Roles of beneficial soil microbes like PGPR and mycorrhizae

Roles of beneficial soil microbes like PGPR and mycorrhizae

Beneficial soil microbes, including Plant Growth-Promoting Rhizobacteria (PGPR) and mycorrhizae, play crucial roles in enhancing plant growth, soil health, and overall ecosystem stability. Here's a detailed look at their roles:

1. Plant Growth-Promoting Rhizobacteria (PGPR)

PGPR are a group of bacteria that colonize plant roots and promote plant growth through various mechanisms. These bacteria are found in the rhizosphere.  The rhizosphere is the zone of soil surrounding a plant root where the biology and chemistry of the soil are influenced by the root, the region of soil directly influenced by root secretions.

 

 (https://images.app.goo.gl/QSLV2Lqw1USjdQo99)

 Roles of PGPR:

1. Nutrient Solubilization and Mobilization:
• Nitrogen Fixation: PGPR, such as Rhizobium species, fix atmospheric nitrogen into readily assimilable form that can be directly absorbed and utilized by plants.
• Phosphate Solubilization: Some PGPR, like Pseudomonas and Bacillus species, solubilize insoluble phosphate compounds in the soil, making phosphorus available to plants.
• Production of Siderophores: PGPR produce siderophores, which are compounds that bind to iron and make it available to plants. This is crucial in iron-deficient soils.
2. Phytohormone Production:
• PGPR produce various plant hormones, such as auxins (e.g., indole-3-acetic acid), gibberellins, and cytokinins, which directly stimulate plant growth and development, by promoting root elongation and branching, seed germination, stem elongation, lateral root formation, and overall plant growth.
• By improving root architecture and health, PGPR increase the efficiency of nutrient uptake by plants, leading to better growth and higher yields.
3. Biocontrol of Plant Pathogens:
• PGPR can protect plants from pathogens through several mechanisms, including the production of antibiotics, lytic enzymes, and hydrogen cyanide, which inhibit the growth of harmful microbes.
• They also compete with pathogens for nutrients and space, effectively reducing the incidence of diseases.
• Some PGPR trigger Induced Systemic Resistance (ISR) in plants, a defense mechanism that enhances the plant's ability to resist pathogens. This involves the activation of defense-related genes and pathways.
4. Stress Tolerance:
• PGPR can help plants withstand abiotic stresses, such as drought, salinity, and heavy metal toxicity, by modulating stress-responsive pathways and producing stress-related compounds.

Applications of PGPR in Agriculture

PGPR are used as biofertilizers to improve crop yields by enhancing nutrient availability, employed as biocontrol agents to protect crops from diseases and play a key role in sustainable farming practices by reducing the need for chemical fertilizers and pesticides.

2. Mycorrhizae

Mycorrhizae are symbiotic associations between fungi and plant roots. These fungi colonize the root system and extend far into the soil, forming a network that benefits both the plant and the fungus.  The term "mycorrhiza" comes from the Greek words "mykes" (fungus) and "rhiza" (root). 

Types of Mycorrhizae

1.      Ectomycorrhizae:

Characteristics: These form an external sheath (or mantle) around the roots and extend into the soil, but they do not penetrate the root cells.  Ectomycorrhizae form an entirely intercellular interface known as the Hartig net, consisting of highly branched hyphae forming a latticework between epidermal and cortical root cells. The external sheath or mantle is usually dense covering around the root surface and can be up to 40 μm thick, with hyphae extending up to several centimeters into the surrounding soil. The hyphal network helps the plant to take up nutrients including water and minerals, and help the host plant to survive adverse conditions.  The fungi are benefitted by the plant by having access to carbohydrates.  In some cases, the hyphae may penetrate the plant cells, and then the mycorrhiza is called an ectendomycorrhiza.

Associated Plants: Common in temperate forests, associated with trees like pines, oaks, and birches.

Function: They significantly increase the surface area for water and nutrient absorption, especially for nitrogen and phosphorus.

Endomycorrhizae                            Ectomycorrhizae 

(https://soil.evs.buffalo.edu/index.php/Ectomycorrhizal_Fungi)

2.      Endomycorrhizae (Arbuscular Mycorrhizae or AM):

Characteristics: These fungi penetrate the cortical cells of the roots of a vascular plant and they form structures like arbuscules (branched, tree-like structures) and vesicles (storage organs) inside the root cells.

