Mycorrhizas: Mutualistic Plant-Fungus Symbioses

Flashcards reviewing endomycorrhizae and ectomycorrhizae definitions, fungal taxonomy, structural anatomy, developmental colonization steps, agricultural benefits, and sources of inoculum.

What is a mycorrhiza?

A symbiotic relationship between a fungus (mycobiont) and the roots of plants (photobiont).

How do endomycorrhizae and ectomycorrhizae differ in how their fungal hyphae interact with host plant root cells?

Endomycorrhizal fungi enter plant root cells and produce intracellular interfaces called arbuscules, whereas ectomycorrhizal fungi grow around the root or between cortex cells but never penetrate the root cells.

Which fungal division is associated with endomycorrhizae?

Glomeromycota (Glomeromycotina).

Which fungal group predominantly represents ectomycorrhizae?

Agaricomycotina.

What primary benefits do the plant and fungus exchange in a mycorrhizal relationship?

The fungal hyphal network accumulates nutrients (especially phosphorus) for the plant, while the plant provides photosynthetic products (carbohydrates) to the fungus.

What are five agricultural benefits of mycorrhizal symbioses?

1. Reduced need for fertilizer (nitrogen and phosphorus)
2. Increased ability to withstand heavy metal and acid rain pollution
3. Improved growth on infertile soils such as mine dumps
4. Better survival against transplant shock
5. Greater resistance to soil-borne diseases

What are the three basic functioning components of a mycorrhizal symbiosis?

1. Fungal mycelium exploring large volumes of soil and retrieving mineral nutrients
2. Plant tissues which produce and store carbohydrates
3. Fungus-plant interface for the exchange of chemicals

What are the three main steps in the colonization process during ectomycorrhiza development?

A) Penetration of hyphae between cells of the root cortex to form a Hartig net B) Establishment of a mantle of hyphae around the outside of the root C) Extension of hyphae from the mantle into surrounding soil

Which important plant host groups form ectomycorrhizal relationships?

Pine, oak, maple, elm trees, and Dipterocarpaceae.

What structure is shown covering the exterior of this ectomycorrhizal root tip?

The fungal mantle (a sheath of hyphae established around the outside of the root).

What is the Hartig net in ectomycorrhizae?

A hyphal network that penetrates intercellularly between cells of the root cortex without entering the cells themselves.

How does reintroducing a mycorrhizal fungus affect pine trees grown in sterilized soil?

It significantly improves the survival rate and growth of the pine tree seedlings compared to those left in sterilized soil without mycorrhizae.

What are the five sources of ectomycorrhizal inoculum?

1. Naturally dispersed spores
2. Colonized soil
3. Mycorrhizal seedlings
4. Basidiomata or spores specifically collected for the purpose
5. Fungal mycelium produced in axenic culture

What is the sequence of steps in arbuscular mycorrhiza (endomycorrhiza) symbiosis development?

1. Fungus enters root hair of plant host
2. Establishes intramatrical phase (hyphae grow between cells)
3. Specialized hyphae penetrate cortical cells and form arbuscules
4. Develops extramatrical hyphal network

Which key crop species serve as important plant partners for endomycorrhizae?

Legumes, grasses, tomatoes, strawberries, peaches, and coffee.

Which plant cell types do endomycorrhizal hyphae NEVER enter?

Meristematic cells or endodermal cells.

What are arbuscules (labeled 'A' in the provided image) and what is their function in endomycorrhizae?

Arbuscules are specialized, highly branched hyphal structures that enter individual cortical cells and serve as the main exchange sites between the fungus and the plant.

What are vesicles in endomycorrhizae and what is their primary function?

Vesicles are lipid-rich structures formed by some fungi within host root cells.

Which specific mineral nutrients are shown being absorbed and transported to the plant root in an endomycorrhizal cross section?

Phosphorus, zinc, copper, nitrogen, and potassium.

Why must arbuscular mycorrhizal (AM) fungi be grown in dual culture with a host plant rather than pure culture?

Because endomycorrhizal fungi are obligate biotrophs and can multiply only in dual culture with a living host plant.

What three materials can be used as inoculum to infect experimental plant roots with arbuscular mycorrhizal fungi?

1. Infected soil
2. Infected roots
3. Spores

What structure is attached to the extramatrical hyphae of Glomus shown in this micrograph?

A chlamydospore.