**4. Phenotypic and molecular bases of maize-AMF symbiosis**

Most terrestrial plants, including maize, interact with AMF under nutrientlimited conditions, mainly phosphorus. The physiological mechanisms underlying AMS establishment are under intense study using model plants, yet little information is currently available about molecular bases. Several studies have been performed for investigating the effect of AMF on gene expression through several approaches in different plant species over the last few years. Transcriptional analyses of few model plants like rice [73], Petunia [74], *Lotus japonicus* [75], *Medicago truncatula* [76], and tomato [77] allowed to identify genes involved in the AMS including genes encoding mycorrhizae-specific transporters. According to Willmann et al. [78], transporter genes are crucial for functional symbiosis. Many authors have defined four distinct stages of AMS based on the morphological analyses of the mutants.

The first step of an AMS is pre-contact signaling. It is characterized by a bi-directional exchange of signaling molecules and metabolic resources between AMF and plants. Indeed, plants produce strigolactone recognized by AMF, which exudes Myc-LCO, leading to deformation of absorbent hairs and nuclear calcium spiking [79]. The calcium oscillations are decoded by calcium and calmodulindependent protein kinases (CCaMK/DMI3), activating CYCLOPS/IPD3. These genes induce various micro RNA, transcription factors, and auxin signaling during AMS as documented by Diedhiou and Diouf [80]. Transcription factors regulate the signaling pathway during mycorrhization through interconnections, not yet clearly defined compared to Rhizobium/legume symbiosis.

Molecular recognition between partners is followed by contact between fungal hyphae and plant roots. This contact triggers a chain of events that starts with the hyphae branching, differentiating into hyphopodium or appressorium on the root surface. This structure prepares the penetration of the fungus into plant cells. Then, hydrolases and other molecules probably make the cell wall more flexible and cause the migration of the nucleus towards the appressorium, rearrangements of the cytoskeleton, and endoplasmic reticulum. These events lead to the subsequent formation of a pre-penetration apparatus (PPA) [57]. This apparatus facilitates the invasion of hyphae on epidermal and the first cortical cells [57]. According to some authors, PPA is responsible for forming a symbiotic interface and a new apoplastic compartment separating AMF and plant. During appressorium formation, defense genes are weakly activated in the plant [81]. Several genes such as VAPYRIN, NSP1/NSP2, and Cbf1/Cbf2 are involved in this step, as documented by Diedhiou and Diouf [80]. However, their precise function in root endosymbiosis remains unclear.

After physical contact between the two partners, hyphae fungal penetrate the cortical cells without damaging their plasma membrane, which invaginates and proliferates around the hyphae that develop inside these cells. This event results in forming an intra-radical mycelium. Intra-radical proliferation extends the colonization area to the intercellular space of the cortical parenchyma and inner cortical cells. Very few specific genes are involved in this step [80, 82]. This low expression supports the hypothesis that very few additional plants genes are activated after successful fungal colonization.

Intra-radical proliferation leads to the formation of arbuscules, which represent the final and most intimate step of the AMS. Indeed, intramatricial hyphae perforate the cell wall and penetrate inside the cell. They branch out to achieve a structure reminiscent of a small tree called an arbuscule which surrounds the cytoplasmic membrane to form the peri-arbuscular membrane (PAM). These modifications induce many changes in genes expression patterns even if their activity on the whole root level largely remains the same. Studies carried out on model plants allowed to identify mainly transcription factors and phosphorus transporters require to form arbuscules [21, 83]. The possible interconnections between these genes are described particularly between RAM1 and PT4/STR [84]. For maize, several analyses focused on identifying phosphate transporters whose Pht1;6 localized to arbuscule-containing cells [78]. It plays a criticssl role in the maintening the arbuscule function. Indeed, loss of function of Pht1;6 reduce root colonization with premature degeneration of the arbuscules. In addition, 13 Pi transporters were identified by Liu et al. [21]. Among them is ZmPt9 gene, which is different from members of the PHT1 gene family. Functional analysis indicates that ZmPt9 promotes the Pi transporter gene induction involved in Pi uptake [85]. Overexpression of ZmPt9 in Arabidopsis plant increases primary root length and lateral root formation. Furthermore, phosphorus content is higher in the transgenic plant compared to the wild type [86]. Recently, Wang et al. [87] showed that ZmPt7 regulates Pi acquisition, and its transport is mediated by phosphorylation.

