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In this article we will discuss about the effect of mycorrhizal fungi on their hosts.
Similar to rhizosphere, mycorrhizosphere (the close vicinity of ectomycorrhizae) shows increased microbial community leading to mycorhizosphere effect. The photosynthates flow into soil through roots and mycorrhizae support a diverse community of soil microorganisms, many of which influence plant growth.
The mycorrhizosphere microorganisms may be facultative anaerobes, extracellular chitinase producers, phosphate solubilizers, and producers of siderophores, antibiotics, hormones, plant growth-suppressors and promotors.
Nutrient uptake and Translocation:
Mycorrhizae increase the absorptive surface of root resulting in increased uptake of water and nutrients from the soil. The ectomycorrhizal fungi translocate phosphorus, nitrogen, calcium and amino acids, and increase translocation of Zn, Na and other minerals to the hosts.
Their hyphae extract N and transport from soil to plant due to increased absorptive surface area. Plant available phosphorus in soil is in small amount (1-15%) of total P content. The by-products of fungi dissolve several insoluble nutrients.
Three mechanism of mycorrhizal activity has been discussed for weathering soil P and transport to host plants:
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(a) The interaction of mycorrhizal fungi and phosphate solubilizing bacteria,
(b) Production of phosphatases by the mycorrhizal fungi, and
(c) Production of organic acids by mycorrhizal fungi.
Translocation of P in fungal hyphae takes place by cytoplasmic streaming. P is stored in the form of polyphosphates due to polyphosphate kinase activity. Then P is transferred to host plant after break down of phosphates by phosphatases and release of inorganic phosphate. P is accumulated in mantle and Hartig net and, thereafter, transferred from Hartig net to host tissue.
Similarly, the VAM fungi also influence growth, exudation and nutrient uptake in host plants. Polyphosphate granules have been found in arbuscules, hyphae and vesicles of VAM fungi. Chitin appears to be the main carbohydrate-related material present in vesicle and hyphal walls. Glomus fasciculatus translocate P over a distance of at least 7 cm, and Rhizopogon luteus to 12 cm.
Transfer of Metabolites from Host to Fungal Symbiont and the other Plants:
The products of photosynthesis (photosynthates) move from host to the fungal symbiont. However, host to host transfer of carbohydrate via a shared fungal symbiont also takes place.
Growth-Hormone/Antibiotic Production:
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Some of the ectomycorrhizal fungi produce indole acetic acid (IAA) which possibly is involved in morphogenesis and longevity of roots. Moreover Leucopaxillus cerealis var. piceina is known to produce growth inhibiting antibiotics.
Plant Protection/Biocontrol of Pathogens:
Both VAM and ectomycorrhizal fungi make the plants drought and frost resistant, increase tolerance to stress against soil temperature, soil toxins, high acidity, and heavy metal toxicity.
VAM and ectomycorrhizal fungi inhibit the infection of pathogens to plant roots. The fungal mantle acts as a passive mechanical barrier influencing either the pathogen or its spread in host tissues.
