Chapter 17: Mycorrhizas: Mutualistic Plant-Fungus Symbioses

Mycorrhizas represent ancient mutualistic partnerships between fungi and plants that fundamentally shaped terrestrial life for over 400 million years, enabling early land plants to establish themselves in nutrient-poor environments by exchanging carbohydrates for mineral acquisition. This chapter explores how the symbiosis operates across two dominant forms: arbuscular mycorrhizas found in over 300,000 plant species and ectomycorrhizas associated with approximately 2,000 woody plants including pines, oaks, and eucalypts. Arbuscular mycorrhizal fungi from the phylum Glomeromycota penetrate root cortical cells to form specialized structures called arbuscules where phosphorus and nitrogen transfer occurs, along with vesicles that serve storage functions, while their obligate dependence on living plant hosts and production of large lipid-rich spores distinguish them as highly efficient but difficult to cultivate in laboratory settings. Ectomycorrhizal fungi, primarily basidiomycetes and some ascomycetes, establish external fungal mantles and form Hartig nets that facilitate nutrient exchange between fungal hyphae and root cortical cells, with notable partners including Amanita muscaria, Boletus edulis, Laccaria laccata, Pisolithus tinctorius, and truffle fungi from the family Tuberaceae. The chapter details how fungal mycelial networks dramatically expand the soil exploration capacity of plant root systems, improving water and nutrient absorption while enhancing stress tolerance, pH buffering, and pathogen protection against organisms like Rhizoctonia and Phytophthora. Underground mycelial networks also facilitate resource transfer between seedlings and mature trees, demonstrating how forests function as interconnected communities. The chapter examines specialized mycorrhizal types including orchid mycorrhizas essential for seed germination, ericoid mycorrhizas in heaths and blueberries, arbutoid mycorrhizas, and monotropoid mycorrhizas in non-photosynthetic plants. Practical applications in forestry and restoration ecology reveal how mycorrhizal inoculation improves seedling survival on degraded sites, reduces fertilizer requirements, and supports cultivation of economically valuable species including truffles, underscoring the ecological and economic significance of these partnerships in global ecosystem productivity.