**1. Introduction**

Rapidly growing human population and modern agricultural system resulted in increased demand for agro-chemicals such as pesticides, fertilizers, preservatives, and disinfectant [1, 2] in which their excessive and indiscriminate use severely affects biodiversity, air, water, and soil healthy [1, 3–5]. They might also exert deleterious effects on human health and exacerbate subsequent socioeconomic effects on communities'

livelihoods by disturbing the ecological balance [2, 3, 5–7]. Furthermore, climate change, an ever-increasing human population, depletion of global rock phosphorus, and growing energy prices make current fertilizer production unsustainable and represent sizeable challenges to global food security [8]. These disastrous consequences promote new strategies that can reduce and/or substitute agrochemicals in sustainable way without jeopardizing human health and ecosystem services [2, 9].

Considering such alarming situations, beneficial microbial inoculants are proposed as a "clean and ecofriendly" option in agriculture sectors for their potential role in food safety and sustainable crop production [3, 10, 11]. They act as biofertilizers, bioherbicide, biopesticides, and biocontrol agents, which minimize the negative impact of chemical input and consequently increase the quantity and quality of produced food [2]. Among soil microbe, arbuscular mycorrhizal fungi (AMF) symbiosis is one of the most promising that partially or fully supplement agrochemicals and reduce their consecutive negative impacts [12, 13].

AMF promote plant growth by bringing morpho-physiological and biochemical changes in host plants by serving as "biofertilizers and bio-protectors" in sustainable way [14, 15] and providing water and mineral nutrients to the plant [16, 17]. This services can occur through the direct pathway (by roots) and by AMF pathway [13, 18–20]. Furthermore, it boosts the health of the subsequent crop by improving soil aggregation, providing nutrients, enhancing abiotic stress tolerance, protection against pathogens [9, 12], and altering the accumulation of contaminants in plants and is essential for the sustainable management of agricultural ecosystems and biodiversity [1, 16, 21–23]. It is the key for production of pesticide-free food crops and ensuring that high yields correspond [23, 24].

In general, it is more effective for improvement of crop yield, growth, and development in areas with low levels of agrochemical inputs (e.g., Africa and South America) [25–27]. Similarly, low soil fertility optimizes the expression of the multiple beneficial effects of AMF in agro-ecosystem and reduces nutrient seepage to the environment [10, 28]. This relationship is the best scenario and alternate technology for both farmers and society to increase the utilization efficiency of scarce nonrenewable fertilizers such as rock phosphate [18, 29], and use of agrochemicals is catastrophically hampered ecosystem [6, 30]. This is also a strategy to enhance the sustainability of agricultural systems through promoting internal regulatory ecosystem processes while reducing chemical fertilizer use without the concomitant loss of crop yield [26, 31]. However, the application of AMF has not been fully adopted by farmers so far [26]. Therefore, to optimize AMF effects on nutrient bioavailability and ecosystem service to achieving future food security in more sustainable agricultural systems, understanding the linkage of AMF with soil and plant nutrient dynamic plays a vital role for benefitting societies and agro-industries. In this regard, this chapter attempted to summarize published results on contribution of AMF in reducing agrochemical use in agriculture for sustainable maintenance of human welfare and ecological service. It also addresses main factor influencing the success of AMF symbioses and inoculation.

## **2. Characteristics of arbuscular mycorrhizal fungi (AMF)**

Mycorrhizal (fungus-root) fungi are specialized members of the vast population of microorganisms, which are morphologically and physiologically diverse in nature that colonize the rhizosphere [16]. Among mycorrhizal symbiosis, AMF symbiosis is *Arbuscular Mycorrhizal Fungi (AMF) in Optimizing Nutrient Bioavailability and Reducing… DOI: http://dx.doi.org/10.5772/intechopen.106995*

one of the most ancient and widespread than other types of mycorrhizal associations [16, 32]. It is belong to the Phylum *Glomeromycota*, *Glomus* spp., which are reproduced asexually and associate with approximately 80% of terrestrial plant species [33], including forest trees, wild grasses, and many crops [16, 34].

AMF is the name given to the endosymbiotic association of a plant root and a fungus from the *Glomeromycota*, which form two unique structures: a highly branched structure formed inside root cells called *arbuscules* that is considered to be the key element of the symbiotic nutrient exchanges and a balloon-like structure called *vesicles* for storage of nutrients within the plant root cortical cells of the host (**Figure 1**) [35]. AMF are classified into three (*Archaeosporomycetes*, *Glomeromycetes*, and *Paraglomeromycetes*) and the five orders*:* Archaeosporales (e.g., *Geosiphon pyriformes*, *Archaeospora trappei*), Diversisporales (e.g., *Scutellospora calospora*, *Acaulospora laevis*, *Entrophospora infrequens*), *Gigasporales* (e.g., *Gigaspora margarita*, *G. rosea*), Glomerales (e.g., *Glomus intraradices*, *G. mosseae*, *G. geosporum*), and Paraglomerales (e.g., *Paraglomus occultum*, *P. laccatum*) [17]*.* They are obligate biotrophs endophytes [34] that completely depend on a host plant for carbon and energy [32] due to lack of the ability to absorb carbohydrates from other source. This carbon cost is offset by the positive effects of AMF on plant growth and soil quality [10]. There are two types of root colonization in AMF symbiosis [36] (1) *Arum*-type, in which the symbiont spreads intercellularly between cortical root cells, forming terminal arbuscules on intracellular hyphal branches (2) while in the *Paris*-type, the fungus grows directly from cell to cell within the cortex and forms intracellular hyphal coils and intercalary arbuscules along the coils [17, 37].

In AM roots the fungus penetrates intercellularly and intracellularly into the root cortex, whereas in ectomycorrhizal (ECM) roots the fungus only penetrates intercellularly into the root cortex (**Figure 1**) [17]. An ECM root is characterized by the presence of three structural components: a sheath or mantle, *Hartig net* (complex intercellular network system), and extraradical mycelium [16]. The mycelium can take nitrate and ammonium from the soil and transfer to the plants [3]. However, AMF mainly uptake ammonium from the soil and transfer to the plants [38, 39]. The host plant root exudates of Strigolactone bring recursive spore germination and

hyphal branching, while *Myc factors* secreted by mycorrhiza perceived by host roots to trigger the signal transduction pathway or common symbiosis (SYM) pathway lead to adhesion of a hyphopodium to the root surface via Ca2+ spiking. Fungal hyphae emerging from this hyphopodium penetrate into the root through the prepenetration apparatus, which guides the fungal hyphae through the root cells toward the cortex. Eventually, a highly branched arbuscule occupies most of the cell volume, forming an extensive surface for nutrient exchange (**Figure 1**) [33].

AMF produce a high number of spores that grow faster even under different stress conditions [40] and produce thick-walled hyphae that penetrate the host root and extend from the roots out into the soil where they interface with soil particles [9]. Then they create a network of extraradical mycelium structure, which increases the fungal absorbing surface and facilitates the translocation of mineral nutrients from soil to host plants (**Figure 1**) [10, 16]. Despite its coenocytic nature, the mycelium that is formed within the root, the intraradical mycelium (IRM) differs morphologically and functionally from the ERM, the mycelium that grows into the soil. The ERM absorbs nutrients from the soil and transfers these nutrients to the host root. The IRM on the other hand releases nutrients into the interfacial apoplast and exchanges them against carbon from the host [17, 18]. Such a fungal structure represents one of the critical elements of the AMF symbiosis and provides increased surface area for nutrient uptake and bridges nutrient depletion zones [10, 37]. These extraradical hyphae acquire phosphate and initiate the colonization of other species [34, 41].
