Specialized symbiotic associations between the roots of plants and fungi are called

Abstract

Mycorrhizal fungi account for about 10% of identified fungal species, including essentially all of the Glomeromycota and substantial fractions of the Ascomycota and Basidiomycota. Several distinct types of mycorrhizal associations exist, including arbuscular, ericoid, orchid and ectomycorrhiza. Although arbuscular mycorrhiza evolved first, ectomycorrhizal and orchid mycorrhizal fungi are the most speciose types, and each of these types arose from multiple independent evolutionary events, followed by convergent evolution. Coevolution between mycorrhizal fungi and plants has played a key role in fungal diversification, but fungal diversification generally has not paralleled plant diversification, and relatively few fungi show strict host specificity.

7.4.2 Functioning of mycorrhizae and agriculture

Mycorrhiza is composed of two terms viz. “myco” refers to “fungi” while “rhiza” refers to the roots system of the plants. It is a symbiotic association between fungi and roots of plants in which both are mutually benefited from each other. Fungi colonize the root tissues of plants acting as host and provide soil mineral nutrition to plants and in reverse fungi find food from plants. In particular cases, mycorrhizae may have parasitic association with host plants (Johnson et al., 1997). These mycorrhizae play a significant role in the field of soil biology, plants’ nutrition, and soil chemistry.

Mycorrhizae are mainly of two types (https://en.wikipedia.org/wiki/Mycorrhiza). One is endomycorrhizae and the other is ectomycorrhizae. Endomycorrhize can be further divided into arbuscular mycorrhiza vesicular-arbuscular mycorrhizae, ericoid mycorrhizae, orchid mycorrhizae, and monotropoid mycorrhizae. In arbuscular mycorrhyzae, hyphae penetrate the cells of plant and produce balloon like vesicles or dichotomously branching invaginations (arbuscules) in order to exchange nutrition. Surface area of contact between hyphae and cell cytoplasm is greatly increased by these arbuscules which make easy nutritional facilitation between them. This type of mycorrhizal association is found in 85% of all plant families and occurs in several crop species (Wang and Qiu, 2006). Glycoprotein glomalin, may be one of the major stores of carbon in the soil, is produced by the hyphae of arbuscular mycorrhizal fungi (International Institute for Applied Systems Analysis, 2019). Ericoid mycorrhizae have intra-radical phase consisting of dense coils of hyphae in the outermost layer of root cells. They have also been shown to keep substantial saprophytic competences which would facilitate plants to obtain nutrients from not-yet-decomposed materials via decomposing actions of their ericoid partners. In orchid mycorrhizal association; hyphae penetrate the cells of root and form coils in order to exchange nutrition. Plants of monotropoid mycorrhiza are mainly heterotrophic or mixotrophic derive their carbon from the fungus partner. This mycorrhizal symbiosis is nonmutualistic parasite types.

Ectomycorrhizae are symbiotic associations between the roots of around 10% of plant families, mostly woody plants including the birch, dipterocarp, eucalyptus, oak, pine, and rose (Wang and Qiu, 2006) families, orchids, (“Orchids and fungi: An unexpected case of symbiosis,” 2011), and fungi belonging to the Basidiomycota, Ascomycota, and Zygomycota (https://en.wikipedia.org/wiki/Mycorrhiza). Nutrients can be shown to move between different plants through the fungal network. Carbon has been shown to move from paper birch trees into Douglas-fir trees thereby promoting succession in ecosystems (Simard et al., 1997).

Mycorrhizal association makes the plants safer from microbial soil borne pathogens and found to be helpful in plant defense either above the ground or below the ground. Enzymes excreted by mycorrhizae are poisonous for the soil born organism-like nematodes (Azcón-Aguilar and Barea, 1997). Mycorrhizae may help in making the plants safe from insects, drought, toxicity, and diseases (https://en.wikipedia.org/wiki/Mycorrhiza).

