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Figure 2. Local macrophage accumulation during inflammation.

For type 1 inflammation, macrophage accumulation involves the enhanced recruitment ofmonocytes from the bloodstream, whereas increased macrophage numbers in type 2 inflam-mation are a result of enhanced local proliferation [1].

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Importantly, this notion is substantiatedby a previous study that reportedincreasedmacrophageaccumulation inthe spleen and liver after continuousapplication of IL-4 using an osmoticmini-pump in mice [11].

These surprising findings pointtowards a completely novelmechanism of local inflammation inthe context of parasite infectionsand wound healing responses inwhich the recruitment of potentiallytissue-destructive neutrophils andinflammatory macrophages is avoided.Instead, under conditions in whichthe immune system does not attemptto kill pathogens but instead tries towall off large parasites and promotehealing processes, the cells requiredfor these processes are generatedfrom the local pool of tissuemacrophages (Figure 2). In conclusion,depending on the situation, acost-effective solution based on theuse and expansion of local, existingresources can be more useful thanrecruiting new, highly specialized

players that potentially cause damageby unnecessarily aggressive andovershooting actions.

References1. Jenkins, S.J., Ruckerl, D., Cook, P.C.,

Jones, L.H., Finkelman, F.D., van Rooijen, N.,

Macdonald, A.S., and Allen, J.E. (2011). Localmacrophage proliferation, rather thanrecruitment from the blood, is a signature ofTH2 inflammation. Science 332, 1284–1288.

2. van Furth, R., Cohn, Z.A., Hirsch, J.G.,Humphrey, J.H., Spector, W.G., andLangevoort, H.L. (1972). The mononuclearphagocyte system: a new classificationof macrophages, monocytes, and theirprecursor cells. Bull. World Health Organ. 46,845–852.

3. van Furth, R., and Cohn, Z.A. (1968). The originand kinetics of mononuclear phagocytes. J.Exp. Med. 128, 415–435.

4. Gordon, S., and Taylor, P.R. (2005). Monocyteand macrophage heterogeneity. Nat. Rev.Immunol. 5, 953–964.

5. Geissmann, F., Jung, S., and Littman, D.R.(2003). Blood monocytes consist of twoprincipal subsets with distinct migratoryproperties. Immunity 19, 71–82.

6. Soehnlein, O., and Lindbom, L. (2010).Phagocyte partnership during the onset andresolution of inflammation. Nat. Rev. Immunol.10, 427–439.

7. Voehringer, D., Shinkai, K., and Locksley, R.M.(2004). Type 2 immunity reflects orchestratedrecruitment of cells committed to IL-4production. Immunity 20, 267–277.

8. Martinez, F.O., Helming, L., and Gordon, S.(2009). Alternative activation of macrophages:an immunologic functional perspective. Annu.Rev. Immunol. 27, 451–483.

9. Gordon, S., and Martinez, F.O. (2010).Alternative activation of macrophages:mechanism and functions. Immunity 32,593–604.

10. Hoffmann, W.H., Petit, G., Schulz-Key, H.,Taylor, D.W., Bain, O., and Le Goff, L. (2000).Litomosoides sigmodontis in mice: reappraisalof an old model for filarial research. Parasitol.Today 16, 387–389.

11. Milner, J.D., Orekov, T., Ward, J.M., Cheng, L.,Torres-Velez, F., Junttila, I., Sun, G., Buller, M.,Morris, S.C., Finkelman, F.D., et al. (2010).Sustained IL-4 exposure leads to a novelpathway for hemophagocytosis, inflammation,and tissue macrophage accumulation. Blood116, 2476–2483.

Institute for Medical Microbiology,Immunology and Hygiene, TechnischeUniversitat Munchen, Munich, Germany.E-mail: laura.helming@mikrobio.med.tum.de

DOI: 10.1016/j.cub.2011.06.005

Mycorrhizal Symbioses: How to BeSeen as a Good Fungus

Plants continually encounter many microorganisms. Some are good, but manyare bad. Two studies show how beneficial fungi tell the plant to let them in andhow the fungus avoids setting off the plant’s defense reaction.

