Rhizospheric microorganisms and effects, PGPR and Mycorrhiza

The rhizosphere is the narrow region of soil around the plant root that is influenced by several factors like the root exudates and the associated soil microorganisms . The term `rhizosphere` was coined by Lorenz Hiltner in 1904 to describe the soil compartment that is affected by plant roots. The prefix `rhiza-` comes from the Greek word for `root`.

The rhizosphere is considered the most active region of soil as it receives the nutrients from the plant, in addition to the microorganisms that are present around the root. It is a dynamic environment that fluctuates with the stages of root growth and senescence. The rhizosphere is an important part of soil microbiology, which is responsible for various metabolic processes occurring in the soil, such as cycling of nutrients and uptake of carbon.

The roots of crop plants create an interface between the plant and the soil environment, thus establishing an enormous reservoir of the microbial community. The area of rhizosphere usually extends a few millimeters from the root surface, where the roots release various compounds like root exudates, mucilage, and sloughed-off root cells that support higher microbial populations and activities than in bulk soil.

The rhizosphere is a complex system that involves interactions between the plant, the soil, and the microorganisms. These interactions can be positive, negative, or neutral, depending on the type and function of the organisms involved. Some of the benefits of rhizosphere microorganisms for plants include:

• Enhancing plant growth and development by producing phytohormones, solubilizing minerals, fixing nitrogen, and increasing water uptake.
• Protecting plants against pathogens by producing antibiotics, antagonizing pathogens, or inducing systemic resistance.
• Modifying soil structure and chemistry by decomposing organic matter, altering pH, and increasing aggregation.

Some of the drawbacks of rhizosphere microorganisms for plants include:

• Competing with plants for nutrients, water, and space.
• Causing diseases or infections by invading plant tissues or producing toxins.
• Interfering with plant signaling or metabolism by producing allelochemicals or degrading plant compounds.

The rhizosphere is a key component of sustainable agriculture and environmental management, as it affects plant productivity, soil quality, and ecosystem functioning. Understanding the rhizosphere processes and their regulation by plant and microbial factors is essential for developing strategies to enhance crop yield and resilience, as well as to conserve soil resources and biodiversity.

The rhizosphere effect is the influence of plant roots on the development of soil microorganisms as a result of the physical and chemical alteration of soil and the release of root secretions and exudates within the rhizosphere . The rhizosphere effect is observed on the basis of the microbial biomass of the rhizosphere when compared to the biomass of the bulk soil. The rhizosphere effect on soil microbial population can be measured by comparing the population density between the rhizosphere soil (R) and the bulk soil (S), for which the “R/ S ratio” is employed. The rhizosphere effect is higher for bacteria > fungi > actinomycetes > protozoa. The microorganism diversity is higher near to the rhizoplane, which then decreases with an increase in distance from the rhizoplane.

The interaction between plant nutrients in soil and plant exudates modifies the microclimate of the rhizosphere. The rhizosphere effect is a result of the interaction between the plant root and the microbial community of the region, where both factors influence each other. In the rhizosphere, microbial activity influences the plant root, and the plant root secretions influence the microbial biomass. Plant roots may exude 20-40% of the sugars and organic acids - photosynthetically fixed carbon. Plant root exudates, such as organic acids, change the chemical structure and the biological communities of the rhizosphere in comparison with the bulk soil or parent soil. Concentrations of organic acids and saccharides affect the ability of the biological communities to shuttle phosphorus, nitrogen , potassium and water to the root cap, and the total availability of iron to the plant and to its neighbors.

The rhizosphere effect has important implications for plant growth and health, as well as for soil quality and ecosystem functioning. Rhizospheric microorganisms play an important role in nutrient cycling, disease suppression, growth promotion, production of antibiotics, geochemical cycling, and plant colonization. Rhizospheric microorganisms can also enhance plant tolerance to abiotic stresses such as drought, salinity, heavy metals, and pH extremes. Furthermore, rhizospheric microorganisms can affect plant interactions with other organisms such as herbivores, pollinators, symbionts, and competitors. Therefore, understanding and manipulating the rhizosphere effect can provide opportunities for improving agricultural productivity and sustainability.

The rhizosphere is the narrow region of soil around the plant root that is influenced by several factors like the root exudates and the associated soil microorganisms. The rhizosphere is considered the most active region of soil as it receives the nutrients from the nutrients, in addition to the microorganisms that are present around the root. It is a dynamic environment fluctuating with the stages of root growth and senescence.

The microbial population in the rhizosphere consists of different groups of microorganisms like bacteria, fungi, parasites, viruses, and algae. The microbial population in the rhizosphere is known as the rhizosphere microbiome and the microbial population in such an area much higher than the bulk soil. In the rhizosphere, there is a microbial population distinct from the rest of the soil.

