Mycorrhiza Study Guide

📖 Core Concepts Mycorrhiza – a mutualistic (often) symbiosis between a green plant and a fungus. Carbon trade – the plant gives the fungus 20 % of its photosynthate (glucose/sucrose) in exchange for water and mineral nutrients (P, N, Fe, Zn, etc.). Cellular interfaces Arbuscular mycorrhiza (AM): fungus penetrates root cells, forming arbuscules (high‑surface‑area structures). Ectomycorrhiza (ECM): fungus stays outside root cells, weaving a Hartig net between cortical cells. Major types – ECM (extracellular) vs. Endomycorrhiza (intracellular). Endomycorrhiza includes AM, orchid, ericoid, arbutoid, monotropoid, and Mucoromycotina fine‑root endophytes. Signal exchange – Plant releases strigolactones → stimulates fungal spore germination & hyphal branching. Fungus releases Myc factors (chitin‑derived molecules) → recognized by plant LysM‑RLKs, activating the common symbiosis signalling pathway (CSSP). Ecological roles – nutrient and water acquisition, soil aggregation, carbon sequestration (via hyphal networks & glomalin), pathogen resistance, drought/salt stress mitigation, and inter‑plant communication (“Wood Wide Web”). --- 📌 Must Remember Prevalence – ≈95 % of plant families, ≈80 % of plant species are mycorrhizal; AM occurs in 78 % of species (≈85 % of families). Carbon cost – plants allocate 20 % of photosynthate to the fungal partner. Host specificity – some ECM fungi are highly specific (e.g., Leccinum), others are generalists (e.g., Amanita). Evolution – AM is the ancestral form; ECM and loss of mycorrhizae evolved independently multiple times. Glomalin – a glycoprotein produced by AM fungi, a major long‑term carbon store in soils. Orchid dependence – orchid seeds lack reserves; they require orchid mycorrhizal fungi for germination. Stress benefits – AM/ECM improve drought tolerance, reduce salt stress, and enhance resistance to soil‑borne pathogens. --- 🔄 Key Processes Symbiosis Initiation Roots exude strigolactones → fungal spores germinate & hyphae branch. Fungal Myc factors bind plant LysM receptors → trigger CSSP. CSSP orchestrates intracellular (AM) or extracellular (ECM) colonization. Colonization AM: hyphae penetrate cortical cells → develop arbuscules. ECM: hyphae envelop root tip → form Hartig net between cells. Nutrient/Water Exchange Plant → fungus: sugars (glucose, sucrose). Fungus → plant: phosphate (P), nitrate/ammonium (N), micronutrients (Fe, Zn), water. Hyphal networks explore soil pores inaccessible to root hairs, increasing absorptive surface. Stress Mitigation (illustrative) Fungi secrete organic acids → solubilize mineral nutrients. Hyphae improve soil moisture retention → drought tolerance. Production of toxic enzymes & priming of plant immunity → pathogen resistance. --- 🔍 Key Comparisons Arbuscular vs. Ectomycorrhiza Location: intracellular arbuscules ↔ extracellular Hartig net. Host range: AM – 78 % of plants; ECM – ≈20 000–25 000 fungal species, 6 000–7 000 plant hosts. Carbon storage: AM produce glomalin; ECM bind heavy metals in extramatrical mycelium. Host‑specific vs. Generalist Fungi Specific: Leccinum (single plant genus). Generalist: Amanita (many tree species). Mutualistic vs. Parasitic Shift Normal: plant supplies carbon, fungus supplies nutrients. Parasitic: certain species or stressful conditions can reverse benefit flow. Plants with vs. without Mycorrhizae With: >90 % of families (e.g., most crops). Without: Brassicaceae, Chenopodiaceae (lost the ability). --- ⚠️ Common Misunderstandings “Mycorrhizae always increase plant growth.” – Benefits depend on nutrient status; in nutrient‑rich soils the carbon cost may outweigh gains. “Fungi give carbon to plants.” – The direction is plant → fungus (photosynthate). “All fungi are mycorrhizal.” – Many are saprotrophs; only specific lineages form mycorrhizae. “Mycorrhizae only help with phosphorus.” – They also deliver nitrogen, micronutrients, water, and improve stress tolerance. “Every plant can form mycorrhizae.” – Some families (e.g., Brassicaceae) lack the necessary genes. --- 🧠 Mental Models / Intuition “Fungal highway” – Imagine the hyphal network as an extension of root hairs that can reach far‑away nutrient “gas stations.” Carbon‑nutrient barter – Think of the plant as a customer paying with sugar for a delivery service of water/minerals. Signal handshake – Strigolactones = “knock”; Myc factors = “doorbell”; CSSP = “door opens” for colonization. --- 🚩 Exceptions & Edge Cases Loss of mycorrhizal ability – Brassicaceae and Chenopodiaceae have independently discarded the symbiosis genes. Parasitic mycorrhizae – Under certain environmental stresses or with specific fungal species, the association can become detrimental to the plant. Multiple origins of ECM – ECM fungi evolved independently many times from saprotrophic ancestors (Basidiomycota, Ascomycota). Heavy‑metal binding – Primarily an ECM trait; AM fungi bind metals less efficiently. --- 📍 When to Use Which | Situation | Preferred Mycorrhizal Type | Reason | |-----------|----------------------------|--------| | Major cereal or legume crops | Arbuscular | High prevalence, works in agricultural soils, improves P/N uptake. | | Temperate forest trees (e.g., pines, oaks) | Ectomycorrhiza | ECM fungi form extensive Hartig nets and excel in organic‑rich, acidic soils. | | Acidic, nutrient‑poor soils with ericaceous shrubs | Ericoid | Specialized for low‑pH, low‑nutrient environments. | | Orchid seed propagation | Orchid mycorrhiza | Seeds lack reserves; require fungal carbon. | | Restoration of barren or heavy‑metal‑contaminated soils | Ectomycorrhiza or AM (depending on host) | ECM bind metals; AM improve overall soil carbon via glomalin. | | High salinity irrigation | AM (most tolerant) | AM shown to alleviate salt stress in many crops. | --- 👀 Patterns to Recognize Strigolactone mention → AM formation (intracellular arbuscules). Hartig net or “extracellular” → ECM (often with forest trees). Glomalin → AM fungi, carbon sequestration cue. Heavy‑metal binding or “extramatrical mycelium” → ECM trait. “Wood Wide Web” or “common mycelial network” → inter‑plant signaling, usually ECM but can apply to AM as well. Seed germination of orchids → orchid mycorrhizal dependency. --- 🗂️ Exam Traps Distractor: “Mycorrhizal fungi provide the plant with carbohydrates.” – Wrong direction of exchange. Distractor: “All plants form mycorrhizae.” – Families like Brassicaceae are exceptions. Distractor: “Mycorrhizae only enhance phosphorus uptake.” – They also supply N, micronutrients, water, and stress tolerance. Distractor: “Ectomycorrhizae always colonize intracellularly.” – By definition ECM remain extracellular. Distractor: “Higher CO₂ always benefits mycorrhizal fungi.” – Elevated CO₂ can help plants, but climate change may also impose temperature stress that reduces fungal activity. Distractor: “Glomalin is a fungal toxin.” – It is a carbon‑rich glycoprotein contributing to soil carbon storage, not a toxin. ---