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Mycorrhiza

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A mycorrhiza (Greek for fungus roots; typically seen in the plural forms mycorrhizae or mycorrhizas) is a symbiotic (occasionally weakly pathogenic) association between a fungus and the roots of a plant.[1] In a mycorrhizal association the fungus may colonize the roots of a host plant either intracellularly or extracellularly.

This mutualistic association provides the fungus with relatively constant and direct access to mono- or dimeric carbohydrates, such as glucose and sucrose produced by the plant in photosynthesis.[2] The carbohydrates are translocated from their source location (usually leaves) to the root tissues and then to the fungal partners. In return, the plant gains the use of the mycelium's very large surface area to absorb water and mineral nutrients from the soil, thus improving the mineral absorption capabilities of the plant roots.[3] Plant roots alone may be incapable of taking up phosphate ions that are immobilized, for example, in soils with an basic pH. The mycelium of the mycorrhizal fungus can however access these phosphorus sources, and make them available to the plants they colonize.[4] The mechanisms of increased absorption are both physical and chemical. Mycorrhizal mycelia are much smaller in diameter than the smallest root, and 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. Mycorrhizae are especially beneficial for the plant partner in nutrient-poor soils.

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

Mycorrhizae form a mutualistic relationship with the roots of most plant species (although only a small proportion of all species have been examined, 95% of all plant families are predominantly mycorrhizal).[5]

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. The absence of mycorrhizal fungi can also slow plant growth in early succession or on degraded landscapes.[6]

Occurrence of mycorrhizal associations

The Rhynie chert is, at around 400 million years old, the earliest fossil assemblage yielding plants preserved in sufficient detail to detect mycorrhizae - and they are indeed observed, at this early time in the evolution in land plants.[7]

Mycorrhizae are present in 92% of plant families (80% of species)[8], with arbuscular mycorrhizae being the ancestral and predominant form,[8] and indeed the most prevalent symbiotic association found in plants at all.[2] The structure of arbuscular mycorrhizae has been highly conserved since their first appearance in the fossil record,[7] with both the development of ectomycorrhizae, and the loss of mycorrhizae, evolving convergently on multiple occasions.[8]

Types of mycorrhizae

Endomycorrhizal wheat
Ectomycorrhizal Eucalyptus
An ericoid mycorrhizal fungus isolated from Woollsia pungens.[9]

The two most common types of mycorrhizae are the ectomycorrhizae and the endomycorrhizae (more commonly known as arbuscular mycorrhizae). The two groups are differentiated by the fact that the hyphae of ectomycorrhizal fungi do not penetrate the cell wall of the plant's root cells, while the hyphae of arbuscular mycorrhizal fungi penetrate the cell wall.

Endomycorrhizae

Arbuscular mycorrhizas, or AM (formerly known as vesicular-arbuscular mycorrhizae), are mycorrhiza whose hyphae enter into the plant cell walls, 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, which are typically associated with the roots of herbaceous plants, but may also be associated with woody plants. Fossil evidence[7] and DNA sequence analysis[10] suggest that this mutualism appeared 400-460 million years ago, when the first plants were colonizing land. Arbuscular mycorrhizae were likely to have been very helpful at that time, protecting plants from adverse conditions such as lack of water and nutrients.

Arbuscular mycorrhizal fungi are quite extraordinary organisms. They have been asexual for many million years. Unusually, individuals can contain many genetically different nuclei (a phenomenon called heterokaryosis).[11]

This type of association is found in 85% of all plant families in the wild, including many crop species such as the grains.[8]

The hyphae of arbuscular mycorrhizal fungi produce glomalin, which may be one of the major stores of carbon in the soil.

Ectomycorrhizae

Ectomycorrhizae, or EcM, typically form between the roots of woody plants and fungi belonging to the divisions Basidiomycota, Ascomycota, or Zygomycota. They are external mycorrhizae that form a cover on root surfaces and between the root's cortical cells.

