Mycorrhiza

A mycorrhiza (typically seen in the plural forms mycorrhizae or mycorrhizas, Greek for fungus roots) is the result of a mutualistic association between a fungus and a plant. This mutualism takes place at the root level, where individual hyphae extending from the mycelium of a fungus colonize the roots of a host plant, either intracellularly or extracellularly.

This mutualistic association provides the fungus with a renewable source of food through access to fixed carbon (sugars) from the plant photosynthate. These are translocated to the root tissues from their source location (usually leaves), and then to the fungal partners. In return, the plant gains the use of the mycelium's tremendous surface area to absorb mineral nutrients from the soil. It is believed that the mycelia of mycorrhizal networks have better mineral absorption capabilities compared to plant roots. An example of this is the manner in which phosphate ions are tightly bound to iron oxides in many soils. Plant roots are generally incapable of accessing these phosphorus sources (which can be large and are termed sinks), yet mycorrhizal mycelia can access these forms of phosphorus. The mechanisms of increased absorption are both physical—mycorrhizal mycelia are much smaller in diameter than the smallest root hair and thus are able to explore a greater volume of soil and have a much larger surface area for absorption—and chemical—the cell membrane chemistry of fungi is different from that of plants. Mycorrhizae are especially beneficial for the plant partner in nutrient poor soils.

Furthermore, 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, perhaps due to the improved water uptake capability of the fungal hyphae.

The cytoplasmic streaming of the mycorrhizal hypha is a mechanism that facilitates the transfer of nutrients from the soil, at relatively remote distances from the root, to the root at rates far exceeding those that would be possible by osmotic flow alone. This has an energy cost to the fungus. In return, the mycorrhizal fungus is rewarded by the "payment" of nutrients in the form of sugars, starches, proteins and lipids from the plant roots. These nutrients, in turn, flow to the whole mycelial network through cytoplasmic streaming.

Mycorrhizas 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). 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.

Early evidence of mycorrhizal associations

The arbuscular mycorrhizal symbiotic relationship formed between plant roots and fungi is one of the most prevalent symbiotic associations found in plants. Some of the earliest fossil plants show evidence of mycorrhizas associated with them. Their structure has been highly conserved since they first colonized the soil about 400 million years ago. This date is important since it corresponds to the transition from aquatic to terrestrial life forms. It is hypothesized that this plant-fungi mutualistic partnership was vital in the colonization of land by plants.

Types of mycorrhizae

The two most common types of mycorrhizas are the ectomycorrhizas and the endomycorrhizas (more commonly known as arbuscular mycorrhizas). 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 mycorrhizas), are an example of a mycorrhiza that involves entry of the hyphae into the plant cell walls to produce 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 mycorrhizas 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 and DNA sequence analysis suggest that this mutualism appeared 400-460 million years ago, when the first plants were colonizing land. Arbuscular mycorrhizas 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. First they have been asexual for many million years and secondly, individuals can contain many genetically different nuclei (a phenomenon called heterokaryosis) .

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

Endomycorrhizal Wheat

Ectomycorrhizae

Ectomycorrhizas, or EcM, typically form between the roots of woody plants and fungi belonging to the divisions Basidiomycota, Ascomycota, or Zygomycota.

These 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.

Found in 10% of plant families, mostly the woody species, including the oak, pine, eucalyptus, dipterocarp, and olive families.

Ectomycorrhizal Eucalyptus

Other forms of mycorrhizae

An ericoid mycorrhizal fungus isolated from Woollsia pungens .

Arbuscular and ecto- mycorrhiza aside many 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 , .

See also

• Glomeromycota
• Basidiomycota
• Ascomycota

External links

• Mycorrhizas – a successful symbiosis Biosafety research into genetically modified barley