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Abstract
Plant roots can establish associations with neutral, beneficial and pathogenic groups of soil organisms. Although it has been recognized from the study of individual isolates that these associations are individually important for plant growth, little is known about interactions of whole assemblages of beneficial and pathogenic microorganisms associating with plants.We investigated the influence of an interaction between local arbuscular mycorrhizal (AM) fungal and pathogenic/saprobic microbial assemblages on the growth of two different plant species from semi-arid grasslands in NE Germany (Mallnow near Berlin). In a greenhouse experiment each plant species was grown for six months in either sterile soil or in sterile soil with one of three different treatments: 1) an AM fungal spore fraction isolated from field soil from Mallnow; 2) a soil pathogen/saprobe fraction consisting of a microbial community prepared with field soil from Mallnow and; 3) the combined AM fungal and pathogen/saprobe fractions. While both plant species grew significantly larger in the presence of AM fungi, they responded negatively to the pathogen/saprobe treatment. For both plant species, we found evidence of pathogen protection effects provided by the AM fungal assemblages. These results indicate that interactions between assemblages of beneficial and pathogenic microorganisms can influence the growth of host plants, but that the magnitude of these effects is plant species-specific.
Highlights
Under natural conditions plant roots interact with different soil organisms, which can be beneficial, neutral or pathogenic
Performance of the plant species We generally found significant arbuscular mycorrhizal (AM) fungal and pathogen effects as well as a significant interaction in both Hieracium umbellatum and Galium verum
We found a significant decrease of these variables in the pathogen treatment compared to the controls for H. umbellatum (Table 4)
Summary
Under natural conditions plant roots interact with different soil organisms, which can be beneficial, neutral or pathogenic. The role of these interactions has been increasingly recognized [e.g., 1– 3]. Beneficial organisms like arbuscular mycorrhizal (AM) fungi may influence plant community structure, diversity [4,5] and productivity [6,7] in a natural ecosystem. While the AM fungus provided protection against Fusarium, it did not increase the nutrient status of the host plant. In natural ecosystems a mycorrhizal plant species is associated with one AM fungus but with an entire AM fungal assemblage [17], and, as a consequence, community-level emergent properties in determining the outcomes of AM fungal-pathogen interactions are poorly understood [18]. It is known that host plants are often exposed to several pathogens simultaneously [19], but there are only relatively few studies with multiple pathogens approaches [e.g., 20–22]
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