Plant pathogen protection by arbuscular mycorrhizas: A role for fungal diversity?
Section snippets
Why consider diversity?
The role of below-ground soil organisms interacting with plant roots has gained increased attention in recent years (e.g. Reynolds et al. 2003; van der Putten 2003; Callaway et al. 2004), and the interactions between beneficial and pathogenic organisms have been identified as being particularly relevant due to their important implications for plant fitness (e.g.Schippers et al. 1987; Fitter and Garbaye 1994; Bever 2003). Most studies, however, focus on interactions between a single pair of
Improved nutrient status of the host plant
It is well known that AM fungi can improve the nutrient status of their host plants (Smith and Read 2008). Several mineral nutrients, especially P, are allocated through the symbiosis to the plant in exchange for carbon (e.g. Pearson and Jakobsen 1993). There is evidence that plants that took up larger amounts of nutrients through their AM fungal symbiont (e.g. Glomus intraradices and Glomus mosseae) have an increased tolerance for pathogenic infections (e.g. Bodker et al. 1998; Karagiannidis
Competitive interactions with pathogenic fungi
Direct (via interference competition, including chemical interactions) and indirect (via exploitation competition) interactions have been suggested as mechanisms by which AM fungi can reduce the abundance of pathogenic fungi in roots. These have generally been proposed in response to observations of negative correlations in the abundance of AM fungal structures and pathogenic microorganisms in roots and soil (St-Arnaud et al. 1994; Bodker et al. 2002; Filion et al. 2003) or on growth medium (
Anatomical or architectural changes in the root system
AM fungal colonization influences root architecture of the host plant in most studies by causing a more profusely branched root system (Price et al. 1989; Yanoet al. 1996; Paszkowski et al. 2002; Olah et al. 2005; Gutjahr et al. 2009).
Interactions between changes in the root system and protection of plant roots from pathogen attack have been demonstrated for several AM species. Matsubara et al. (1995) have shown that eggplants colonized by Glomus etunicatum or Gigaspora margarita contained
Microbial community changes in the rhizosphere
Different factors (e.g. altered exudation patterns, putative direct AM fungal effects, different root size and architecture, altered physiology) may contribute to quantitative and qualitative microbial community changes in the mycorrhizosphere caused by AM fungi (Andrade et al. 1998; Hodge 2000; Artursson et al. 2006; Toljanderet al. 2007; Finlay 2008). Changes due to the influence of single AM fungi on mycorrhizosphere microbial communities may in turn lead directly to the reduction of fungal
Activation of plant defense mechanisms
The recognition dialogue established between the host plant and AM fungi even before physical contact, results in specific shifts in the host's gene regulation (Genre et al. 2009; Oldroyd et al. 2009) with the consequent production of specific multifunctional compounds (i.e. both involved in transduction pathways and capable to confer disease resistance) (Pozo and Azcón-Aguilar 2007; Liu et al. 2007; van Wees et al. 2008). It is possible that the processes of AM fungal establishment rather than
Conclusions
Our discussion above suggests that different AM fungal species may confer protection of host plants from root pathogenic fungi via different mechanisms, although there is sparse evidence to either support or disprove this. Most studies do not only focus on single isolates but only on a few Glomus species (mostly G. intraradices and G. mosseae); this is likely not representative of the whole range of interactions between AM fungi and pathogens. There is presently only one report (Maherali and
Acknowledgements
We acknowledge funding through the Dahlem Center of Plant Sciences at Freie Universität Berlin during the preparation of this paper. P.M.A. acknowledges funding through the EU Marie-Curie IRG program.
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