Climate-Smart Maize Breeding: The Potential of Arbuscular Mycorrhizal Symbiosis in Improving Yield, Biotic and Abiotic Stress Resistance, and Carbon and Nitrogen Sink Efficiency

Abstract

Maize is part of the essential food security crops for which yields need to tremendously increase to support future population growth expectations with their accompanying food and feed demand. However, current yield increases trends are sub-optimal due to an array of biotic and abiotic factors that will be compounded by future negative climate scenarios and continued land degradations. These negative projections for maize yield call for re-orienting maize breeding to leverage the beneficial soil microbiota, among which arbuscular mycorrhizal fungi (AMS) hold enormous promises. In this chapter, we first review the components relevant to maize-AMF interaction, then present the benefits of arbuscular mycorrhizal symbiosis (AMS) to maize growth and yield in terms of biotic and abiotic stress tolerance and improvement of yield and yield components, and finally summarize pre-breeding information related to maize-AMF interaction and trait improvement avenues based on up-to-date molecular breeding technologies.

1. Introduction

By 2050, around 9.9 billion people will be living on earth, with an expected concurrent doubling of the global food demand; hence agricultural production must increase at the same rate [1, 2]. This exponential population growth comes with an expected doubling of meat consumption; hence, demand for cereals-based feeds such as maize (Zea mays L.) will follow the same trend [3]. Therefore, modern breeding programs must double current genetic gains [2, 4]. However, a 2013 study alerted that current yield increase trends of most staple crops, including maize, are insufficient to meet the 2050 food demands [5]. Several biotic and abiotic factors are determinant limitations to crop yield despite breeding efforts, and climate disturbances will further compound these yield-reducing stressors [2, 6].

With this background, it is apparent that crop improvement programs targeting only the inherent genetic makeups of plants to increase yields are not efficient enough to increase crop production sufficiently to meet future food and feed demands [4]. Furthermore, it is necessary to achieve the desired crop production without increasing production surfaces since land degradation will likely worsen shortly [3]. This alarming situation calls for re-orienting plant breeding programs to adopt new climate-smart breeding approaches to boost crop productivity sufficiently to levels that would match the current and expected population growth rates [4, 7]. Instead of just focusing on increasing crop yields through improving its inherent genetic makeup, plant breeders should explore the immediate growing environment of the crops they seek to improve.

One of the components of the immediate growing environments of crops is the soil microbiota composed of microorganisms including fungi, archaea, and bacteria that have co-evolved with plants, among which some form highly beneficial symbiotic relationships with plants helping them in nutrient uptake, growth regulation, biotic and abiotic resistance, which ultimately results in increased yields [8, 9, 10]. The soil microbiota is relevant since most plant traits of interest, including nutrient use efficiency, tolerance to drought, salt, pest, and diseases, and yield, are part of a system comprising complex plant-associated microorganisms [11]. Plants have developed the ability to modulate the composition and activity of their microbiota through the secretion molecules and other signaling compounds [12]. Plant breeders would greatly benefit from understanding the genetic basis of this interaction and its influence on traits of interest and using this knowledge during selection and stability studies [11].

Among these microorganisms, arbuscular mycorrhizal fungi (AMF) are significant elements of the soil–plant system and constitute 5 to 50% of the microbial biomass of soils (Olsson et al., 1999). Arbuscular mycorrhizal symbiosis (AMS) is the most widespread and oldest terrestrial symbiosis [13], formed by 80–90% of terrestrial vascular plants, including grasses such as maize [14]. Plants benefit from AMS better mineral nutrition, especially phosphorus uptake [15], which improves mycorrhized plants biomass compared to non-mycorrhizal plants [13, 16], through colonization of plant roots, production of large networks of extra-root mycelium in the soil, and aid in the uptake of mineral nutrients by hosts in exchange for carbohydrates [17]. Elements such as nitrogen, magnesium, calcium, potassium, and trace elements such as copper [18], zinc, or even iron, are better absorbed by the plant through the AMF symbiosis [19, 20].