4.3 MYCORRHIZAS AS NUTRITIONAL MUTUALISMS

Except for orchid and monotropoid associations, mycorrhizas involve plant exchange of photosynthates in return for fungal exchange of mineral nutrients. The convergence of so many unrelated forms of mycorrhizas is a testament for the mutual benefits of these trading partnerships. To understand the dynamics of resource exchange in mycorrhizas, we must examine the mechanisms by which resources are acquired by both partners. Mycorrhizal fungi improve nutrient uptake for plants, in part, by exploring the soil more efficiently than plant roots. Mycorrhizal fungal hyphae occupy large volumes of soil, extending far beyond the nutrient depletion zone that develops around roots. Simard et al. (2002) estimated that, on average, the external hyphae of EM fungi produce a 60-fold increase in surface area. The small diameter of fungal hyphae allows them to extract nutrients from soil pore spaces too small for plant roots to exploit (van Breemen et al. 2000). Recent studies on phosphate and ammonium uptake also reveal that mycorrhizal fungi improve uptake kinetics through reductions in Km and increases in Vmax (van Tichelen and Colpaert 2000).

Most mycorrhizal fungi depend heavily on plant photosynthate to meet their energy requirements; AM fungi are obligate biotrophs while EM and ericoid fungi are biotrophs with some saprotrophic abilities. The carbon cost of mycorrhizas is difficult to accurately estimate, but field and laboratory studies suggest that plants allocate 10–20 percent of net primary production to their fungal associates (Smith and Read 1997). Root colonization by mycorrhizal fungi often increases rates of host plant photosynthesis. This effect has been attributed to mycorrhizal enhancement of plant nutritional status in some systems (Black et al. 2000) and a greater assimilate sink in other systems (Dosskey et al. 1990).

Mycorrhizal fungi are a significant carbon sink for their host plants, and if nutrient uptake benefits do not outweigh these carbon costs, then both plant and fungal growth can be depressed (Peng et al. 1993; Colpaert et al. 1996). Mycorrhizal biomass has been shown to both increase and decrease with increasing availability of soil nitrogen (Wallenda and Kottke 1998; Johnson et al. 2003a). Treseder and Allen (2002) proposed a conceptual model to account for this apparent contradiction (Figure 4.2a). The model is based on three premises:

Both plants and mycorrhizal fungi have minimum N and P requirements and plants have a higher total requirement for these nutrients than fungi.

Biomass of mycorrhizal fungi is limited by the availability of plant carbon allocated belowground.

Plants allocate less photosynthate belowground when they are not limited by nitrogen and phosphorus; thus, mycorrhizal growth decreases when availability of these nutrients is high.

At very low soil nitrogen and phosphorus availability, both plants and mycorrhizal fungi are nutrient limited, so enrichment of these resources will increase mycorrhizal growth. At very high nitrogen and phosphorus availability, neither plants nor fungi are limited by these elements; consequently mycorrhizal biomass is reduced as plants allocate relatively less photosynthate belowground and more aboveground to shoots (shaded area in Figure 4.2a). This model is useful because it provides a simple heuristic framework for understanding how the relative availability of below- (minerals) and aboveground (photosynthate) resources control mycorrhizal biomass. Considering the interplay between nitrogen and phosphorus availability may further enhance the predictive value of this model. Because mycorrhizal fungi generally acquire phosphorus more readily than their host plants, we predict that the mutualistic value of mycorrhizal associations to plants will be highest at high soil N:P ratios and diminish as N:P ratios decrease.

Two lines of evidence suggest that mycorrhizal plants have evolved mechanisms to actively balance photosynthate costs with mineral nutrient benefits. First, environmental factors that reduce photosynthetic rates, such as low light intensity, lead to reductions in mycorrhizal development (e.g., Gehring 2003). Secondly, plant allocation to root structures is sensitive to mycorrhizal benefits. This is observed at both a gross taxonomic level and within ecotypes of the same plant species. Plant taxa with coarse root systems (low surface area) are generally more dependent upon mycorrhizas than those with fibrous root systems (high surface area). This suggests that for highly mycotrophic plant taxa, it is more adaptive to provide a fungal partner with photosynthates than to maintain fibrous root systems (Newsham et al. 1995). Also, it appears that mycotrophic plants have evolved a certain degree of plasticity in their allocation to roots in response to their mycorrhizal status. Mycorrhizal plants often have reduced root:shoot ratios compared to non-mycorrhizal plants of the same species grown under identical conditions (Mosse 1973; Colpaert et al. 1996; Figure 4.1).

