Some of the most important benefits of Mycorrhizal Fungi are as follows: 1. Uptake of Phosphorus 2. Uptake of Minor Elements 3. Role of AMF in Nitrogen Absorbtion 4. Enhanced Water Absorption 5. Improves Soil Structure 6. Biocontrol against Plant Pathogens 7. Synergistic Effect with PGPRs 8. Enhanced Tolerance against Abiotic Stress 9. Bioremediation 10. Reclamation of Wasteland.
Benefit # 1. Uptake of Phosphorus:
Mycorrhizal fungi greatly enhance the ability of plants to take up phosphorus and other nutrients that are relatively immobile and exist in low concentration in the soil solution. Arbuscular mycorrhizal fungi can play a significant role in P nutrition of crop, increasing total uptake and P use efficiency.
This may result in increased growth and crop yield. Due to faster uptake of phosphate by roots from soil, a phosphate depletion zone is formed around the roots. The extensive hyphal network of AMF extending out into the soil bridges this zone of nutrient depletion enabling the plant to have access to Phosphate beyond its rhizosphere.
The reasons for higher uptake of phosphate by arbuscular mycorrhizal fungi are:
i. Increase in surface absorbing area of the root.
ii. AMF has phosphatases activity which is able to mineralize organic P sources making it available to the host plant.
iii. Enhanced translocation of Phosphate into host plants through arbuscules.
iv. Efficient transfer of Phosphate to plant roots, effective utilization of P within plants and increased storage of absorbed P.
Benefit # 2. Uptake of Minor Elements:
It has become increasingly apparent that arbuscular mycorrhizal fungi can be important in tike uptake of other nutrients by the host plant. The uptake of copper, iron, potassium, calcium, magnesium and zinc has been reported to be enhanced under conditions of low soil nutrient availability. Zinc nutrition is most commonly reported as being influenced by the association.
The higher absorption by AMF is attributed to the uptake and transport by external hyphae due to wider exploration of soil by the extended fungal hyphae. The enhanced iron (Fe) uptake may be due to specific Fe chelators and the uptake from low concentration solutions is due to the siderophores formed by AMF. However, the uptake and concentration of manganese (Mn) in plants may not be affected by AMF and more often it may be lower in mycorrhizal plants.
AMF also affect the supply of organic mineral nutrients. Some litter-inhabiting mycorrhizal fungi produce proteases. They provide soluble amino compounds through hyphal networks into the host root. Recently, Glomus has been shown to transport the amino acids glycine and glutamine from soil into wheat plant.
Benefit # 3. Role of AMF in Nitrogen Absorbtion:
Nitrogen uptake and transport of inorganic and organic nitrogen in arbuscular mycorrhizal fungi colonized plants is high. However, AMF themselves do not fix nitrogen. They interact with other microbes and enhance the nitrogen availability to plants by stimulating the number and activity of free living/ symbiotic nitrogen fixing bacteria.
The different approaches adopted by the association for enhanced N-fixation are:
i. AMF act as prerequisite for effective nodulation in some legumes.
ii. They enhance effectiveness of Rhizobium in converting elemental nitrogen to ammonical form. AMF and Rhizobium exert a synergistic effect on the enhancement of rhizobial activity and efficiency. Dual inoculation with AMF and Rhizobium enhances rates of mycorhhizal colonization and nitrate reeducates activity as well as increased biomass of the plant.
iii. AMF facilitates the host plant to meet high energy demand for this energy intensive N-fixation process by providing adequate P-supply.
Benefit # 4. Enhanced Water Absorption:
The AMF association ensures better survival of plants in extreme dry conditions. They improve water uptake by altering the plant physiology to reduce the stress response to soil drought. AMF enhances the hydraulic conductivity of the roots and permeability of cell membrane to water. They also change leaf elasticity and improve water and turgor potential of leaf as well as increase root length and depth.
The probable reasons for the enhanced water and nutrient uptake rates by mycorrhizal plants are:
i. Extensive distribution of absorbing hyphal network.
ii. More favorable geometry of hyphae in comparison to roots.
iii. Greater surface area and faster extension rate.
iv. Increased functional longevity.
v. Chemical alteration in soil rhizosphere and altered rhizosphere microbial population.
vi. Favorable water uptake kinetics.
vii. Lower transpiration rates per unit leaf area.
Benefit # 5. Improves Soil Structure:
Mycorrhiza improves the physical, chemical and biological nature of the soil. Mycorrhizae cause soil particles to aggregate improving water infiltration. They increase soil air space and encourage other beneficial microbes to build humus, fix nitrogen and improve soil structure.
