1. The drivers behind plant competition and diversity have long been debated, and there is acceptance that soil micro-organisms may act as key drivers in plant interactions and community structure. The evidence for a microbial role in shaping plant interactions and communities will be considered here with emphasis on symbionts and pathogens. 2. Microbial populations are themselves strongly influenced by the plant community via the ‘rhizosphere’ effect. The rhizosphere community includes microbes both beneficial and detrimental to the plant. Both the ability of plants to cultivate different rhizosphere microbial populations and the resulting impact upon other plant species have largely been studied via ‘plant–soil’ feedback studies, a proxy necessitated by the fact the majority of soil microorganisms are unculturable, but which nevertheless has rarely been used in conjunction with modern techniques to identify and quantify the micro-organisms involved. 3. Both microbial symbionts and pathogens can affect plant diversity and productivity, but the direct evidence for impacts on competitive interactions is surprisingly scarce. Evidence comes from biological invasions, the unintentional introduction of microbial pathogens to native plant communities and manipulative experiments under both field and controlled conditions. Pathogens generally have direct effects on plants, reducing their growth and so rendering them less competitive, whereas other symbionts may act by altering the availability of resources, with more subtle effects on competitive interactions. 4. Some of the best evidence for indirect effects comes from studies on arbuscular mycorrhizal (AM) fungi. New developments in our understanding of nutrient exchange in the arbuscular mycorrhizal symbiosis emphasize the need to view the fungal partners not as mere extensions of the plant. The suggestion that AM fungi may act to share resources among connected plants however remains to be proved. 5. Although plant competitive interactions can be driven by key microbial groups including symbionts and pathogens, knowledge gaps in the basic biology of these micro-organisms has hindered a full mechanistic understanding of these processes. If ecologists now embrace new technologies, significant advances in this area should be forthcoming.
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