Mycorrhizal fungi: These can be called a symbiont, associate, mycobiont, inhabitant, etc., but it is usually sufficient to call them fungi. Mycorrhizal fungi should not be called endophytes to avoid confusion with another major category of plant inhabiting fungi.¹

Colonization: Neutral term ‘colonisation’ is preferential to infection (implying disease) when describing mycorrhizal fungus activity and the resulting fungal structures can be defined as colonies.¹

Inoculum: Propagules of fungi capable of dispersing or initiating contact with plants.¹

Vascular plants: Higher plants with conducting elements for water and nutrients, differentiated leaves and roots, with a dominant sporophyte.¹

Roots: Plant organ responsible for nutrient uptake, mechanical support, storage, etc. that is usually subterranean.¹

Fungi: Members of the fungus kingdom are eukaryotic, heterotrophic organisms with a tubular body that reproduce by spores.¹

Minerals: The basic form of substances required for life (N, P, K, etc. excluding gases).¹

1.2. Definition of Mycorrhizas:

The name mycorrhizas which literally means fungus-root, invented by Frank (1885) as non-pathogenic symbiotic association between root and fungi. “Mycorrhizas are symbiotic associations essential for one or both partners, between a fungus (specialized for life in soils and plants) and a root (or other substrate-contacting organ) of a living plant, that is primarily responsible for nutrient transfer. Mycorrhizas occur in a specialized plant organ where intimate contact results from synchronized plant-fungus development.”¹

Table no. 1: Key characteristics of mycorrhiza

Fungus	Symbiosis	Plant
Soil inhabitant	Intimate contact at interface for nutrient transfer	Control of association
Plant inhabitant	Essential for one or both partners	Specialized organ
Specialized hyphae	Synchronized development	Root or stem

1.3. Some important fact about mycorrhiza:

1. The structure and development of mycorrhizal fungus hyphae is substantially altered in the presence of roots of host plants. These root-borne hyphae are distinct from hyphae which are specialised for growth in soil.¹

2. All mycorrhizas have intimate contact between hyphae and plant cells in an interface where nutrient exchange occurs.¹

3. The primary role of mycorrhizas is the transfer of mineral nutrients from fungus to plant. In most cases there also is substantial transfer of metabolites from the plant to fungus.¹

4. Mycorrhizas require synchronised plant-fungus development, since hyphae only colonise young roots (except orchid mycorrhizas and exploitative VAM).¹

5. Plants control the intensity of mycorrhizas by root growth, digestion of old interface hyphae in plant cells (AM, orchid), or altered root system form (ECM).¹

6. Roots evolved as habitats for mycorrhizal fungi. Mycorrhizas normally occur in roots, but can be hosted in stems in some cases (e.g. some orchids).¹

7. All mycorrhizas have intimate contact between hyphae and plant cells in an interface where nutrient exchange occurs.¹

8. The primary role of mycorrhizas is the transfer of mineral nutrients from fungus to plant. In most cases there also is substantial transfer of metabolites from the plant to fungus.¹

9. Mycorrhizas require synchronised plant-fungus development, since hyphae only colonise young roots (except orchid mycorrhizas and exploitative VAM).¹

10. Plants control the intensity of mycorrhizas by root growth, digestion of old interface hyphae in plant cells (AM, orchid), or altered root system form (ECM).¹

11. Roots evolved as habitats for mycorrhizal fungi. Mycorrhizas normally occur in roots, but can be hosted in stems in some cases (e.g. some orchids).¹

1.4. Types of mycorrhiza:

Mycorrhizas are commonly divided into ectomycorrhizae and endomycorrhizae. The two groups are differentiated by the fact that the hyphae of ectomycorrhizae 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.²

1.4.1. Endomycorrhiza:

Endomycorrhizae are variable and have been further classified as arbuscular, ericoid, arbutoid, monotropoid, and orchid mycorrhizae.³ 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 hyphae and the cell cytoplasm to facilitate the transfer of nutrients between them.