Furthermore, AMF are also a significant component of soil fertility improvement [21], playing an essential role in the soil’s physical, chemical, and biological components. They increase soil water holding capacity by improving its structure and inherent enzymatic activity by activating other microorganisms such as nitrogen-fixing bacteria [22]. AMF also participate in the nitrogen, phosphorus, and carbon cycles while correcting soil acidity [23, 24]. Regarding plant bio-protection, AMF play an essential role [25, 26] through, for instance, helping plants to thwart root-damaging nematodes [27, 28], pathogenic fungi such as verticillium wilt [29], and various pathogenic bacteria [30]. In addition, AMS provides plants with better resistance to abiotic stresses such as water and salt stress or the presence of heavy metals [31, 32].

Maize (Z. mays L.) is one of the essential staple food crops, therefore, essential to food security [33, 34]. Along with rice and wheat, it provides at least 30% of the food calories to more than 4.5 billion people in 100 countries [35, 36]. Maize is grown in more than 166 countries in the world, including tropical, subtropical, and temperate regions from mean sea level to 3000 m AMSL [37], and among cereals, it ranks highest in terms of grain yield per hectare worldwide [38]. Furthermore, besides its revenue generation as a cash crop, maize utilization is diversifying and joining new markets such as biofuel production and livestock feed [33, 34]. Therefore, maize productivity and production need to increase exponentially to support future food and feed demands as a food security crop. This increase will be required to occur under less inorganic input, severed land degradation, and aggravated climate disturbances that will profoundly disadvantage maize in its interaction with its biotic and abiotic environments. Considering the tremendous benefits of AMS to crops such as maize, we believe future maize research and breeding should adopt a more climate-smart approach by focusing more on understanding maize-AMF interaction and improving symbiotic capacity to boost yields. Therefore, in this chapter, we first review the components relevant to maize-AMF interaction, then present benefits of AMS in terms of biotic and abiotic stress tolerance and improvement of yield and yield components, and finally summarize pre-breeding information related to maize-AMF interaction and trait improvement avenues based on up-to-date molecular breeding technologies.

2. Arbuscular mycorrhizal fungi (AMF)

AMF are significant elements of the soil–plant system [13]. Mycorrhizae from the Greek “myco” for fungus and “rhize” for root essentially refers to the symbiotic association between fungi and plants’ roots. AMF constitute 5 to 50% of the microbial biomass of soils. Mycorrhizal hyphae biomass can vary from 54 to 900 kg per hectare [39], or nearly 200 meters of hyphae per gram of soil [40]. AMS is the most widespread and oldest terrestrial symbiosis [41], formed by more than 80–90% of terrestrial vascular plants [14]. AMF inhabit all continents, from the subarctic islands to the Antarctic Peninsula [19, 42]. According to Wang et al. [9], AMF has co-evolved with plants for at least 400 million years, allowing plants the colonization of lands by plants through improved hydro-mineral nutrition.

AMF belong to the phylum of Glomeromycota [43], with a taxonomic classification of AMF originally based on morpho-anatomical observations of spores [44]. However, the advances in biomolecular tools have allowed the use of Polymerase Chain Reaction (PCR) in the classification of AMF through amplification of ribosomal regions (18S), which permits a better definition of species or even molecular taxa [45]. More than 250 species of Glomeromyceta are currently described, with a constantly updated taxonomy updated, resulting in new species and higher taxa being regularly introduced [46, 47]. AMF are obligate symbiont because of their inability to develop without a host plant [48], with a reproductive system that can be either clonal or asexual through spore or coenocyte formation [49]. Several studies have shown the existence of hyphal fusions, called anastomoses which lead to exchanges of nuclei and cytoplasm between species of the same genus [50, 51, 52], hence participating in the conservation of diversity and the complex genetics of AMF [53]. Some AMF species are homokaryotic, with identical nuclei in each spore; thus, the genetic variation is present in each spore, resulting in several different copies of the same gene [54]. Heterokaryotic species are characterized by different nuclei in each spore, resulting in the distribution of the genetic variation among the different nuclei inside a spore. In other words, the existence of several different genomes in each spore helps AMF to adapt to different environments [55]. Furthermore, there are four phases in the life cycle of AMF [56]: the spore germination and hyphae growth in the rhizosphere phase, the root infection phase by hyphae, stimulated by carbon dioxide (CO₂), and root exudates, which propagates in the root, the phase of root colonization with the formation of arbuscules and vesicles, and the phase of the development of external hyphae, resulting in an increased volume of soil explored by the roots and production of spores [57].

