizal root colonization and spore density were positively correlated with fine root length, specific root length, root hair length, root hair incidence, and bulk density and negatively correlated with fine root diameter and concentration of some nutrients both in soil and root tissues. Nutrient concentration in root tissue and in soil was positively correlated with fine root diameter and negatively correlated with specific root length, root hair length, and root hair incidence. These results suggest different adaptation strategies of plant roots for soil exploration and mineral acquisition among the different successional stages. Early successional stages displayed plants with fine root morphology and AM fungi colonization to improve the root functional efficiencies for uptake of nutrients and faster soil resource exploration. Late successional stages displayed plants with fine root morphology and mycorrhizal symbiosis for both a lower rate of soil proliferation and soil exploration capacity to acquire nutrients.     

拉杰沙希大学校园药用植物菌根研究

Forty different medicinal plants were investigated for arbuscular mycorrhizal association in the Rajshahi University Campus in Bangladesh. The results indicated that 35 different plants were infected by AM (arbuscular mycorrhizal) fungi as found by trypan blue staining procedure. The percentage of root colonization by AM fungi varied from 13.3% to 100%. Mangifera indica and Morus indica have maximum percentage of colonization (100%). The intensity of root colonization were abundant in the plants belonging to the families Anacardiaceae, Asclepiadaceae, Moraceae, Leguminosae and Apocynaceae whereas the intensity of colonization of crop roots were moderate and poor belonging to Gramineae and Leguminosae. The presence of greater number of spore in soil was always associated with the incidence of abundant mycelia. In plant roots the formation of spore and mycelia was restricted by low pH. Number of mycorrhizal fungus spores ranged between 35 to100 per 100g air dried soil in different family respective soils. The frequency of mycorrhizal fungus infection showed positive correlation with soil pH, moisture, water holding capacity, texture, total nitrogen, organic carbon, phosphorus, calcium, potassium, and magnesium. Especially phosphorus and nitrogen in the soil greatly influenced the plant root infection by AM fungi.     

Root fungal symbionts interact with mammalian herbivory, soil nutrient availability and specific habitat conditions

Herbivory, competition and soil fertility interactively shape plant communities and exhibit an important role in modifying conditions for host-dependent fungal symbionts. However, field studies on the combined impacts of natural herbivory, competition and soil fertility on root fungal symbionts are rare. We asked how mammalian herbivory, fertilization, liming and plant–plant competition affect the root colonization of arbuscular mycorrhizal fungi (AMF) and dark septate endophytic (DSE) fungi of the dicot herb, Solidago virgaurea. The 2-year full-factorial experiment was conducted in two contrasting habitats: non-acidic and acidic mountain tundra. We found that herbivory increased arbuscular colonization (i.e. the site of resource exchange) at fertile non-acidic sites, where vegetation was rich in species having AMF symbionts, whereas at infertile acidic sites, where plants having AMF symbiont are scarce, the response was the opposite. Herbivory of the host plant negatively affected DSE hyphal and sclerotial colonization in unfertilized plots, possibly due to reduced carbon flow from the host plant while there was no effect of herbivory in fertilized plots. DSE colonization was highest in unfertilized exclosures where soil nutrient concentrations were also lowest. Liming had a negative effect on DSE hyphal colonization, and its effect also interacted with herbivory and the habitat. Biomass removal of the neighboring plants did not affect the root colonization percent of either arbuscules or DSE. Our results show that the impacts of aboveground mammalian herbivory, soil nutrient availability and specific habitat conditions on belowground root fungal symbionts are highly dependent on each other. Arbuscule response to herbivory appeared to be regulated by specific habitat conditions possibly caused by differences in the AMF availability in the soil while DSE response was associated with availability of host-derived carbon. Our result of the relationship between herbivory and soil nutrients suggests an important role of DSE in ecosystem processes.     

Interaction between Earthworms and Arbuscular Mycorrhizal Fungi in Plants: A Review

Different kinds of soil animals and microorganisms inhabit the plant rhizosphere, which function closely to plant roots. Of them, arbuscular mycorrhizal fungi (AMF) and earthworms play a critical role in sustaining the soil-plant health. Earthworms and AMF belong to the soil community and are soil beneficial organisms at different trophic levels. Both of them improve soil fertility and structural development, collectively promoting plant growth and nutrient acquisition capacity. Earthworm activities redistribute mycorrhizal fungi spores and give diversified effects on root mycorrhizal fungal colonization. Dual inoculation with both earthworms and AMF strongly magnifies the response on plant growth through increased soil enzyme activities and changes in soil nutrient availability, collectively mitigating the negative effects of heavy metal pollution in plants and soils. This thus enhances phytoremediation and plant disease resistance. This review simply outlines the effects of earthworms and AMF on the soil-plant relationship. The effects of earthworms on root AMF colonization and activities are also analyzed. This paper also summarizes the interaction between earthworms and AMF on plants along with suggested future research.     

Mycorrhizosphere interactions to improve plant fitness and soil quality

Arbuscular mycorrhizal fungi are key components of soil microbiota and obviously interact with other microorganisms in the rhizosphere, i.e. the zone of influence of plant roots on microbial populations and other soil constituents. Mycorrhiza formation changes several aspects of plant physiology and some nutritional and physical properties of the rhizospheric soil. These effects modify the colonization patterns of the root or mycorrhizas (mycorrhizosphere) by soil microorganisms. The rhizosphere of mycorrhizal plants, in practice a mycorrhizosphere, harbors a great array of microbial activities responsible for several key ecosystem processes. This paper summarizes the main conceptual principles and accepted statements on the microbial interactions between mycorrhizal fungi and other members of rhizosphere microbiota and discusses current developments and future trends concerning the following topics: (i) effect of soil microorganisms on mycorrhiza formation; (ii) mycorrhizosphere establishment; (iii) interactions involved in nutrient cycling and plant growth; (iv) interactions involved in the biological control of plant pathogens; and (v) interactions to improve soil quality. The main conclusion is that microbial interactions in the rhizosphere of mycorrhizal plants improve plant fitness and soil quality, critical issues for a sustainable agricultural development and ecosystem functioning. This revised version was published online in August 2006 with corrections to the Cover Date.