Arbuscular mycorrhizae are formed only by fungi in the division Glomeromycota. Fossil evidence⁴ and DNA sequence analysis⁵ 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.⁶ 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).⁷

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.

1.4.2. Ectomycorrhiza:

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 ^[6] 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.⁸

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.^9,10 The first genomic sequence for a representative of symbiotic fungi, the ectomycorrhizal basidiomycete Laccaria bicolor, has been published.¹¹ 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.

1.4.3. Ericoid mycorrhiza:

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.^[2]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.^[12]

1.5. Images of mycorrhiza:

1.5.1. Arbuscular mycorrhizas:

Fig.1. Part of a clover root (upper part of the image) naturally infected by an arbuscular mycorrhizal fungus. The image shows part of the external network of fungal hyphae, bearing several large (up to 1 mm) spores of the fungus.^[13]

Fig. 2. Higher magnification of a similar root treated with hot alkali to destroy the plant cell contents, then stained with trypan blue to the fungal structures. Some hyphae are seen radiating from the root surface; others are within the root tissues.^[13]

Fig. 3. Still higher magnification, showing the fungal hyphae which run longitudinally between the root cortical cells. These hyphae produce swollen vesicles in the root tissues, and tree-like branching structures (arbuscules, seen here as blue fuzzy areas) within individual root cells.^[13]

Fig.4. Very high magnification of two arbuscules within root cortical cells.^[13]

1.5.2. Ectomycorrhiza:

Fig. 5. Ectomycorrhizal roots. The terminal branches of the root system are highly modified - the roots are short and stumpy, covered with a mantle (sheath) of fungal tissue and there are few or no root hairs. The fungus takes over the normal nutrient-absorbing role of the root hairs.¹³

Fig. 6. The fungal mantle is less conspicuous than in the previous image, but the fuzzy appearance of the roots is due to many fungal hyphae growing from the mantle into the soil. Such roots are seen easily if the undecomposed, surface litter is scaped away from the forest floor to reveal the decomposing litter containing a mass of mycorrhizas and their fungal networks.¹³

Fig.7. Cross-section of a pine mycorrhiza, showing the substantial fungal sheath that encases the root. The section was stained to show phenolic compounds (red) that often are formed in pine roots in response to mycorrhizal infection. They might have a role in limiting the fungal invasion of the tissues.^[13^]

Fig. 8. Higher magnification of the sheath (left side) composed of a tightly packed fungal 'tissue'. From the inner side of the sheath, the fungus grows between the root cortical cells, forming a network termed the Hartig net. The section was stained to show phenolic compounds (red) that often are formed in pine roots in response to mycorrhizal infection. They might have a role in limiting the fungal invasion of the tissues. ^[13]

Fig. 9. Scanning electron micrograph of part of an ectomycorrhizal root tip, showing the multilayered fungal sheath (left) and extension of the Hartig net between some of the outer cortical cells of the root.^[13]

1.5.3. Mycelial cords:

Mycelial cords (also termed mycelial strands) are specialised differentiated structures consisting of linear aggregations of hyphae, usually bound together in an extracellular matrix and consolidated by hyphal fusions. They develop usually in response to nutrient stress and, in their mature regions, are composed of wide, empty "vessel hyphae" surrounded by narrower "sheathing hyphae". Mycelial cords are capable of conducting nutrients over long distances - for example, to channel nutrients from a hyphal network to a developing fruitbody, or to enable wood-rotting fungi to grow through soil from an established food base in search of new food sources. For ectomycorrhizal fungi they also can serve to channel water from the deeper, moister soil zones to the roots nearer the soil surface, and they can help to spread infection by growing from established clusters of mycorrhizas to uninfected parts of the root system. ^[13]

Fig.10. Mycelial cord of the ectomycorrhizal fungus Lactarius pubescens, growing across a nutrient-poor agar plate. Note that many hyphae are aggregated into a thick, cord-like structure, but the hyphae fan-out from this cord in search of nutrients at the margin of the agar colony.^[13]

