**2. Symbiotic association between mycorrhizal fungi and agricultural crops**

Crop production is the first step to produce foods, so it is essential to study the factors that directly affect this production, such as the biological ones. Initially, it is important to mention that mycorrhizal fungi are one of the varieties of fungi of extreme importance for agricultural production due to their great contribution to the nutrient absorption by plants and several other functions. It is known that some fungi species can interact with others organisms leading to the establishment of symbiotic associations, and mycorrhizae are one of these associations [6].

When a fungus establishes an association with the roots of certain plant species, we have the so-called mycorrhizae, which are divided into two major categories: ectomycorrhizae and endomycorrhizae. This classification is based according to the morphological and anatomical aspects of the fungal colonization of plant roots [6]. We can observe this difference in **Figure 1**.

*Introductory Chapter: Mycorrhizal Fungi – A Current Overview on Agricultural Productivity… DOI: http://dx.doi.org/10.5772/intechopen.109021*

#### **Figure 1.**

*Ectomycorrhizae and endomycorrhizae scheme of colonization of roots. Source: [7].*

Ectomycorrhizae are formed mainly by basidiomycetes and ascomycete fungi, representing about 3% of phanerogams [8], which phanerogams is mainly a subkingdom of the plant kingdom which produce seeds to reproduce. Whereas, in colder regions with cooler temperate, around 90% of forest species present mutualism with this fungus [8].

In tropical regions like Brazil, ectomycorrhizae are more studied and found mainly on economically exploited species such as pine, eucalyptus, and acacia. In ectomycorrhizae, the fungi associated with the root do not penetrate the living cells of the root; and the hyphae grow between the cells of the root cortex, forming a characteristic structure, the Hartig's web (**Figure 1**). The roots of plants associated with ectomycorrhizae are devoid of hairs and their function is performed by fungal hyphae [9].

Endomycorrhizae are more common than ectomycorrhizae and occur in about 80% of vascular plants. The fungi that best represent this association are zygomycetes, which is a class of fungi with more than 1000 known species. The Endomycorrhizae penetrate the cortical cells of the plant roots, where they form very branched structures and chlamydospore (**Figure 1**), which is the defined as a thickwalled, non-deciduous, intercalary or terminal, asexual spore formed by the rounding of a cell or cells whose primary function is perennation, not dissemination [10]. Their hyphae extend for several centimeters in the soil, significantly increasing the amount of nutrients and phosphates essential for plant development.

Many studies have reported that root colonization by mycorrhizal fungi significantly increases the productivity of various plants in low-fertility soils [9]. This is due to the greater uptake of nutrients such as phosphorus, zinc, and copper which are essential for plant development. In turn, the fungus benefits greatly from this association since it can feed on the sugars, amino acids, and other organic sources (photoassimilates) produced by plants through photosynthesis [9].

As already well discussed, mycorrhiza is a symbiosis between plants and soil fungi in which both parties are mutually aided along the way. In this symbiotic relationship, first, organic carbon flows from the host plant to the fungi and then the inorganic elements flow from the fungi structure to the plant, so both parties benefit [11]. Mycorrhizal fungi can only survive when combined with plants because the fungi cannot produce its own food and although it decomposes organic compounds for energy acquisition, it is not enough, it needs to grow and develop together with plants [11].

In crops context, this symbiotic relationship is important as it determines the ability to stimulate plant growth due to higher nutrient acquisition and protection from pathogens attacks such as bacteria and fungi, protecting against diseases. As observed, microorganisms have great potential to increase crop productivity. Dark septate fungi, Pseudomonas bacteria, and bacteriocins are some of the studied microorganisms and substances that can lead to new agricultural bioproducts, as growth promoters or for the control of agricultural pests and diseases [11].

Researchers have demonstrated excellent results with the use of microorganisms in the soil to improve the agronomic efficiency of crops. Some selected bacteria have the ability to solubilize the phosphorus present in the soil and to promote it available to the plants, while others have the function of fixing nitrogen from the air into the roots. In general, soils in tropical regions are mostly deficient in nitrogen and phosphorus, and these fertilizers usually add up to more than 50% of the cultivation cost [11].

As we know, not all applied fertilizer is absorbed by plants, maize absorbs only 55–60% N [12], around 20% P [13], 50–70% potassium (K) [14], and 33% sulfur (S) [15]. One strategy to improve the plants' ability to absorb nutrients and consequently reduce fertilizer application, especially of P, is to inoculate AM fungi. The final response will be a maize plant well-nourished and presenting full growth and development.

