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INDEX

 A. Sponsors
 B. Introduction
 C. Symbiosis and Mutualism
     1. Plant-Fungus Symbioses

 D. Definition of Mycorrhizas
 E. Classifying Mycorrhizas
 F. Morphology of Associations
     1. Arbuscular Mycorrhizas
     2. Ectomycorrhizas
     3. Orchid Mycorrhizas
     4. Ericoid Mycorrhizas
     5. Subepidermal Association

 G. Host plants
 H. Mycorrhizal Fungi
 I. Terminology

© Mark Brundrett 2008


MYCORRHIZAL ASSOCIATIONS: THE WEB RESOURCE


SECTION 1. INTRODUCTION TO MYCORRHIZAS


A. SPONSORS

 * Australian Center for International Agricultural Research
 * The University of Western Australia, School of Plant Biology
 * Lotterywest

Web Hosting:
Digital Pacific

 


B. INTRODUCTION

This site was developed as an online textbook to provide current information
about mycorrhizal associations. Information about this site, instructions,
acknowledgements and site history information is provided in Section 13.

All images on this site are protected by copyright and were taken by the author
unless another photographer is named.

Your browser must allow interactive content for animations and detailed images
provided as roll-overs to be visible (hover mouse over images for interaction)

MAJOR DATA SOURCES

Topic Major Sources Definition and classification of mycorrhizal associations
Brundrett 2004 More information about mycorrhizas: Smith & Read 1997, Peterson
et al. 2004
other mycorrhizal books International Mycorrhizal SocietyInternational
Mycorrhizal Society

 


C. SYMBIOSIS AND MUTUALISM

The terms symbiotic and mutualistic have been used interchangeably to describe
mycorrhizal associations and parasitic fungi have also been called symbiotic,
but many scientists now only call beneficial associations symbiotic (Lewis 1985,
Paracer & Ahmadjian 2000). Symbiosis is defined broadly as “two or more
organisms living together” and in most cases both partners benefit (Lewis 1985).
There are many types of symbiosis evolving different combinations of plants,
fungi, microbes and animals. Only plant-fungus associations are considered in
detail here, but several others are illustrated below.

Fungal symbioses have been defined as “all associations where fungi come into
contact with living host from which they obtain, in a variety of ways, either
metabolites or nutrients” (Cook 1977). However, this definition excludes
mycorrhizal associations of myco-heterotrophic plants, where plants are
nutritionally dependant on fungi (Brundrett 2004). Only the broadest definition
of symbiosis - “living together of two or more organisms”, applies universally
to mycorrhizal associations (Lewis 1985, Smith & Read 1997, Brundrett 2004).

Mutualistic associations are a subset of symbioses where two or more different
living organisms receive mutual benefits, as illustrated in the diagram below.

Examples of Symbiotic Associations Between Animals and Algae

Diverse coral reef community on the Great Barrier Reef (Heron Island). Corals
are symbiotic associations between an animal (coral polyp) and photosynthetic
algae (zooxanthellae) inside the polyp.

Giant Clam (Tridacna gigas) in the Great Barrier Reef. Note algae in mantle
(rollover image shows greater detail).

Other Examples of Symbiotic Associations

Nitrogen fixing symbiotic association of the cycad Macrozamia riedlei. These
coralloid roots at the soil surface which contain cyanobacteria that fix
nitrogen. Other nitrogen fixing associations include bacteria in nodules of peas
(the Fabaceae) and actinomycete nodules in several other plant families.

The ash bolete (Gyrodon merulioides), which occurs under ash trees in North
America (Fraxinus americana), has a symbiotic association with aphids (seen in
cross section). See Section 10 for more information.

 


1. PLANT-FUNGAL SYMBIOSES

Mycorrhizas are the most important type of symbiotic plant-fungus associations,
but there are a wide diversity of other associations between plants and fungi,
as illustrated in the diagram below (pdf version). The relationship between
mycorrhizas and other types of plant-fungus associations, such as parasitic or
endophytic associations, are also shown below.

This diagram compares types of plant-fungus interactions and each is explained
separately below (after Brundrett 2004).

Mutualistic associations occupy the mutual benefit (+ +) quadrant in diagrams
contrasting the relative benefits (+) or harm (-) to two interacting organisms
(Boucher 1985, Lewis 1985). This is a phase plane diagram that describes
biological interactions according to a cost-benefit model, where mutualism is an
isocline showing both partners are more successful together than they are alone
(Boucher 1985, Lewis 1985, Tuomi et al. 2001).

EXPLANATIONS

The vertical axis is a continuum of fungal harm or benefits.
The horizontal axis is a plant harm-benefit continuum.

