More About Mycorrhizae
By David Rust
Mycorrhizal symbiosis is a seemingly simple concept: plants provide carbohydrates; fungi gather nutrients. That’s once the mycorrhizal connection is made, but what’s involved in the process of colonization? How do these underground connections come about at the structural level of hyphae and the root system? What environmental factors are in play to facilitate this connection?
How do fungi and plants make a mycorrhizal connection?
The initial formation of fungus-plant root connection resembles diplomacy. Nutrients that plants have taken in and used for photosynthesis (or building tissue) get transformed into new organic compounds which may be toxic or no longer needed. At the cellular level, organic molecules and catalyst nutrients are moved in and out of the cell to prevent a buildup of toxins. Plants dump photosynthetic waste compounds and the byproducts of respiration through their root system into the soil (rhizosphere).
These organic compounds change soil pH and overall soil biology, and their presence can signal that the plant is seeking a fungal partner. Prior to initiating an interaction, a plant may respond to fungal signals and accept or reject potential partners based on their identities or anticipated benefits (Bogar, 2019). Given that a particular vascular plant or tree may have multiple species of fungi associated with its root system, colonization and re-colonization by fungi is constant as new roots emerge. The concept of plants as “hosts” in mycorrhizal relationships is often overstated, but they definitely have a say in which fungi they will accept as partners.
Chemical signaling may attract the attention of nearby hyphae; it may also induce germination of a spore in the soil bank. Colonization can take place by either a nearby spore or from the approach of compatible hyphae in the rhizosphere (Smith and Read, 2008).
In return, fungal signals drive physiological changes in the potential root partner. As the hypha approaches a root, the root epidermis thickens in anticipation of fungus-root contact. The hypha accumulates acidic polypeptides (enzymes), creates an entry point into the epidermis, and then drills through this outer wall and penetrates between the cortical cells. A flat disk called an “appressorium” is formed, which firmly attaches the hypha to the plant cell wall. Through continued gene expression of symbiosis-related polypeptides, the process of nutrient exchange begins (Smith and Read, 2008). During this initial period, the plant produces stress and defense related proteins, indicating resistance to colonization. This threat response subsides over a period of 14-21 days, and may have originated as a natural defense against pathogens.
Colonized roots are stimulated to produce side branching, which is why mycorrhizae look so gangly.
Another Important Discovery: Mycorrhization Role of Soil Bacteria
Research has identified another piece to the puzzle of fungus-plant root colonization: the influence and actions of nearby soil bacteria, a concept known as Mycorrhization Helper Bacteria (Duponnois and Garbaye, 1991).
The wide range of lineages of Mycorrhization Helper Bacteria identified so far belong to many groups and bacterial genera, such as Gram-negative Proteobacteria (Agrobacterium, Azospirillum, Azotobacter, Burkholderia, Bradyrhizobium, Enterobacter, Pseudomonas, Klebsiella and Rhizobium), Gram-positive Firmicutes (Bacillus, Brevibacillus, and Paenibacillus) and Gram-positive actinomycetes (Rhodococcus, Streptomyces, and Arthrobacter) (Rigamonte, 2010).
These bacteria have created a niche whereby they are key to the process of mycorrhizal colonization. Mycorrhization Helper Bacteria influence receptivity of the root to the fungus, engaging both partners to assist root-fungus recognition; influence and stimulate fungal growth; promote stabilization of rhizospheric soil; and stimulate the germination of fungal propagules. Release of organic material by the root system of the plant is a natural source of nutrients which get recycled by mycorrhizae. These root exudates serve as substrates for soil bacteria. The relative soil pH also determines the composition of genera of bacteria which can live in this environment. It just so happens that many of these genera of bacteria are also Mycorrhization Helper Bacteria. As noted above, plant roots clearly influence the microorganisms in their rhizosphere. Bacteria which thrive in this organic “soup” help the plant ward off pathogens, and adapt to stress conditions concerning water and mineral nutrition (Lynch, 1990). Some of these same bacteria may act as rhizobia, fixing nitrogen and sourcing additional nitrogen and phosphorus for the plant root system.