plant surface microbiology.pdf

19 Functional Diversity of Arbuscular Mycorrhizal
Fungi on Root Surfaces
M. Zakaria Solaiman and Lynette K. Abbott
1 Introduction
Arbuscular mycorrhizal (AM) fungi can promote host plant growth by
increasing phosphorus (P) uptake from soil while simultaneously obtaining
carbon (C) from the photosynthate of the host plant. However, reductions in
plant growth associated with AM fungi have also been recorded (e.g. Graham
and Eissenstat 1998; Graham and Abbott 2000) which can be linked to carbon
and phosphorus exchange (Koide and Elliott 1989). Both growth promotion
and reduction depend upon the particular plant–fungal combination (Johnson
et al. 1997) and soil conditions. P and C exchange between the host plant
and mycorrhizal fungus also depends on environmental and biological variables
(Jakobsen 1998). The combined effects of P uptake and transfer to the
plant and C release to the fungus are important considerations for the functioning
of arbuscular mycorrhizas. While the mechanisms of nutrient
exchange between AM fungi and the host plant remain speculative (Schwab et
al. 1991; Saito 2000; Smith et al. 2001), more is known about the plant genes
involved in P transfer (Harrison 1999) than the fungal genes (Rausch et al.
2001). Symbiotic exchange of nutrients in arbuscular mycorrhizas, especially
transport along hyphae and transfer to the host plant, has been reviewed
(Saito 2000; Smith et al. 2001) and it has been pointed out that the mechanisms
of symbiotic nutrient exchange may be more diverse than originally
expected (Saito 2000).
AM fungi occur in soil and in association with roots as communities of
organisms that may simultaneously interact with the roots of one or several
co-existing plant species. Species of AM fungi differ in their mode of colonisation
and their capacity to form hyphae in soil and within the root (Abbott et
al. 1992).
Although hyphal characteristics may be distinctive for some fungi (Dodd
et al. 2000), they are not usually present as discrete organisms and are difficult
to distinguish from one another within and on the surface of roots. Although
the fungi may have markedly different characteristics, they appear to function
Plant Surface Microbiology
A.Varma, L. Abbott, D. Werner, R. Hampp (Eds.)
© Springer-Verlag Berlin Heidelberg 2004