Bacteria in the rhizosphere are larger and have higher proportions of Gram-negative and denitrifying bacteria than those in the bulk soil. Rhizosphere fungal populations, abundant in both pathogenic and mycorrhizal species, can be 10 to 20 times higher than those in the non-rhizosphere. Protozoa and other microfauna also thrive in the rhizosphere because that is where food is most plentiful.

The type and population of microorganisms in the rhizosphere are highly influenced by the type of plant grown on the soil. Microbes in the bulk soil often experience long periods of nutrient deprivation; they have different survival strategies in dealing with starvation and stress. The rhizosphere bacterial community is recruited from the main reservoir of microorganisms present in the soil.

Next to the recruitment of specific soil microbes into the rhizosphere microbiome, plant roots also influence specific functions of the microbiome. Some of the examples of microorganisms found in the rhizosphere region include Bacillus, Arthrobacter, Pseudomonas, Agrobacterium, Alcaligenes, Clostridium, Flavobacterium, Corynebacterium, Micrococcus, Xanthomonas, Amanita, Tricholoma, Torrendia, Descomyces, Thelephora, Verticillium, Phytophthora, Rhizoctonia, Micromonospora, Thermoactinomycetes, Amycolaptosis, Actinomadura, etc.

The rhizosphere microbiome plays an important role in the ecological fitness of the plant and the soil. Important microbial processes like plant protection, growth promotion, production of antibiotics, geochemical cycling, and plant colonization take place in the rhizosphere. The rhizosphere microbiome can also affect plant-pathogen interactions by acting as a barrier to pathogen invasion or inducing plant systemic resistance.

Plant Growth Promoting Rhizobacteria (PGPR) are a group of bacteria that colonize the root surface or the rhizosphere of plants and enhance their growth and development by various direct and indirect mechanisms . The term PGPR was first coined by Joseph W. Kloepper in the late 1970s and has become widely used in scientific literature . PGPR are considered as biofertilizers, biocontrol agents, and biostimulants that can improve crop productivity and soil health in a sustainable manner .

Direct mechanisms of PGPR

PGPR can directly promote plant growth by providing nutrients, phytohormones, or enzymes to the plants. Some of the direct mechanisms are:

• Nitrogen fixation: Some PGPR can fix atmospheric nitrogen into ammonia, which can be used by plants as a source of nitrogen. Nitrogen fixation is catalyzed by the enzyme nitrogenase, which requires anaerobic conditions and a supply of oxygen from the plant . Examples of nitrogen-fixing PGPR are Rhizobium, Azospirillum, Azotobacter, and Burkholderia .
• Phosphate solubilization: Some PGPR can solubilize insoluble phosphate compounds in the soil and make them available to plants. Phosphate solubilization is achieved by the production of organic acids, chelating agents, or phosphatases by the bacteria . Examples of phosphate-solubilizing PGPR are Bacillus, Pseudomonas, Enterobacter, and Serratia .
• Siderophore production: Some PGPR can produce siderophores, which are low-molecular-weight compounds that bind and transport iron from the soil to the plants. Iron is an essential micronutrient for plants, but it is often limiting in alkaline or calcareous soils. Siderophores can increase iron availability and prevent iron deficiency chlorosis in plants . Examples of siderophore-producing PGPR are Pseudomonas, Bacillus, Azospirillum, and Rhizobium .
• Phytohormone production: Some PGPR can produce phytohormones, which are plant growth regulators that modulate various physiological processes in plants. Phytohormones can stimulate root growth, cell division, cell elongation, flowering, fruiting, seed germination, and stress tolerance in plants . Examples of phytohormones produced by PGPR are indole-3-acetic acid (IAA), gibberellic acid (GA), cytokinins, ethylene, and abscisic acid (ABA) . Examples of phytohormone-producing PGPR are Pseudomonas, Bacillus, Azospirillum, and Burkholderia .
• ACC deaminase production: Some PGPR can produce ACC deaminase, which is an enzyme that degrades 1-aminocyclopropane-1-carboxylate (ACC), the precursor of ethylene in plants. Ethylene is a phytohormone that induces senescence, abscission, and stress responses in plants. By lowering ethylene levels, ACC deaminase can reduce the negative effects of stress on plant growth and development . Examples of ACC deaminase-producing PGPR are Pseudomonas, Bacillus, Azospirillum, and Enterobacter .

