Industrial Applications of Soil Microbes: Volume 4 E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Serie: Industrial Applications of Soil Microbes
- Sprache: Englisch
Industrial Applications of Soil Microbes is a compilation of reviews on the industrial usage of soil microorganisms. Readers will be updated about recent applications of soil bacteria, fungi and viruses in sectors such as agriculture, biotechnology, environmental management.
Volume 4 includes review on mycorrhizal fungi, endophytes and a range of microbial chemicals and processes beneficuall at industrial scale. The 19 chapters start with an overview of mycorrhizae as biofertilizers, their symbiosis with plants, and their applications in improving crop yield, stress management, and soil health. Case studies on Lycopersicon esculentum highlight practical benefits. Soil microbes, endophytes, and microbial proteases are discussed for their role in biocontrol, disease management, and crop improvement. The volume also explores eco-friendly nematicides, viruses in temperate fruit crops, mushrooms’ nutritional value, and metagenomics for bioinoculants. Overall, the volume emphasizes sustainable practices and future prospects involving microbes and microbe-assisted processes.
Readership
General readers, microbiology and biotechnology enthusiasts, science students and industrial trainees
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Seitenzahl: 842
Veröffentlichungsjahr: 2024
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FOREWORD
I, being an academician and researcher, feel very happy to write a foreword for the present book “Industrial Applications of Soil Microbes” volume 4. In the present scenario of environmental conditions, plant health and humans need for a sustainable life, therefore, it has become a need to find out solutions to increase healthy crop yield, medicines, food, etc.
In the past, we have used enormous chemical fertilizers to increase the crop yield to feed the increasing population of the world. But it has caused a loss to the physical structure of the soil and its chemistry. These chemicals have also caused chemical pollution to the soil and the environment. Therefore, there is a rise in various plant, animal and human diseases. Therefore, there is a need to revive the soil and the microorganisms living in a harmonious environment.
I am sure that the chapters written by various eminent experts in their field on mycorrhizae, fungi, bacteria and soul-borne viruses will not only provide basic information about these microorganisms but also present the recent development in this field. The metagenomic approach is also an important way to find out soil microbes in a very less time and is helpful in the study of these microbes. An interesting part is that metal-nanoparticle interaction with plants and soil microbes could also be a new source to improve crop yield.
I am sure that this book will be helpful to all the academicians, researchers, graduate and postgraduate students of agriculture as well as biotechnology and industrialists. I wish good luck to all the authors of the book and the editors.
PREFACE
Soil microbes, including algae, fungi, bacteria, and nematodes, can cause significant plant diseases, resulting in substantial yield losses. Conversely, healthy soil contains a diverse array of microbes in a delicate balance. These microscopic and submicroscopic organisms are crucial not only for plant health but also for the well-being of animal life and the overall environment. Over the past 30 to 35 years, the field of soil microbiology has blossomed, particularly after key discoveries regarding nutrient cycles involving these microbes. The presence or absence of such microorganisms can influence nutrient cycling, soil conditions, and environmental health. A single gram of soil can host a multitude of microbes that interact with one another and their surroundings, driving various biogeochemical cycles. Mycorrhiza is a prime example of this interaction, where fungi partner with plant roots to enhance mineral nutrient uptake. Plants with mycorrhizal associations exhibit improved growth, development, and yield under favorable environmental conditions.
The discovery of secondary metabolites produced by microbes during their interactions with plants and the environment has revealed their significant benefits to human life. These metabolites are now employed across various industries, including pharmaceuticals, food and beverages, and textiles. Advances in biotechnology and genome sequencing have deepened our understanding of these microbes and their interactions with both plants and the environment. This growing knowledge continues to enhance our ability to harness the potential of these secondary metabolites for various applications.
This book's chapters delve into the dynamics of soil microbes, including bacteria, fungi, and mycorrhizae, while also highlighting the latest advancements in the field. The volume is divided into two parts: the first focuses on fundamental and advanced knowledge of mycorrhizae and their interactions with plants, while the second addresses other soil microbes, such as fungi, bacteria, and soil-borne viruses. Additionally, it explores the effects of metal nanoparticles on fungal and bacterial populations at the molecular level, aiming to enhance plant health, growth, and yield. Microbial genetic diversity plays a crucial role in the soil environment, and metagenomic analyses can uncover novel molecules for therapeutic, biotechnological, and sustainable agricultural applications.
