Microbial Ecology of the Arbuscular Mycorrhiza

Home , Arbuscular mycorrhiza

FEMS Microbiology Ecology 32 (2000) 91^96 www.fems-microbiology.org MiniReview Microbial ecology of the arbuscular mycorrhiza

Angela Hodge *

Department of Biology, The University of York, P.O. Box 373, York YO10 5YW, UK

Received 14 January 2000; received in revised form 13 March 2000; accepted 14 March 2000

Abstract

Arbuscular mycorrhizal (AM) fungi interact with a wide variety of organisms during all stages of their life. Some of these interactions such as grazing of the external mycelium are detrimental, while others including interactions with plant growth promoting rhizobacteria (PG PR) promote mycorrhizal functioning. Following mycorrhizal colonisation the functions of the root become modified, with consequences for the rhizosphere community which is extended into the mycorrhizosphere due to the presence of the AM external mycelium. However, we still know relatively little of the ecology of AM fungi and, in particular, the mycelium network under natural conditions. This area merits attention in the future with emphasis on the fungal partner in the association rather than the plant which has been the focus in the past. ß 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.

Keywords: Arbuscular mycorrhiza; Fungal mycelium network; Soil microbial ecology; Interaction; Rhizosphere and mycorrhizosphere

1. Introduction the most ancient and probably aided the ¢rst land plants to colonise by scavenging for phosphate [3]. The AM as- Mycorrhizal associations between a fungus and a plant sociation is so called because of the formation of highly root are ubiquitous in the natural environment. The asso- branched intracellular fungal structures or `arbuscules' ciations themselves can be further classi¢ed into one of which are believed to be the site of phosphate exchange seven di¡erent types (i.e. arbuscular, ectomycorrhiza, ec- between fungus and plant. Vesicles which contain lipids tendomycorrhiza, ericoid, arbutoid, orchid and monotro- and are thought to be carbon storage structures may poid) based on the type of fungus involved and the range also form in some cases, although this will depend on of resulting structures produced by the root^fungus com- the fungal symbiont as well as environmental conditions bination (see [1]). Common to all types of mycorrhizal [1]. association, however, is the movement of carbon, gener- Approximately two-thirds of all land plants form the ally, but not always, in the direction from plant to fungus. AM type of association, in sharp contrast with the rela- The association may not be obviously mutualistic at all tively small numbers of fungi involved, all of which are points in time and this together with the range of func- members of the order Glomales (Zygomycotina) compris- tions thus far identi¢ed for the association (i.e. defence, ing only approximately 150 described taxa [1]. Conse- nutrient uptake, soil aggregation stability, drought resis- quently, the AM association is generally assumed to tance) has posed problems in producing a clear de¢nition have no, or at least very low, speci¢city. More recently to best describe the association. Currently, the most useful however, van der Heijden et al. [4] demonstrated that de¢nition is perhaps that of ``a sustainable non-pathogenic the biomass of several plant species in microcosms con- biotrophic interaction between a fungus and a root'' as taining four native AM fungal taxa was approximately proposed by Fitter and Moyersoen [2], although this equal to biomass production in treatments that included does not emphasise the importance of the presence of the single fungal taxa that induced the largest growth re- both intra- and extraradical mycelia in the association. sponse. This indicated that plants may be able to at least By far the most common type of association is that of select the AM fungus which may bene¢t them the most. the arbuscular mycorrhiza (AM). The AM association is However, the bulk of knowledge of the AM symbiosis derives from microcosm experiments using a small number of plant and fungal taxa, and with little or no attention * Tel.: +44 (1904) 432878; Fax: +44 (1904) 432860; paid to the other soil biota with which they must interact. E-mail: [email protected] The purpose of this review is to focus on the ecology of