Ian R. Sanders

Plants form a variety of differentassociations with microorganisms.They are continually challenged byfungal pathogens but also form

mutualistic associations with beneficialmicroorganisms. Of these beneficialinteractions the mycorrhizal symbiosis,an association between plant roots andsoil fungi, is the most abundant and isof major importance for plant ecology,

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plant growth, agriculture and forestry[1]. There are several types ofmycorrhizal symbiosis and at least 80%of plant species worldwide form suchassociations. By far the commonest isthe arbuscular mycorrhizal symbiosis(AM) that occurs between the majorityof plants and arbuscular mycorrhizalfungi (AMF). Most plants form the AMsymbiosis; it is found in most terrestrialecosystems and occurs with allglobally important food plants.Ectomycorrhizal symbioses (EM) areformed by other groups of fungi andoccur on most trees in temperate andboreal forests. Both of thesesymbioses are of global importancebecause they help plants to accessessential nutrients such as inorganicnitrogen and phosphate, improve plantgrowth, promote plant diversity andplay an important role in global nutrientcycling. The problem for plants is thattheir roots continually encounter a vastarray of microorganisms, especiallyfungi. Some of these fungi, like the AMand EM fungi, may be beneficial, butmany of them are potentially harmful.So the problem for both plants andmycorrhizal fungi is how to tell eachother to establish a symbiosis and howto do that without setting off some ofthe plant’s complex defense reactions.While considerable progress has beenmade over the last decade onunderstanding which plant genes areinvolved in the establishment of themycorrhizal symbiosis, much lessprogress has been made on the fungalside [2,3]. It is, therefore, exciting that inthis issue of Current Biology twoindependent studies by Plett et al. [4]and Kloppholz et al. [5] on the EMand AM symbioses shed light onmutualistic fungal effector proteins,showing that they allow symbiosisformation and allow the fungus tomanipulate the plant’s defenseresponse, respectively.

Fungal pathogens of plants areknown to release effectors, often smallproteins that are usually encoded byavirulence genes [6]. Effector proteinsplay a part in pathogen attack and acteither in the plant extra cellular spaceor after entering host plant cells [7].It was long suspected that suchproteins would also likely occur inmutualistic fungi such as AM and EMsymbionts, although owing todifficulties of studying organismsthat live together in symbiosis suchproteins remained elusive. In 2008,genome-wide transcriptome studies

on the symbiosis between theectomycorrhizal fungus Laccariabicolor and poplar revealed a largenumber of small proteins that weresuspected to have a role in signalling.One of the genes that was highlyup-regulated in L. bicolor in thesymbiotic state encodes a proteinof 68 amino acids that accumulatesin mycorrhizal root tips [8]. The protein,known as Mycorrhizal induced SmallSecreted Protein7 (MiSSP7), wasshown to occur only when the fungusmade the symbiosis and not whenthe fungus was in a free-living state.Excitingly, Plett et al. [4] found thatif the fungus was grown near a rootbut not allowed contact with the root,MiSSP7 was produced by the fungus.Interestingly, this also occurred inthe presence of the plant Arabidopsisthaliana which is not capable offorming any type of mycorrhizalsymbiosis, either EM or AM.This indicates that diffusibleplant-produced signals must bereleased by the root but that theyare not specific. Combinationsof immunofluorescent labelingand inhibitors of endocytosisand endosome vesicular trackingwere used to demonstrate that MiSSP7enters plant cells, probably byendocytosis, and accumulatesin the plant nucleus. Interestingly,accumulation occurs in the absenceof the fungus, as shown by introducingMiSSP7 to plant cell suspensions.These experiments demonstratethat it is the plant that actively takesup MiSSP7. One very exciting findingof this study is that MiSSP7 also seemsto be critical for the formation of theso-called Hartig net, a symbioticstructure that creates a high surfacearea between the fungus and plantcells and allows an efficientbi-directional exchange of nutrientsbetween the plant and fungus. TheHartig net is, thus, a critical structurethat has to be formed for a functionalmutualistic symbiosis to occurbetween the plant and the fungus.Homologous gene replacement has notbeen achieved in L. bicolor and so Plettet al. [4] used RNA silencing to lowerthe production of MiSSP7, resultingin very low percentages of mycorrhizalroot tip formation and only a verypoorly developed Hartig net in thoseinstances where some mycorrhizaldevelopment had occurred. MiSSP7appears to control Hartig net formationby inducing transcripts involved in cell