We encourage students in biology, ecology, biogeochemistry, and soil science, as well as participants in online courses, engineers, foresters, biogeochemists, agronomists, biotechnologists, bioscience educators, and researchers worldwide to engage critically with our volumes for insights relevant to their respective fields.
With optimism, the editors of this volume welcome suggestions and comments to enhance the content for future editions.
List of Contributors
An Overview of Mycorrhizae: Nature's Own Biofertilizers
Lalit Mohan1,Vandana Gupta1,*Abstract
Mycorrhizae are mutualistic associations between plant roots and fungi, conferring several advantages to plants, improving their survival and growth even under harsh soil conditions such as drought, acidic pH, the presence of toxic compounds, low nutrient availability, the presence of soil pathogens, etc., and hence act as nature’s own biofertilizers. The importance of mycorrhizal associations is signified by the fact that almost all the plant species on our planet carry these associations at least for some part and typically for most of their life cycle. In this chapter, our focus is to provide undergraduate and graduate students with an overview of three different types of mycorrhizae, namely endo-mycorrhizae, ectomycorrhizae, and ectendomycorrizae, based primarily on their macro- and microscopic structures. Further classification of endomycorrhizae into vesicular arbuscular mycorrhizae (VAM), arbuscular mycorrhizae, orchid, and ericoid mycorrhizae and classification of ectendomycorrhizae into monotropoid and arbutoid mycorrhizae are based on further details of microscopic features and the fungal and plant species involved. This chapter also aims at providing the reader with an insight into the advantages conferred by the fungal partner to the plants and the accelerated use of these fungi as inoculants for various applications such as agriculture, afforestation, and reclamation of waste lands.
INTRODUCTION
Albert Bernard Frankin (1885) discovered a special association between plant roots and microorganisms. For the first time, he introduced the Greek term “mycorrhiza,” meaning “fungus roots”. He reported that these were widespread on the roots of many woody plants. He also suggested that these mycorrhizae represent a mutualistic association in which the fungus counterpart absorbs minerals from the soil and humus and transfers or translocates them to the
plants, which in turn provide nutrition to the fungus [1]. The discovery by A.B. Frank remained a topic of controversy for almost 40 years, but several related observations and experiments discarded all the controversies, establishing the existence of mycorrhizal associations with a wide range of plants. The existence of mycorrhizae has also been confirmed by fossil records. The records also suggest the evolution of plants along with mycorrhizae [1]. Though Frank was not the first to discover the mycorrhizae (now known as ectomycorrhizae), he is credited because he did his research and his interpretations led him to logical conclusions [2]. Frank was also the first to study in detail the stages of development of ectomycorrhizae, starting from the point of contact of the hypha with the roots to their complete development [1].
The most common fungal partners forming mycorrhizal associations are members of Zygomycetes, Basidiomycetes, and Ascomycetes. There are at least seven different kinds of mycorrhizal associations depending on the type of host plant, the fungus partner, and the nature of interaction between the host plant and the fungal partner.
The term “colony of mycorrhizae” refers to the hyphae of the mycorrhizal fungus originating from one entry point inside the root or one propagule in the soil. The term colonization refers to the extent or degree of the occupation or coverage of the roots by the mycorrhizal fungus. The features of the host plant, the mycorrhizal fungus, as well as the soil and environmental conditions, regulate the mycorrhizal associations (Fig. 1) [3].
Fig. (1)) Three-way interaction involved in the development of the mycorrhizal associations.The factors influencing the occurrence of the effective mycorrhizal association are: (a) Properties of the roots of the host plant; (b) Climatic factors; (c) Host-fungus compatibility; (d) Disturbances in the soil; and (e) Organisms present in the soil. The mycorrhizal fungi constitute a dominant component of the soil microflora with restricted saprophytic abilities. The effectiveness of mycorrhizal association, such as the amount of soil hyphae produced compared to the root hyphae and physiological characteristics such as nutrient uptake and translocation, is mainly governed by the endophytic properties of the mycorrhizal fungus. Steps in the formation, maturation, and senescence of mycorrhizal associations are depicted in Fig. (2).