3. Survey of AMF that colonize maize

Host specificity of AMF is a longtime debate among researchers. Although many authors argued that AMF have no host-plant specificity [58], several studies tend to demonstrate the preference of some AMF genus to some plant species [59]. Crops like maize have a relatively high mycorrhizal dependency for plant growth and nutrient uptake [60]. Mycorrhizal dependency is an intrinsic property of every plant species that depend on the AMS. The type of crop, the soil properties, and the effect of the cropping system characterize mycorrhizal dependency [61]. There are three categories of plant species according to their mycorrhizal dependency. First, non-mycotrophic plants are capable of developing without the intervention of mycorrhizal fungi. Secondly, facultative mycotrophs require that reproduction occurs in the presence of mycorrhizal symbiosis only when the environment in which they grow is nutrient-limited. Lastly, obligate mycotrophs can only complete their development cycle associated with AMF. To study mycorrhizal dependency, the percentage of root colonized by AMF is a critical index. For example, the root colonization rate is 0% for Brassica napus and between 50 and 70% for Z. mays [62].

Furthermore, researchers use different methods such as trap cultures, wet sieving, morphological identification of spores, and molecular tools to determine AMF communities associated with maize. Based on such methods, many authors showed that maize crops could associate with different AMF species belonging to various genera (Table 1). Paraglomeraceae, Aucolosporaceae, Gigasporaceae, Glomeraceae, Archaeosporaceae, and Paraglomeraceae are the AMF families that associated with maize; however, the Glomus group preponderant [63, 64, 66, 68, 69, 71]. Although it is well established that the genus Glomus has the most widespread dispersion, maize genotypes and agricultural systems influence the mycorrhizal community. Evaluation of maize varieties with different genomes revealed colonization levels depend largely and continuously on maize genotypes within each germplasm [70]. Maize monoculture also reduces AMF diversity [68, 71, 72]. For instance, Archaeosporaceae and Paraglomeraceae group are not colonizers of maize grown in monoculture [69].

AMF family	AMF species	Habitat	References
Acaulosporaceae	Acaulospora sp	Benin, Thailand	[63, 64, 65]
Acaulosporaceae	Acaulospora longula	Brazil	[66]
Acaulosporaceae	Acaulospora rugosa	Brazil	[66]
Acaulosporaceae	Acaulospora scrobiculata	Brazil	[66]
Acaulosporaceae	Acaulospora morrowiae	Brazil	[66]
Glomeraceae	Glomus caledonium	Japan	[67]
Entrophosporacae	Entrophospora sp	Thailand	[65]
Glomeraceae	Glomus sp	Benin, Hungary, Japan, Thailand	[63, 64, 65, 68, 69, 70]
Glomeraceae	Glomus intraradices	Germany, Switzerland	[71]
Glomeraceae	Scutellospora sp	Benin, Thailand	[63, 65]
Archaeosporaceae	Archaeospora sp	Hungary	[66, 69]
igasporaceae	Gigaspora sp	Benin, Hungary	[63, 64, 69]
Entrophosporaceae	Entrophospora schenckii	Thailand	[65]
Glomeraceae	Glomus mosseae	Germany, Switzerland	[65, 71]
Paraglomeraceae	Paraglomus sp	Hungary	[69, 71]
Gigasporaceae	Scutellospora fulgida	Thailand	[65]
Glomeraceae	Glomus geosporum	Thailand	[65]

Table 1.

Examples of AMF species associated with maize.

4. Phenotypic and molecular bases of maize-AMF symbiosis

Most terrestrial plants, including maize, interact with AMF under nutrient-limited 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 calmodulin-dependent 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, 94, 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 [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, 124, 125, 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, 135, 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 chitin-like 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, 140, 141, 142, 143, 144, 145], opening avenues for breeding of climate-considerate 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 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%.

