**3. Mechanism of suppression of mycorrhizae bioprotectants**

The production of healthy and disease-free plant and plant products with corresponding higher yield can directly or indirectly be linked with microorganisms in the soil rhizosphere. Due to the number of environmental concerns regarding the use of different biological control agents, and increasing pathogens resistant to pesticides, more stable and environmentally friendly alternatives are now been considered. AM fungi are not only useful as biofertilizers, but also as bio-stimulants due to their antagonistic capabilities against plant pathogens [20]. They are known to established symbiotic relationship with more than 80% of the plant species [21, 22]. In plant disease management, AMF has been considered as one of the reliable and available options as it is found to serve as a bioprotectant and plant stimulant in sustainable food production and ensure reduction in plant pathogen population to acceptable level without harmful effect to the environment. AMF has been used as a biological control agent in the reduction of incidence and severity of bacteria such as *Pseudomonas syringae, Erwinia carotovora* [23] and fungi such as *Fusarium* spp. [24], *Pythium* sp. [25], *Verticillium* sp. [26], *Sclerotinia* sp., *Phytophthora* sp., *Macrophomina* sp. [27], and nematodes such as *Radophulus* sp. [28].

There are numerous pathogens in the soil that cause diseases to plants and result in substantial reduction in plant yields. These pathogens have to be controlled to ensure food security around the world. Among the new and sustainable control alternatives is biological control that involve the use of antagonistic organisms to suppress damage activities of other organisms that cause diseases to plants [29]. Among the most promising biological control agents is the rhizosphere-competent fungi called mycorrhizae, which is capable of suppressing the activity of disease-causing organisms both major and minor beside their role in stimulating plant growth response. The roots of most plants are in symbiotic association with certain soil fungi and this association is called mycorrhiza [30]. The mycorrhiza has number of functions that include enhancement of nutrient uptake, improvement of soil structure and plant establishment, protection of plants against environmental stresses and suppression of plant diseases [31].

Plant roots colonization by AMF usually results in the decrease of the incidence and severity of the diseases caused by pathogens. The reduction in damage by AMF maybe as a result of changes in the morphology and plant root growth, biochemical and physiological changes in the plant, histopathological changes in the plant root, mycorrhizosphere effects that results in the modification of microbial population density, activation of host defense mechanisms, parasitism of nematodes by AMF and competition for photosynthetic products and colonization sites [32]. Among the various proposed biocontrol mechanisms for the plant diseases, the most effective biocontrol scheme could either be the result of all the mechanisms working together

or as a separate entity. The major limitation in the use of AMF as a biocontrol agent could culminate from its obligate nature, the role of environmental influence on the various mycorrhiza symbiotic associations and limited understanding of the mechanism involved in the interaction processes. The objective of this chapter is therefore to throw more light on the mechanism of suppression of mycorrhizae bioprotectants.

Protection of plants by mycorrhizal fungi against disease causing organisms involves multiple mechanisms that include: production and changes in the exudation pattern, formation of physical barrier (fungal mantle) around the roots and synthesis of anti-fungal compounds by the plant roots in response to mycorrhiza symbiotic association [33]. For example, *Paxillus involutus* (Ectomycorrhizal fungi) was reported to successfully controlled *Fusarium moniliforme* and *Fusarium oxysporum* causing rot disease in *Pinus resinosa* as well as *Pisolithus tinctorius* (Ectomycorrhizal fungi) in controlling *Phytophthora cinnamomi* causing disease in sand pine [34]. Specific form of disease suppression may result from the activity of one or few antagonistic microbes.

The symbiotic association of AMF has been reported to induce plant host defense response both at early and later stage of invasion by the pathogen [35]. There have been reports of quick response in terms of plant host defense to pathogens by the mycorrhizal associated plants compare to those devoid of this symbiotic relationship and for that matter, AMF colonization has been proposed to act as a priming scheme for the pathogen resistance process [36, 37].

In a related defensive mechanism against plant pathogens, AM fungi have been involved in the activation of the plant defense response against pathogens and this include the expression of number of genes with their matching proteins (e.g., phenolics, cellulose deposition, chitinases, hydroxyproline-rich glycoproteins, phytoalexins, peroxidases and proteins relating to pathogenicity) [38, 39]. Both localized and systemic resistance to *Phytophthora parasitica* has been reported in tomato root system [40], Pathogenesis-related proteins are involved in triggering of the Systemic acquired resistance (SAR) defense mechanism [41]. The pathogen *Aphanomyces euteiches* causing disease on garden pea was biologically controlled after pre gene activation of the host defense response by mycorrhiza-related chitinolytic enzymes [38].

Mycorrhization have been recorded to change plant root exudation pattern and these alterations could indirectly affect the pathogen through alteration of the pH of soil environment or through production of inhibitory products. In a study involving symbiotic association of strawberry with mycorrhiza fungi, exudates released by the roots of the strawberry had shown to suppress the growth and sporulation of *Phytophthora fragariae* [42], as well as affect the germination of microconidia produced by *Fusarium oxysporum* in a related experiment [43, 44]. There is also evidence of direct antagonistic action by AMF against pathogens in the soil rhizosphere [45, 46].