AMF produce glomalin, a glycoprotein, which along with the AMF hyphae form soil microaggregates and finally macroaggregates, making backbone for soil aggregation and soil stabilization. AMF also releases exudates in the soil promoting aggregate stability as well as favors growth of other microorganism.
Benefit # 6. Biocontrol against Plant Pathogens:
AMF have also been reported to be used as bio control agent against plant root pathogens. It controls plant diseases in an eco-friendly manner.
The mechanisms possibly involved are as follows:
i. Both AMF and root pathogens have same infection site, thus, early colonization of roots by AMF may compete out the root pathogen.
ii. Restricting the entry of root pathogens by facilitating cell wall thickening by lignification or callose deposition in the host plant.
iii. AMF colonized roots have actinomycetes which are antagonistic to many root pathogens.
iv. Better nutritional status of plants colonized by mycorrhiza than non-mycorrhizal plants confer resistance against the diseases.
v. Increased S-containing amino acids, sugars, phenolic compounds and enzymatic activities in mycorrhizal roots may also induce disease resistance in host plant.
AMF have also been shown to inhibit endoparasitic nematodes of plants. An antagonistic effect of AMF on nematodes has been reported in tomato by Glomus fasciculatum. In addition AMF may also inhibit plant parasitic nematodes by stimulating the parasitic microbes such as Pasteuria penetrans and Verticillium chlamydosporium against them.
Benefit # 7. Synergistic Effect with PGPRs:
AMF promotes the Plant Growth Promoting Rhizobacteria (PGPR) like Actinoplanes, Agrobacterium, Azospirillum, Azotobacter, Bacillus and Pseudomonas, etc. The mechanisms of promotion of PGPRs by AMF may include production of antibiotics or siderophores that suppress deleterious rhizosphere microorganisms (DRMO) and production of phytohormones or other growth enhancing compounds.
Often, dual inoculations of AMF with PGPR provide greater efficacy than applied singly. The high yield in Triticum aestivum by inoculation of Pseudomonas striate followed by Glomus fasciculatum, significantly increase the dry biomass.
Benefit # 8. Enhanced Tolerance against Abiotic Stress:
AMF play an important role in imparting tolerance against many abiotic stress caused by salinity, drought etc. Many AMF like Agaulospora, Glomus, Sclerocystis and Gigaspora have been found tolerant against extreme salinity.
Possible mechanisms by which AMF helps to overcome salinity stress include:
i. Exclusion of Na or equilibrium of Na/K rate.
ii. Better osmotic adjustments by accumulation of proline, glycine, betaine or soluble sugars.
iii. The stimulation of root growth.
iv. AMF may also sometimes alter root morphology improving water uptake.
AMF also enhance drought resistance in host by drought avoidance or drought tolerance by the following:
i. AMF improves the uptake of nutrients like N and P in water stressed conditions.
ii. They alter nonhydraulic root-to-shoot signaling of soil drying which is involved in the water stress.
iii. The mycorrhizal fungal hyphae increase the absorptive surface area of the roots.
iv. The hyphae penetrating the root cortex upto the endodermis provide a low resistance pathway for water movement across the root.
Benefit # 9. Bioremediation:
Many human activities lead to accumulation of toxic elements particularly heavy metals in the environment. They reduce the activities of soil microflora and microfauna. A number of plants and microorganisms including AMF show resistance to heavy metals by restricting metal uptake or by surviving in the presence of high internal metal concentration.
They can be used for cost effective and ecofriendly approach for removing the toxic elements and remediation of the contaminated land. AMF have shown to have potential to be used for bioremediation for heavy metal contaminated soils. Different strains of AM fungi show different sensitivity to metal toxicity.
i. Immobilization of metal compounds by AMF.
ii. Precipitation of polyphosphate granules in the soil and chelation of heavy metal in the AMF.
iii. Mycorrhizae reduce translocation of heavy metals to shoots by binding of the heavy metals to the cell walls of the fungal hyphae in roots.
Benefit # 10. Reclamation of Wasteland:
AMF have shown immense potential in recovery and reclamation of wastelands formed due to various natural and anthropogenic activities. Inoculation of wastelands with AMF improves the growth and survival of the plants on the disturbed lands.
AMF are thought to provide better survival of plants by enhancing the phosphorus and other micronutrients uptakes ability providing nutritional advantage to them. They also provide resistance to low soil pH, heavy metal toxicants and high temperature.
Other Beneficial Effects of AMF:
In addition to the above discussed roles of mycorrhiza, there are several other benefits that AMF imparts to the host plant.
They can be listed as:
i. AMF can be used as a bio hardening agent that can be used in micro propagation of woody plants.
ii. Colonization by mycorrhizal fungi has been shown to increase the vase life of cut ornamental flowers.
iii. AMF has also been reported to control weeds. They can change the nature and composition of weed communities.