plant surface microbiology.pdf

19 Functional Diversity of Arbuscular Mycorrhizal Fungi on Root Surfaces 333
2.1 Establishment of the Symbiosis
The establishment of a functional symbiosis between AM fungi and host
plants involves a sequence of recognition events between the fungus and the
plant (Giovanetti et al. 1993a; Giovannetti and Sbrana 1998). Mycorrhizal
colonisation has several phases (Tester et al. 1987; Gianinazzi-Pearson and
Gianinazzi 1989) including spore germination, hyphal growth in soil, hyphal
attachment to roots, appressorium formation, intraradical penetration and
intraradical growth involving the formation of arbuscules and coils (Smith
and Read 1997; Wegel et al. 1998). The developmental stages of fungal interaction
with the plant are associated with plant signals inducing gene expression
and recognition between the two partners of the symbiosis (Giovanetti and
Sbrana 1998).
2.2 Spore Germination and Hyphal Growth
Spores of AM fungi can germinate under appropriate storage and environmental
conditions. A host signal is not necessary for this step in the life cycle.
The first response of a fungus to a host root is stimulation of hyphal growth.
It is well documented that host roots can promote hyphal growth of AM fungi
and induce changes in hyphal growth pattern and morphology by stimulating
branching and inducing the formation of hyphal fans (Giovannetti et al.
1993b). Hyphal contact with the surface of the host root occurs at random and
the increased branching of a fungus near the root surface would increase the
probability of root interception. Root architecture and root density also influence
the likelihood of root and hypha interception (Abbott and Robson 1984).
2.3 Role of Plant Root Exudates
Although hyphal growth can be increased in response to root exudates from
host plants (Mosse 1962, 1988; Koske and Gemma 1992), there is no direct evidence
for the release of inhibitory compounds from non-host roots. Indirect
evidence has raised this as a possibility (Ocampo et al. 1980; Holliday 1989).
The exudates from non-hosts appeared to lack factors which induced hyphal
growth (Giovannetti and Sbrana 1998; Nagahashi 2000).
Hyphal elongation of G. fasciculatus was enhanced by exudates from Trifolium
repens when the plants were grown under phosphate-deficient conditions
(Elias and Safir 1987). This effect was reduced when phosphorus was
added. Root exudates from both non-mycorrhizal and mycorrhizal peas
inhibited hyphal growth of Gigaspora margarita (Balaji et al. 1994). In contrast,
mycorrhizal Pisum sativum and its non-mycorrhizal isogenic mutant
did not form root exudates that had different effects on Glomus mossae (Gio-