plant surface microbiology.pdf

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M. Zakaria Solaiman and Lynette K. Abbott
in a similar manner, but with different levels of efficiency depending on their
abundance as well as their intrinsic characteristics. Furthermore, their symbiotic
response depends on environmental conditions and the relative abundance
of other AM fungi associated with the roots of the same plant.
The purpose of this review is to discuss the functional diversity of AM
fungi and its significance in the context of interactions at root surfaces and
the potential consequences of this for plant growth and plant community
structure.
2 Mycorrhiza Formation and Ecological Specificity
Arbuscular mycorrhizal fungi live symbiotically with the roots of approximately
60 % of terrestrial plants (Brundrett and Abbott 2002). About 200
species of AM fungi have been described so far in the Glomales, but unequivocal
evidence of their capacity to form mycorrhizas is not available for many
species (Walker and Trappe 1993). A putative zygosporic stage has only been
reported for the life cycle of Gigaspora decipiens (Tommerup and Sivasithamparam
1990). Knowledge of genetic diversity of AM fungi is poorly defined.
Associations between hosts and symbionts are usually non-specific in AM
symbiosis (Mosse 1975). Most of the evidence for non-specificity in these
associations has been demonstrated by inoculating roots with propagules of
species of AM fungi in separate pot cultures (Smith and Read 1997). However,
it may be possible for a plant grown in field soil to be preferentially colonised
by one of the species of the AM fungi present. This could result from differences
in the infectivity and/or quantity of propagules of each species, or from
differences in the susceptibility of roots to colonisation by each fungus. This
phenomenon has been defined as ‘ecological specificity’ by McGonigle and
Fitter (1990).
Arbuscular mycorrhizal fungi are obligate symbionts and they depend on
the formation of mycorrhizas to take up carbon from the root for completing
their life cycles. Fungal growth does not continue much in axenic culture in
the absence of the host plant. The obligate status of AM fungi, the coenocytic
nature of their spores (Becard and Pfeffer 1993) and the lack of demonstration
of recombination (Rosendahl and Taylor 1997) limit the opportunities
for fundamental research on their interactions with plant roots. Molecular
techniques based on DNA analysis provide a number of possibilities to
develop specific probes for AM fungi for determining phylogenetic relationships
and diversity and for their identification in soil and plant roots (Jacquot
et al. 2000).