Besides the mantle formed by the mycorrhizae, most of the biomass of the fungus is found branching into the soil, with some extending to the apoplast, stopping short of the endodermis.

Ectomycorrhizae are found in 10% of plant families, mostly the woody species, including the oak, pine, eucalyptus, Birch , dipterocarp, and olive families.[8]

Other forms of mycorrhizae

Arbuscular and ecto- mycorrhiza associating with plants belonging to the order Ericales form ericoid mycorrhiza, while some Ericales form arbutoid and monotropoid mycorrhiza. All orchids are mycoheterotrophic at some stage during their lifecycle and form orchid mycorrhiza with a range of basidiomycete fungi.

Research by Klironomos and Hart at the University of Guelph, Ontario has found that the mycorrhizal fungus Laccaria bicolor can lure springtails and kill them (probably with a toxin); the fungus utilises the nitrogen from the decaying springtails, and some of this nitrogen also becomes available to the plant that the fungus forms mycorrhizae on (in this study, the Eastern White Pine). Klironomos found that up to 25% of the plant nitrogen came from springtails or insects.[12][13]

See also

References

  1. ^ Kirk, P.M., P.F. Cannon, J.C. David & J. Stalpers 2001. Ainsworth and Bisby’s Dictionary of the Fungi. 9th ed. CAB International, Wallingford, UK.
  2. ^ a b Harrison MJ (2005). "Signaling in the arbuscular mycorrhizal symbiosis". Annu Rev Microbiol. 59: 19–42. PMID 16153162.
  3. ^ Selosse MA, Richard F, He X, Simard SW (2006). "Mycorrhizal networks: des liaisons dangereuses?". Trends Ecol Evol. 21: 621–628. PMID 16843567.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ Li H, Smith SE, Holloway RE, Zhu Y, Smith FA. (2006). "Arbuscular mycorrhizal fungi contribute to phosphorus uptake by wheat grown in a phosphorus-fixing soil even in the absence of positive growth responses". New Phytol. 172: 536–543. PMID 17083683.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ^ Trappe, J. M. 1987. Phylogenetic and ecologic aspects of mycotrophy in the angiosperms from an evolutionary standpoint"". Ecophysiology of VA Mycorrhizal Plants, G.R. Safir (EDS), CRC Press, Florida
  6. ^ Jeffries, P; Gianinazzi, S; Perotto, S; Turnau, K; Barea, J-M (2003). "The contribution of arbuscular mycorrhizal fungi in sustainable maintenance of plant health and soil fertility". Biol. Fertility Soils. 37: 1–16. http://cat.inist.fr/?aModele=afficheN&cpsidt=14498927.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  7. ^ a b c Remy W, Taylor TN, Hass H, Kerp H (1994). "4 hundred million year old vesicular-arbuscular mycorrhizae". Proc. National Academy of Sciences. 91: 11841–11843. PMID 11607500.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ^ a b c d e Wang, B. (2006). "Phylogenetic distribution and evolution of mycorrhizas in land plants" (PDF). Mycorrhiza. 16 (5): 299–363. doi:10.1007/s00572-005-0033-6. Retrieved 2008-01-21. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  9. ^ Midgley, DJ, Chambers, SM & Cairney, JWG. 2002. Spatial distribution of fungal endophyte genotypes in a Woollsia pungens (Ericaceae) root system. Australian Journal of Botany 50, 559-565
  10. ^ L Simon, J Bousquet, RC Lévesque, M. Lalonde (1993) Origin and diversification of endomycorrhizal fungi and coincidence with vascular land plants. Nature, 363, 67-69
  11. ^ Hijri M & Sanders IR. 2005. Low gene copy number shows that arbuscular mycorrhizal fungi inherit genetically different nuclei Nature 433:160-163
  12. ^ Fungi kill insects and feed host plants 24hourscholar.com
  13. ^ Klironomos, J. N. and Hart, M. M. 2001. Animal nitrogen swap for plant carbon. Nature, 410: 651-652.
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