Associated Plants: Found in a wide variety of plants, including crops like wheat, maize, and rice.

Function: AM fungi help plants to absorb nutrients such as phosphorus, sulfur, nitrogen and micronutrients from the soil.

 (https://en.wikipedia.org/wiki/Arbuscular_mycorrhiza)

3.      Ericoid Mycorrhizae:

Characteristics: Associated with plants in the Ericaceae family and several lineages of mycorrhizal fungi.  This symbiosis helps the plant to adapt to acidic and nutrient poor soils.  Ericoid mycorrhizal fungi form fungal coils in the epidermal cells of the fine hair roots of ericaceous species. The fungi establish loose hyphal networks around the outside of hair roots, then they penetrate the walls of cortical cells and form intracellular coils, but the fungi do not penetrate plasma membranes.

Associated Plants: Plants of the Ericaceae family, such as blueberries and rhododendrons.

Function: These fungi help plants survive in nutrient-poor, acidic soils by breaking down organic matter to release nutrients.

4.      Orchid Mycorrhizae:

Characteristics: These are endomycorrhizal fungi which develop symbiotic relationships with the roots and seeds of plants of the family Orchidaceae. Orchids form a unique symbiosis with mycorrhizal fungi, especially during seed germination.  Orchid mycorrhizae are critically important during orchid germination, as an orchid seed has no energy reserve and obtains sufficient nutrients from the fungal symbiont.  The first stage in the life cycle of Orchid is the non-germinated orchid seed, the next stage is the protocorm, followed by the adult orchid stage. Orchid seeds are very small, has an undifferentiated embryo and does not have enough nutritional support to grow since it lacks endosperm.   It gets nutrients needed for germination from the fungal symbiont.  Many adult orchids retain their fungal symbionts throughout their life.

Associated Plants: Plants of the family Orchidaceae

Function: The fungi provide essential nutrients to the developing orchid seedling, which is critical for its survival and growth.

 Roles of Mycorrhizae:

1. Enhanced Nutrient Acquisition:  Mycorrhizae increase the surface area for nutrient absorption, particularly for immobile nutrients like phosphorus, zinc, and copper. This is especially crucial in nutrient-poor soils
• Phosphorus Uptake: Mycorrhizal fungi are particularly effective at absorbing phosphorus from the soil and transferring it to plants.
• Nitrogen Uptake: Mycorrhizae can also improve the uptake of nitrogen, especially organic nitrogen, which is less accessible to plants.
• Micronutrient Uptake: They enhance the availability of other essential nutrients, such as zinc and copper, by extending the root's absorptive capacity.
2. Improved Water Absorption:

° The extensive fungal network helps plants absorb water more efficiently, improving drought tolerance.

3. Soil Structure Improvement:
• Mycorrhizal networks help stabilize soil structure by binding soil particles together with fungal hyphae, leading to better soil aggregation. 
• Mycorrhizae produce glomalin, a glycoprotein that binds soil particles together, enhancing soil aggregation and reducing erosion and help to stabilize soil structure
• Improved soil structure enhances water retention, reduces erosion, and facilitates root growth.
4. Stress Tolerance:
• Mycorrhizal associations help plants cope with abiotic stresses, such as drought and salinity, by improving water uptake and modulating stress responses.
• They can also help plants tolerate heavy metal contamination by sequestering toxic metals within the fungal network.
5. Protection Against Pathogens:
• Mycorrhizal fungi can compete with soil pathogens for space and resources, thereby reducing root infections.
• Some mycorrhizae also induce systemic resistance in plants, similar to PGPR, enhancing the plant's overall defense mechanisms.

By facilitating better nutrient and water uptake and protecting against pathogens, mycorrhizae contribute to overall plant health, leading to higher productivity and better quality crops.

Synergistic Effects of PGPR and Mycorrhizae:

PGPR and mycorrhizae can have synergistic effects on plant growth and soil health. PGPR can enhance mycorrhizal colonization, while mycorrhizae can improve the efficacy of PGPR.  This combined approach can be used for sustainable agricultural practices, reducing the need for chemical fertilizers and pesticides while improving crop yields and resilience.

The roles of beneficial soil microbes like PGPR and mycorrhizae are multifaceted and integral to sustainable agriculture. They not only enhance plant growth and yield but also contribute to soil health, environmental sustainability, and ecosystem resilience and can be used for more efficient and eco-friendly farming practices.