**4. Arbuscular mycorrhizae**

Endomycorrhiza i.e. Arbuscular Mycorrhizal (AM) symbiosis is formed by approximately 80% of all terrestrial plant species. Even the roots of some aquatic plants are colonized by AM fungi [19, 20]. AM fungi belong to the class Zygomycetes, order Endogonales, family Endogonaceae, and phylum Glomeromycota. The mycorrhizal associations are formed by the six genera of fungi belonging to Endogonaceae. These are Glomus, Gigaspora, Acaulospora, Entrophospora Sclerocystis and Scutellospora. Their common characteristic are spores and sporocarps which are formed mostly in the soil surrounding the roots and rarely inside the roots [21].

The most visible AM structure is the hyphal network. Hyphae are thin from 2 μm in diameter to >20 μm, hollow tubes of fungi having only few cross walls and distinct angular projections [22]. In search of the roots of host plants, these tubes originally grow from fungal spores, extending short distances into the soil.

Hyphae that penetrates a host root form a structure called an appressorium. It penetrates the cell wall of the root by mechanical pressure or through the enzymes that degrade the cell wall. Hyphae that enter host roots through these infection points can form networks both inside the root and throughout the soil surrounding the root. As the name suggests, the AM fungus colonizes the root cortex forming a mycelial network and characteristic bladder-like structures called as vesicles and branched finger-like hyphae called as arbuscules. Arbuscules are short-lived structures meant for nutrient transfer and absorptive function. The hyphal branch that penetrates the plant cell wall forms the arbuscules trunk. This arbuscule trunk branches repeatedly and is surrounded by the plasma membrane. The terminal swellings of the hyphae forms vesicles on both intercellular and intracellular surfaces, and have storage as function [23, 24].

#### **4.1 As a tool for sustainable agriculture**

#### *4.1.1 Benefits from tripartite relationship*

In the mutualistic association, the plant provides the fungus with photosynthetically derived carbohydrate, while the fungus supplies the plant roots with nutrients. Also, in this symbiotic association, there is a third component i.e., a bacterium that seems to be having a loose or tight association with the plants and the mycorrhizal fungi and play an important role in mycorrhizal function. So, there is a tripartite relationship among host plant, AM fungi and bacteria. This bacterium has been termed as 'helper bacteria' because it supports mycorrhizal establishment [25].

For the establishment of a symbiotic relationship with the nitrogen fixing rhizobium bacteria, the AM fungi releases a 'myc factor' which is a diffusional factor responsible for activating the nodulation factor's inducible gene MtEnod11. This gene is involved in establishing symbiotic relationship with the nitrogen fixing rhizobial bacteria [26–28]. Under natural conditions, this bacterium live in the cytoplasm as endobacteria or colonize the surface of extraradical hyphae [29].

#### *4.1.2 Natural growth regulators*

AM fungi are used as bio-inoculants, and as prominent natural growth regulators in sustainable crop productivity. Also, the stomatal conductance, leaf water potential, relative water content, photosystem II efficiency, and carbon dioxide assimilation are improved by AM inoculation that contribute greatly to organic culturing for growth promotion and yield maximization [30–32].

#### *4.1.3 Bio-fertilizer*

For fulfilling the fertilizer requirements of plants in areas of marginal fertility and to reduce the harmful effects of chemical fertilizer, AM have a potential use as a biofertilizer. Bio- fertilizers are a mixture of naturally occurring substances for improving soil fertility [33]. Various problems and damaging impact on the quality of food products, soil health, and air and water systems are associated with the continuous use of inorganic fertilizers, herbicides, and fungicides. Reports showed the AM can possibly lower down the use of chemical fertilizers up to 50% for best agricultural production [34].