There is evidence that local ecotypes of plants and mycorrhizal fungi co-adapt to each other and to their local soil environment (Figure 4.3a). A comparison of Andropogon gerardii ecotypes from phosphorus-rich and phosphorus-poor prairies show that each ecotype grew best in the soil of its origin. Furthermore, the A. gerardii ecotype from the phosphorus-poor soil was three times more responsive to mycorrhizal colonization and had a significantly coarser root system than the ecotype from the phosphorus-rich soil (Schultz et al. 2001). These results suggest that the genetic composition of plant populations evolve so that mycorrhizal costs are minimized and benefits are maximized within the local soil fertility conditions.

Introduction

Mycorrhizas with many of the characteristics of ectomycorrhizas, but also exhibiting a high degree of intracellular penetration, have been described at various times in the last century in various species of tree. These ectendomycorrhizal structures appear to be quite distinct from ectomycorrhizas where a few cells only are penetrated by the fungus, or where the senescent cortex becomes fully colonized by hyphae in the late Hartig net zone (see Chapter 6). Ectendomycorrhizas are also distinct from ‘pseudomycorrhizas’ described in Pinus by Melin (1917, and later) as forms of colonization by septate fungi which did not form sheath and Hartig net.

The term ‘ectendomycorrhiza’ should be used as a purely descriptive name for tho se mycorrhizal roots which exhibit some of the structural characteristics of both ectomycorrhizas and endomycorrhizas, and it implies no functional significance. The symbioses described here, occurring mainly in conifers, are distinct from ageing ectomycorrhizas and from mycorrhizas in some members of the Ericales in which a considerable degree or a specialized kind of intracellular penetration

Mycorrhizas in Arbutus and Arctostaphylos

The fungi mycorrhizal with Arbutus and other plants in the Arbutoideae were long believed to be basidiomycetes because of the structural similarities between ECM and arbutoid mycorrhizas. This has been confirmed both by synthesis experiments and by the descriptions of dolipore septa in fungi associated with mycorrhizas of Arctostaphylos (Duddridge, 1980; Scannerini and Bonfante-Fasolo, 1983; Read, 1983). The work of Zak (1973, 1974, 1976a, 1976b) who both traced mycelium and performed synthesis experiments, showed that mycorrhizas in Arbutus menziesii and Arctostaphylos uva-ursi are formed by fungi which also form ectomycorrhizas. The fungi involved included Hebeloma crustuliniforme, Laccaria laccata, Lactarius sangufluus, Poria terrestris var. subluteus, Rhizopogon vinicolor, Pisolithus tinctorius, Poria terrestris, Thelephora terrestris, Piloderma bicolor and Cenococcum geophilum. Similarly, Molina and Trappe (1982a) tested the ability of 28 ECM fungi to form mycorrhizas with Arbutus menziesii and Arctostaphylos uva-ursi in pure culture. All but three produced arbutoid mycorrhizas with both species. The conclusion here must be that the plant plays an important part in regulating the development of mycorrhizas, with the consequence that different structures are produced in different taxa of plant.

There is no evidence which will allow comparison of function in arbutoid and ectomycorrhizas, but the assumption is that they operate similarly. The plants are all woody and photosynthetic and, since mycorrhizas are the common form of absorbing organ of members of the Arbutoideae, an important and ecologically significant group of species, the symbiosis must be assumed to be of selective advantage. This is even more likely because the sheath on the roots, as in ectomycorrhizas, may not only have a storage function, but also separates the plant from the soil. Hence, the fungus calls the tune in absorption by the short roots, and everything absorbed by them must pass through it. It seems extremely likely that the mycelium and rhizomorphs in soil are important in nutrient scavenging.