Indirect mechanisms of PGPR

PGPR can indirectly promote plant growth by protecting plants from pathogens or pests, or by improving soil structure and fertility. Some of the indirect mechanisms are:

• Biocontrol: Some PGPR can suppress or inhibit the growth or activity of plant pathogens or pests by various means. Biocontrol mechanisms include competition for nutrients or space, production of antibiotics or lytic enzymes, induction of systemic resistance or priming in plants, interference with quorum sensing or biofilm formation of pathogens, or parasitism or predation on pests . Examples of biocontrol PGPR are Pseudomonas, Bacillus,

  Factors influencing the growth and activities of Rhizospheric microorganisms

Rhizospheric microorganisms are the soil microorganisms that live in close association with plant roots and are influenced by root exudates and secretions. The growth and activities of these microorganisms depend on various factors, such as:

• Nutrients: The availability and quality of nutrients in the soil and the root exudates affect the diversity and abundance of rhizospheric microorganisms. The root exudates provide a rich source of carbon and energy for the microorganisms, as well as other organic and inorganic compounds that can stimulate or inhibit their growth . The rhizosphere is enriched in nitrogen, phosphorus, potassium, iron, zinc, and other essential elements for plant and microbial nutrition. However, the nutrient supply is not uniform and varies with the plant species, root growth stage, soil type, and environmental conditions.

• Physiochemical factors of soil: The physical and chemical properties of the soil, such as soil moisture, soil temperature, soil pH, soil texture, soil structure, soil aeration, and soil salinity, also influence the rhizospheric microorganisms. Soil moisture affects the diffusion of nutrients and oxygen in the soil, as well as the microbial activity and mobility . Soil temperature affects the metabolic rates and enzyme activities of the microorganisms, as well as their adaptation to different temperature regimes . Soil pH influences the solubility and availability of nutrients and metals, as well as the microbial diversity and community structure . Soil texture affects the water retention and drainage, as well as the pore size and distribution that determine the habitat and colonization of microorganisms . Soil structure affects the aggregation and stability of soil particles, as well as the formation of microsites and niches for microbial growth . Soil aeration affects the oxygen concentration and redox potential in the soil, which influence the aerobic and anaerobic processes mediated by microorganisms . Soil salinity affects the osmotic potential and ion balance of the soil solution, which affect the water uptake and stress tolerance of plants and microorganisms .

• Interactions: The interactions among different groups of microorganisms, as well as between microorganisms and plants, also affect their growth and activities in the rhizosphere. The interactions can be positive (mutualism, commensalism, synergism), negative (competition, predation, parasitism, amensalism), or neutral (coexistence) . Positive interactions enhance the survival and performance of both partners, such as plant growth-promoting rhizobacteria (PGPR) that provide benefits to plants by producing phytohormones, solubilizing phosphate, fixing nitrogen, producing siderophores, inducing systemic resistance, or suppressing pathogens. Negative interactions limit or reduce the growth or activity of one or both partners, such as plant pathogens that cause diseases or reduce plant fitness by producing toxins, enzymes, or elicitors. Neutral interactions do not affect either partner significantly but may influence their diversity or distribution in the rhizosphere.

Introduction to Mycorrhiza

Mycorrhiza is a term that describes a symbiotic association between a fungus and a plant root. The word mycorrhiza literally means "fungus-root" . Mycorrhiza is a common and widespread phenomenon in nature, as about 90% of all land plants form some type of mycorrhizal relationship with fungi . Mycorrhiza can be beneficial for both the plant and the fungus, as they exchange nutrients and help each other survive in different environmental conditions. However, some mycorrhizal associations can also be parasitic or commensal, depending on the balance of costs and benefits for each partner.

Mycorrhiza can be classified into two main types: ectomycorrhiza and endomycorrhiza. Ectomycorrhiza are fungi that form an external sheath around the plant root and penetrate between the root cells, but do not enter the cells. Ectomycorrhiza are mostly associated with woody plants, such as trees and shrubs. Endomycorrhiza are fungi that invade the root cells and form specialized structures inside them, such as arbuscules and vesicles. Endomycorrhiza are more common and diverse than ectomycorrhiza, and are found in many herbaceous plants, crops, and grasses.

Mycorrhiza play important roles in plant nutrition, soil biology, and soil chemistry. They can enhance the uptake of water and minerals, especially phosphorus, by the plant roots. They can also protect the plants from pathogens, drought, salinity, and heavy metals. They can also influence the composition and activity of other soil microorganisms, such as bacteria and nematodes. They can also affect the cycling of carbon, nitrogen, and other elements in the soil.

Mycorrhiza are fascinating examples of how plants and fungi have co-evolved to form mutually beneficial relationships that shape the terrestrial ecosystems.

Types of Mycorrhiza

Ectomycorrhiza

Ectomycorrhiza is a type of mycorrhiza where the fungal hyphae form a dense sheath or mantle around the root surface, but do not penetrate the root cells. Instead, they grow between the cells of the root cortex, forming a network called the Hartig net. The Hartig net allows the exchange of nutrients and signals between the fungus and the plant.