FEMSEC 1124 10-5-00 92 A. Hodge / FEMS Microbiology Ecology 32 (2000) 91^96 the AM association in terms of interactions with other ing in enhanced ¢xation levels and consequently improved organisms and the implications of these interactions for N status of the plant thus promoting plant growth and mycorrhizal development and functioning. functioning which in turn also bene¢ts mycorrhizal development (reviewed for legumes by [17]; see [18] for Frank- ia). 2. Interactions in the mycorrhizosphere 2.2. AM in£uences on soil microbial interactions 2.1. Soil microbial interactions on AM fungi Once mycorrhizal colonisation has occurred, subsequent Soil micro-organisms in£uence AM fungal development exudation release by the root may be modi¢ed both and symbiosis establishment but no clear pattern of re- through the mycorrhizal fungus acting as a considerable sponse has been found: positive [5^7], negative [8,9] and carbon sink for photoassimilate and through hyphal exu- neutral [10] interactions have all been reported. Negative dation. This may be expected to lead to changes in both impacts upon the AM fungi include a reduction in spore the qualitative and quantitative release of exudates into germination and hyphal length in the extramatrical stage, the mycorrhizosphere. AM colonisation generally de- decreased root colonisation and a decline in the metabolic creases root exudation [19] although not always [20] and activity of the internal mycelium. Which e¡ect is the more may be in£uenced by the species of fungus present [21]. In predominant has been found to be in£uenced by the tim- addition, a reduction in sugar and amino acid release has ing of addition of the micro-organisms, the type of AM been reported in some studies [19,22] but there is no clear fungus present and the plant species which the AM fungus pattern as to the consistency of this phenomenon. Simi- has colonised [9,11,12]. These factors together with the larly the reported impact on the mycorrhizosphere com- complex and dynamic nature of the soil environment munity is equally inconsistent with, for example, £uores- mean that it is di¤cult to draw any useful generalisations. cent pseudomonads showing a decrease, increase or no Indeed, even the same genus has been shown to have ei- e¡ect following AM colonisation [10,21,23,24]. Meyer ther a bene¢cial, negative or neutral e¡ect upon AM fungi, and Linderman [25] observed no alteration in the total as has been reported for both Trichoderma and Pseudomo- number of bacteria or actinomycetes isolated from the nas spp. [5,8,10,12^14]. Recent advances in both biochem- rhizosphere of Zea mays and Trifolium subterranean L. ical and molecular techniques should provide more useful colonised by the AM fungus Glomus fasciculatum. How- insights into the nature of the interactions between AM ever, there was a change in the