What is mycorrhiza?

• Mycorrhiza (plural, -ae or –as) - Greek - means “fungus+ root”

• A symbiotic association between a plant and nonpathogenic fungus that colonizes the cortical tissue of roots during periods of active plant growth.

• These symbioses are characterized by movement of nutrients - carbon flows to the fungus and inorganic nutrients move to the plant, thereby providing a critical linkage between the plant root and soil.

• Few higher plants do not form mycorrhizal associations, 10-20% including some aquatic vascular plants and members of the Brassicaceae, Cyperaceae, and Juncaceae

History of mycorrhiza

• Fossil records show mycorrhizae have been around for 460 million years old. In 460 million year-old fossils the underground parts have mycorrhizal fungi very similar to those found in today’s plants

• Robert Hartig (1840) first illustrated the mycorrhizal fine roots of a pine, but did not recognize them as a separate being.

• S. Reissek (1847) described and recognized fungal cells associated with orchids.

• A. B. Frank (1885) described fungus-root structure and showed increased growth in plant when mycorrhizal.

History of mycorrhiza

Earliest experiments - 1880’s German botanist Frank - grew pines in sterilized and non-sterilized soil - described mycorrhizae on roots - “big plant/little plant syndrome”

Type of mycorrhizae

• There are at least seven major types of mycorrhizal associations, Vesicular-arbuscular mycorrhizae (VAM), ectomycrorrhizae, ectendomycorrhizae, ericoid mycorrhizae, orchid mycorrhizae, arbutoid mycorrhizae, and monotropoid mycorrhizae.

• Most common association is the VAM symbiosis (sometimes called AM),and ectomycorrhizae areprobably the next most common type symbiosis.

Type of mycorrhizae

• The fungal mycelia extend inward, between root cortical cells, to form a network (Hartig net) and outward into the surrounding soil. Usually the fungal hyphae also form a mantle on the surface of the root.

• Hyphae do not penetrate into cells but contact with roots is very close and metabolites are transferred in both direcions.

• Some ectomycorrhizae produce large above ground sporocarps or mushrooms which facilitate dispersal of spores along with underground fruiting bodies.

• Found on many woody plants ranging from shrubs to forest trees. Host plants belong to the families Pinaceae, Fagaceae, Betulaceae and Myrtaceae and a few others but no grasses.

Ectomycorrhizae (EM)

Type of mycorrhizaeEctomycorrhizae (EM)

pine - two roots joined by sheath fungal sheath around pine root

pine - Hartig net - fungus around cells

thick mantle (M) and Hartig net hyphae (arrows).

Type of mycorrhizaeEctomycorrhizae (EM)

Type of mycorrhizae

• Mycorrhizal association with intracellular penetration of the host root cortical cells by the fungus as well as outward extension into the surrounding soil.

• The fungus initially grows between cortical cells, but soon penetrates the host cell wall and grows within the cell.

• Common in must species of herbaceous angiosperms, flowering plants, annual and perennial crops, and many of the gymnosperm genera.

Endomycorrhizae

Type of mycorrhizae

• Consists primarily of vesicular-arbscular mycorrhizae (VAM), especially common in grasslands.

• Vesicles are rounded intercellularstructures which act as storageorgans

• Arbuscles are branched intercellular structures, which serve as the site of nutrient transfer

between the fungus and the root.

Endomycorrhizae

Spores

• Swollen structures with one or more subtending hyphae that form in the soil or in roots. Spores usually develop thick walls, which often have more than one layer. They can function as propagules. Spores of VAM fungi are sometimes called chlamydospores or azygospores.

• The tiny spores of EM fungi are usually borne in above ground (i.e. mushrooms) or below ground (i.e. truffles) reproductive structures. They are dispersed by wind, water, and animals that may eat the fungal fruiting body.

• The much larger spores of VAM fungi are often borne directly in the soil, formed inside the root, or clustered in various ways with other spores.VAM spores are less likely than EM spores to be dispersed by wind.