Ectomycorrhiza is mainly found in woody plants, such as pine, oak, birch, beech, willow, spruce, and fir. It accounts for about 5 to 10% of all mycorrhizal associations. Ectomycorrhizal fungi belong to various groups, such as basidiomycetes, ascomycetes, zygomycetes, and fungi imperfecti. Some examples of ectomycorrhizal fungi are Amanita, Tricholoma, Boletus, Russula, Lactarius, Cortinarius, and Pisolithus.

Ectomycorrhiza helps plants to access water and nutrients from the soil, especially nitrogen and phosphorus. It also protects plants from pathogens, drought, salinity, and heavy metals. Ectomycorrhiza can also influence soil structure and decomposition by producing extracellular enzymes and organic acids.

Endomycorrhiza

Endomycorrhiza is a type of mycorrhiza where the fungal hyphae penetrate the root cells and form specialized structures for nutrient exchange. These structures can be vesicles (spherical storage organs), arbuscules (branched tree-like organs), or coils (hyphal loops). The fungal hyphae also extend outside the root into the soil, forming an external mycelium that enhances soil exploration.

Endomycorrhiza is more widespread than ectomycorrhiza, as it occurs in about 80% of plant species, including many crops, grasses, flowers, fruits, and trees. Endomycorrhizal fungi are mostly arbuscular mycorrhizal fungi (AMF), which belong to a single group of zygomycetes called Glomeromycota. Some examples of AMF are Glomus, Gigaspora, Acaulospora, Scutellospora, and Funneliformis.

Endomycorrhiza enhances plant growth by improving phosphorus uptake, as well as other nutrients such as zinc, copper, iron, and sulfur. It also confers resistance to biotic and abiotic stresses, such as pathogens, drought, salinity, and heavy metals. Endomycorrhiza can also modulate plant hormones, metabolism, and gene expression.

Endomycorrhiza can be further divided into subtypes based on the host plants and the morphology of the symbiosis. These subtypes are:

• Arbuscular mycorrhiza: The most common type of endomycorrhiza that forms arbuscules inside the root cells. It occurs in most herbaceous plants and some woody plants.
• Ericoid mycorrhiza: A type of endomycorrhiza that forms coils inside the root cells. It occurs in plants of the family Ericaceae (heathers), which grow in acidic soils with low nutrient availability.
• Orchid mycorrhiza: A type of endomycorrhiza that forms coils or pelotons (hyphal aggregates) inside the root cells. It occurs in orchids (Orchidaceae), which depend on fungi for seed germination and nutrition.
• Arbutoid mycorrhiza: A type of endomycorrhiza that forms arbuscules and vesicles inside the root cells, as well as a mantle around the root surface. It occurs in plants of the family Ericaceae that have woody roots, such as Arbutus and Arctostaphylos.
• Monotropoid mycorrhiza: A type of endomycorrhiza that forms coils inside the root cells, as well as a mantle around the root surface. It occurs in plants of the family Ericaceae that are non-photosynthetic and parasitic on other mycorrhizal plants, such as Monotropa and Pterospora.

Differences between Ectomycorrhiza and Endomycorrhiza

Ectomycorrhiza and endomycorrhiza are two types of mycorrhizal fungi that form symbiotic associations with plant roots. However, they differ in several aspects, such as:

• Hyphal penetration: Ectomycorrhiza form a sheath or a mantle around the root surface, but do not penetrate the cortical cells of the root. Endomycorrhiza, on the other hand, enter the cortical cells of the root and form intracellular structures such as arbuscules and vesicles .
• Hyphal network: Ectomycorrhiza produce a network of hyphae called the Hartig net between the cells of the root cortex, which facilitates nutrient exchange. Endomycorrhiza do not produce a Hartig net .
• Fungal diversity: Ectomycorrhiza belong to various fungal phyla, such as Basidiomycota, Ascomycota, and Zygomycota. Endomycorrhiza are mainly represented by the phylum Glomeromycota, especially the arbuscular mycorrhiza (AM), which are the most common and widespread type of mycorrhiza .
• Plant diversity: Ectomycorrhiza are less prevalent and form symbiosis with about 10% of plant families, mostly woody plants such as conifers, birch, oak, and eucalyptus. Endomycorrhiza are more prevalent and form symbiosis with about 80% of plant families, including many herbaceous plants, crops, and grasses .
• Morphological changes: Ectomycorrhiza induce morphological changes in the root system, such as increased branching and swelling of root tips. Endomycorrhiza do not cause significant changes in the root morphology .

These differences reflect the different modes of interaction and function of ectomycorrhiza and endomycorrhiza in relation to their host plants and soil environment. Both types of mycorrhiza play important roles in enhancing plant growth, nutrition, and stress tolerance by facilitating nutrient uptake, especially phosphorus, nitrogen, and water .

