plant surface microbiology.pdf

een described as a plant-growth-promoting root endophyte (Varma et al.
1999). However, because this chapter deals only with rhizobacteria, these, and
other fungal endophytes, will not be dealt with further.
Plant symbiotic bacteria and fungi are recognized and can either include
pathogens or mutualistic organisms. Mycorrhizal fungi and nitrogen (N 2)-fixing
bacteria are the main mutualistic symbionts (Barea 1997). This chapter
will focus only on mycorrhizal fungi and PGPRs.
3 Mycorrhizal Fungi
20 Mycorrhizal Fungi and Plant Growth Promoting Rhizobacteria 353
The roots of most plant species associate with certain soil fungi and establish
what are known as mycorrhiza (Smith and Read 1997). Mycorrhizal functions
include improvement of plant establishment, enhancement of nutrient
uptake, protection against cultural and environmental stresses, and the
improvement of soil structure (Barea et al. 1997).
Mycorrhizal symbiosis can be found in nearly all types of ecological situations,
and most plant species are able to form this symbiosis naturally, the
most common type involved in the normal cropping systems is the arbuscular
mycorrhizal (AM) type (Smith and Read 1997). The responsible AM fungi
belong to the order Glomales in the Zygomycetes (Morton and Redecker
2001), and are a very common group of soil-borne fungi whose origin and
divergence date back 100–400 million years ago (Simon et al. 1993; Morton
2000; Redecker et al. 2000). However, a new fungal phylum, the Glomeromycota
have recently been proposed (Schübler et al. 2001). Because this chapter
focuses only on arbuscular mycorrhizas, the term “AM fungi” will be used to
refer to “arbuscular mycorrhizal fungi”.
During the process of AM formation (Giovannetti 2000), in which the plant
“accepts” the fungal colonization without any significant rejection reaction
(Dumas-Gaudot et al. 2000), a series of root–fungus interactions allows the
integration of both organisms. The establishment of the symbiosis is the
result of a continuous molecular “dialogue” between plant and fungus, as
exerted through the exchange of both recognition and acceptance signals
(Vierheilig and Piché 2002). The result of this dialogue will finally depend on
the genome expression of both partners (Gianinazzi-Pearson et al. 1996;
Franken and Requena 2001).
After the biotrophic colonization of the root cortex, AM fungi develop an
external mycelium which is a bridge connecting the root with the surrounding
soil microhabitats. Such mycorrhizal (fungal-root) symbiosis is critical in
nutrient cycling in soil–plant systems (Smith and Read 1997). In cooperation
with other soil organisms, the external AM fungal mycelium forms water-stable
aggregates necessary for good soil tilth (Miller and Jastrow 2000; Requena
et al. 2001). The AM symbiosis also improves plant health through increased
protection against biotic and abiotic stresses (Bethlenfalvay and Linderman