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Mycorrhizal Effects on Host Plant Physiology

The word "Mycorrhiza" is given to a mutualistic association between a fungus (Myco) and the roots (rhiza) of the plants. This ascociation is symbiotic because the relationship is advantageous for both organisms. The macrosymbiont (the plant) gains increased exploration of the soil (rhizo sphere) with the intrincate net of hyphae that increases the uptake of water and nutrients from the soil interphase. The microsymbiont (the fungus) uses the carbon provided by the plant for its physiological functions, growth and development.



MYCORRHIZA at TAMU The Nursery Crop Physiology lab at the Department of Horticulture has been working over the past 25 years with ectomycorrhizal and arbuscular mycorrhizal fungi under severe outplanting conditions (Texas lignite coal, strip mines, and highway revegetation sites). They have also characterized mycorrhizal associations in herbaceous and woody plant species and some of the mechanisms of enhanced drought, nutrient relations and temperature stress tolerance of mycorrhizal plants. Mycorrhizal plants can also be used to phytoremediate soil contaminated with petroleum and heavy metals such as chromium (Cr).

Benefits of Mycorrhiza: Enhanced plant efficiency in absorbing water and nutrients from the soil.

Reducing fertility and irrigation requirements.

Increased drought resistance

Increased pathogen resistance/protection.

Enhancing plant health and vigor, and minimizing stress.

Enhanced seedling growth. Enhanced rooting of cuttings.

Enhanced plant transplant establishment.

Improved phytoremediation of petroleum and heavy metal contaminated sites.

Mycorrhiza enhances Drought Resistance





Mycorrhizal Plants in Revegetaion site Advantages of Mycorrhiza: Produce more stress resistant plants during production and for landscape.

Potentially less pesticide usage.

Plants are more drought and nutrient tolerant in the landscape.

Potentially higher transplanting success and faster establishment.

Value added: Marketing landscape plants with greater stress tolerance.





Different Types of Mycorrhiza:

ARBUSCULAR MYCORRHIZA

The name arbuscular mycorriza (AM) comes from the structure that characterizes the symbiotic associations; an intracelular finely branched hyphae (haustoria) that is called "arbuscule" where the metabolic exchanges between the fungus and the host plant takes place. The others structures that are produced by some genera of arbuscular mycorrhiza are the vesicles for lipid storage and reproductive structures present in intracellular spaces.



Morton and Benny (1990) classified the Arbuscular Mycorrhiza in the Order Glomales and in two suborders: Glomineae with vesicles in the roots and formation of asexual spores (Chlamydospores) and the Order Gigasporineae with absence of vesicles in the roots and formation of auxiliary cells and azygospores.



The suborder Glomineae was divided into two families:



Glomaceae.-With the genus Glomus and Sclerocystis.



Acaulosporaceae.- With the genus Acaulospora and Entrophospora.



The suborder Gigasporineae family Gigasporiaceae are divided into two genera Scutellospora with a germination shield and Gigaspora with bigger spores.

Arbuscule





Vesicle ECTOMYCORRHIZAS

Ectomycorrhizal synbiosis is comon in the families Pinaceae, Fagaceae, Myrtaceae and Betulaceae.Most of these fungi are Ascomycetes and Basidiomycetes and there are several species of Zygomycetes. The colonization is characterized by an external coat of fungal hyphae that may completely cover the host root (fungal mantel) and the presence of hyphae between root cortical cells that constitute a structure called "Harting Net".

Pinaceae



ERICACEOUS MYCORRHIZAS

The symbiosis between mycorrhizas and the plants of the order Ericales range from ecto-types throught ect-endomycorrhiza to pure endomycorrhiza.



The hyphae in the root can penetrate cortical cells but no arbuscules are formed. The colonization is a specializad form of mycotrophy which enable the plants to survive in poor soils.



Three major forms of Ericaceous mycorrhiza have been described:



Ericoid- This symbiosis are found on plants such as Callanu and Rhododendron that have very fine root systems and grow in acid soil.



Arbutoid- These associations are found on Arbutus, Arctostaphylos and several species of the Pyrolaceae. The fungi involved in the association are basidiomycetes.



Monotropoid- The fungi colonize achlorophyllous plants in Monotropaceae.





Callanu





Rhododendron



ORCHID MYCORRHIZAS

The Orchids species are epiphytes, which are mainly distributed in the tropics and subtropics; these plants have very small seeds with little nutrient supply that are consumed at the first stage of development the plant becomes colonized by Mycorhizal fungus (Basidiomycetes), which supply the carbon and energy necesary for the development of the embryo.

In autotrophic species (photosynthetic) the microsymbiont provides the supply of minerals, but heterotrophics species are obligate mycotrophs because they depend on mycorrhiza for the carbon supply.



The colonization of orchids by mycorrhiza is a endoinfection characterized by the invagination of the host cell membrane and formation of hyphae coils within the cell.

Orchids