Fig. (2)) Sequential steps in the development of mycorrhizal associations, maturation of the association, senescence, and propagule formation.The mycorrhizal association helps to improve plant productivity and connects the plants to the soil via a network of hyphae. The association plays an important role not only in the uptake of various key elements such as nitrogen, phosphorus, iron, calcium, and carbon in the plants, which have lost their photosynthetic capabilities and parasitize mycorrhizal fungi associated with neighbouring photosynthetic plants to fulfil their carbon requirement; approximately 400 plant species form these types of associations. Not only uptake, but mycorrhizal associations also facilitate the solubilization of at least some minerals like phosphorus and iron, thus increasing their bioavailability for plants. The mycorrhizal association has an impact on the decomposition of the litter, seedling establishment, soil aggregation, and soil formation. Studies even show that mycorrhizal fungi can provide an additive competitive advantage to the host plants [4]. Due to the enormous advantages posed by mycorrhizal fungi, their importance has been increasing in agriculture and forestry. They are now being considered the best candidate in the biofertilizer's category. The mycorrhizal fungus, when missing from the soil, leads to an inefficient functioning of the ecosystem. The establishment of natural-level associations can help replace conventional fertilization practices and thus enable us to achieve sustainable agricultural goals [5].
TYPES OF MYCORRHIZAE
The classification of the mycorrhizal association as depicted in Fig. (3) depends upon three factors, namely anatomical differences, evolutionary differences, and their respective functions [6]. The microscopic differences in the different types of mycorrhizae are illustrated in Fig. (4).
Fig. (3)) Different types of mycorrhizal associations formed between the plant roots and the mycorrhizal fungi. The type of the mycorrhizal association developed depends upon the specific nature of the host plant and the fungal species forming mycorrhizal association.Endomycorrhizae
In this type of association, the fungal structures are located within the cortical cells of the host root following intracellular penetration; thus, the name “endo” is given to this type of mycorrhiza. The fungal hyphae colonize the epidermal and cortical cells of the plant roots and generally do not lead to any observable macroscopic changes in the root. The hyphae of the endomycorrhizal fungi extend from the infected roots in the surrounding soil and form a vast hyphal network. These extensive networks help the plants fetch nutrients from faraway sites as the hyphae invade the soil much more extensively than the root hair and otherwise remain out of the reach of the plant roots.
Vesicular Arbuscular Mycorrhizae (VAM)
These endomycorrhizal fungi produce peculiar structures such as vesicles (structures resembling bladders) and arbuscules (structures resembling branched finger-like hyphae) within the cortical cells of infected roots. Because of the presence of these structures, this type of mycorrhiza is known as vesicular arbuscular mycorrhizae (VAM). The mycelia of the VAM fungi are either septate or aseptate, branching out intracellularly in the root’s cortical cells, forming arbuscules, and causing a certain degree of low-level damage to the surrounding tissues. Arbuscules increase the surface area for nutrient exchange significantly and probably are associated with absorptive functions, while vesicles are defined as the terminal swollen part of the hyphae containing lipids, which has a storage function (Fig. 4) [7].
Fig. (4)) A transverse section of the root depicting various mycorrhizal associations formed with the plant roots, the various microscopic changes that occur in the host plant roots upon the formation of such associations, and the comparison of the uninfected root with the mycorrhizal infected roots.The VAM colonizes approximately 80% of the total plant species. There are approximately 150 different fungal species forming VAM, which prominently include the genera Scutellospora, Gigaspora, Sclerocystis, Glomus, Entrophospora, and Acaulospora [8]. The range of host plants extends from Bryophyta to the most advanced angiosperms. The fossil evidence has proven that the VAM has been present since the history of vascular plants and has undergone very little or no genetic variation.
