Mycorrhiza

A mycorrhiza (Gk.,: fungus roots[1], pl mycorrhizae, mycorrhizas) is a symbiotic (generally mutualistic, but occasionally weakly pathogenic) association between a fungus and the roots of a vascular plant. [2]

In a mycorrhizal association, the fungus colonizes the host plants' roots, either intracellularly as in arbuscular mycorrhizal fungi (AMF), or extracellularly as in ectomycorrhizal fungi. They are an important component of soil life and soil chemistry.

Contents

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] The introduction of alien mycorrhizal plants to nutrient-deficient ecosystems puts indigenous non-mycorrhizal plants at a competitive disadvantage.[16]

Resistance to toxicity

Fungi have been found to have a protective role for plants rooted in soils with high metal concentrations, such as acidic and contaminated soils. Pine trees inoculated with Pisolithus tinctorius planted in several contaminated sites displayed high tolerance to the prevailing contaminant, survivorship and growth. One study discovered the existence of Suillus luteus strains with varying tolerance of zinc. Another study discovered that zinc-tolerant strains of Suillus bovinus conferred resistance to plants of Pinus sylvestris. This was probably due to binding of the metal to the extramatricial mycelium of the fungus, without affecting the exchange of beneficial substances.[16]

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.[17]

Mycorrhizae are present in 92% of plant families (80% of species),[18] with arbuscular mycorrhizae being the ancestral and predominant form,[18] 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,[17] with both the development of ectomycorrhizae, and the loss of mycorrhizae, evolving convergently on multiple occasions.[18]

Types of mycorrhiza

Arbuscular mycorrhizal wheat

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[17] 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.[18] 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

Ectomycorrhizal beech

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[18] 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 paper birch trees into Douglas-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]

The first genomic sequence for a representative of symbiotic fungi, the ectomycorrhizal basidiomycete Laccaria bicolor, has been published[26]. An expansion of several multigene families occurred in this fungus, suggesting that adaptation to symbiosis proceeded by gene duplication. Within lineage-specific genes those coding for symbiosis-regulated secreted proteins showed an up-regulated expression in ectomycorrhizal root tips suggesting a role in the partner communication. Laccaria bicolor is lacking enzymes involved in the degradation of plant cell wall components (cellulose, hemicellulose, pectins and pectates), preventing the symbiont from degrading host cells during the root colonisation. By contrast, Laccaria bicolor possesses expanded multigene families associated with hydrolysis of bacterial and microfauna polysaccharides and proteins. This genome analysis revealed the dual saprotrophic and biotrophic lifestyle of the mycorrhizal fungus that enables it to grow within both soil and living plant roots.

Ericoid mycorrhiza

An ericoid mycorrhizal fungus isolated from Woollsia pungens[27]

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. [28]

Ericoid mycorrhizae have also been shown to have considerable saprotrophic capabilities, which would enable plants to receive nutrients from not-yet-decomposed materials via the decomposing actions of their ericoid partners. [29]

See also

References

  1. ↑ AB Frank (1885) Über die auf Würzelsymbiose beruhende Ehrnährung gewisser Bäum durch unterirdische Pilze. Berichte der Deutschen Botanischen Gesellschaft 3, 128-145.
  2. ↑ 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.
  3. ↑ 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
  4. ↑ 4.0 4.1 Harrison MJ (2005). "Signaling in the arbuscular mycorrhizal symbiosis". Annu Rev Microbiol. 59: 19–42. doi:10.1146/annurev.micro.58.030603.123749. PMID 16153162. 
  5. ↑ Selosse MA, Richard F, He X, Simard SW (2006). "Mycorrhizal networks: des liaisons dangereuses?". Trends Ecol Evol. 21 (11): 621–628. doi:10.1016/j.tree.2006.07.003. PMID 16843567. 
  6. ↑ 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 (3): 536–543. doi:10.1111/j.1469-8137.2006.01846.x. PMID 17083683. 
  7. ↑ http://cropsoil.psu.edu/sylvia/mycorrhiza.htm
  8. ↑ http://www.biologie.uni-hamburg.de/b-online/e33/33b.htm
  9. ↑ http://www.springerlink.com/content/qnrwj34011g842p2/
  10. ↑ http://www.usask.ca/biology/kaminskyj/arctic.html
  11. ↑ http://aggie-horticulture.tamu.edu/Faculty/davies/research/mycorrhizae.html
  12. ↑ http://www.jstor.org/stable/2557434
  13. ↑ http://journals.cambridge.org/action/displayAbstract;jsessionid=5C15AC557F5360499373FD8790442EBB.tomcat1?fromPage=online&aid=162271
  14. ↑ Root fungi turn rock into soil Planet Earth Online 3 July 2009
  15. ↑ 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. 
  16. ↑ 16.0 16.1 David M. Richardson (2000). Ecology and biogeography of Pinus. London: Cambridge University Press. p. 336. ISBN 0521789109. 
  17. ↑ 17.0 17.1 17.2 Remy W, Taylor TN, Hass H, Kerp H (1994). "4 hundred million year old vesicular-arbuscular mycorrhizae". Proc. National Academy of Sciences 91 (25): 11841–11843. doi:10.1073/pnas.91.25.11841. PMID 11607500. 
  18. ↑ 18.0 18.1 18.2 18.3 18.4 Wang, B.; Qiu, Y.L. (2006). "Phylogenetic distribution and evolution of mycorrhizas in land plants". Mycorrhiza 16 (5): 299–363. doi:10.1007/s00572-005-0033-6. PMID 16845554. http://www.springerlink.com/index/X7151P60502078U1.pdf. Retrieved 2008-01-21. 
  19. ↑ Allen, Michael F. 1991. The ecology of mycorrhizae. Cambridge University Press, Cambridge. Harley, J.L. and S.E. Smith 1983. Mycorrhizal symbiosis (1st ed.). Academic Press, London.
  20. ↑ Peterson, R.L., H.B. Massicotte and L.H. Melville (2004) Mycorrhizas: anatomy and cell biology. National Research Council Research Press. ISBN 978-0-660-19087-7
  21. ↑ 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
  22. ↑ Hijri M & Sanders IR. 2005. Low gene copy number shows that arbuscular mycorrhizal fungi inherit genetically different nuclei Nature 433:160-163
  23. ↑ http://www.nature.com/nature/journal/v388/n6642/full/388579a0.html Nature 388, 579-582 (7 August 1997) Net transfer of carbon between ectomycorrhizal tree species in the field. Suzanne W. Simard, David A. Perry, Melanie D. Jones, David D. Myrold, Daniel M. Durall and Randy Molina
  24. ↑ Fungi kill insects and feed host plants BNET.com
  25. ↑ Klironomos, J. N. and Hart, M. M. 2001. Animal nitrogen swap for plant carbon. Nature, 410: 651-652.
  26. ↑ Martin, F. et al. 2008. The genome of Laccaria bicolor provides insights into mycorrhizal symbiosis. Nature 452: 88–92.
  27. ↑ 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
  28. ↑ Allen, Michael F. 1991. The ecology of mycorrhizae. Cambridge University Press, Cambridge. Harley, J.L. and S.E. Smith 1983. Mycorrhizal symbiosis (1st ed.). Academic Press, London.
  29. ↑ Read, D.J. and J. Perez-Moreno. 2003. Mycorrhizas and nutrient cycling in ecosystems - a journey towards relevance? New Phytologist (2003) 157:475-492.

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