plant surface microbiology.pdf

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M. Zakaria Solaiman and Lynette K. Abbott
4.2 Relationship Between Hyphae in the Root and in the Soil
The quantity of hyphae in the vicinity of roots associated with mycorrhizal
roots can vary greatly (Sylvia 1986) and change with time (Bethlenfalvay et al.
1982). Hyphae of AM fungi play key roles in the formation and functioning of
mycorrhizas (Abbott et al. 1992). Hyphae in soil, originating from either an
established hyphal network or from other propagules (spores, vesicles and
root fragments), lead to the recognition and subsequent colonisation of roots.
The distribution of hyphae and associated sporulation will determine where
propagules are located in relation to newly formed roots. The roles of the
hyphae in both P uptake and soil stabilisation are dependent on their distribution
within the soil matrix and their interaction with the root surface.
Abbott and Robson (1984) hypothesised for G. invermaium that low initial
levels of infective hyphae in the soil would lead to small amounts of hyphae in
soil relative to the amount formed inside the root. For high initial densities of
infective hyphae of this fungus in soil, the exponential phase of colonisation
of roots was expected to occur in parallel with extensive development of
hyphae in the soil. In contrast, there was no similar relationship between the
formation of hyphae in soil and within the root expected for S. calospora
which consistently produced large amounts of hyphae in soil, irrespective of
the density of hyphae within the root.
There is relatively little information about the longevity of hyphae in soil
(Sylvia 1988; Hamel et al. 1990). This would be important for predicting the
activities of hyphae for both colonisation and P uptake. The majority of studies
of mycorrhizas measure the extent of colonisation of roots at one point in
time. This measure is of little value for understanding functional diversity of
AM fungi because the AM fungi within the root may either be highly active or
have ceased activity for some time. Most routine techniques for assessing
mycorrhizas do not assess any functional attribute of the fungus. Therefore,
care is required in extrapolating from levels of mycorrhizal root colonised to
functional diversity of communities of AM fungi present in soil.
5 Role of Arbuscular Mycorrhizal Fungi Associated with
Roots in Soil Aggregation
The AM fungal hyphae can enhance soil aggregation by using more than one
mechanism. In clayey soils, entanglement of soil particles by hyphae can
occur (Tisdall and Oades 1982; Oades 1984). There is insufficient hyphal
length to extend around particles of sand and a more likely mechanism is
cross-linking of particles by hyphae in sandy soils (Degens 1997). AM fungal
hyphae may bind soil aggregates by exuding a glycoprotein (Wright et al.
1996; Wright and Upadhyaya 1998). Some information on species differences
in soil aggregation is available which indicates that AM fungi commonly pro-