Arbuscular Mycorrhizae (AM)
AM is the most common type of endomycorrhizal association found in the ecosystem and is characterized by the formation of arbuscules in the cortical cells. The lifespan of arbuscules is about 4–15 days; after completing their lifespan, they break down and the root cells assume their normal shape [9]. Arbuscules are the site for nutrient exchange, as in VAM.
The AM fungi belong to the phylum Glomeromycota. The fungal genera include Acaulospora, Ambispora, Archespora, Intraspora, Diverspora, and Glomus. The AM fungi show no particular specificity for particular host species and are known to have evolved in early land plants in order to improve carbon-nutrient exchange using the arbuscules.
VAM and AM fungi produce chlamydospores (large perennating structures) in the soil in association with plant roots. These spores then germinate near the new plant roots and initiate the infection process. The hyphae grow and penetrate the host root tissue, form appressoria in the root cortex, and eventually penetrate the root cells. As a result of cell penetration by the hyphae, the plasmalemma invaginates and starts to grow. The VAM and AM fungi are termed “obligate biotrophs,” as they have a definite requirement for a host for their survival. VAM and AM fungi are known to increase plant growth by augmenting water and phosphate uptake abilities in addition to other mineral elements such as Zn, K, N, S, Ca, and Mg. They enhance draught and salinity tolerance, provide resistance to disease, and help host plants tolerate high metal concentrations found in certain soils [10]. They are a potential candidate for bio-control agents as they control the root infections and produce different kinds of hormones needed by the plant for proper growth [7]. These associations also improve soil structure and aggregation. It has recently been demonstrated that AMF reduces the N2O emission from soil and is thus useful in maintaining climatic changes [10]. N2O is produced as a result of dissimilatory nitrate reduction. It rises to the stratosphere and leads to ozone depletion.
Ericoid Mycorrhizae (ErM)
In ericoid mycorrhizae, the fungal hyphae penetrate the host root cells intracellularly, but there is no mantle or Hartig net formation [11, 12]. The absence of root hairs is one of the distinctive features of this association [12]. The intracellular penetration by fungal hyphae is restricted to mature epidermal cells [9] (Fig. 4). The main function of the ErM is to detoxify peat and help improve plant nutrient uptake [12]. Members of Ascomycota such as Phoma, Hymenoscyphus, Myxotrichum, and Gymnascella are the fungal members forming such associations [12, 16]. The plant taxa involved in such associations are Ericales and Bryophyta [11].
Orchid Mycorrhizae
These are the mycorrhizal associations formed with the orchids. The orchid mycorrhizal association is highly specific, formed only by a narrow range of fungi. Every orchid relies on mycorrhizal fungi at some point in its life cycle for nutrient supply. Fungal partners mainly provide the host with carbohydrates, as the host is non-photosynthetic. It has also been observed that most orchid seeds fail to germinate unless infected with a suitable fungus. For the first year of life, the host is completely dependent on the fungus for its nutrition requirements [9]. The orchid mycorrhizal fungus belongs to the Basidiomycota, particularly the genera Rhizoctonia [9, 11, 13].
The fungal hyphae penetrate the embryonic cells, the plasma membrane of the host cells invaginates, and there is a thin layer of cytoplasm formed around the hyphae. The hyphae form pelotons, which are coil-like structures that increase the surface area for exchange between the host and the fungus. The peloton has a short life span, and it degrades to be used by the host plant as a nutrient source [9].
The fungal infection of seeds doesn’t always lead to germination; it may lead to parasitic infection of the seeds, due to which the embryo dies, or it may also lead to the rejection of the fungal species [9, 11].