Functions of Mycorrhiza

Mycorrhiza is a symbiotic association between a fungus and a plant root that provides various benefits to both partners. Some of the main functions of mycorrhiza are:

• Enhancing nutrient and water uptake by plants: Mycorrhizal fungi extend their hyphae into the soil and increase the surface area for absorbing water and mineral nutrients, such as phosphorus, nitrogen, potassium, calcium, magnesium, iron, zinc, and copper. The fungi also help to solubilize or mobilize nutrients that are otherwise unavailable or inaccessible to plants. The plants provide the fungi with organic carbon compounds, such as sugars, that are produced by photosynthesis.

• Improving soil structure and fertility: Mycorrhizal fungi produce various substances, such as glomalin, polysaccharides, and humic acids, that bind soil particles together and form aggregates. This improves soil porosity, aeration, water retention, and resistance to erosion. The fungi also contribute to soil organic matter formation and decomposition, which affects soil nutrient cycling and availability.

• Protecting plants from biotic and abiotic stresses: Mycorrhizal fungi can act as biocontrol agents against plant pathogens, such as nematodes, bacteria, and fungi, by competing with them for space and resources, producing antibiotics or toxins, inducing systemic resistance in plants, or enhancing plant immune responses. The fungi can also help plants to tolerate environmental stresses, such as drought, salinity, heavy metals, acidity, or alkalinity, by modulating plant water relations, nutrient status, hormone levels, antioxidant systems, or gene expression.

• Facilitating plant diversity and ecosystem functioning: Mycorrhizal fungi can influence plant community composition and dynamics by affecting plant growth, survival, reproduction, competition, facilitation, or succession. The fungi can also affect plant interactions with other organisms, such as pollinators, herbivores, decomposers, or other symbionts. Mycorrhizal fungi play a key role in ecosystem processes, such as primary productivity , carbon sequestration , nutrient cycling , and biodiversity .

Positive effects of Rhizospheric microorganisms on Plants

Rhizospheric microorganisms play an important role in the ecological fitness of the plant and the soil. They are involved in various processes that enhance plant growth, health, and productivity. Some of the positive effects of rhizospheric microorganisms on plants are:

• Nutrient acquisition: Rhizospheric microorganisms increase the supply of mineral nutrients from the soil to the plant by solubilizing, mobilizing, or fixing them. For example, plant growth-promoting rhizobacteria (PGPR) can solubilize phosphate, fix nitrogen, produce siderophores, and chelate iron and zinc . Mycorrhizal fungi can extend the root surface area and improve the uptake of phosphorus, nitrogen, sulfur, and other micronutrients.
• Plant growth promotion: Rhizospheric microorganisms can stimulate plant growth directly by producing phytohormones such as auxin, cytokinin, gibberellin, ethylene, and abscisic acid . These hormones can regulate various aspects of plant development such as root initiation, elongation, branching, flowering, fruiting, and senescence. Some rhizospheric microorganisms can also produce volatile organic compounds (VOCs) that can modulate plant growth and metabolism.
• Plant protection: Rhizospheric microorganisms can protect plants from pathogens by direct antagonistic interactions or by inducing systemic resistance. Direct antagonism involves the production of antibiotics, lytic enzymes, hydrogen cyanide, or other inhibitory substances that can suppress or kill the pathogens . Induced systemic resistance (ISR) involves the activation of plant defense mechanisms by microbial signals such as lipopolysaccharides, flagellin, N-acyl homoserine lactones, or salicylic acid. ISR can enhance the plant`s ability to resist biotic and abiotic stresses.
• Soil health improvement: Rhizospheric microorganisms can improve the soil structure, fertility, and functioning by decomposing organic matter, cycling nutrients, aggregating soil particles, and increasing water retention . They can also degrade pollutants and toxins in the soil and contribute to phytoremediation processes.
• Stress tolerance enhancement: Rhizospheric microorganisms can enhance the tolerance of plants to various abiotic stresses such as drought, salinity, heat, cold, heavy metals, and nutrient deficiency. They can do so by modulating plant hormone levels, osmotic adjustment, antioxidant systems, gene expression, and membrane stability . For example, some PGPR can produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase that can lower ethylene levels in plants under stress conditions .

In summary, rhizospheric microorganisms have multiple positive effects on plants that can improve their growth and yield under normal and stressed environments. These effects are mediated by various mechanisms involving biochemical, physiological, molecular, and ecological interactions between the plant roots and the microbial community.

