In the Part 1 and Part 2 of this series, I described what mycorrhizae are and how they can benefit the growth and health of plants. I described the three main types of mycorrhizae.

Just as a refresher, ectomycorrhizae are associated with the roots of pines, firs, oaks, eucalyptus, hazelnut and birch. Ericoid mycorrhizae are associated with ericaceous plants like rhododendron, blueberry and azalea. Arbuscular mycorrhizae (AM) make up the largest group and are associated with the most different plants on the planet, including most crop plants.

Given that AM have been helping plants grow for some 460 million years, they definitely have proven their worth. Also remember that some plants, like cabbage, broccoli, beets, turnips, radishes and carnations simply don't form mycorrhizae.

Mycorrhizae provide many benefits to their host plant partner, including improved root development, transplant success, soil structures, tolerance to soil drought and soil toxicities and fertilizer-use efficiency. All of this leads to increased yield and quality, which might leave you wondering, how does mycorrhizal fungi do all that? In this final part of the series, I want to explain what this symbiotic relationship is, and how it develops and functions. Such a discussion embraces something called the mycorrhizosphere phenomenon.

Mycorrhizae Formation

As you might recall, AM fungi begin their association with plant roots when spores in the soil germinate near roots and colonize the roots, penetrating the root cortex but without causing any damage to the cells. The fungal and plant cells are communicating as this symbiotic relationship develops in the root. The root allows the fungus to grow between the cells and into the cells without causing any damage, much like pressing your hand into a balloon without breaking it.

That close interface between the fungus and cell cytoplasm is where chemical exchanges take place—mineral nutrients provided by the AM fungus to the plant, carbohydrates from the plant to the fungus. Eventually, the AM fungus grows out into the soil to mine for mineral nutrients that it can then contribute to its plant host partner.

That fungal growth is dependent on the carbohydrate nutrients that the plant partner provided. Eventually, the fungal growth into the soil results in the AM fungus, reproducing itself by making new spores that allow it to survive through time and until a new host plant root comes near enough to start the cycle all over again.

But the story is just beginning. The AM fungus has now colonized the soil, producing an amazing amount of biomass in the form of hyphae (tiny tubes) that are connected to the host root, but are also interfacing with a myriad of other microbes in the soil.

Those hyphae exude or leak nutrients into the soil, as do the roots themselves, and the combination of fungal and root exudates are available for other microbes to use. Some will like the exudates and thrive, others will not. Thus a new equilibrium of microbes will become a team that can grow and function in tandem. And so, the mycorrhizosphere is created.

How the Mycorrhizosphere Works

In the mycorrhizosphere, there is a new set of microbial players that have formed a team. Those players were recruited from the bulk soil by the AM fungus and the altered root exudation pattern. The team leader is the AM fungus, like the quarterback of the football team. The other members of the team have different and various functions, some of which can affect plant growth and health.

They perform their functions by growing and producing metabolites, as by-products of their growth, and those materials can be absorbed by the AM fungus and transported to the roots. Once in the root, the plant can respond to those chemicals, directly or indirectly, in ways that optimize plant growth and health.

Some of those chemicals may trigger genetic responses by the host plant. For example, host disease resistance genes may be turned on. Some of the action also takes place in the mycorrhizosphere directly between different microbes.

For example, some bacteria that have been favored by the AM fungal exudates may produce antibiotics that inhibit the growth of deleterious root pathogens, averting potential diseases. Other microbes produce hormones that can stimulate more root development. A plant with a bigger, healthier root system will produce more yield than a plant with a smaller, weaker (possibly root rot infected) root system.

Still other microbes may capture atmospheric nitrogen and convert it into plant-usable nitrogen fertilizer. Some produce enzymes can destroy the cell walls of fungal pathogens, thus preventing root infections. Some produce acids that can dissolve phosphorous precipitates, making the phosphorous readily available for uptake and plant use.

In most soils, phosphorous is tightly bound and therefore immobile in the soil, so it cannot flow to the root. AM fungi can grow into the tiny spaces of the soil and mine it for such bound elements like phosphorous. But, the real miners are the phosphate-solubilizers that associate with the AM fungal hyphae.

We have measured increased population of phosphorous-solubilizing bacteria directly within the soil aggregates created by AM fungal hyphae in soil (Fig. 2). What this means is that the fungal exudates, some of which are sticky and bind the soil particles, release nutrients into the soil that favor the buildup of phosphorous solubilizers. The latter dissolve the phosphorous, making it now available for the AM fungus to transport it to its plant partner.

How cool is that?

Recruiting Microbial Associates

Where do the good players in the mycorrhizosphere come from? In time, they can be recruited from the bulk soil, assuming the soil has some good ones. But what if there are no or few good players to be recruited? Soilless media or potting mixes are likely to have none; fumigated soil may have few as well. Microbial diversity in soilless potting mixes is likely to be low, and soils with low organic matter will also have low microbial diversity.

If you inoculate your plant roots with a mycorrhizal product that has been produced in a soilless medium, or produced in vitro, what are the chances for recruiting any good players in time to help the plant grow? The AM fungi in those products may colonize the roots, but without good teammates, a quarterback cannot function well. Many commercial mycorrhizal products that were not produced in soil often are laced with other microbes, added at the last step before bagging. Some may be good ones, but might not fit well into the team. Many will not even survive or are at low population levels, too low to affect plant growth.

If you want a full-functioning microbial team that can function in tandem, you want one that has grown up together. You want a holistic product. Using a mycorrhizal product that has a good blend of AM fungal species—an organic matrix that provides microbial diversity of good microbial players—and organic materials that are known to stimulate root development is the way to go.

Mycorrhizal products for Maximum Yield

If you are interested in maximizing yield from any crop, and having the product be the highest quality possible, consider everything you might do to make that happen. I have read and reviewed research papers and much of what I have said about the benefits and mode of action of mycorrhizal fungi has been gleaned from those readings, in addition to my own work with students and colleagues. In general, all those studies pooled together indicate that maximizing yield and quality comes as a result of all the efforts to increase soil quality.

Soil quality is maximized by balancing the three main soil components: soil chemistry, soil physics and soil microbiology. Of course, environmental conditions that provide appropriate levels of light and temperature are critical, too.

Outdoors, we have to rely on what Mother Nature gives us. Indoors, we can manage nearly, if not all the variables. The last component of soil microbiology seems to be the most difficult. We have to suppress the bad guys, and encourage the good guys. That means keeping pathogens at bay, and encouraging those beneficial microbes to do their thing. With mycorrhizae, that means inoculating young plants with the best possible product, properly placed to contact roots early, and not doing the things that may suppress them. A plant with no root or foliage diseases, with balanced fertilizer, growing in a well-aerated soil or medium, and with the aid of mycorrhizal fungi and their microbial associates, should be the path to success.

A good example is the production of giant pumpkins, an internationally occurring competition where growers try to optimize all the parameters they can to grow larger and larger pumpkins. Genetically-superior seed is used, environmental conditions are modified as much as possible, chemical products are applied to prevent diseases and everything possible is done to enhance the quality of the soil. When mycorrhizal fungi were first applied, in part as a result of my interaction with a grower from Rhode Island, Ron Wallace, the results were amazing. The world record was broken the first year with a pumpkin he grew that weighed 1,502 lbs. Wallace has continued to refine his techniques, and this year he established a new world record of more than 2,000 lbs. Mycorrhizae were a part of that story!