Fungus benefits are linked to plant benefits in balanced mycorrhizas.
Obligate associations require greater investment from both partners than
facultative mycorrhizas.

Exploitative mycorrhizas (myco-heterotrophs) are parallel to the vertical axis -
plant benefit occurs at expense of fungi.

Parasitic plant-fungal associations are those where fungal benefits are linked
to plant harm.

Endophytic plant-fungus associations (no plant harm or benefit).

Other categories of plant-fungus interactions include antagonism of fungi by
plants or plants by fungi (causing harm to another organism without gaining
direct benefits).

 


D. DEFINITION OF MYCORRHIZAS

The name mycorrhizas, which literally means fungus-root, was invented by Frank
(1885) for non-pathogenic symbiotic associations between roots and fungi. A
revised definition that includes non-mutualistic mycorrhizas and excludes other
plant-fungus associations is provided below (Brundrett 2004). You should refer
to review articles and books on mycorrhizas for further information about these
associations.

Definition of Mycorrhizas Mycorrhizas are symbiotic associations essential for
one or both partners, between a fungus (specialised for life in soils and
plants) and a root (or other substrate-contacting organ) of a living plant, that
is primarily responsible for nutrient transfer. Mycorrhizas occur in a
specialised plant organ where intimate contact results from synchronised
plant-fungus development.

Key characteristics of mycorrhizas Fungus Symbiosis Plant Soil inhabitant1
Intimate contact at interface for nutrient transfer2 Control of association5
Plant inhabitant1 Essential for one or both partners3 Specialised organ6
Specialised hyphae1 Synchronised development4 Root or stem6

NOTES

 1. The structure and development of mycorrhizal fungus hyphae is substantially
    altered in the presence of roots of host plants. These root-borne hyphae are
    distinct from hyphae which are specialised for growth in soil.
 2. All mycorrhizas have intimate contact between hyphae and plant cells in an
    interface where nutrient exchange occurs.
 3. The primary role of mycorrhizas is the transfer of mineral nutrients from
    fungus to plant. In most cases there also is substantial transfer of
    metabolites from the plant to fungus.
 4. Mycorrhizas require synchronised plant-fungus development, since hyphae only
    colonise young roots (except orchid mycorrhizas and exploitative VAM).
 5. Plants control the intensity of mycorrhizas by root growth, digestion of old
    interface hyphae in plant cells (AM, orchid), or altered root system form
    (ECM).
 6. Roots evolved as habitats for mycorrhizal fungi (see Section 2). Mycorrhizas
    normally occur in roots, but can be hosted in stems in some cases (e.g. some
    orchids).

 


E. CATEGORIES OF MYCORRHIZAL ASSOCIATIONS

Consistent definitions of mycorrhizal associations are required for accurate
communication of data. The flowchart below groups similar types of mycorrhizas
together using categories regulated by the host and morphotypes caused by
different fungi (pdf version). Categories and subcategories are defined in the
subsequent table.

Association Categories Morphotypes
Arbuscular
Mycorrhizal
Associations
Ectomycorrhizal
Associations Definitions See Table below See ECM and VAM sections

HIERARCHICAL CLASSIFICATION SCHEME FOR MYCORRHIZAL ASSOCIATIONS (BRUNDRETT 2004)