Negative effects of Rhizospheric microorganisms on Plants

Although rhizospheric microorganisms have many beneficial effects on plants, they can also have some negative effects. Some of the negative effects of rhizospheric microorganisms on plants are:

• Competition: Rhizospheric microorganisms compete with plants for water, nutrients, and space in the soil. This can reduce the availability of these resources for plant growth and development. For example, some rhizospheric bacteria can consume nitrate and ammonium, which are essential for plant nitrogen metabolism.
• Pathogenicity: Some rhizospheric microorganisms can act as plant pathogens, causing various diseases and disorders in plants. These pathogens can invade the plant tissues through wounds or natural openings, or by producing toxins or enzymes that degrade the plant cell wall. Some examples of plant pathogens in the rhizosphere are fungi (e.g., Phytophthora, Rhizoctonia, Fusarium), bacteria (e.g., Agrobacterium, Xanthomonas, Pseudomonas), nematodes (e.g., Meloidogyne, Heterodera), and viruses (e.g., Tobacco mosaic virus, Tomato spotted wilt virus).
• Negative feedback: Rhizospheric microorganisms can also influence the plant community structure and diversity by creating negative feedback loops. This means that the presence of certain microorganisms can inhibit the growth of the same or similar plant species, thus preventing the dominance of a single species in the population. This can affect the stability and productivity of the plant ecosystem. For example, some rhizospheric fungi can produce allelopathic compounds that suppress the germination and growth of other plants.

Therefore, rhizospheric microorganisms have a complex and dynamic relationship with plants, which can be positive or negative depending on various factors such as the type of microorganism, the type of plant, the soil conditions, and the environmental stresses. Understanding these interactions is important for developing sustainable and effective strategies for enhancing plant health and productivity.

Mycorrhiza is a term that describes a symbiotic association between a fungus and a plant root. The word mycorrhiza literally means "fungus-root" . Mycorrhiza is a common and widespread phenomenon in nature, as about 90% of all land plants form some type of mycorrhizal relationship with fungi . Mycorrhiza can be beneficial for both the plant and the fungus, as they exchange nutrients and help each other survive in different environmental conditions. However, some mycorrhizal associations can also be parasitic or commensal, depending on the balance of costs and benefits for each partner.

Mycorrhiza can be classified into two main types: ectomycorrhiza and endomycorrhiza. Ectomycorrhiza are fungi that form an external sheath around the plant root and penetrate between the root cells, but do not enter the cells. Ectomycorrhiza are mostly associated with woody plants, such as trees and shrubs. Endomycorrhiza are fungi that invade the root cells and form specialized structures inside them, such as arbuscules and vesicles. Endomycorrhiza are more common and diverse than ectomycorrhiza, and are found in many herbaceous plants, crops, and grasses.

Mycorrhiza play important roles in plant nutrition, soil biology, and soil chemistry. They can enhance the uptake of water and minerals, especially phosphorus, by the plant roots. They can also protect the plants from pathogens, drought, salinity, and heavy metals. They can also influence the composition and activity of other soil microorganisms, such as bacteria and nematodes. They can also affect the cycling of carbon, nitrogen, and other elements in the soil.

Mycorrhiza are fascinating examples of how plants and fungi have co-evolved to form mutually beneficial relationships that shape the terrestrial ecosystems.

Ectomycorrhiza

Ectomycorrhiza is a type of mycorrhiza where the fungal hyphae form a dense sheath or mantle around the root surface, but do not penetrate the root cells. Instead, they grow between the cells of the root cortex, forming a network called the Hartig net. The Hartig net allows the exchange of nutrients and signals between the fungus and the plant.

Ectomycorrhiza is mainly found in woody plants, such as pine, oak, birch, beech, willow, spruce, and fir. It accounts for about 5 to 10% of all mycorrhizal associations. Ectomycorrhizal fungi belong to various groups, such as basidiomycetes, ascomycetes, zygomycetes, and fungi imperfecti. Some examples of ectomycorrhizal fungi are Amanita, Tricholoma, Boletus, Russula, Lactarius, Cortinarius, and Pisolithus.

Ectomycorrhiza helps plants to access water and nutrients from the soil, especially nitrogen and phosphorus. It also protects plants from pathogens, drought, salinity, and heavy metals. Ectomycorrhiza can also influence soil structure and decomposition by producing extracellular enzymes and organic acids.

Endomycorrhiza

Endomycorrhiza is a type of mycorrhiza where the fungal hyphae penetrate the root cells and form specialized structures for nutrient exchange. These structures can be vesicles (spherical storage organs), arbuscules (branched tree-like organs), or coils (hyphal loops). The fungal hyphae also extend outside the root into the soil, forming an external mycelium that enhances soil exploration.

Endomycorrhiza is more widespread than ectomycorrhiza, as it occurs in about 80% of plant species, including many crops, grasses, flowers, fruits, and trees. Endomycorrhizal fungi are mostly arbuscular mycorrhizal fungi (AMF), which belong to a single group of zygomycetes called Glomeromycota. Some examples of AMF are Glomus, Gigaspora, Acaulospora, Scutellospora, and Funneliformis.