Massicotte et al. (1993) carried out a detailed analysis of the structure and histochemistry of arbutoid mycorrhizas, synthesized in growth pouches between Arbutus menziesii and the basidiomycetes Pisolithus tinctorius and Piloderma bicolor. The morphology of the mycorrhizas was strongly influenced by the identity of the fungal symbiont. In the case of plants colonized by P. tinctorius, repeated pinnate branching of first and second-order lateral roots produced a complex structure (Figure 7.5), similar to that originally described by Rivett (1924) in Arbutus unedo. This pattern of branching, which has been observed in associations between Arbutus spp. and a range of other fungi (Molina and Trappe, 1982a, 1982b; Giovannetti and Lioi, 1990), appears to arise from precocious initiation of individual lateral roots, rather than by dichotomy of the apical meristem of the root, as is typically seen in ectomycorrhizas of Pinus (Piché et al., 1982; see Chapter 6). Each of the rootlets colonized by P. tinctorius is ensheathed in a well-developed mantle, from the outer layer of which an extensive system of rhizomorphs develops. This is a feature previously recorded in associations between Arbutus and several other species ECM fungi (Zak, 1976b; Molina and Trappe, 1982b).

Mycorrhizas formed by P. bicolor, in contrast, were largely unbranched and had a thin or non-existent mantle, sparse surface hyphae being embedded in mucilage in a manner similar to that seen in ectendomycorrhizas formed in Pinus by Wilcoxina spp. (Piché et al., 1986; Scales and Peterson, 1991a).

A longitudinal section of the pinnately branched type of mycorrhiza formed by P. tinctorius (Figure 7.6) reveals a thick mantle, intercellular development of mycelium to produce a Hartig net and penetration of some epidermal cells by fungal hyphae which proliferate to form dense hyphal complexes (Massicotte et al., 1993). The combined presence of mantle, Hartig net and intracellular proliferation are diagnostic features of arbutoid mycorrhizas which can only be revealed by anatomical investigation. There are reports (e.g. Largent et al., 1980) that Arbutus spp. are ECM, but these are based only on superficial recognition of the presence of a mantle. Clearly, in the absence of more detailed structural analyses such reports must be regarded with suspicion.

It was observed by Rivett (1924) and subsequently confirmed (Fusconi and Bonfante-Fasolo, 1984; Münzenberger, 1991; Massicotte et al., 1993) that the Hartig net in Arbutus is of the para-epidermal kind typically found in ectomycorrhizas in the majority of angiosperms (Brundrett et al., 1990; Chapter 6). Massicotte et al. (1993) propose that deeper penetration may be prevented by deposition of suberin lamellae and a Casparian strip in radial walls of the outer tier of cortical cells, so forming an exodermis. The epidermal cell walls contain phenolic substances but no suberin and clearly do not inhibit fungal penetration. The physiological activity and potential storage role of the fungal tissue is indicated by the presence of glycogen rosettes and of polyphospate (polyP) (Ling Lee et al., 1975).

ROOT ENDOPHYTES

Mycorrhizal fungi are also endophytes. However, because they are primarily macrofungi that form well-characterized, specialized symbioses with their hosts, they are considered separately (see Chapter 15) from more generalized root endophytes. Root endophytes as we describe them refer to nonmycorrhizal microfungi that infect roots or associate with mycorrhizae. Roots of forest trees are colonized by a variety of nonmycorrhizal endophytes, although detailed investigations of healthy roots exist only for a few hosts. Soil fungi, saprobic rhizosphere fungi, fungal root pathogens, and endophytes overlap considerably, although certain taxa appear to be isolated repeatedly and preferentially as symbionts from living roots. Nonmycorrhizal microfungi isolated from serially washed mycorrhizal roots of Picea mariana (Summerbell 1989) were primarily sterile strains of Mycelium radicis atrovirens and Penicillium species. Holdenrieder and Sieber (1992) similarly used serial washing to compare populations of endophytic fungi colonizing roots of Picea abies in relation to site and soil characteristics. Of the 120 taxa recovered, Mycelium radicis atrovirens, Penicillium species, Cylindrocarpon destructans, and Cryptosporiopsis species were isolated most frequently.