### **5. Impact of AMF on maize resistance to biotic and abiotic stress**

Maize is one of the essential sources of carbohydrate globally [88]; however, abiotic stresses and plant pests and diseases are significant threats in maize production worldwide, and future climate disturbances will further compound these scenarios [89]. AMS improves plant growth, hydro-mineral nutrition, and physiology under various environmental stress conditions like salinity, drought, and the presence of heavy metals [90], as well as resistance to biotic stresses such as pests, diseases, pathogen and weeds [91]. The benefits of AMF to plant partners vary depending on the type of stress [92].

AMF adapt to biotic and abiotic stresses independently of its host plant [14] and respond to stresses such as pests, diseases, pathogen, weeds, drought, extreme temperatures, salinity, and heavy metals [93–95]. Extensive evidence shows that AMF can control plant fungal, viral, and bacterial diseases (Himaya et al., 2021). The adaptation mechanisms of AMF to these biotic stresses are generally linked to pathogen resistance, including competition for colonization sites and improvement of the defense system of the plants [14]. Gerlach et al. [96] reported changes in leaf's elemental concentration, resource reallocation, especially for carbohydrates and amino acids, and expression of defense-related genes under maize-AMF symbiosis. Patanita et al. [97] demonstrated the benefits of mycorrhization in the control of *Magnaporthiopsis maydis* also called *Harpophora maydis* [98], the cause of late wilt disease of maize, which causes up to 50% grain yield losses in many countries

*Climate-Smart Maize Breeding: The Potential of Arbuscular Mycorrhizal Symbiosis… DOI: http://dx.doi.org/10.5772/intechopen.100626*

[99, 100]. In addition, *Fusarium* and *Aspergillus* are two of the most dominant fungal pest species of maize, causing acute diseases and yield losses and majorly responsible for deterioration and losses on maize plants [101, 102]. Olawuyi et al. [103] investigated the effect AMF on Aspergillus niger and revealed that *Glomus deserticola* was an effective biocontrol agent against *Aspergillus niger*, the soilborne pathogen of maize. *Glomus clarum* and *Glomus deserticola* a have biocontrol potential against *Fusarium verticillioides* [104]. Downy mildew disease caused by *Peronosclerospora* is responsible for decreasing maize production (Soenartiningsih and Talanca 2010). The combination of botanical fungicides (Turmeric rhizome and betel leaves) with AMF (*Enthropospora* sp., *Gigaspora* sp., and *Glomus* sp.) and *Trichoderma asperellum* can reduce the incidence of downy mildew by extending the incubation period and increasing the dry weight of maize shoots [105]. *Striga hermonthica* is one of the most critical biotic constraints affecting maize crops in sub-Saharan Africa. The high infestation of this parasitic plant has forced many poor farmers to abandon their farms [106]. Several studies have demonstrated that AMF can inhibit or suppress Striga germination, especially on cereal crops such as maize [107, 108]. Othira [109] carried out a study that confirmed the effectiveness of AMF in protecting maize against Striga infestation, promoting crop growth, and reducing *Striga* plant incidence, plant biomass, and phosphate content. He evidenced that AMF (*Gigaspora margarita*) enhanced the performance of the maize plant host, allowing it to resist better *Striga* damage [109].

