The Rhizosphere: Molecular Interactions Between Microorganisms and Roots

Roots constitute important plant organs for water and nutrient uptake. They, however, also release a wide range of carbon compounds of lowmolecular weight. This release can amount up to 30% of total net fixed carbon (Smith and Read 2008; Rovira 1991) and forms the basis for an environment inhabited by a highly diverse and active microbial community, the rhizosphere (Hiltner 1904; Hartmann et al. 2008), which is defined as soil compartment influenced by living roots. The release of assimilates by plant roots results in a greater microbial density and activity in the rhizosphere than in the bulk soil. The specific conditions lead to the selection of distinct microbial communities, where fungi play an important role (Frey-Klett et al. 2005). Especially, symbiotic fungi (ecto/arbuscular mycorrhiza) release a substantial amount of plant-derived carbon to the soil, creating another sphere, the mycorrhizosphere. These organic carbon enriched spheres are highly attractive for other microorganisms. For example, the rhizosphere/bulk soil ratio for Gram-negative bacteria reaches from 2 to 20, for actinomycetes from 5 to 10, and for fungi from 10 to 20 (Morgan et al. 2005). The diversity and structure of bacterial communities are plant specific and vary over time (Smalla et al. 2001; Barriuso et al. 2005; Berg and Smalla 2009; Hartmann et al. 2009). Bacteria can have a negative, neutral, or beneficial effect to plant fitness. Detrimental effects are caused by