MYCORRHIZAE

The name mycorrhiza(e) from the Greek words: Mykes = fungus Rhiza = root

Mycorrhizae Mycorrhizae are modified roots consisting of symbiotic associations of fungi and roots. Mycorrhizae are not unusual and this plant-fungus symbiosis might have been one of the evolutionary adaptations that made it possible for plants to colonize land in the first place.

Symbiosis The symbiosis of mycorrhizae is mutualistic, meaning both parties benefit. The fungus, which cannot photosynthesize benefits from a hospitable environment and a steady supply of sugar donated by the host plant. The fungus provides increased surface area for water and nutrient uptake. It also secretes a growth hormone that stimulates roots to grow and branch. The fungus also produces antibiotics that help protect the plant from disease.

Mycorrhizae Mycorrhizae have a greater surface area than the roots, so they are able to absorb soil nutrients more efficiently and pass them onto the roots. Almost every plant has some type of mycorrhizae to enhance nutrient uptake. In return, the fungi get photosynthetic products from the plants.

Fungal Basics A fungus consists of fine, thread-like filaments called hyphae. Most hyphae are 2-10 µm, so they are so fine that they cannot be seen by the naked eye. A hyphal mass is referred to as a mycelium. Hypha

Septate VS Nonsepate Hyphae If the hyphae contain cross-walls, it is called septate hyphae. Non-septate hyphae contain no crosswalls. One of the ways fungi are classified is according to their hyphae. Septum

Mycorrhizae There are seven different types of mycorrhizae, and can be distinguished by their positions along two gradients, from a lack of penetrations of cortical cells (ectomycorrhizae) to penetration (vesicular-arbuscular and arbuscular mycorrhizae) and from enclosed (ectomycorrhizae) to open root (vesicular-arbuscular mycorrhizae). Any natural ecosystem normally contains a mixture of types of mycorrhizal associations. The two main types of mycorrhizae are: Endomycorrhizae and Ectomycorrhizae

Ectomycorrhizae In ectomycorrhizae, the mycelium form a dense sheath or mantle, over the surface of the root, but does not enter the root cell. Hyphae extend from the mantle into the soil. Fungal hyphae grow into the cortex of the root, making the root much thicker, shorter, and more branched. The hyphae may aggregate to form coarse hyphal strands or rhizomorphs.

Ectomycorrhizae Ectomycorrhizae do not form root hairs. Ectomycorrhizae are especially common in woods plants, including trees of the pine, spruce, oak, walnut, birch, willow, and eucalyptus families. The fungi often form mushrooms or truffles.

Endomycorrhizae Endomycorrhizae are much more common than ectomycorrhizae, and are found in over 90% of plant species.

Endomycorrhizae Endomycorrhizae are within the root cell. They do not have a dense mantle ensheathing the root, although microscopic hyphae extend into the soil. The hyphae extend inward by digesting small patches of the root cell wall.

Endomycorrhizae hyphae A hypha does not actually pierce the plasma membrane and enter the cytoplasm of the host cell, but instead grows into a tube formed by invagination of the root cell’s membrane. To the unaided eye, endomycorrhizae look like normal roots with root hairs.

Arbuscular Mycorrhiza Once the hyphae have penetrated the cell, some of the fungal hyphae become highly branched to form dense knot-like invaginations called arbuscles.

Vesicular-arbuscular mycorrhizae All but 2 genera of arbuscular mycorrhizae form vesicles. Vesicles are lipid filled, terminal swelling of hyphae. This primary function is storage.

AM Fungi The hyphae of AM fungi can make up to 70% of the soil’s microbial biomass. Under the microscope, the hyphae are usually recognizably distinct from other soil fungi. These septate hyphae are relatively large, with smaller side branches that arise from bumps and angle on the main trunk hyphae.

Other Mycorrhizae Arbutoid mycorrhizae-look like ectomycorrhizae and similar fungi, but are technically endomycorrhizae because the outer cortical cells and hyphae fill the cells Ectendomycorrhizae-related to ectomycorrhizae, except the fungus enters the root cells. Monotropoid mycorrhizae-found on certain plants without chlorophyll, share a fungus with a nearby tree and are parasites of the tree through the fungus. Ex: Indian Pipe

Other Mycorrhizae Ericoid mycorrhizae - fungus grows loosely over lateral “hair” roots of the host plant and the hyphae penetrate the single layer of cortical cells. Ex: blueberries Orchid mycorrhizae – for all or part of their life cycle, orchids are obligately dependent on their mycorrhizal partner.

Out of It There are four major plant families that do not form mycorrhizae: Amaranthaceae (Pigweed) Brassicaceae (Mustard) Chemopodiaceae (Goosefoot) Zygophyllaceae (Caltrop)

References 1. http://scitec.uwichill.edu.bb/bcs/mycorrhizas 1. http://scitec.uwichill.edu.bb/bcs/mycorrhizas 2. Campbell, N.A. and Jane B. Reese. 2002. Biology 6th Ed., Benjamin Cummings, San Francisco, CA. 3. Madigan, M.T., Martinko, J.M., Parker, J. 2003. Brock Biology of Microorganisms, 10th Ed., Pearson Education, Upper Saddle River, NJ