Fig. 11. Scanning electron micrograph of a freeze-fractured mycelial cord of the ectomycorrhizal fungus Leccinum scabrum. This image shows both the surface of the mycelial cord with individual nutrient-absorbing hyphae extending from it (arrow) and a cross section of the cord. Some of the hyphae in the centre of the cord are wider than others, and presumably serve as vessel hyphae (vh) for conducting water or mineral nutrients. Narrower sheathing hyphae (sh) surround the vessel hyphae. Image courtesy of Dr Frances Fox [see Fox, F. M. (1987) Transactions of the British Mycological Society 89, 551-560]^[13]

Fig. 12. Transmission electron micrograph of part of a mature region of a mycelial cord of Leccinum scabrum, showing empty vessel hyphae (vh) with extremely thick walls and narrower sheathing hyphae (sh), some of which have protoplasmic contents. All of these hyphae are embedded in an "extracellular" matrix which provides cohesion to the mycelial cord. Image courtesy of Dr Frances Fox [see Fox, F. M. (1987) Transactions of the British Mycological Society 89, 551-560]^[13]

1.5.4. Orchid Mycorrhiza:

Fig. 13. Cross-section of the outer part of the protocorm of an orchid, Neottia, stained to reveal the masses of fungal hyphae (intense red staining) in the outer cortical cells of the protocorm^.[^13]

Fig. 14. Part of a section at much higher magnification. The cells of the orchid are filled with coils of fungal hyphae but, significantly, the plant cells are still alive and they contain nuclei. The fungal coils will only last a few days or weeks before they are digested (those in the nucleate cell on the right appear to be degenerate) and the process of invasion and digestion will begin again. ^[13]

1.6. Categories of Mycorrhizal Associations

Consistent definitions of mycorrhizal associations are required for accurate communication of data. The flowchart below groups similar types of mycorrhizas together using categories regulated by the host and morphotypes caused by different fungi. Categories and subcategories are defined in the subsequent table no. 2 ^[1]