Endomycorrhiza enhances plant growth by improving phosphorus uptake, as well as other nutrients such as zinc, copper, iron, and sulfur. It also confers resistance to biotic and abiotic stresses, such as pathogens, drought, salinity, and heavy metals. Endomycorrhiza can also modulate plant hormones, metabolism, and gene expression.

Endomycorrhiza can be further divided into subtypes based on the host plants and the morphology of the symbiosis. These subtypes are:

• Arbuscular mycorrhiza: The most common type of endomycorrhiza that forms arbuscules inside the root cells. It occurs in most herbaceous plants and some woody plants.
• Ericoid mycorrhiza: A type of endomycorrhiza that forms coils inside the root cells. It occurs in plants of the family Ericaceae (heathers), which grow in acidic soils with low nutrient availability.
• Orchid mycorrhiza: A type of endomycorrhiza that forms coils or pelotons (hyphal aggregates) inside the root cells. It occurs in orchids (Orchidaceae), which depend on fungi for seed germination and nutrition.
• Arbutoid mycorrhiza: A type of endomycorrhiza that forms arbuscules and vesicles inside the root cells, as well as a mantle around the root surface. It occurs in plants of the family Ericaceae that have woody roots, such as Arbutus and Arctostaphylos.
• Monotropoid mycorrhiza: A type of endomycorrhiza that forms coils inside the root cells, as well as a mantle around the root surface. It occurs in plants of the family Ericaceae that are non-photosynthetic and parasitic on other mycorrhizal plants, such as Monotropa and Pterospora.

Ectomycorrhiza and endomycorrhiza are two types of mycorrhizal fungi that form symbiotic associations with plant roots. However, they differ in several aspects, such as:

• Hyphal penetration: Ectomycorrhiza form a sheath or a mantle around the root surface, but do not penetrate the cortical cells of the root. Endomycorrhiza, on the other hand, enter the cortical cells of the root and form intracellular structures such as arbuscules and vesicles .
• Hyphal network: Ectomycorrhiza produce a network of hyphae called the Hartig net between the cells of the root cortex, which facilitates nutrient exchange. Endomycorrhiza do not produce a Hartig net .
• Fungal diversity: Ectomycorrhiza belong to various fungal phyla, such as Basidiomycota, Ascomycota, and Zygomycota. Endomycorrhiza are mainly represented by the phylum Glomeromycota, especially the arbuscular mycorrhiza (AM), which are the most common and widespread type of mycorrhiza .
• Plant diversity: Ectomycorrhiza are less prevalent and form symbiosis with about 10% of plant families, mostly woody plants such as conifers, birch, oak, and eucalyptus. Endomycorrhiza are more prevalent and form symbiosis with about 80% of plant families, including many herbaceous plants, crops, and grasses .
• Morphological changes: Ectomycorrhiza induce morphological changes in the root system, such as increased branching and swelling of root tips. Endomycorrhiza do not cause significant changes in the root morphology .

These differences reflect the different modes of interaction and function of ectomycorrhiza and endomycorrhiza in relation to their host plants and soil environment. Both types of mycorrhiza play important roles in enhancing plant growth, nutrition, and stress tolerance by facilitating nutrient uptake, especially phosphorus, nitrogen, and water .

Mycorrhiza is a symbiotic association between a fungus and a plant root that provides various benefits to both partners. Some of the main functions of mycorrhiza are:

• Enhancing nutrient and water uptake by plants: Mycorrhizal fungi extend their hyphae into the soil and increase the surface area for absorbing water and mineral nutrients, such as phosphorus, nitrogen, potassium, calcium, magnesium, iron, zinc, and copper. The fungi also help to solubilize or mobilize nutrients that are otherwise unavailable or inaccessible to plants. The plants provide the fungi with organic carbon compounds, such as sugars, that are produced by photosynthesis.

• Improving soil structure and fertility: Mycorrhizal fungi produce various substances, such as glomalin, polysaccharides, and humic acids, that bind soil particles together and form aggregates. This improves soil porosity, aeration, water retention, and resistance to erosion. The fungi also contribute to soil organic matter formation and decomposition, which affects soil nutrient cycling and availability.

• Protecting plants from biotic and abiotic stresses: Mycorrhizal fungi can act as biocontrol agents against plant pathogens, such as nematodes, bacteria, and fungi, by competing with them for space and resources, producing antibiotics or toxins, inducing systemic resistance in plants, or enhancing plant immune responses. The fungi can also help plants to tolerate environmental stresses, such as drought, salinity, heavy metals, acidity, or alkalinity, by modulating plant water relations, nutrient status, hormone levels, antioxidant systems, or gene expression.