8. Pre-breeding and breeding perspectives to maize-AMF symbiosis

One of the pivotal paths towards climate resilience and reversing the predicted negative impacts on food security is the adoption of climate-smart breeding approaches to design of high-yielding crops adapted to climate disturbances, such as increased abiotic and biotic stress [155, 156, 157]. Considering the tremendous positive impact of AMF symbiosis on maize yield, biotic and abiotic stress tolerance, Carbon and Nitrogen sink capacity, greenhouse gas mitigation [10, 14, 90, 91, 141, 158, 159, 160, 161], it is crucial to improve the symbiotic response capacity in the crop as a contribution to food security in the face of the changing climate scenarios and continuous land degradation [117, 162]. Besides, beneficial microorganisms such as AMF are not just traits influencers but rather are part of the phenotypic expression of plants in an integrated system started from nutrient mobilization to resource allocation to different plant functions, including resistance to biotic and abiotic tolerance, and nutrient efficiency and accumulation of assimilates in plant reserve organs which translates into yield [11, 163]. The current scientific information is more inclined towards a non-antagonistic impact of modern breeding activities on the mycorrhizal symbiotic capacity of maize [70, 96, 164, 165, 166], indicating the feasibility of efficient incorporating AMF-maize collaboration into ongoing breeding programs [165, 167, 168]. Therefore, the necessary continuous increase in crop yield to meet future food demands requires breeding efforts to improve symbiosis between critical crops such as maize with AMF [11, 166, 168].

For breeding to be efficient and achieve the expected genetic gains and progress, it is necessary to generate a host of useful pre-breeding information and devise breeding strategies based on state-of-art technologies selected through careful consideration of the factors that control maize-AMF interactions. In the coming sections, we review the critical pre-breeding information generated so far regarding maize symbiotic response to and interaction with AMF and identify areas necessitating further research to support breeding activities. We also devise breeding strategies to improve maize varieties’ symbiotic response and interaction to AMF based on the current knowledge of maize-AMF interactions.

8.1 Genetic diversity and inheritance of maize response to AMF symbiosis

The first and foremost step towards climate-smart crops is harnessing genetic diversity to allow the selection of superior material for breeding [169]. The maize genotype affects AMF abundance in the soil [170]. Maize has high genetic variance in response to AMF colonization [11, 70, 164, 170], indicating the possibility of selection in breeding. However, little information is available regarding the type of gene action controlling the maize response to AMP symbiosis, hence the need for more research in this area to judge opportunities for trait improvement through breeding. The high genetic diversity revealed by the few studies that investigated this pre-breeding characteristic of the maize-AMF interaction should be verified in several other genetic and environmental backgrounds to confirm promises of fast breeding progress. However, breeding progress might be hampered by the generally low to moderate heritability of maize responsiveness to AMF colonization [171].

Expectations of slow breeding progress are especially true for the traditional phenotypic selection, which strongly relies on phenotyping, increasing cost and time for budget-constrained breeding programs. Also, information related to the genetic control of maize-AMF interactions as to the proportion of additive and non-additive gene action involved in maize symbiotic response to AMF colonization [11]. Understanding the genetic control of any trait is crucial to designing effective breeding strategies for its improvement [172]. Therefore, besides the need to conduct more studies targeting maize levels of genetic diversity in response to AMF colonization, determining the preponderance of additive vs. dominance or epistasis genetic control on the trait needs to be elucidated better to inform future breeding strategies. New breeding approaches, especially those relying on advances in marker technologies such as marker-assisted selection (MAS) and genomic selection (GS), but also transgenic and genome editing (GE) techniques, could help to accelerate the improvement of maize responsiveness to AMF colonization and increase yield and stress tolerance [173, 174, 175, 176].

8.2 Genetic architecture of maize response to AMF

One of the first steps into implementing molecular breeding for maize response to AMF to increase yield performance is identifying genetic polymorphisms controlling the final maize benefit from the symbiosis [177]. However, very few studies undertook to map genomic regions associated with maize response to AMF symbiosis [168, 171]. Kaeppler et al. [164] conducted the first quantitative trait loci (QTL) mapping for maize interaction with AMF in a population generated from a cross between B73 and Mo17. They identified one QTL controlling maize responsiveness to AMF, and such a low number of QTL was attributable to the low heritability of the trait in their study. Twenty years later, Ramírez-Flores et al. [161] undertook another study to identify QTL that determined maize benefits from AMF symbiosis. Several QTL were identified in this study, suggesting a polygenic nature in the control of the trait, contrary to the monogenetic direction indicated by the earlier study [168, 171]. Considering the molecular complexity involved in the symbiosis process, from the recognition between AMF and plant to the effective establishment of the symbiotic relationship [158], the polygenic nature of Maize-AMF interaction is more plausible. However, more studies are required to confirm this hypothesis further.