• More often, they are moved with soil or sediment in water, or carried by earthworms and other vectors.

Hyphae

• hyphae: thread or filament forming the mycelium of fungus.

• The hyphae of mycorrhizal fungi can make most of the soil's microbial biomass.

• The hyphae of VAM fungi are usually recognizably distinct from other kinds of soil fungi. These hyphae are relatively large, with smaller side branches that arise from characteristic bumps and angles on the main trunk hypha. The hyphae have few cross-walls (septa) and the larger ones persist in the soil long after they are dead.

• Hyphae of EM fungi and some others may aggregate to form coarse hyphal strands (rhizomorphs). These can carry water and nutrients a considerable distance toward the plant.

Mycorrhizae inoculum

• Mycorrhizal inoculum is material that carries mycorrhizal fungi in a usable form to the intended host plants.

• In the case of VAM fungi, inoculum may be spores, hyphae, and colonized root fragments.

• The quality of mycorrhizal inoculum is not easy to assess, but the best way is an expression of the number of propagules per unit volume of material. Propagules include spores, hyphae, and root fragments.

Functions of Mycorrhizal

• Fungus uses organic nutrients (including sugars) produced by plant ECM are not obligate mutualists but VAM are.

• Fungi impart benefits to plant by increasing: nutrient absorption, especially phosphorus, water uptake, tolerance of harsh environmental conditions, including polluted environments

(e.g. acts as shield against acidity, elemental toxicity and pathogens).

Factors that can affect of mycorrhizal formation

• > pH of soil----------------------------------------(-)

• > soil moisture------------------------------------(-)

• > soil depth----------------------------------------(-)

• > amount of above ground plant cover--------(+)

• > grazing-------------------------------------------(-)

• > pesticides----------------------------------------(-)

• > soil fertility--------------------------------------(-)

Benefits of Mycorrhizae

• Increased uptake of nutrients– Hyphae explore the soil for nutrients, increase surface area for nutrient

absorption transport them back to the plant.

– The nutrients P, Zn, C, N, Cu and S have been shown to be absorbed and translocated to the host by mycorrhizal fungi

• Increased rootlet size and longevity– Mycorrhizal plants have larger roots than nonmycorrhizal plants

regardless of whether mycorrhizal fungi are present.

Benefits of Mycorrhizae

• Water relations– Hyphae explore the soil for water and increase surface area for

absorption– Some mycorrhizae alter the plant’s physiology, increasing stomatal

resistance, resulting in less water loss.

• Improved growth rate

Benefits of Mycorrhizae

Mycorrhizal vs non-mycorrhizal junipers - 6 mo old

Mycorrhizal and non-mycorrhizal pines - grown in sand with low P content

Benefits of Mycorrhizae

• Tolerant of harsh conditions

– fungi are more tolerant of acidity, elemental toxicity and high soil temperatures than are higher plants and able to, in some cases (ectomycorrhizae), shield the root from these condition.

– Lower levels of heavy metals generally found in mycorrhizal plants than nonmycorrhizal plants.

Benefits of Mycorrhizae

• Increased seedling survival– Mycorrhiza promotes plant survival, whether new seedlings or

out-planted container stock.

– Survival of inoculated plants can be up to five times the survival of uninoculated plants.

– Improved survival is no doubt due to a combination of mycorrhizal benefits, including faster growth to help overtop weeds, protection from pathogens, and improved drought tolerance.

Mycorrhizae and Plant diversity

• Biodiversity of belowground fungal symbionts increases biodiversity of above ground plants

• Increased access to nutrients becomes restricted under competitive conditions

• Differences in functional capacity of a specific fungus-plant combination appear to explain the effect

• In ecosystems, increased functional capacity allows one plant species to perform better than others

• Restored plant communities have been found to be more diverse when mycorrhizal fungi are present when both inoculated and uninoculated areas receive the same seed mix.

