Mycorrhizae Research Information

by Dr. John Klironomos,University of Guelph

 Eighty-five percent of all plants, all trees included, have root infections called mycorrhizas. The term “mycorrhiza” is greek for “fungus – root,”and as the term implies, this type of infection results when certain microscopic fungi invade the fine roots of plants. Luckily, these infections are not parasitic or pathogenic, but rather, mutualistic. This means that both the fungus and plant benefit from the infection. How can trees benefit from such an infection? Well, these fungi infect the roots up to up to a few cells deep, and then extend their thin, tubular bodies out into the soil to a distance of several centimeters. In essence, they act as extensions to the roots themselves and pick up valuable mineral nutrients, in particular phosphorous and nitrogen.They then transport these nutrients back to the plant. These fungal tubes, called hyphae, are much thinner than even the finest roots, and so can penetrate the tiniest pores in soil. Most books still teach us that roots are responsible for mineral uptake. However, this is quite misleading. It is the fungi that fulfill this role in nature. Roots are not well designed to pick up nutrients from soil, and these fungi are necessary for this. Also, these fungi can protect roots from infection by other harmful fungi and bacteria, and there are many pathogenic microorganisms in soil waiting to invade. Why do these mycorrhizal fungi go to all this trouble to help plant out? Well, they certainly do not work for free.There is a significant cost to the plant for maintaining this mutualism. In exchange for improved nutrition, the plant gives the fungus a significant portion of its carbon supply. Remember, plants have the unique ability to photosynthesize. In other words, they can convert CO2 from the atmosphere into usable sugars and it is these sugars that the fungus is after. Actually, depending on the tree species, the plant gives up anywhere from 10-85 percent of its newly synthesized sugars to the fungus, in return for mineral nutrients. This may sound quite expensive for the plant, but in nature most plants would simply not survive in the absence of mycorrhizas. Mycorrhizas have been around for a long time. There is fossil evidencethat suggests that they are 400 million years old. At that time the world was a very different place, and plants were just migrating from aquatic to terrestrial habitats. Plants had pathetic root systems and soils were not yet developed. Rather, the terrestrial environment was made up mostly of rock, a difficult habitat for plants to establish. Fungi were also making the transition onto land at the same time, and it is believed these two groups of organisms formed these intimate associations because they simply could not make the transition and establish on their own. In today’s world, mycorrhizal fungi exist everywhere. It is very difficult to find a gram of soil without these mutualists. However, many species are sensitive and are strongly affected by disturbance, while others are”weedy” and proliferate under similar conditions. Disturbance can be mechanical, such as soil tillage or compaction, or chemical, such as the fertilizer and pesticide application. Typically, only the weedy and aggressive fungi do well under high disturbance, and unfortunately,those are not the best partners for trees. They tend to ask for a lot of sugar, in exchange for very little return in mineral nutrients. Urban trees, in particular, are subject to many environmental stresses. Many harmful chemicals are deposited on the trees and directly on the soil, and the ground they are growing on is typically heavily compacted. Also, let’s face it, they don’t exactly grow in great quality soil. Furthermore, young tree seedlings and sapling growing in nurseries are typically fertilized, which further eliminates many beneficial mycorrhizal fungi. Some of our preliminary research in the cities of Guelph and Toronto shows that trees in streets capes are heavily infected by mycorrhizal fungi (a good thing), but fungaldiversity is very low (not a good thing). Many species normally associated with these trees in forests have become extinct from urban environments. In a recent field-trial we tried to bring back some of these fungal species. We inoculated black maples with a cocktail of mycorrhizalfungi to see if this would improve their growth. During the first year,there was no sign that inoculation helped at all. All trees were similaris size and overall health, and we began re-evaluating our story. Butthen came the big summer drought of 1998. This placed a lot of stresson our experimental trees, and right now, the inoculated trees are the only ones that have survived. This suggests that things may be fine during normal years, but that this diversity of fungi may be important during years with extremes in climate. As climate is expected to be less predictable in the coming years, mycorrhizal inoculations may prove to be more necessary.