A Symbiotic Relationship

Sept. 1, 2003
When Alexander Fleming discovered penicillin, he was trying to perfect an antiseptic formula based on nasal mucus. The nasal mucus formulation never did materialize (we all can breathe a sigh of relief!), but his unforeseen discovery of antibiotics changed the world. Great discoveries often have been unanticipated or appreciated only years after the initial findings. Walter Alvarez was attempting to figure out the geologic origins of Italy’s Apenine Mountains; years later, he realized that he had stumbled upon evidence that dinosaurs had been rendered extinct by a large comet or asteroid. When scientists were trying to find an alternative form of Freon, they invented Teflon. Rayon and nylon were botched experiments. Clearly serendipity has played a role in many great discoveries.
Photo 1: Mycorrhizal fungal filaments radiating from a mycorrhizal colonized rootThis brings us to a recent discovery in a Wisconsin roadcut (Redecker et al., 2000). The roadcut contained newly discovered fossilized roots containing the threads and microscopic spores of fungi some 460 million years old. At first, scientists believed these ancient fungi had a negative effect on the plant. After all, fungi are parasites and saprophytes and are likely robbing the plant roots of their energy or rotting them like a moldy piece of bread. Upon closer examination, however, this group of fossilized fungi clearly was different. They were mycorrhizal fungiÑthe same mycorrhizal fungi that form a beneficial association with the fine roots of terrestrial plants growing throughout the world today. These specialized fungal filaments form a symbiotic organ called the mycorrhiza (photo1). Through this structure, the plant provides energy for the fungus, and in return the fungus provides the nitrogen, phosphorous, and other minerals and water necessary for plant establishment and growth. The mycorrhizal fungus also promotes fine root development and soil structure and protects plant roots from diseases and soil toxicities (Trappe and Fogel, 1977; Harley and Smith, 1983).The discovery in the Wisconsin cutbank is further evidence that fungi and green plants moved from water onto land together, bolstering the theory that these specialized fungi were the key factor that allowed plants to successfully invade the land. The newly discovered fossils push the origin of land-based fungi back some 55 million to 60 million years to about the same era that green plants first grew on land. The mycorrhizal fungi attached to the roots of aquatic plants allowed plants to access water and nutrients on the relatively harsh earth surface. Before the mycorrhizal arrangement, these aquatic plants could not survive on land. The plant and fungus mycorrhizal association is widespread today, with 85-90% of all green plants forming mycorrhizae. By far, the most common mycorrhizal type is the arbuscular mycorrhizae (AM), which penetrate the plant root cells but do not modify the external appearance of the root. This type of mycorrhiza forms with more than 100 fungal species in the family Endogonaceae; predominates in grasses, forbs, bryophytes, pteridophytes, and most tropical tree genera; and is associated with most nursery and agricultural crops. Most undisturbed natural areas today contain an abundance of mycorrhizal fungi. However, areas disturbed by human activity often lack sufficient mycorrhizal populations that are important for plant establishment and growth (Amaranthus and Perry, 1994; Amaranthus et al., 1996; Dumroese et al., 1998). Disturbance, compaction, invasion by weeds, erosion, removal of topsoil, mixing, land clearing, and frequent pesticide use can adversely affect beneficial mycorrhizae. The ancient Wisconsin fossils provide insight into establishing plants in the disturbed and hostile environments of today. A good example is a recent study of the McDonald Basin, a chronic erosion-prone site in the Siskiyou Mountains of southwest Oregon.McDonald BasinPhoto 2: Planting Agrostis plug seedlings at McDonald basin McDonald Basin is a denuded, high-elevation site that contributes large quantities of granitic sand-size sediments to the Applegate watershed. The need for soil cover by native plants has been identified as a resource objective for the area for nearly four decades. Debbie Whiteall, hydrologist for the United States Department of Agriculture Forest Service, has worked in the region for more than 15 years and describes the area’s condition: “McDonald Basin is certainly one the more disturbed areas along the Siskiyou crest. The site’s elevation, aspect, and prevailing weather patterns contribute to the naturally occurring, highly erosive, and nutrient deficient condition. For over a hundred years, human impacts, including grazing and road building, have exacerbated the condition. The basin is situated in the headwaters of the Little Applegate watershed and provides water for domestic uses and endangered salmon downstream and is a priority for restoration activities. Many years of operational plantings and studies of native grasses and trees in McDonald Basin have yielded little improvement in soil cover.”ObjectiveAbove: Photo 3a: A McDonald basin area before planting
