Chapter 11 Chemical Signalling in the Arbuscular Mycorrhizal Symbiosis: Biotechnological Applications Juan A. Lo ´pez-Ra ´ez and Marı ´a J. Pozo 11.1 Introduction Plants are living organisms that continuously communicate with other organisms, including microorganisms, present in their environment. Unlike animals, plants are sessile organisms that largely rely on chemicals as signalling molecules to perceive and respond to environmental changes. For instance, plants use molecules to recognize potential pathogens and defend themselves as well as to establish mutu- alistic beneficial associations with certain microorganisms in the rhizosphere. These beneficial associations belowground can affect plant growth and develop- ment, change nutrient dynamics, susceptibility to disease, tolerance to heavy metals and can help plants in the degradation of xenobiotics (Morgan et al. 2005). As a result, these plant–microorganism interactions have considerable potential for biotechnological exploitation. The rhizosphere is the narrow soil zone surrounding plant roots and constitutes a very dynamic environment. It harbours many different microorganisms and is highly influenced by the root and the root exudates (Bais et al. 2006; Badri et al. 2009). Plants produce and exude through the roots a large variety of chemicals including sugars, amino acids, fatty acids, enzymes, plant growth regulators and secondary metabolites into the rhizosphere, some of which are used to communi- cate with their environment (Siegler 1998; Bertin et al. 2003; Bais et al. 2006). Moreover, the release of root exudates together with decaying plant material provides carbon sources for the heterotrophic soil biota. On the other hand, micro- bial activity in the rhizosphere affects rooting patterns and the supply of available nutrients to plants, thereby modifying the quantity and quality of root exudates (Barea et al. 2005). Of special interest for rhizosphere communication are the so-called secondary metabolites from root exudates, which received this name J.A. Lo ´pez-Ra ´ez (*) • M.J. Pozo Department of Soil Microbiology and Symbiotic Systems, Estacio ´n Experimental del Zaidı ´n (CSIC), Profesor Albareda 1, 18008 Granada, Spain e-mail: juan.lopezraez@eez.csic.es; mariajose.pozo@eez.csic.es R. Aroca (ed.), Symbiotic Endophytes, Soil Biology 37, DOI 10.1007/978-3-642-39317-4_11, © Springer-Verlag Berlin Heidelberg 2013 215