Table no. 2. Categories of Mycorrhizal Associations

Association

Categories

Morphotypes

Arbuscular
Mycorrhizal
Associations

Ectomycorrhizal
Associations

1.6.1. Hierarchical Classification Scheme for Mycorrhizal Associations (Brundrett 2004)

Table no. 3. Hierarchical Classification Scheme for Mycorrhizal Associations

No.	Category	Definition	Hosts	Fungi
1	Arbuscular mycorrhizas ^[1]	Associations formed by Glomeromycotan fungi in plants that usually have arbuscules and often have vesicles (also known as vesicular-arbuscular mycorrhizas, AM, VAM).	Plants	Glomeromycota
1.1	Linear VAM^[1]	Associations that spread predominantly by longitudinal intercellular hyphae in roots (formerly known as Arum series VAM).	Plants	As above
1.2	Coiling VAM^[1]	Associations that spread predominantly by intracellular hyphal coils within roots (formerly known as Paris series VAM).	Plants	As above
1.2.1	Beaded VAM^[1]	Coiling VAM in roots, where interrupted root growth results in short segments divided by constrictions.	Woody plants	As above
1.2.2	Inner cortex VAM^[1]	Coiling VAM with arbuscules in one layer of cells of the root inner cortex.	Plants	As above
1.2.3	Exploitative VAM^[1]	Coiling VAM of myco-heterotrophic plants, usually without arbuscules.	Achlorophyllous plants	As above
2	Ecto-mycorrhiza (ECM)^[1]	Associations with a hyphal mantle enclosing short lateral roots and a Hartig net of labyrinthine hyphae that penetrate between root cells.	Hosts	Higher fungi (asco-, basidio- and zygo- mycetes)
2.1	Cortical^[1]	Hartig net hyphae penetrate between multiple cortex cell layers of short roots	Most are gymnosperm trees	As above
2.2	Epidermal^[1]	Hartig net fungal hyphae are confined to epidermal cells of short roots	Angiosperms (most are trees)	As above
2.2.1	Transfer cell^[1]	Epidermal Hartig net with transfer cells (plant cells with wall ingrowths)	Pisonia (Nyctaginaceae). See Peterson et al. 2004 for others	Tomentella spp. in Pisonia (Chambers et al. 2005)
2.2.2	Monotropoid^[1]	Exploitative epidermal ECM of myco-heterotrophic plants in the Ericaceae where individual hyphae penetrate epidermal cells.	Ericaceae (Monotropa, Pterospora, Sarcodes)	Basidiomycetes
2.2.3	Arbutoid^[1]	ECM of autotrophic plants in in the Ericaceae where multiple hyphae penetrate epidermal Hartig net cells.	Ericaceae (part only)	Basidiomycetes
3	Orchid^[1]	Associations where coils of hyphae (pelotons) penetrate within cells in the plant family Orchidaceae.	Hosts	Most are basidiomycetes in Rhizoctonia alliance
3.1	Orchid Root^[1]	Associations within a root cortex.	Orchidaceae	As above
3.2	Orchid Stem^[1]	Associations within a stem or rhizome.	Orchidaceae	As above
3.3	Exploitative Orchids^[1]	Associations of myco-heterotrophic orchids.	Orchidaceae (fully or partially achlorophyllous)	Orchid, ectomycorrhizal, or saprophytic fungi
4	Ericoid^[1]	Coils of hyphae within very thin roots (hair roots) of the Ericaceae.	Ericaceae (most genera)	Most are Ascomycetes
5	Sub- epidermal ^[1]	Hyphae in cavities under epidermal cells, only known from an Australian monocot genus.	Thysanotus spp. (Laxmaniaceae)	Unknown

1.7.

Symbiosis and Mutualism

The terms symbiotic and mutualistic have been used interchangeably to describe mycorrhizal associations and parasitic fungi have also been called symbiotic, but many scientists now only call beneficial associations symbiotic (Lewis 1985, Paracer and Ahmadjian 2000). Symbiosis is defined broadly as “two or more organisms living together” and in most cases both partners benefit (Lewis 1985). There are many types of symbiosis evolving different combinations of plants, fungi, microbes and animals. Only plant-fungus associations are considered in detail here, but several others are illustrated below. ^[1]Fungal symbioses have been defined as “all associations where fungi come into contact with living host from which they obtain, in a variety of ways, either metabolites or nutrients” (Cook 1977). However, this definition excludes mycorrhizal associations of myco-heterotrophic plants, where plants are nutritionally dependant on fungi (Brundrett 2004). Only the broadest definition of symbiosis - “living together of two or more organisms”, applies universally to mycorrhizal associations (Lewis 1985, Smith and Read 1997, Brundrett 2004).^[1]

1.7.1. Plant-Fungal Symbioses

Mycorrhizas are the most important type of symbiotic plant-fungus associations, but there are a wide diversity of other associations between plants and fungi, as illustrated in the diagram below. The relationship between mycorrhizas and other types of plant-fungus associations, such as parasitic or endophytic associations, are also shown below.^[1]

Fig. 15. Plant-Fungal Symbioses.

Fig. 16. The vertical axis is a continuum of fungal harm or benefits The horizontal axis is a plant harm-benefit continuum.^[1]

Fig. 17. Fungus benefits are linked to plant benefits in balanced mycorrhizas.Obligate associationsrequire greater investment from both partners than facultative mycorrhizas.^[1]

Fig.18. Exploitative mycorrhizas (myco-heterotrophs) are parallel to the vertical axis - plant benefit occurs at expense of fungi^[^1]

Fig. 19.Parasitic plant-fungal associations are those where fungal benefits are linked to plant harm.^[^1]

Fig. 20. Endophytic plant-fungus associations (no plant harm or benefit).^[1]

Fig. 21. Other categories of plant-fungus interactions include antagonism of fungi by plants or plants by fungi (causing harm to another organism without gaining direct benefits).^[1]

1.8. 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).^[14]