Dark septate fungi are promising microorganisms that promote plant growth. At this fungus contact, rice plants grew shoots and roots 30% faster. In addition, tillers, which are new branches of the plant, increased by about 50%, which means greater nutrient uptake and grain production capacity [16]. As we observed, this fungus group also improves the nutritional state of the plants. In tests performed on tomatoes, nutrients such as nitrogen, phosphorus, and potassium accumulated to a greater extent compared to plants without the presence of microbes. In this way, it is possible to optimize plant nutrient use and obtain more vigorous crops [16].

When the farmer decides to use arbuscular mycorrhizal fungus, the plant must be inoculated, as it grows the fungus slowly develops and multiply, which can take many days for them to become fully established. Conversely, with the dark septum fungus use, the inoculum will be established in a few days. Another interesting feature of dark septum is its resistance to water stress. Researchers have observed that in the presence of this fungus, even with a lack of water, the plant developed similarly to another plant grown under normal conditions (without drought stress), confirming the full potential of this microorganism [16].

Research has shown that fungi and bacteria are effective and responsive in the restoration of highly degraded soils. Legume plants associated with these microorganisms have been successfully applied in several regions of mining and impoverished soils to lead to their recovery. As we have seen, microorganisms increase the absorption capacity of the roots, making them more resistant to environmental stress. This technique allows rapid revegetation, even when the subsoil has been exposed. While

*Introductory Chapter: Mycorrhizal Fungi – A Current Overview on Agricultural Productivity… DOI: http://dx.doi.org/10.5772/intechopen.109021*

bacteria provide plants with the nutrient nitrogen, mycorrhizal fungi help plants uptake other nutrients, especially phosphorus and water.

The researchers selected the bacterial strains most effective at nitrogen fixation for each plant species and using microbiological multiplication techniques in the laboratory, it is possible to increase these more resistant and nitrogen-self-sufficient strains.

### **3. Agronomic practices that negatively impact mycorrhizal fungi**

Agronomic practices such as intensive monocrop, soil use with recurrent harrowing, and subsoiling lead to a significant degradation and decrease in soil microbial biomass. There are other important impacts that we should highlight such as intensive grazing, indiscriminate use of pesticides and fertilizers, and mining activities. All these activities are responsible for sick, impoverished, and unproductive soil.

It is important to emphasize that fungi do not always act in a positive way, sometimes these microorganisms are the cause of a large number of diseases. Therefore, it is important to be careful with agronomic practices since if a person without chemical and biological knowledge recommends some wrong strain for inoculation, it may not help, but greatly hinder the production [16]. It is common that many field technicians with the intention of fertilizing the soil to invest in mycorrhizal fungi inoculation, however, it is necessary for a specific professional in the area to make such a symbiotic association between mycorrhizal fungi and agricultural crops.

It is important to say that the control of the disease caused by fungi requires mainly preventive measures since it is necessary to pay attention to the choice of the area to be cultivated until the harvests. This is due to the fact that for chemical control of diseases there are several products available on the market, from different companies.

### **4. Agronomic practices that positively impact mycorrhizal fungi**

The different types of crop residues are very important for healthy soil, not only for supplying nutrients via biogeochemical cycling, but also for functions directly affecting physical, chemical, and biological properties.

Microorganisms are vital to agriculture, especially in two biological processes: (i) Biological efficiency − In this process, certain types of fungi act as solubilizers of soil nutrients, optimizing yields. (ii) Biocontrol − The process by which a group of fungi can do the work of bio-pesticides and biocides to facilitate the management. In addition to their use in agriculture, they can also be used in the medical field, for drug development [17] due to their bactericidal properties. They also can be used to make antibiotics, such as penicillin, whose function is to fight infectious diseases caused by bacteria. Another area where fungi excel is food, some are edible and can be used as part of human consumption [17].

The no-till system (NTS) and crop rotation are essential practices for maintaining good biological soil quality. The no-till farming system is considered a conservationist and sustainable practice, this agricultural practice is a form of soil management that involves techniques recommended to increase productivity while conserving or continuously improving the cropping environment. The main practices are: Absence or minimal soil disturbance. In relation to crop rotation, we can say that is the practice of alternating the plant species grown each season over the years for a more productive

system. This conservation agriculture technique aims to reduce soil exhaustion. The crop rotation can benefit soil macro- and micro-fauna, since the richness and abundance of edaphic organisms are determined, among other factors, by the quantity and quality of aerial and root phytomass added to the soil [18].

The crop rotation system combined with no-till farming promotes an improvement in soil quality, as there are erosion reductions, increases in nutrient cycling, and, consequently, better biological activity. The straw has a fundamental function in the no-till farming principle and is highly effective in protecting the soil, avoiding the impact of raindrops, wind, and solar rays. It is also responsible for reducing temperature and water amplitudes, favoring biological activity. In addition, it increases the soil's chemical and physical characteristics such as organic matter content, cation exchange capacity (CEC), and water availability for plants and other organisms.