Ectomycorrhizae (ECM)
ECM fungi are widespread in nature but can form associations with only 3% of vascular plants [12]. The ECM fungi belong to the phyla Ascomycota (7 families) and Basidiomycota (25 families) and a few members of Zygomycetes. Prominent genera include Laccaria, Rhizopogon, Scleroderma, Amanita, Boletus, Tuber, Paxillus, Leucopaxillus, Hebeloma, Pisolithus, Suillus, Tylopilus, Boletopsis, Thelephora, Densospora, Endogone, etc. The host plant belongs to either gymnosperms or angiosperms [11, 13]. The host range of ECM fungi is mainly the temperate forest trees such as pines and conifers [14], such as pine, fir, spruce, cedar, larch, golden larch, Douglas fir, etc., and other important forestry trees such as Eucalyptus, tea tree, Leptospermum, Acacia, alder, birch, hornbeam, hazel, chestnut, beech, oak, poplar, willow, etc. The ECM fungi have specific host preferences; they are metabolically more diverse and respond differently to habitat conditions. Theodor Hartig, in the year 1840, was the first to describe the structure of ECM in pine. He illustrated the fungal mantle and the intercellular network of hyphae [1]. These fungi have no intracellular penetration inside the host root cells; hence, they are called ectomycorrhizal fungi.
The ECM formation initiates when hyphae infect the secondary or tertiary roots of the host plants. The hyphae start to grow behind the root cap and form a bulky sheath over the root surface (called the mantle), and then these hyphae start to penetrate in between the epidermal cells and reach the cortex, where they form an extensive network within the intercellular spaces in the cortical cells, looking like a net when the transverse section of the infected roots is stained with cotton blue or any other fungal stain. This net-like structure was later given the name “Hartig Net” in order to recognize Theodor Hartig’s contribution. The Hartig net surrounds the cortical cells completely in such a way that the cell loses contact with the surrounding cells and increases the surface area for the exchange of nutrients between the host plant and the fungi [9].
The fungal infection changes the host root growth pattern and morphology; the fungal sheath around the roots reduces the frequency of cell division at the root tip, which in turn slows the elongation of roots. The cortical cells also tend to grow radially, due to which the fungal-infected roots appear to be shorter and thicker as compared to the uninfected roots. The development of the fungal sheath leads to the disruption of the root epidermis and also to the loss of the root hairs; thus, all the nutrient uptake is through the fungal hyphae [9].
The succession pattern of the mycorrhizal fungi changes along with the host's life span. The ‘early stage’ fungi are the primary colonizers of the host roots, and when the host reaches maturity, the primary fungi are gradually replaced by the ‘late stage’ fungi. The different carbohydrates released by the host during its lifetime drive these successional changes [9]. ECM fungi produce fruiting bodies, which are nothing but ascocarps or basidiocarps of the fungi. Many times these are edible, for example, the fruiting bodies of Boletus edulis and Tuber melanosporum, which form ECM associations. This adds to the advantages of ECM fungi. Moreover, ECM fungi are cultivable and hence can be grown in large fermenters to produce mass inoculum for use in the forestry plants for afforestation and reforestation practices. EM fungi are of particular use in the reclamation of acidic soils with high heavy metal concentrations around mining areas [5].
Ectendomycorrhizae
The ectendomycorrhizae were previously known to be ectendotrophic mycorrhizae [15]. The ectendomycorrhizae have the characteristics of both the endo- and the ectomycorrhizae. The ectendomycorrhizae form the Hartig net structure and the mantle, which is reduced in size compared to the ECM. They also have the capability of intracellular penetration [11]. The ectendomycorrhizal associations are commonly formed by the members of Basidiomycota, Ascomycota, or Zygomycota, such as Wilcoxina mikolae, W. rehmii, Sphaerosporella brunnea, Phialophora finlandia, and Chloridium paucisporum, with the roots of plants belonging to angiosperms and gymnosperms, particularly with plants such as Picea, Pinus, and Larix. The fungal species identified to form the ectendomycorrhizal associations do not form fruiting bodies when present in the association, but some tend to form fruiting bodies in the culture conditions [9, 11, 15].
Arbutoid Mycorrhizae
Arbutoid mycorrhizae is an ectendomycorrhizae with a well-formed and distinctive Hartig net, mantle, and prolific extrametrical mycelium, as seen in ECM. Additionally, intracellular penetration of hyphae occurs, forming hyphal coils within the cortical cells [11]. The nutrients captured by the mycelium and rhizomorphs from the soil pass through the hyphal sheath and then to the short roots of the host plants. The fungal sheath also stores important nutrients and releases them during times of scarcity faced by the plants [9]. The fungal species form associations with hosts belonging to the members of Ericales, namely, Arctostaphylos and Arbutus species. The fungi forming these associations are member of the Basidiomycetes and can also form ECM with other hosts [11].