1.8.1. 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.^[15] 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 absorptive capacity for water and mineral nutrients (due to comparatively large surface area of mycelium: root ratio), thus improving the plant's mineral absorption capabilities.^[16]

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.^[17]

1.8.1.1. 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 ^[18]). Mycorrhizae are especially beneficial for the plant partner in nutrient-poor soils.^[19]

1.8.2. Disease resistance

Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogens,^[20],[21] and are also more resistant to the effects of droug^{[22],[23],[24].]}

1.8.3. 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.^[25] The absence of mycorrhizal fungi can also slow plant growth in early succession or on degraded landscapes.^[26] The introduction of alien mycorrhizal plants to nutrient-deficient ecosystems puts indigenous non-mycorrhizal plants at a competitive disadvantage.^[27]

2.8.4. 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.^[27.]

2. ROLE OF MYCORRHIZA:

2.1. Mineral nutrition

2.1.1 Phosphorus

The major role of VA-fungi is to supply infected plant roots with phosphorus, because phosphorus is an extremely immobile element in soils.Even if phosphorus was added to soil in soluble form soon, it becomes immobilized as organic phosphorus, calcium phosphates, or other fixed forms^[29][30].VA-fungiare known to be effective in increasing nutrient uptake, particularly phosphorus and biomass accumulation of many crops in low phosphorus soil ^[31]. Several investigators indicated that there is a beneficial effect of VA-fungi inoculation on nutrient uptake and on plant growth especially in sterilized soils ^{[32],[33].[34][35]}.In white clover (Trifolium repens L.), mycorrhizal inoculation doubled the concentration of phosphorus in shoots and roots of infected plants and increased their dry weight ^[36]. Also Al-Karaki et al., ^[37].indicated that shoot dry matter, shoot phosphorus and root dry matter were higher for mycorrhizal infected wheat (Triticum aestivum L.). On the other hand, mycorrhizal infection has been shown depress plant growth in soils with optimum phosphorusnts availability, these effect were attributed to competition for carbon between the host plant and the mycorrhizal for carbon between the host plant and the mycorrhizal fungi^[31].

2.1.2. Nitrogen and micronutrients

The enhanced effect of VA-fungi on the uptake of nitrogen and micronutrient uptake may be attributed to two situations. In the first one is mycorrhizal hyphae act as extension to plant root, increasing root surface area and exploring larger soil volume, which will increase the chance of more micronutrient uptake. Mycorrhizal association with plant root may also enhance translocation between root and shoot of the infected plant, hence enhancing the plant growth ^[31]. At low phosphorus-levels in soil, mycorrhizae substantially increase copper and zinc contents of the shoot. However, it was found in case of soybean (Glysine max L.), grown in high phosphorsu-levels soils, the mycorrhizae decreases copper and zink contents of infected plants ^[38]. Peanut (Arachis hypogaea L.) plants grown in sterilized soil without VA-fungi inoculation developed visible symptoms of phosphorus and zinc deficiency. ^[39]

2.1.3. Water relationship

Although most of the work done with VA-fungi has concentrated on their effects in plant nutrition, there is an increasing interest also on drought resistance of mycorrhizal plants ^[40]. VA-fungi infection has been reported to increase nutrient uptake in water stressed plants ^[41], enable plant to use water more efficiently and to increase root hydraulic conductivity^[42]. Few studies however are available on the effect of water-stress on the fungi themselves, displayed by the number of spores in the soil and the root infection percentage.

2.2. Protection against pathogen

Few investigations were made about the importance of endomycorrhizal and ectomycorrhizal fungi in protecting host plants from phytopathogens.