• Facilitating plant diversity and ecosystem functioning: Mycorrhizal fungi can influence plant community composition and dynamics by affecting plant growth, survival, reproduction, competition, facilitation, or succession. The fungi can also affect plant interactions with other organisms, such as pollinators, herbivores, decomposers, or other symbionts. Mycorrhizal fungi play a key role in ecosystem processes, such as primary productivity , carbon sequestration , nutrient cycling , and biodiversity .

Rhizospheric microorganisms play an important role in the ecological fitness of the plant and the soil. They are involved in various processes that enhance plant growth, health, and productivity. Some of the positive effects of rhizospheric microorganisms on plants are:

• Nutrient acquisition: Rhizospheric microorganisms increase the supply of mineral nutrients from the soil to the plant by solubilizing, mobilizing, or fixing them. For example, plant growth-promoting rhizobacteria (PGPR) can solubilize phosphate, fix nitrogen, produce siderophores, and chelate iron and zinc . Mycorrhizal fungi can extend the root surface area and improve the uptake of phosphorus, nitrogen, sulfur, and other micronutrients.
• Plant growth promotion: Rhizospheric microorganisms can stimulate plant growth directly by producing phytohormones such as auxin, cytokinin, gibberellin, ethylene, and abscisic acid . These hormones can regulate various aspects of plant development such as root initiation, elongation, branching, flowering, fruiting, and senescence. Some rhizospheric microorganisms can also produce volatile organic compounds (VOCs) that can modulate plant growth and metabolism.
• Plant protection: Rhizospheric microorganisms can protect plants from pathogens by direct antagonistic interactions or by inducing systemic resistance. Direct antagonism involves the production of antibiotics, lytic enzymes, hydrogen cyanide, or other inhibitory substances that can suppress or kill the pathogens . Induced systemic resistance (ISR) involves the activation of plant defense mechanisms by microbial signals such as lipopolysaccharides, flagellin, N-acyl homoserine lactones, or salicylic acid. ISR can enhance the plant`s ability to resist biotic and abiotic stresses.
• Soil health improvement: Rhizospheric microorganisms can improve the soil structure, fertility, and functioning by decomposing organic matter, cycling nutrients, aggregating soil particles, and increasing water retention . They can also degrade pollutants and toxins in the soil and contribute to phytoremediation processes.
• Stress tolerance enhancement: Rhizospheric microorganisms can enhance the tolerance of plants to various abiotic stresses such as drought, salinity, heat, cold, heavy metals, and nutrient deficiency. They can do so by modulating plant hormone levels, osmotic adjustment, antioxidant systems, gene expression, and membrane stability . For example, some PGPR can produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase that can lower ethylene levels in plants under stress conditions .

In summary, rhizospheric microorganisms have multiple positive effects on plants that can improve their growth and yield under normal and stressed environments. These effects are mediated by various mechanisms involving biochemical, physiological, molecular, and ecological interactions between the plant roots and the microbial community.

Although rhizospheric microorganisms have many beneficial effects on plants, they can also have some negative effects. Some of the negative effects of rhizospheric microorganisms on plants are:

• Competition: Rhizospheric microorganisms compete with plants for water, nutrients, and space in the soil. This can reduce the availability of these resources for plant growth and development. For example, some rhizospheric bacteria can consume nitrate and ammonium, which are essential for plant nitrogen metabolism.
• Pathogenicity: Some rhizospheric microorganisms can act as plant pathogens, causing various diseases and disorders in plants. These pathogens can invade the plant tissues through wounds or natural openings, or by producing toxins or enzymes that degrade the plant cell wall. Some examples of plant pathogens in the rhizosphere are fungi (e.g., Phytophthora, Rhizoctonia, Fusarium), bacteria (e.g., Agrobacterium, Xanthomonas, Pseudomonas), nematodes (e.g., Meloidogyne, Heterodera), and viruses (e.g., Tobacco mosaic virus, Tomato spotted wilt virus).
• Negative feedback: Rhizospheric microorganisms can also influence the plant community structure and diversity by creating negative feedback loops. This means that the presence of certain microorganisms can inhibit the growth of the same or similar plant species, thus preventing the dominance of a single species in the population. This can affect the stability and productivity of the plant ecosystem. For example, some rhizospheric fungi can produce allelopathic compounds that suppress the germination and growth of other plants.

Therefore, rhizospheric microorganisms have a complex and dynamic relationship with plants, which can be positive or negative depending on various factors such as the type of microorganism, the type of plant, the soil conditions, and the environmental stresses. Understanding these interactions is important for developing sustainable and effective strategies for enhancing plant health and productivity.

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