In addition, AMF also helps maize plants cope with abiotic stresses such as salinity, drought, extreme temperature, and heavy metal. Various mechanisms explain abiotic stress biological regulation through AMF, such as increased hydromineral nutrition, ion selectivity, gene regulation, production of osmolytes, and the synthesis of phytohormones and antioxidants [14]. For instance, *Rhizophagus irregularis,* an AMF species, can improve maize drought tolerance through enhancing apoplastic water flow [110]. According to Mathur and Jajoo [111], *Glomus Funneliformis* can help maize resist extreme temperatures by regulating the photosystem (PS) II heterogeneity. Studies carried out by Estrada et al. [94] demonstrated that AMF species such as *R. irregularis*, *Septoglomus constrictum,* and *Claroideoglomus etunicatum* improve maize tolerance to salinity. The authors showed that these AMF species improve K+ and Na+ homeostasis, shoot and root dry weights, shoot K concentration, and reduced Cl and Na contents in shoots. AMF can also play a role in maize tolerance to heavy metals. Indeed, maize inoculation with some Glomus isolates can improve maize dry weight and contents of essential elements (K, P, and Mg) [93].

Drought is also one of the significant stresses that can reduce maize productivity [112]. Water constraints decrease the photosynthetic activity of plants, which close their stomates to minimize water loss, decreasing productivity [113, 114]. Several studies demonstrated that AMF improves crops performance under drought stress [115]. Mycorrhizal maize deals with water deficit through drought mitigation and drought tolerance [116]. A drought mitigation strategy is mediated by indirect AMF benefits and enhanced water uptake. In contrast, drought tolerance involves a combination of direct AMF benefits that improve the innate ability of the plant to cope with stress [117]. Furthermore, inoculation with AMF improves strigolactone and auxin responses to drought stress Ruiz-Lozano et al. [118]. These two critical hormones in plant resilience to abiotic stress [119].

### **6. Impact of AMF on maize carbon and nitrogen sink efficiency**

Carbon (C) and nitrogen (N) are indispensable mineral elements for plant growth and development. AMF plays a vital role in maintaining soil quality by increasing

carbon mineralization. After inoculating by *Glomus etunicatum*, the contents of dissolved organic carbon (DOC), microbial biomass carbon (MBC), and readily oxidizable carbon (ROC) increase in the soil rhizosphere of maize [120]. *Rhizoglomus intraradices* colonization improves the active carbon pools such as water-soluble carbon, hot water-soluble carbon, biomass carbon up to 305 mg.kg − 1, and passive pools such as soil organic carbon up to 4.31 mg.g − 1 compared to the control [121].

Plants can uptake nitrogen from the soil in the form of organic or chemical fertilizers [122] or establish beneficial associations with microbes that facilitate plant N acquisition [123–126]. Microbes convert different forms of N that plants can use following chemical reactions carried out by living microorganisms such as bacteria, archaea, and fungi. Bacteria like Rhizobia and Frankia are the leading nitrogen suppliers to legumes and actinorhizal plants, respectively [124, 125, 127]. Symbiotic mycorrhizal associations can also enhance plant N acquisition through endomycorrhizae or ectomycorrhizae [128]. AMF mobilizes N in the surrounding rhizosphere and provides it to the host plant [129, 130]. Indeed, AMF develop interconnected structures such as arbuscules, intraradical and extraradical mycelium that allow the N uptake [131] through up-regulating genes coding for NO3– and NH4+ transporters, including AMT3.1 [132]. ATM3.1 is the primary driver of NH4+ transfer to the plant colonized by AMF.

Another way to enhance plant nutrition, particularly N uptake, is to develop tripartite associations with bacteria and mycorrhizal fungi, even if they are not well characterized yet [133]. Indeed, bacteria of the genus Paenibacillus have been identified inside Laccaria bicolor cells and can stimulate in vitro production of *R. irregularis* spore and mycorrhizal plant colonization by *Glomus mossea* [134–136]. This stimulating effect enhances fungal growth that could favor the establishment of more efficient fungal and N2-fixing symbioses. Nevertheless, the contributions of AMF to nitrogen acquisition are little be known, even intercropping system between maize and nitrogen-fixing plant. In an intercropping between maize and soybean, common mycorrhizal networks (CMNs) regulate Nitrogen allocation to plant roots [137]. Co-inoculation with AMF and rhizobia transferred more than 54% more nitrogen from soybean to maize than inoculation with AMF alone [137]. Furthermore, a recent study questioned the relevance of the chitinlike N source, an organic N source for the AMF, in the N supply to plants. Experiments showed that only *R. irregularis* hyphae can access a significant fraction (>20%) of the organic N supplied as chitin into a pot zone but not *Andropogon gerardii* roots, and this Ni was transferred to the plants within as little time as five weeks [138].

