mycorrhiza

A mycorrhiza (Greek for fungus roots coined by Frank, 1885^[1]; typically seen in the plural forms mycorrhizae or mycorrhizas) is a symbiotic (generally mutualistic, but occasionally weakly pathogenic) association between a fungus and the roots of a plant.^[2] In a mycorrhizal association, the fungus may colonize the roots of a host plant, either intracellularly (arbuscular mycorrhizal fungi--AMF) or extracellularly (ectomycorrhizal fungi). They are an important part of soil life.

Mutualist dynamics

Mycorrhizae form a mutualistic relationship with the roots of most plant species (and while only a small proportion of all species has been examined, 95% of these plant families are predominantly mycorrhizal).^[3]

Sugar-Water/Mineral exchange

This mutualistic association provides the fungus with relatively constant and direct access to carbohydrates, such as glucose and sucrose supplied by the plant.^[4] The carbohydrates are translocated from their source (usually leaves) to root tissue and on to fungal partners. In return, the plant gains the benefits of the mycelium's higher absorbtive capacity for water and mineral nutrients (due to comparatively large surface area of mycelium:root ratio), thus improving the plant's mineral absorption capabilities.^[5]

Plant roots alone may be incapable of taking up phosphate ions that are demineralized, for example, in soils with a basic pH. The mycelium of the mycorrhizal fungus can, however, access these phosphorus sources, and make them available to the plants they colonize.^[6]

Mechanisms

The mechanisms of increased absorption are both physical and chemical. Mycorrhizal mycelia are much smaller in diameter than the smallest root, and thus can explore a greater volume of soil, providing a larger surface area for absorption. Also, the cell membrane chemistry of fungi is different from that of plants (including organic acid excretion which aids in ion displacement^[7]). Mycorrhizae are especially beneficial for the plant partner in nutrient-poor soils^[8].

Disease resistance

Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogens^[9][10], and are also more resistant to the effects of drought^[11][12][13]. These effects are perhaps due to the improved water and mineral uptake in mycorrhizal plants.

Colonization of barren soil

Plants grown in sterile soils and growth media often perform poorly without the addition of spores or hyphae of mycorrhizal fungi to colonise the plant roots and aid in the uptake of soil mineral nutrients.^[14] The absence of mycorrhizal fungi can also slow plant growth in early succession or on degraded landscapes.^[15]

Occurrence of mycorrhizal associations

At around 400 million years old, the Rhynie chert contains the earliest fossil assemblage yielding plants preserved in sufficient detail to detect mycorrhizae - and they are indeed observed in the stems of Aglaophyton major.^[16]

Mycorrhizae are present in 92% of plant families (80% of species),^[17] with arbuscular mycorrhizae being the ancestral and predominant form,^[17] and indeed the most prevalent symbiotic association found in all the plant kingdom.^[4] The structure of arbuscular mycorrhizae has been highly conserved since their first appearance in the fossil record,^[16] with both the development of ectomycorrhizae, and the loss of mycorrhizae, evolving convergently on multiple occasions.^[17]

Types of mycorrhiza

Arbuscular mycorrhizal wheat

Ectomycorrhizal beech

An ericoid mycorrhizal fungus isolated from Woollsia pungens.^[18]

Mycorrhizas are commonly divided into ectomycorrhizas and endomycorrhizas. The two groups are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate individual cells within the root, while the hyphae of endomycorrhizal fungi penetrate the cell wall and invaginate the cell membrane. A third group known as Ericoid mycorrhizae is also ecologically significant. ^[19]

Endomycorrhiza

Endomycorrhiza are variable and have been further classified as arbuscular, ericoid, arbutoid, monotropoid, and orchid mycorrhizae ^[20]. Arbuscular mycorrhizas, or AM (formerly known as vesicular-arbuscular mycorrhizas, or VAM), are mycorrhizas whose hyphae enter into the plant cells, producing structures that are either balloon-like (vesicles) or dichotomously-branching invaginations (arbuscules). The fungal hyphae do not in fact penetrate the protoplast (i.e. the interior of the cell), but invaginate the cell membrane. The structure of the arbuscules greatly increases the contact surface area between the hypha and the cell cytoplasm to facilitate the transfer of nutrients between them.

Arbuscular mycorrhizae are formed only by fungi in the division Glomeromycota. Fossil evidence^[16] and DNA sequence analysis^[21] suggest that this mutualism appeared 400-460 million years ago, when the first plants were colonizing land. Arbuscular mycorrhizas are found in 85% of all plant families, and occur in many crop species.^[17] The hyphae of arbuscular mycorrhizal fungi produce the glycoprotein glomalin, which may be one of the major stores of carbon in the soil. Arbuscular mycorrhizal fungi have (possibly) been asexual for many millions of years and, unusually, individuals can contain many genetically different nuclei (a phenomenon called heterokaryosis).^[22]

Many plants in the order Ericales form ericoid mycorrhizas, while some members of the Ericales form arbutoid and monotropoid mycorrhizas. All orchids are mycoheterotrophic at some stage during their lifecycle and form orchid mycorrhiza with a range of basidiomycete fungi.

Ectomycorrhiza

Ectomycorrhizas, or EcM, are typically formed between the roots of around 10% of plant families, mostly woody plants including the birch, dipterocarp, eucalyptus, oak, pine, and rose^[17] families and fungi belonging to the Basidiomycota, Ascomycota, and Zygomycota. Ectomycorrhizas consist of a hyphal sheath, or mantle, covering the root tip and a hartig net of hyphae surrounding the plant cells within the root cortex. In some cases the hyphae may also penetrate the plant cells, in which case the mycorrhiza is called an ectendomycorrhiza. Outside the root, the fungal mycelium forms an extensive network within the soil and leaf litter. Nutrients can be shown to move between different plants through the fungal network (sometimes called the wood wide web). Carbon has been shown to move from birch trees into fir trees thereby promoting succession in ecosystems.^[23]

The ectomycorrhizal fungus Laccaria bicolor has been found to lure and kill springtails to obtain nitrogen, some of which may then be transferred to the mycorrhizal host plant. In a study by Klironomos and Hart, Eastern White Pine inoculated with L. bicolor was able to derive up to 25% of its nitrogen from springtails.^[24][25]

Ericoid mycorrhiza

Ericoid mycorrhizas are the third of the three more ecologically important types, They have a simple intraradical (grow in cells) phase, consisting of dense coils of hyphae in the outermost layer of root cells. There is no periradical phase and the extraradical phase consists of sparse hyphae that don't extend very far into the surrounding soil. They might form sporocarps (probably in the form of small cups), but their reproductive biology is little understood. ^[26]

See also

Fungi portal

• Glomeromycota
• Basidiomycota
• Ascomycota
• Mucigel - substance that creates symbiotic environment for fungi
• Rhizobia - bacteria which fix nitrogen in legumes

References

External links

• Orchid mycorrhiza extraction & culture on the CultureSheet
• Mycorrhizal Associations: The Web Resource Comprehensive illustrations and lists of mycorrhizal and nonmycorrhizal plants and fungi
• Mycorrhizas – a successful symbiosis Biosafety research into genetically modified barley
• International Mycorrhiza Society International Mycorrhiza Society