ABSTRACT
Plants are surrounded by complex microbial communities (Tiedje et al., 1999; Torsvik et al., 1996). Direct interactions between plants and microbiota are well-known in the form of symbioses and pathogenesis and are discussed in other chapters of this volume. Of interest here is that microbial communities also exert indirect effects on plants. Indirect effects include such phenomena as soil formation, nutrient cycling (especially phosphorous and nitrogen mobilisation), acidification, disease suppression, detoxification and many more (Zhou et al., 2002). These phenomena are emergent properties of microbial activity in the plant environment rather than any specific plant-microbe interaction (Kent and Triplett, 2002; Robertson et al., 1997). Whether direct or indirect interactions are involved there are obviously numerous opportunities for feedback interactions between plant and microbial communities (Bever, 2003).
