EFB530 Plant Physiology

Role of mycorrhizae in nutrient uptake

Mycorrhizae = fungus + root = an organ

Mycorrhizal fungi colonize most plant roots in nature (83% dicots, 79% monocots, 100% Gymnosperms)

• Mutualism: these fungi aid in the uptake of P, Cu, Zn, water
• in return they receive sugars and amino acids
• Ancient association: 400 year old Rhynie cherts show GM; recently set back to 550 mya (Ordovician); first land plants were mycorrhizal
• the association apparently does not induce plant defense responses

Non-mycorrhizal plants:

• Brassicaceae, Cenopodiaceae, Proteaceae, Probably evolutionary loss of association
• plants under high nutrients (ag system) or primary succession; Ecological influence

Arbuscular mycorrhizae (AM, formerly called Vesicular-Arbuscular Mycorrhizae or endomycorhizae)

• Formed by fungi in the Glomales= primitive, microscopic
• colonize crop plants, fruit trees, shrubs, vines, grasses, ferns, liverworts, redwoods, maple
• hyphae grow between cells
• vesicles ( = storage unit, lipid rich) and arbuscles (= site of cell-to-cell contact and exchange) penetrate individual cortical cells
• relatively small number of species (100-200), so each species has broad host range

Ectomycorrhizae (EM)

• formed by basidiomycetes and ascomycetes; derived, macroscopic fleshy fungi
• colonize temperate forest dominant trees, Pinaceae, Fagaceae, Betulaceae, many Fabaceae, Juglandaceae, Salicaceae, Myrtaceae, Ericaceae
• form a sheath of growth around the root of equal mass as the root itself (Mantle = storage unit)
• hyphae extend in between cortical cells = Hartig net = network of hyphae inside root, site of exchange
• No penetration of root cell walls, thus the 'ecto-' prefix
• large number of species (~6000), each with narrow host range

Also: ericoid mycorrhizae, orchid mycorrhizae

Under natural conditions soil is very heterogeneous in terms of nutrient pools

• Rhizosphere (µm-mm area around roots) become depleted for nutrients rapidly (uptake and competition with microorganism
• most nutrients are cycling through the microbial pools before they get to the plant, i.e. mycorrhizal fungi
• hyphae are less costly to maintain than roots, can penetrate smaller pore spaces than roots, and have different biochemical pathways for nutrient acquisition

Mycorrhizal fungi reduce water stress on plants

• Fungi with extensive mycelium and thick rhizomorphs enable plants to sustain high y_xylem at low y_soil

Stomatal conductance is a function of [P] in leaves, a fertilizer or mycorrhizal effect

• P is very dilute in soil, very patchy
• occurs in inorganic or organic pools

Fungi access sources of P unavailable to plants:

• alter pH in soil, releasing P
• produce acid phosphatases to release phosphate
• these enzymes have lower Km for P than roots
• at low [P], better uptake of P was correlated with mycorrhizal fungi

Nitrogen occurs in nature as NO₃^- , NH₄⁺ , and amino acids

• NO₃^- predominates in agricultural settings
• NH₄⁺ predominates in forests
• fungi can uptake both forms, as well as amino acids cleaved from polypeptides
• metabolic pathway depends on environment, species, and strain
• GS-GOGAT  and GDH function to provide NH₄⁺ for translocation from fungus to plant

Other nutrients: Zn, Cu, K, S, Mg, etc (Hatch 1937)

Toxic levels of nutrients: Zn, Cu, Fe, Co, Ti, Ba, Arsenate

• fungi can be employed to sequester heavy metals
• Zn, Cu, Fe, Al
• siderophores (chelators) are under investigation
• again, species and strain specific responses
• arsenate tolerance is related to P transport system

N-deposition since the industrial revolution (got SUVs?)

• severe reduction in diversity of EM fungi observed in Europe based on sporocarp records
• studies based on root tips show major impacts below ground also

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