Below: Photo 3b: after planting
From a management standpoint, the restoration objective at McDonald Basin is to establish an adequate soil cover over time. This can occur in several ways: directly through the presence of live or dead vegetation or indirectly through more seed yield or a better environment for seed germination and establishment. Any measures that improve soil cover should improve watershed conditions. Poor fertility at the McDonald Basin site often has been cited as one of the primary reasons for the mortality of planted vegetation. The use of time-release fertilizer in the nursery soil of containerized plantings could be a mechanism for maintaining plant vigor in the harsh outplanting environment. New information from the Wisconsin cutbank also has sparked interest in using the primary relationship between plants and mycorrhizal fungi in restoration planting. This study investigated the role that AM and slow-release fertilizers in the nursery play in establishing grass seedlings outplanted at McDonald Basin. MethodsWe sowed native grass seed, Agrostis pallens (dune bent grass) in 4-in.3 containers in late May 1999. Prior to sowing, we mixed mycorrhizae and slow-release fertilizers into the media in the following combinations:Mycorrhiza onlyMycorrhiza and slow-release fertilizerSlow-release fertilizer onlyNo mycorrhiza or slow-release fertilizer (standard greenhouse practices)The mycorrhiza inoculum was composed of the species Glomus intraradices and had a concentration of 130 propagules per gram. We applied it at a rate of 1.2 grams inoculum per seedling. We used Osmocote 18-6-12 with an eight- to nine-month longevity for the slow-release fertilizer. After the seeds were sown, we placed the containers in a greenhouse and grew the seedlings for two months. By that time, the snows had melted on the slopes of McDonald Basin and we could assess the site.We planted the seedlings in mid-July in a random block outplanting design, where each block included each treatment randomly placed (photo 2). Although the growing season is short at this elevation, the planted seedlings became established and grew before the snows covered the slopes in late October and remained covered until the following summer (photos 3a and 3b).ResultsWe revisited the study site in mid-September to take measurements and collect seedling samples. Within a two-month period we found dramatic differences in the survival rate between mycorrhizal inoculated grass seedlings and noninoculated seedlings (Figure 1). Grass seedlings that had been inoculated with mycorrhiza in the nursery had 100% survival. Seedlings that had been inoculated with mycorrhiza in combination with slow-release fertilizer application survived at an 81.4% rate. Seedlings fertilized with slow-release fertilizer, but not inoculated with mycorrhiza, resulted in 26.4% survival rate. Seedlings grown under standard nursery practices of soluble fertilizer, but without mycorrhizal inoculation and slow-release fertilizer, had only a 17.2% survival rate.From each treatment, we excavated five seedlings and brought them back to the lab to measure the dry weights of roots and leaves and also to evaluate the amount of mycorrhizal colonization on the root systems of each seedling. We found that mycorrhizal-inoculated seedlings had significantly greater root biomass and mycorrhizal colonization after one year in the field compared to seedlings receiving fertilizers only (Figures 2 and 3). Very little mycorrhizal colonization occurred on seedling roots with fertilizer-only treatments. Seedlings inoculated with mycorrhizae in the nursery also had high nutrient concentrations compared to other treatments. Foliar levels of phosphorous, potassium, calcium, and sulfur levels were all significantly higher (Figure 4).DiscussionThe results of this study highlight basic differences between the function of mycorrhizal fungi and fertilizers. “Can’t I just fertilize?” is a common question from