Soil and Root Testing for Mycorrhizal Fungi in California

Mycorrhizae, derived from the Greek words "mykes" (fungus) and "rhiza" (root), represent a symbiotic association between fungi and plant roots. These relationships are ancient, dating back to the early colonization of land by plants approximately 450 million years ago. Mycorrhizal fungi are integral to the health and productivity of most terrestrial ecosystems. They enhance plant nutrition, improve soil structure, and increase plant resistance to pathogens. This article delves into the types of mycorrhizal associations, their benefits to plants, the variety of plants involved, and methods for detecting these associations.

Types of Mycorrhizae

There are several types of mycorrhizal associations, categorized primarily based on their structural characteristics and the specific fungi involved. The major types include:

1. Arbuscular Mycorrhizae (AM)

   • Fungi Involved: Glomeromycota
   • Structure: These fungi penetrate plant root cells, forming structures called arbuscules (branched, tree-like structures) and vesicles (storage organs).
   • Plants Involved: Predominantly herbaceous plants, including crops like wheat, maize, and legumes.

2. Ectomycorrhizae (ECM)

   • Fungi Involved: Basidiomycota, Ascomycota
   • Structure: These fungi form a sheath around the root, called a mantle, and extend into the root cortex intercellularly, forming a Hartig net.
   • Plants Involved: Mostly woody plants, such as pine, oak, birch, and eucalyptus.

3. Ericoid Mycorrhizae (ERM)

   • Fungi Involved: Ascomycota (primarily)
   • Structure: The fungi form loose coils inside the root cells.
   • Plants Involved: Heath family (Ericaceae), including heather and blueberries.

4. Orchid Mycorrhizae

   • Fungi Involved: Basidiomycota
   • Structure: Fungi form pelotons (coiled structures) within the root cells.
   • Plants Involved: Orchids

5. Monotropoid Mycorrhizae

   • Fungi Involved: Basidiomycota
   • Structure: Similar to ECM but specifically associated with non-photosynthetic plants in the Monotropoideae subfamily.
   • Plants Involved: Monotropes, such as Indian pipe.

Benefits to Plants

Mycorrhizal associations offer a multitude of benefits to plants, which can be broadly categorized into nutritional, physiological, and ecological advantages.

Nutritional Benefits

• Enhanced Nutrient Uptake: Mycorrhizal fungi extend the root system through their hyphal networks, increasing the surface area for nutrient absorption. This is particularly beneficial for the uptake of phosphorus, nitrogen, and micronutrients like zinc and copper.
• Improved Water Absorption: The extensive hyphal network also aids in water uptake, making plants more resilient to drought conditions.

Physiological Benefits

• Increased Stress Tolerance: Mycorrhizal associations help plants cope with various environmental stresses, including salinity, heavy metals, and drought. The fungi can detoxify these elements and sequester them, reducing their impact on the plant.
• Enhanced Root Growth: Mycorrhizal fungi secrete growth-promoting substances that can stimulate root branching and elongation, further improving nutrient and water absorption capabilities.

Ecological Benefits

• Soil Structure Improvement: Mycorrhizal fungi produce glomalin, a glycoprotein that helps bind soil particles together, improving soil structure and stability. This enhances soil aeration, water retention, and reduces erosion.
• Disease Resistance: Mycorrhizal fungi can protect plants from soil-borne pathogens through competitive exclusion, production of antifungal compounds, and induction of plant defense mechanisms.
• Biodiversity Support: Mycorrhizal networks can connect different plants, facilitating nutrient exchange and communication, which can enhance ecosystem stability and biodiversity.

Plant Associations

Mycorrhizal associations are widespread and occur in nearly 90% of all land plants. Different types of mycorrhizae associate with various plant groups, as outlined previously. Here are some examples:

• Arbuscular Mycorrhizae: Most crops (e.g., wheat, maize, rice), many vegetables (e.g., tomatoes, carrots), and many herbaceous plants.
• Ectomycorrhizae: Many forest trees (e.g., pine, oak, spruce), woody shrubs, and some herbaceous plants.
• Ericoid Mycorrhizae: Plants in the Ericaceae family (e.g., rhododendrons, blueberries, cranberries).
• Orchid Mycorrhizae: All orchids, which rely on these fungi for seed germination and nutrient supply, especially in the early stages of growth.
• Monotropoid Mycorrhizae: Non-photosynthetic plants like Indian pipe and other members of the Monotropoideae subfamily.

Detection of Mycorrhizal Associations

Detecting and studying mycorrhizal associations involves various methods, including visual microscopy and plate counts.

Visual Microscopy

Microscopy is a powerful tool for examining mycorrhizal structures within plant roots. The process involves several steps:

1. Sample Collection: Roots are collected from the field or greenhouse, ensuring minimal disturbance.
2. Clearing: Roots are cleared of pigments and cellular contents using a solution of potassium hydroxide (KOH) to make fungal structures more visible.
3. Staining: Cleared roots are stained using dyes like trypan blue, acid fuchsin, or Schaeffer black ink, which selectively stain fungal structures.
4. Mounting: Stained roots are mounted on microscope slides and examined under a compound microscope.

For different types of mycorrhizae, specific structures are identified:

• Arbuscular Mycorrhizae: Look for arbuscules, vesicles, and hyphal networks within root cells.
• Ectomycorrhizae: Identify the mantle around the roots and the Hartig net.
• Ericoid Mycorrhizae: Look for hyphal coils within root cells.
• Orchid Mycorrhizae: Identify pelotons within root cells.

Plate Counts

Plate counts are used to quantify and identify mycorrhizal fungi in soil or root samples. This method involves culturing fungi on selective media and counting the resulting colonies. The process includes:

1. Sample Preparation: Soil or root samples are collected and homogenized.
2. Dilution: Samples are serially diluted to reduce the density of fungal spores and hyphae.
3. Plating: Diluted samples are spread onto agar plates containing selective media that favor the growth of mycorrhizal fungi while inhibiting other microorganisms.
4. Incubation: Plates are incubated under conditions suitable for fungal growth, typically at 25-30°C for several days to weeks.
5. Counting: Colonies that develop on the plates are counted and identified based on morphological characteristics. For precise identification, molecular techniques like DNA sequencing may be used.

Conclusion

Mycorrhizal associations are fundamental to plant health and ecosystem functioning. By forming symbiotic relationships with plants, mycorrhizal fungi enhance nutrient uptake, improve water absorption, increase stress tolerance, and protect against pathogens. The different types of mycorrhizae, including arbuscular, ectomycorrhizae, ericoid, orchid, and monotropoid mycorrhizae, associate with a wide range of plant species, from crops to forest trees and orchids. Detecting these associations through visual microscopy and plate counts provides valuable insights into the diversity and functionality of mycorrhizal fungi. As we continue to explore these intricate relationships, the potential for harnessing mycorrhizal fungi in sustainable agriculture and ecosystem management becomes increasingly apparent.