Giving back to the community: microbial mechanisms of plant–soil interactions

The role of both plants and soil microbes on ecosystem functioning has been long recognized, but the precise feedback mechanisms between them are more elusive. Definition of these interactions is critical if we aim to achieve an integral understanding of ecosystem functioning, and ultimately explain natural, agricultural and synthetic systems. Advances in genomic technologies and the development of more appropriate statistical, mathematical and computational frameworks enable researchers to almost fully describe and measure the diversity of microbial communities in soil, rhizosphere and plant tissues. Under the scaffold of community ecology, we integrate the observed patterns of microbial diversity with current mechanistic understanding of plant–microbe mutualistic and pathogenic interactions, and propose a model in which plant microbial communities are shaped by different ecological forces differentially through the plant life cycle. The same genomic technologies, applied on natural and reconstructed systems, establish that plant genotype has a small, but significant, effect on the microbial community composition in, on and around plant organs. Despite these advances, technical limitations are still important and only a handful of studies exist where a precise genetic element definitively participates in these interactions. Studies at the field or ecosystem level are dominated by agricultural settings, examining microbial species and communities effects on plant productivity; and conversely, that plant genetics and agricultural practices can potentially impose selective pressures on specific microbes and microbial communities. Revitalized interest in plant–soil microbial feedbacks requires researchers to systematically pose and evaluate more complex hypotheses with increasingly more realistic microbial settings. Despite the advances reviewed here, most studies focus on one aspect of plant, microbe and soil interactions. Experiments that simultaneously and methodically manipulate multiple components are necessary to establish the ecological principles, and molecular mechanisms, which drive microbially mediated plant–soil interactions. This knowledge will be critical to predict how environmental changes affect microbial and plant diversity, and will guide efforts to improve agricultural and conservation practices.