2.3. VA-Mycorrhizal association with legume cropes

Legume crop are generally cultivated in poor environment,even recently bred cultivars are selected to grown in such a poor enviorment and associated with its Rhizobium and an associated microflora Legume crops have a high (P) requirement for nodule formation, nitrogen fixation and optimum growth. Mycorrhizal condition of legume crops Legume crops are generally cultivated in poor environments, even recently bred cultivars are selected to grow in such a found to increase its vegetative growth and seed yield in addition to improve nodulation on it’s root system ^[38] . Nair et al.,reported that higher level of VA mycorrhizal infection was beneficial for plant growth of cowpea (Vigna unguiculata L.) under field condition. Hamel and Smith ^[60] reported that mixture growth of both corn (Zea mays L.) and soybean plants was greatly enhanced when inoculated with mycorrhizal fungi.Although more N appeared to be transferred from soybean to corn when plants were mycorrhizal, growth enhancement was attributed mainly to a better phosphorus uptake by mycorrhizal plants. Jackson and Mason ^[29] found positive relationships among (P) availability, VA mycorrhizal infection and pod yield in groundnut (Arachis hypogaea L.). It was indicated that mycorrhizal colonization in several cowpea genotypes was host dependent and heritable ^{[61] .} Alloush ^[62] found that chickpea plants inoculated with mycorrhizal fungus Glomus versiforme had higher number of nodules, shoot phosphorus content, shoot dry weight and grainyield than uninoculated chickpea plants.

2.4. Effect of soil amendment with orgenic wastes on mycorhhizal colonization

The materials we refer as organic wastes are merely those which are not put to use in our existing technological system. Once we begin to use them, theym will no longer be called wastes and if they are in demand,we may even seek to increase their production. Organic wastes are really resources out of place. Farmers historically have applied animal manure and human wastes to the land, both treated and untreated, for crop production. Animal and crop plant wastes are different in their chemical and biological composition depending on the source of the material. Kale et al., ^[43] found that mycorrhizae in roots of a summer crop was 2.85% in soil previously received chemical fertilizers compared to 10% in the soil with half the recommended dosage of chemical fertilizers and organic matter (OM) amendment.Inoculation with VA-fungus did not significantly affect seed yield of pea (Pisum sativum L.) plants in soil which is rich in OM and phosphorus^. On the contrary, seed yield was significantly enhanced with VA-fungi inoculation in soil which is poor in OM and phosphorus ^[63]. In mycorrhizae treatments, sludge showed inhibition of the mycorrhizal infection. This inhibition was persistent and apparently due to suppression of mycorrhizal fungi by toxic levels of NH₄ ^[38]. Also, both VA mycorrhiza spore density and root colonization were found to be higher under wastewater irrigated old field soils than in non-irrigated ^[44]. Large quantities of olive mill by-products are obtained when oils are extracted after mechanical and chemical treatments of olive yields ^[45]. The olive milling industry by-products; solid portion known as (Jift) or the liquids called (Zebar) could be used as soil OM amendment as Jift material is a nitrogen rich organic waste ^[46]. Although there are high levels of phytotoxic compounds found in fresh Jift which may inhibit seed germination or reduce plant growth, it contains no chemical contaminates like heavy metals ^[47]. On the other hand, Al Sakit and Al-Momani ^[48] found a positive relationship between fresh Jift amendment, olive seedling growth and association with mycorrhizae. There are no previous reports about the influence of the olive mill by-products, jift and zebar on the VA-fungi and its ecology and significance to commercial legume crops.

2.5. Effect of soil sterlization and fungicide treatment on mycorrhizal infection

2.5.1. Fungicide treatment

The effects of biocide use on non target organisms, such as VA-fungi, are of interest to agriculture, since inhibition of beneficial organisms may counteract benefits derived from pest and disease control. Most of the fungicides which have been used to study their effect on VA mycorrhizal fungi were found to be deleterious, but some were quite compatible with VA mycorrhizal fungi. Sreenivasa and Bagyaraj ^[64] were studied the effect of nine fungicides on root colonization with VA mycorrhizal fungi and indicated that reduction from 10 to 20% of root infection percentages were recorded when the recommended level of fungicides were used. While some fungicides were significantly increased the percentage root colonization at half the recommended level. In an experiment studied the effect of different fungicides on VA-fungi infection and population, it was concluded that application of fungicide to soil reduced sporulation and the root length colonized by VA-fungus although interaction of VA-fungi and fungicide were observed to be highly variable depending on fungicide combination and on environmental conditions.