#### *4.1.4 Plant yield*

AM Fungi can also have potential to enhance the dietary quality of crops and to increase the levels of secondary metabolites and production of carotenoids and certain volatile compounds. There are reports that showed beneficial effects of AM fungi *Glomus versiforme* on the increased contents of sugars, organic acids, vitamin C, flavonoids, and minerals resulting in enhanced citrus fruit quality. It enhances plant yield for a healthy food production chain by increasing the accumulation of anthocyanins, chlorophyll, carotenoids, total soluble phenolics, tocopherols, and various mineral nutrients. The field production of maize, yam, and potato, has been significantly increased using AM fungi [34–36].

#### *4.1.5 Mineral nutrition cycle*

The performance of most agricultural crops becomes better and is more productive in the presence of AM fungi. Mycorrhiza develops symbiosis with roots to obtain essential nutrients from the host plant and consequently provide mineral nutrients in return, for example, N, P, K, Ca, Zn, and S. This symbiosis increases the micronutrient uptake and growth of their plant host [37]. It has an important function in promoting the mineral cycling by maintaining an efficient and closed nutrient cycle of natural ecosystems, thereby changing the ecology of surrounding environment. An increase in the accumulation of biomass is also observed by the inoculation of AM fungi. This is because AM fungi increases the concentration of various macro-nutrients significantly, leading to increased photosynthate production [38, 39]. Thus, even under inappropriate conditions it provides nutritional support to the plants.

**23**

*Arbuscular Mycorrhizal (AM) Fungi as a Tool for Sustainable Agricultural System*

The AM fungi are important to their hosts as they enhance the ability of plants to absorb phosphorus from soil, which is relatively inaccessible to the plants. The arbuscules of the fungi assist in exchange of inorganic minerals and the compounds

A part from the macronutrients, AM fungi association has been reported to increase the phyto-availability of micronutrients like zinc and copper. Also, it helps the plants to take up nutrients from the nutrient-deficient soils. It is also responsible for the uptake of almost all essential nutrients, specifically phosphate, in plants. It was also reported to increase the absorption of trace elements, such as boron and

On the other hand, it also decreases the uptake of sodium and chlorine thereby stimulating the plant growth. Increased nitrogen content in plants evidently results in higher chlorophyll contents that can effectively trap nitrogen. Maintenance of calcium ion and sodium ion ratio helps improve the overall plant performance. It

• In mycorrhizal chickpea, improved growth and levels of protein, iron, and zinc

• In the mycorrhizal roots of *Lotus japonicus*, an enhanced activity of a potassium

• AM fungi when inoculated in tomato plants have shown increased leaf area, and nitrogen, potassium, calcium, and phosphorus contents, showing an

• In *Pelargonium graveolens* L., mycorrhizal symbiosis increased the concentra-

• In *Euonymus japonica,* improved levels of P, Ca, and K under salinity stress due to instant fungus attachment were reported. In another study, AMF-inoculated Pistachio plants exhibited high levels of P, K, Zn, and Mn under drought stress [54].

• In *Chrysanthemum morifolium* plant tissues, improvement in P and N contents

• In *Leymus chinensis*, an increased seedling weight by improving water content

• *Glomus mosseae* and *Rhizophagus irregularis* showed improved heavy metal

and intercellular CO2, P, and N contents was reported [56].

tions of Nitrogen, Phosphorus, and Iron under drought stress [53].

also improves the surface absorbing capability of host roots [46–48].

Some examples of enhancement of mineral nutrition:

of carbon and phosphorus imparting a considerable strength to host plants. Therefore, it significantly boosts the phosphorus concentration in both root and shoot systems. Also, under phosphorus-limited conditions, the association improves phosphorus supply to the infected roots of host plants. For phosphorus uptake, the crops that are poor at seeking out nutrients in the soil are dependent on AM fungi. It has significant effects on different plant communities, particularly on invasive plants and the fungal-mediated transport of phosphorus and nitrogen to plants [40–42].

*DOI: http://dx.doi.org/10.5772/intechopen.94442*

*4.1.7 Phyto-availability of micronutrients*

molybdenum [43–45].

were found [49].

were reported [55].

transporter was reported [50].

increased plant growth [51, 52].

translocation in the shoot [57, 58].

*4.1.6 Transport of phosphorus and nitrogen to plants*