Phialocephala fortinii, P. dimorphospora, P. finlandia, Oidiodendron species, Geomyces species, and Scytalidium vaccinii (Dalpe et al. 1989) are common components of a guild of endophytes forming root associations with alpine ericoid and other perennial hosts. Mycelium radicis atrovirens, generally regarded as a heterogeneous taxon, is the name commonly applied to sterile dematiaceous isolates. Roots colonized by these fungi have a unique morphology, particularly when associated with ericoid hosts; consequently, they sometimes are termed ericoid mycorrhizae, although the fungi apparently have a much broader host range (Stoyke and Currah 1991; Stoyke et al. 1992). Dark, septate endophytes dominated the mycobiota isolated from the fine roots of several species of forest trees and shrubs in Europe and western Canada. A large proportion of those isolates proved to be Phialocephala fortinii, a root-inhabiting fungus with a very broad host distribution and geographic range (Ahlick and Sieber 1996).

Hyphae in roots appear rhizoctonialike with “monilioid hyphae” and frequently produce a loose weft on the outer root surface. Root colonization is relatively extensive, but intracellular colonization of outer cortical cells is limited. Coiled or branched hyphae and intracellular microsclerotia may be present. In contrast, hyphae associated with conifer hosts form ectomycorrhizalike structures in which the intercellular colony resembles a Hartig net (Wilcox and Wang 1987; O'Dell et al. 1993). In culture, the fungi characteristically have thick-walled, dark-pigmented, septate hyphae and are usually sterile or very slow to sporulate.

Although root endophytes are apparently quite common, with wide geographic and host distributions, the ecological role of most species is unknown, although some may form mycorrhizae or be root pathogens. In addition to the roots of their ericoid hosts, Phialocephala fortinii and Mycelium radicis atrovirens commonly are isolated from roots of hardwoods (Fagus sylvatica), conifers (Abies alba, Picea abies, Pinus sylvestris, P. resinosa, P. contorta), and various alpine perennials (Wang and Wilcox 1985; Holdenrieder and Sieber 1992; Stoyke et al. 1992; O'Dell et al. 1993; Ahlick and Sieber 1996). Root morphology, depending on the extent of fungal infection, is described as ectomycorrhizal, ectendomycorrhizal, pseudomycorrhizal, nonmycorrhizal, or possibly pathogenic (Wilcox and Wang 1987). Species designations are based on morphotypes, which are not very informative. A current trend, therefore, is to use biochemical or genetic markers to distinguish host- or site-specific strains. This approach is exemplified by the restriction-fragment-length polymorphism analyses of sterile P. fortinii isolates from various alpine hosts (Stoyke et al. 1992) and of E-strain mycorrhizal fungi, a relatively uniform morpho-group that produces chlamydospores on and within infected roots (Egger and Fortin 1990; Egger et al. 1991).

The number and identities of species are uncertain. Repeatedly reported taxa in the group are Rhizoctonia species, Phialocephala species, Phialophora species, and Chloridium species. Scytalidium vaccinii, Gymnascella dankaliensis, Myxotrichum setosum, and Pseudogymnoascus roseus also may form ericoid root associations (Stoyke and Currah 1991). Species of Exophiala, Hormonema, Monodictys, and Phaeoramularia have been reported from the roots of several forest trees (Ahlick and Sieber 1996). Although most appear to have affinities among the orders of Discomycetes (Monreal et al. 1999), too little is known to generalize about the possible involvement of basidiomycetes. Sterile, basidiomycetous root endophytes have been reported (e.g., Ahlick and Sieber 1996); typically they are not melanized. The paucity of morphological characters and difficulty of inducing sporulation in root fungi contribute to the difficulty of identification. Inoculation experiments on host responses to infections or pathogenicity of Mycelium radicis atrovirens have led to contradictory and inconclusive results ranging from beneficial to pathogenic reactions. Host range is apparently broad, based on inoculation studies (Wilcox and Wang 1987). Roots and other tissues of various tropical epiphytes have been examined by Dreyfuss and Petrini (1984), Petrini and Dreyfuss (1981), and Richardson and Currah (1995).

