Figure 14A. On a calm day, experienced collectors may smell the maple sugar/fenugreek/curry odor of Lactarius helvus many meters away. Photo Jim Cornish.
of plants that begins when a fungus and a plant use volatiles to locate one another.20
Once found, the fungus
releases other volatiles that suspend the plants’ immune responses, enabling hyphae to surround or penetrate root cells and begin the exchange of water and soil nutrients for the plant’s carbohydrates. Tis mutualistic symbiosis lasts throughout the lifetime of both partners and comes with many other benefits (Fig. 17). Fungal volatiles are also known to improve resistance to pathogens, to act like growth hormones in shaping root architecture and to promote apical root development and lateral root growth in the partnering plants. Because fungal volatile diffusion can be widespread, they can also affect growth similarly in non-partnering plants.17,21
Competitive
interactions Fungi often compete against each
other for space and resources. To gain an advantage, some fungi produce and diffuse inhibitory volatiles that slow or halt competitor growth long before physical contact. When rival fungi meet, volatiles exuded by one or both competitors can cause changes in morphology, secondary metabolite production, pigment deposition, accumulation of oxidants, and enzyme activity in each other. Tese battles often tax resources and energy, forcing some fungi to sacrifice growth to synthesize
Figure 14B. Hydnellum suaveolens is another of our forest mushrooms whose anise/peppermint smell can be detected long before it is sighted. Photo Andrus Voitk.
the volatiles necessary to maintain their competitive advantages. Tese volatile- mediated competitions can end in a stalemate or one species can overtake some or all the territory and resources of its competitor.19
Pathogenic
interactions Pathogenic fungi are parasitic and
obtain their nutrients by feeding on living host cells. Pathogens usually “sniff” for specific plant odors to locate susceptible species and then inject them with volatiles to disarm or manipulate their defenses. Once the pathogen is established, volatiles are used to initiate physiological and developmental changes that appear as wilting leaves, lesions, cankers, and shriveled fruit in hosts (Fig. 19).15,5 Some fungi are invertebrate pathogens
and use volatiles to seek out vulnerable insects. Fungal pathogenesis often results in fungal growths extending from the insect’s exoskeleton and an invasion of their bodies by hyphae (Fig. 20).5
Defensive
interactions Odors act as “swords and shields”
in defending both fungi and their mycorrhizal partners against other organisms. Below ground, odors from mycorrhizal fungi are known to activate plant defenses against pathogens and
Fall 2021 FUNGI Volume 14:4 49
Figure 15A (above). Armillaria solidipes, the honey mushroom, is a parasitic fungus that consists mainly of fused masses of genetically identical mycelia connected by black bootlace-like rhizomorphs (Fig. 15B, below) that spread throughout the substrate in search of new hosts. Photo Jim Cornish.
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