In terms of improvement in the nutritional status or reduction of plant root damage by the pathogen, the increase supply of nutrient by mycorrhiza fungi to plants have been suggested to enhanced their tolerance level to pathogen damage and carbon drain from plants to the pathogen. AMF absorb nutrient via the external network of fungi hypha by solubilizing both macro and micro elements like Mn, Ca, Zn, Cu, N and P [47–49]. This nutrient uptake ensures healthy growth of the plant due readily or available nutrients supply to the plant that enhances the tolerance or resistance level of the plant to the pathogens [50]. The mycorrhizal fungi increase the rate at which phosphorous is absorbed by increasing the surface area, number of roots, growth and development of plant root hairs. The increase in phosphorous uptake in

#### *Role of Mycorrhizae in Crop Protection DOI: http://dx.doi.org/10.5772/intechopen.109020*

plant-mycorrhizal symbiotic relationship constitute the major mechanism for the AMF-mediated biocontrol [51].

With regards to the morphological alteration of the plant roots, mycorrhization has been reported to cause some changes in the morphology of the roots in spatial, structural, temporal and quantitative way [52, 53]. The AM produce arbuscles and vesicles both inter and intracellularly within plant root. Any pathogen that encounters with ectomycorrhizal fungus has to first of all deal with the external and multilayer network of hyphae known as mantle and inner cortical cells which serve as physical barrier to invasive pathogens and play a critical role in enhancing the population of the useful microorganisms in the soil with corresponding production of growth promoting elements by PGPRs that increase the plant resistance to pathogens [54–56]. In aromatic plant (e.g., basil), the root length and toot tip numbers, level of branching and fresh weight of the plant have been reported to be altered independently based on the type of AMF involving in the colonization process [53].

Plant roots colonized by AM fungi have enlarged length and diameter with profuse branches [57, 58]. Plant roots were found to accumulate an increased deposition of lignin and chitinases content [59] as well increase the resistance of plant root system to pathogens when in association with AM fungi. Incidence and severity of diseases caused by *Phytophthora parasitica* were found to decrease in AMF association with plant as compare with non-mycorrhizal roots [22]. AMF associated plants produced a lot of arginine that were found to suppress Thielaviopsis spore formation and large amount of proteins, phytoalexins and peroxidases [58, 60, 61] that induce plant resistance to pathogens.

AMF is found to prevent infection of the root during root colonization by decreasing the access sites to the pathogen as well as stimulate plant host defense mechanism as it was reported in reducing the incidence and severity of root-knot nematodes [62]. Number of mechanisms have been reported to increase stress tolerance of plants by AM fungi and this include the formation of a complex network hypha by AM fungi around the plant roots that block intruding pathogens. In an apple seedling trial, an apple replant disease triggered by phytotoxic myxomycetes has been successfully suppressed by AM fungi such as *Glomus fasciculatum* and *G. macrocarpum* [63]. AMF are also known to provide protection to plants against pathogenic bacteria that affect roots in the soil. Disease caused by *P. syringae* on tomato plant have been drastically reduced in plant-mycorrhiza symbiotic association [23, 33]. The various protective and suppressive mechanisms involve in this include: indirect effects (chemical interactions; physical protection); and indirect mechanisms e.g., isoflavonoids, increase nutrition uptake by plants; changes in the morphology of the plant roots by increased lignification.

Competition by AMF with pathogens for infection site on the plant root is well documented. In the competition for the site, AMF usually inhabit the location on the plant root surface where the pathogen require to penetrate the root or it preestablishes itself in the cells so that the site cannot be occupied by any new invasive pathogen [40, 64]. In other cases, Mycorrhizal fungi and pathogens causing plant diseases, more often than not live in the same niche that bring them into physical contact to compete for the limited resources (nutrient and space) in the rhizosphere [65]. AMF is also known to compete with the other pathogens for carbon. The AMF colonize the roots of the plants and make use of the carbohydrate from the plant, thereby leaving limited amount of carbon to be utilized by the competing pathogen and this explains the rationale behind the biocontrol strategy implore by AM fungi [36, 66, 67]. There are diverse AMF species that show different carbon sink strength in the roots of plants associated with mycorrhiza and thus have shown different inhibitory or antagonistic effect against plant pathogens [68, 69]. For example, in nematode trial, *Meloidogyne incognita* reproduction factor was found to be reduced when in association with AMF prior to inoculation [70]. Elucidation and protective capability of the mycorrhizal symbiotic association with variable expression of the traits in relation to their ability to protect plants have been well documented [16]. AM fungi in association with plants results in biochemical changes in host tissues, reduction in plant stress, uptake of phytonutrients, changes in plant root anatomy and morphology, trigger systemic resistance, and competition for the limited resources such as nutrient and space [40].