functional groups of these fungi and other soil micro-organisms. For example, organisms including more facultative anaerobic bacteria in although the presence of Trichoderma harzianum decreased the rhizosphere of AM colonised T. subterraneum but few- root colonisation and, when an organic nutrient source er £uorescent pseudomonads and chitinase-producing ac- was added, external hyphal density of the AM fungus tinomycetes in the rhizosphere of AM-colonised Z. mays. Glomus intraradices, the living AM mycelial biomass The total number of bacteria isolated from the rhizoplane (measured as the content of a membrane fatty acid, of both T. subterraneum and Z. mays increased as a result PFLA16:1g5) did not decrease nor did AM hyphal trans- of AM colonisation although total numbers of actinomy- port of 33P [15]. cetes were una¡ected. In addition, leachates from Z. mays Positive in£uences on the AM symbiosis after addition rhizosphere soil reduced production of zoospores and of plant growth promoting rhizobacteria (PGPR), which sporangia by Phytophthora cinnamomi when colonised by include £uorescent pseudomonads and sporulating bacilli, G. fasciculatum than non-mycorrhizal Z. mays rhizosphere are frequently reported. For example, dual inoculation of leachates indicating a potential mechanism by which AM a PGPR (Pseudomonas putida) and an AM fungus induced colonisation may aid pathogen resistance [25]. However, an additive growth enhancement of subterranean clover the chitinolytic producing actinomycete population may when added together rather than singly [5]. Inoculation act as general biocontrol agents, thus, the reduction in with the PGPR also increased root colonisation by the this population may mean chitin containing pathogens be- AM fungus initially (i.e. measured at 6 weeks) although come more important. Using three di¡erent AM fungi later (at 12 weeks) colonisation levels were similar regard- (Glomus etunicatum, Glomus mosseae or Gigaspora rosea) less of the presence of the PGPR [5]. Enhanced mycelial Schreiner et al. [26] observed di¡erences in bacterial growth from Glomus mosseae spores by a PGPR has also groups (i.e. Gram-negative or Gram-positive) depending been reported [16]. Thus, PGPR appear sometimes to pro- on which fungus had colonised the roots of Glycine max mote both mycorrhizal development and functioning. In L. (soybean). The AM fungus G. mosseae produced the addition, the mycorrhizal and nodulated (i.e. Frankia, Rhi- greatest amount of external hyphae (i.e. 8.1 m g31 soil). zobium and Bradyrhizobium) symbioses are generally syn- The other two AM fungi did not di¡er in the amount of ergistic. It is believed that the AM symbiosis relieves P external hyphae they produced but soil sampled from pots stress for the plant which in turn has bene¢ts for the containing G. etunicatum had higher amounts of Gram-

