A. Symbionts of Plants
Ninety-eight percent of all plant species have
symbiotic fungal partners associated with their root systems. Fungal-root
associations are called mycorrhizae (i.e., myco=fungus;
rhiza=root). For many years
mycorrhizae were thought to be parasitic, but early in this century a
mutualistic relationship was clearly demonstrated (see Harley & Smith.
1983. Mycorrhizal Symbiosis. Academic Press. N.Y.).
There are two main groups of mycorrhizae, depending on the fungus-host cell association. In Endomycorrhizae, the fungus mycelium enters the cortical cells of roots in some manner without causing damage to the host cells. In Ectomycorrhizae the fungus does not enter the cell but invades the intracellular spaces (Fig. 14-34). In mycorrhizal fungi, unlike pathogenic species, the fungus does not penetrate the central core of vascular tissue.
Fig. 14-34. An illustration of the three basic types of mycorrhizae; A) ectomycorrhizae, B) ectendomycorrhizae, and C) endomycorrhizae. (Adapted from Deacon, 1997, Modern Mycology, Blackwell Sci., London)
One of the most common and most abundant endomycorrhizal groups is what is call the vesicular-arbuscular mycorrhizae (VAM), because most produce distinctive mycelial vesicles (Fig. 14-35) in the intercellular spaces and arbuscules (Fig. 14-36) (i.e. highly branched, finely rooted hyphae) within the host cell.
Fig. 14-35. Vesicles of an AM fungus in the roots of a plant.
Fig. 14-36. Arbuscules within a root cell.
Recently, it has been shown that many do not form vesicles and the acronym is now AM fungi. Members of the order Gigasporales in the Zygomycetes form vesicular-arbuscular mycorrhizae. Common genera include Scutellospora, Glomus (Fig. 14-37), Gigaspora (Fig. 14-38), and Acaulospora (Fig. 14-39).
Fig. 14-37. Soybean root completely covered with spores of Glomus.
Fig. 14-38. A large azygospore of a species of Gigaspora.
Fig. 14-39. A cluster of spores of Acaulospora.
AM fungi have been shown to provide protection to the plant from root pathogens, reduce drought stress, and provide a type of biological fertilization. Phosphorus is a vital mineral for normal growth of plants. However, phosphorus is immobile in the soil and a young seedling will quickly develop a “phosphorus deficient zone” around its root system, and unless replenished by fertilization, the plant will be stunted. When mycorrhizal fungi are present, the mycelial system of these fungi can extend great distances into the soil; thus, providing essential water and minerals.
Other types of endomycorrhizae include the arbutoid which form on bear berry in the Pacific Northwest, and the ericoid type that formed inside root cells of members of the Ericaceae (the blueberry family). These have been termed ectendomycorrhizae because their features are somewhat intermediate between the endo and ectomycorrhizae. The discomycete, Hymenoscyphus, is a common mycorrhizal fungus of blueberries (Fig. 14-40).
Fig. 14-40. Apothecia of Hymenoscyphus, the chief mycorrhizal fungus on blueberries.
A large percent of the fleshy mushrooms and boletes are ectomycorrhzal, as well as the Tuberales (truffles) and gasteromycetes. This is the reason why during rainy seasons one can usually find an abundance of mushrooms beneath trees (Fig. 14-41).
Fig. 14-41. A cluster of Amanita, one of the most common ectomycorrhizal groups.
If one removes a tree seedling from the soil, extensive mycelium can be found radiating into the soil (Fig. 14-42).
Fig. 14-42. A young pine seedling with extensive ectomycorrhizal growth.
In both ecto- and endomycorrhizae, the fungus does not invade the central vascular system of the host, but ectomycorrhizae form a tight reticulum of cells around cortical cells called the hartig net (Fig. 14-43).
Fig. 14-43. Cortical cells of an ectomycorrhizal host completely ensheathed by a hartig net.
Ectomycorrhizal roots are ensheathed by hyphae called a mantle (Fig. 14-44).
Fig. 14-44. A close up of a root showing an extensive mantle with mycelium extending outward.
Ectomycorrhizae occur predominantly on members of the Pinaceae (pines), Fagaceae (oaks), and Betulaceae (beech). Alexopoulos et al. (1996, Introductory Mycology, 4th Ed., John Wiley & Sons, N.Y.) list several genera of mushrooms and related fungi that are mycorrhizal. Slides of many of these will be shown during the lecture period. Ectomycorrhizal genera of fungi include species of the mushroom genera Amanita (Fig. 14-45), Boletus, Cantharellus (Fig. 14-46), Cortinarius, Entyloma, Russula (Fig. 14-47) Lactarius (Fig. 14-48), Suillus, Tylopilus, and a number of subterranean gasteromycetes such as Rhizopogon (Fig. 14-49), Octaviana, Hymenogaster, and Hydnangium, as well as the truffles (subterranean Ascomycetes) Tuber, Barssia, Geopora, Hydnobolites, and the false truffle Elaphomyces.
Fig. 14-45. Amanita verna, one of the common ectomycorrhizal species.
Fig. 14-46. Cantharellus cibarius is not only an ectomycorrhizal species, but a choice edible one.
Fig. 14-47. Russula emetica is ectomycorrhizal, but unfortunately causes nausea when eaten.
Fig. 14-48. Lactarius piperatus (white) and Lactarius corrugis (brown) are two of a large group of ectomycorrhizal Lactarius species.
Fig. 14-49. A basidocarp of Rhizopogon, one of may groups of underground gasteromycetes that are ectomycorrhizal.
Ectomycorrhizae have a great impact on the survival and growth of seedlings. Significant research has been done to establish which fungus partner is best suited for a particular host plant. Plants grown aeroponically are especially good to test the application of mycorrhizae (Fig. 14-50).
Fig. 14-50. The use of aeroponic technology in the production of mycorrhizal inoculum.
c. Orchid Mycorrhizae:
Essentially all species of the Orchidaceae develop a symbiotic relationship with fungi. These have been referred to as orchid mycorrhizae (Fig. 14-51).
Fig. 14-51. A large number of fungi are mycorrhizal on orchids, and are absolutely essential for their growth and development.
Most of the fungal partners belong to the Basidiomycetes. In many orchids, chlorophyll is never produced or production is delayed for long periods of time after seed germination. Without photosynthesis they must derive nutrients elsewhere. Most such achlorophyllous orchids become associated with Rhizoctonia, Stereum, and a number of wood-rotting fungi from which they can derive their nutrients. Through enzyme action, these wood-rotting fungi break down cellulose, pectin, and lignin from their plant substrates and share part of it with the orchid. Some of the mycorrhizal partners on orchids, such as species of Rhizoctonia, are also pathogenic to various plants.