Monotropoid Mycorrhizae
These associations are formed by the achlorophyllous plant species Monotropa and the fungal species Boletus. These Monotropa species grow near trees such as pine, oak, fir, and spruce. The fungus forms EM associations with these trees, and the same fungus also forms monotropoidal associations with the Monotropa species and transfers carbohydrates from the trees to the Monotropa species with the help of fungal networks between the two. The root cells of Monotropa are surrounded by dense hyphal networks forming the Hartig net, which has limited intracellular penetration [9, 16].
The hypha invaginates into the cortical cells, forming fungal pegs. Due to the extensive growth, the surface area of the pegs increases, leading to the formation of a structure resembling the transfer cell. Eventually, these transfer cells elongate and burst. A membranous sac extends from the peg in the cell cytoplasm, through which all the contents of transfer cells pass into the host cytoplasm. The process of bursting provides nutrients to the plant for seed production [9].
Earlier, these associations were classified under the arbutoid category, but due to their distinctive feature of not penetrating the plant cell walls, they were placed under the new category of monotropoid mycorrhizae [9].
BENEFITS CONFERRED BY MYCORRHIZAL ASSOCIATIONS TO PLANTS
The mycorrhizal association provides many added advantages to the host plants and to the surrounding niche. Some of the benefits posed by the mycorrhizal associations are:
Nutrient Mobilization
The mycorrhizal associations help to continuously mobilize and absorb different nutrients within the ecosystem. They help in carbon cycling, which in turn enhances the host plant productivity; nitrogen cycling by helping plants in the uptake of nitrogen from the soil; and phosphorus cycling by solubilizing the insoluble forms of phosphate present in the soil [4].
It has been demonstrated that plants allocate approximately 15% of their carbon reserves to the associated mycorrhizal fungi. The mycorrhizal fungi act as a key regulator in the carbon cycle as they store and degrade carbon compounds at the same time [4].
The mycorrhizal fungi make a significant amount of P and N available to the plants even in nutrient-deficient soil by assimilating these nutrients from the soil and transferring them to the host. The mycorrhizal fungi contribute up to 90% of their host’s phosphate requirements [4].
Water Uptake and Drought Tolerance
The mycorrhizal hyphae help in better absorption of water as they can penetrate deep inside the soil where even the host roots fail to penetrate, thus exploiting the deeply situated water reservoir. It also stores excess amounts of water and makes them available during the times of scarcity. The fungal hyphae act as a buffer system for water, modulating the water flow within the host. This decreases stressful conditions and helps mycorrhizal plants thrive better than non-mycorrhizal ones [17].
Better Growth
Mycorrhizal associations increase the plant's productivity and growth by many folds by fetching even the most distant nutrients. The enhancement in growth is also a species-specific phenomenon; plants with thick roots rely more on mycorrhizal associations, whereas those with thin root structures depend less on mycorrhizal associations. The conferred benefits vary according to the changes during the life cycle. The benefits posed to the seedlings are much greater as compared to any other stage of the lifecycle [4]. It promotes growth by reducing nutritional deficiencies. The mycorrhizal hyphae dig out nutrients such as Cu, Zn, Mg, and P that are out of the reach of host plant roots [14].
Tolerance to Salinity, Acidic pH, Adverse Temperature, Presence of Heavy Metals and Toxins
Studies have depicted that mycorrhizal associations protect the host from salt and micronutrient toxicity by capturing and storing these toxic compounds within themselves in vacuoles and thus making them unavailable to the host plants [14]. The AM symbiosis helps to improve the plant’s tolerance to temperature stress. The possible mechanisms are: increasing the water and nutrient uptake by the plant; improving the photosynthetic ability and efficiency; protecting the host plant from oxidative damage; and increasing the accumulation of various osmoregulatory compounds [17].
Growth Hormones
The mycorrhizae produce many growth hormones similar to those of plant origin, such as auxins, cytokinin (CKs), ethylene (ET), gibberellic acid (GAs), abscisic acid (ABA), jasmonic acid (JA), and salicylic acid (SA). These hormones control the overall development of the plants and act as an important molecule in signalling during biotic and abiotic stress [18].