growers. Inoculated Agrostis seedlings grown without additions of time-release and soluble fertilizers had higher foliar concentrations of important macronutrients. It is well established that mycorrhizal fungi facilitate the capture and uptake of nutrients from the soil and produce enzymes important for the extraction of minerals from soil particles. Studies have shown that grass species in the family Poaceae benefit greatly from mycorrhizal colonization in terms of growth and nutrient acquisition (Gemma and Koske, 1989; Hall et al., 1984). Mycorrhiza also perform many important activities beyond the capture and uptake of nutrients that might improve plant establishment. For example, mycorrhizal fungi are important for disease suppression, drought protection, improved soil structure, enhanced leaf chlorophyll levels, and tolerance of nutrient imbalances (Linderman, 1994). Research studies have shown that mycorrhizae can enhance the ability of grasses to avoid water stress (Koske et al., 1995; Auge et al., 1995; Allen, 1991). Recent studies indicate that creeping bent grass inoculated with the mycorrhizal fungus Glomus intraradices tolerated drought conditions significantly longer than nonmycorrhizal turf (Gemma et al., 1997). It is also well documented that inoculating grasses with mycorrhizal fungi in soil with low phosphorous concentrations can produce greater shoot and root biomass (Hall et al., 1984; Petrovic, 1984; Hetrick et al., 1986). Further research on mycorrhizae has shown that mycorrhizae promotes the development of early successional tallgrass prairie communities. A significant increase in the percent cover of native species Andropogon gerardii, Panicum virgatum, and Sorghastrum nutans was observed (Smith et al., 1998). Mycorrhizal fungi can tap into the soil resource by extending their hyphae far from the roots’ surface. Mycorrhizal filaments are two to five times smaller than the roots themselves and have a greater surface area per unit volume. This allows the filaments to explore more soil volumes and smaller spaces in the soil not accessible by roots. Once in these small spaces, the mycorrhizal fungi excrete enzymes and chelates that access immobile nutrients.“In nature, most plants require mycorrhizae to compete effectively; without mycorrhizal fungi they are competitively inferior,” says Jim Trappe, Ph.D., professor of botany at Oregon State University. “In stressful environments this means the difference between life and death.” “Some of the biggest success stories have been the use of mycorrhizal inoculants on mine reclamation sites,” remarks Efren Cazares, assistant professor of forest science at Oregon State University. Over the years he has been able to study the effects of human activities on mycorrhizal activity. “While mycorrhizal inoculants should not be considered a silver bullet, it is clear that linking plants to the soil resource via the appropriate mycorrhizal fungi greatly increases the chances for a successful revegetation project,” says Cazares.A common question from erosion control practitioners is “When should I apply mycorrhizal fungi?” Mycorrhiza can be incorporated during propagation, nursery production, or transplanting, or to existing trees and shrubs. Common methods include incorporating granules into soil-less bark/sand media at the nursery or banding or rototilling into soils at field sites. Other methods include using mycorrhizal seed coats, sprinkling granular materials around root balls, and injecting soluble powder blends into soils surrounding existing plants. Still other methods include inserting mycorrhizal tablets into the rooting zone and using hydromulchers to spray mycorrhizal inoculum onto sites. Whatever the method, mycorrhizal fungi need access to roots, and as with any living organisms, mycorrhizal fungi prefer some living situations to others, so diverse species mixes are best. Mycorrhizal fungi have been critical to the establishment of plants for more than 460 million years, but it has been only in the last 40 years that scientists truly have appreciated the importance of the relationship. Today there are more than 50,000 scientific papers on mycorrhiza and their function. The significant difference in plant survival and root growth in the McDonald Basin is part of a growing body of scientific studies that demonstrate that mycorrhizal inoculation in disturbed environments can help plants become established more effectively and with less chemical inputs. Microscopic root fossils in a Wisconsin cutbank remind us of a simple and obvious restoration tool that lies right under our feet.