No. Category Definition Hosts Fungi 1 Arbuscular mycorrhizas Associations formed
by Glomeromycotan fungi in plants that usually have arbuscules and often have
vesicles (also known as vesicular-arbuscular mycorrhizas, AM, VAM). Plants
Glomeromycota (see Section 4) 1.1 Linear VAM Associations that spread
predominantly by longitudinal intercellular hyphae in roots (formerly known as
Arum series VAM). Plants As above 1.2 Coiling VAM Associations that spread
predominantly by intracellular hyphal coils within roots (formerly known as
Paris series VAM). Plants As above 1.2.1 Beaded VAMCoiling VAM in roots, where
interrupted root growth results in short segments divided by constrictions.
Woody plants As above 1.2.2 Inner cortex VAMCoiling VAM with arbuscules in one
layer of cells of the root inner cortex. Plants As above 1.2.3 Exploitative
VAMCoiling VAM of myco-heterotrophic plants, usually without arbuscules.
Achlorophyllous plants As above   2 Ecto-mycorrhiza (ECM) Associations with a
hyphal mantle enclosing short lateral roots and a Hartig net of labyrinthine
hyphae that penetrate between root cells. hosts Higher fungi (asco-, basidio-
and zygo- mycetes) - see Section 9 2.1 CorticalHartig net hyphae penetrate
between multiple cortex cell layers of short roots Most are gymnosperm trees As
above 2.2EpidermalHartig net fungal hyphae are confined to epidermal cells of
short roots Angiosperms (most are trees) As above 2.2.1 Transfer cell Epidermal
Hartig net with transfer cells (plant cells with wall ingrowths) Pisonia
(Nyctaginaceae). See Peterson et al. 2004 for others Tomentella spp. in Pisonia
(Chambers et al. 2005) 2.2.2Monotropoid Exploitative epidermal ECM of
myco-heterotrophic plants in the Ericaceae where individual hyphae penetrate
epidermal cells. Ericaceae (Monotropa, Pterospora, Sarcodes) Basidiomycetes
2.2.3ArbutoidECM of autotrophic plants in in the Ericaceae where multiple hyphae
penetrate epidermal Hartig net cells. Ericaceae (part only) Basidiomycetes  
3OrchidAssociations where coils of hyphae (pelotons) penetrate within cells in
the plant family Orchidaceae. hosts Most are basidiomycetes in Rhizoctonia
alliance (listed below). 3.1Orchid RootAssociations within a root cortex.
Orchidaceae As above 3.2Orchid StemAssociations within a stem or rhizome.
Orchidaceae As above 3.3Exploitative OrchidsAssociations of myco-heterotrophic
orchids. Orchidaceae (fully or partially achlorophyllous) Orchid,
ectomycorrhizal, or saprophytic fungi   4EricoidCoils of hyphae within very thin
roots (hair roots) of the Ericaceae. Ericaceae (most genera) Most are
Ascomycetes (listed below)   5 Sub- epidermalHyphae in cavities under epidermal
cells, only known from an Australian monocot genus. Thysanotus spp.
(Laxmaniaceae) Unknown

 


F. MORPHOLOGY OF MYCORRHIZAL ASSOCIATIONS

The mycorrhizal association types defined in the table above are briefly
described and illustrated below. More information on arbuscular mycorrhizas and
ectomycorrhizas are provided in dedicated Sections of this site.

 

1. ARBUSCULAR MYCORRHIZAS

Arbuscular mycorrhizas (Vesicular-Arbuscular Mycorrhizas, VAM or AM) are
associations where Glomeromycete fungi produce arbuscules, hyphae, and vesicles
within root cortex cells. These associations are defined by the presence of
arbuscules. Fungi in roots spread by linear hyphae or coiled hyphae. VAM
associations are described in detail in Section 4.

Arbuscule of a Glomus species in a root cortex cell. More information.

Vesicles of a Glomus species in a root cortex.
More information.

1.1. Linear association in root of Allium porrum with arbuscules (A) and
vesicles (B) on longitudinal hyphae near entry point (arrow). More information.

1.2. Coiling association with arbuscules (A) on coiling hyphae (arrow) in a root
of Erythronium americanum. More information.

1.2.1. Beaded roots (arrows) of Sugar Maple (Acer saccharum) - a VAM host. More
information

1.2.2. Coiling association with arbuscules (A) only in the inner cortex layer of
Asarum canadense roots.

1.2.3. MYCO-HETEROTROPHIC "ARBUSCULAR" MYCORRHIZAS

1.2.3. Coils of hyphae in the rhizome of Psilotum nudum a whisk fern. This is a
type of VAM association without arbuscules from a young sporophyte with green
shoots that is not fully myco-heterotrophic. See Section 2 for more information
on mycorrhizas of primitive plants.



Cleared and stained rhizome
C = coil, V = vesicle
Roll-over animation shows a single coil.

 

2. ECTOMYCORRHIZAS

Ectomycorrhizas (ECM) are associations where fungi form a mantle around roots
and a Hartig net between root cells. These associations are defined by Hartig
net hyphae which grow around cells in the epidermis or cortex of short swollen
lateral roots. ECM associations are described in Section 4. The former category
of ECM is a morphotype (defined by fungi not hosts). Characteristics of this ECM
morphotype are summarised by Yu et al. (2001).

2.1. Cortical Hartig net of Pinus ECM root.

Highly magnified view of cleared and stained section
More information

2.2. Epidermal Hartig net of Populus ECM.

Highly magnified view of cleared and stained section
More information.

Betula root system showing thicker branched or unbranched ECM roots borne on
thinner lateral roots. ECM root systems are described in Section 5.




Magnified view of ECM root system
(grid = 1 mm).