19.6 Stress response and pigments/phenolics in ECM fungi

Mycorrhizal fungi are exposed to all or many of the environmental stresses that other fungi may experience. These include extremes of temperature and pH, anoxia, water stress, physical fragmentation, toxic metals, and other pollutants, as well as anthropogenic stresses arising from applications of fertilizers, lime, and wood ash. Hence, stress response in ECM fungi has attracted the attention of various researchers. Fungi can respond to these stresses by altering their morphology, modifying their external environment, or adapting their internal metabolism [72]. When any stress is applied, the most prominent physiological reactions are the production of a set of novel proteins or an increase in the number of certain types of existing proteins. Stress proteins or heat shock proteins (sHSPs) synthesized could be the high-molecular size (69–120 kDa), medium-molecular size (39–68 kDa), or low-molecular size (<38 kDa). The sHSPs ranging from 12 to 42 KDa are synthesized ubiquitously in eukaryotic and prokaryotic cells in response to heat and other stresses [23]. Synthesis and accumulation of protective organic compounds, such as sucrose, glycogen, and trehalose, are one of the tolerance mechanisms found in fungi to live in adverse environmental conditions [73]. Ferreira et al. [24] have demonstrated an accumulation of intracellular trehalose and an increase in trehalase activity in the mycelium of Pisolithus sp. RV82 in response to heat shock at 42⁰C. The methodology employed is protein profile of ECM fungus in optimum and stressed conditions that are carried by sodium dodecyl sulfate - polyacrylamide gel electrophoresis (SDS-PAGE) followed by protein staining by silver staining. Better resolution can be achieved by carrying out 2D electrophoresis; Isoelectric focusing (IEF) followed by SDS-PAGE. Recently, Ramesh et al. [74], characterized two metallothionine (MTs) genes, HcMT1 and HcMT2, from the ECM fungus Hebeloma cylindrosporum under metal stress by performing competitive reverse transcription-PCR analysis. They observed that ECM fungi encode different MTs with particular expression pattern that could help the host plant to survive and grow in heavy metals contaminated ecosystems. The authors have identified a heat-shock protein gene from L. bicolor (Lbhsp) that could probably have a significant role in the establishment of functional ECM. The methodology included characterizing this gene and gene product is cDNA cloning, sequencing, and phylogenetic analysis, Northern and Southern analysis. The results suggest that Lbhsp mRNA is about 611 nucleotides long and codes for ∼17 kDa protein and could play a key role in Ras-mediated mycorrhizal signaling pathway [73]. Metabolome profiling reveals the formation of stress-related metabolites like amino acids [75].

ECM fungi produce various pigmented compounds. Pigments in higher fungi are well documented [76,77]. The fruiting bodies of some macrofungi elaborate hues ranging from beautiful shades of pink, blue, yellow, red, and brown (e.g., Boletus, Cortinarius, Hypholoma, Hydnellum, Hygrocybe, spp.) [78]. Several varieties of Dermocybe species yield pink, salmon, and red dyes, Gymnopilus spectabilis yields butter yellow dye, and Omphalotus olivascens yields gray, green, and purple dyes [31].

The chromophores of mushroom dyes contain a variety of fascinating organic compounds. Their pigmentation may vary with the age and some undergo distinctive color changes on bruising. Many of the pigments of higher fungi are quinones or similar conjugated structures. Recently, the presence of naphthoquinone, a naphthalenoid pulvinic acid (pisoquinone), has been demonstrated in a white-skinned variant of the ectomycorrhizal gastromycete Pisolithus arhizus [79]. Phenolic compounds produced by ECM fungi are antimicrobial and involved in the protection against invasion by pathogens and competitive organisms. Antimicrobial phenolic compounds are synthesized in fruiting bodies, soil hyphae, and the mantle in ECM [80]. This probably explains host root protection that has been observed in ECM symbiosis. Experimentation is usually carried out by testing the antagonistic activity of cell-free culture broth or pigments extracted from fruiting bodies against phytopathogenic cultures by disc diffusion or bore well method [29].