2.5.2. Solarization treatment:

Soil solarization was shown to be cost reducing, compatible with other pest management tactics, readily integrated into standard production systems and a valid alternative to preplant fumigation with methyl bromide ^[49]. It also reported that soil solarization induced better growth response in plants even when no pathogen is present in the soil ^[50]. In field experiment, it was reported that solarization of soil by covering it with transparent plastic sheets resulted in reduction or complete elimination of soil pathogens between 0 and 25 cm depth in soil covered for 30-60 days ^[51]. In other experiment it was observed that covering the soil with a clear plastic sheet resulted in complete elimination of endomycorrhizal fungi at 10 and 20 cm soil depths ^[52]. It was also reported that root nodulation, infection by mycorrhizal fungi and yield of cowpea were higher in plants grown in solarized soil when compared to control treatment without solarization ^[53]. Stapleton and DeVay ^[50] indicated that the beneficial response of plant growth to soil solarization might have resulted from the effects of better root nodulation, enhanced VA mycorrhizal association and the increased availability of some of the macro and micro nutrients in soil solution due to solarization.

2.5.3. Methyl bromide treatment

Although there was a grave environmental concern about the application of methyl bromide and it’s toxicity to mammals, it is still recommended for soil disinfection. Great reduction or complete elimination of all living organisms in the soil after methyl bromide gas fumigation of soil is well documented ^[62][49]. Soil disinfection by methyl bromide fumigation or steam is often used to eliminate soil-borne plant pathogens, but such treatments can reduce VA mycorrhizal fungi as well ^[35]. Several studies have indicated that plant stunting following soil fumigation treatments may be due to elimination of VA mycorrhizae ^[54],[55].

2.6. Effect of soil fertility on mycorrhizal infection

Most authors report extensive colonization to occur mainly in plants growing in soils of low fertility ^[56][57]. Field and greenhouse studies demonstrated that crops growing in nutrient-poor soils had higher levels of mycorrhizal colonization than crops growing in better soils ^[57]. Vesicular- arbuscular mycorrhiza inoculation in combination with phosphorus increased dry and fresh shoot weight, leaf area and leaf number of strawberry compared to application of phosphorus alone ^[58].

2.7. Disease resistence

Mycorrhizal plants are often more resistant to diseases, such as those caused by microbial soil-borne pathogens,^[20],[21] and are also more resistant to the effects of droug^{[22],[23],[24].]}

2.8. Resistence to toxicity

2.9. Effect on production of secondary metabolite (alkaloid, glycoside)

Mycorrhizal fungus has been shown to increase the amino acid, protein production in plant. Mycorrhizal fungus has been shown to increase the rate of secondary metabolites pathway and hence, production of secondary metabolite (alkaloid, glycoside) will also increases. These secondary metabolite (alkaloid, glycoside) are very important plant active constituent for prevention and treatment of various disorder.

3. CONCLUSION:

After discussion on mycorrhiza we can conclude that mycorrhiza is one of the important fungs which is mutualistically associated with plant. There are different types of mycorrhiza as we have seen; these are ecto-, endo-, orchid-, ericoid- etc. which are categories on the basis of their morphological association with plant. Fungus takes carbohydrates such as glucose, sucrose from plant which is important for their life and in return it increases the absorption of water, minerals (phosphorus, nitrogen etc) due to their high surface area of mycelium. It also increases plant resistance to toxicity, disease, and attack of pathogen. Another important role of mycorrhiza is that it increases the yield of product of plant like wheat and legume, as well as production of secondary metabolite such as alkaloid and glycosides.

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Received on 25.10.2010

Accepted on 12.11.2010

Research J. Science and Tech. 2(6): Nov. -Dec. 2010: 117-128

Raipur Institute of Technology, RITEE, Chhatauna, Mandir Hasaud, Raipur (C.G.) 492101 India