Confirming the genetic architecture of maize-AMF interaction is pivotal since effective conventional or molecular breeding strategy design will depend on the number, siege of QTL controlling the traits and their interactions [178, 179]. These studies should be conducted in a wide variety of germplasm and geographical backgrounds to discover a comprehensive number of QTL that could accurately determine the genetic architecture of the trait through meta-analyses and other integrative studies [180].

8.3 Research perspectives and breeding strategies for improved maize response to AMS

Plant breeders generally are biased towards direct phenotypes, ignoring that most of these traits are mediated by beneficial microorganisms [11, 163, 167]. Although evidence points more towards a positive co-existence between modern plant breeding activities and practices, it is necessary to ascertain this status on target environments and maize populations. It is evident that response to AMF colonization is dependent on available resources such as soil phosphorus, crop species, and genotype [181]. The quantity and quality of soil phosphorus available to a particular crop are parameters that determine the maintenance of the diversity and quantity of the AMF community and their symbiotic capacity with maize [182]. Reports exist about the possibility of inhibition of the symbiosis between maize and AMF after artificial fertilization through the addition of external Phosphorus [183], making it necessary to adapt crop improvement for symbiotic capacity to target environments and cropping systems. Figure 1 shows the cascade of pre-breeding and breeding activities that should be involved in a strategic crop improvement program targeting improved maize response to AMF colonization and symbiosis.

Figure 1.

Pre-breeding and breeding pipeline for improved maize-AMF symbiosis.

A typical breeding program for any trait should identify adequate germplasm, possibly including wild relatives, exotic accessions, and landraces, as a base population for breeding through careful mating designs and accelerate genetic gains towards possible variety release [184, 185]. The base population should be both phenotyped for target traits and genotyped with molecular markers to allow measuring phenotype and marker-based genetic diversity and population structure to optimize downstream research and breeding activities such as parent selection and cross designs for pre-breeding activities such as inheritance and genetic control studies, QTL mapping, and selection techniques such as phenotypic selection (PS), MAS, GS [186, 187, 188, 189]. Phenotypic selection is a group of breeding methods basing the selection of superior genotypes for the next generation of for variety release on their observed phenotypic values. Phenotypic selection is best for highly heritable and easy-to-measure traits. Both MAS and GS are based on selecting genotypes using molecular markers, albeit they are essentially different. MAS relies on mapped QTLs for a particular trait, of which it uses associated markers to select desired phenotypically unobserved lines. MAS works best when the trait is monogenic or oligogenic (controlled by one or a few large-effect QTL) [178, 190]. GS uses whole-genome markers to compute genomic-estimated breeding values of phenotypically unobserved genotypes as a basis for selection. GS performs best on polygenic traits that are controlled by multiple small-effect QTL, which characterizes most traits that plant breeders investigate [191, 192, 193].

In the case of maize interaction with AMF, phenotyping should be done with a control (non-mycorrhized plants) experiment to allow direct estimation of benefits offered by AMF symbiosis on traits of interest as in several studies [109, 194, 195, 196]. A comprehensive number of phenotypic, biochemical, and omic traits should be selected for phenotyping based on their direct or indirect involvement in or them being influenced by maize-AMF interactions to run univariate and multivariate analyses for genotype ranking, estimation of AMS effect on target traits, and strength and direction of relationships among traits [197]. Where possible, high-throughput phenotyping (HTP) techniques should be used to precisely measure and allow the accurate estimation of genetic and genomic parameters, including genetic control and inheritance, marker-trait association, and genomic prediction accuracies [198, 199, 200]. During the last decade, HTP technologies served to precisely measure the shoot biomass of tomato, barley [201], and Medicago [201, 202] growth trends under AMF colonization and to estimate nitrogen use efficiency of tomato, barley, and Medicago plants [203].