 

2.2.2 MONOTROPOID

Monotropoid mycorrhizas are ECM associations of a few genera of
myco-heterotrophic plants in the Ericaceae. These associations are characterised
by limited hyphal penetration into epidermal cells. Information on structure of
associations and the identity of mycorrhizal fungi in Monotropa, Pterospora,
Sarcodes, etc. is provided by Robertson & Robertson (1982), Castellano & Trappe
(1985) and Bidartondo et al. (2000).

2.2.2. Monotropa root with epidermal Hartig net (H) and mantle (M) in a
cross-section viewed with UV light. Roll-over - hypha projecting into an
epidermal cell (arrow) in stained root section.

Monotropa uniflora (Canada) is a myco-heterotrophic plant lacking chlorophyll
that is entirely dependant on ECM fungi linked to nearby trees.

2.2.3 ARBUTOID MYCORRHIZAS

Arbutoid mycorrhizal associations are variants of ECM found in certain plants in
the Ericaceae characterised by hyphal coils in epidermal cells. These
mycorrhizal roots are described by Largent et al. (1980), Molina & Trappe (1982)
and Massicotte et al. (1998, 2005a). Gaultheria and Kalmia have ericoid
mycorrhizas as well as arbutoid associations (Massicotte et al. 2005b).

2.2.3. Arbutus unedo root with Hartig net (arrows), coils (C) and mantle (M) of
stained or unstained hyphae.



Highly magnified views of sectioned and cleared roots
(Bar = 20 µm)

 

3. ORCHID MYCORRHIZAS

Orchid mycorrhizas consist of coils of hyphae within roots or stems of
orchidaceous plants. Details of Orchid mycorrhizal associations are not provided
here, but Australian Orchids found to have mycorrhizas are listed.

3.2. Orchid mycorrhizas with hyphae in trichomes and hyphal coils in stem of
Pterostylis vittata


Highly magnified views of cleared and stained hand section.

3.1. Hyphal coils from orchid mycorrhizas in Epipactis helleborine root.


Highly magnified views of cleared and stained hand section.
More information

3.3. Hyphal coils (pelotons) of an exploitative mycorrhizal association in a
myco-heterotrophic orchid. Coils are white of brown fuzzy balls. This rhizome of
the Western Underground Orchid (Rhizanthella gardneri) is 5 mm wide.

Seedlings of Rhizanthella gardneri germinated by a mycorrhizal fungus linked to
ECM roots of a shrub (Melaleuca sp.). These subterranean seedlings are 2-10 mm
long with a zone of brown hyphal coils clearly visible at their base.

 

4. ERICOID MYCORRHIZAS

Ericoid mycorrhizas have hyphal coils in outer cells of the narrow "hair roots"
of plants in the family Ericaceae. These associations are not described in
detail here, but Australian plants with these mycorrhizas are listed.

Ericoid mycorrhizas with hyphal coils in hair roots of Leucopogon verticillatus

Highly magnified views of cleared and stained roots.
More Information

 

5. SUBEPIDERMAL ASSOCIATION OF THYSANOTUS

The Australian lilies in the genus Thysanotus (Laxmaniaceae) have unique
mycorrhizas where fungus hyphae grow in a cavity under epidermal cells. Other
members of this family have VAM or have NM roots (Section 8).



Highly magnified views of a sectioned and stained root of Thysanotus sp.
Arrows point to hyphae under the epidermis (E)

 


G. HOST PLANTS

Comprehensive lists of Australian mycorrhizal plants, as well as plant families
which are ECM hosts, or have NM roots are presented in Sections 5, 6 and 8 of
this site. A summary of mycorrhizal associations in flowering and primitive
plants are also provided in Section 2.

Field surveys have found that plants with mycorrhizal associations predominate
in most natural ecosystems, as summarised the in table below.

Association Occurrence Vesicular Arbuscular Mycorrhizal (VAM) Plants
 * Plants with VAM are common in most habitats
 * It is easier to say where they are not found

Ectomycorrhizal (ECM) Plants

See Section 5
 * Trees with ECM are dominant in coniferous forests, especially in cold boreal
   or alpine regions
 * ECM trees and shrubs common in many broad-leaved forests in temperate or
   mediterranean regions
 * ECM trees also occur in some tropical or subtropical savanna or rain forests
   habitats

Nonmycorrhizal (NM) Plants

See Section 6
 * NM plants are most common in disturbed habitats, or sites with extreme
   environmental or soil conditions
 * NM plants appear to be more common in Australia than in other continents.