wall remodeling and auxinhomeostasis. It seems, therefore,that MiSSP7 really is a fungal effectornecessary for ectomycorrhizaformation.Since it has proved difficult to find

mycorrhizal fungal effectors, it issurprising to see a second studyappearing at exactly the same timeand also demonstrating the existenceof such effectors [5]. However, what isso significant about this second studyis that it has been found in AM fungi,fungi that diverged from other fungallineages hundreds of millions of yearsbefore the evolution of EM fungi andmost fungal pathogens of plants.Arbuscular mycorrhizal fungi areexceptionally difficult to work with atthe molecular level for a number ofreasons, including the lack of a stabletransformation system and the inabilityto grow the fungus without plant roots[3]. Because of the lack of a completeAMF genome or publishedtranscriptome data, the authors useda modified version of the yeastsecretion sequence trap method tocollect proteins secreted by the AMfungus Glomus intraradices. A smallprotein that they called secretedprotein 7 (SP7) was identified thatcontained a signal domain, a nuclearlocalization domain and tandemhydrophilic repeats. SP7 was found toaccumulate in the fungus when incontact with a plant root. Becausethere is no known stable transformationsystem in G. intraradices, or any otherAM fungus, SP7 was expressed in thefilamentous fungus Aspergillusnidulans in order to show that, similar toMiSSP7, it would locate to the nucleus.Transgenic plants expressing SP7fused to green flourescent protein alsoshowed that the protein was localizedin the plant nucleus. The truly excitingaspect of this study came when theauthors found that SP7 interacts witha transcription factor called ERF19 inthe plant Medicago truncatula and thatERF19 normally activates theexpression of defense proteins inplants. Kloppholz et al. [5] generatedtransformed plant roots thatconstitutively express the fungalprotein SP7. In those roots, the AMfungus could more efficiently colonizethe roots. Interestingly, ERF19 isnormally expressed in plant rootschallenged with the fungal pathogenColletotrichum trifolii but suchexpression is halved when the rootsexpress the fungal protein SP7,

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indicating that SP7 interacts withERF19 to suppress its activity.

An intriguing part of the study wasthat when plants were inoculated witha transgenic form of the hemibiotrophicfungal pathogen Magnaporthe oryzaethat expressed SP7, the detrimentaleffects of the pathogen were less thanin plants infected with the wild-typefungus. The authors suggest thatSP7-mediated suppression of defensepathways in the plant allows the fungusto remain in a biotrophic state. This, inturn, allows the fungus to keep theplant alive for longer. It does not seemvery intuitive that a protein thatsuppresses plant defense could bebeneficial for the plant. However, itcould allow a fungus like M. oryzae tosimply remain undetected inside thehost for a longer period until the planthas grown larger and more resourcescould be obtained from the host.Additionally, defense suppressioncould open up the possibility for otherfungi to attack the plant. Such resultsalso raise the question of whethera plant is more or less susceptible tofungal pathogens when colonized by

an AMF that is expressing SP7 and,thus, suppressing part of the plant’sability to defend itself. Indeed, reportsabout whether AMF increase ordecrease the ability of plants to protectthemselves against fungal pathogensare rather contradictory. Nevertheless,the finding of an AMF effector proteinthat switches off parts of the plantdefense mechanism is indeed a majormilestone in understanding the AMsymbiosis.

Given the global importance of themycorrhizal symbiosis, coupled withthe difficulty of working with the fungalpartners, these two studies identifyingthe role of fungal effector proteins thatallow mycorrhiza formation and allowthe fungus to switch off the plant’sdefense responses truly further ourknowledge of these plant–fungalassociations.

References1. Smith, S.E., and Read, D.J. (2008). The

Mycorrhizal Symbiosis (San Diego: AcademicPress).