Mycorrhizae and Plant diversity

Basis for fungal species richness on plant biodiversity and production

No symbionts

Mycorrhizae and Plant diversity

Basis for fungal species richness on plant biodiversity and production

No symbionts One symbiont

Increasing diversity

Mycorrhizae and Plant diversity

Basis for fungal species richness on plant biodiversity and production

No symbionts One symbiont Two symbionts

Increasing diversity

Increasing productivity

Basis for fungal species richness on plant biodiversity and production

No symbionts One symbiont Two symbionts Four symbionts

Increasing diversity

Increasing productivity

Mycorrhizae and Plant diversity

Effects of mycorrhizal fungal species richness on plants

Van der Heijden et al. 1998

Mycorrhizae and pathogens

• Mycorrhizal fungi have long been known to help defend roots against soil-borne disease.

• Mycorrhizae promote beneficial bacteria that may be directly responsible for protection against root pathogens.

• Mycorrhizal fungi may reduce the incidence and severity of root diseases. The mechanisms proposed to explain this protective effect include:

– development of a mechanical barrier-especially the mantle of the EM-to infection by pathogens

– production of antibiotic compounds that suppress the pathogen, – competition for nutrients with the pathogen, including production of siderophores – induction of generalized host defense mechanisms.

Mycorrhizae and pathogens

Disadvantage of Mycorrhizae

• When the nutrient levels in the soil are so low and the fungus cannot extract extra nutrients and no benefit accrues to the plant in return for carbon transferred to fungus parasitism may be occur.

• Sometimes plant growth suppression has been attributed to mycorrhizal colonization, but usually this occurs only under unusual circumstances that affect plant photosynthesis (low light and cold temperatures) or high-phosphorus

Mycorrhizae and Succession

• Many below-ground processes, including plant-microbial interactions influence the rate and direction of succession, whether primary or secondary.

• Formation of mycorrhizae as one of the ten major successional processes that determines change in species composition during succession.

• Mycorrhizal fungi alter succession and community composition by altering the relative success of plants, both in the seedling stage and during later development

Mycorrhizae and Succession

• Succession on arid and semiarid lands is influenced in part by the relative density of mycorrhizal inoculum.

• The nonmycorrhizal of the early stages of succession are initially replaced by facultative mycotrophs and finally by obligate mycotrophs.

Mycorrhizae less common in the early seral stages More prevalent in low fertile sites and harsher environments

Many climax species are dependent on mycorrhizae

Mycorrhizae and Soil Structure

• Soil structure is tremendously important in the health of the vegetation, because it facilitates water infiltration, aeration, root growth, and movement of soil animals.

• Structure refers to the arrangement of soil particles into aggregates. Mycorrhizal fungi are key players in soil structure

• by their hyphae. Pore space allows ready movement of water, air, roots, and animals. Soils without pore space are very unfriendly to plant growth.

• Both ECM and VAM fungi facilitate soil structure by binding soil aggregates together, to each other, and to plant roots

• Soil bacteria cement soil particles and contribute importantly to soil structure, and mycorrhizal hyphae help supply some of the fuel that keeps the soil bacteria alive

Mycorrhizae and Weeds

• Healthy native ecosystems are resistant to weed "ruderal" invasion; an effect promoted by vigorous, mycorrhizal native plants.

• Many weeds do not need to become mycorrhizal, and can grow in places where natives cannot become mycorrhizal. In such locations, weeds win the competitive struggle greatly incapacitated natives.

• Weeds usually require relatively higher soil fertility than native species. Healthy, mycorrhizal natives rapidly remove mobile nutrient ions from the soil, making the site inhospitable to weeds.

• Some weeds react to mycorrhizal fungi as they would to root pathogens. Seedlings of Russian thistle, an important weed in the western US, soon die if the soil is full of mycorrhizal hyphae. Other weed species that are intolerant of mycorrhizal fungi may be discovered in the future.

Management of Mycorrhizae

• Most of the mycorrhizal fungi are in the top 15 cm of the soil

• Preserve topsoil and microbial activity

• Do not use chemicals that kill fungi

• Do not over-fertilize

• Keep native plants