Mycorrhiza

Many mycorrhizal fungi are known to adapt to adverse environments, hence, they can prove beneficial to plants under stressed conditions (Mosse et al., 1981). Interestingly, when drought stress is perceived by most of the plants, they ask AMF for help, by secreting a signaling molecule into the rhizosphere called as “strigolactone” a class of phytohormones (Oldroyd, 2013). At the same time, mycorrhizal hyphae extensively improves soil structure as well as its water holding capacity (Miller and Jastrow, 2000), hence, improves overall growth of the plants in comparison to their non-mycorrhizal counterparts (Augé, 2004). In addition, AMF can also form hyphal fusion (anastomosis) which is a vital mechanism for AMF persistence and to enhances their chances of survival under water deficit conditions (Giovannetti et al., 2015). Previous studies have found that, host plant’s association with mycorrhiza increases the water status of the plant, as shown by relative leaf water content (RLWC) (Wu et al., 2017; Barros et al., 2018). Plants colonized by AMF show enhanced biosynthesis of osmolytes such as sugars and proline, which lowers the water potential of drought stressed plants (Latef et al., 2016; Ruiz-Lozano, 2003), and this lowered water potential allow plants to maintain turgor and carry on various physiological activities such as growth, photosynthesis and stomatal opening (Smith et al., 2010). There are many studies which have reported that inoculation of AMF enhances water content in the leaves of Zucchini plants leaves (Colla et al., 2008), water potential and nutrients status in maize plants (Feng et al., 2000) and chlorophyll content of plants, for example, Sesbania aegyptica, S. grandiflora and Lotus glaber (Colla et al., 2008; Giri and Mukerji, 2004; Sannazzaro et al., 2006).

Ouledali et al. (2018) revealed that AMF colonization enhances root growth and architecture which overall develops a greatly functional root system and this leads to the uptake of water and nutrients. In a study on Lactuca sativa it was reported that plants infected by AM fungi showed higher content of water in comparison to non-infected control plants under water deficit conditions (Marulanda et al., 2003). Root-shoot dry weights, fruit fresh weight, chlorophyll content, carotenoids content showed a significant increase under drought stress in Capsicum annuum plants colonized by AMF in comparison to non-AMF plants (Mena-Violante et al., 2006). Li et al. (2019) found that AMF colonization sufficiently reduced the adverse effects of water deficit conditions on plant growth and enhanced plant biomass, photosynthetic rate, stomatal conductance, intrinsic water use efficiency (iWUE), superoxide dismutase (SOD) activity and reduced malondialdehyde (MDA) levels in Leymus chinensis (C3 plant) under light and moderate drought and in Hemarthria altissima (C4 plant) AMF colonization increased the aboveground biomass and CAT activity in light and moderate drought stress. In another experiment, when the effects of three different water regimes on lettuce and tomato plant were examined in AMF and non-AMF plants it was found that AMF infected plants showed a better growth and efficiency of photosystem II (PS II) in comparison to non-AMF plants from very early stages of plant colonization (Ruiz-Lozano et al., 2016). Regarding its effect on drought, in different crops studied, AMF colonization alleviates the deleterious effects of the stress by making the host plant more tolerant to drought (Aroca et al., 2012; Bárzana et al., 2014; Augé et al., 2015; Moradtalab et al., 2019).

