Mycorrhizae – Part I
In the fall of 2012 my wife and I were planting trees, lots of trees. If you were living in Central Virginia at the time, you may recall we had had a little wind storm the previous June. Actually, this was no little storm! It was the first time I had ever heard the word derecho. As a result of this unusual storm, we lost many trees, so the following autumn, we began the chore of replanting.
We were planting our first replacement, a native dogwood, and I noticed my wife sprinkling a gray powdery substance on the root ball and in the hole. Hmmm. I just had to ask, “What’s that stuff you’re adding?” Well, I got that familiar stare, and then she replied, “Mycorrhizae.” Well, I needed a little more information about this mystery substance. “Mircorissen what?” I asked. And after being married for some forty odd years, I got the standard response. “Yep, mycorrhizae and it’s pronounced: my-kuh-ri-zee.”
“Okay,” I responded, “what is that mycorrhizae stuff you are dumping in the hole?” With a look of disbelief, she replied, “Mycorrhizae is derived from two Greek words, Mykos meaning fungus and Riza meaning roots.” Maybe I shouldn’t have asked. I started shaking my head in disbelief, and before I could close my mouth, it just rolled out — “Well, that’s just fantastic — you’re infecting our new tree with a root fungus. That’s just great! At least it will be easy digging when we replace the dead tree. What are you thinking?” I really wasn’t interested in her thought process at that moment, but I wanted to throw in a little barb. She slowly responded, “Last week I was reading about the benefits of mycorrhizae to trees. Perhaps you should do a little research and stop complaining.” Well, I figured she was right. I needed to do a little research before we proceed to kill the new trees we were planting!
Well, by now I know you are wondering what this has got to do with the edible garden, and I promise we’ll get to that in just a bit. But first I needed to do a little fungi research before my wife killed all our new trees — all 28 of them.
As it turns out, mycorrhizal fungi have been around for about 400 million years and they are native to most soils. However, the concept of mycorrhizal fungi is a fairly recent notion. In 1881 a Polish mycologist, Franciszek Kamienski described an association between a fungus and the roots of a plant. In 1885, A.B. Frank, a German botanist, was commissioned by the King of Prussia to discover if truffle production could be increased. He never succeeded in his mission, but described root structures on trees closely associated with truffles. Mr. Frank hypothesized that there was a mutually beneficial relationship — called a symbiotic relationship — in which a fungus and host plant rely on each other: the fungus extracts nutrients from the soil and transmits them to the host plant; the host plant in turn nourishes the fungus. Frank is given the credit for coining the term mycorrhiza (this is the singular; the plural is mycorrhizae), meaning fungus root. Back in the 1880’s the concept that a fungus could be good for a plant was a revolutionary idea and flew in the face of the conventional thinking at the time. I must admit on that day when my bride was adding fungi to our new dogwood tree, I too did a little head scratching.
As it turns out, after 130 years and thousands of research papers, mycorrhiza has become an accepted scientific concept, and the symbiotic relationships between fungus and roots have been exploited for applications in agriculture. This interaction results in a recognizable fungal structure — tubular filaments — on or within the roots, and today it is estimated the approximately 90% of all vascular plants live in some association or symbiotic relationship with mycorrhizal fungi.
Mycorrhizal fungi are generally classified into two general types based on the position of the thread-like filaments of the fungus: ectomycorrhiza and endomycorrhiza. (Brady &Weil)
Ectomycorrhiza (Ecto=outside) group includes hundreds of different fungal species associated with trees and shrubs such as pine, birch, hemlock, beech and oak. The fungi grows around and between the root cells, the fungus doesn’t actually penetrate the root cells. The fungus also forms a considerable mass in the soil surrounding the plant roots. The fruiting, or reproductive, bodies of these fungi are sometimes visible, and we recognize them as mushrooms! Ectomycorrhiza are commonly associated with forest trees, pasture fields, lawns, and open woodlands and in places next to water. They are host specific, have many species and can disperse far and quickly. (Brady & Weil)
Endomycorrhia, (Endo=inside), the most important and most widespread of the endomycorrhia group are called arbuscular mycorrhizae (AM). These fungi actually reside inside the cells of the plant root. Once inside the root cell, they form small, highly-branched structures called arbuscules. These structures serve to transfer mineral nutrients from the fungi to the host plants and sugars from the plant to the fungus. The fungus also forms a considerable mass of hyphae in the soil surrounding the plant roots. This mass of hyphae is often referred to as the mycelia. (Brady & Weil)
Root hairs absorb plant nutrients dissolved in soil water, often called the soil solution. However, there are some essential nutrients such as phosphorus, zinc and copper that have limited mobility in the soil solution and are often unavailable to the root hairs. AM fungi function as an extension of a plant’s root system. In addition to growing within the root, much of the body of the fungus called hyphae is in the soil. These filamentous structures of the fungus give the root more “surface area” and are more effective than root hairs at exploring the soil for nutrients such as phosphorus, copper and zinc. The fungus picks up these nutrients and transports them back to the root where they are released by the arbuscules and can be used by the plant. Simply put, mycorrhizae extend the plant’s reach, allowing it to get more than it needs to survive, making the plant stronger, especially during drought periods. A stronger individual plant also means the community of plants is more resilient to disturbance. (Plaster)
Most native plants and agricultural crops that have the capacity to form mycorrhizal associations perform poorly in unfertile soil, especially soils with low phosphorus levels and the absence of mycorrhizal fungi. Many plants in the Legume family are especially dependent on mycorrhizal relationships, not only to obtain sufficient phosphorus and other nutrients, but also to enhance their nitrogen-fixing symbiosis with the rhizobia bacteria.