Protection from Pathogens
The mycorrhizal fungi reduce the probability of attack by pathogens responsible for causing root diseases. The mycorrhizal fungi act as a competition for the root pathogen as they do not provide the site of attachment to the pathogen and also consume the root exudates, due to which the pathogens are not able to get the required nutrition. The fungi also thicken the host root cells, thus barring the entry of pathogens inside the cell [14].
Enhanced Fruiting and Flowering
Due to the decreased stress conditions, the plants have the ability and resources to produce a large number of fruits and flowers as compared to the non-associated plant species [14]. This confers economic benefits to the farmers who grow flower plants and fruit trees.
APPLICATIONS OF MYCORRHIZAL FUNGUS (MF) AS BIOFERTILIZERS
The factors governing a successful inoculation are species compatibility, as different plant species will respond differently to the inoculated MF; the degree of competition faced by the MF by other soil microbes present in the target soil; other soil parameters such as pH; the presence of inhibitory compounds; and the season of inoculation. Fumigation of nursery soil prior to MF inoculation reduces microorganisms that can colonize introduced inocula and those that damage roots (root pathogens), thereby improving mycorrhizal development. A number of commercially available formulations of endomycorrhizal inoculants, ECM inoculants, and endo in combination with ECM fungi are now available for different applications. These commercial preparations usually contain a combination or consortium of many mycorrhizal fungi for a wide coverage of an assortment of host plants.
The Endomycorrizal Fungal Inoculants
The challenge of using endomycorrhizal fungi as a biofertilizer is that they cannot be cultivated in pure form, which makes large-scale production of inoculum difficult [5]. Being an obligate symbiont, only the propagules raised along with the host plant can be used for their further propagation and ultimate use as biofertilizer in the fields. There are several ways in which the inoculants can be prepared:
The soil collected from near the infected roots of plants harbouring AMF can be used as an inoculum.The spores extracted from the soil using sieves with different pore sizes are used to prepare crude inoculum, after which, based on the compatibility of AMF, the host is identified, which is grown together in the soil. Later on, this soil can be used for large-scale production of the inoculum.The small pieces of AMF-infected root can also serve as a source of inoculum [5].Different Forms of ECM Inoculants
ECM inoculants [19] that can be used in various formulations for applications in the field are discussed below:
Natural inoculum (humus, ectomycorrhizal roots, and soil): Initially, efforts were made to inoculate forest tree seedlings with ECM fungi by gathering several types of organic matter, including soil, rotten wood, plant litter, humus, and ectomycorrhizae, from the forests. Though easy, in this method it’s not possible to control the presence of ECM fungal species in the inoculum. Additionally, this type of inoculum might harbour weeds and harmful microorganisms, along with ECM fungi. Preferably, it should be collected from individual trees inoculated with the desired species of ECM fungus.Basidiospores: Sporocarps, which are a rich source of spores, can be exploited as almost uniform inoculants. The benefits of spores are that spore inoculum is light, has a long shelf life, and can endure storage from one season to another. Basidiospores can be applied either blended with water or as a dry spore preparation.Mycelial suspension (slurry): Mycelial slurry from the lab- or fermenter-grown ECM fungi such as Suillus granulates, Pisolithus tinctorius, etc., can be prepared by blending mycelial mats from liquid cultures with distilled water at high speed.Application Techniques for ECM Inoculant
Vermiculite grown with ECM fungi: autoclaved vermiculite inoculated with suitable ECM fungi and incubated at room temperature for about 15 weeks allows fungal growth on the vermiculite particles. It can be applied directly to the soil, or the seedlings can be first allowed to grow in this, and after proper colonization of the roots, they can be planted in the soil.Mycelium blended and entrapped in alginate beads survives longer and has been shown to be a better ECM inoculant. The seedlings can be inoculated before or after the sowing of the seeds and planting of the seedlings.Basidiospores can also be applied after mixing with a moistened carrier, such as vermiculite, kaolin, or sand, and then mixing into the nursery soil.Dusting dry spores onto the soil.Suspending in water and irrigating. Natural and vegetative inoculum must be placed in the rooting zone of seedlings. Three methods of its application include: Mixing with the rooting medium.Layering it below the seeds or seedlings.Poured onto seedlings or dipping seedlings into the slurry before planting.An artificial mycorrhization of seedlings is mandatory as a reforestation and afforestation management tool considering the extensive area of hostile forest sites and barren lands [19].