It is noteworthy that the inherent low allele diversity and low recombination rates arising from the bi-parental nature of such mapping populations and the short timespan from their generation to advanced stages used for mapping are critical limits to most of these studies based on genetic linkage based QTL mapping methods [204, 205, 206, 207]. The low statistical power is because all the genetic and allele diversity only comes from the two parents crossed to generate the mapping population. The low resolution of QTL is caused by the short time for creating such populations, which, even with recombinant inbred lines, is still too little to allow enough recombination in the genome of the lines [206, 207]. These limitations lead to low statistical power for QTL discovery and low resolution of the genomic regions mapped [206, 207]. These shortcomings could have partly explained the low QTL number mapped by Kaeppler et al. [164], and that results from Ramírez-Flores et al. [161] might not have comprehensively captured the genetic architecture of maize-AF interaction. Genome-wide association studies (GWAS) is an alternative and complementary technique to pipe rental population-based QTL mapping from which it differs by the reliance on populations composed of diverse lines with historical recombination events. Consideration of genome-wide association studies (GWAS) should allow complementing traditional bi-parental QTL analyses, especially in Joint Linkage Association Mapping (JLAM), a technique that combines the strengths of both GWAS and biparental QTL mapping to alleviate their respective weaknesses [205]. Also, GWAS will increase the statistical power and resolution of the resulting QTL [204, 205, 208].

One of the main challenges breeders face is combining several traits of interest in elite lines due to the pervasive pleiotropic effect and close-linkage of genes controlling these traits, two genetic phenomena that yield similar phenotypic outcomes but are difficult to distinguish between each other unless specific analyses are performed [185, 209]. A prerequisite for efficiently achieving multiple-trait selection is delineating the genetic basis of the correlations among traits through multivariate analyses [210]. Several multivariate GWAS and GS exist in the perspective of genomics-aided multi-trait selection for maize response to AMF symbiosis. Multivariate methods allow leveraging shared genetic information among traits and possibly environments to increase statistical power and accuracy [210, 211]. Also, GWAS could complement GS by including GWAS-discovered QTL as fixed effects in GS models, which is reported to improve prediction accuracy, thereby increasing genetic gains per unit time [173].

Since its invention, GWAS has evolved, moving from single-locus single-trait mixed linear models proposed by Yu et al. [212] to multi-locus multi-traits algorithms, which, unlike the former, jointly test associations between several traits and all genome-wide markers [213, 214]. Single-trait mixed linear models suffer from several weaknesses, including high rates of false-negative associations caused by multiple testing issues that require stringent Bonferroni thresholds [215]. In contrast, multi-locus multi-traits algorithms have better statistical power by avoiding correcting for multiple testing [214, 216]. However, these methods are still inefficient in differentiating between the two causes of trait correlations [216]; instead, integration of structural equation modeling (SEM) to GWAS is necessary [217]. Several GWAS packages that incorporate SEM are available for use in the case of maize-AMF interactions, for instance, GW-SEM [218], SEM-GWAS [219], GenomicSEM [220]. A more advanced software package is the multi-trait multi-locus Structural Equation Modeling (mtmlSEM) that considers, besides the multi-trait framework, a multi-locus approach to model associations between multiple traits and all loci simultaneously using SEM [221]. Also, GWAS results should be complemented with a robust candidate gene discovery and In Silico and lad-based prioritization steps to allow selection of high-confidence trait-associated genes that could be used in molecular breeding techniques such as MAS GE [222, 223, 224, 225]. GE is a novel molecular breeding technique that, after mapping a genome region with an unfavorable genetic effect or with the potential of improving a trait, is used to precisely modify, insert, replace, or delete DNA in a genome [226].

Determining the genetic architecture of maize-AMF interactions will allow breeders to decide what selection approach would yield better genetic gains in a shorter time with a competitive budget requirement. However, considering the complexity of the molecular basis of symbiosis (see Section 7 of this chapter) and the probable polygenic nature of the phenomenon [168], it is expected that PS or MAS might not be efficient [227, 228, 229, 230]. GS, especially combined with HTP technologies, should accelerate genetic gains while reducing overall variety development costs [231]. For complex and polygenic traits such as maize-AMF interactions subject to several non-genetic influences, multi-trait GS models, especially those considering multi-environment trials (MET) such as R packages BMTME [232], would be of tremendous benefit. Multi-trait multi-environment GS methods are being routinely used for diverse traits of diverse crops, including maize [233, 234, 235, 236].

Acknowledgments

The Authors thank the Carnegie Corporation of New York for funding the Post-Doctoral Fellowship of A.B. through the Regional Universities Forum for Capacity Building in Agriculture (RUFORUM), Grant number: RU-NARO/2020/Post-Doc/02. A.F.F is supported by the Regional Academic Exchanged for Enhanced Skills in Fragile Ecosystem Management (REFORM) Program. I.D is supported by DST-FICCI sponsored by CV Raman International Fellowship for African Researchers.

Conflict of interest

The authors declare no conflict of interest.