Data are from Brundrett (1991)

 


H. MYCORRHIZAL FUNGI

Members of the fungus kingdom obtain nutrition from many sources, including
decomposition of organic substrates, predation and parasitism, and involvement
in mutualistic associations (Christensen 1989, Kendrick 1992). Mycorrhizal fungi
are a major component of the soil microflora in many ecosystems, but usually
have limited saprophytic abilities (Tanesaka et al. 1993, Hobbie et al. 2001).
They are considered to have many important roles in natural and managed
ecosystems as explained in Section 7. These fungi are introduced in the table
below.

 

Mycorrhiza Phylum Families Anamorphs Teliomorphs References Arbuscular
Glomeromycota Glomaceae, Acaulosporacae, etc. Glomus, Scutellospora,
Acaulospora, etc. none Listed in Section 4 Ecto- mycorrhiza (ECM) Basidiomycota,
Ascomycota, Zygomycota Many families including Amanitaceae, Cortinariaceae,
Boletaceae, etc. Most ECM fungi lack anamorphs, but Cenococcum is one example
Many genera including Amanita, Cortinarius, Russula, etc. Listed in Section 9
Monotropoid ECM Basidiomycota Russulaceae, etc. NA Russula, Tricholoma,
Rhizopogon, etc. Bidartondo et al. 2000, Bidartondo & Bruns 2001, 2002, Leake et
al. 2004 Orchid:
not myco-
heterotrophic Basidiomycota (Ascomycete) Ceratobasidiaceae, Tulasnellacea,
Sebacinaceae (related to Chanterellaceae?) (also many others are reported)
Sterile hyphae: Rhizoctonia alliance: Epulorhiza, Ceratorhiza, Tulasnella, etc.
as well as Fusarium, etc. Ceratobasidium, Thanatophorus, Sebacina, etc. Zelmer
et al. 1996, Rasmussen 2002, McCormick et al. 2004, Bayman  & Otero 2006, 
Dearnaley 2007, Taylor & McCormick 2007 Orchid:
myco-
heterotrophic Basidiomycota Russulaceae, Telephoraceae, etc. NA unrelated clades
of ECM, orchid and saprophytic fungi See lists by Batty et al. 2002, Bidartondo
et al. 2004, Dearnaley 2007 Ericoid Ascomycota (Basidiomycota) Helotiaceae
(Sebacinaceae) NA Hymenoscyphus, Rhizoscyphus, (Sebacina) Allen et al. 2003,
Berch et al. 2003,  Bougoure & Cairney 2005, Selosse et al. 2007

 


I. TERMINOLOGY

Symbiosis Refers to intimate associations between two or more different living
organisms. Only a broad definition of symbiosis - living together of two or more
organisms includes all types mycorrhizal associations (Lewis, 1985; Smith &
Read, 1997). Mutualism A category of symbiotic associations where both partners
benefit (see Fig. 1 below). Mycorrhizas of myco-heterotrophic plants are not
mutualistic as the fungus is being Mycorrhiza, Mycorrhizas, Mycorrhizal These
were defined by Frank (1885) as symbiotic associations between fungi and roots
that are not pathogenic (i.e. intimate root-fungus associations without disease
symptoms). Frank named these associations mycorrhizas which means fungus-root
(they were formerly called mycorrhizae). A comprehensive definition of
mycorrhizas is provided above. Myco-heterotrophic mycorrhizas Non-mutualistic
mycorrhizal associations where plants are parasitic on fungi (see Leake 1994).
These plants are sometimes referred to as saprotrophic, cheating,
myco-parasitic, etc. However, these should be referred to as myco-heterotrophic
(fungus feeding) or exploitative associations. Host Plant A plant containing a
fungus of any type. Mycorrhizal fungi These can be called a symbiont, associate,
mycobiont, inhabitant, etc. , but it is usually sufficient to call them fungi.
Mycorrhizal fungi should not be called endophytes to avoid confusion with
another major category of plant inhabiting fungi. Colonisation neutral term
‘colonisation’ is preferential to infection (implying disease) when describing
mycorrhizal fungus activity and the resulting fungal structures can be defined
as colonies. Inoculum propagules of fungi capable of dispersing or initiating
contact with plants. Vascular plants "Higher plants" with conducting elements
for water and nutrients, differentiated leaves and roots, with a dominant
sporophyte. Roots Plant organ responsible for nutrient uptake, mechanical
support, storage, etc. that are usually subterranean. Fungi Members of the
fungus kingdom are eukaryotic, heterotrophic organisms with a tubular body that
reproduce by spores. Mineral nutrients The basic form of substances required for
life (N, P, K, etc. excluding gases). Photosynthesis Solar energy capture by
plants and conversion into organic carbon.

Version 2 © Mark Brundrett 2008