OTHER BENEFITS AND LIMITATIONS OF MYCORRHIZAE
The mycorrhizal fungi have a wide range of ecosystem advantages, such as litter decomposition, soil aggregation, and soil formation, in addition to the advantages conferred on plant growth and development. It has also been proposed that AM fungi help host plants extend their ecological niches and help plants by providing them with a competitive advantage over others inhabiting the same niche [4]. At the same time, one major problem associated with mycorrhizae is their ability to store excess minerals within themselves, which sometimes leads to heavy metal accumulation and toxicity in the plants.
CONCLUDING REMARKS
Advantages conferred by the MF on its host plant are numerous in return for carbohydrates, which it derives from the host plant that account for about 15% of the total carbohydrates synthesized by the plant. There are a variety of inoculum types, inoculum preparation methods, and inoculation techniques to initiate the development of mycorrhizae in agriculture, horticulture, and forest tree seedlings. Almost all plant species, including gymnosperms as well as angiosperms, are naturally associated with one or another type of mycorrhizal fungi, indicating the importance of these associations in natural ecosystems. Therefore, MF inoculants are extensively commercially exploited as biofertilizers in organic farming and forestry, including the bioremediation of barren lands.
Though a lot of work has been done, further research is necessary to screen possible host fungus species and host fungus-environment interactions to augment the outcome of MF on plants. Further, scientists might be able to simplify the application of MF as inoculants.
ACKNOWLEDGMENT
The authors would like to thank Prof. Rakesh Kumar Gupta, Principal, Ram Lal Anand College, for providing all the necessary infrastructure and support in compiling this chapter.
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Ekta Narwal1,*,Amar P. Garg2,Jairam Choudhary3,R.K. Naresh4Abstract
Mycorrhizae are mutualistic symbiotic associations between fungi and plants. Mycorrhizal associations are believed to be established between the Ordovician and Devonian periods. The mycorrhizal association is prevalent in almost all ecosystems with a high degree of host specificity. About 40,000–50,000 fungal species colonize the roots of nearly about 250,000 plant species. These symbiotic relations benefit associated plants by providing up to 80% of N and P and also help in plant growth and fitness by different mechanisms. A look into the recent literature suggests that mycorrhizal fungi are not only involved in improving crop yield but also increase the quality of products through the increase in antioxidants, vitamins, and essential trace elements in plants. Due to eco-friendly and sustainable aspects, widespread research and industrial applications of AM fungi are trending in today’s world. During recent years of urbanization and industrialization, the concentration of trace elements has increased in soil and water. Recovery of contaminated areas is very crucial as it may get into the food chain and the process is generally complex. For this, mycorrhizae have evolved as an efficient and sustainable aspect. Ecological restoration of mining sites using AM fungi is considered necessary and useful.
AMF displays significant positive effects, such as increased plant survival under unfavourable growth conditions, enhanced growth and nutrition, improved soil structure and quality, and greater plant re-establishment. Implementation of various molecular techniques and advanced scientific knowledge on AM fungal symbioses, mycorrhizal biotechnology has reached various application domains such as horticulture, agriculture, soil reclamation, bioremediation, gardening, landscaping, and other areas of the plant market.
INTRODUCTION
Mycorrhiza is a mutual symbiotic association between certain soil fungi and roots of the higher plants. AM symbiosis in plants is approximately 480 Mio years old and it is found in the majority of land plants in most taxa and virtually all ecological niches. Research findings suggest that mycorrhizal associations with plants were established in the Ordovician and Devonian periods [1]. This symbiotic association is known to play a key role in the evolution of land plants in the reducing and harsh environment prevalent at that time [2