N2-¢xing nitrogenase system of the other symbiont, result- positive bacteria, measured as colony-forming units per g

FEMSEC 1124 10-5-00 A. Hodge / FEMS Microbiology Ecology 32 (2000) 91^96 93 of dry soil, than corresponding samples from G. rosea. Soil ever, the feeding of the collembola Folsomia candida on an from G. etunicatum pots also contained higher counts of exclusive diet of the AM fungi Acaulospora spinosa, Scu- Gram-negative bacteria than those counted from G. mos- tellospora calospora and Gigaspora gigantea actually re- seae. These results would seem to imply that the hypho- duced the reproductive capacity of this collembola. In sphere (the volume of soil in£uenced by the external my- contrast, two other AM fungi (G. intraradices and G. etu- celium of the AM fungus) of di¡erent AM fungi may nicatum) although less palatable than T. harzianum were as in£uence certain bacterial groups however, it should also pro¢table in terms of reproductive success [34]. Thus, be noted that where external mycelium production was although active grazing of AM fungal hyphae may not greatest (i.e. in the case of G. mosseae) the in£uence on be an important feature under ¢eld conditions, collembola overall counts was less than from G. etunicatum which may reduce the e¡ectiveness of the mycorrhizal symbiosis produced a less extensive mycelium. Indeed, low P status in other ways. For example, although the collembola of the soil had a greater e¡ect on total bacterial, and in F. candida did not actively graze on hyphae of the AM particular Gram-positive, counts, than did mycorrhizal fungus G. intraradices, they did bite and sever the external treatments. Alternatively, the more extensive mycelium AM hyphae from the root before grazing on their pre- could have inhibited bacterial populations as a means of ferred food source (conidial fungal hyphae) present at reducing competition for nutrients in the mycorrhizo- the same time. This severing of AM hyphal networks sphere [15,24]. Other studies speci¢cally testing the hypho- was as much as 50% at the highest populations of collem- sphere soil have found no quantitative change in bacterial bola studied [35]. Although under some circumstances numbers [27,28]. However, whereas Andrade et al. [27] grazing by soil organisms such as earthworms, collembola found variations in bacterial composition which depended and other organisms will be bene¢cial (e.g. as spores can on the AM fungus present, Olsson et al. [28] found no survive ingestion thus grazing and deposition elsewhere such changes in composition or activities of the bacterial will aid in dispersal) the grazing or cleavage of external community. hyphae may have more important consequences on the Filion et al. [29] examined the release of soluble uniden- e¡ectiveness of the mycorrhizal symbiosis [33]. The inter- ti¢ed substances by the external mycelium of Glomus intra- nal colonisation will remain intact and thus represent a radices on the conidial germination of two fungi and the carbon drain on the plant, but with reduced bene¢ts due growth of two bacteria. Conidial germination of Tricho- to a reduction in the external hyphal length. Re-establish- derma harzianum and growth of Pseudomonas chlororaphis ment of the external mycelium will again require more were stimulated whereas growth of Clavibacter michiga- plant carbon to be invested. nensis subsp. michiganensis was una¡ected and conidial germination of Fusarium oxysporum f.sp. chrysanthemi was reduced. These observed e¡ects were generally corre- 3. From microcosm to ¢eld lated with the extract concentration. The authors [29] suggested that this was a possible means by which the AM The majority of the studies discussed above have in- mycelium may alter the microbial environment so that it volved investigating the interactions between soil micro- was detrimental to pathogens. In contrast, Green et al. [15] organisms and, generally, a single AM fungal inoculum also examining the interaction between G. intraradices and added to microcosm units. Such studies are a ¢rst approx- T. harzianum, observed no e¡ect of the AM external my- imation to understanding the complex interactions which celium on the population density of T. harzianum, except can occur using controlled conditions which would be im- in the presence of an organic substrate when population possible in the ¢eld. However, the ecology of AM in the densities and metabolic activity of T. harzianum were ac- ¢eld may be quite di¡erent. It certainly is more complex, tually reduced. The di¡ering results reported on the in£u- with diversity of AM fungi in the root systems of plants ence of AM fungi upon soil micro-organisms therefore are ranging from two common taxa in an arable ecosystem to probably not only due to the type of AM fungus present ca 11 in a woodland system [36] and ca 23 morphospecies but also the conditions, such as soil nutrient availability, being described from a single farm in Canada [4]. In an in which the interaction is studied. arable situation where plants are grown as crops then removed before re-sowing, the AM fungus is continually 2.3. Grazing of AM fungi having to re-establish itself. This is similar to the situation in microcosm units where the fungal inoculum is generally AM spores and external mycelium are subject to grazing added as colonised root fragments or spores, thus here by larger soil organisms such as collembola (or spring- also the AM fungus has to endure a period of develop- tails), earthworms and mammals as well as other fungi ment and establishment. Clearly however, other factors and actinomycetes (see [30]). For example, although not impact on AM formation in arable systems such as pesti- their preferred food source [31], collembola can graze on cide and fertiliser usage. However, in natural undisturbed the spores and extraradical mycelium of AM hyphae as ecosystems the fungus forms a permanent external myce- shown by examination of their gut contents [32,33]. How- lium network and plants are linked by a common mycelial

