Microbial Interactions in the Rhizosphere

The diverse genetic and functional groups of the extensive soil microbial populations are known to carry out activities exerting a critical impact on soil functions (Barea et al., 2005b; Avis et al., 2008). Among other activities, soil rnicroorganisms propel the biogeochemical cycling of nutrients and organic matter and improve plant performance and soil quality, key issues for agroecosystem selfsustainability (Gianinazzi et al., 2010; Jeffries and Barea, 2012). These topics are relevant not only for optimizing the stability and productivity of the agroecosystems but also to prevent erosion and to rninimize negative cultural and environmental stresses (Buscot, 2005). Micobial activities are particularly re1evant in the root-soi1 interface rnicrohabitat, known as the rhizosphere, a dynamic environment where rnicroorganisms interact with p1ant roots and soi1constituents (Bowen and Rovira, 1999). Strategic and app1ied research has demonstrated that microbial interactions in the rhizosphere can be managed, as a 10w input biotechno1ogy, to he1p sustainab1e environmentally friendly agrotechno1ogical practices (Ramos-Solano et al., 2009; Azcón and Barea, 2010). Thus, the formation, deve1opment, significance, functioning, and managing of rhizosphere rnicrobia1 populations are topics of current research interest, which have been reviewed recently (Berg and Smalla, 2009; Jones et al., 2009; Lambers et al., 2009; Hartmann et al., 2009; Dessaux et al., 2010). Particu1ar1y, the molecular determinants invo1ved in rhizosphere formation and functioning are receiving much attention (Matilla et al., 2007; Ferrer et al., 2008; Leitner et al., 2008; Faure et al., 2009; Mathesius, 2009; Tarkka et al., 2009; López-Ráez et al., 2012; Christensen and Kolomiets, 2011; Jousset et al., 2011; Santoro et al., 2011). The soi1 rnicrobiota is often separated into the so-called rnicroorganisms and the larger "microfauna" (Bowen and Rovira, 1999). Although it is acknowledged that microfauna affect plant growth and above-ground food webs (Scheu et al., 2005), this review will concentrate on rnicroorganisms and discuss the eco1ogical and molecular aspects of rhizosphere microbial interactions, analyzing (i) trophic and functional groups of microorganisms involved in rhizosphere interactions; (ii) direct rnicrobe-microbe interactions benefiting agroecosystems; and (iii) rnicrobial interactions invo1ving arbuscular mycorrhiza, the omnipresent fungal-plant (root) symbiosiso The research trends of this thematic area are thus outlined. As these topics were reviewed by us in Barea et al. (2005b), most information concems new advances in rhizosphere interactions.