As always in the plant world, there are exceptions to the rule: two plant groups that do not form mycorrhizae are the Cruciferae family (mustards, cabbage, rapeseed) and the Chenopodiaceae family (beet and spinach). The presence of AM fungi are most important where soils are low in nutrients, especially phosphorus. (Brady & Weil)
Mycorrhizal fungi obtain carbohydrates (simple sugars) from the host plant. It is estimated that plants “infected” by mycorrhiazae shunt about 15 percent of sugars produced by their leaves to their mycorrhizal symbiont. In return the host plant gains a number of benefits:
- Roots are better able to absorb phosphorus — probably the most certain and important benefit.
- Roots are better able to absorb water, making plants more drought resistant and able to absorb zinc, copper and other nutrients.
- Rootlets that are “infected” by mycorrhizal fungi live longer than uninfected ones.
- Some mycorrhizae protect roots from disease and probably from low levels of toxins, including aluminum and heavy metals.
- Mycorrhizal fungi are particularly effective at aggregating soil particles, so they improve the physical condition of soil for plants. A complex, sticky gooey substance, called glomalin, is produced by mycorrhizae that binds and protects soil aggregates. (Plaster)
Today the beneficial properties of mycorrhizae have been exploited for applications in agriculture, forestry, horticulture, declamation and biocontrol.
In addition to gathering nutrients and water for plants, mycorrhizae can infect and connect neighboring plants, allowing nutrients and transfers between the two plants. Even plants of different species may be connected through hyphae bridges or the mycelium network between the roots of different plants. An example is the Indian Pipe wildflower (Monotropa uniflora), which is unable to obtain food through photosynthesis; instead, its roots are infected by a mycorrhizae from a nearby tree and the tree feeds the fungus, which in turn transfers some of the sugar to the Indian pipe. (Plaster)
In addition to these benefits, new research is surfacing to suggest that the mycelium network of a myccorhizae may be viewed as a “biological internet” and that plants are able to exchange information using the mycorrhizae network. One research project performed on broad beans supports this notion.
When a pest damages a broad bean plant, the level of plant “volatiles” (methyl salicylate) is increased. The volatiles act not only to repel pests but also to attract enemies such as parasitoids. Increased levels of volatiles were not only observed in a plant under attack from aphids, but also higher volatiles were measured in the unattacked plants connected to the mycorrhizae network of the plant being attacked. No increased levels of volatiles were noted in adjacent plants that were not connected to the “biological internet.” The study concluded that the aphid-attacked plant was sending out a message about the attack via the mycorrhizae network to the other plants on the network.
Several years ago I was taking a soil class at the local community college and the professor remarked that the emphasis in soil science was changing from soil chemistry to soil biology. Well, my mycorrhizae journey may have just reaffirmed his statement. And as for my bride, well, she may already have made that transition from chemicals to biology.
Thanks for stopping by The Garden Shed and we hope to see you again next month when we go into detail on how mycorrhizal fungi fit into the organic vegetable garden, its effect on individual vegetables, and how to maintain a healthy mycorrhizae population.
The Nature and Properties of Soils, (Brady & Weil, 14th ed.rev. 2008), pp.472-475.
Soil Science & Management (Plaster,E., 2009), pp. 109-117.
Remy, Winfried, Thomas, Taylor N., Hass, H., & Kerp, Hans, Plant Biology, “Four Hundred-Million year-old Vesicular Arbuscular, Mycorrhizae”, Vol. 91, pp, 11841-11843, December, 1994
“Hidden Partners: Mycorrhizae Fungi and Plants,” The New York Botanical Garden, http://sciweb.nybg.org/science2/hcol/mycorrhizae.asp.html
“Symbiosis: Mycorrhizae and Lichens,” Department of Botany, University of Hawaii at Manoa http://www.botany.hawaii.edu/faculty/wong/BOT135/Lect26.htm
“Life Cycle, Significance and Structures of Arbuscular Mycorrhizae” http://www.zor.zut.edu.pl/Glomeromycota/Life cycle, significance and properties of AM.html (Blaszkowski, James, University of Szczecin)
Encyclopedia of Plant Pathology, “Mycorrhizae,”(Vol. II, 2001), http://pnwhandbooks.org/plantdisease/node/1805/print
Trappe, J, M., US National Library of Medicine National Institutes of Health, “Mycorrhizae”, 2005, June: 15 (4) 277-81 Epub 2004 Oct. 19 PMID 15503185 http://www.ncbi.nlm.nih.gov/pubmed/15503185
Babikova, Z., Gillbert, L., Bruce, T., Burkett, M., Caulfield, J., Woodcock, C., Pickett, J., Johnson, D., Ecology Letters, “ Underground signals carried through common mycelia networks warn neighboring plants of aphid attack”,http://scholar.harvard.edu/files/cmhart/files/2013_media_messaging_-_all_papers_merged.pdf