FEMSEC 1124 10-5-00 94 A. Hodge / FEMS Microbiology Ecology 32 (2000) 91^96 network (CMN). This CMN probably becomes the pri- due to the essential role the plant plays in ensuring con- mary source of inoculum by which plants become colon- tinued growth and functioning of the fungus. However, ised. However, we know relatively little of the ecology of the fungus should not be out of mind. From the fungal this network such as the distances to which it can extend, viewpoint, the linking of plants by this CMN makes stra- how many plants may be linked, the di¡ering ability of tegic sense. It allows fungal spread through the soil and AM fungal taxa to produce such networks and their in- maximises carbon capture by active colonisation of roots teraction with each other let alone the interactions with it encounters ensuring continued growth and activity. Fu- the other soil biota. Some data are available, for example ture research emphasis needs to be placed on the fungal the spread of hyphae of G. fasciculatum through unplanted symbiont in the association adopting a more mycocentric soil has been estimated to occur at a rate of 1.66 mm approach as suggested by Fitter et al. [46]. day31 [37] but again such estimates generally come from Recent development of new techniques such as the sta- microcosm studies. In a ¢eld study, Chiariello et al. [38] ble-isotope probing method [47] and £uorescent in situ applied 32P to the leaves of a donor Plantago erecta plant hybridization combined with microautoradiography [48] present in a serpentine annual grassland and detected high and tracking of labelled substrate uptake [49] now make levels (i.e. s 40% above background counts per min) in it possible to directly follow the active populations of bac- the shoots of neighbouring plants at a distance of ca 45 teria (rather than just culturable organisms) in soil. In mm after 6^7 days. However, neither the type or size of addition, analysis of appropriately selected phospholipid the neighbouring plants nor the distance between donor fatty acid (PLFA) pro¢les can indicate the amount of and receiver were indicators of the amount of 32P trans- fungal and bacterial biomass present and, when coupled ferred. Thus, there is an urgent need to investigate the with radio- or stable-isotope analysis, can indicate alter- ecology of the symbiosis under a range of ¢eld conditions ations in the activity of this biomass (see [15]). PLFA in order to more fully understand the context dependence techniques have recently been used to investigate the in- of the data obtained in relation to mycorrhizal functioning teraction between AM fungi and other soil micro-organ- and the nature of the interactions with other soil biota. isms and have shown considerable promise [15,29]. These For the plant, being linked into the CMN may help to new methods in soil microbial ecology together with ad- reduce the uncertainty of soil heterogeneity with the fungal vances in molecular techniques to identify AM fungi both mycelium being able to locate, access and exploit the nu- in colonised roots and the external phase mean that fol- trient-rich zones or patches which occur naturally in all lowing AM fungal ecology and their interaction with the soils due to organic matter inputs (see [39]) more e¡ec- soil biota is now possible. Indeed, molecular methodolo- tively than plant roots. It is well established that when gies have already shown that spore diversity found in the roots of some plant species encounter such organic patches vicinity of the root is not readily translated into diversity they proliferate roots within them [39]. This proliferation found in the actual colonised root [50]. The combined is believed to be a foraging response to the heterogeneous application of these new techniques in the future should nature of the environment. AM fungi can also proliferate enable valuable insights into the ecological role of inter- hyphae within nutrient-rich organic patches [40,41]. Al- acting groups of soil micro-organisms and AM fungi and lowing fungal hyphal proliferation instead would be their subsequent impact upon carbon dynamics and nu- more carbon cost e¤cient for the plant (see [42]). Further- trient translocation under a range of di¡ering soil, and more, because of their size, AM fungal hyphae should be ultimately ¢eld, conditions. better able to compete with the indigenous soil biota for the microbially released nutrients. The AM fungi may also be able to access organic sources directly (i.e. without the Acknowledgements prior need for microbial mineralisation) as has been demonstrated for nitrogen under both ¢eld [43] and laboratory I am very grateful to Alastair Fitter for detailed com- conditions [44]. The importance and ubiquity of this up- ments and suggestions on earlier drafts. A.H. is funded by take of intact organic compounds by AM fungi remains a BBSRC David Phillips Fellowship. controversial (see [1]) and may depend on the competitive ability of AM fungi in comparison with the other soil biota present. However, the distances over which nutrients References can e¡ectively be transported among plants via the network may be small (see review by [45]). The reason why [1] Smith, S.E. and Read, D.J. (1997) Mycorrhizal Symbiosis. Academic we know so little about the CMN is partially the di¤cul- Press, London. ties associated with studying it particularly under ¢eld [2] Fitter, A.H. and Moyersoen, B. (1996) Evolutionary trends in root- conditions. Furthermore, in the past most emphasis in microbes symbioses. Phil. Trans. R. Soc. Lond. B 351, 1367^1375. [3] Simon, L., Bousquet, J., Levesque, R.C. and Lalonde, M. (1993) mycorrhizal research has been placed upon the plant Origin and diversi¢cation of endomycorrhizal fungi and coincidence rather than the fungus or indeed the symbiotic state. with vascular land plants. Nature 363, 67^69. This is particularly true in the AM association probably [4] van der Heijden, M.G.A., Klironomos, J.N., Ursic, M., Moutoglis,

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