2. Parniske, M. (2008). Arbuscular mycorrhiza: themother of plant root endosymbioses. Nat. Rev.Microbiol. 6, 763–775.

3. Sanders, I.R., and Croll, D. (2010). Arbuscularmycorrhiza: The challenge to understand thegenetics of the fungal partner. Annu. Rev. Genet.44, 271–292.

4. Plett, J.M., Kemppainen, M., Kale, S.D.,Kohler, A., Legue, V., Brun, A., Tyler, B.M.,Pardo, A.G., and Martin, M. (2011). A secretedeffector protein of Laccaria bicolor is requiredfor symbiosis development. Curr. Biol. 21,1197–1203.

5. Kloppholz, S., Kuhn, H., and Requena, N. (2011).A secreted fungal effector of Glomusintraradices promotes symbiotic biotrophy.Curr. Biol. 21, 1204–1209.

6. Stergiopoulos, I., and de Wit, P.J.G.M. (2009).Fungal effector proteins. Annu. Rev.Phytopathol. 47, 233–263.

7. Ellis, J.G., Rafiqi, M., Gan, P., Chakrabarti, A.,and Dodds, P.N. (2009). Recent progress indiscovery and functional analysis of effectorproteins of fungal and oomycete plantpathogens. Curr. Opin. Plant Biol. 12,399–405.

8. Martin, F., Aerts, A., Ahren, D., Brun, A.,Danchin, E.G.J., Duchaussoy, F., Gibon, J.,Kohler, A., Lindquist, E., Pereda, V., et al. (2008).The genome of Laccaria bicolor providesinsights into mycorrhizal symbiosis. Nature452, 88–92.

Department of Ecology and Evolution,Biophore Building, University of Lausanne,1015 Lausanne, Switzerland.E-mail: ian.sanders@unil.ch

DOI: 10.1016/j.cub.2011.06.022

Cell Biology: Actin KeepsEndocytosis on a Short Leash

High-resolution structural analysis of branched actin networks at the sites ofclathrin-mediated endocytosis sheds light on the role of actin in endocytosisand mechanisms controlling actin assembly.

Vladimir Sirotkin

The dynamic actin cytoskeleton playsmany important roles in fundamentalcellular processes, including cellmotility, cell division, control of cellshape and endocytosis. Studies inorganisms ranging from yeast to manhave revealed a burst of actin assemblyat the endocytic site or patch, peakingat the internalization of an endocyticvesicle [1–3]. Actin plays an obligatoryrole in endocytosis in yeast, while therole of actin in endocytosis in animalcells remains a subject of debate.Given that force generated by actinassembly is sufficient to deform cellmembranes and to move particleswithin dense cytoplasm, actin wasproposed to participate at multiplestages of endocytosis, including

membrane invagination, scission andpropulsion of the endocytic vesicle.In this issue of Current Biology, Collinset al. [4] provide remarkable electronmicroscopy images of the actinnetwork around clathrin-coatedstructures in animal cells. Theorganization of the network helps todefine the role of actin in endocytosisand provides important clues aboutthe mechanisms controlling actinassembly at the endocytic sites.

The exact mechanism for the roleof actin in endocytosis depends onthe arrangement of filaments aroundthe endocytic sites (Figure 1A). Severalmodels have been proposed [1–3,5].Work in budding yeast favorsactin filament organization withfast-growing, barbed ends facing theplasma membrane and slow-growing,

pointed ends anchored at theendocytic vesicle coat. In a secondmodel, filament barbed ends areoriented towards the tip of endocyticinvagination, so that growing filamentsforce elongation of the neck of thebudding vesicle and propel the vesicleafter the scission from plasmamembrane. The images in the presentstudy [4] support a third model, inwhich actin filaments form a collar-likestructure around the neck of theendocytic vesicle. Growing filamentsare oriented towards the vesicle neck,providing force for neck elongation,vesicle scission and propulsion.Collins et al. [4] used platinum replica

electron microscopy and electrontomography to produce strikingimages of the actin network at clathrin-coated structures in cultured mousecells that were either ‘de-roofed’ bysonication or extracted with detergent.In the de-roofed preparations, manyof the clathrin-coated structures areat the early stages of invaginationand are surrounded by a branchedactin network. Electron tomographyhas revealed that the actin networkis located at the neck of the endocytic