6.2.2.1 Mycorrhizal fungi

The word mycorrhiza itself explain it as the symbiotic association between plant root and fungi since this word is derived from the Greek word “mikos” meaning fungi and “rhiza” meaning roots (Bonfante & Anca, 2009; Pathan et al., 2020). Mycorrhiza is classified into two groups as ectomycorrhiza and endomycorrhiza on the basis of their anatomical location in plant root. Ectomycorrhiza remains extracellular colonizing in intercellular spaces of epidermal and cortical cells of root forming hartig net associate mainly with woody plants like trees and shrubs, while endomycorrhiza is intracellular penetrating the hyphae inside the root cells forming hyphopodia (or appressoria), coils, and arbuscule. The endomycorrhiza is further classified into three subgroups as AMF, ericoid, and orchid. The AMF is most predominant associated with more than 80% of land plants including most agricultural crops (Akiyama et al., 2005; Daniell et al., 2001; Garcia et al., 2015), while ericoid mycorrhiza is restricted to the order Ericales and orchid to the family Orchidaceae. The extensive studies have been done primarily on two types of mycorrhiza: ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) due to their ecological and economic importance (Garcia et al., 2015).

Their functions include beneficial effects in establishment of plant, nutrient uptake, against biotic and abiotic stresses, as biocontrol agent, and the improvement of soil structure (Barea et al., 2005; Ocón et al., 2007).

Mycorrhiza colonize plant tissues through several steps like host recognition, spore germination, penetration of the epidermis, and tissue colonization (Petrini, 1991). They invade plant tissues with extensive branching in the vicinity of host root and penetrate the cuticle and epidermal cell wall. The infection may be inter or intracellular limiting to a single or multiple cells around the penetration site (Akiyama et al., 2005).

The sporulation and hyphal growth of mycorrhiza is initiated by signal molecules of root exudates. For example, Rutin, a flavonoid signal, in Eucalyptus globulus spp. bicostata root exudates initiates the ECM hyphal growth (Lagrange et al., 2001). Similarly, abietic acid in Pinus sylvestris root exudates induces germination of Suillus spp. spores (Garcia et al., 2015). Mycorrhiza also produces symbiotic signals like Myc factors that helps it to proliferate in diverse families like Fabaceae, Asteraceae, and Umbelliferae of the plants. Such discovery of Myc factors open the door for molecular and cellular dissection of plant root endosymbiosis (Maillet et al., 2011).

These fungal symbionts form a bridge between soil and plant roots termed “wood-wide web” which create network among different plants resulting horizontal nutrient movement (Simard et al., 1997). These networks promote aggregation and stability of soil that significantly affects soil quality and increase plant productivity (Rillig & Mummey, 2006). Many achlorophyllous heterotrophic plants obtain organic carbon through this network for their growth.

Nutritional Characteristics

All orchid mycorrhizal fungi are able to obtain carbohydrate from outside the orchid, although there is some diversity of source. Most are relatively fast-growing saprophytes which, in culture, can use complex polymers such as starch, pectin and cellulose and occasionally lignin, as well as soluble sugars (Burgeff, 1936; Harley, 1969; Perombelon and Hadley, 1965; Smith, 1966; Hadley and Ong, 1978). Others, such as Rhizoctonia soiani and Armillaria mellea, grow saprophytically in culture but are better known as parasites. There is also mounting evidence of mycelial links between orchids and ectomycorrhizal plant species. Warcup (1985, 1991) has demonstrated that a Rhizoctonia which forms orchid mycorrhizas with Rhi-zanthella is also ectomycorrhizal with Melaleuca uncinata, while Zelmer and Currah (1995) have shown that Corallorhiza trifida is linked to ectomycorrhizal Pinus contorta by a slow-growing, bright yellow, clamp-forming fungus. In these examples, as well as the case of Cephalanthera austinae mentioned above, the implication is that both fungus and orchid are using organic C from the photosynthetic mycorrhizal species.

Orchid fungi have unspecialized requirements for nutrients other than C sources. Most can use a wide range of N compounds, at least in pure culture (Hollander, in Burgeff, 1936; see also Arditti, 1979, 1992) and it seems likely that organic sources of N, and possibly also P, are important for these fungi growing in soil. although direct investigations have not been carried out. The significance in nature of the effects of B vitamins, yeast extract and root exudates which have been observed in culture, has not been followed up, although it may have relevance to the horticultural production of orchids by symbiotic methods (Vermeulen, 1946; de Silva and Wood, 1964; Perombelon and Hadley, 1965; Hadley and Ong, 1978).