plant surface microbiology.pdf

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M. Zakaria Solaiman and Lynette K. Abbott
vanetti et al. 1993a). Generally, the amount, type and form of root exudates
have the potential to influence fungal growth before the fungus meets the root
as well as after the hyphae contact the root. In addition, the formation of
appressoria and hyphal penetration of the root surface may involve recognition
processes (Koske and Gemma 1992).
3 Functioning of Arbuscular Mycorrhizas in Nutrient
Exchange
Nutrient exchange in arbuscular mycorrhizas can occur between the hyphae
and host root cells. External hyphae penetrate the root surface and proceed
into the root cortex forming arbuscules and coils (Saito 2000). The arbuscule
is a complex intracellular hyphal structure formed in Arum-type mycorrhizas
(Smith and Smith 1997). It has been assumed that the arbuscule is a likely site
for symbiotic nutrient transfer in Arum-type mycorrhizas (Bonfante-Fasolo
1987; Smith and Smith 1997), but carbon exchange may also occur across
walls of non-arbuscular hyphae (Smith and Read 1997). Variation in arbuscule
development in Arum-type mycorrhizas could reflect different characteristics
of roots and fungi (Smith and Dickson 1991).
Arbuscular mycorrhizal fungi with a range in demand for phosphorus have
been isolated from south-western Australia. In particular, an isolate of S.
calospora either increased or decreased plant growth depending on the phosphate
status of the soil (Thomson et al. 1986). This fungus has a high demand
for carbon from the plant relative to P transfer (Pearson et al. 1994), and forms
considerably more external hyphae than some other fungi that are highly
effective at enhancing P uptake across a range of P supply (Abbott and Robson
1985). Indeed, under some circumstances, it can be inefficient in P transfer
(Smith et al. 2000). In addition, S. calospora can interact with Glomus invermaium
during colonisation of roots, restricting growth of G. invermaium in
other parts of the same root system during some stages of its colonisation
(Pearson et al. 1993). However, colonisation and activity of S. calospora can be
stopped once sporulation has taken place (Pearson and Schweiger 1994),
resulting in resumption of colonisation (and presumably P uptake and transfer)
by G. invermaium. This example demonstrates the dynamics of activities
of two AM fungi on root surfaces, but the extent to which this occurs generally
for all isolates of these or other species is not known. Unfortunately, relatively
few isolates have been investigated for most species of AM fungi in any environment
so the extent to which generalisations can be made, even for species,
is unknown (Morton and Bentivenga 1994).
In field soils, as several species of AM fungi would generally be involved in
co-colonisation of roots, P uptake and transfer into the plant might be
affected if one fungus interfered with colonisation by another. The outcome
would depend on the functional diversity of the species present, i.e. their effi-