Overall, the presented evidence suggests that AMF significantly impacts N use efficiency by mycorrhizal/rhizobial plants, and carbon allocation is effective even with a cereal-legume cropping system. Understanding these mechanisms in a climate change context is critical for introducing symbiotic microorganisms as organic fertilizer in both croplands and forests while taking care of the ecosystem services rendered by microbial symbionts. Moreover, there are few evience that AMS could mitigate greenhouse gase emission in several cropping systems through diverse mechanisms [139–145], opening avenues for breeding of climateconsiderate crop varieties. In that regard, AMS was reported to mitigate N2O emissions in several crop-soil systems such as maize [139], tomato [143, 144], rice [145], and grassland [142, 144].

### **7. Impact of AMF on maize yield and allied traits**

Currently, AMF are critical organic components in cropping systems. Their interaction with crops increases yields by promoting plant growth and nutrition capacity [10]. Several experiments investigated the effects of AMF inoculation

### *Climate-Smart Maize Breeding: The Potential of Arbuscular Mycorrhizal Symbiosis… DOI: http://dx.doi.org/10.5772/intechopen.100626*

on maize yield. According to Cozzolino et al. [146], inoculation by *Rhizophagus irregularis* increases maize stalk and leaf dry weight and grain yields compared to non-inoculated plants. They also found that colonization of maize by *R. irregularis* increases available soil phosphorus (P) concentrations suggesting that inoculated roots mobilize more P and water than wild type. Recently, Assogba et al. [147] revealed mixed effects of Glomeraceae and Acaulosporaceae groups on the growth of maize seedlings under greenhouse conditions. Glomeraceae group improve significantly fresh above and underground biomass to 54.97% and 42.94%, respectively, and 55.23% for the leaf area compared to the control. Moreover, maximum plant heights and number of leaves were obtained with the Acaulosporaceae group, having 20.55% and 17.04%, respectively, compared to 11.77% for the control.

Studies were also conducted to reveal the effects of AMF on maize yield under abiotic stresses such as drought, salinity, heavy metals. Drought is one of the significant stresses that negatively affect maize yield [112]. AMF applications under water deficit improve the maize yield in different irrigation regimes. *Rhizophagus irregularis* enhances shoot dry weight (SDW) between 26 and 35% under drought conditions [148, 149]. Limited irrigation causes a two-fold decrease of the dry shoot weight (SDW) in AMF-maize plants as compared to non-AMF plants (17% vs. 37%, respectively) [149]. Furthermore, co-inoculation of *Funneliformis mosseae* and *Pseudomonas fluorescens* (phosphate solubilizing bacteria) on maize improves vegetative and reproductive traits, root colonization, grain yield under water deficit while preserving natural resources such as P stocks [150]. According to Celebi et al. [151], *R. irregularis* significantly improves agro-morphological parameters even in restricted irrigation conditions and increases leaf and stem ratios. Like for drought, AMF can increase resistance to salinity through several mechanisms, thus improving yield. Zhang et al. [152] reported that Trichoderma and Stachybotrys could promote maize growth in saline soil. Indigenous AMF improve maize growth in saline fields by significantly increasing biomass production and promoting leaf proline accumulation and a higher K+/Na + ratio [21]. Besides, *Glomus tortuosum* remarkably ameliorates dry mass and leaf area and enhances photosynthetic capacity by improving chlorophyll content and efficiently allowing light energy utilization, gas exchange, and rubisco activity under salinity stress [153].

Furthermore, several studies indicate that AMF can facilitate the revegetation of heavy metal contaminated soils and improve yields. Inoculation of maize by *Claroideoglomus etunicatum* in soils spiked with Lanthanum (La) significantly enhanced dry shoot weight and increased K, P, Ca, and Mg content in maize shoots between 27.40 and 441.77% [154]. Also, *C. etunicatum* decreased shoot La concentration by 51.53% in maize while root La concentration increased by 30.45%.
