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Endomicorrizas Arbuscurales


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Endomicorrizas Arbuscurales

  2. 2. Introduction arbuscular mycorrhizal
  3. 3. Introduction The arbuscular mycorrhizal symbiosis is viewed as a classic mutualism, an interaction in which both partners benefit. The fungi appear to acquire their entire carbon supply from the plant, and although colonisation of roots by AM fungi (AMF) can confer a wide range of benefits to the plant
  4. 4. Introduction Phosphorus acquisition. The fungal hyphae can explore a large volume of soil and acquire P beyond the phosphate depletion zone that rapidly builds up around the root
  5. 5. Introduction • The first land plants had rhizomes and rhizoids, but no root systems • Its evidence reveals that these early plants had fungal structures strikingly similar tomodern AM structures AM in Erythronium americanum
  6. 6. Morphology of AMF colonisation AMF ARUM PARIS
  7. 7. Arum Type • Series associations where hyphae proliferate in the cortex by growing longitudinally between host cells. • This occurs because hyphae grow through longitudinal intercellular air spaces that are present
  8. 8. Paris Type • series where hyphae spread by forming coils within cells because there are no continuous longitudinal air spaces.
  9. 9. Carbon • The major fluxes in the AM symbiosis appear to be of C from plant to fungus and of P, and possibly N, from fungus to plant. • The mechanisms of these fluxes are not yet well understood. • Even the location of the carbon flux is obscure, withthe best evidence – from activity of ATPases – suggesting that it occurs in the Arum type at the inter- cellular hyphae.
  10. 10. Phosphate • Phosphate is takenup by high-affinity phosphate transporters in the mycelium. and once in the hyphae the long chains are hydrolysed, facilitating transfer to the plant
  11. 11. N AMF can certainly transport N to roots: • AM exposed to NO3 or NH4 became highly labelled and this N was subsequently translocated to the roots. • N is translocated in the hyphae as arginine but probably broken down to urea and ultimately transferred to the plant as NH4 with the resulting C skeletons from arginine breakdown being re-incorporated into the fungal.
  12. 12. Plant–fungus reciprocity: who drives whom? These fluxes underlie the operation of the symbiosis, but we do not know how the exchange is managed. Is there some reciprocity between the C supplied by the plant and the P (or N). supplied by the fungus? If so, there will have been powerful selection on the operation of this mechanism, with potentially conflicting pressures on the two partners
  13. 13. Plant–fungus reciprocity • Plants will have been under powerful selection to discriminate among fungal partners on the basis of symbiotic effectiveness. • One possible mechanism for that would be a set of molecular signals specific to each fungus. • The establishment of the symbiosis involves just such a recognition process
  14. 14. Nutrient acquisition by AM fungi • Unlike other mycorrhizal associations, where at least some of the fungi involved can be grown in pure culture and we can measure the capability of the fungus acting alone to decompose organic materials and take up the products of decomposition • AMF, however, are not saprotrophic and therefore are reliant on saprotrophic microorganisms to decompose organic matter and release inorganic ions for capture by AM hyphae (Smith & Read 2008)
  15. 15. Nutrient acquisition by AM fungi • The external AM mycelium phase is the fungal phase which is in contact with the soil and thus responsible for nutrient acquisition and transport to the internal mycelium inside the root before any transfer to the plant occurs
  16. 16. • In return, triacylglycerides can be transported from the internal to the external mycelium phase to support the glyoxyl cycle for metabolic activity
  17. 17. Nutrient acquisition by AM fungi • N capture by AM fungi was previously believed to have little ecological relevance. An important and influential article by Read (1991) argued that AM associations tend to dominate in systems where nitrification is favoured and the main form of inorganic N will consequently be nitrate
  18. 18. • As NO3, unlike phosphate, is highly mobile in soils and depletion zones around roots can be measured in centimetres rather then millimetres, plants should not require AM fungi in order to enhance capture of NO3
  19. 19. Nutrient acquisition by AM fungi • NH4 uptake may be less energetically expensive for the fungus as NO3 first has to be reduced to NH4 prior tincorporation into amino acids.
  20. 20. CONCLUSIONS • An increased acceptance of the independent but interacting roles of plant and fungus in nutrient transfers allows a new and necessary focus on the nutritional needs of the fungus itself
  21. 21. CONCLUSIONS • All fungi have a high nitrogen content and hence potentially a high N demand. • AMF are among the most abundant fungi on earth, their role in global N and P cycles would repay close attention.
  22. 22. CONCLUSIONS • AMF can acquire N (and presumably also P) from decomposing organic material and transfer it to the plant. The N and P transfer to the plant may, if the model proposed above is correct, be a consequence of the fungal demand for nutrients, with both host plant and fungus evolving transporters to take advantage of localised increases in nutrients.
  23. 23. CONCLUSIONS The mechanisms by which these fungi actively forage in soil for both N and P remain unclear. What is certain is that we need to pay much more attention to the biology and ecology of the extra- radical phase of these fungi if we are to understand how they operate in soil and the roles that they play in ecosystems