Role of Mycorrhizae in Agriculture

 Mycorrhizas are mutualistic relationships formed by soil fungus and plant roots. Almost all plant species benefit from advantageous relationships formed by specialised root-inhabiting fungus. These fungi infiltrate feeder root tissues and generate modified roots known as mycorrhizae (fungus roots), which considerably improve nutrient and water absorption efficiency. In natural soils, most plants require mycorrhizae for normal growth and development. Members of the fungal kingdom (Zygomycetes, Ascomycetes, and Basidiomycetes) and most vascular plants are partners in this interaction. Arbuscular and ectomycorrhizae are the most frequent and ubiquitous of the seven varieties of mycorrhizae (arbuscular, ecto, ectendo, arbutoid, monotropoid, ericoid, orchidaceous mycorrhizae). 

Ectomycorrhizas: The hyphae of ectomycorrhizal fungi do not penetrate individual root cells in ectomycorrhizas. Ectomycorrhizas consist of a hyphal membrane, or mantle, that covers the root tip and a Hartig net of hyphae that surrounds the plant cells within the root cortex. In certain instances, the hyphae may also penetrate the plant cells; in this case, the mycorrhiza is referred to as an ectomycorrhiza.

Endomycorrhizas: Hyphae of endomycorrhizal fungi penetrate the cell wall and invade the cell membrane in endomycorrhizas. Hyphae enter plant cells and produce either balloon-like structures (vesicles) or invaginations with dichotomous branching (arbuscules). To facilitate the transmission of nutrients between the hypha and the cell cytoplasm, the structure of the arbuscules significantly increases the contact surface area between the hypha and the cell cytoplasm. 

 Role of Mycorrhiza in Agriculture : 

The widespread presence of various mycorrhizal fungi on crops and trees in natural ecosystems, as well as their effects on mineral nutrition and growth, led to the early recognition that different mycorrhizal symbioses could be manipulated to increase crop yields in various types of primary production systems. Most agricultural and horticultural plants, as well as some forest species, develop arbuscular mycorrhizas (AM), however other mycorrhizal forms are useful in certain circumstances: Ectomycorrhizas (ECM) are used in forest products and reforestation programmes. Ericoid mycorrhizas (ERM) for blueberries and orchid mycorrhizas for improved propagation, especially for conservation. All mycorrhizal species, as soil biota components, have the potential to be crucial in the restoration of areas damaged by mining or forestry operations. 

 (1) CONTROL PLANT DISEASE:

  Plant diseases can be managed by adding antagonists or manipulating indigenous bacteria to minimise disease-producing propagules. AM fungus and their interactions with plants help to decrease plant pathogen damage. With the rising cost of pesticides, as well as the environmental and public health risks connected with pesticides and pesticide-resistant diseases, AM fungus may provide a more viable and ecologically acceptable option for sustainable agriculture and forestry. The interactions between various AM fungus and plant diseases varied depending on the host plant and the cultural system. Furthermore, the protective impact of AM inoculation may be systemic as well as localised. AM fungal colonisation of the root often decreases the severity of plant pathogen-caused illnesses.Changes in root growth and morphology; histopathological changes in the host root; physiological and biochemical changes within the plant; changes in host nutrition; mycorrhizosphere effects that modify microbial populations; competition for colonisation sites and photosynthates; activation of defence mechanisms; and nematode parasitism by AM fungi may all contribute to reduced damage in mycorrhizal plants. The obligatory character of AM fungus, a lack of knowledge of the processes involved, and the significance of environmental conditions in these interactions are all obstacles to establishing biocontrol through the use of AM fungi. Cropping sequences, fertilisation, and plant-pathogen control practises all have an impact on AM fungal propagules in soil as well as their consequences on plants. To employ AM fungi in sustainable agriculture, it is necessary to understand the elements that impact AM fungus, such as fertiliser inputs, pesticide usage, and soil management practises. Furthermore, effective inoculants should be developed and used as bio-fertilizers, bio-protectants, and bio-stimulants in agriculture and forestry for long-term sustainability. 

 2. ACQUISITION OF PHOSPHATE

 The AM route for phosphorus acquisition begins with the absorption of Pi-free in the soil by fungal extra-radical hyphae. These fungal hyphae grow outside the host root system, allowing for phosphate absorption from a larger soil volume. Furthermore, AM colonisation induces physiological responses in the host, such as root growth and phosphatase production, which enhance phosphate absorption indirectly. 

 3. NITROGEN UPTAKE 

Nitrogen, like phosphate, is a critical limiting nutrient of plant development, particularly during cereal crop production. Nitrogen is present in the soil as ammonium (NH4+) and nitrate (NO3). Despite the fact that the concentration of ammonium in soil is 10-1,000 times lower than that of nitrate, ammonium is the preferred form of nitrogen absorbed when plants are subjected to nitrogen deficit or grown in water-logged or acidic soils. Mycorrhizal fungi's extra-radical mycelium may absorb ammonium nitrate and amino acids, and the importance of mycorrhizal nitrogen supply is becoming more well recognised. The bulk of nitrogen is assumed to be absorbed in the form of ammonium by a fungal partner. 

 4. SOIL HEAVY METAL STRESS ALLEVIATION 

Heavy metal elements such as Cu, Fe, Mn, Ni, and Zn are required for normal plant growth and development. These metals are needed for a variety of enzyme-catalyzed or redox processes, electron transport, and structural activities in nucleic acid metabolism. Metals such as Cd, Pb, Hg, and As, on the other hand, are not required and may be hazardous to plants even at extremely low quantities in soil. Specific absorption mechanisms deliver essential heavy metals into the root, but at high quantities, they also enter the cell via nonspecific transporters. Heavy metals interfere with important enzymatic functions at high concentrations by changing protein structure or substituting a necessary element, resulting in deficiency symptoms. As a consequence, toxicity symptoms such as chlorosis, growth retardation, browning of roots, effects on both photosystems, cell cycle arrest, and others are observed. AM fungus play an important role in the cleanup of polluted soil as buildup. AM fungi's exterior mycelium allows for larger investigation of soil volumes by expanding outside the root exploration zone, allowing access to bigger amounts of heavy metals contained in the rhizosphere. Metal concentrations are also higher in mycorrhizal structures in the root and fungal spores. AM fungus may be found in the soil of almost every habitat, even contaminated soils. They improve their hosts' nutritional condition by obtaining phosphate, micronutrients, and water and giving a part of it to them. Heavy metals are similarly taken up by the fungal hyphae and delivered to the plant. Soil chemical and physical variables impact the plant-fungus-heavy metal combo. In many circumstances, AM fungi act as a filtration barrier, preventing heavy metal ions from being transferred from roots to shoots. The protection and improved mineral absorption capabilities result in increased biomass production, which is required for effective cleanup. AM isolates found naturally in heavy metal-polluted soils are more metal-tolerant than isolates found in non-polluted soils and have been shown to colonise plant roots efficiently in heavy metal-stressed situations. 

 5. Increase in Agricultural yield:

Grain and oil seed crop growth has been an important part of the agricultural economy for years, and continuous increases in demand and prices have led farmers to use highly intensive agricultural management practises in order to increase crop productivity. Fertiliser usage is a typical agricultural management practise, yet it has a detrimental influence on ecosystems. Arbuscular mycorrhizal symbioses have been found to aid the growth of numerous field crops, owing to extensive hyphal network formation in soil, more effective nutrient exploitation, and improved plant absorption. AM symbiosis also boosts tolerance to biotic and abiotic stressors and lowers disease incidence, making it an important component of sustainable agriculture. The efficient application of AM fungus may allow for acceptable production levels with little fertiliser dosage while also lowering expenses and the danger of environmental damage. This is a potential method for achieving high yields with little fertiliser inputs to assist sustainable agriculture systems. Numerous field crops have been shown to benefit from arbuscular mycorrhizal symbioses due to the extensive hyphal network development in soil, more efficient nutrient exploitation, and increased plant absorption. In addition to increasing resistance to biotic and abiotic stresses and decreasing disease incidence, AM symbiosis is a crucial component of sustainable agriculture. The efficient use of AM fungi may enable for the attainment of acceptable yield levels with minimal fertiliser dose, thereby reducing costs and environmental contamination risk, all while minimising environmental impact. This is a promising strategy for achieving high yields with minimal fertiliser inputs in support of sustainable agricultural systems .